Development of One-Step Post-Polymerisation Methods for ...

Development of One-Step

Post-Polymerisation Methods

for Semiconducting Polymers

Adam David Creamer

Submitted in partial fulfilment of the requirements for the degree of

Doctor of Philosophy in Chemistry

Imperial College London

September 2017

ii

“In this house we obey the laws of thermodynamics!”

Homer Simpson

iii

Declaration

The work described in this thesis was carried out in the Department of Chemistry, Imperial

College London and CSIRO Melbourne under the supervision of Professor Martin Heeney

and Dr. Fiona Scholes. The content of this thesis is the author’s original work, unless stated

otherwise.

Adam Creamer, September 2017

iv

Contributions

This work would not have been possible without the help and contribution of several

people:

In Chapter 2, density field theory calculations were performed with assistance from Adam

Marsh and Dr. Abby Casey.

In Chapter 3, natural transition orbital calculations were carried out by Dr. Karl Thorley.

Polymers 3.4-F and 3.5-F were synthesised by Dr. Abby Casey and 3.7-Copolymer was

synthesised by Shengyu Cong. Photoluminescence quantum yield (PLQY) was measured by

Iain Hamilton. MALDI was performed by Dr. Lisa Haigh.

The work in Chapter 4 was performed in collaboration with Dr. Chris Wood and processing

of time correlated single photon counting data in Chapter 4 was performed by Dr. Robert

Godin. Scanning transmission electron microscopy measurements in Chapter 4 were carried

out by Dr. Claire Burgess. Nanoparticle tracking analysis was carried out by Dr. Chris Wood.

Atomic force microscopy and optical microscopy measurements in Chapter 5 were

performed with the help of Dr. Yang Han.

Organic photovoltaic devices were fabricated by Dr. Pabitra Shakya Tuladhar in Chapter 5

and the inverted devices of 2.9 were fabricated by Dr. Munazza Shahid in Chapter 2.

Elemental Analysis was carried out by Dr. Stephen Boyer at London Metropolitan University.

v

Copyright

The copyright of this thesis rests with the author and is made available under a Creative

Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to

copy, distribute or transmit the thesis on the condition that they attribute it, that they do

not use it for commercial purposes and that they do not alter, transform or build upon it.

For any reuse or redistribution, researchers must make clear to others the licence terms of

this work.

vi

Acknowledgements

Firstly, I would like to thank my supervisor Professor Martin Heeney for the opportunity to

carry out my PhD within your group. You’ve always found the time to meet on a regular

basis and you made me feel the work I was doing was valued.

Thank you to CSIRO for providing the funding and allowing me to visit the labs twice over

the course of my studies. I would also like to thank Scott, Fiona, Chris, Regine, Anthony,

Dooji and the rest of the team for making me feel incredibly welcome on the other side of

the world. Also, a big thank you to Mei for teaching me how to make solar cells and Kallista

for helping with those painful EQE measurements.

I would like to thank the members of the McCulloch and Heeney groups for a great

Harwood lab experience. Thank you Abby, Josh, Pierre, Sam, Jess, Mike, Bob, Christian,

Alex, Iain M, Marsh, Tom, Notina, Iain A., Cameron, Mark, Karl, Matt, Andy, Fei, Brett….the

list goes on. I would also like to thank Pabitra for her tireless work on device fabrication in

the early stages of my PhD. Thank you to Dr. Chris Wood for an enjoyable final year in the

Stevens’ lab and Dr. Phil Howes for helping set up the collaboration.

I am also grateful to Pete Haycock and Dick Shephard for providing such a smooth running

NMR service and to Lisa Haigh for her excellent work in the mass spectrometry lab.

Thanks to my dad for always being on the other end of the phone and for persuading me to

pursue a career in science. Also, thanks to my brother Alex, for the great times in Australia

and New Zealand. I’d like to thank my London-based friends for making my four years here

such a wonderful experience. To my friends back in the North West, thank you for providing

a great distraction from my studies, I could not have wished for a greater group to be a part

of. Finally, I would like to give a massive thank you to my girlfriend Emma. Without your

continued love and support this work would not have been possible.

vii

Abstract

Organic semiconductors have several advantages over their inorganic counterparts. For

example, the solution processability allows for the fabrication of flexible, lightweight and

low-cost semiconducting devices. However, much work is needed to compete with the

high-performance of inorganics.

The first half of this work focuses primarily on nucleophilic aromatic substitution (SNAr)

reactions with molecules and polymers containing the fluorinated 2,1,3-benzothiadiazole

(BT) unit. Chapter 2 explores the effect the type of heteroatom on the BT unit (N, O and S)

of a carbazole-based polymer has on the optical properties and organic photovoltaic (OPV)

performance. The following chapter focuses on post-polymerisation SNAr reactions, in

which thiols were found to displace fluorine groups on a variety of polymers, including

polymers containing fluorinated benzotriazole (BTz) and thienothiophene (TT) units instead

of BT. The reaction was then taken a step further, by substituting fluorine atoms on

P(F8fBT) with a diverse range of end-functionalised thiols, thioacetates and alcohols. It was

also found that the amount of thiol substitution could be finely tuned, which led to the

development of multifunctionalised polymers.

The second half of this thesis explores the possible applications of the BT SNAr reaction

towards semiconducting polymer nanoparticles (SPN) and OPVs. In Chapter 4, SPNs from

polymers functionalised with azide and carboxylic groups were synthesised and the surface

was found to be reactive to strained-alkynes and amines, respectively. In the final chapter,

a trimethoxysilane-functionalised OPV polymer was synthesised and was found to increase

the thermal stability of devices via cross-linking, with no additives required.

viii

Table of contents

Declaration ................................................................................................................................ iii

Contributions ............................................................................................................................ iv

Copyright .................................................................................................................................... v

Acknowledgements ................................................................................................................... vi

Abstract .................................................................................................................................... vii

Table of contents .................................................................................................................... viii

Publications ............................................................................................................................... xi

Abbreviations ........................................................................................................................... xii

Chapter 1 Introduction ......................................................................................................... 1

1.1 Semiconducting polymers ........................................................................................... 2

1.1.1 Why semiconducting polymers? .......................................................................... 3

1.2 Conjugation and band-gap .......................................................................................... 5

1.3 Chemical design........................................................................................................... 8

1.3.1 Increasing planarity .............................................................................................. 8

1.3.2 Donor-acceptor approach .................................................................................. 10

1.4 Optical properties ..................................................................................................... 12

1.4.1 Absorption ......................................................................................................... 13

1.4.2 Emission ............................................................................................................. 15

1.4.3 Solvatochromism ............................................................................................... 18

1.4.4 Aggregation effects ............................................................................................ 18

1.5 Organic photovoltaics ............................................................................................... 20

1.5.1 The active layer .................................................................................................. 22

1.5.2 Characterising OPV performance ...................................................................... 22

1.5.3 Device architecture ............................................................................................ 27

1.6 Semiconducting polymer nanoparticles ................................................................... 29

1.6.1 Synthesis of SPNs ............................................................................................... 31

1.6.2 Characterisation of size ...................................................................................... 33

1.6.3 Optical properties of SPNs ................................................................................. 34

ix

1.6.4 Förster resonance energy transfer (FRET) ......................................................... 37

1.7 Thesis aims and overview ......................................................................................... 40

Chapter 2 Systematic tuning of 2,1,3-benzothiadiazole acceptor strength by mono-functionalisation with alkylamine, thioalkyl or alkoxy groups in carbazole donor-acceptor polymers ..……………………………………………………………………………………………………………………..…..47

2.1 Introduction............................................................................................................... 48

2.2 Synthesis of monomers and polymers ...................................................................... 52

2.3 Absorption spectra .................................................................................................... 56

2.4 Photoluminescence spectra ...................................................................................... 64

2.5 OPV performance ...................................................................................................... 68

2.6 Conclusion ................................................................................................................. 72

2.7 Experimental ............................................................................................................. 73

2.7.1 OPV Device Fabrication. ..................................................................................... 73

2.7.2 DFT and TD-DFT calculations ............................................................................. 74

2.7.3 Synthesis of monomers and polymers .............................................................. 75

Chapter 3 Post-polymerisation functionalisation of semiconducting polymers containing fluorinated electron deficient units ......................................................................................... 87

3.1 Introduction............................................................................................................... 88

3.2 Post-polymerisation reaction .................................................................................... 93

3.3 Control of thiol loading ........................................................................................... 103

3.4 Relating absorption properties to structure using DFT and TD-DFT ....................... 107

3.5 Post-polymerisation reaction with end-functionalised reagents ........................... 114

3.6 Synthesis of multifunctionalised polymers ............................................................. 124

3.7 Conclusion ............................................................................................................... 130

3.8 Experimental ........................................................................................................... 131

3.8.1 Fluorinated-polymer synthesis ........................................................................ 131

3.8.1 Alkylthiol substitution ...................................................................................... 137

3.8.2 Functionalised thiol, thioacetate and alcohol reactions with P(F8fBT) ........... 142

Chapter 4 The use of semiconducting polymers in nanoparticles towards surface functionalisation .................................................................................................................... 151

4.1 Introduction............................................................................................................. 152

4.2 Synthesis and characterisation of SPNs .................................................................. 154

4.3 Azide functionalised SPNs ....................................................................................... 160

x

4.4 Carboxylic acid functionalised SPNs ........................................................................ 170

4.5 Multifunctionalised SPNs ........................................................................................ 173

4.6 Conclusions.............................................................................................................. 179

4.7 Experimental ........................................................................................................... 180

4.7.1 Nanoparticle synthesis ..................................................................................... 180

4.7.2 Azide-DBCO click .............................................................................................. 180

4.7.3 EDC coupling of Amine-Biotin to SPN-COOH ................................................... 181

4.7.4 Multifunctional - EDC coupling ........................................................................ 182

4.7.5 Multifunctional - DBCO click ............................................................................ 183

4.7.6 Multifunctional – EDC coupling and DBCO click in tandem ............................. 183

Chapter 5 Post-polymerisation modification of semiconducting polymers towards improving stability of organic photovoltaics ......................................................................... 185

5.1 Introduction............................................................................................................. 186

5.2 Synthesis of polymers ............................................................................................. 190

5.3 Cross-linking study .................................................................................................. 197

5.4 Stability of cross-linked polymer OPVs ................................................................... 200

5.5 Study of morphology ............................................................................................... 206

5.6 Conclusion ............................................................................................................... 210

5.7 Experimental ........................................................................................................... 211

5.7.1 Monomer and polymer synthesis .................................................................... 211

5.7.2 Device fabrication ............................................................................................ 214

References ............................................................................................................................. 217

General Experimental ............................................................................................................ 231

Appendix ................................................................................................................................ 235

Chapter 2 ............................................................................................................................ 235

Chapter 3 ............................................................................................................................ 237

Chapter 4 ............................................................................................................................ 251

Chapter 5 ............................................................................................................................ 257

Permissions ............................................................................................................................ 260

xi

Publications

Publications relating to the work carried out in this thesis are as follows:

Chapter 2:

1) Adam Creamer, Abby Casey, Adam V. Marsh, Munazza Shahid, Mei Gao, and Martin

Heeney, Macromolecules, 2017, 50 (7), 2736–2746

Manuscripts for the work carried out in Chapters 3 and 4 are currently being drafted.

Other publications (this work is not discussed in this thesis):

2) Jonathan Marshall, Jake Hooton, Yang Han, Adam Creamer, Raja Shahid Ashraf, Yoann

Porte, Thomas D Anthopoulos, Paul N Stavrinou, Martyn A McLachlan, Hugo Bronstein,

Peter Beavis, Martin Heeney, Polymer Chemistry, 2014, 21 (5), 6190-6199.

xii

Abbreviations

° Degrees 18-crown-6 1,4,7,10,13,16-hexaoxacyclooctadecane AFM Atomic force microscopy AIE Aggregation-induced emission Amine-Biotin O-(2-Aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol Anal. Elemental analysis BHJ Bulk heterojunction BT 2,1,3-Benzothiadiazole BTz Benzo[d][1,2,3]triazole CHCl3 Chloroform Ð Dispersity index DBCO Dibenzocyclooctyne DCM Dichloromethane DFT Density functional theory DLS Dynamic light scattering DMF N,N-Dimethylformamide DMSO Dimethylsulfoxide EA Electron affinity EDC 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide EDX Energy-dispersive x-ray EFRET FRET efficiency Eg Band-gap EI Electron ionisation EQE External quantum efficiency ES Excited State ESI Electrospray ionisation eV Electron volts EWG Electron withdrawing group F8BT Poly(9,9-dioctylfluorene-alt-benzothiadiazole) FBS Fetal bovine serum FF Fill factor FRET Förster resonance energy transfer GPC Gel permeation chromatography GS Ground state h Hours HOMO Highest occupied molecular orbital HTL Hole transport layer

xiii

ICT Intramolecular charge transfer IP Ionisation potential ISC Intersystem crossing ITO Indium tin oxide JSC Short circuit current kDa Kilodalton LDPE low density polyethylene LUMO Lowest unoccupied molecular orbital m/z Mass to charge ratio

MALDI-TOF Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry

MgSO4 Magnesium sulfate min Minutes Mn Number averaged molecular weight Mpt Melting point MS Mass spectrometry MsOH Methanesulfonic acid Mw Weight averaged molecular weight NBS N-Bromosuccinimide NFA Non-fullerene acceptor NIR Near-infrared nm Nanometre NMR Nuclear magnetic resonance NTA Nanoparticle tracking analysis NTO Natural transition orbital OPV Organic photovoltaic PC61BM [6,6]-Phenyl-C₆₁-butyric acid methyl ester PC71BM [6,6]-Phenyl-C7₁-butyric acid methyl ester PCE Power conversion efficiency Pd(PPh3)4 Tetrakis(triphenylphosphine)palladium(0) PEDOT:PSS Poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate PEG Polyethylene glycol PESA Photoelectron spectroscopy in air PL Photoluminescence PLQY photoluminescence quantum yield PTFE Polytetrafluoroethylene PVC polyvinyl chloride POM Polarised optical microscopy ppm Parts per million PSMA Poly(styrene-co-maleic anhydride) QD Quantum dot rpm Revolutions per minute

xiv

SPN Semiconducting polymer nanoparticle SNAr Nucleophilic aromatic substitution STEM Scanning transmission electron microscopy Sulfo-NHS N-hydroxysulfosuccinimide TBAF Tetra-n-butylammonium fluoride TCSPC Time-correlated single photon counting TD-DFT Time-dependent density functional theory TFA Trifluoroacetic acid THF Tetrahydrofuran TT Thieno[3,4-b]thiophene UHMWPE Ultra-high weight polyethylene UV-Vis Ultraviolet/visible v:v Volume ratio VOC Open circuit voltage α Absorption coefficient δ Chemical shift Δλ Stokes shift λ Wavelength μ Micro τ Fluorescence lifetime

1

Chapter 1 Introduction

2

1.1 Semiconducting polymers

Synthetic polymers have changed the world we live in. In 1907, Baekeland synthesised the

first synthetic polymer, Bakelite.1 A huge range of materials have been developed since

then, a huge industry has been built around the ability to design and synthesise new

materials with controlled properties by polymerisation of monomeric building blocks.

Control of the polymerisation conditions can result in incredible changes in physical

properties from the high strength fibres of ultra-high weight polyethylene (UHMWPE) to the

flexibility of low density polyethylene (LDPE). Control of the chemical structure of the

building blocks can also afford a huge range of physical properties, from the durable

polytetrafluoroethylene (PTFE) used in industrial coatings to the stretchy plastic wrap made

from polyvinyl chloride (PVC).

In 1977 Heeger, MacDiarmid, Shairakawa and co-workers showed that films of trans-

polyacetylene could be doped with halogens to give a marked enhancement in

conductivity.2 This was the first example of a synthetic polymer employing semiconducting

properties. Much like the explosion of new materials after the synthesis of Bakelite 70 years

previous, this discovery led to the development of a huge variety of conducting and

semiconducting polymers and the creation of entirely new field based on organic

semiconductors. Heeger, MacDiarmid and Shairakawa were awarded the Nobel Prize in

2000 as a result.3

3

1.1.1 Why semiconducting polymers?

A semiconductor is a material with conductivity between that of a metal and an insulator.

In a metal there is no separation in energy between the valence (occupied energy levels)

and conduction bands (unoccupied energy levels), therefore electrons are free to move

throughout the material. In an insulator the separation between the valence and

conduction band, known as the band-gap, is too large for the electrons to occupy the

conduction band. However, in a semiconductor, the band-gap is small enough to allow for

conductivity under controlled circumstances. The majority of commercial semiconducting

devices are made from ultra-pure crystalline silicon, in which the conductivity is controlled

by chemical doping. Although they are widely used, silicon semiconductors do have some

drawbacks.

The first limitation is the high manufacturing cost to obtain the high purity of silicon

required for efficient operation.4 While there is no shortage of silicon atoms on earth, an

enormous amount of energy is required to isolate them and form single crystals. This is

usually achieved in an electronic arc furnace which reaches temperatures of over 1500 °C.

The environmental impact of such high energy generation is also a consideration. Once

pure silicon is produced, the manufacture of devices also incurs high costs. For example,

computer chips require a highly complex process involving etching and doping which have

to be performed under very controlled conditions, to avoid further contamination.

4

A second drawback is the mechanical properties of silicon. In its crystalline form it is very

brittle, making the majority of semiconducting devices only suitable for flat substrates. This

has led to the development of flexible devices constructed from amorphous silicon.5

However, this comes at the cost of a drop in device performance.

A third drawback is the lack of tuning of the optical properties of silicon, this is particularly

important in photovoltaic and light emitting applications. This has led to the development

of semiconducting alloys, such as GaAs or CdTe, in order to tune the band-gap.6 However,

these are not without their own issues, such as high processing costs, toxicity and

abundance of raw materials.

The development of organic semiconductors offers an alternative to silicon (and other

inorganic) semiconductors. The production of semiconducting polymers (and small

molecules) has the potential to be at a relatively low cost.7 In addition, polymers can be

solution processed, potentially lowering device manufacturing costs enormously. Polymers

can also be processed onto flexible substrates, allowing for the development of light-weight,

flexible electronic devices.8

This introduction will focus on how a band-gap is formed in semiconducting polymers and

how this can be manipulated through chemical design. The optical properties of

semiconducting polymers will then be discussed, followed by a description of two

applications which require very different optical properties from semiconducting polymers:

organic photovoltaics (OPV) and fluorescent semiconducting polymer nanoparticles (SPNs).

5

1.2 Conjugation and band-gap

The valence shell of a carbon atom has two electrons in the 2s orbital and a further two

electrons in the three 2p orbitals (px, py and pz). As the energies of the 2s and 2p orbitals are

similar they can undergo hybridisation, where the 2s orbital mixes with between one and

three of the 2p orbitals to give a occupied hybrid orbitals with an overall lower energy. The

number of orbitals that mix determines the type of hybridisation that occurs and in-turn the

orientation and type of bonds around the carbon atom. To illustrate this, hybridisation in

ethane, ethene and ethyne are considered (structures in Figure 1.1).

Figure 1.1: Orbital diagram for ethane, ethene and ethyne with sp3, sp

2 and sp hybridisation, respectively.

On the carbon atoms in ethane, all four orbitals mix, meaning both carbons have four sp3

hybrid orbitals each. These orbitals have an electron in each, which arrange in a tetrahedral

geometry around the carbons and form σ-bonds with the other carbon (and three further σ-

bonds with hydrogen). In ethene, the 2s orbital mixes with the px and py orbitals meaning

each carbon has three sp2 hybrid orbitals. In this case these three orbitals arrange in a

6

trigonal planar arrangement around the carbon, which again can bond to another carbon

(and a further two hydrogens each). This leaves two un-hybridised pz orbital perpendicular

to the other bonds. These orbitals can overlap, forming a delocalised π-bond above and

below the plane of the molecule, i.e. a double bond (Figure 1.1). The final example is

ethyne, which employs sp hybridisation. The pz and py orbitals are left un-hybridised, which

overlap to form two π-bonds. This results in a triple bond between the carbon atoms.

In conjugated molecules, the bonding consists of alternating single and double (or triple)

bonds. The un-hybridised pz orbitals overlap across neighbouring atoms, forming an

extended π network. When two pz orbitals overlap they generate a bonding (π) and anti-

bonding (π*) molecular orbital, lower and higher in energy respectively. The two bonding

electrons occupy the bonding orbital and leave the anti-bonding orbital vacant, known as

the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular

orbital) respectively. This generates an energy gap between the HOMO and LUMO, which at

7.6 eV happens to be too large for ethene to be considered a semiconductor (Figure 1.2).

If the chain is extended by an ethene unit, forming butadiene, the result is four pz orbitals

that can overlap. This generates two new π-orbitals of slightly higher and lower energy,

than in ethene, which the electrons fully occupy. This increases the energy of the HOMO

slightly. The analogous case occurs for the π* orbitals, which decreases the energy of the

LUMO slightly. The formation of butadiene decreases the energy gap between the HOMO

and LUMO (Figure 1.2). This trend continues for hexatriene and beyond. Effectively by

7

increasing number of overlapping pz orbitals, the energy gap between the HOMO and LUMO

decreases.

Figure 1.2: Illustration of the decreasing band-gap with increasing number ethene units, due to the increasing

number of π-bonds.

In theory, applying this trend to much longer ethene chains (i.e. polyacetylene), the HOMO

and LUMO energies would be expected to eventually merge, effectively creating a metallic

polymer. However, this is not the case due to the phenomenon known as Peierls instability,

which states that a one-dimensional metal must undergo a perturbation in bond lengths to

decrease the energy of the system.9 This effectively causes an intrinsic energy gap (or band-

gap) to form between the HOMO and LUMO. The band-gap is now small enough at 1.5 eV

for polyacetylene to be considered a semiconductor (Figure 1.2).

8

The point at which the band-gap no longer decreases with additional repeat units is known

as the effective conjugation length (ECL) and this is typically reached by approximately ten-

twenty repeat units.10 At this point the band-gap becomes independent of the length of the

polymer chain (the molecular weight),11 however, the band-gap can be altered further by

chemical design.

1.3 Chemical design

The control of the band-gap is crucial for the applications of semiconducting polymers, for

example, the band-gap in organic light emitting diodes (OLED’s) largely determines the

colour of the light emitted. The importance of band-gap for organic photovoltaics will be

discussed in more detail in section 1.5.

1.3.1 Increasing planarity

In order for semiconducting polymers to be solution processable, aliphatic side-chains are

usually required. As well as improving solubility, they are also useful in influencing the

band-gap. They can also be used to influence morphology, but this will be discussed further

in section 1.4.4. In general, the more planar a polymer-backbone is the more efficient the

overlap of the pz orbitals and the smaller the band-gap becomes as a result. In

polythiophene, for example, if the side-chains are positioned pointing towards each other

(head-to-head) a steric clash causes the polymer to twist. If the side-chains are positioned

9

away from each other (head-to-tail) the polymer can keep its planar form and the band-gap

is smaller as a result.12–14 This is illustrated in Figure 1.3a.

There are other ways of manipulating the planarity without using aliphatic side-chains. For

example, fluorine and sulfur have been shown to interact favourably through space.15 This

in turn has led to the development of fluorinated aromatic units adjacent to thiophene

units. The favourable interaction results in a more planar structure, compared to the non-

fluorinated analogue, illustrated in Figure 1.3b.16

Figure 1.3: Examples of strategies employed to form a more planar backbones: a) side-chain positioning to reduce torsional strain

17, b) fluorination of backbone

16 and c) forcing quinoidal character by ring fusing with 3-

fluorothieno[3,4-b]thiophene as an example.18

Another way to favour planarity is to increase the quinoidal character of the repeat units i.e.

decrease the aromaticity.18,19 This causes the single bonds between repeat units (where

twisting can occur) to have more double bond character (in which twisting is more

10

restricted). This can increase planarity and decrease the band-gap. This can be achieved in

thiophenes, for example, by fusing the thiophene ring with a further ring such that there is

an energetic advantage of restoring aromaticity in the fused ring, at the expense of reducing

aromaticity in the thiophene.20 An example of this is illustrated in Figure 1.3c, with 3-

fluorothieno[3,4-b]thiophene.18

1.3.2 Donor-acceptor approach

Another way of reducing the band-gap is to copolymerise an electron-deficient monomer

with an electron-rich comonomer.21–24 The resulting polymer is referred to as a donor-

acceptor polymer. The electron-deficient monomer (acceptor) will typically have a lower

lying HOMO and LUMO than the electron-rich (donor) analogue. Much like before, these

molecular orbitals can mix to form hybrid molecular orbitals (Figure 1.4). The mixing of the

HOMO orbitals of the respective monomers results in two filled orbitals, higher and lower in

energy than the original orbitals. This generates a new HOMO, higher in energy. The

analogous process occurs for the LUMO orbitals - they mix, generating two vacant hybrid

orbitals and a new LUMO, lower in energy. The overall band-gap (ΔE) is therefore smaller

than the analogous polymers consisting of just donor or acceptor.

The donor-acceptor approach often results in a ‘dual-band’ absorption profile, consisting of

a low energy transition (from the HOMO-LUMO in Figure 1.4) and a high energy transition

from the HOMO to a higher-lying LUMO. For example, F8BT (depicted in Figure 4) exhibits

two absorption bands. Theoretical calculations have shown that the low energy band is

11

from the HOMO to LUMO transition and the high energy band is a result a transition from

the HOMO to the 9th LUMO level.23 In this case the orbitals are both delocalised along the

polymer backbone, which is often referred to as the π – π* transition.

Figure 1.4: Molecular orbital diagram illustrating the formation of hybrid orbitals from the overlap of the HOMO and LUMO of the donor and acceptor. Illustration of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) included as an example of a donor-acceptor polymer.

From the molecular orbital diagram (Figure 1.4) it can be seen that the hybrid HOMO more

closely resembles the HOMO of the isolated donor, than the acceptor. Conversely, the

hybrid LUMO more closely resembles the isolated acceptor LUMO. This means that altering

chemical structures of the donor and acceptor independently can give rise to excellent

control of the HOMO and LUMO levels, respectively. This is crucially important as the

12

position of the LUMO and HOMO, relative to the energy levels of other components in a

device (such as the electrodes), is essential for a working device. Fine-tuning of the HOMO

and LUMO is often done by manipulation of the electron density of the donor and acceptor

moieties.12,25,26

1.4 Optical properties

A semiconducting polymer’s associated band-gap is key to the way it interacts with light.

Upon absorption of a photon with energy greater than the band-gap, a π-electron from the

HOMO can jump to the LUMO (or to higher states). This generates an electron-hole pair,

bound by Coulombic forces, called an exciton. An exciton can either collapse

(recombination of electron and hole) or dissociate into a free hole and electron.27 They do

not readily dissociate because of the attract Coulombic forces, weakly shielded by the small

dielectric constant of organic semiconductors.28 This is in contrast to inorganic

semiconductors where excitons can dissociate easily, due to the high dielectric constant of

the materials.29

A number of different phenomena must be understood in order to explain various aspects

of semiconducting polymers such as absorption, emission, solvatochromism and

aggregation effects.

13

1.4.1 Absorption

On formation of an excition, the bonds located near the exciton need to rearrange in order

to stabilise the quasi-particle. This means that the nuclei positions in the ground state are

different to the excited state. However, the time scale of absorption of a photon (10-15 s) is

much faster than the rearrangement of the bonds (10-12 s),30 therefore, an electronic

transition must occur before a structural rearrangement can. This is an application of the

Frank-Condon principle. A simplified illustration of this is can be seen in Figure 1.5. An

optical transition from HOMO to LUMO occurs vertically (i.e. with no change in nuclear

positions) from the vibronic ground state of the HOMO to excited vibrational energy levels

in the LUMO (i.e. not to the vibronic ground state of the LUMO). Theoretically these distinct

transitions should result in sharp defined absorption peaks for each vibronic state, however

in reality the large amount of disorder in polymers causes the absorption peaks to broaden

and form a relatively featureless absorption band.

Each vibrational energy level has a wavefunction associated with it. This can be thought of

as a standing wave oscillating between the walls of the electronic energy level. The

efficiency of the overlap between the wavefunctions of the HOMO and LUMO vibrational

energy levels determines the efficiency of the transition and hence the magnitude of the

absorption peaks. In Figure 1.5, the ground state has been arbitrarily chosen to have the

most efficient overlap with the third vibrational energy level of the LUMO, resulting in the

14

tallest absorption peak. After formation of an exciton, the bonds can then rearrange

(vibrationally relax) to the local electronic minima.

It should be noted that the fundamental band-gap (the energy difference between the

HOMO and LUMO) is actually larger than the optical band-gap (energy required to promote

energy to the excited state) due to the Coulombic forces which hold the exciton together.31

Figure 1.5: Illustration of the process which occurs upon the absorption of a photon. An electron from the HOMO is promoted vertically (no change in nuclear position) to the descrete vibrational energy levels in the LUMO. The absorption graph of this system is depicted, with an example of a polymer absorption band underneath.

As discussed previously, in donor-acceptor polymers there are often two dominating

absorption bands. The lowest energy band is the HOMO-LUMO transition from the hybrid

orbitals, referred to as the intramolecular charge transfer band (ICT). The higher energy

band is the transition from the HOMO to a higher lying LUMO. It is often referred to as the

15

π- π* band as both orbitals are often distributed over the polymer backbone (resembling a π

bond).

1.4.2 Emission

Once the exciton has formed and stabilised, it can decay through a number of mechanisms:

fluorescence, phosphorescence and by non-radiative pathways. These are illustrated in the

Jablonski diagram in Figure 1.6. The solid arrows denote processes which occur via emission

of a photon (radiative).

Figure 1.6: Jablonski diagram illustrating the decay path upon the formation of an exciton. Solid lines denote radiative pathways, dashed lines show the non-radiative.

Under fluorescence the electron and hole recombine and release the extra energy as a

photon. Similar to absorption, the electron falls vertically to the excited vibrational energy

levels of the electronic ground state as a result of the Frank-Condon principle (Figure 1.7).

This results in the emitted photon exhibiting an overall lower energy (higher wavelength)

16

that the absorption. The magnitude of this difference in energy is known as the Stokes shift.

The graph in Figure 1.7 shows the absorption and emission bands as mirror images.

However, in polymeric systems, this is often not the case due to exciton migration to an

area of lower energy or by forming excitons across polymer chains.32

Figure 1.7: Illustration of the process which occurs upon the emission of a photon. An electron from the LUMO falls vertically (no change in nuclear position) to the descrete vibrational energy levels in the HOMO. The absorption and emmision graph of this system is depicted, with an example of a polymer emission band underneath.

Another mechanism of exciton decay is phosphorescence. Excitons can take two forms,

triplets or singlets, labelled as Tx and Sx (where x denotes the electronic state occupied).

Triplets occur when the overall spin of the electron-hole pair has a value of 1 (where each

spin as a value of ½), this occurs when the spins are parallel (and also when they are a linear

superposition of anti-parallel states, which is beyond the scope of this thesis). Singlet

excitons occur when the overall spin has a value of zero. The vast majority of excitons

17

formed via optical excitation are singlets because the electrons occupying the HOMO have

their spins paired in the ground state (with overall spin of zero), due to the Pauli Exclusion

Principle. Promotion of an electron keeps the spins paired (as a change of spin is forbidden)

which preserves the overall spin. However, the spins can flip after formation of the exciton

in a process called intersystem crossing (ISC), forming a triplet state (Figure 1.6). The triplet

state formed (T1) has a lower energy than the corresponding singlet (S1) due to Hund’s Rule,

which states that the lowest energy state is one that achieves a maximum total spin number

i.e. when the spins are parallel. The triplet state can then decay to the ground state via

phosphorescence (where a spin must again flip), emitting a photon of lower energy (longer

wavelength) than from the fluorescent equivalent. Due to the forbidden processes

required, phosphorescence is much less common than fluorescence in conventional organic

semiconductors and occurs in the order of microseconds or longer.30

Alternatively, the exciton can decay via a non-radiative path (black line, Figure 1.6). The

electron can fall to the ground state through vibrations and interactions with neighbouring

molecules, including solvents. In this case, the excess energy is dissipated as heat.

Controlling the way excitons behave upon formation is crucial to the application. For

example, in organic photovoltaic devices the collapse of excitons is undesired and the device

is engineered to increase the chances of forming free electrons and holes, generating

current. On the other hand, in semiconducting polymer nanoparticles the fluorescent decay

of an exciton is desired and polymers are designed to preferentially decay via this radiative

pathway. These two examples will be discussed in more detail later.

18

1.4.3 Solvatochromism

As discussed previously, in donor-acceptor polymers, hybrid HOMO and LUMO’s are formed

in order to lower the band-gap. In donor-acceptor polymers these orbitals are usually

distributed differently along the polymer backbone. For example, in the case of poly(9,9-

dioctylfluorene-alt-benzothiadiazole) (F8BT, Figure 1.4), the LUMO is localised on the

benzothiadiazole (BT) unit and the HOMO is distributed along the polymer backbone.23 This

means that when an exciton is formed, the electron it is located primarily on the acceptor

unit whereas the hole in the HOMO is delocalised along the polymer backbone. This

separation of charges gives the excited state charge-transfer characteristics. The excited

state is polar and can therefore be stabilised by polar solvents. This results in emission of a

photon lower in energy (more red-shifted) than if the polymer was dissolved in a less polar

solvents.33 This phenomenon is known as solvatochromism.

1.4.4 Aggregation effects

A polymer chain can be thought of as a series of chromophores (i.e. π-electrons conjugated

over different numbers of repeat units) separated by kinks and other defects in the polymer

chain. When individual polymer chains start to interact with neighbouring chains the

chromophores can overlap, causing aggregation effects. Aggregation has been shown to

increase the chances of non-radiative decay of excitons, and decrease the efficiency of

fluorescence as a result, known as aggregation induced quenching.34–36 Aggregation occurs

more readily when the aromatic units of separate polymer chains can efficiently stack. This

19

has been demonstrated by the work of Han et al., in which the removal of bulky side-chains

increased the extent of aggregation.37

Figure 1.8: Illustration of the overlap of chromophores yielding two distinct energy levels resulting in different allowed transitions, depending on the type of aggregation between chromophores. Red and blue lines represent the allowed absorption and emission pathways, respectively. It should be noted that the energy levels of J- and H-aggregates would differ in reality, but this has been overlooked for simplicity.

It may come as no surprise that there are exceptions to the above. Aggregation can also

increase the chances of radiative decay in certain circumstances. When two chromophores

come in close proximity, the molecular orbitals can overlap and split into two distinct energy

levels of higher and lower energy.38 Depending on the alignment of the chromophore

dipoles, either H- or J-aggregation can occur (Figure 1.8). Under H-aggregation the

transition to the higher energy level is allowed, resulting in a lower wavelength of

absorption (when compared to the non-aggregated analogue). This forms an exciton of

higher energy (technically, any exciton formed in an aggregated state should be referred to

as an excimer or exciplex (depending on the type of overlap) but for simplification ‘exciton’

20

has been used here). The radiative decay to the ground state has to occur from the lowest

energy level of the excited state. Therefore, forbidden transitions must occur in order to

achieve this, resulting in weak emission from H-aggregate excitons. However, in J-

aggregation, the transition to the lower energy level is now allowed (resulting in a longer

wavelength of absorption) (Figure 1.8). The exciton can now freely decay radiatively to the

ground state, resulting in strong emission from J-aggregates excitons, with a longer

wavelength (lower energy) than the non-aggregated excitons.39 H- and J-aggregation are

terms which were originally used to describe small-molecule interactions,40 therefore

exactly how H- and J-aggregates manifest in real polymeric systems is relatively poorly

understood.

1.5 Organic photovoltaics

Organic photovoltaics (OPVs) have received a great deal of attention over the last two

decades.41 The solution processable components have allowed for the development of

flexible, light-weight and low-cost devices. Although conventional single-crystal silicon cells

exhibit efficiencies much greater than OPVs, they are relatively expensive to fabricate,

inflexible, heavy and opaque to majority of the the solar spectrum. OPVs can therefore

succeed in tasks that conventional devices would be unsuitable for. A good example of this

is the African Union Peace and Security building in Addis Ababa, which has the glass roof

decorated with semi-transparent OPVs in the shape of the African continent.42

21

The efficiency of OPVs has steadily increased over the years from 1% in 199543 to 11.7% in

2017.44 However, these efficiencies were measured on lab-scale cells (approximately the

size of a postage stamp). Large-scale devices with efficiencies now approaching 5% have

been achieved.45

Although this thesis primarily focuses on donor-acceptor organic photovoltaic devices, a

number of other alternatives to silicon have been developed over the last thirty years.

Perovskite and dye sensitised solar cells have been heavily researched, as they have many of

the desirable properties of OPVs. A perovskite solar cell consists of a heavy-metal halide

hybrid, commonly solution processed via a methylammonium lead trihalide intermediate, in

which the halide can be changed to tailor the band-gap. Unlike OPVs, perovskite solar cells

have achieved efficiencies rivalling that of silicon devices (22.1%).46 However, there are

doubts about the toxicity of the heavy-metal components.47 Dye-sensitized solar cells have

received similar attention to OPVs and have presented similar efficiencies (11.9%).48

However, device efficiencies have not shown much improvement in the past two decades.46

One big issue is that both perovskites and dye solar sensitised cells have major stability

issues in the presence of UV-light, water or oxygen.49 A relatively recent development in

solar technologies has also been quantum dot solar cells, which have efficiencies already

exceeding OPVs (13.4%) in under 10 years of development.46 However, again the toxic

nature of the heavy-metal components (commonly cadmium) could be a barrier to

commercial application.50

22

1.5.1 Characterising OPV performance

The efficiency of a solar cell can be represented by the power conversion efficiency (PCE),

which simply is the useful power output of the device (electrical energy) as a percentage of

the power input (PIn) where power is the product of the voltage and current. The power

input can be represented as the product of three components: the open circuit voltage

(VOC), short circuit current (JSC) and fill factor (FF). Therefore, the PCE can be represented as:

𝑃𝐶𝐸 (%) =𝐽𝑆𝐶𝑉𝑂𝐶𝐹𝐹

𝑃𝐼𝑛 × 100

The VOC is the maximum voltage generated at a current of zero. It is closely related to the

energy difference between the HOMO of the p-type and LUMO of the n-type material

(labelled ΔE2 in Figure 1.10) and is sensitive to the loss mechanisms discussed previously.51

The JSC refers to the current generated when no voltage is applied. This value is heavily

dependent on the illumination area and intensity, therefore JSC is usually represented as a

function of the unit area and devices are illuminated under standardised conditions. The JSC

is heavily dependent on how efficiently excitons can dissociate and free charges are

collected.52 A difficult compromise in OPV devices is the need for a low band-gap, to

maximise the number of photons absorbed (and as a result the JSC), whilst keeping a low

lying HOMO, to maximise the VOC.53

23

Figure 1.9: J-V curve of a typical OPV device under illumination.

The final component is the fill factor (FF). Qualitatively it represents how closely the power

generated under device illumination reflects the maximum power possible. It is

represented as the maximum power generated when the device is illuminated divided by

the maximum power the device could generate (i.e. the product of the JSC and VOC). The

current and voltage at the point of maximum power (JMP and VMP, respectively) can be

calculated by measuring current as a function of a sweeping voltage, under device

illumination, known as a J-V curve (Figure 1.9). The fill factor is therefore represented as:

𝐹𝐹 = 𝐽𝑀𝑃𝑉𝑀𝑃

𝐽𝑆𝐶𝑉𝑂𝐶

24

An additional useful parameter is the external quantum efficiency (EQE).54 This is a measure

of the number of charges collected at each wavelength. Under ideal circumstances, the EQE

at each wavelength would be 1, indicating all photons have generated free charges,

however this is rarely observed due to the loss mechanisms discussed previously. Also,

photons with energy lower than the optical band-gap of the polymer (at high wavelengths)

cannot generate charges so exhibit an EQE of zero.

1.5.2 The active layer

Typical OPV devices consist of an electron accepting (n-type) and electron donating (p-type)

material, referred to as the active layer, sandwiched between two electrodes. Commonly,

the p-type material is a semiconducting polymer and the n-type is a fullerene derivative.

Fullerenes have week absorption properties so it is assumed that light mainly interacts with

the polymer material.55 As discussed previously, upon the absorption of a photon, an

exciton is generated where it can then diffuse approximately 10 nm before it decays.32 If

the exciton meets a junction between the acceptor and donor (p-n junction) the electron

can transfer from the LUMO of the polymer to that of the fullerene (Figure 1.10).56 This

process can only occur if the difference in the LUMO energies (labelled ΔE1, Figure 1.10) is

large enough to overcome the Coulombic forces between electron and hole.57 If this

dissociation does occur the free charges can then travel to their respective electrodes,

generating a current. However, this does not always occur resulting in loss mechanisms.51

Excitons can simply decay before having reached a p-n junction, but also even if the electron

25

and hole have separated at a p-n junction they can still recombine at that same junction -

this is known geminate recombination.58 Free electrons and holes can also recombine in the

bulk layer, known as non-geminate recombination.59

Figure 1.10: Simplified illustration of the energy levels in an OPV device with electron-hole separation at the p-n junction followed by transport to the relevant electrodes.

Due to the short diffusion length of excitons (~10 nm), blending the n- and p-type materials

into a bicontinuous network has been the most popular strategy, forming a bulk

heterojunction (BHJ) device.43,60 This strategy maximises the interfacial area between

acceptor and donor but requires careful control of the morphology of the two

components.61 The domains of acceptor and donor need to have percolation pathway for

charges to meet their respective electrodes. They also need to be small enough in size to

allow a maximum number of generated excitons to diffuse to a p-n junction before

decaying.62

26

Fullerenes have been the acceptor of choice for years, mainly due to the low-lying LUMO

and HOMO. The most common fullerene used is phenyl-C61-butyric acid-methyl ester

(PC61BM) (structure depicted in Figure 1.11), based on a mono-substituted C60. This

molecule is almost perfectly symmetrical which results in a very small absorption of light, an

undesired property for an active component of a solar cell. An asymmetrical alternative is

available, phenyl-C71-butyric acid-methyl ester (PC71BM) in which the fullerene sphere is

distorted along one axis. This results in an improvement in light absorption, usually

improving device performance when used instead of PC61BM.55 However, both fullerene

derivatives are relatively expensive and PC71BM is approximately three times the price of

PC61BM.41 Another issue is that fullerenes have been shown to crystallise out of the BHJ

over time, resulting in dramatic drops in device efficiency (this topic will be discussed

further in Chapter 5).63 This has led to the development of non-fullerene acceptors (NFAs),

commonly small molecules which have similar LUMO levels to fullerenes. They can absorb

light more effectively, have a potential low-cost and cause less stability issues.64 Also it is

important to note that using non-fullerene acceptors allows for careful tailoring of the

HOMO and LUMO levels, which has been performed on donor materials for years. The use

of non-fullerene acceptors in devices has recently shown to outperform the fullerene-

containing analogues.65–67

27

Figure 1.11: Chemical structure of phenyl-C61-butyric acid-methyl ester (PC61BM).

1.5.3 Device architecture

OPV device consists of an active layer sandwiched between two electrodes of which one

electrode must be transparent to allow the absorption of light. Between each electrode and

the active layer there is a transport layer which facilitates the transport of either holes or

electrons to the neighbouring electrode.

There are two ways of arranging these electrodes, either using the conventional

architecture or the inverted architecture (Figure 1.12). Under a conventional setup a

transparent anode with a high workfunction is deposited onto a transparent substrate

(usually glass in lab scale devices). The vast majority of conventional devices use indium tin

oxide (ITO) for this purpose. The ITO is then coated with a hole-transporting layer, typically

poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS), which effectively

blocks electron transport to the anode. The active layer blend (in BHJ devices) is then

deposited onto the hole-transporting layer. A reflective cathode is then deposited (typically

28

via evaporation) onto the active layer. The metal used has a low work function such as

aluminium. Calcium or lithium fluoride is often used as an interlayer to improve electron

injection.

Figure 1.12: Illustration of the conventional and inverted device architecture. HTL and ETL refer to hole and electron transport layers, respectively.

The conventional architecture is prevalent in the literature and is used prominently in this

thesis. However, it is not without its issues. PEDOT:PSS is acidic in nature and has been

shown to degrade both ITO and the active layer.68 Due to the low work function, the

cathodes are susceptible to oxidation which can be detrimental to device performance.69

The formation of pin holes in conventional devices is also common,70 which allow for the

active layer to react with water.71

An alternative to this architecture is to invert the structure (Figure 1.12). The ITO still acts

as the transparent electrode but is coated with a transparent (inorganic) n-type

semiconductor such as zinc oxide or titanium oxide. The interlayer facilitates the transport

29

of electrons to the ITO. This allows for a high workfunction metal to be used as the top

electrode such as silver and gold, which are less susceptible to oxidation than aluminium.

The hole transporting interlayer, between the active layer and metal is a transition metal

oxide, typically molybdenum oxide, which facilitates hole transport. The absence of low

workfunction metals and PEDOT:PSS causes a marked improvement in device stability.72

Devices with the inverted architecture often also exhibit greater efficiencies. However,

conventional devices require less expensive electrodes so were used in this thesis.

1.6 Semiconducting polymer nanoparticles

Semiconducting nanoparticles (often referred to as conjugated polymer nanoparticles and

Pdots) are highly emissive polymer spheres formed from fluorescent semiconducting

polymers. The synthesis of which will be discussed in more detail later in this section. They

have received extensive research as potential alternatives to single-molecule fluorescent

markers used for bioimaging applications.73–75

The use of single-molecule fluorescent markers for bioimaging has resulted in significant

advances in the understanding of biological processes at the molecular level76 and in living

cells.77,78 However, the low photostability of single-molecule dyes makes them unsuitable

for long-term cell studies. This has led to the development of various labelling alternatives.

One of these is semiconducting quantum dots (QDs), which have received a great deal of

attention due to their broad absorption, narrow emission and increased photostability.79,80

30

However, the toxicity of the heavy-metal components (usually cadmium) could potentially

cause serious damage to the host.81 Another commonly used alternative to single-

molecules dyes is to load silica nanoparticles with multiple dye molecules.82 This forms a

nanoparticle with increased brightness and photostability. However, an issue with this

approach is the limitation in the dye concentration due to aggregation induced quenching of

the fluorescence83 and potential leaching of the dye over time.84 Interestingly, the rare

phenomenon of aggregation induced emission (AIE) has also led to a branch of fluorescent

nanoparticles.85–87 In this case, a twisted chromophore can only restore π-conjugation and

fluorescent emission upon aggregation. AIE nanoparticles have shown low cytotoxicity and

good photostability.88

Fluorescent semiconducting polymer nanoparticles (SPNs) have also received a great deal of

interest in recent years. Their high fluorescent brightness and excellent photostability has

made them a promising alternative to single-molecule fluorescent dyes and quantum

dots.75,89–93 They have also shown to exhibit low cytotoxicity,94–97 however, more research is

needed to fully assess the efficient excretion of these materials.98

SPNs are also relatively straight-forward to synthesise and the absorption and emission can

be tuned by synthetic design of the polymer. However, SPNs do have some drawbacks

when compared to the alternatives, with the emission of SPNs tending to be broader due to

the degrees of disorder in a polymer compared to the well-defined emission seen for

quantum dots and small-molecules. This being the case, research has shown that an overall

narrow emission can be achieved by the introduction of a dye with narrow emission into the

31

system, to which energy from the polymer can cascade.99,100 Control of size is also more

challenging with SPNs. In conventional QD synthesis, for example, the reaction time or

temperature can carefully dictate the size of nanoparticles.101,102 However, the size of SPNs

is dependent on many variables including concentration, molecular weight and poly-

dispersity of the polymer used.101 It should be noted that effort has been made to solve this

issue by the synthesis of nanoparticles from non-linear polymers103 (which do not strain

under nanoparticle formation, so consistently make small nanoparticles) and block-

copolymers,104 to give a couple of examples. Finally, conjugated modification of the surface

of SPNs can be challenging, this will be discussed in more detail in Chapter 4.

1.6.1 Synthesis of SPNs

There are two main methods of forming SPNs, the miniemulsion method105 and the

nanoprecipitation method (Figure 1.13).106 Both strategies essentially involve dissolving the

polymer in a good solvent and injecting the solution into water under sonication.

Under the miniemulsion method, the polymer is dissolved in a water-immiscible solvent and

the solution is added to a solution of an aqueous surfactant. The solution is then rapidly

mixed under sonication. This forms a miniemulsion consisting of small polymer-solution

droplets, coated with surfactant. The organic solvent is evaporated, leaving surfactant-

coated polymer nanoparticles in water. The surfactant is used to prevent coalescence and

flocculation of droplets.107 In this case, coalescence is the merging of droplets into an

overall larger droplet and flocculation is the joining of two droplets, which do not coalesce.

32

Another undesired phenomenon is known as Oswald ripening, where large droplets

preferentially form over smaller ones.108 However, this effect can be quelled by the addition

of hydrophobic agents such as hexadecane.109

Figure 1.13: Illustration of the main two ways to synthesise SPNs: miniemulsion and nanoprecipitation.

Under the nanoprecipitation method, the polymer is dissolved in a water-miscible solvent.

The solution is rapidly injected into a relatively large amount of water and the organic

solvent is evaporated. Under these conditions the polymer coils into a sphere to minimise

the contact with the now ‘bad’ solvent (mostly water). This technique does not require the

use of surfactants and hydrophobic reagents. However, if a surfactant is desired, this

method can be altered in order to coat the nanoparticles in surfactant by dissolving the

33

surfactant in the organic or aqueous solvent before injection.110,111 The nanoprecipitation

method also boasts greater size control due to the absence of the droplet effects discussed

earlier and was the preferred synthetic route used in this thesis as a result.112

1.6.2 Characterisation of size

The estimation of the size of nanoparticles can be challenging, especially in polydisperse

samples. There are a number of ways of determining particles size. The most common

method used is dynamic light scattering (DLS), in which the size is determined by

fluctuations in the scattering intensity. 113 There are three main methods used to represent

a size distribution in DLS: intensity, volume and number. The scattering intensity of a

particle is proportional to its size to this sixth power. This means that intensity distributions

can result in a small amount of larger particles dominating the DLS spectrum. Volume and

number intensity distributions are a correction to the intensity distribution, reducing the

contribution from the larger particles. It is important to note that the mathematics behind

the DLS distributions assumes a much simpler system than maybe present. For example,

volume distributions are calculated, assuming the system is a homogeneous arrangement of

spherical particles.114

The size of nanoparticles can also be determined by scanning transmission electron

microscopy (STEM). 115 Nanoparticle solutions are deposited and allowed to dry on a carbon

mesh grid. A high-energy electron beam is then focused onto a single point on the sample

grid, in which a proportion of the electrons are transmitted or scattered through the

34

sample. The resulting electrons are then magnified by a series of electromagnetic lenses to

generate an image. As the nanoparticles will scatter electrons differently compared to the

mesh grid, a contrast image can be generated when the electron beam is scanned across the

sample surface. Furthermore, under the bombardment of electrons, the atoms can emit X-

rays, which can be analysed using energy dispersive X-ray (EDX) spectroscopy to give

elemental composition as a function of position. This technique is much more expensive

and time consuming compared to DLS.

Nanoparticle tracking analysis (NTA) is another technique that can be used to determine

particle size.116 A nanoparticle solution is irradiated with a laser and the light scattered from

the particles is continuously recorded, generating a video of the particles moving in solution.

The Brownian motion of each particle is then analysed by the software and a size

distribution is calculated. This technique also allows for the concentration of nanoparticles

to be determined, as the volume of the solution under irradiation is known.

1.6.3 Optical properties of SPNs

As discussed previously, upon the absorption of a photon, an exciton forms along the

semiconducting polymer backbone where it can then diffuse through the material. Studies

have shown that (in single polymer chains) the excitons drift to specific low-energy positions

on the polymer backbone.117 At these regions the exciton can either decay via fluorescence

(a radiative process) or non-radiative pathways. In some cases the exciton can also

dissociate into free charges but, due to the low dielectric of semiconducting polymers, this is

35

a rare phenomenon (hence why a p-n junction is required for OPVs). The efficiency can be

represented as the proportion of incident photons that result in a radiative decay and is

known as the photoluminescence quantum yield (PLQY).

As discussed previously, the way in which polymer chains interact can have an important

influence on the PLQY of a polymeric system, such as nanoparticles or thin films, due to

aggregation effects.36 This has led to the development of highly emissive semiconducting

polymers, in which polymers are designed with bulky side-chains and non-planar backbones

to discourage aggregation.118 Moreover, in thin-film applications, such as organic light

emitting diodes (OLED’s), processing techniques have been developed to discourage

aggregation.119

Interestingly, unlike with QDs, the size of SPNs have been shown to have relatively small

influence of the optical properties, with small shifts in the absorption and emission peaks

observed in some cases. For example, Kurokawa et al. observed a 30 nm red-shift in

emission when the particles size was increased tenfold.106 Increased particle size has,

however, been shown to decrease the PLQY.89

As discussed previously, SPNs have excellent photostability when compared to single-

molecule dyes.89 For example, the highly stable single molecule dye Rhodamine 6G can

absorb approximately one million photons before irreversible photo-bleaching, whereas

SPNs have been shown to increase this number to approximately one billion. 112 This makes

them more suitable for long-term biological studies.

36

The tuneable nature of semiconducting polymers has led to the synthesis of highly emissive

SPNs with fluorescence colours spanning the visible spectrum.75,89 An example of a range of

polymers and the approximate colour of the corresponding SPNs can be seen in Figure 1.14.

Figure 1.14: Chemical structures of common semiconducting polymers, with the emission colours (in their nanoparticle form) spanning the visible spectrum.

75,89

Near infrared (NIR) light can penetrate tissue further than visible light.120–122 As a result,

SPNs with emission at the near-infrared is highly desirable for the study of biological

processes in vivo.122 In order to achieve the low band-gaps required for the low energy

emission, rigid and planar polymer backbones are required. As discussed previously, this

increases the chances of aggregation induced emission and can result in low quantum yields

as a result.92 However, Chen et al. have shown how the quantum yield can be increased in

37

NIR-emitting SPNs by introducing a torsional strain to small percentages of the polymer

backbone, reducing aggregation.123

Another approach has been to embed a small-molecule NIR dye into the SPN, where the

polymer absorbs the photon and the energy cascades to the dye, which then emits in the

NIR.124 The advantage of this technique is the high quantum yields and very narrow

emission spectra. However, leaching of the dye over time could be an issue in biological

studies. A way around this has been to covalently bond a NIR-emitting molecules onto a

small percentage of the polymer backbone, as not to disrupt the bulk properties of the

polymer.100,125,126 This yields bright, photostable, NIR emitting SPNs with high quantum

yields. However, this strategy does add more synthetic complexity to the materials, which

could impede their commercial viability. Very recently, Crossley et al. have shown how a

simple borylation of BT-containing polymers, post-polymerisation, can result in a huge red-

shift of the emission into the near-IR. This provides a simple method for synthesising NIR

SPNs from commercially available polymers.127

1.6.4 Förster resonance energy transfer (FRET)

Although fluorescence allows the location and movement of nanoparticles to be analysed in

a biological system, it gives very little information about the molecular processes occurring

in that system. Energy transfer from one species to another however, can be incredibly

useful when investigating biological processes.128

38

Förster resonance energy transfer (FRET) describes the phenomenon in which non-radiative

energy transfer occurs between two different chromophores. In order for this process to

occur, the emission spectra of one chromophore must overlap efficiently with the

absorption of the other (the chromophores are labelled as the donor and acceptor

respectively). They must also be in close proximity, approximately less than 10 nm. Under

irradiation the exciton formed in the donor chromophore can be treated as an oscillating

dipole. This oscillation can form resonance with the nearby acceptor molecule. This causes

the excitation energy of the exciton to transfer to the acceptor, in a non-radiative fashion.

The acceptor exciton can then either decay via the radiative or non-radiative processes

depicted in Figure 1.6.

The FRET efficiency (EFRET) can be represented as follows:

𝐸𝐹𝑅𝐸𝑇 = 1

1 + (𝑟 𝑟0⁄ )6

Where 𝑟 is the distance between the donor and acceptor chromophores and 𝑟0 is the

distance where the FRET efficiency drops to half of its maximum value. This equation shows

that the efficiency is very sensitive to the separation of donor and acceptor. Illustrating why

this is a useful technique for visualising the interactions of biological matter.129

The efficiency of FRET can be visualised in many ways. Under photoluminescence

spectroscopy, for example, the intensity of the emission maxima will decrease for the donor

and increase for the acceptor as the FRET efficiency increases (Figure 1.15a). The lifetime of

the excited state should also decrease with increasing FRET efficiency. The fluorescence

39

lifetime refers to the lifetime of an excited state, before decaying to the ground state. The

lifetime of an excited state is typically measured by time-correlated single photon counting

(TCSPC). This is a technique that accurately measures the time delay between the

absorption and emission of a photon, resulting in photon count (or intensity) as a function

of time, known as the decay trace (Figure 1.15b).

Figure 1.15: Illustration of the characteristics of FRET transfer observed in a) photoluminescence spectroscopy (pulsed at donor absorption) and b) TCSPC, with decreasing donor-acceptor separation distances (r).

As depicted in the Jablonski diagram in Figure 1.6, three processes occur in the formation

and decay of an exciton: absorption, relaxation and radiative or non-radiative decay to the

ground state. Upon absorption of a photon (by the donor chromophore), an electron is

promoted from the electronic ground state to the excited state, typically over femtosecond

timescales. This electron then relaxes down to the local minima, which occurs over

picosecond timescales. Finally, the electron decays down to the ground state, typically in

the order of nanoseconds (ignoring the rarely observed phosphorescence).30 The final step

40

takes the longest and therefore can be treated as the rate determining step and the

preceding steps can be ignored. The rate constant associated with decay can be split into

contributions from the radiative decay paths (KR) and non-radiative decay paths (KNR) to the

ground state. As the lifetime of the donor fluorescence lifetime (τ) is inversely proportional

to the rate constant it can be stated that:

𝜏 ∝1

𝐾𝑅 + 𝐾𝑁𝑅

Therefore, as FRET becomes more efficient (a non-radiative process) the contribution from

𝐾𝑁𝑅 will increase, resulting in a shorter lifetime of the excited state of the donor.

1.7 Thesis aims and overview

This thesis will predominantly focus on the modification of the widely-used acceptor unit,

2,1,3-benzothiadiazole (BT) through nucleophilic aromatic substitution (SNAr) reactions.

As discussed previously, reducing the band-gap is desirable for OPV applications in

particular, as light of higher wavelengths can be absorbed. Fluorine groups have been

shown to be useful for controlling the band-gap and aggregation.16,130 They are strongly

electronegative, therefore when affixed to the acceptor unit of a donor acceptor polymer

the LUMO and band-gap decreases.131 Also, as already stated fluorination of backbones can

increase backbone planarity through favourable F-S interactions.16 As a result acceptor

41

units in donor-acceptor polymers are often fluorinated in high-performance

polymers.64,132,133

SNAr reactions occur on electron deficient aromatic units, in which a nucleophile displaces a

leaving group (typically a halide).134 The reaction is a two-step process: firstly the

nucleophile attacks the carbon centre, breaking the aromaticity. This is then followed by

the loss of the leaving group, restoring aromaticity. Fluorine is generally a poor leaving

groups due to the strong C-F bond. However, under SNAr conditions the attack of the

nucleophile is the rate determining step, due to the loss in aromaticity, therefore the leaving

ability does not factor into the rate of reaction. Fluorine is very electron withdrawing and

this can stabilise the anionic intermediate, so is an excellent leaving group for SNAr

reactions. The other requirement for the SNAr reaction to work is additional electron

withdrawing groups (EWG) on the ring, which aid in further stabilisation of the anionic

intermediate.

Figure 1.16: Proposed SNAr mechanism for fluorinated benzothiadiazole.

The BT unit satisfies the criteria to undergo SNAr, it is often fluorinated and has the electron

withdrawing nitrogen-sulfur-nitrogen bridge (the additional lone-pair on nitrogen is

42

perpendicular to the delocalised π-network, so cannot mesomerically donate into the BT

ring, therefore it acts as an EWG). A proposed mechanism of SNAr on fluorinated BT is


Chapter 2 focuses on the substitution of a monofluorinated-BT with octylamine, octanol and

octanethiol. This reaction occurs on the monomers, pre-polymerisation. The resulting

monomers are then copolymerised with a carbazole comonomer. The influence of varying

the heteroatom on the optical properties and OPV performance of the resulting polymers

are then investigated. The chemical structures are depicted in Figure 1.17.

Figure 1.17: Chemical structures of alkylamine, alkoxy and thioalkyl substituted polymers.

The BT SNAr reaction is then applied to the backbone of fluorinated polymers in Chapter 3.

The reaction is shown to work on a variety of BT containing polymers but also gratifyingly is

shown to work on fluorinated benzotriazole (BTz) and thienothiophene (TT) based polymers

(structures in Figure 1.18).

43

Figure 1.18: Schematic of SNAr reactions on fluorinated benzothiadiazole (BT), benzotriazole (BTz) and thienothiophene (TT) containing polymers.

Chapter 3 then goes onto the explore the BT SNAr reaction through post-polymerisation

functionalisation of poly(9,9-dioctylfluorene-alt-5-fluoro-benzothiadiazole) P(F8fBT) whilst

varying the nucleophile used. The reaction is found to be successful with a variety of

functionalised alcohols, thiols and thioacetates (where the thiolate species is generated in

situ). The amount fluorine substitution (with octanethiol) could also be finely tuned. This

ultimately led to the synthesis of multifunctionalised polymers, in a one-pot synthesis

(example in Figure 1.19).

44

Figure 1.19: Multifunctional nanoparticles which can undergo two bioorthogonal click-reactions with amines and strained-alkynes.

The novel reaction provides a very simple path for synthesising functionalised polymers.

Covalent surface modification of SPNs is synthetically challenging by existing methods.

Chapter 4 focuses on the incorporation of functionalised P(F8fBT) in fluorescent SPNs for

use in bioconjugation reactions. This chapter concludes with the synthesis of

multifunctional nanoparticles with azide and carboxylic acid groups at the surface. The

nanoparticles could undergo bio-orthogonal click reactions on the same nanoparticle (Figure

1.19).

45

Figure 1.20: Chemical structure of trimethoxysilane functionalised polymer.

Finally, Chapter 5 shows how trimethoxysilane groups are incorporated onto the backbone

of poly-[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(5-

fluoro-[2,1,3]-benzothiadiazole)] (P(CPDTfBT)), utilising the substitution reaction. The

chemical structure of the functionalised polymer is depicted in Figure 1.20. The cross-

linking ability of the resulting polymer was then investigated and was found to be

accelerated by both heat and PEDOT:PSS. When the polymer was incorporated into OPV

devices they showed an initial reduced performance compared to devices consisting of un-

substituted (P(CPDTfBT)). However, the cross-linked devices did show impressive thermal

and ambient stability.

47

Chapter 2 Systematic tuning of 2,1,3-

benzothiadiazole acceptor strength by mono-

functionalisation with alkylamine, thioalkyl or alkoxy

groups in carbazole donor-acceptor polymers

48

Figures and text in this chapter adapted with permission from (‘Systematic Tuning of 2,1,3-

Benzothiadiazole Acceptor Strength by Monofunctionalization with Alkylamine, Thioalkyl, or

Alkoxy Groups in Carbazole Donor–Acceptor Polymers’ Adam Creamer, Abby Casey, Adam

V. Marsh, Munazza Shahid, Mei Gao, and Martin Heeney, Macromolecules, 2017, 50 (7),

2736–2746). Copyright (2017) American Chemical Society.

2.1 Introduction

There has been much effort in the development of low band-gap conjugated polymers for

their interesting optoelectronic properties. For example, low band-gap conjugated

polymers have been demonstrated to display promising ambipolar behaviour in thin-film

transistor devices,135 whereupon either holes or electrons can be transported under

appropriate gate biasing. They have also shown near-IR emission in organic

electroluminescent devices (OLEDs)136–138 and have attracted much attention as donor

materials in organic photovoltaic (OPV) devices.44 There have been many approaches

reported to the development of low band-gap polymers, but one of the most successful has

been the donor-acceptor approach, in which an electron-accepting monomer is

copolymerized with an electron-donating monomer (discussed in section 1.3.2).139 The

strength of the electron accepting comonomer has a strong correlation to the optical band-

gap and polymer energetics, and as such the development of new electron accepting

comonomers continues to attract much interest.140–142

49

Figure 2.1: Structure of 2,1,3-benzothiadiazole with numbering system.143

The ideal acceptor should be straightforward to synthesize, have a readily tuneable electron

affinity and convey sufficient solubility to render the resulting copolymer processable. In

the context of acceptor comonomers, 2,1,3-benzothiadiazole (BT) meets many of these

requirements, and its derivatives have been extensively investigated as comonomers in the

development of low band-gap polymers.144 One attractive feature of the BT unit is the

opportunity to introduce substituents on the 5 and/or 6 positions to tune its properties (see

Figure 2.1 for structure). For example, the incorporation of fluorine onto the 5/6 positions

has been shown to significantly boost the solar cell efficiency of low band-gap polymers

containing the BT unit.145–147 This has been related to a combination of the highly

electronegative fluorine increasing both the ionization potential and electron affinity of the

polymer, which helps to increase the open circuit voltage (Voc) of solar cell blends, as well as

promoting aggregation of the polymer via increased inter- and intramolecular

interactions.148,149 The incorporation of strongly electron accepting cyano groups results in

an even more pronounced increase in electron affinity, such that the polymers become

dominant n-type semiconductors.150 The number of cyano groups can be used to tune the

electron affinity of the polymer and therefore solar cell performance.151 Similarly fusing

50

additional aromatic units to the BT groups, such as an additional thiadiazole or pyrazine unit

can result in a substantial reduction in polymer band-gap.152 However, in many of these

derivatives the lack of a solubilizing alkyl side-chain on the BT group can be an issue such

that the resultant polymer exhibits low solubility.

In order to try and solve this solubility issue, alkyl chains have been incorporated onto the

5,6 positions of the benzothiadiazole. However, this resulted in significant torsional

twisting with the adjacent thienyl groups, and therefore a significant decrease in the

effective conjugation length of the polymer.153 Functionalization of the BT unit has also

been extensively explored with alkoxy substituents. The introduction of one-alkoxy group in

combination with one fluorine, reduced the negative (from a solar cell perspective)

decrease in ionization potential, and high solar cell efficiencies have been reported for

copolymers containing this acceptor.154,155 The incorporation of two octyloxy groups onto

the BT unit of the carbazole copolymer poly(2,7-carbazole-alt-dithienylbenzothiadiazole)

P(CdTBT) gave little compromise in efficiency whilst vastly improving the solubility of the

polymer.156,157 Work primarily by another member of the group, Dr. Abby Casey, expanded

on this by introducing thioalkyl groups onto the same polymer. The thioalkyl groups were

found to significantly affect the optical properties in comparison with the analogous alkoxy

polymer. This was attributed to the larger size of the thioalkyl substituents introducing

torsional twisting with the adjacent thienyl group, decreasing backbone planarity and

increasing the optical band-gap.25 In blend organic photo voltaic (OPV) devices, this polymer

gave a very high open circuit voltage (VOC) of over 1 V but a relatively low photocurrent (JSC).

51

The use of alkoxy groups afforded a more planar backbone, than the thioalkyl polymer, but

donation of electron density into the BT unit also reduced the ionization potential in

comparison to P(CdTBT), the non-substituted analogue.52 This resulted in a lower VOC but a

much higher JSC, compared to the thioalkyl analogue. Thus, the position, length and type of

substituent on the BT core clearly have an important role in the opto-electronic properties

of the resulting polymer.

Surprisingly however, very little work has been performed with mono-substituted BT

acceptors and with only one example of a copolymer containing a BT with a single alkoxy

substituent reported in the literature.158 Previous to this work, there have been no

systematic studies looking at the effect of different mono-functional substituents on the

acceptor strength of BT in donor-acceptor polymers.

In this chapter, the ready synthesis of a series of mono-functionalized BT comonomers with

mono-alkoxy, thioalkyl and alkylamino groups is reported. All three comonomers are

available in two steps from a common precursor, and the thioalkyl and alkylamino

functionalized BT’s are reported for the first time. In order to probe the properties of these

substituents, all three acceptors were copolymerised with a common carbazole comonomer

to afford donor-acceptor copolymers. The effect of heteroatom substituent on the

optoelectronic properties is then explored and related to the structural and electronic

changes using DFT calculations. Finally, the OPV device performance of all three polymers

with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) is reported, demonstrating how

the VOC and JSC can be tuned with addition of the various functional groups.

52

2.2 Synthesis of monomers and polymers

Work previously reported in the group showed that the fluorine substituents of 5,6-difluoro-

4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (dTdfBT) could be substituted by alkylthiol or

cyanide nucleophiles via a SNAr type mechanism.25,150 Further publications in the group

have shown that the mono-fluorinated BT derivative 5-difluoro-4,7-di(thiophen-2-yl)-2,1,3-

benzothiadiazole (dTFBT) is also receptive to nucleophilic substitution with cyanide, despite

the reduced electron deficicency compared di-fluorinated analogue.151 Here that work is

built upon to demonstrate that the mono-fluorinated BT analogue is also reactive to

nucleophilic substitution with alkylthiols, alkylalcohols and alkylamines.

Scheme 2.1: Synthetic route to precursor 2.2 from commercially available 5-fluoro-2,1,3-benzothiadiazole.

The synthesis of all monomers was possible from a common precursor, 4,7-bis(thiophen-2-

yl)-5-fluoro-2,1,3-benzothiadiazole (2.2). This precurser was prepared via the bromination

of 5-fluoro-2,1,3-benzothiadiazole in hydrobromic acid, yielding 1.1, followed by the Negishi

coupling of 2-thienylzinc bromine with 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole

(Scheme 2.1).

53

Scheme 2.2: Synthetic route to monomers: 2.6, 2.7 and 2.8. i) NBS, chloroform, ii) C8H17OH, THF, KtOBu, iii)

C8H17SH, DMF, K2CO3; iv) C8H17NH2, NMP.

The precurser (2.2) was then used to synthesise alkylamine, thioalkyl and alkoxy monomers

(Scheme 2.2). Treatment of 2.2 with octanol in THF in the presence of KOtBu afforded 71%

of the substituted BT (2.4), whereas treatment with octanethiol in DMF in the presence of

Na2CO3 gave almost quantitative substitution (92%) of the fluoride to yeild 2.5. Both

materials could be readily brominated with N-bromosuccinimide (NBS) at room temperature

in reasonable yield (62% and 74% respectively) to afford 2.7 and 2.8 (Scheme 2.2).

However, 2.7 was found to not be pure enough for polymer synthesis so was recrystalised

from heptane to yield pure 2.7, with a lower overall yeild of 41%.

A similar reaction was attempted with octylamine, initially in THF, but the reaction was slow

under these conditions. Attempts to increase the rate of reaction using DMF as an aprotic

54

cosolvent at 90 ⁰C, resulted in some byproduct resulting from the substitution with

dimethylamine, presumably arising from the decomposition of DMF.159 Changing the

solvent to DMSO solved this issue, and the octylamino product was isolated in 98% yield.

However, attempted bromination of this monomer proved problematic, with the reaction

affording a complex mixture of products under a variety of conditions. It appeared that

undesired oxidation of the amine was a competing process. Therefore an alternative route

was developed, in which precurser 2.2 was initally brominated to afford (4,7-bis(5-

bromothiophen-2-yl)-5-fluoro-2,1,3-benzothiadiazole) (2.3). Reaction of 2.3 with one

equivalent of octylamine in a mixed solvent of toluene and NMP afforded the desired

product 2.6 but in relatively low yield (21%), due to competing substitution of the bromine

substituents with octylamine.

Scheme 2.3: Synthesis of 2.9, 2.10 and 2.11 by Suzuki polymerization.

Polymers were prepared by the Suzuki polymerisation of monomers 2.6, 2.7 and 2.8 with 9-

(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester using Pd(PPh3)4 in a

mixed solvent of toluene and aqueous sodium carbonate (Scheme 2.3). The reaction was

55

performed in a sealed tube at 120 °C for 3 days, using a phase transfer agent to help mixing

of the two phases. The polymers were end-capped with bromobenzene and phenylboronic

acid before precipitation into methanol. Low weight oligomers were removed by washing

with methanol, acetone and hexane. The polymer was then extracted in chloroform before

washing with sodium diethyldithiocarbamate dihydrate to sequester Pd residues.160 A final

re-precipitation afforded 2.9, 2.10 and 2.11 in good yields (85-89%).

Table 2.1: Molecular weights and degree of polymerization measured using gel permeation chromatography (against polystyrene standards) in chlorobenzene at 80 °C.

Polymer Mn (kg mol-1) Mw (kg mol-1) DPn DPw Ð

2.9 19 36 23 43 1.90

2.10 28 49 34 59 1.75

2.11 24 53 28 62 2.25

All polymers had good room temperature solubility in non-chlorinated solvents such as

toluene and THF. The molecular weight of the final polymer was determined by gel-

permeation chromatography (GPC) (against polysterene standards) in chlorobenzene at 80

⁰C. Both 2.10 and 2.11 had a comparable molecular weight and dispersity values, whereas

crude 2.9 had a lower molecular weight (Mn) of 15 kg mol-1. To try to limit molecular

weight issues in the subsequent studies, 2.9 was fractionated using preparative GPC to give

a higher molecular weight fraction of 19 kg mol-1 (see Table 2.1). Since preparative GPC may

also remove some very low weight impurities or catalyst residues,161 2.10 and 2.11 were

56

also passed through the preparative GPC but not fractionated, to enable a fair comparison.

Note the molecular weight was not changed by this treatment.

2.3 Absorption spectra

The absorption spectra of 2.9, 2.10 and 2.11 in chloroform solution are shown in Figure 2.2

and summariszed in Table 2.1. All three polymers exhibit the classic ‘dual band’ absorption

observed for most donor-acceptor type polymers.140 The two peaks are often explained to

result from a higher energy -* transition located on the donor, and a lower energy peak

resulting from a (partial) charge transfer between donor and acceptor comonomers.162,163

Alternative explanations suggest the peaks originate from hybridisation of the frontier

molecular orbitals to afford low- and high-lying energy bands spread over the donor and the

acceptor.164 In the current case, the molar absorptivity for all three polymers in solution is

shown in Figure 2.2a, as estimated using the molecular weight of the repeat unit. Although

such an estimation does not account for the fact that the effective conjugation length of the

chromophore is longer than the repeat unit, it does allow for a ready comparison between

the three polymers. It is immediately evident that the nature of the substituent has a

significant impact on the strength of the low energy absorption around 500 nm. As

discussed above this peak is usually ascribed to an intramolecular charge transfer (ICT)

band,165 and this assignment is supported by fact that the photoluminensce emission

wavelength varies according to solvent polarity (see below). Accordingly the energy of the

ICT transition would be expected to vary according to the strength of the acceptor, with

57

weaker acceptors leading to a higher energy transition (blue shift). In the current example,

all three polymers are blue shifted when compared to the non-substituted analogue,

P(CDTBT), which has absorbtion maxima at 547 and 392 nm in chloroform.156 2.10 has a

small blue shift of 6 nm in the low energy band, whilst 2.9 and 2.11 have larger blue shifts of

46 and 43 nm respectively.

Figure 2.2: UV-Vis absorption spectra of 2.9, 2.10 and 2.11 in a) chloroform solution (1.67 x 10-2

g dm-3

) and b) normalised as-spun thin films

With the exception of the thioalkyl polymer, this trend can be qualitatively explained by a

consideration of the electron donating/withdrawing effects of the substituent. The effects

of a substituent can be indicated by the Hammett parameters. The more negative the

parameter the more electron-donating a substituent is and conversely, the more positive a

value the more electron-withdrawing a substituent is. Alkylamine is generally considered to

be a stronger electron donor than an alkoxy group, as supported by the Hammett

parameters for –OMe and -NHMe (σp -0.27, -0.7 & σm 0.12, -0.21 respectively. σp and σm

58

refer to the Hammett parameters at the para and meta positions of benzoic acid,

respectively).166 Substitution of the BT with the alkylamine clearly reduces the electron

accepting nature of the BT, increasing the energy of the ICT band as well as reducing its

intensity. Interestingly the higher energy band around 390 nm is hardly affected by the

nature of the substituent in agreement with the suggestion it is largely associated with the

donor.

However, this explanation does not hold for the thioalkyl substituted polymer, since a

thioalkyl group is generally considered to be a very weak donor on the basis of its Hammett

parameters (-SMe, p 0.0 and m 0.15)166 and therefore might be expected to give the BT

more electron accepting character than either of the other groups. The fact that this is not

observed is in agreement with work previously reported in the group, in which a di-thioalkyl

substituted BT containing polymer was found that the thioalkyl groups introduced

considerable torsional strain along the polymer backbone. This reduced backbone

delocalisation and weakening the intensity of the ICT transition.

59

Table 2.2: Optical properties of 2.9, 2.10 and 2.11 in chloroform solution and as thin films. Δλ is the difference between the low energy absorption peak and the emission peak.

Polymer

Solution Thin film

Eg(opt), eV

I.P., eV (PESA)

HOMO, eV

(DFT) λabs,max , nm (α, M-1 cm-1)

λem

max , nm

Δλ, nm (eV)

λabs,max , nm

λem,

max , nm

Δλ, nm (eV)

2.9 389 (2.9 x 104), 501 (2.0 x 104)

705 204

(0.72) 386, 516

702 186

(0.64) 1.91 -5.29 -4.73

2.10 391 (3.2 x 104), 541 (3.3 x 104)

693 152

(0.50) 396, 564

727 163

(0.49) 1.91 -5.29 -4.67

2.11 377 (2.7 x 104), 504 (1.8 x 104)

682 178

(0.64) 384, 534

707 173

(0.57) 1.96 -5.39 -4.90

Upon film formation, the absorption peaks of all three polymers showed a red shift, with a

larger shift observed for the ICT band versus the high energy absorption (Figure 2.2b). The

red shift can be related to enhanced backbone planarity as the polymer enters the solid

state, as well as a possible increase in intermolecular ICT transitions.167

To further rationalise the role of the different substitutents on the optical properties of the

polymers, density function theory (DFT) calculations were performed to calculate the

optimised ground state geometries, electron density plots and frontier molecular orbital

energies of all polymers using a B3LYP168 functional and a 6-31G(d) basis set. In order to

simplify calculations, trimers of the carbazole-dTBT repeat unit was used, with methyl

groups instead of longer side-chains.

60

Polymer Average dihedral angle (⁰)

α β γ δ

2.9 28 39 6 27

2.10 27 1 6 26

2.11 29 44 4 28

Figure 2.3: a) Structure of trimer for DFT analysis with key bond angles labelled as α, β, γ and δ; b) structures illustrating the alignment of flanking thiophene units of the lowest energy conformer for each trimer for 2.9, 2.10 and 2.11. Only the central thiophene-benzothiadiazole-thiophene unit is shown for simplification; c) dihedral angles around key bonds in the central thiophene-benzothiadiazole-thiophene unit. Due to slight puckering of the benzothiadiazole unit, recorded angles are an average of the two dihedral angles around each key bond (labelled in section a))

The discussion is initially focused on the planarity of the central thiophene-

benzothiadiazole-thiophene (T-BT-T) unit. All trimers were initially allowed to relax to their

equilibrium geometry starting from either cis (sulfur atom on flanking thiophene groups

61

pointing toward thiadiazole group) or trans (sulfur on thiophene pointing away from

thiadiazole group) geometry, and the lowest energy conformations are shown in Figure

2.3b. The thiophene was found to be preferentially in the trans-trans conformation for 2.10

and trans-cis for 2.11 and 2.9. The dihedral angles of the central BT unit of the optimised

trimer with the adjacent thiophene on the substituted and non-substituted side were

measured (labelled as α, β, γ and δ in Figure 2.3a). The bond angles between the thiophene

and BT unit on the non-substituted side of the BT unit were 4-6° in all cases (angle γ in

Figure 2.3a). However, there was a noticeable difference in planarity between thiophene at

the substituted side of the BT unit (angle β, Figure 2.3a). The side-on DFT images can be

seen in Figure 2.4 with angle β displayed. 2.10 was almost completely planar with a bond

angle of ca. 1° whereas 2.9 and 2.11 were more twisted at 39° and 44° respectively. The

appreciable deviation from planarity and change in conformation suggests that the NH and S

groups introduce steric hindrance with the adjacent thiophene. This is expected to reduce

effective conjugation along the polymer backbone, resulting in a wider band-gap and blue

shifted absorption. This suggests that the reduction in the acceptor strength for the

aminoalkyl substituted BT is a combination of electronic and steric effects, whereas for the

thioalkyl polymer the effect is mainly steric. As 2.10 is almost coplanar, the observed blue

shift in comparison to the unsubstituted polymer P(CdTBT) is likely to be predominantly due

to electron donation from the –OR group reducing the strength of the electron accepting BT

unit. Finally, it is noted that the bond angles between the carbazole and thiophene units

were similar for all three polymers at 26°-28° (bond angles α and δ in Figure 2.3a).

62

Figure 2.4: Structure of the trimer used in estimating the conformation of polymers 2.9, 2.10 and 2.11 with key bond angles labelled. Below this is the side view of the central T-BT-T unit for the trimer for 2.9, 2.10 and 2.11 with the T-BT bond angle (bond β) at DFT with a B3LYP level of theory and a basis set of 6-31G(d).

It is worth noting that the introduction of additional backbone torsion does not always

result in a wider band-gap. For example, it has been shown that introducing additional

steric bulk onto the flanking thiophenes rather than the BT unit itself can subtly decrease

backbone coplanarity, and result in an increased polymer ionisation potential without a

significant change in optical absorption.169,170 In these examples the substitutents (either

hexyl or cyclohexyl) were introduced to the 4-position of the flanking thiophene (i.e.

pointing away from the BT unit and towards the thiophene of the donor comonomer).

The frontier molecular orbitals for the trimer are shown in Figure 2.5. It can be seen that

the highest occupied molecular orbital (HOMO) is effectively delocalised over both the

carbazole and the T-BT-T unit of the backbone in all polymers, whereas the lowest

unoccupied molecular orbital is mainly located on the BT and the adjacent thiophenes. As

such the substituent clearly influences the predicted energy level of the HOMO (Table 2.2).

Notably the HOMO of 2.10 is predicted to be higher lying than 2.9, despite the amino group

63

being a stronger donor than alkoxy. This is, again, rationalised on the basis of the increase

in torsional twisting that the aminoalkyl group induces in the backbone, which somewhat

reduces effective conjugation along the backbone. The calculated HOMO levels are in

reasonable agreement with the measured ionisation potentials (IP) (Table 2.2) of 2.9, 2.10

and 2.11. Measurements were performed on thin films using photoelectron spectroscopy in

air (PESA) with an error of ±0.05 eV.

Figure 2.5: HOMO and LUMO electron density plots and optimised geometries of 2.9, 2.10 and 2.11 by DFT

with a B3LYP level of theory and a basis set of 6-31G(d).

64

2.4 Photoluminescence spectra

Figure 2.6: Normalized absorbance and photoluminescence of a) 2.9, b) 2.10 and c) 2.11 in chloroform solution (1.67 x 10

-2 g dm

-3).

The photoluminescence spectra (PL) of 2.9¸ 2.10 and 2.11 in chloroform solution are shown

in Figure 2.6 and emmision maxima are detailed in Table 2.2. All three polymers are red

emitters with emission maxima at 705, 693 and 682 nm for 2.9¸ 2.10 and 2.11 respectively,

resulting in a Stokes shift of 0.72 eV for 2.9, 0.50 eV for 2.10 and 0.64 eV for 2.11.

Interestingly the trend in emission wavelength now appears to track with the electron

65

donating ability of the substitutent, with 2.9 exhibiting the longest wavelength emission and

2.11 the shortest. The emission spectra of the three polymers was also measured in toluene

and compared to chloroform at the same concentration (Figure 2.7). All polymers exhibited

a blue-shifted emission maxima in toluene; a less polar solvent. The emission

solvatochromism suggests that the excited state has polar character, in agreement with

lowest energy absorption and emission having ICT character.

Figure 2.7: Photoluminescence spectra of a) 2.9, b) 2.10 and c) 2.11 in chloroform and toluene solution

(1.67 x 10-2

g dm-3

).

In order to try to understand the observed trend and to investigate the unusually large

Stokes shifts of these polymers, the excited state (ES) and ground state (GS) geometries of

66

the polymer repeat units were calculated using TD-DFT (ES) and DFT (GS) respectively, with

a CAM-B3LYP functional and a basis set of 6-31G(d). Optimization and frequency

calculations were carried out on single polymer repeat units (i.e. donor-acceptor repeat

unit) instead of trimers due to computational restrictions. The GS and ES geometries of the

different polymer monomers (donor-acceptor repeat unit) were therefore used as estimates

of the polymer ES and GS. The CAM-B3LYP functional was used as it provides long range

corrections allowing more accurate modelling of electron excitations.171 Similar to the

trimer calculations of the ground state (GS), methyl-hetero groups were used on the BT unit

and 2-propyl groups were used on the carbazole unit instead of the full alkyl chains in order

to simplify calculations. The structure of the monomers used for DFT analysis can be seen in

Figure 2.8a.

67

Angle 2.9 2.10 2.11

GS ES ΔGS-ES GS ES ΔGS-ES GS ES ΔGS-ES

ε 44.5 18.7 25.7 0.2 0.6 -0.3 49.2 20.4 28.8

ω 31.2 19.1 12.1 31.3 16.9 14.4 31.6 17.8 13.9

Figure 2.8: a) Structure of monomer used in DFT analysis with key bond angles labelled; b) Average dihedral angle around the key bonds calculated using DFT for Ground state (GS) and TD-DFT for Excited State (ES). Due to slight puckering of the benzothiadiazole unit, recorded angles are an average of the two dihedral angles around each key bond (labelled in section a)).

One factor that contributes to the Stokes shift is the conformational change that the

molecule undergoes between the GS and the ES. Assuming the conformation of the

monomer is reflective for that of the polymer, it could be expected that the larger the

difference in conformation between the ground and excited state, the larger the Stokes shift

associated with emission. The dihedral angles around the carbazole-thiophene bond (bond

ω) showed very similar change from ground to excited state for all three polymers, at 12°-

15° (Figure 2.8). Whereas, a large difference in conformation occurs around the thiophene-

BT bond (bond ε) between the three polymers. All polymers were more planar in the

excited state, suggesting the excited state has quinoidal character, as shown in Figure 2.9.

68

However, 2.10 shows very little change in conformation from ground to excited state whilst

2.9 and 2.11 both exhibit a dihedral angle change of over 25°. This shows agreement with

the trend in Stokes shift observed as both 2.9 and 2.11 have a larger shift of 0.72 eV and

0.64 eV compared to the lower shift of 0.50 eV for the alkoxy polymer. Note that the

dihedral angles are slightly different when compared to the trimer calculations in Figure 2.4,

this is due to the fact that only one repeat unit is calculated in this case.

Figure 2.9: Ground state (GS) and Excited State (ES) optimized geometries for donor-acceptor monomers of 2.9, 2.10 and 2.11.

2.5 OPV performance

The photovoltaic performance of all polymers was investigated in blends with PC71BM, due

to the enhanced light harvesting ability when compared to PC61BM. All polymers were

69

tested in devices with the configuration of glass/ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al.

Fabrication conditions were optimised over three batches. In the first batch of devices were

made from solutions of 2.11 and PC71BM in chlorobenzene (at a polymer concentration of

10 mg/mL), with weight ratios of 1:2, 1:3 and 1:4 (polymer:PC71BM) at speeds of 1000, 2000

and 3000 rpm, upon spin coating. The optimized blend was found to be at 1:3 with a spin

speed of 3000 rpm. In the second batch, spin speeds of 3000 and 4000 rpm were test with

and without annealing for 10 min at 120 °C. The final optimised conditions were found to

be at 4000 rpm with a post-fabrication anneal (annealing before electrode evaporation was

found to decrease device performance). These optimised conditions were then applied to

2.9, 2.10 and 2.11 in the final batch (this included a further test at spin speeds of 5000 rpm,

which were found to negatively affect device performance). Figure 2.10 shows the J-V

device curves and external quantum efficiency (EQE) for the best devices made from

solutions of 2.9, 2.10 and 2.11. The performance of the devices is summarized in Table 2.3.

Table 2.3: Performance parameters of 2.9, 2.10 and 2.11 in a device configuration of glass/ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al.

Polymer Voc (V) Jsc (mAcm-2) FF (%) PCE (%)

2.9 0.77 ±0.02 (0.79)b 2.95 ±0.05 (-3.00)b 30.18 ±0.30 (30.61)b 0.68 ±0.02a (0.71)b

2.10 0.87 ±0.00 (0.87)b 7.70 ±0.08 (-7.83)b 43.59 ±0.45 (44.17)b 2.92 ±0.02a (2.94)b

2.11 0.93 ±0.01 (0.95)b 6.34 ±0.08 (-6.41)b 36.31 ±0.48 (37.00)b 2.14 ±0.06a (2.20)b

aAverage device efficiency over five devices.

bBest device efficiency.

Thioalkyl polymer (2.11) exhibited the largest open circuit voltage (Voc) of 0.93 V, which was

expected from the large ionization potential (~5.4 eV). Whereas, alkoxy polymer (2.10) had

70

a lower Voc of 0.87 V, the difference between these values is comparable to the difference

between ionization potentials (0.1 V) for the polymer films. However, 2.9 gave a lower Voc

of 0.77 V, which was somewhat unexpected as it shares the same ionization potential as

2.10. The Voc could be reduced due to increased charge recombination or energetic

disorder leading to high trap density,172 as well as possible differences in polymer

conformation for the pristine and blend devices.173 Jsc was highest for 2.10, which is in

agreement with its low band-gap and high extinction coefficient in the 400-600 nm region

(Table 2.2), compared to 2.11 and 2.9, which both exhibited a lower Jsc. The fill factor was

also highest for 2.10, this could be due to the largely planar nature of the polymer leading to

more efficient π-interactions, compared to the more twisted 2.9 and 2.11.

Figure 2.10: a) JV curves and b) external quantum efficiency (EQE) for 2.9 (blue), 2.10 (red) and 2.11 (black).

Previous studies have shown that surface protonation of basic groups in the polymer

backbone by the excess polystyrene sulfonic acid (PSS) present in the PEDOT:PSS hole-

71

transporting layer can lead to reduced device performance as a result of interfacial traps

and poor charge extraction174,175 To investigate that possibility for 2.9, which contains a

basic secondary amine group, inverted devices using a glass/ITO/ZnO/polymer blend

/MoO3/Ag structure was fabricated (by Dr. Munazza Shahid) (Appendix Figure 1). A modest

increase in PCE to 1.1% was observed, mainly as a result of increased photocurrent although

the devices still had low FF (0.3) and Voc (0.78 V). To further investigate the possibility of

surface protonation, the UV-Vis absorption of polymer solution was measured in the

presence of methanesulfonic acid (MsOH) as a proxy for PSS. Both 2.11 and 2.10 exhibited

no shift in absorption maxima upon the addition of an excess of MsOH (Figure 2.11b).

However, 2.9 exhibited a gradual colour change from orange to pink (a 10 nm red shift in

absorption maxima) upon the addition of 0.5 and 1 eq. of MsOH (Figure 2.11a and c);

polymer concentration kept constant at 1.67 x 10-2 g dm-3). The red shift is most likely due

to (partial) protonation of the secondary amine, which would change the amine from an

electron donating to an electron accepting substituent, resulting in a smaller band-gap. The

change in absorption was fully reversible upon addition of a base, in this case excess tetra-n-

butylammonium fluoride (TBAF), as shown in Figure 2.11. The sensitivity of 2.9 to acid and

the reversibility of the reaction could be potentially interesting as a sensing mechanism for

acidic or basic species.

72

Figure 2.11: Normalized UV-Vis absorption of a) 2.9 with increasing quantities of MsOH and b) 2.11 and 2.10 with excess MsOH; c) image of chloroform solutions of 2.9 with and without MsOH (1.67 x 10

-2 g dm

-3).

2.6 Conclusion

A systematic study on the mono-functionalization of a 2,1,3-benzothiadiazole based

monomer by alkylamine, thioalkyl or alkoxy side-chains has been reported. A simple

synthetic procedure to all three monomers from a common fluorinated BT precursor (2.2)

73

by nucleophilic substitution of the fluorine group was developed. Donor-acceptor

copolymers of all three comonomers were prepared by Suzuki polymerization with an

electron rich carbazole monomer. All polymers exhibited the classic dual band absorption

associated with donor-acceptor polymers. The wavelength and intensity of the long

wavelength intramolecular charge transfer peak was found to vary significantly depending

on the nature of the substituent. This was ascribed to a combination of electronic and steric

factors, with alkylamine and thioalkyl substituents in particular calculated to result in

significant torsional twisting of the conjugated backbone. Solar cell devices were fabricated

from blends of all three copolymers with PC71BM. The nature of the substituent was found

to have a significant impact on solar cell performance, significantly impacting open circuit

voltage of the devices. Finally the optical spectra of novel alkylamino substituted BT

copolymers were found to be reversibly sensitive to presence of acid, suggesting a possible

pathway to the development of sensing materials.

2.7 Experimental

2.7.1 OPV Device Fabrication.

The solar cells were prepared on commercial glass slides coated with patterned indium tin

oxide (25mm × 25mm patterned ITO glass, sheet resistance of 15 Ω/sq from Kintech, HK),

which were cleaned with ultrasonic bath in turns of using detergent solution, deionized

water, acetone and isopropanol. Then, an ~100 nm thick active layer was spin-coated on

74

top of a ~40 nm thick PEDOT:PSS (Clevious P VP AI 4083 from H.C.Starck) layer on the

cleaned patterned ITO glass substrate from a chlorobenzene solution of Polymer:PC71BM

with a weight ratio of 1 : 3 at spin speed of 4000 rpm. Finally, 20 nm of Ca followed by 100

nm of Al was thermally deposited in a vacuum of 1.5 x 10-7 Torr to form the top electrode.

Devices were then annealed for 10 min at 120 ⁰C. The working area of each cell was 0.10

cm2.

2.7.2 DFT and TD-DFT calculations

The HOMO and LUMO molecular orbital energies, electron density plots and optimized

geometries of trimer forms of 2.9, 2.10 and 2.11 were calculated using DFT (density

functional theory) with a basis set of 6-31G(d) and a B3LYP168 level of theory. Calculations

were performed using gaussian 09 software (revision d.01).176 For each structure trans

(sulfur on thiophene pointing away from thiadiazole group), cis (sulfur atom on flanking

thiophene groups pointing toward thiadiazole group) and cis-trans conformations were

allowed to relax to the equilibrium geometry; the lowest energy conformations are shown.

Frequency calculations were performed on the lowest energy conformations to ensure an

energy minimum had been reached. This method was also used to determine the optimised

geometries of the ground state (GS) monomers. TD-DFT calculations, with the CAM-B3LYP

level of theory and a basis set of 6-31G(d), were used to predict excited state (ES)

geometries of monomers. CAM-B3LYP uses a long-range corrected function, making it

appropriate for modeling electron excitations to higher orbitals.171

75

2.7.3 Synthesis of monomers and polymers

2.1 (4,7-Dibromo-5-fluoro-2,1,3-benzothiadiazole) - modified literature procedure177

To a solution of 5-fluoro[2,1,3]benzothiadiazole (4 g, 26 mmol) in 48 % hydrobromic acid (50

mL) was added molecular bromine (14 mL, 272 mmol) drop wise. The resulting mixture was

refluxed for 48 h. The reaction was allowed to cool to room temperature and diluted with

chloroform and deionized water. The bi-phasic mixture was separated from the organic,

washed several times with water, rinsed with saturated Na2SO3 and rinsed with saturated

Na2CO3. Organics were collected and dried over MgSO4, filtered and the solvent removed

under reduced pressure. The crude solid showed presence of starting material under

analysis with TLC. The crude product was taken through the same reaction conditions,

resulting in a crude solid with no sign of starting material on TLC. The crude mixture was

dissolved in methanol and filtrate was concentrated under reduced pressure and the

resulting solid was dissolved in chloroform (50 mL) and precipitated into methanol (400 mL).

The resulting precipitate was filtered and dried under vacuum overnight to afford a white

solid. Yield: 5.44 g (67 %) (literature 57 %); Mpt. 162°C; 1H NMR (CDCl3): δ 7.78 (d, 1H, J =

8.3 Hz); 19F NMR (CDCl3): δ -102.25, -102.28.

76

2.2 (5-Fluoro-4,7-di(thiophen-2-yl)- 2,1,3-benzothiadiazole) - modified literature

procedure.25

5-Fluoro-4,7-dibromobenzo[c][1,2,5]thiadiazole (1) (0.500 g, 1.603 mmol) and Pd(PPh3)4

(104 mg, 0.090 mmol) were added to a 20 mL high pressure microwave reactor vial, with a

stirrer bar. The vial was then sealed with a septum, degassed and 2-thienylzinc bromide

solution in THF (7.04 mL of a 0.5 M solution, 3.527 mmol) was added. The solution was

flushed for 20 min before the reaction was heated for 30 min at 100 °C in a microwave

reactor. The reaction mixture was allowed to cool to room temperature, diluted with THF

and passed through a silica plug (10×5×5 cm). Solvent was removed under reduced

pressure, the residue was purified by column chromatography using hexane/THF 4:1 (v:v).

The product was then recrystallized from chloroform to afford 2.2 as an orange solid (443

mg, 1.39 mmol). Yield 87%; Mpt. 115°C; 1H NMR (400 MHz, CDCl3) δ 8.25 (d, J = 3.7 Hz, 1H),

8.11 (dd, J = 3.7, 1.2 Hz, 1H), 7.74 (d, J = 12.8 Hz, 1H), 7.56 (dd, J= 5.1, 1.2 Hz, 1H), 7.50 (dd, J

= 5.1, 1.2 Hz, 1H), 7.26 – 7.22 (m, 1H), 7.21 (dd, J = 5.1, 3.7 Hz, 1H); 13C NMR (101 MHz,

CDCl3) δ 158.98 (d, J = 254.0 Hz), 153.56 (d, J = 10.9 Hz), 149.86 (s), 138.07 (s), 132.60 (d, J =

5.4 Hz), 130.23 (d, J = 8.1 Hz), 128.51 (s), 128.25 (s), 128.07 (s), 128.02 (s), 127.36 (s), 126.01

77

(d, J = 11.3 Hz), 117.08 (d, J = 32.5 Hz), 111.32 (d, J = 15.3 Hz); 19F NMR (377 MHz, CDCl3) δ -

108.35 (d, J = 12.8 Hz); MS (EI): m/z = 318 [M+].

2.3 (4,7-Bis(5-bromothiophen-2-yl)-5-fluoro-2,1,3-benzothiadiazole) - modified literature

procedure.25

To a solution of 2.2 (430 mg, 1.350 mmol) in chloroform (200 mL) was added N-

bromosuccinimide (NBS) (532 mg, 2.97 mmol) and the reaction stirred in the dark for 16 h.

The reaction mixture was then added to a saturated solution of sodium sulfite to remove all

residual bromine and extracted with chloroform. The organics were combined, dried

(MgSO4), filtered, and concentrated under reduced pressure. The residue was recrystallized

from heptane to afford the product (2.3) as a red solid (307 mg, 0.644 mmol). Yield 48%;

Mpt. 196.5 °C; 1H NMR (400 MHz, CDCl3) δ 8.05 (d, J = 4.1 Hz, 1H), 7.83 (d, J = 4.1 Hz, 1H),

7.72 (d, J = 12.9 Hz, 1H), 7.25 – 7.18 (m, 2H); 19F NMR (377 MHz, CDCl3) δ -108.03 (d, J = 12.9

Hz); MS (EI): isotopic cluster at m/z = 476 [M+].

2.4 (4,7-Bis(thiophen-2-yl)-5-octyloxy-2,1,3-benzothiadiazole)

78

To a solution of 2.2 (473 mg, 1.49 mmol) and KOtBu (200 mg, 1.78 mmol) in THF (50 mL)

under argon was added 1-octanol (1.17 mL, 7.45 mmol). The mixture was heated at reflux

for 3 days. The resulting mixture was diluted with DCM and washed with water (3 x 100

mL). The organic phase was dried (MgSO4), filtered and the solvent removed under reduced

pressure. The resulting solid was disolved in hexane and passed through a silica plug

(10×5×5 cm) to removed excess octanol, the product was then extracted with EtOAc. The

solvent was removed under reduced pressure yielding 2.4 as a red solid (430 mg, 1.05

mmol); Yield: 71%; Mpt 69-70 °C; 1H NMR (400 MHz, CDCl3) δ 8.58 (dd, J = 3.8, 1.2 Hz, 1H),

8.10 (dd, J = 3.7, 1.2 Hz, 1H), 7.75 (s, 1H), 7.48-7.46 (m, 2H), 7.25-7.20 (m, 2H), 4.32 (t, J = 6.6

Hz, 2H), 2.08 – 1.95 (m, 2H), 1.45 – 1.23 (m, 10H), 0.94 – 0.86 (m, 3H); 13C NMR (101 MHz,

CDCl3) δ 154.91, 154.21, 148.83, 139.04, 134.87, 129.68, 128.06, 127.87, 127.02, 126.56,

125.35, 116.16, 112.12, 71.04, 31.82, 29.57, 29.35, 29.22, 26.15, 24.60, 22.68, 14.14; MS

(EI): m/z = 429 ([M+H]+).

2.5 (4,7-Bis(thiophen-2-yl)-5-octylthio-2,1,3-benzothiadiazole)

79

To a suspension of 2.2 (401 mg, 1.26 mmol) and Na2CO3 (650mg, 6.13 mmol) in DMF (6 mL)

under argon was added 1-octanethiol (1.3 mL, 7.50 mmol). The reaction mixture was stirred

at 60 °C overnight, left to cool and added to cold water. The product was extracted with

chloroform, dried (MgSO4), filtered, and solvent was removed under reduced pressure. The

residue was purified by column chromatography using hexane, yielding 2.5 as a yellow solid

(513 mg, 1.15 mmol); Yield: 92 %; Mpt 74 °C; 1H NMR (400 MHz, CDCl3) δ 8.13 (dd, J = 3.7,

1.2 Hz, 1H), 7.94 (s, 1H), 7.58 (dd, J = 5.1, 1.2 Hz, 1H), 7.50 – 7.47 (m, 2H), 7.25 – 7.22 (m,

2H), 3.03 – 2.98 (m, 2H), 1.71 – 1.63 (m, 2H), 1.46 – 1.37 (m, 2H), 1.46 - 1.38 (m, 2H), 1.33 –

1.20 (m, 8H), 0.89 – 0.83 (m, 3H); 13C NMR (101 MHz, CDCl3) δ 155.70, 150.85, 139.37,

138.87, 136.01, 130.44, 128.34, 128.28, 127.65, 127.28, 126.93, 126.34, 124.53, 76.84,

34.28, 31.89, 29.40, 29.26, 29.23, 28.99, 22.76, 14.23; MS (EI): m/z = 445 [M+].

2.6 (4,7-Bis(5-bromothiophen-2-yl)-5-octylamino-2,1,3-benzothiadiazole)

To a solution of 2.3 (307 mg, 0.645 mmol) in anhydrous toluene (4 mL) and anhydrous NMP

(1 mL) under argon was added 1-octylamine solution in NMP (2 mL of a 0.323 M solution).

The mixture was stirred at 90 °C for 18 h. After cooling, the toluene was removed under

reduced pressure. The resulting mixture in NMP was poured into cold water and the

product was extracted with DCM. The DCM was removed under reduced pressure,

80

dissolved in hexane and passed through a silica plug (10×5×5 cm) followed by EtOAc to yield

the product. The product was then recrystalised from heptane to yield 2.6 as a dark red

solid (80 mg, 0.137 mmol); Yield: 21%; Mpt 66-67 °C; 1H NMR (400 MHz, CDCl3) δ 7.82 (d, J =

4.0 Hz, 1H), 7.46 (s, 1H), 7.17 (dd, J = 5.7, 3.9 Hz, 2H), 7.07 (d, J = 3.7 Hz, 1H), 4.84 (m, 1H),

3.34 (q, J = 6.7 Hz, 2H), 1.66 (p, J = 7.2 Hz, 2H), 1.44 – 1.24 (m, 10H), 0.88 (m, 3H); 13C NMR

(101 MHz, CDCl3) δ 156.15, 146.94, 146.81, 140.66, 137.42, 130.84, 130.39, 128.87, 128.03,

126.67, 115.11, 114.87, 113.59, 102.72, 44.36, 31.94, 29.94, 29.42, 29.37, 27.13, 22.82,

14.29; MS (EI): isotopic cluster at m/z = 584 [M+].

2.7 (4,7-Bis(5-bromothiophen-2-yl)-5-octyloxy-2,1,3-benzothiadiazole)


bromosuccinimide (NBS) (208 mg, 1.170 mmol) and the reaction stirred in the dark for 18 h.

The reaction mixture was then poured into a saturated solution of sodium sulfite to remove

all residual bromine and extracted with chloroform. The organics were combined, dried


from methanol/toluene 9:1 (v:v) followed by heptane to afford the product (2.7) as a red

solid (146 mg, 0.249 mmol); Yield: 41 %; Mpt 83.3 °C; 1H NMR (400 MHz, CDCl3) δ 8.43 (d, J =

4.1 Hz, 1H), 7.76 (d, J = 4.0 Hz, 1H), 7.59 (s, 1H), 7.15 (dd, J = 4.1, 2.5 Hz, 2H), 4.29 (t, J = 6.6

81

Hz, 2H), 2.05 – 1.96 (m, 2H), 1.47 – 1.27 (m, 10H), 0.96 – 0.86 (m, 3H); 13C NMR (101 MHz,

CDCl3) δ 154.62, 153.45, 140.19, 136.46, 130.73, 129.93, 129.44, 127.58, 124.48, 115.06,

114.84, 114.60, 111.52, 110.00, 71.13, 31.82, 29.46, 29.34, 29.22, 26.13, 22.69, 14.14; MS

(EI): isotopic cluster at m/z = 585 ([M+H]+).

2.8 (4,7-Bis(5-bromothiophen-2-yl)-5-octylthio-2,1,3-benzothiadiazole)


bromosuccinimide (NBS) (345 mg, 1.94 mmol) and the reaction stirred in the dark overnight.

The reaction mixture was then poured into a saturated solution of sodium sulfite to remove

all residual bromine and extracted with chloroform. The organics were combined, dried


from hexane to afford the product (2.8) as a yellow solid (451 mg, 0.749 mmol); Yield: 74 %;

Mpt 85 °C; 1H NMR (400 MHz, CDCl3) δ 7.87 (s, 1H), 7.86 (d, J = 4.0 Hz, 1H), 7.36 (d, J = 3.9

Hz, 1H), 7.20 (m, 2H), 3.07 – 3.01 (m, 2H), 1.70 (p, J = 7.4 Hz, 2H), 1.44 (d, J = 7.7 Hz, 2H),

1.29 (d, J = 9.4 Hz, 8H), 0.91 – 0.86 (m, 3H); 13C NMR (101 MHz, CDCl3) δ 155.70, 150.85,

139.37, 138.87, 136.01, 130.44, 128.34, 128.28, 127.65, 127.28, 126.93, 126.34, 124.53,

76.84, 34.28, 31.89, 29.40, 29.26, 29.23, 28.99, 22.76, 14.23; MS (EI): (isotopic cluster at)

m/z = 603 ([M+H]+).

82

2.9 Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-5-octylamino-4,7-di(thiophen-2-yl)-

2,1,3-benzothiadiazole)-5,5-diyl]

2.6 (62.8 mg, 0.107 mmol), 9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol)

ester (70.6 mg, 0.107 mmol), Pd(PPh3)4 (3.2 mg, 0.003 mmol) and a stirrer bar were added

to a 5 mL high pressure microwave reactor vial. The vial was then sealed with a septum and

flushed with argon, before degassed toluene (1.7 mL), degassed aqueous 1 M Na2CO3 (0.3

mL) and a drop of aliquot 336 was added. The resulting solution was degassed for 30 min

before the reaction was heated to 120 °C for 3 days. A solution of phenyl boronic acid (3 mg

in 0.1 mL toluene, 0.021 mmol) was injected and the reaction stirred for 2 h at 120 °C.

Bromobenzene (7 mg, 0.043 mmol) was then added and the resulting mixture heated for a

further 2 h. The reaction was then cooled to room temperature, precipitated in methanol

(60 mL), stirred for 30 min and filtered through a Soxhlet thimble. The polymer was then

extracted (Soxhlet) using methanol, acetone, hexane and chloroform in that order under

Argon. The chloroform fraction was collected and concentrated to ~70 mL, to which a

solution of aqueous sodium diethyldithiocarbamate dihydrate solution (~100 mg in 70 mL)

was added. The two layers were stirred vigorously at 60 °C for 60 min, with a condenser

attached, to extract the palladium. The chloroform layer was extracted and washed

83

thoroughly with water (3 x 100 mL). The organic layer was dried (MgSO4), filtered and

concentrated to ~10 mL before being precipitated into methanol (60 mL), stirred for 30 min

and filtered. This precipitation was repeated again to yield 2.9 as a black solid (76 mg, 85%).

The polymer (76 mg) was then fractionated using a preparative GPC running in

chlorobenzene to obtain 24 mg of 2.9 with Mn of 19 kDa, 36 Mw of kDa, Mw/Mn (Ð) = 1.90;

1H NMR (400 MHz, CDCl3) δ 8.20 (br, 1H), 8.12 (br, 2H), 7.90 (br, 1H), 7.74 – 7.51 (m, 6H),

7.39 (br, 1H), 5.07 (br, 1H), 4.68 (br, 1H), 3.46 (s, 2H), 2.39 (br, 2H), 2.23 (s, 2H), 1.75 (br,

2H), 1.42 – 1.05 (m, 34H), 0.87 (d, J = 7.6 Hz, 3H), 0.79 (t, J = 6.7 Hz, 6H); Anal. Calcd. for

C53H64N4S3 C 71.87, H 7.78, N 6.76, found: C 71.68, H 7.05, N 6.70.

2.10 Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-5-octyloxy-4,7-di(thiophen-2-yl)-


2.7 (142.5 mg, 0.243 mmol), 9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid

bis(pinacol) ester (159.8 mg, 0.243 mmol), Pd(PPh3)4 (5.6 mg, 0.005 mmol) and a stirrer bar

were added to a 5 mL high pressure microwave reactor vial. The vial was then sealed with a

septum and flushed with argon, before degassed toluene (3 mL), degassed aqueous 1 M

84

Na2CO3 (0.6 mL) and a drop of aliquot 336 was added. The resulting solution was degassed

for 30 min before the reaction was heated to 120 °C for 3 days. A solution of phenyl boronic

acid (6 mg in 0.2 mL toluene, 0.050 mmol) was injected and the reaction stirred for 2 h at

120 °C. Bromobenzene (15 mg, 0.09 mmol) was then added and the resulting mixture

heated for a further 2 h. The reaction was then cooled to room temperature, precipitated in

methanol (100 mL), stirred for 30 min and filtered through a Soxhlet thimble. The polymer

was then extracted (Soxhlet) using methanol, acetone, hexane and chloroform in that order

under Argon. The chloroform fraction was washed with aqueous sodium

diethyldithiocarbamate dehydrate as above. The chloroform layer was extracted and

washed thoroughly with water (3 x 100 mL). The organic layer was dried (MgSO4), filtered

and concentrated to ~10 mL before being precipitated into methanol (60 mL), stirred for 30

min and filtered. This precipitation was repeated again to yield 2.10 as a black solid (176

mg, 89 %). The polymer (150 mg) was then fractionated using a preparative GPC running in

chlorobenzene to obtain 50 mg of 2.10 with Mn of 28 kDa, Mw of 49 kDa, Mw/Mn (Ð) =

1.75; 1H NMR (400 MHz, CDCl3) δ 8.68 (br, 1H), 8.21 (br, 1H), 8.11 (br, 2H), 7.89 (br, 2H),

7.73 (br, 1H), 7.62 (br, 2H), 7.55 (br, 2H), 4.70 (br, 1H), 4.46 (br, 2H), 2.43 (br, 2H), 2.14 (br,

2H), 2.04 (br, 2H), 1.71 (br, 2H), 1.43 – 1.06 (m, 32H), 0.89 (br, 3H), 0.79 (br, 6H); Anal.

Calcd. for C51H63N3S4 C 73.78, H 7.65, N 5.06, found: C 73.69, H 7.48, N 5.12.

2.11 Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-5-octylthio-4,7-di(thiophen-2-yl)-


85

2.8 (202.1 mg, 0.335 mmol), 9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid

bis(pinacol) ester (220.6 mg, 0.335 mmol), Pd(PPh3)4 (7.8 mg, 0.007 mmol) The vial was then

sealed with a septum and flushed with argon, before degassed toluene (3.6 mL), degassed

aqueous 1 M Na2CO3 (0.8 mL) and a drop of aliquot 336 was added. The resulting solution

was degassed for 30 min before the reaction was heated to 120 °C for 3 days. A solution of

phenyl boronic acid (8 mg in 0.2 mL toluene, 0.066 mmol was injected and the reaction

stirred for 2 h at 120 °C. Bromobenzene (21 mg, 0.132 mmol) was then added and the

resulting mixture heated for a further 2 h. The reaction was then cooled to room

temperature, precipitated in methanol (120 mL), stirred for 30 min and filtered through a

Soxhlet thimble. The polymer was then extracted (Soxhlet) using methanol, acetone,

hexane and chloroform in that order under Argon. The chloroform fraction was washed

with aqueous sodium diethyldithiocarbamate dehydrate. The chloroform layer was

extracted and washed thoroughly with water (3 x 100 mL). The organic layer was dried

(MgSO4), filtered and concentrated to ~10 mL before being precipitated into methanol (60

mL), stirred for 30 min and filtered. This precipitation was repeated again to yield 2.11 as a

black solid (250 mg, 89 %). The polymer (100 mg) was then fractionated using a preparative

GPC running in chlorobenzene to obtain 90 mg of 2.11 with Mn of 24 kDa, Mw of 53 kDa,

86

Mw/Mn (Ð) = 2.25; 1H NMR (400 MHz, CDCl3) δ 8.23 (br, 1H), 8.13 (br, 2H), 8.05 (br, 1H),

7.90 (br, 1H), 7.73 (br, 1H), 7.60 (br, 3H), 7.55 (br, 2H), 4.68 (br, 1H), 3.12 (br, 2H), 2.40 (br,

2H), 2.02 (br, 2H), 1.80 – 1.72 (m, 2H), 1.36 – 1.10 (m, 34H), 0.86 – 0.82 (m, 3H), 0.79 (t, J =

6.5 Hz, 6H); Anal. Calcd. for C53H63N3S4 C 72.38, H 7.50, N 4.97, found: C 72.24, H 7.63, N

5.13.

87

Chapter 3 Post-polymerisation functionalisation of

semiconducting polymers containing fluorinated

electron deficient units

88

3.1 Introduction

Side-chain engineering of semiconducting polymers is prevalent in the literature. Altering

the side-chains allows for the structural characteristics of the polymer to be manipulated,

whilst keeping the semiconducting properties of the polymer relatively preserved. Side-

chain engineering has been shown to be a useful way of manipulating the reactivity,

morphology and surface energy of polymeric systems.118 There are many examples in a

variety of fields in organic electronics. Polymers with cyano-terminated and fluorinated

side-chains have shown to alter the surface energy of thin-films.178,179 Siloxane-terminated

and ethylene-glycol side-chains have shown to increase electron mobility of polymer thin-

films.180,181 Quaternary ammonium and other salts have been incorporated onto polymer

backbones to create water-soluble polyelectrolytes.182,183 Polymers with cross-linkable

groups on the side-chain have shown to increase the thermal stability of OPVs.184–189

Affixing reactive group terminated side-chains have also allowed for the click of complex

structures onto polymer backbones such as ferrocene, dyes and even quantum dots.190,191

89

Scheme 3.1: Typical synthetic route yielding polymers with desired functional group ‘Y’ on the backbone. Final step is optional if functional group required can withstand polymerisation conditions.

The most common way of introducing functional group terminated side-chains onto

polymer backbones has been to first synthesise a monomer with functional group-

terminated side-chains. The modified monomer can be copolymerised with other

monomers to yield a functionalised polymer. The ratio of monomers can be adjusted to

give the desired loading of functional group on the polymer backbone. In many cases the

functional group required cannot survive (or could perturb) the polymerisation process. In

this case, a monomer is synthesised with a stable precursor to the functional group required

and the polymer is formed as before. The groups on the side-chains are then converted to

the required functional group through post-polymerisation reactions. This strategy is

illustrated in Scheme 3.1.

An example of this process is the synthesis of an azide-functionalised poly(3-

hexylthiophene) (P3HT) polymer by Nam et al..185 The polymer was synthesised by the

copolymerisation of a thiophene monomer with a protected-alcohol terminated side-chain

(synthesised over two steps) with a hexyl-thiophene comonomer. The resulting protected-

90

alcohol polymer underwent a deprotection step followed by a Mitisnobu-type reaction to

yield the azide-functionalised polymer. In this study 5% of the polymer backbone was

functionalised with azide. Chiu et al. also applied a similar strategy to polymer

nanoparticles.192 They introduced carboxylic acid groups on backbone of a poly(9,9-

dioctylfluorene-alt-benzothiadiazole) (F8BT) based polymer and under the formation of

nanoparticles the acid groups coated the surface, making them available for bioconjugation

reactions. However, this study highlights an important limitation of this synthetic strategy.

A systematic study of different loadings of the carboxylic acid-functionalised side-chains was

performed. This resulted in a synthesis of separate polymer batches for each loading of

carboxylic acid side-chains and led to varying molecular weights and dispersity from batch to

batch. This can therefore make the establishment of structure-property relationships

problematic due to potentially competing effects.

In the previous chapter the nucleophilic aromatic substitution (SNAr) of fluorine substituents

of a fluorinated benzothiadiazole monomer with octanethiol, octanol and octylamine was

discussed. The scope of this reaction was then developed directly onto the backbone of

semiconducting polymers, Preliminary work by Dr Abby Casey has shown that

dodecanethiols can readily substitute fluorine substituents on two benzothiadiazole based

polymers, 3.5-F and 3.4-F (structures in Table 3.1).193

Scheme 3.2 shows the reaction overview which will be discussed in this chapter. In general,

a polymer containing a fluorinated electron-deficient unit can undergo nucleophilic

aromatic substitution with a thiol or alcohol. The nucleophile can also be terminated with a

91

functional group (labelled Y). In theory, this reaction allows for functional groups to be

introduced to a polymer backbone, in one step, from common semiconducting polymers.

This reaction has many advantages over the monomer synthesis strategy discussed

previously. Firstly, the product can be easily purified. In the majority of cases it was found

that the reaction mixture could be precipitated directly into methanol and unreacted thiol

could be removed under Soxhlet purification (with acetone or methanol) before extraction

of the polymer with chloroform. The synthesis of functionalised monomers before polymer

synthesis (Scheme 3.1) would most likely be much harder to purify, especially as Stille and

Suzuki polymerisations require a high level of monomer purity for high molecular weight

polymers.194 This has been illustrated in the previous chapter; the synthesis of thioalkyl

monomer 2.8 required purification by column chromatography and recrystallisation to yield

a monomer pure enough for the subsequent polymerisation reaction. Also, this synthesis

involved a simple thioalkyl group, other synthetic challenges would most likely occur if the

thiol group was terminated with a functional group.

Another advantage of the post-polymerisation reaction is the quantity of material needed to

yield a functional polymer. This reaction has been found to be successful on as little as 3 mg

of fluorinated polymer, meaning that test reactions can be economical with the resources

available. Finally, the percentage of fluorine atoms substituted with nucleophile can be

carefully controlled. This negates the need for a separate polymer synthesis for each

desired loading of functional group, and means that a batch with identical molecular weight

and dispersity can be utilised.

92

Scheme 3.2: Overview of post-polymerisation reaction discussed in this chapter. Reaction occurs with thiols, thioacetates and alcohols on polymers containing the fluorinated electron deficient units depicted. BT = benzothiadiazole, Tz = benzotriaziole, TT = thienothiophene.

In this chapter the work by Dr Abby Casey is built upon, showing the impressive versatility of

this substitution reaction. The substitution reaction was found to be successful on polymers

containing fluorinated benzotriazole (Tz) and thienothiophene (TT) units, as well as

benzothiadiazole (BT) units (Scheme 3.2). The substitution reaction was also successful with

thioacetates and alcohols under altered reaction conditions. Finally, the impressive

functional group tolerance of the reaction by reacting poly(9,9-dioctylfluorene-alt-5-fluoro-

benzothiadiazole) (P(F8fBT)) with a variety of functionalised thiols, thioacetates and alcohols

is discussed. Polymers 3.4-F and 3.5-F were synthesised by Dr Abby Casey and 3.7-

Copolymer was synthesised by Shengyu Cong.

93

3.2 Post-polymerisation reaction

The reaction of simple alkyl-thiols with a range of semiconducting polymers will be

discussed first. From initial test reactions, the substitution reaction was found to proceed in

a reliable fashion under the following conditions: the polymer is dissolved in a mix of

chlorobenzene and DMF (3:1, v:v) with an excess of K2CO3. An excess of thiol is then added

and the reaction is heated at 120 °C for 30 minutes, in a microwave reactor.

The first step in validating this reaction was to ensure that the reaction conditions yielded

the same polymer when synthesised from the thiol-substituted monomer. This is to ensure

that all other bonds in the polymer backbone remain intact under the reaction conditions.

The thiol-substituted polymer from the previous chapter (2.11) was chosen as the desired

target polymer as the fluorine and thiol-substituted monomers (2.2 and 2.8 respectively)

had both been synthesised and characterised fully.

94

Scheme 3.3: Synthetic routes to thiol-substituted polymer; A) 120 °C 30 min (microwave), excess K2CO3, 3:1 (v:v) CB:DMF; B) 9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester, Pd(PPh3)4, toluene, Na2CO3 (aq), 120 °C 3 days.

The fluorinated polymer (poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-5-fluoro-4,7-

di(thiophen-2-yl)-2,1,3-benzothiadiazole)-5,5-diyl], 3.1) was synthesised by first

copolymerising the carbazole monomer (9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic

acid bis(pinacol) ester) with the fluorinated comonomer (2.2) (Scheme 3.3). However, the

resulting polymer was found to be sparingly soluble and only 5 mg (2% yield) of polymer was

obtained under Soxhlet extraction. This was just enough material to characterise and react

further.

This polymer was then heated in a microwave reactor at 120 °C for 30 minutes in a solution

of excess K2CO3 and octanethiol dissolved in a 3:1 (v:v) mix of chlorobenzene and DMF

(Scheme 3.3). The UV-Vis absorption spectra and proton NMR of 3.2 and 2.11 were found

to be very similar (Figure 3.1). This suggests that the SNAr reaction successfully substitutes

95

the fluorine atoms on the polymer, without adversely affecting other parts of the polymer

backbone. As discussed in the previous chapter, the blue shift in UV-Vis absorption occurs

due to the twisting of the backbone due to the large thioalkyl group introduced, reducing

the band-gap as a result.

Figure 3.1: a) Normalised UV-Vis absorption and b) NMR spectra of 2.11 and 3.2.

The versatility of this reaction with other high-performing semiconducting polymers was

then investigated. Chemical structures and UV-Vis spectra for each polymer can be found in

Table 3.1. For each polymer, the 1H and 19F NMR was recorded before and after the

substitution reaction (see Appendix). In all cases, after completion of the reaction, the

absence of peaks in 19F NMR was observed. Each polymer was heated in a microwave

reactor at 120 °C for 30 minutes in a solution of excess K2CO3 and alkyl-thiol, in a 3:1 (v:v)

mix of chlorobenzene and DMF.

96

Scheme 3.4: Synthesis of 3.3-F from commercially available 4,7-dibromo-5,6-difluoro-2-(2-butyloctyl)-2H-benzotriazole; i) 2-thienylzinc bromide, Pd(PPh3)4, THF; ii) NBS, THF; iii) 6-bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophene-2-yl)-benzo[1,2-b;4,5-b']dithiophene, Pd2(dba)3, o-xylene. Structure of 3.3-F in Table 1.1

P(BDTdTdFTz) (3.3-F) is a novel benzotriazole (BTz) based polymer. Polymers with similar

structures have exhibited power conversion efficiencies exceeding 11% in organic

photovoltaic (OPV) devices.132 The synthesis of the BTz comonomer (3.3-M2) was based the

synthesis of 2.3 in the previous chapter (Scheme 3.4). The starting material, 4,7-dibromo-

5,6-difluoro-2-(2-butyloctyl)-2H-benzotriazole, was converted to 3.3-M1 via a Negishi

coupling with 2-thienylzinc bromide. 3.3-M2 was then synthesised by bromination with N-

bromosuccinimide (NBS) at room temperature. Both reactions proceeded with yeilds

exceeding 87%. 3.3-M2 was then reacted with the comonomer, 6-bis(trimethyltin)-4,8-

bis(5-(2-butyloctyl)thiophene-2-yl)-benzo[1,2-b;4,5-b']dithiophene, under standard Stille

conditions, yeilding 3.3-F. BTz groups are a less electron deficient than BT but are a useful

tool as they allow for solubilising chains to be affixed to the electron deficient unit.

Gratifyingly, the substitution reaction was shown to work with this polymer. The

alkylthiopolymer (3.3-SR) exhibited a large blue shift and a loss of the shoulder peak in the

UV-Vis. This is most likely due to the addition twisting of the backbone leading to less

aggregation in solution and lowering of the conjugation length.

97

Thiol-substitution reactions have both been previously reported for 3.4-F and 3.5-F by Dr

Abby Casey.193 They are included in this chapter to illustrate the versatility of this reaction

and to add to a discussion of structure-property relationships. Interestingly, polymer 3.4-F

exhibited very little changes in the UV-Vis spectra upon thiol-substitution. This has been

attributed to the vinyl units between the thiophene and benzothiadiazole units preventing

steric clash between thioalkyl and alkyl chains. The reaction on the indenofluorene based

polymer 3.5-F was accompanied by the familiar drop in ICT and blue shift.

Table 3.1: Structures of polymers before and after thiol-substitution, with corresponding UV-Vis absorption.

Polymer UV-Vis

Mn / Mw (kDa)

Before substitution

After substitution

38 / 104 37 / 80

51 / 83 28 / 48

12 / 22 9 / 14

21 / 67 20 / 66

300 350 400 450 500 550

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lis

ed

ab

so

rpti

on

Wavelength (nm)

3.7-F

3.7-SR

44 / 144 60 / 99

101

Polymer 3.6, P(BDTTT-EFT) (often referred to as PTB7) is a high performance polymer which

has shown to give power conversion efficiencies exceeding 11% in OPV devices.195

Interestingly the fluorinated thienothiophene group was also shown to readily undergo

substitution with thiols under the same reaction conditions. The final polymer in the series

is the highly emissive poly(9,9′-dioctylfluorene-5-fluoro-2,1,3-benzothiadiazole) polymer

3.7-F. The non-fluorinated analogue is a commercially available material commonly used in

light emitting diodes, transistors and semiconducting polymer nanoparticles (SPNs).75,196–199

The reaction of the polymer, with excess octanethiol, yielded a product with no blue shift

under UV-Vis, a relative lowering of the ICT intensity and the emergence of a new band at

360 nm (discussed in section 3.4).

A batch of 3.7-F with a lower molecular weight (Mn) was also synthesised. This allowed for

the molecular mass of the repeat unit to be determined by matrix-assisted laser

desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF). This technique was

found to be unsuccessful on high molecular weight analogues. The mass of the repeat unit

can usually be determined by calculating the difference in the major m/z peaks (Δ1 and Δ2,

Figure 3.2). The low molecular weight polymer was reacted with an excess dodecanethiol as

before and isolated under precipitation. The fluorinated and thioalkyl polymers were both

analysed by MALDI-TOF (Figure 3.2 a) and b), respectively). The difference between major

peaks, Δ1 and Δ2 was found to be 541 and 724 m/z respectively, which shows good

agreement with the predicted mass of repeat units (541 and 723 g mol-1, respectively). It

should be noted that the MALDI spectra for the thiol-substituted polymer (Figure 3.2b) also

102

shows m/z peaks with intensity close to that of the major peaks. The smaller peak at lower

m/z values than each major peak (indicated with a †) indicate a loss of 201 mass units,

which is equal to the mass of a S-C12H25 chain. The peaks to the right of the three major

peaks (indicated with a *) indicate a gain of 190 mass units (or a loss of 538 from the larger

major peak), which could not be attributed any plausible structural changes upon ionisation.

Figure 3.2: MALDI spectra of a low molecular weight batch of a) 3.7-F and b) 3.7-SR (dodecanethiol substituted). Molecular weight of repeat units of 3.7-F and 3.7-SR are 541 and 723 g mol

-1 respectively.

The molecular weights of each polymer can be found in Table 3.1 (Mn and Mw). In most

cases, both the Mn and Mw were found to decrease upon thiol substitution. This shows

that the polymer is interacting with polystyrene stationary phase differently to the

fluorinated analogue, most likely due to structural changes in the polymer.

103

3.3 Control of thiol loading

The control of the percentage of fluorine substitution was then investigated. P(F8fBT) (3.7-

F) was chosen as the material to test further due to its excellent solubility in chloroform,

which resulted in good quality NMR spectra. Five separate reactions were performed with

the following conditions: 5 mg of polymer was added to an excess of K2CO3, then dissolved

in 2 mL of a 3:1 (v:v) mix of chlorobenzene and DMF under argon. The desired mol% of

dodecanethiol was then added to the reaction mixture, which was then degassed for 30 min

(to deter disulfide formation). The resulting mixture was then heated in the microwave

reactor at 150 °C for 60 min. The resulting solution was precipitated in MeOH and filtered

through a Soxhlet thimble. Residual DMF was then removed using Soxhlet apparatus with

acetone. The polymer was then extracted with chloroform. After removing the solvent

under vacuum the 1H NMR and UV-Vis was recorded. A longer reaction time and higher

temperatures were used to ensure all the thiol had reacted.

104

Figure 3.3: Control of loading of docadecanethiol loading onto polymer 3.7-F: a) Structure of substituted polymers; b) normalised UV-Vis absorption spectra of each polymer, in chlorobenzene solution; c) Stacked

1H

NMR spectra with increasing mol% of thiol, aromatic peaks integrated to 7H for each spectrum. Integration value for HA (𝑥) listed in table for each mol% of thiol reacted.

The mol% of dodecanethiol was gradually increased for each reaction, starting with 20 mol%

then 40, 60 and 80 mol%. Finally an excess of thiol was reacted to give the fully substituted

polymer. Figure 3.3 shows the 1H NMR and normalised UV-Vis spectra, with increasing

105

alkyl-thiol content. The polymer UV-Vis showed a gradual decrease in the relative intensity

of the ICT band whilst simultaneously increasing the intensity of the new band at 360 nm

(discussed in section 3.4). The level of fluorine displacement was estimated by integrating

the new -SCH2- proton signals (at 3.03 ppm) against the aromatic proton signals in each 1H

NMR spectrum, which should always integrate to 7H. As expected, the integration of the -

SCH2- proton signals was 2H when an excess of thiol was reacted. When 20 mol% of thiol

was reacted the integration was found to be 0.4H, indicating that 20% of the fluorine atoms

had been substituted on the polymer backbone. All of the reactions proceeded in this

fashion, showing that excellent control of thiol loading had been achieved with the post-

polymerisation reaction.

106

Figure 3.4: Photoluminescence quantum yield (PLQY) of P(F8fBT) (3.7-F) with increasing percentage of thiol substitution in THF solution (black) (1.67 x 10

-2 g cm

-2), thin film (red) and in nanoparticle suspensions in water

(blue). Measured using an integrating sphere.200

The photoluminescence quantum yield (PLQY) of the series of polymers was then recorded,

using an integrating sphere using the method developed by de Mello et.al..200 In THF

solution, the PLQY was found to decrease with increasing thiol content from 95% for the

fully fluorinated polymer to 55% for the fully thiol-substituted polymer (Figure 3.4). In thin

films the thiol content was found to have a similar effect on the PLQY. The fully fluorinated

material presented a PLQY of 24% whereas the fully thiol-substituted polymer gave 16%.

However, in this case, a trend in the PLQY with thiol substitution was less clear. This is most

likely due to subtle changes in the morphology, which has been shown to lead to changes in

the PLQY of polymer thin films.201,202 An aqueous solution of polymer nanoparticles of the

fully fluorinated and thiol-substituted polymers were also synthesised. Interestingly, the

thiol-substitution has a much smaller effect on the PLQY of the nanoparticle solutions, only

107

decreasing from 54 to 42% in this case. The synthesis, characterisation and properties of

polymer nanoparticles of P(F8fBT) will be discussed in much more detail in Chapter 4.

3.4 Relating absorption properties to structure using DFT and TD-

DFT

As discussed previously, the substitution of fluorine with thioalkyl groups on 3.7-F results in

the formation of a new band in the UV-Vis spectrum (at ~360 nm, Figure 3.3b). This was

investigated further by DFT analysis, using similar methodology described in section 2.3.

The optimised ground state of trimer molecules, with methyl instead of octyl groups, was

calculated. The central BT unit, with surrounding monomers for both polymers, are


Figure 3.5: Central units of the trimer of each polymer for DFT calculations. Methyl groups were used to simplify calculations. Dihedral angles around BT unit are labelled.

The dihedral angles were measured around the central BT unit (labelled in Figure 3.5). The

angle on the non-substituted side (labelled ‘Ψ’) showed a small change of ~2°. However, on

108

the substituted side the dihedral angle (labelled ‘θ’) showed a large change of ~20°.

Interestingly, as can be seen from absorption spectra in Figure 3.6, there is a very small blue

shift upon thiol substitution (~2 nm). This is contrary to the findings for carbazole polymer

3.2 (section 2.3) where large torsional twisting of 44° resulted in a large blue shift of 43 nm

(Figure 2.2) (note, DFT analysis was performed on the fluorinated precursor (3.1) and was

found to be almost completely planar). This suggests that backbone twisting is not the only

contributing factor to the observed change in band-gap from thiol substitution.

Once the optimised ground state geometries were determined, the excitation energies and

associated oscillator strengths for transitions could be calculated. In general, the greater

the oscillator strength, the more allowed a transition is and the greater absorption

coefficient observed in UV-Vis spectra.203 The first twelve excitation energies were

calculated using the TD-DFT methodology with CAM-B3LYP functional with a 6-31G(d) basis

set.176 Solvent effects were simulated using a polarisable continuum model using

chloroform as the solvent of choice. The oscillator strengths as a function of wavelength

can be found in Figure 3.6.

109

Figure 3.6: UV-Vis spectra (in chloroform) for 3.7-F and 3.7-SR overlaid with the: a) calculated oscillator strength and b) adjusted oscillator strengths. Oscillator strengths are adjusted to match the λmax of the low energy transition. Calculated oscillator strengths for 3.7-F and 3.7-SR were adjusted by 29.5 and 41.3 nm respectively.

Gratifyingly, the fluorinated polymer (3.7-F) exhibited two transitions with large oscillator

strengths (above 1) whilst the other transitions all had values of less than 0.25. The two

transitions were predicted to occur at approximately 410 nm and 290 nm. The difference in

wavelength between the two major transitions showed very good agreement with the

difference between the λmax of the two absorption bands (the ICT and π-π* band) in the UV-

Vis spectrum (~120 nm). However, the absolute energy was overestimated by 30 nm which

can be clearly seen by overlaying the UV-Vis absorption with calculated oscillator strengths

(Figure 3.6a). The offset is most likely because DFT calculations were performed on a

trimer, which will presumably not be reflecting the effective conjugation length of the

polymer. Hence, the predicted transition energies will be larger than that experimentally

observed. To accommodate for this the wavelengths of the lowest energy transition was

300 350 400 450 5000.0

0.2

0.4

0.6

0.8

1.0

No

rma

lis

ed

Ab

so

rpti

on

Wavelength (nm)

0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Os

cilla

tor

Str

ne

gth

3.7-F

3.7-SR

300 350 400 450 5000.0

0.2

0.4

0.6

0.8

1.0

No

rma

lis

ed

Ab

so

rpti

on

Wavelength (nm)

0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Os

cilla

tor

Str

ne

gth

3.7-F

3.7-SR

a) b)

110

matched with the low energy λmax. The corrected oscillator strengths and UV-Vis spectrum

as a function of wavelength are plotted in Figure 3.6b.

In the case of 3.7-SR there was also two transitions with large oscillator strengths (at 395

and 285 nm), in which the difference in energy again showed excellent agreement with that

observed in the UV-Vis spectrum (~120 nm). Interestingly, there was also another transition

(between the two major transitions) with a comparable oscillator strength of 0.5 and a

wavelength of 310 nm. The absolute values were again overestimated, illustrated by the

overlayed UV-Vis and oscillator strengths (Figure 3.6a). When the oscillator strengths were

adjusted to match the UV-Vis spectrum (as before) the new transition (now at 355 nm)

showed excellent alignment with the new band observed in the UV-Vis spectrum (Figure

3.6b). These calculations show that the substitution of fluorine with sulfur gives access to a

new (or previously unallowed) transition, which is most likely responsible for the new band

observed in the UV-Vis. They also correctly predict a relative drop in intensity of the ICT

band.

111

Figure 3.7: Natural transition orbitals (NTO’s), of the ground and excited state, for the transition predicted at 310 nm for 3.7-SR and 3.7-OR.

The calculations discussed above also yield a list of all of the orbitals which contribute to

each transitions. Natural transition orbital (NTO) calculations condense these orbital

transitions into a set of ground and excited state orbitals to allow for a qualitative analysis

of the transitions involved. The NTO’s were calculated for each of the major transitions

discussed above. The predicted transitions responsible for the ICT and π-π* band observed

in the UV-Vis spectrum showed very similar NTO’s for both polymers (see Appendix Figure

5-6). The NTO’s associated with the new transition in 3.7-SR are depicted in Figure 3.7.

Interestingly, the ground state orbitals show a large contribution from the orbitals located

on the sulfur atom, whereas the excited state orbitals were mostly located on the BT ring.

This suggests that the allowed transition is a direct result of the sulfur atom.

As will become apparent later in this chapter, the substitution of fluorine atoms on 3.7-F is

also possible with alcohols (see 3.7-OR in Scheme 3.5 for the chemical structure). Curiously,

112

the UV-Vis of 3.7-OR (Figure 3.8a) does not exhibit the new band at 360 nm, but does

exhibit the relative lowering of the ICT band. This was investigated further using the same

computational methods discussed above. Also, the emission spectra of 3.7-SR and 3.7-OR

exhibited a small red-shift of 17 nm when compared to 3.7-F (see Appendix Figure 7).

Polymer 3.7-OR was found to be more planar, with a dihedral angle change of ~7°, than the

thioalkyl analogue. The UV-Vis spectra and oscillator strengths can be seen in Figure 3.8a

and the corrected data can be seen in Figure 3.8b. Interestingly, there are now two

transitions observed at low wavelengths (300 and 280 nm, Figure 3.8a) which could explain

why the π-π* band (at ~330 nm) is red-shifted compared to the fluorinated analogue.

However, there are no other transitions that compete in oscillator strength. The NTO’s for

the transition at 310 nm (with a low oscillator strength of 0.25) showed similarities with

those depicted for the new transition in 3.7-SR (also at ~310 nm). Both NTO’s are depicted

for the ground and excited state in Figure 3.7. The orbitals in the excited state are very

similar, whereas the ground state orbitals of 3.7-OR are less localised on the heteroatom

and more diffuse across the backbone than 3.7-SR.

113

Figure 3.8: UV-Vis spectra (in chloroform) for 3.7-F and 3.7-OR overlaid with the: a) calculated oscillator strength and b) adjusted oscillator strengths. Oscillator strengths are adjusted to match the λmax of the low energy transition. Calculated oscillator strengths for 3.7-F and 3.7-SR were both adjusted by 29.5 nm.

The computational analysis, discussed in this section, points to two possible reasons for the

observed new band in 3.7-SR. The first is a result of the larger sulfur orbitals, which can

contribute more to the orbitals responsible for the optical transitions. This new interaction

allows for previously non-existent or forbidden transitions to occur under optical excitation.

The second possible reason is a result of twisting of the backbone. As shown, 3.7-SR is more

twisted than both 3.7-OR and 3.7-F. Moreover, Figure 3.7 shows how the ground state

orbitals are more localised for the new transition NTO’s of 3.7-SR than 3.7-OR. Perhaps the

twisting restricts delocalisation of orbitals across the polymer backbone which in turn

causes a new transition to become more allowed.

300 400 5000.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

No

rma

lis

ed

Ab

so

rba

nc

e

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Os

sila

tor

Str

en

gth

300 400 5000.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

No

rmalised

Ab

so

rban

ce

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Oss

ilato

r S

tren

gth

3.7-F

3.7-SR

3.7-F

3.7-SR

a) b)

114

3.5 Post-polymerisation reaction with end-functionalised

reagents

The versatility of the post-polymerisation reaction with a selection of functionalised thiols

on P(F8fBT) (3.7-F) was investigated. The chemical structures of all polymers synthesised

are depicted in Scheme 3.5. The solution UV-Vis and full NMR spectra for each polymer are

included in the Appendix.

This post-polymerisation reaction was utilised to incorporate an azide-terminated side-chain

directly onto the backbone of 3.7-F. As briefly discussed, azides have been incorporated

onto semiconducting polymers before.19–22 In all of these references, a monomer with a

bromine-terminated side-chain is synthesised which is converted to an azide group, post-

polymerisation. Due to the relative low-cost of S-(3-azidopropyl)thioacetate (when

compared to the thiol analogue) the reaction conditions were altered to perform an in situ

deprotection of the thioacetate followed by substitution onto the polymer backbone. The

polymer was dissolved a solution of a 3:1 (v:v) mix of chlorobenzene and DMF, heated to

100 °C. An excess of NaOH was then added and the reaction was heated for 10 minutes. An

excess of the thioacetate was then added and heated for a further 10 minutes. The azide

functionalised polymer (3.7-N3) had the same UV-Vis as the fully substituted polymer (3.7-

SR, Table 3.1). Successful incorporation of thiol was evident from the -SCH2- peak (δ 3.02

ppm) and -CH2N3- peak (δ 3.34 ppm) in the 1H NMR spectra (Figure 3.13). Under IR analysis

115

the substituted polymer exhibited a distinctive peak at ~2100 cm-1 (Figure 3.12), in the

expected absorption range for organic azides.208

Alkene functionalised semiconducting polymers have also been an area of interest in the

literature, due to their reactivity in thiol-ene reactions.209–212 The same conditions used to

synthesise 3.7-N3 were also used to react S-(10-undecenyl) thioacetate to form 3.7-Alkene.

Successful incorporation of thiol was evident from the -SCH2- peak (at ~3.0 ppm) and the

distinctive alkene protons at ~5.8 and ~4.9 ppm in the 1H NMR spectra (Figure 3.13). The

solution UV-Vis spectrum was very similar to 3.7-N3.

116

Figure 3.9: Trimmed NMR spectra and structure of 3.7-SAc, with key peaks labelled.

Thiol-functionalised conjugated polymers have been reported in the literature.213,214 For

example, P3HT functionalised with thiols have been shown to self-assemble onto gold and

silver surfaces.215 The polymer was synthesised via a brominated P3HT polymer in which

bromine groups were converted to thiosulfates, post polymerisation. The thiol groups were

then formed in situ in order for the polymer to self-assemble onto silver and gold. P(F8fBT)

with thioacetate (a protected thiol) terminated chains (3.7-SAc) was synthesised using a

variation on the reaction discussed above. 8-Octanedithiol diacetate was added in excess

whilst the amount of base was carefully controlled, to avoid crosslinking. The polymer (3.7-

F) and octanedithiol diacetate was heated to 100 °C in a solution of a 3:1 (v:v) mix of

117

chlorobenzene and DMF. NaOH (35 mol%) of was added to the reaction mixture, the

reaction was heated for 10 minutes before a further 35 mol% of NaOH was added. This

reaction resulted in a polymer with approximately 35% of the fluorines displaced by thiols,

estimated from integration of the -SCH2- protons (δ 2.96 ppm) in the NMR spectra (Ha,

Figure 3.9). The spectra also showed the distinctive -CH2SAc- peak (Hb) and -SCOCH3- peak

(Hc) which gave the expected integration values. Under IR analysis the substituted polymer

exhibited a peak at ~1720 cm-1 and a broad peak a ~3330 cm-1, both previously reported for

thioacetate groups (Figure 3.12).216 Two equivalents of base were needed in this reaction,

relative to the thioacetate reacted, as during the deprotection step thioacetic acid is

produced which can protonate the formed thiolate ion. Hence, a second equivalent of base

was required to form the reactive thiolate species. Full substitution of the polymer was

attempted but this resulted in an insoluble product.

As touched on previously, carboxylic acid groups have been incorporated onto the backbone

of semiconducting polymers to allow for EDC coupling reactions to occur on the surface of

polymer nanoparticles.217,218 Carboxylic acid functionalised polymer (3.7-COOH) was

synthesised by reacting an excess of 11-mercaptoundecanoic acid with 3.7-F at 150 °C for 60

min under microwave irradiation, in the presence of an excess of K2CO3. The resulting

polymer had a UV-Vis resembling full substitution of the polymer but was sparingly soluble

in hot chlorobenzene. Therefore the amount of thiol was lowered to 30 mol%. The amount

of fluorine substituted on the polymer backbone was estimated to be 25% from NMR

spectroscopy, estimated from integration of the -SCH2- protons (δ 2.97 ppm) in the NMR

118

(Figure 3.15). The difference of 5 mol% between thiol added and reacted was most likely

due to disulfide impurities in the 11-mercaptoundecanoic acid. Under IR analysis the

substituted polymer exhibited a distinctive peak at ~1710 cm-1, in the expected absorption

range for carbonyl groups (Figure 3.12).

Scheme 3.5: Reaction web of P(F8fBT) (3.7-F) with functionalised thiols, thioacetates and alcohols resulting in functional polymers. All reactions in CB:DMF (3:1, v:v) solvent. i) S-(3-Azidopropyl)thioacetate, KOH; ii) (3-mercaptopropyl)trimethoxysilane, K2CO3; iii) poly(2-ethyl-2-oxazoline), K2CO3; iv) 11-mercaptoundecanoic acid, K2CO3; v) 1,8-octanedithiol diacetate, NaOH; vi) S-(10-undecenyl)thioacetate, KOH; vii) 2-ethyl-1-hexanol, KOH; viii) Triethylene glycol monomethyl ether, KOH.

120

Silane functionalised polymers have been synthesised previously by Behrendt et al..219 In

this example a fluorene monomer with alcohol-terminated side-chains is synthesised,

polymerised and is then converted to triethoxysilane. This material is then used to fabricate

hybrid silica-polymer nanoparticles. Trimethoxysilane functionalised P(F8fBT) (3.7-Silane)

was synthesised by reacting an excess (3-mercaptopropyl)trimethoxysilane with 3.7-F at 120

°C for 30 min under microwave irradiation, in the presence of an excess of K2CO3, under

anhydrous conditions.. The NMR of the key peaks is shown in Figure 3.10. Integration of the

-SCH2- protons (Ha), with respect to the seven aromatic protons, gave the expected value of

2H. Integration of the silane proton peaks (Hb) gave rise to a value close to 9H, however,

the peak showed several smaller peaks either side. This is presumably from hydrolysis of

methoxy groups, changing the chemical shift of the nearby Hb protons. The peak labelled

with a * indicates the presence of a small amount of thiol starting material, this could not be

removed with the purification steps.

121

Figure 3.10: Trimmed NMR spectra and structure of 3.7-Silane, with key peaks labelled.

Graft copolymers with a semiconducting component have featured in the literature in an

effort to control the microstructure of polymer films.220,221 For example, Subbiah et al.

synthesised P3HT, by Grignard metathesis, end-capped with a norbornene terminated

group. The norborene groups were then polymerised under ring-opening metathesis to

yield an aliphatic polymer with semiconducting polymer side-chains.222 A graft copolymer

based on P(F8fBT), with aliphatic polymer side-chains (3.7-Copolymer) was synthesised

under the standard conditions, with an excess of thiol terminated poly(2-ethyl-2-oxazoline)

(Mn ca. 2000). The resulting polymer showed excellent solubility in methanol. Under IR

analysis the substituted polymer exhibited a distinctive peak at ~1632 cm-1, in the expected

absorption range for carbonyls (Figure 3.12). The NMR of the key peaks is shown in Figure

3.11. The aromatic peaks on the conjugated polymer backbone were integrated to 7H. The

122

terminal phenyl and -PhCH2- protons (Hb) of the aliphatic polymer gave integration values of

5H and 2H respectively, indicating full substitution of fluorine atoms.

Figure 3.11: Trimmed NMR spectra and structure of 3.7-Copolymer, with key peaks labelled.

The substitution reaction was also found to be successful with functionalised alcohols. 2-

Ethyl-1-hexanol was added to a solution of 3.7-F in a 3:1 (v:v) mix of chlorobenzene and

DMF, with excess KOH. The resulting mixture was then heated in the microwave reactor at

150 °C for 30 min. The resulting polymer (3.7-OR) exhibited a slightly different UV-Vis

spectrum to the thiol equivalent as discussed previously (Figure 3.8). The PLQY of 3.7-OR

was also recorded in solution and thin film and gave a value of 88 and 28% respectively.

This shows a greater stability of PLQY with substitution, compared to the thiol-subsumed

123

analogue. However, 3.7-OR was synthesised from a different batch of 3.7-F so a direct

comparison in PLQY values is not conclusive due to different molecular weights.201

Figure 3.12: Infra-red spectra of a) 3.7-SAc, b) 3.7-N3, c) 3.7-COOH and d) 3.7-Copolymer with 3.7-F as a reference. Unique peaks are labelled.

This reaction was then applied to incorporate ethylene glycol side-chains onto the polymer

backbone of P(F8fBT). Ethylene glycol chains have shown to be an effective biocompatible

surface coating for nanomaterials as they suppress non-specific adsorption of biological

matter.95,223,224 Conjugated polymers with glycol chains have been synthesised and have

shown to improve ion diffusion when applied to electrochemical transistors.225 Triethylene

124

glycol monomethyl ether was reacted with 3.7-F under the same conditions to form 3.7-

PEG. The NMR spectrum, with key peaks labelled, is shown in Figure 3.15. Successful

incorporation of alcohol was evident from the -OCH2- peak (~4.3 ppm), terminal methyl

group (~4.3 ppm) and distinctive glycol peaks (3.9-3.4 ppm) in the 1H NMR spectra.

3.6 Synthesis of multifunctionalised polymers

The substitution reaction was then applied to the synthesis of multifunctionalised polymers.

Multifunctionalised polymers are rarely reported in the literature, perhaps due to the

synthetic challenges involved. For example, He et al. synthesised a fluorescent

polyelectrolyte with FRET-acceptor dye and quaternary ammonium salts on the same

backbone.226 The dye became a FRET-acceptor in the presence of peroxide whilst the

ammonium salts allow for the polymer to be water soluble. The synthesis of this polymer

required a monomer functionalised with bromine-terminated side-chains and another with

a protected amine group side-chains. The monomers were copolymerised and the terminal

groups were converted to the ammonium salt and dye through three post-polymerisation

reactions. This work highlights how challenging it can be to synthesise multifunctionalised

polymers.

125

Figure 3.13: NMR spectrum and structure of 3.8 with key proton peaks labelled with integration values. Includes NMR spectra of 3.7-N3 and 3.7-alkene for reference.

A simple one-pot synthesis of F8BT with 50% azide and 50% alkene groups was first

attempted (labelled 3.8, structure in Figure 3.13). The fluorinated precursor (3.7-F) was

reacted with 50 mol% of S-(10-undecenyl)thioacetate for 10 min at 100 °C, in the presence

of excess KOH, followed by the addition of excess S-(3-azidopropyl)thioacetate. After a

further 10 minutes of heating, the polymer was isolated and characterised. Under IR

analysis the polymer exhibited the distinct azide peak at ~2100 cm-1 (see appendix).

The NMR spectra of 3.8 with key peaks can be seen in Figure 3.13, alongside the spectra of

3.7-N3 and 3.7-alkene for reference. To estimate the ratio of azide to alkene-terminated

126

side-chains, the key peaks were integrated relative to the aromatic protons (set to 7 protons

at 8.2 – 7.6 ppm). The N3CH2- protons (Hb) were found to integrate to one proton. Since

these protons equate to two in the fully modified polymer, the percentage of azide-

terminated side-chains was estimated to be 50%. Equally, the terminal alkene protons (He)

also integrated to one proton, indicating a 50% loading of alkene groups. This reaction

shows how a dual-functionalised polymer can readily be synthesised utilising the post-

polymerisation method.

UV-Vis spectroscopy was also used to track the reaction (Figure 3.14a). After the

incorporation of alkene side-chains a small aliquot of the reaction was extracted and diluted

in 3 mL chloroform. The resulting UV-Vis resembled a spectrum between 40 and 60%

modified 3.7-F (Figure 3.3b). After the addition of excess S-(3-azidopropyl)thioacetate, the

UV-Vis closely resembled the fully substituted 3.7-F. The use of UV-Vis spectroscopy allows

for very quick tracking of the reaction, and the UV-Vis spectra obtained can be compared to

known loading of alkylthiols to give an approximate substitution percentage.

127

Figure 3.14: Normalised UV-Vis spectra tracking synthesis of multifunctional polymers a) 3.8 and b) 3.9 with sequential addition of functional groups. Polymer dissolved in chloroform.

The post-polymerisation reaction was then applied to a more complicated system. A tri-

functionalised polymer was synthesised with carboxylic acid, azide and triethylene glycol

side-chains (labelled 3.9, Figure 3.15). 11-Mercaptoundecanoic acid (25 mol%) was reacted

onto the polymer backbone of 3.7-F. The polymer was worked up as before, analysed by

NMR and was found to contain 20% of carboxylic acid side-chains. The polymer was then

reacted with 60 mol% of triethylene glycol monomethyl in excess KOH for 10 minutes

before adding an excess of S-(3-azidopropyl)thioacetate. After a further 10 minutes of

heating the polymer was then worked up as before. The NMR spectra can be seen in Figure

3.15, with spectra of 3.7-COOH, 3.7-N3 and 3.7-PEG for reference.

The percentage loading of each substituent was again determined via the integration of the

aromatic peaks in the proton NMR (Figure 3.15). The -OCH2- protons on the ethylene glycol

chain (Hc) were found to integrate to 1.3 protons, since these equate to two protons the

128

percentage of ethylene glycol chains could be estimated to be 65%. There is overlap of

azide protons (Ha and Hd) with other proton peaks. However, the integration of -SCH2- on

the carboxylic acid was found to be 0.4H before the final step took place, therefore it be

inferred that the additional 0.2H at 3.0 ppm is due to the azide -SCH2- groups and estimate

that there is 10% azide-terminated side-chains on the polymer backbone. The azide

percentage can be estimated another way, the terminal ethylene glycol protons (He)

overlap with the -CH2N3 protons (Hd). The three He protons should integrate to 1.95H

(0.65H x 3), therefore, the additional 0.5H found at 3.3ppm can be attributed to the

additional azide protons. However this puts the percentage azide content at 25%. This

illustrates accuracy issues when using integration to estimate side-chain percentage. Even

so, the spectra show that the substitution reaction can be used to functionalise a polymer

with multiple functional groups. To achieve the same result by pre-polymerisation methods

would be synthetically much more difficult.

UV-Vis spectroscopy was again used to track the reaction (Figure 3.14b). Aliquots were

taken before the addition of each functionalised reactant. The relative intensity of the ICT

band (at 450 nm) was found to drop with the addition of each reactant.

129

Figure 3.15: NMR spectrum and structure of 3.9 with key proton peaks labelled with integration values. Includes NMR spectra of 3.7-COOH, 3.7-N3 and 3.7-PEG for reference.

A one pot synthesis of 3.9 was also achieved, starting from 3.7-F. The polymer was

dissolved in the solvent mixture with a pellet of KOH and 25 mol% of 11-

mercaptoundecanoic acid. The mixture was heated for one hour at 150 °C in a microwave

reactor. The reaction was then heated at 100 °C in an oil bath and 60 mol% triethylene

glycol monomethyl was added before heating for 10 minutes. An excess of S-(3-

azidopropyl)thioacetate was then added and the reaction was heated for a further 10

minutes. This yielded a polymer with a consistent NMR compared to the polymer

130

synthesised step-wise (see Appendix Figure 24). However, estimation of the loading of each

group was not possible due to the overlap of key peaks.

3.7 Conclusion

In this chapter a novel post-polymerisation reaction has been developed. Under moderate

heat and in the presence of base, polymers containing fluorinated electron-deficient units

can undergo nucleophilic substitution with alcohols, thioacetates and thiols. The versatility

of the reaction of thiols with polymers containing benzothiadiazole, benzotriazole and

thienothiophene units has been shown. Excellent control of the extent of polymer

modification could be afforded by carefully controlling the amount of alkylthiol added to the

reaction mixture, using P(F8fBT) (3.7-F) as the base polymer. The post-polymerisation

reaction also allowed for the introduction of functionalised side-chains onto the backbone

of P(F8fBT). A large selection of functionalised thiols, alcohols and thioacetate was reacted

with P(F8fBT) (3.7) to afford polymers monofunctionalised with azide, carboxylic acid,

thioacetate, alkene, trimethoxysilane and ethylene glycol groups. The reaction was also

used to synthesise a graft copolymer.

Finally, polymers with multiple functional groups on the same backbone were synthesised.

All of the modified polymers in this chapter were synthesised in a simple one-step reaction,

from the fluorinated polymer. The purification was also very simple, each reaction mixture

was precipitated and solid precipitate was washed with solvent to remove impurities. The

131

following chapters will explore the application of this reaction in the field of semiconducting

polymer nanoparticles and organic photovoltaics.

3.8 Experimental

3.8.1 Fluorinated-polymer synthesis

3.1 – P(CdTfBT)

5-Fluoro-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (2.2) (149.3 mg, 0.315 mmol), 9-(9-

heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester (206.1 mg, 0.315 mmol),

Pd(PPh3)4 (7.2 mg, 0.006 mmol) and a stirrer bar were added to a 5 mL high pressure

microwave reactor vial. The vial was then sealed with a septum and flushed with argon,

before degassed toluene (3 mL), degassed aqueous 1 M Na2CO3 (0.6 mL) and a drop of

aliquot 336 were added. The resulting solution was degassed for 30 min before the reaction

was heated to 120 °C for 3 days. The reaction was cooled to room temperature,

precipitated in methanol (100 mL), stirred for 30 min and filtered through a Soxhlet thimble.

The polymer was extracted (Soxhlet) using methanol, acetone, hexane and chloroform in

that order under Argon. The chloroform fraction was concentrated to ~1 mL before

precipitation into methanol (10 mL). The suspension was stirred for 30 min, filtered,

132

redissolved in chloroform and precipitated into methanol again to yield 3.1 as a black solid

(5 mg, 2%). Mn of 3.8 kDa, Mw of 4.9 kDa, Mw/Mn (Ð) = 1.29; 1H NMR (400 MHz, CDCl3) δ 8.37

– 7.43 (m, 11H), 4.81 – 4.55 (br, 1H), 2.50 – 2.33 (br, 2H), 2.11 – 1.96 (br, 2H), 1.42 – 1.07

(m, 22H), 0.90 – 0.74 (m, 6H).

3.3-M1 - 4,7-bis(thiophen-2-yl)-2-(2-butyloctyl)-5,6-difluoro-2H-benzo[d][1,2,3]triazole –

methods based on synthesis of 2.2 (Chapter 2)

4,7-Dibromo-5,6-difluoro-2-(2-butyloctyl)-2H-benzotriazole (2.2 g, 4.58 mmol) and Pd(PPh3)4

(297 mg, 0.256 mmol) were added to a 20 mL high pressure microwave reactor vial, with a

stirrer bar. The vial was then sealed with a septum, degassed and 2-thienylzinc bromide

solution in THF (20 mL of a 0.5 M solution, 10.05 mmol) was added. The solution was

flushed for 10 min before the reaction was heated for 30 min at 100 °C in a microwave

reactor. The reaction mixture was allowed to cool to room temperature, solvent was

removed under reduced pressure. The resulting viscous oil was dissolved in a 20% solution

of acetone in hexane and passed through a silica plug (10×5×5 cm). Solvent was removed

under reduced pressure, the residue was purified by column chromatography using 15%

DCM in hexane. The resulting product was purified by vapour diffusion recrystallization; the

product was dissolved in minimal amount of chloroform (at room temperature) in a small

133

sample vial. The vial was placed in a larger vial containing methanol, the tube was sealed

and left overnight. The resulting solid was filtered, washed with methanol and dried to

afford the product as a yellow solid (1.93 g, 3.97 mol); Yield: 87%; 1H NMR (400 MHz, CDCl3)

δ 8.33 (dd, J = 3.8, 1.1 Hz, 2H), 7.55 (d, J = 5.1, 1.1 Hz, 2H), 7.25 – 2.23 (m, 2H), 4.73 (d, J =

6.5 Hz, 2H), 2.29 (hept, 1H), 2.34 – 1.23 (m, 16H), 0.90 (t, J = 7.2 Hz, 3H), 0.85 (t, J = 6.9 Hz,

3H); 13C NMR (101 MHz, CDCl3) δ 148.65, 148.46, 146.14, 145.94, 137.75, 132.42, 130.18,

128.05, 128.02, 127.98, 127.45, 110.18, 110.13, 110.08, 110.04, 60.13, 39.23, 31.97, 31.59,

31.32, 29.70, 28.63, 26.38, 23.09, 22.80, 14.22; 19F NMR (400 MHz, CDCl3) δ 134.08; MS (EI):

m/z = 488.2 [M+].

3.3-M2 - 4,7-bis(5-bromothiophen-2-yl)-2-(2-butyloctyl)-5,6-difluoro-2H-

benzo[d][1,2,3]triazole - method based on literature procedure227

To a solution of (1.91 g, 3.92 mmol) in THF (40 mL) was added N-bromosuccinimide (NBS)

(1.54 g, 8.65 ,mol) and the reaction stirred in the dark for 18 h. The reaction mixture was

then poured into a saturated solution of sodium sulfite to remove all residual bromine and

extracted with chloroform. The organics were combined, dried (MgSO4), filtered, and

concentrated under reduced pressure. The resulting product was purified by vapour

diffusion recrystallisation as above. The resulting solid was filtered, washed with methanol

134

and dried to afford the product as a yellow solid (2.34 g, 3.62 mol); Yield: 92%; 1H NMR (400

MHz, CDCl3) δ 7.96 (d, J = 4.1 Hz, 2H), 7.12 (d, J = 4.1 Hz, 2H), 4.68 (d, J = 4.1 Hz, 2H), 2.23

(hept, 1H), 7.98 – 7.93 (m, 16H), 0.92 (t, J = 7.2 Hz, 3H), 0.86 (t, J = 6.7 Hz, 3H); 13C NMR (101

MHz, CDCl3) δ 148.37, 148.17, 145.85, 145.65, 137.13, 133.90, 130.40, 130.32, 116.11,

116.06, 116.02, 109.49, 109.45, 109.40, 109.35, 59.98, 39.26, 32.00, 31.58, 31.34, 29.73,

28.63, 26.37, 23.13, 22.83, 14.25; 19F NMR (400 MHz, CDCl3) δ 133.96; MS (EI): isotopic

cluster at m/z = 644.0 ([M]+).

3.3-F - P(BDTdTdFTz) – method based on previous work published in the group228

4,7-Bis(5-bromothiophen-2-yl)-2-(2-butyloctyl)-5,6-difluoro-2H-benzo[d][1,2,3]triazole

(505.9 mg, 0.784 mmol), 2,6-bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophene-2-yl)-

benzo[1,2-b;4,5-b']dithiophene (796.9 mg, 0.784 mmol), Pd2(dba)3 (14.3 mg, 0.016 mmol),

P(o-tol)3 (19.2, 0.063) and a stirrer bar were added to a 5 mL high pressure microwave vial.

The vial was then sealed with a septum and flushed with argon, before degassed o-xylene (5

mL) was added. The solution was degassed for 10 min under argon. The vial was heated by

microwave irradiation to 100 °C for 2 min, 140 °C for 2 min, 160 °C for 2 min, 180 °C for 10

min and 200 °C for 25 min. The solution was allowed to cool to room temperature,

chlorobenzene (3mL) was added to fully dissolve the polymer and the solution was

135

precipitated into methanol (200 mL). The resulting suspension was stirred for 30 min and

filtered through a Soxhlet thimble. The polymer was then extracted (Soxhlet) using

methanol, acetone, hexane and chloroform in that order under Argon. The chloroform

fraction was concentrated to ~20 mL before being precipitated into methanol (200 mL),

stirred for 30 min and filtered to yield 3.3-F as a black solid (760 mg, 86%); Mn of 37.9 kDa,

Mw of 103.6 kDa, Mw/Mn (Ð) = 2.73; 1H NMR (400 MHz, TCE-d2, 403K) δ 8.40 – 6.95 (m, 10H),

5.01 – 4.58 (m, 2H), 3.21 – 2.97 (m, 4H), 2.51 – 2.31 (m, 1H), 2.05 – 1.85 (m, 2H), 1.66 – 1.38

(m, 48H), 1.14 – 0.87 (m, 18H).

3.4 – P(DTG-dTdVdfBT)

This polymer was synthesised by Dr Abby Casey using the method reported in thesis.193 Mn:

51.4 kDa, Mw: 82.6 kDa, Mw/Mn (Ð): 1.61; 1H NMR (400 MHz, CDCl3, 50°C) δ 8.66 – 8.07 (br,

4H), 7.14 – 6.78 (br, 4H), 2.85 – 2.66 (br, 2H), 1.81 – 1.16 (m, 112H), 0.97 – 0.78 (m, 18H);

3.5-F - P(IF-dTdfBT)

This polymer was synthesised by Dr Abby Casey using the method reported in thesis.193 Mn

= 11.6 kDa, Mw = 22.2 kDa, Mw/Mn (Ð) = 1.91. 1H NMR (400 MHz, CDCl3) δ 8.40 – 8.28 (m,

136

2H), 7.85 – 7.53 (m, 10H), 2.20 – 1.97 (m, 8H), 1.67 – 1.47 (m, 8H), 1.20 – 1.00 (m, 40H), 0.79

– 0.71 (m, 12H).

3.6-F – P(BDT-TT)

Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-

ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] was purchased from

Ossila; Mn: 21 kDa, Mw: 67 kDa, Mw/Mn (Ð): 3.3.

3.7-F – P(F8fBT)

4,7-Dibromo-5-fluoro-2,1,3-benzothiadiazole (290.1 mg, 0.930 mmol), 9,9-dioctyl-9H-

fluorene-2,7-diboronic acid bis(pinacol) ester (597.7 mg, 0.930 mmol), Pd(PPh3)4 (21.5 mg,

0.019 mmol) and a stirrer bar were added to a 20 mL high pressure microwave reactor vial.

The vial was then sealed with a septum and flushed with argon, before degassed toluene (8

mL), degassed aqueous 2 M Na2CO3 (5 mL) and a drop of aliquot 336 was added. The

resulting solution was degassed for 30 min before the reaction was heated to 120 °C for 3

137

days. The reaction was cooled to room temperature, precipitated in methanol (100 mL),

stirred for 30 min and filtered through a Soxhlet thimble. The polymer was then extracted

(Soxhlet) using methanol, acetone, hexane and chloroform in that order under Argon. The

chloroform fraction was concentrated to ~10 mL before precipitation into methanol (100

mL). The suspension was stirred for 30 min, filtered, redissolved in chloroform and

precipitated into methanol again to yield 3.7-F as a yellow solid (413 mg, 82%); 1H NMR (400

MHz, CDCl3) δ 8.15 – 7.78 (m, 7H), 2.34 – 1.95 (m, 4H), 1.24 – 1.11 (m, 24H), 0.84 – 0.78 (m,

6H). Multiple batches synthesised with various molecular weights, batch 1: Mn: 44 kDa, Mw:

144 kDa, Mw/Mn (Ð): 2.56; batch 2: Mn: 39.9 kDa, Mw: 72.8 kDa, Mw/Mn (Ð): 1.82; batch 3:

Mn: 39.7 kDa, Mw: 62.5 kDa, Mw/Mn (Ð): 1.57; batch 4: Mn: 120 kDa, Mw: 206 kDa, Mw/Mn

(Ð): 1.72.

3.8.1 Alkylthiol substitution

3.2

3.1 (3 mg, 0.004 mmol) and K2CO3 (100 mg, 0.72 mmol) were added to a 2 mL high pressure

microwave vial. The vial was sealed with a septum and degassed with argon, before

anhydrous chlorobenzene (0.75 mL) and DMF (0.25 mL) were added. Octanethiol (0.1 mL,

0.58 mmol) was then added and the solution was degassed with argon. The solution was

138

heated in a microwave reactor at 120 °C for 30 min. After cooling the solution was

precipitated into methanol (10 mL), stirred for 30 min and filtered through a Soxhlet

thimble. Unreacted thiol was removed by washing (Soxhlet) with acetone and the polymer

was extracted with CHCl3. The CHCl3 fraction was concentrated down to 1 mL and

precipitated into MeOH and filtered, resulting in a black solid (1.6 mg, 46%), Mn: 5.8 kDa,

Mw: 6.2 kDa, Mw/Mn (Ð): 1.07; 1H NMR (400 MHz, CDCl3) δ 8.26 – 7.48 (m, 11H), 4.67 (br,

1H), 3.17 – 3.01 (m, 2H), 2.40 (br, 2H), 2.02 (br, 2H), 1.39 – 1.02 (m, 36H), 0.90 – 0.74 (m,

9H).

3.3-SR

3.3-F (10 mg, 0.009 mmol) was reacted using the same method for 3.2, with dodecanethiol

instead of octanethiol to yield a black solid (6.7 mg, 51%). Mn: 37 kDa, Mw: 80 kDa, Mw/Mn

(Ð): 2.17; 1H NMR (400 MHz, TCE-d2, 403K) δ 8.13 – 7.99 (m, 2H), 7.86 – 7.75 (m, 2H), 7.50

– 7.35 (m, 4H), 7.05 – 6.96 (m, 2H), 4.80 – 4.60 (m, 2H), 3.05 – 2.92 (m, 8H), 1.91 – 1.24 (m,

90H), 1.04 – 0.89 (m, 25H).

3.4-SR

139

This polymer was synthesised by Dr Abby Casey using the method reported in thesis.193 Mn:

28 kDa, Mw: 48 kDa, Mw/Mn (Ð): 1.69; 1H NMR (400 MHz, CDCl3) δ 8.77 (d, J = 15.9 Hz, 2H),

7.91 (d, J = 15.9 Hz, 2H), 7.23 – 7.14 (m, 2H), 7.04 – 6.94 (m, 2H), 3.00 – 2.89 (m, 4H), 2.84 –

2.74 (m, 2H), 1.78 – 1.66 (m, 4H), 1.64 – 1.53 (m, 6H), 1.50 – 1.37 (m, 12H), 1.34 – 1.13 (m,

130H), 0.91 – 0.79 (m, 24H).

3.5-SR

3.5-F (9.5 mg, 0.009 mmol) was reacted using the same method as above. Yielded a dark

red solid (5.3 mg, 41%). Mn: 9 kDa, Mw: 14 kDa, Mw/Mn (Ð): 1.58; 1H NMR (400 MHz, CDCl3)

δ 7.81 – 7.49 (m, 12H), 2.94 – 2.79 (m, 4H) 2.19 – 1.95 (m, 8H), 1.32 – 1.01 (m, 88H), 0.89 –

0.76 (m, 18H).

3.6-SR

140

Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-

ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (3.6-F) (9.7 mg, 0.011

mmol) was reacted using the same method for 3.3-SR, to yield a black solid (4.9 mg, 42%).

Mn: 20 kDa, Mw: 66 kDa, Mw/Mn (Ð): 3.3 ; 1H NMR (400 MHz, TCE-d2, 403K) δ 8.23 – 6.81

(m, 6H), 4.37 (br, 2H), 3.05 – 2.86 (m, 4H), 2.72 (br, 2H), 1.88 – 1.74 (m, 4H), 1.61 – 1.16 (m,

44H), 1.08 – 0.87 (m, 21H).

3.7-SR

3.7-F (94 mg, 0.174 mmol, batch 1) was reacted using the same method as 3.2. Yielded a

yellow solid (85 mg, 71%). Mn: 60.0 kDa, Mw: 98.9 kDa, Mw/Mn (Ð): 1.65; 1H NMR (400 MHz,

CDCl3) δ 8.07 – 7.91 (m, 5H), 7.70 – 7.60 (m, 2H), 3.02 – 2.93 (m, 2H), 2.24 – 1.93 (m, 4H),

1.42 – 1.09 (m, 44H), 0.92 – 0.73 (m, 9H). Anal. Calcd. for C43H58N2S2 C 77.42, H 8.46, N 4.20,

found: C 77.33, H 8.64, N 4.33.

Substitution of P(F8fBT) (3.7-F) with increasing thiol content.

3.7-F (5 mg on average, 9.24 μmol (variation for each reaction, see below)) and K2CO3 (10

mg, 0.072 mmol) were added to a 2 mL high pressure microwave vial. The vial was sealed

with a septum and degassed with argon, before anhydrous chlorobenzene (0.75 mL) and

DMF (0.25 mL) were added. Dodecanethiol solution (varied amount, see below) was then

141

added and the solution was degassed with argon. The solution was heated in a microwave

reactor at 150 °C for 60 min. After cooling the solution was precipitated into methanol (10

mL), stirred for 30 min and filtered through a Soxhlet thimble. Unreacted thiol and DMF

was removed by washing (Soxhlet) with methanol and the polymer was extracted with

CHCl3. The CHCl3 was removed under reduced pressure 1 mL resulting in a yellow solid. 1H

NMR (400 MHz, CDCl3) δ 8.07 – 7.60 (m, 7H), 3.02 – 2.93 (m, varied (see below)), 2.24 – 1.93

(m, 4H), 1.42 – 1.09 (m, varied), 0.92 – 0.73 (m, 9H).

20 mol%: 3.7-F (5.15 mg, 9.52 μmol), dodecanethiol (0.46 μL, 1.9 μmol); δ 3.02 – 2.93 (m,

0.4H).

40 mol%: 3.7-F (11.34 mg, 20.96 μmol), dodecanethiol (2.02 μL, 8.38 μmol); δ 3.02 – 2.93

(m, 0.8H).


1.2H).


1.6H).

100 mol%: 3.7-F (4.40 mg, 8.13 μmol), dodecanethiol (0.1 mL, 0.42 mmol); δ 3.02 – 2.93 (m,

2H).

142

3.8.2 Functionalised thiol, thioacetate and alcohol reactions with

P(F8fBT) (3.7-F)

3.7-N3 - reaction performed in air

3.7-F (31 mg, 0.057 mmol, batch 1) and a stirrer bar was added to a 20 mL high pressure

microwave vial. Chlorobenzene (4.5 mL) and DMF (1.5 mL) were added under ambient

atmosphere and the solution was heated to 100 °C, when the polymer was fully dissolved a

pellet of potassium hydroxide was then added. The solution was stirred for 10 min before

adding S-(3-azidopropyl)thioacetate (20 μL, 0.14 mmol). The solution was heated for a

further 10 min in the dark. The resulting solution was allowed to cool to room temperature

before the addition of 5 mL CHCl3. The organics were washed with water (2 x 30 mL),

concentrated to 3 mL and precipitated into methanol (50 mL) and stirred for 30 min.

Unreacted thiol was removed by washing (Soxhlet) with acetone and the polymer was

extracted into CHCl3. The CHCl3 fraction was concentrated down to 1 mL and precipitated

into MeOH and filtered, resulting in a yellow solid (31 mg 86%). Mn: 26.3 kDa, Mw: 42.4 kDa,

Mw/Mn (Ð): 1.61. 1H NMR (400 MHz, CDCl3) δ 8.11 – 7.86 (m, 5H), 7.72 – 7.63 (m, 2H), 3.40 –

3.27 (m, 2H), 3.08 – 2.96 (m, 2H), 2.24 – 1.94 (m, 2H), 1.91 – 1.80 (m, 2H), 1.24 – 1.08 (m,

26H), 0.86 – 0.76 (m, 6H); IR 2094 cm-1 (-N=N=N).

143

3.7-Alkene - reaction performed in air

3.7-F (20.5 mg, 0.038 mmol, batch 3) was reacted using the same method as above, with S-

(10-undecenyl) thioacetate instead of S-(3-azidopropyl)thioacetate to afford a yellow solid

(13.5 mg, 46%). Mn: 51.2 k8Da, Mw: 86.3 kDa, Mw/Mn (Ð): 1.6. 1H NMR (400 MHz, CDCl3) δ

8.11 – 7.86 (m, 5H), 7.72 – 7.63 (m, 2H), 5.89 – 7.74 (m, 1H), 5.06 – 4.87 (m, 2H), 3.06 – 2.90

(m, 2H), 2.15 – 1.96 (m, 4H), 1.47 – 1.12 (m, 40H), 0.97 – 0.81 (m, 6H).

3.7-COOH

3.7-F (13.4 mg, 0.025 mmol, batch 1), 11-mercaptoundecanoic acid (1.7 mg, 7.78 μmol) and

K2CO3 (100 mg, 0.72 mmol) were added to a 5 mL high pressure microwave vial. The vial

was sealed with a septum and degassed with argon, before anhydrous chlorobenzene (2.25

mL) and DMF (0.75 mL) were added. The solution was heated in a microwave reactor at 150

°C for 1 h. The solution was allowed to cool to room temperature before a few drops of

acetic acid was added. The solution was then diluted with CHCl3 and washed with saturated

aqueous ammonium chloride (20 mL) followed by water (20 mL). The solution was

144

concentrated to 3mL under reduced pressure and precipitated into methanol (10 mL),

stirred for 30 min and filtered. The polymer was washed with acetone and hexane to

remove disulfide impurity and dried to afford a yellow solid (12.9 mg, 86%). Mn: 27.4 kDa,

Mw: 57.1 kDa, Mw/Mn (Ð): 2.08. 1H NMR (400 MHz, CDCl3) δ 8.18 – 7.59 (m, 7H), 2.97 (br,

0.5), 2.26 – 2.00 (m, 4H), 1.37 – 0.89 (m, 30H), 0.87 – 0.79 (m, 6H). IR 1704 cm-1 (C=O).

3.7-SAc - reaction performed in air

3.7-F (5.8 mg, 0.011 mmol), 1,8-octanedithiol diacetate (30 mg, 0.130 mmol) and a stirrer

bar was added to a 2 mL high pressure microwave vial. Chlorobenzene (0.75 mL) and DMF

(0.25 mL) were added and the solution was heated to 100 °C. 188 mM NaOH in methanol

(20 uL, 3.8 μmol) was added and the solution was heated for 10 min. A further 20 uL was

then added and the solution was heated for a further 10 min. The resulting solution was

precipitated into methanol (20 mL) and stirred for 30 min. Unreacted thioacetate was

removed by washing (Soxhlet) with acetone and the polymer was extracted with CHCl3. The

CHCl3 fraction was concentrated down to 1 mL and precipitated into MeOH and filtered,

resulting in a yellow solid (3mg, 45%). 1H NMR (400 MHz, CDCl3) δ 8.18 – 7.63 (m, 7H), 3.02

– 2.94 (m, 0.70H), 2.84 (t, J = 7.4 Hz, 0.70H), 2.31 (s, 1H), 2.27 – 1.97 (m, 2H), 1.32 – 1.00 (m,

30H), 0.85 – 0.73 (m, 6H); IR 1750 cm-1 (C=O).

145

3.7-Silane

3.7-F (30 mg, 0.055 mmol), and K2CO3 (500 mg, 3.62 mmol) were added to a 20 mL high

pressure microwave reactor vial. The vial was sealed with a septum and degassed with

argon, before anhydrous chlorobenzene (4.5 mL) and DMF (1.5 mL) were added. (3-

Mercaptopropyl)trimethoxysilane (0.1 ml, 0.54 mmol) was added and the solution degassed

for 30 min. The solution was heated at 120 °C for 30 min in the microwave. After cooling

the solution was precipitated into MeOH 100 mL and filtered. The solid was washed with

MeOH and hexane to remove excess thiol and K2CO3. The yellow solid was then dried under

vacuum to leave a yellow solid (30 mg, yield 76%); 1H NMR (400 MHz, CDCl3) δ 8.13 – 7.83

(m, 5H), 7.71 – 7.57 (m, 2H), 3.53 – 3.49 (m, 9H), 3.04 – 2.89 (m, 2H), 2.27 – 1.86 (m, 4H),

1.82 – 1.72 (m, 2H), 1.38 – 0.65 (m, 47H).

3.7-Copolymer

146

3.7-F (30 mg, 0.055 mmol), and K2CO3 (64 mg, 0.452 mmol) were added to a 5 mL high

pressure microwave reactor vial. The vial was sealed with a septum and degassed with

argon, before anhydrous chlorobenzene (2.25 mL) and DMF (0.75 mL) were added. Poly(2-

ethyl-2-oxazoline) (α-benzyl, ω-thiol terminated, average Mn 2000) (451.33 mg, 0.126 mmol)

was then added and the solution was heated at 120 °C for 60 min in the microwave. After

cooling the solution was precipitated into water 20 mL and filtered. Unreacted thiol

polymer was removed by washing (Soxhlet) with acetone and the polymer was extracted

into CHCl3. The CHCl3 fraction was concentrated down to 5 mL and precipitated into MeOH

and filtered, resulting in a yellow solid (66 mg, yield 48%); 1H NMR (400 MHz, CDCl3) δ 8.19 –

7.56 (m, 7H), 7.42 – 7.31 (m, 3H), 7.2 – 7.1 (m, 2H), 4.61 – 4.49 (m, 2H), 3.67 – 3.2 (m, 100H),

3.15 – 2.97 (br, 2H), 2.48 – 2.22 (m, 47H), 2.17 – 2.08 (br, 4H), 1.24 – 0.98 (br, 100H), 0.83 –

0.76 (br, 6H); IR 1632 cm-1 (C=O).

3.7-OR

3.7-F (23 mg, 0.043 mmol, batch 2), and a pellet of KOH were added to a 5 mL high pressure

microwave reactor vial. The vial was sealed with a septum and degassed with argon, before

anhydrous chlorobenzene (2.25 mL) and DMF (0.75 mL) were added. 2-ethyl-1-hexanol

(0.067 mL, 0.430 mmol) was then added and the solution. The solution was heated at 130

147

°C for 60 min in the microwave. After cooling the solution was precipitated into MeOH 20

mL and filtered. Unreacted thiol was removed by washing (Soxhlet) with acetone and the

polymer was extracted into CHCl3. The CHCl3 fraction was concentrated down to 4 mL and

precipitated into MeOH and filtered, resulting in a yellow solid (19 mg, yield 69%). Mn: 35

kDa, Mw: 71 kDa, Mw/Mn (Ð) 2.02 1H NMR (400 MHz, CDCl3) δ 8.29 – 7.65 (m, 7H), 4.17 –

4.05 (br, 2H), 2.30 – 1.95 (br, 4H), 1.79 – 0.90 (m, 49H), 0.89 – 0.76 (m, 12H).

3.7-PEG

3.7-F (10 mg, 0.018 mmol, Batch 2), and KOH (1 pellet) were added to a 2 mL high pressure

microwave reactor vial. The vial was sealed with a septum and degassed with argon, before

anhydrous chlorobenzene (1.5 mL) and DMF (0.5 mL) were added. Triethylene glycol

monomethyl ether (10uL, 1.6 mmol) then added and the solution. The solution was heated

at 120 °C for 30 min in the microwave. After cooling the solution was precipitated into

MeOH 20 mL, filtered and washed with hot MeOH. The polymer was then dried under

vacuum overnight to leave a yellow solid (10.2 mg, yield 81%). Mn: 15.4 kDa, Mw: 28.4 kDa,

Mw/Mn (Ð) 1.84. 1H NMR (400 MHz, CDCl3) δ 8.13 – 7.77 (m, 7H), 4.38 – 4.31 (br, 2H), 3.85 –

3.78 (br, 2H), 3.70 – 3.59 (m, 6H), 3.54 – 3.48 (br, 2H), 3.36 – 3.33 (br, 3H), 2.24 – 1.90 (br,

4H), 1.24 – 1.04 (m, 24H), 0.83 – 0.76 (br, 6H).

148

3.8 – synthesised in one-pot.

3.7-F (17 mg, 0.031 mmol) and a stirrer bar was added to a 5 mL high pressure microwave

vial. Chlorobenzene (3 mL) and DMF (1 mL) were added under ambient atmosphere and the

solution was heated to 100 °C, when the polymer was fully dissolved a pellet of potassium

hydroxide was then added. The solution was stirred for 10 min before S-(10-undecenyl)

thioacetate (4 μL, 0.016 mmol) was added. The solution was heated for a further 10 min. S-

(3-Azidopropyl)thioacetate (10 μL, 0.07 mmol) was then added and the solution was heated

for a further 10 min. The resulting solution was allowed to cool to room temperature

before precipitation into methanol (20 mL) and stirred for 30 min. Unreacted thiols were

removed by washing (Soxhlet) with acetone and the polymer was extracted into CHCl3. The

CHCl3 fraction was concentrated down to 1 mL and precipitated into MeOH and filtered,

resulting in a yellow solid (11.7 mg, 56%). 1H NMR (400 MHz, CDCl3) δ 8.08 – 7.58 (m, 7H),

5.86 – 5.73 (m, 0.5H), 5.04 – 4.86 (m, 1H), 3.34 (br, 1H), 3.07 – 2.92 (m, 2H), 2.07 – 1.98 (m,

1H), 2.07 – 1.98 (m, 1H), 1.43 – 1.03 (m, 35H), 0.85 – 0.77 (m, 6H).

3.9 – synthesised over two steps

149

25 mol% of 11-mercaptoundecanoic acid was reacted with 3.7-F following the method used

to synthesis 3.7-COOH. This yielded a polymer with 20 mol% carboxylic acid groups on the

backbone (estimated from NMR analysis). The resulting polymer (15 mg, 0.026 mmol) and a

stirrer bar were added to a 5 mL high pressure microwave vial. Chlorobenzene (2.25 mL)

and DMF (0.75 mL) were added under ambient atmosphere and the solution was heated to

100 °C, when the polymer was fully dissolved a pellet of potassium hydroxide was then

added. The solution was stirred for 10 min before adding triethylene glycol monomethyl

ether (2.6 μL, 0.015 mmol). The solution was heated for a further 10 min (an aliquot (10 μL)

of reaction mixture was then added to CHCl3 (3 mL) for UV-Vis analysis). S-(3-

Azidopropyl)thioacetate (10 μL, 0.07 mmol) was then added to the reaction mixture and

was heated for an additional 10 minutes, in the dark. The resulting solution was allowed to

cool to room temperature before the addition of 5 mL CHCl3. The organics were washed

with water (2 x 30 mL), concentrated to 3 mL and precipitated into methanol (20 mL),

filtered and washed with hot MeOH (3 x 20 mL). The polymer was dried under vacuum,

resulting in a yellow solid (9.5 mg, 53%). 1H NMR (400 MHz, CDCl3) δ 8.11 – 7.56 (m, 7H),

4.35 (br, 1.3H), 3.81 (br, 1.3H), 3.70 – 3.57 (m, 3.9H), 5.13 (br, 1.3H), 3.40 – 3.28 (m, 2.4H),

3.07 – 2.91 (m, 0.6H), 2.20 – 1.81 (m, 4H), 1.36 – 0.59 (m, 34H).

3.9 – synthesised in one-pot.

150

3.7-F (19 mg, 0.035 mmol), 11-mercaptoundecanoic acid (1.7 mg, 7.78 μmol) and KOH (one

pellet) were added to a 5 mL high pressure microwave vial. The vial was sealed with a

septum and degassed with argon, before anhydrous chlorobenzene (3 mL) and DMF (1 mL)

were added. The solution was heated in a microwave reactor at 150 °C for 1 h. The solution

was allowed to cool to room temperature, the septum was removed and an aliquot (10 μL)

of reaction mixture was then added to CHCl3 (3 mL) for UV-Vis analysis). The reaction

mixture was heated (in an oil bath) to 100 °C a further pellet of KOH was then added. The

solution was stirred for 10 min before adding triethylene glycol monomethyl ether (3.0 μL,

0.017 mmol). The solution was heated for a further 10 min (an aliquot (10 μL) of reaction

mixture was then added to CHCl3 (3 mL) for UV-Vis analysis). S-(3-azidopropyl)thioacetate

(20 μL, 0.14 mmol) was then added to the reaction mixture and was heated for an additional

10 minutes, in the dark. The resulting solution was allowed to cool to room temperature

before the addition of 5 mL CHCl3. The organics were washed with water (2 x 30 mL),

concentrated to 3 mL and precipitated into methanol (20 mL), filtered and washed with hot

MeOH (3 x 20 mL). The polymer was dried under vacuum, resulting in a yellow solid (18 mg,

74%). 1H NMR (400 MHz, CDCl3) δ 8.11 – 7.60 (m, 7H), 4.35 (br, 1.4H), 3.81 (br, 1.4H), 3.70 –

3.57 (m, 4.2H), 5.13 (br, 1.4H), 3.40 – 3.28 (m, 2.7H), 3.07 – 2.91 (m, 0.6H), 2.23 – 1.83 (m,

4H), 1.36 – 0.70 (m, 34H).

151

Chapter 4 The use of semiconducting polymers in

nanoparticles towards surface functionalisation

152

4.1 Introduction

As discussed in section 1.6, SPNs are a promising alternative to QDs and dye-loaded silica

nanoparticles. A key requirement for any nanoparticle system used in bioimaging is control

of the surface chemistry. For example, presenting functional groups at the surface of

nanoparticles can facilitate targeted cellular imaging,92,100,192,229–231 biosensing,79,232–234 drug

delivery223,235 and cancer treatment.236–239 Furthermore, encapsulation of nanoparticles

with polyethylene glycol (PEG) chains has shown to improve cell-uptake, colloidal stability

and reduce non-specific binding.95,224 Whilst SPNs have impressive photostability and

brightness, functionalising the surface of SPNs is not without its challenges as, unlike QDs

and silica nanoparticles, the surfaces of conventional SPNs are inherently unreactive.

Several strategies have been developed to functionalise the surface of SPNs. An example

being the encapsulation of the nanoparticle in a silica shell which can then be modified with

functionalised silanes.230,240 Another popular approach is to co-precipitate the

semiconducting polymer with an aliphatic polymer functionalised with the desired reactive

group, commonly poly(styrene-co-maleic anhydride) (PSMA) affording carboxylic acid-

functionalised nanoparticles.196,241–244 A further strategy is to synthesise a semiconducting

polymer with the desired functionality covalently linked directly on the backbone of the

polymer.217,218 All of the methods discussed above yield polymer nanoparticles with

reactive groups at the surface. The first two methods described typically involve

commercially available reagents whereas the latter example requires organic synthesis of

153

bespoke polymers for each application. However, the covalent linkage can provide more

stable surface functionalisation. Work by Yu et al. illustrated this instability in the co-

precipitation of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-(2,1’,3)-thiadiazole)]

(F8BT) with PSMA.241 In this work, a dye was coupled to PSMA prior to co-precipitation with

F8BT, allowing for FRET from the polymer to dye to occur. Over time the FRET efficiency

decreased as PSMA leached from the nanoparticles.

As shown in the previous chapter, a novel methodology for functionalising polymer

backbones has been developed. The desired reactive group can be added to the polymer in

a one-step reaction, from commercially available reagents. This avoids tailored monomer

synthesis and multiple polymerisation reactions. This methodology potentially bridges the

gap between relatively straightforward non-covalent methods and the synthetically involved

covalent methods for functionalising the surface of SPNs.

The use of these materials in the preparation of SPNs was explored, ultimately leading to

nanoparticles with surfaces capable of undergoing bioorthogonal click reactions. In this

chapter the synthesis, characterisation and surface reactivity will be discussed in turn for

azide, carboxylic acid, silane and multifunctionalised nanoparticles (prepared from polymers

3.7-N3, 3.7-COOH, 3.7-Silane and 3.9, respectively).

Calculations of Förster resonance energy transfer (FRET) efficiency from photoluminescence

(PL) and time-correlated single photon counting (TCSPC) experiments was performed by Dr.

Robert Godin.

154

4.2 Synthesis and characterisation of SPNs

All nanoparticles in this study were synthesised using the nanoprecipitation method (Figure

4.1).245 The polymer was dissolved in THF (0.1 mg/mL), 1 mL of which was then rapidly

injected into 9 mL of Milli-Q water, under sonication. The THF was then removed by

flushing with N2 for one hour at 60 °C. This yields a colloidal solution of nanoparticles in

water. Nanoparticles of 3.7-N3, 3.7-COOH, 3.7-Silane and multifunctionalised polymers 3.9

were all synthesised and labelled SPN-N3, SPN-COOH, SPN-Silane and SPN-Multi

respectively (Figure 4.1).

155

Figure 4.1: Illustration of the nanoprecipitation method to form surface functionalised SPNs

As discussed in section 1.6.2, there are multiple ways of measuring the size of nanoparticles.

In this chapter, sizes were determined by a combination of dynamic light scattering (DLS),

scanning transmission electron microscopy (STEM) and nanoparticle tracking analysis (NTA).

The intensity, volume and number distributions from DLS analysis are displayed in Figure

4.2a, b and c respectively. In all cases the modal particle sizes decreases from intensity to

volume to number distribution. This is due to a reduction in the contribution from the

larger particles in solution to the volume and number distribution. The mean particle sizes

were calculated from each DLS distribution and are displayed in Table 4.1. It should be

noted that DLS is a reliable technique for measuring the sizes of relatively monodisperse

nanoparticle solutions, whereas accuracy issues can occur in polydisperse samples, such as

156

typical SPN solutions.246 However, the convenience of DLS makes it the most widely used

size-determining technique for SPNs.

In contrast to the monofunctionalised nanoparticles, SPN-Multi nanoparticles present a

bimodal distribution in the intensity plot (Figure 4.2a), indicating there is a greater number

of large particles compared to the monofunctionalised analogues. The mean size from both

peaks were found to be 269 and 35 nm. Under volume and number calculations however, a

normal distribution is restored and present a mean size of 29 and 21 nm, respectively. This

indicates that the vast majority of particles are around 20 – 30 nm in size.

The discussion above highlights the difficulty of measuring the particle size of polydisperse

solutions from DLS data alone. The raw intensity distributions obtained often overestimate

the contribution from large particles. As a result, volume and number distributions are

often reported in the literature. However, each distribution involves different assumptions

of the system and therefore give differing mean size values.247 As a result, DLS is often used

alongside other size-determining techniques.

157

Figure 4.2: Dynamic light scattering of SPNs from three different distribution modes: a) intensity, b) volume and c) number.

The size of SPN-N3 nanoparticles was also measured using scanning transmission electron

microscopy (STEM), in which the diameter of one hundred particles was measured. The size

distribution is depicted in Figure 4.3a and an example of a STEM image of SPN-N3 can be

found in Figure 4.3b. The measurements indicate a mean size of 120 nm (± 33 nm) and

reflect the mean value from the DLS intensity more closely than volume and number

distributions (Table 4.1). However, the measurements are of dry particles on a surface, so

the size and shape may differ from that in solution.

158

Figure 4.3: a) Size distribution from scanning transmission electron microscopy (STEM) images of SPN-N3; b) STEM image of SPN-N3; c) STEM image followed by energy-dispersive x-ray EDX images highlighting the carbon (C), silicon (Si) and sulfur (S) composition of SPN-Silane nanoparticles; d) Nanoparticle tracking analysis (NTA) image of a solution of SPN-N3 nanoparticles.

STEM can also be used in tandem with energy-dispersive x-ray (EDX) spectroscopy. Once an

image is generated from STEM, EDX can be used to probe the elemental composition of the

nanoparticles. This technique was used with SPN-Silane nanoparticles. Under STEM-EDX

the nanoparticles exhibited a large concentration of silicon and sulfur at the nanoparticle,

159

compared to surrounding environment (Figure 4.3b). Since the silicon atoms are unique to

the polymer side-chains this gives strong evidence that the observed nanoparticles are

composed of the modified polymer.

Table 4.1: Mean particle size of nanoparticles from three different methods: dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and scanning transmission electron microscopy (STEM). DLS: mean size calculated from Zetasizer software for each distribution, value shown is averaged over 2 separate DLS measurements (3 measurements when labelled with a *). NTA: mean calculated from merging size distributions calculated from five separate videos; STEM: mean size of 100 particles measured. †mean size obtained from bimodal distribution of two peaks (intensity ratio ~1:1) with mean size of 269 and 35 nm independently.

SPN

Mean Size (nm)

DLS NTA STEM

Intensity Volume Number

SPN-N3 136 41 25 77 (±22) 121 (±33)

SPN-COOH 117 79 53 56 (±24) -

SPN-Silane* 143 64 38 - -

SPN-Multi* 151† 29 21 83 (±34) -

Nanoparticle tracking analysis (NTA) was also used to measure particle size. A snapshot of a

video of SPN-N3 can be seen in Figure 4.3d. For each nanoparticle system, five videos were

recorded and the average mean size was calculated (Table 4.1). The mean sizes do not

appear to correlate with the sizes obtained from DLS but the disparity between DLS and NTA

in polydisperse samples has been reported previously.248 The raw NTA data can be found in

the appendix.

160

Even with multiple size-determining techniques, obtaining an absolute particle size from

polydisperse samples is challenging. Each technique has limitations which could affect the

accuracy of the data obtained. Whilst much more work is needed to narrow the

polydispersity and measure size more accurately, the synthesis of nanoparticles was

encouraging. As a result, the surface reactivity of the nanoparticles was assessed.

4.3 Azide functionalised SPNs

Azide groups will undergo click cycloaddition reactions with alkynes (copper-catalysed) or

strained alkynes (copper-free). Wu et al. utilised this property by co-precipitating F8BT with

PSMA to generate fluorescent nanoparticles with free carboxylic acid groups on the

surface.192 The nanoparticles were then reacted with an azide-terminated amine using the

standard carboxyl–amine coupling catalysed by 1-ethyl-3-(3-dimethylaminopropyl)-

carbodiimide (EDC). This yielded azide-functionalised SPNs which could then undergo

cycloadditions with alkyne-functionalised proteins. Azide-coated nanoparticles exhibited no

shift in emission maxima when compared to 3.7-SR (dissolved in THF) (see Appendix Figure

7).

Alkyne-azide click chemistry was chosen as an appropriate reaction to investigate the

reactivity of the surface of azide functionalised nanoparticles (SPN-N3). Strained alkynes

have been extensively developed for use in bioconjugation reactions as they allow for azide-

alkyne click reactions to occur without the need for cytotoxic copper (I) catalysts.249 A large

161

library of dibenzocyclooctyne (DBCO) functionalised dyes are commercially available. A

rhodamine-based dye (DBCO-MB 594, purchased from Kerafast, see Figure 4.4 for structure)

was used as it exhibits complimentary absorption with the fluorescence of SPN-N3

nanoparticles (Figure 4.4b). The efficient overlap of the emission from the nanoparticle with

the absorption of the dye can allow for Förster resonance energy transfer (FRET) between

the dye and the polymer (discussed in section 1.6.4). The efficiency of FRET is inversely

proportional to the inter-chromophore distance to the sixth power. Therefore, efficient

FRET transfer should only occur when the dye (FRET acceptor) is closely bound to the

surface of the nanoparticle (FRET donor), making it a useful tool for determining the surface

reactivity of the nanoparticles.

162

Figure 4.4: a) Schematic of the reaction of DBCO-MB 594 with azide groups at the surface of the nanoparticle, allowing for efficient FRET transfer from the nanoparticle to the dye; b) UV-Vis absorbance and emission spectroscopy of SPN-N3 and DBCO-MB 594 dye. Nanoparticle and dye were pulsed at their absorbance maxima to generate emission spectra.

The concentration of nanoparticles was calculated using NTA and was found to be 0.56 nM.

The volume of nanoparticle solutions was kept constant whilst the amount of dye reacted

was gradually increased. The initial reaction was performed with 423 equivalents of dye

(500 equivalents were chosen as the initial amount of dye but a mistake in the NTA

calculations resulted in a revision of the equivalents, after the study had been completed).

For simplification, the initial 423 equivalents will be given the value of ‘𝑥’ from now on. The

equivalents were gradually increased from 𝑥 to 16𝑥 (𝑥, 2𝑥, 4𝑥, 8𝑥, 12𝑥 and 16𝑥

equivalents). In each case, DBCO-MB 594 dye (in DMSO) was added to a solution of SPN-N3

163

and left to react at 4 °C overnight, whilst maintaining a consistent nanoparticle

concentration. As a control, a solution of DBCO-MB 594 was pre-reacted with a large excess

of azidoacetic acid (4 hours at 4 °C) to yield a dye incapable of undertaking further click

reactions. The unreactive dye was then added to a nanoparticle solution as above, in 𝑥, 2𝑥,

4𝑥 and 8𝑥 equivalents.

The photoluminescence (PL) spectra of the resulting solutions were then recorded (samples

were pulsed at 450 nm, the λMAX of the nanoparticle). The nanoparticles reacted with the

active dye exhibit a significant quenching of the nanoparticle (donor) and enhancement of

the dye (acceptor) fluorescence intensity with increasing dye equivalence (Figure 4.5a). The

analogous reaction with the inactive dye showed a comparatively insignificant donor

quenching and acceptor enhancement (Figure 4.5b). This suggests that energy is

transferring more efficiently from the nanoparticle to the dye in the case of the reactive

dye, likely via a FRET mechanism.

The energy transfer could also be seen by eye when the solutions were irradiated with UV

light (365 nm). Samples with active dye reacted were red in colour (i.e. energy is

transferring to the dye, which then emits) and the control reactions were more

green/yellow in colour (i.e. nanoparticle fluorescence dominating). Images of the colours

seen in a related system can been found in Figure 4.13c.

DLS measurements of the dye-coated nanoparticles were not possible due to the absorption

profile of the dye overlapping with the energy of the laser pulse. Therefore, to ensure the

164

nanoparticles were preserved, STEM images were analysed. Gratifyingly, nanoparticles

were found to be of a similar size and shape (see Appendix Figure 32).

Figure 4.5: Photoluminescence (PL) spectra of SPN-N3 with increasing equivalence of a) reactive dye and b) unreactive dye. Equivalence is defined here as number of dye molecules relative to number of nanoparticles in solution (x = 423).

FRET efficiencies (EFRET) were calculated from the PL spectra using the equation:

𝐸𝐹𝑅𝐸𝑇 = 1 −𝐼𝐷𝐴

𝐼𝐷

Where I refers to the maximum fluorescence intensity of the donor in the presence (DA) or

absence (D) of acceptor. EFRET as a function of the dye equivalence can be found in Figure

4.9 (red data points). For the reactive dye, the efficiency starts to plateau at 8𝑥 equivalents

165

as the value approaches the maximal FRET efficiency (see below for how maximal FRET

efficiency was calculated). However, in the case of the unreactive dye, saturation of the

EFRET is not observed. This suggests that in the case of the unreactive dye, the acceptors are

free in solution and are often too far from the donor nanoparticles for efficient FRET to

occur.

To provide further evidence of that a FRET mechanism was occurring, the lifetime of the

excited state (of the donor) with increasing acceptor equivalence was recorded using time-

correlated single photon counting (TCSPC). The lifetime of the donor’s excited state should

decrease as the FRET efficiency increases.

166

Figure 4.6: Time-Correlated Single Photon Counting (TCSPC) decay trace of SPN-N3 nanoparticles with a) 4x eq. and b) 8x eq. of reactive and unreactive dye. Here x = 423.

The decay traces of the reaction solutions, with increasing reactive and unreactive dye, were

measured. As an example, the decay traces of SPN-N3 with 4𝑥 and 8𝑥 equivalents of

reactive and unreactive dye can be found in Figure 4.6a and b, respectively (decay traces at

other equivalences can be found in the Appendix Figure 30). It can be clearly seen that the

excited state decays faster in the presence of the reactive dye, compared to the unreactive

dye analogue.

167

Figure 4.7: TCPSPC decays of nanoparticles with a) reactive dye and b) unreactive dye, normalized at 10 ns (x = 423).

It can also be seen that the decay traces of the nanoparticles with dye converge with those

of the free nanoparticle at times approximately longer than 10 ns (Figure 4.6). This is

illustrated for all equivalents of dye by the plot of decay traces normalised at 10 ns, where

all systems showed similar decay paths regardless of dye concentration beyond 10 ns

(Figure 4.7). As a result, the FRET component could be extracted by subtracting the free

nanoparticle trace from each decay trace and isolating the first 5 ns of decay. The resulting

spectra (normalised at t = 0) can be found in Figure 4.8. The shape of the decay traces can

be seen to be independent of dye equivalents and only dependant on the type of dye used

(reactive or unreactive). The FRET efficiency of the extracted data was calculated using a

stretched exponential fit of each of the decay traces, the details of which can be found in

Appendix Figure 31. This gave a maximal FRET efficiency value of 0.80 ± 0.02 for the active

dye and 0.68 ± 0.01 for the inactive dye. The higher maximal FRET efficiency observed for

168

the reactive dye highlights the success of the click reaction and intimate interaction through

formation of a covalent linkage.

Figure 4.8: Extracted decays of the FRET component for reactions with a) reactive dye and b) unreactive dye at different equivalents of dye (x = 423).

The lifetime of the excited state at each dye concentration was calculated using a stretched

exponential fit of each decay trace (the details can can be found in Appendix Figure 29). The

obtained lifetimes were then used to calculate the FRET efficiency using a similar equation

to that used for PL:

𝐸𝐹𝑅𝐸𝑇 = 1 −𝜏𝐷𝐴

𝜏𝐷

Here 𝜏𝐷𝐴 and 𝜏𝐷 refer to the donor fluorescence lifetime in the presence or absence of the

acceptor, respectively. The FRET efficiencies as a function of dye equivalence are plotted in

Figure 4.9. It can be clearly seen that the efficiencies start to plateau at 8𝑥 equivalence of

reactive dye and the levels of efficiency are much lower with unreactive dye. EFRET does not

169

rival the maximal FRET efficiencies, especially in the unreactive case, because of the slower

decay processes are competing with the FRET.

The trends observed in EFRET with increasing acceptor concentration, calculated from PL and

TCSPC data, also show excellent agreement. Note that there is some level of discrepancy

between the photoluminescence and time-resolved measurements. This is most likely due

to the simplification in calculating the average lifetime from the stretched exponential fit.

Nevertheless, substantial differences are seen between the reactive and unreactive dye

reactions, consistent with the formation of a covalent linkage.

0.0

0.2

0.4

0.6

0.8

16x12x8x4x2x

TCSPC

PL

Maximal EFRET

Unreactive dye

E FRET

Dye Equivalents

Reactive dye

x 8x4x2xx

Figure 4.9: FRET efficiency (EFRET) as a function of equivalents of active and inactive dye from PL spectra (red)

and time-correlated single photon counting (TCSPC) (black) (x = 423).

It could be argued that the difference in FRET efficiency with active and inactive dye is a

result of the additional carboxylic acid group on the inactive dye (structure in Figure 4.5) and

not due to the formation of a covalent linkage. As a result, a further control reaction was

conducted. Nanoparticles of octanethiol-substituted polymer (3.7-SR) were prepared

170

(labelled SPN-C8). A large excess of reactive dye was added to the azide-free nanoparticle

solution, under the same conditions as before. The analogous reaction was then performed

on the azide nanoparticles (SPN-N3). The PL was then recorded for both reaction mixtures

(Figure 4.10). A substantial quenching of donor fluorescence was observed with the azide

nanoparticles, relative to the control. This provides further evidence that the enhanced

FRET efficiency is due to click reactions on the surface of the azide nanoparticles and not as

a result of non-specific binding.

Figure 4.10: PL spectra of azide (SPN-N3) and octane (SPN-C8) functionalised nanoparticles with an excess of reactive dye.

4.4 Carboxylic acid functionalised SPNs

Carboxylic acid-coated nanoparticles have been used in 1-ethyl-3-(3-

dimethylaminopropyl)carbodiimide (EDC) coupling reactions with various functionalised

amines. To give a few examples, carboxylic acid functionalised SPNs have been

171

functionalised with streptavidin100, β-cyclodextrin233 (for cholesterol sensing) and

antibiotics250 (vancomycin and polymyxin B). The first two examples introduced carboxylic

acid functionality by co-precipitation with PSMA. The final example did so by synthesising a

tert-butyl-protected fluorene monomer, which was copolymerised with benzothiadiazole

and unfunctionalised fluorene comonomers. Carboxylic acid groups were then generated

by a reaction with trifluoroacetic acid (TFA), post-polymerisation.

SPN-COOH nanoparticles were reacted with EDC and N-hydroxysulfosuccinimide (sulfo-NHS)

with varying amounts of O-(2-aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol

(Amine-Biotin), a water-soluble amine functionalised with biotin (structure in Figure 4.11a).

Nanoparticle solutions (200 μL) were pre-reacted with EDC and sulfo-NHS (50 μL of a 1

μg/mL solution of each) for 15 minutes before adding 15 μL of a 1, 10, 100 and 200 ug/mL

solution of Amine-Biotin and left to react over two days. As a control, Amine-Biotin was

reacted with SPN-COOH as before, without EDC present. Each reaction mixture was

incubated in fetal bovine serum (FBS) and TWEEN®20 (a detergent, 0.02 vol%) to mitigate

non-specific binding. Each resulting mixture was run up a nitrocellulose-based lateral flow

strip with a test line 4 cm from the base (purchased from Mologic). Located at the test line

are bound streptavidin proteins. Since biotin bonds very strongly with streptatividin251, the

presence of biotin on the nanoparticle surface could be confirmed if particles were found to

bind to the test line (see Figure 4.11b for illustration of a lateral flow strip).

172

Figure 4.11: a) Schematic of the EDC coupling of Amine-Biotin onto the surface of SPN-COOH nanoparticles; b) Illustration of nitrocellulose based lateral flow strips with a poly-streptavidin test line 4 cm from the base; c) image of nitrocellulose based lateral flow strips with a poly-streptavidin test line 4 cm from the base, under UV exposure (365 nm). Before running samples up each strip, SPN-COOH nanoparticles were reacted with i) 1, ii) 10, iii) 100 and iv) 200 μg/mL solutions of amine-biotin (O-(2-Aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol) with and without EDC present. The reaction solutions were then run up the strips and allowed to dry.

The greatest fluorescence intensity, at the test line, was observed for particles reacted with

10 ug/mL solutions of PEG-biotin (Figure 4.11c, observed under UV irradiation (365 nm)).

No binding was observed in all control reactions (without EDC present). This provides

strong evidence that the carboxylic acid-functionalised nanoparticles allow for EDC coupling

reactions to occur on the surface.

173

4.5 Multifunctionalised SPNs

Semiconducting polymer nanoparticles with multiple functional groups conjugated to the

surface are rare in the literature. This is most likely due to the synthetic challenges that

arise from synthesising a multifunctionalised polymer backbone by conventional methods.

For example, Xing et al. synthesised a polythiophene-based polymer with pendant

ammonium salt and porphyrin units, which allowed for water solubility and singlet oxygen

generation.236 However, seven steps in total were required to synthesise the dual-

functionalised polymer which could potentially act as a barrier to commercialisation.

As discussed in the previous chapter, the unique control of the novel post-polymerisation

reaction allows for straightforward synthesis of multifunctional polymers. This offers the

exciting possibility of creating fluorescent nanoparticles that can be functionalised with

multiple groups via orthogonal coupling reactions.

174

Scheme 4.1: Schematic of the reaction of SPN-Multi nanoparticles with both DBCO-MB 594 and Amine-Biotin (O-(2-Aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol) independently.

SPN-Multi nanoparticles contain both azide and carboxylic acid functional groups. The first

step was to ensure that SPN-Multi nanoparticles reacted in the same way as both SPN-

COOH and SPN-N3 in independent reactions (Scheme 4.1). Multifunctionalised

nanoparticles were reacted with Amine-Biotin in an identical reaction to that used for SPN-

COOH nanoparticles. The particles were found to interact with the streptavidin strips in the

same fashion as SPN-COOH, with optimum binding at 10 μg/mL of Amine-Biotin whilst all

control reactions showing no binding (Figure 4.12c).

In separate reactions, SPN-Multi nanoparticles were mixed with an excess of the reactive

and unreactive dye (Scheme 4.1). The PL spectra of the resulting solutions were then

recorded (Figure 4.12a, red lines). Interestingly, the relative quenching of the donor (at 550

175

nm) and enhancement of the acceptor (at 650 nm) was not as substantial as that observed

for the analogous reactions with SPN-N3. The control, with unreactive dye, was appearing

to show some evidence of efficient FRET transfer. It was speculated that the carboxylic acid

groups on the surface of the multifunctionalised nanoparticles were interacting with the

dye. The MB-593 dye is zwitterionic in nature (see Scheme 4.1 for structure), therefore it is

feasible that the polar carboxylic acid groups could interact favourably with the dye,

allowing for efficient FRET transfer via a non-covalent linkage. In an attempt to block this

intermolecular interaction, THF (10 vol%) was added to the reaction solutions and the PL

spectra were recorded once more (Figure 4.12b, red lines). Upon addition of THF the

relative quenching of nanoparticle (at 550 nm) and enhancement of dye (at 650 nm) was

only observed with the reactive dye mixture. The PL spectra of the unreactive dye solutions

resembled the control reactions observed with SPN-N3.

To investigate whether the enhanced FRET efficiency was a result of carboxylic groups on

the nanoparticles surface, SPN-COOH nanoparticles were mixed with reactive and

unreactive dye as before. As there are no azide groups on the nanoparticle surface, no

covalent linkage should be possible with either dye. The PL spectra of both reaction

solutions were recorded (Figure 4.12a, black lines). The reactive dye solution exhibited a

ratio of donor to acceptor peaks similar consistent with efficient FRET transfer, even though

covalent linkage should not be possible. Upon addition of THF, both solutions (with reactive

and unreactive dye) resembled a system consisting of dye free from nanoparticle surface. It

176

is speculated that here the THF is blocking the surface of the nanoparticles and preventing

intermolecular interactions with dye molecules.

Figure 4.12: PL spectra of SPN-COOH (black) and SPN-Multi (red) with excess reactive and unreactive dye in a) pure water and b) water with THF (10 vol%); c) Photo of nitrocellulose based lateral flow strips with a poly-streptavidin test line 4 cm from the base, under UV exposure (365 nm). Before running samples up each strip, SPN-Multi nanoparticles were reacted with i) 1, ii) 10, iii) 100 and iv) 200 μg/mL solutions of Amine-Biotin (O-(2-Aminoethyl)-O′-[2-(biotinylamino)ethyl]octaethylene glycol) with and without EDC present. The reaction solutions were then run up the strips and allowed to dry.

The ability of both DBCO dye and Amine-Biotin to react on the same nanoparticle was then

investigated (Figure 4.13a). It was found that adding DBCO dye first inhibited sequential

EDC coupling reactions i.e. no binding on the test line of streptavidin strips was observed.

The EDC coupling reaction was found to be equally unsuccessful if all reactants were added

177

at once. In both cases, this failure is most likely due surface bound dye blocking further

reactions. However, a positive result occurred when the EDC coupling was allowed to occur

first, prior to the reaction with DBCO dye (Figure 4.13a). Amine-Biotin was reacted with

SPN-Multi under the optimised conditions discussed previously (with 10 μg/mL of Amine-

Biotin), forming biotin-functionalised nanoparticles. With no purification, excess reactive

dye and unreactive dye were added to separate solutions of the biotin-functionalised

nanoparticles. The PL spectra of the solutions displayed evidence of non-covalent binding of

the dye to the nanoparticle surface. However, as before, adding THF (10 vol%) gave rise to

the familiar PL spectra resembling the reactive dye covalently bound to the surface of the

nanoparticle (Figure 4.13b). This also gave rise to a red colour, also observed with SPN-N3,

when the reactive dye solutions were irradiated with UV light (365 nm) (Figure 4.13c).

178

Figure 4.13: a) Schematic of the surface functionalisation of SPN-Multi nanoparticles with Amine-Biotin, followed by DBCO-dye; b) PL spectra of biotin-functionalised particles with excess reactive and unreactive dye added (in 10 vol% THF solution); c) Images of lateral flow strips run with unreactive and reactive dye nanoparticle solutions alongside the solutions (with 10 vol% THF added) all irradiated with UV light (365 nm).

Gratifyingly, running the solution containing the reactive dye up a lateral flow strip

produced a red test line under UV irradiation (365 nm) (Figure 4.13c). This indicates that

particles functionalised with both biotin and dye, were binding at the test line. In contrast,

control solutions with unreactive dye treated in the same way presented a green/yellow

coloured test line (Figure 4.13c).

A similar result could also be achieved by reacting DBCO dye directly on the biotin-

functionalised particles bound to the test line (Figure 4.14a). SPN-Multi nanoparticle

suspensions, functionalised with biotin under the same reaction conditions as before, were

run up two streptavidin strips and dried. An aqueous solution of reactive and unreactive

179

dye was run separately up a streptavidin strip. The strips were again allowed to dry and free

dye was washed away by running the strips in pure water. The image of the strips under UV

irradiation can be found in Figure 4.14b. The reactive dye strip (labelled i) showed the

familiar red colour whilst the unreactive strip (ii) exhibited a green colour.

Figure 4.14: Schematic of click reaction directly on test line of nitrocellulose strips with image of strips reacted with i) reactive and ii) unreactive MB-594 dye (irradiated with UV light (365 nm). *solutions are incubated in fetal bovine solution (FBS) with TWEEN®20 (0.02 vol%) prior to the adding of the strip.

4.6 Conclusions

The work in this chapter demonstrates an important application of the novel post-

polymerisation reaction developed in the previous chapter. This reaction allowed for the

preparation of a range of functionalised polymer nanoparticles with reactive groups

covalently bound to the surface. The surface reactivity of azide and carboxylic acid

nanoparticles was probed by utilising DBCO click chemistry and EDC coupling respectively.

180

This method was then applied to the synthesis of multifunctional nanoparticles which could

undergo dual-functionalisation with DBCO click and EDC coupling reactions on the same

surface. This work opens up many opportunities for multi-modal nanoparticles for various

applications in diagnostic and therapeutic medicine.

4.7 Experimental

4.7.1 Nanoparticle synthesis

Polymer solutions in THF (0.1 mg/mL) were made from 3.7-N3, 3.7-COOH, 3.7-Silane and 3.9

polymers. Each solution was then filtered through a 0.2 μm PTFE filter. The solution (1 mL)

was then injected rapidly into 9 mL of water under sonication and left under sonication for 3

min. The resulting nanoparticle suspensions were heated to 60 °C and bubbled with N2 for 1

h to remove THF. Resulting solutions were then filtered through a 0.2 μm cellulose filter to

remove large particulates.

4.7.2 Azide-DBCO click

Concentrations of SPN-N3 were found to be 3.4 x 1011 particles/mL (0.56 nM) using

nanoparticle tracking analysis (NTA).

To prepare the unreactive dye solution, 10 μL of DBCO-MB 594 (500 μM, in DMSO) was

reacted with azidoacetic acid (1 μL, 0.013 mmol) at 4 °C in the dark in for 4 h. This resulted

in a 454.6 μM solution of unreactive dye. Reactive dye solutions were made up to same

181

concentration by adding 1 μL DMSO to 10 μL of DBCO-MB 594 (500 μM, in DMSO). These

solutions were both then diluted to a concentration of 1 μM by adding 1 μL of dye DMSO

solution to 453.55 μL of water.

To solutions of SPN-N3 (10 μL, 5.6 fmol) was added to an increasing quantity of reactive dye

solutions (see table below). Solutions were left at 4 °C for 18 h in the dark. Each solution

was then made up to 100 μL with water. The PL spectra and TCSPC of each solution was

then recorded.

Equivalents Reactive dye Unreactive dye

423 (x) 2.4 μL, 2.4 nmol 2.4 μL, 2.4 nmol

827 (2x) 4.8 μL, 4.8 nmol 4.8 μL, 4.8 nmol

1714 (4x) 9.6 μL, 9.6 nmol 9.6 μL, 9.6 nmol

3429 (8x) 19.2 μL, 19.2 nmol 19.2 μL, 19.2 nmol

5143 (12x) 28.8 μL, 28.8 nmol -

6857 (16x) 38.4 μL, 38.4 nmol -

4.7.3 EDC coupling of Amine-Biotin to SPN-COOH

1 μg/mL solutions of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)

and N-hydroxysulfosuccinimide (Sulfo-NHS) were made up in water. O-(2-aminoethyl)-O′-

182

[2-(biotinylamino)ethyl]octaethylene glycol (Amine-Biotin) solutions were made up to 1, 10,

100 and 200 μg/mL in water.

To 4 solutions of SPN-COOH (100 μL) was added EDC solution (50 μL, 0.26 nmol) and Sulfo-

NHS solution (50 μL, 0.23 nmol). These solutions were left to react at room temperature for

15 min. The 4 different solutions of Amine-Biotin (15 μL) was then added to each reaction

mixture. The mixtures were then left to react for 48 h at room temperature whilst shaking.

To a further 4 solutions of SPN-COOH (100 μL) was added Sulfo-NHS solution (50 μL) and

water (50 μL) and were treated in the same way to act as controls for non-specific binding.

10 μL of each resulting solution was added to a mixture of fetal bovine serum (FBS) and

TWEEN®20 (0.02 vol%) (50 μL) and incubated for 5 min. To each mixture, poly-streptavidin

test strips were added so the solution which was allowed to run up the test strip under

capillary action and then left until dry. Strips were illuminated under UV light (365 nm) and

photographs taken.

4.7.4 Multifunctional - EDC coupling

1 μg/mL solutions of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)

and N-hydroxysulfosuccinimide (Sulfo-NHS) were made up in water. O-(2-aminoethyl)-O′-

[2-(biotinylamino)ethyl]octaethylene glycol (Amine-Biotin) solutions were made up to 1, 10,

100 and 200 μg/mL in water.

To four solutions of SPN-Multi (100 μL) was added EDC solution (50 μL, 0.26 nmol) and

Sulfo-NHS solution (50 μL, 0.23 nmol). These solutions were left to react at room

183

temperature for 15 min. The four different concentrations of Amine-Biotin (15 μL) was then

added to each solution. The mixtures were then left to react for 48 h at room temperature

whilst shaking. To a further 4 solutions of SPN-Multi was added Sulfo-NHS solution (50 μL)

and water (50 μL) and were treated in the same way to act as controls for non-specific

binding. 10 μL of each solution was added to a mixture of fetal bovine serum (FBS) and

TWEEN®20 (0.02 vol%) (50 μL) and incubated for 5 min. To each mixture, poly-streptavidin

test strips were added so the solution which was allowed to run up the test strip under

capillary action and then left until dry. Strips were illuminated under UV light (365 nm) and

photographs taken.

4.7.5 Multifunctional - DBCO click

To 12.9 pM solutions (calculated using NTA, and then equalised using water) of SPN-Multi

and SPN-COOH (50 μL, 0.6 fmol) was added 0.2 μL of reactive dye (454.6 μM, in DMSO) or

0.2 μL of unreactive dye (454.6 μM, in DMSO) solutions (see above for method of making

dye solutions). Solutions were left to react for 18 h at 4 °C. Reaction mixtures were then

made up to 100 μL and the PL spectra were recorded.

4.7.6 Multifunctional - EDC coupling and DBCO click in tandem

SPN-Multi were reacted with Amine-Biotin (15 μL of 10 μg/mL) using the method described

above, to yield a biotin-functionalised nanoparticle solution. To aliquots of this solution

(49.8 μL) was added reactive dye solution or unreactive dye solution (454.6 μM, 0.25 μL).

184

Solutions were left to react for 18 h at 4 °C. The solutions were irradiated under UV-light

(365 nm) and photographs taken. 10 μL of each solution was added to a mixture of fetal

bovine serum (FBS) and TWEEN®20 (0.02 vol%) (50 μL) and incubated for 5 min. To each

mixture, poly-streptavidin test strips were added so the solution which was allowed to run

up the test strip under capillary action and then left until dry. Strips were illuminated under

UV light (365 nm) and photographs taken. The remaining reaction solutions were made up

to 100 μL with water and the PL spectra was recorded with and without 10 vol% of THF.

185

Chapter 5 Post-polymerisation modification of

semiconducting polymers towards improving stability

of organic photovoltaics

186

5.1 Introduction

The active layer of organic photovoltaics (OPVs) consists of a blend of electron donor and

acceptor materials, commonly semiconducting polymers and fullerenes respectively. The

microstructure of such blends is one of the key factors in OPV device performance, with the

domain size and purity having a critical influence.62,252 As such, it is important that a high

performance OPV device maintains the optimal microstructure over time. In an OPVs

lifetime, devices will be subject to large fluctuations in temperature.253 This can drive

crystallisation of either component in the active layer, disrupting the microstructure and

leading to a reduced device performance.254–256

There have been two main strategies for improving thermal stability: by using

compatibilisers and cross-linking agents. In this context, compatabilisers are typically block

copolymers consisting of a block of fullerene-functionalised polymer fused to a block of

standard polymer.257–261 However, there are other examples of compatabilisers without

fullerene present.262–264 When added to the fullerene-polymer blend, the block copolymers

can act as a surfactant between the domains in the active layer and stabilise the

morphology.265

Another popular approach in stabilising the blend microstructure has been to cross-link the

components together, ideally fixing the morphology in an optimised form, after solution

processing (Figure 5.1). Several polymer-to-polymer, fullerene-to-fullerene and polymer-to-

fullerene cross-linking methods have been reported in the literature.63,266–268

187

Figure 5.1: Illustration of the morphological changes that can occur over time during device operation with and without cross-linking groups on the polymer backbone.

A common strategy in polymer cross-linking has been to design polymer side-chains

terminated with functional groups, which become reactive upon treatment with external

stimuli.266,269,270 Examples of this include, azide-terminated side-chains185 and brominated

side-chains,268 which both cross-link under ultraviolet light and vinyl-terminated side-chains

which cross-link under thermal conditions.271 A possible problem with these methods is

that they require harsh conditions to cross-link, which can degrade the active layer and any

other components in the cell. A different approach has been the addition of a catalytic

amount of a photoacid to the blend which reacts with the functionalised side-chains after a

less intense UV exposure. Examples of this include expoxide-terminated272 and oxetane-

terminated side-chains,273 which both cross-link in acidic conditions. Although the UV-light

188

intensity required is relatively mild, due to the small concentration of the photoacid, the

additive will reduce the purity of the active layer. On the other hand, UV-triggered cross-

linking does give the added option of patterning films by masking.273 Tournebize et al. have

also shown how cross-linking can occur with no additives or functional groups in a

carbazole-based polymer by photochemical reactions between the carbazole monomer and

fullerene.274

Alkoxysilanes are widely used in industry due to their low cost and the mild conditions in

which they react. The labile nature of the alkoxy groups makes the silanes very reactive to

pendant hydroxyl groups, which makes them useful adhesion promoters275 and coupling

agents.276 They are also very reactive to water, where the alkoxysilane will hydrolyse

forming hydroxysilanes which can then react with other alkoxysilanes, making them very

useful as cross-linking agents277 and moisture scavengers.278

Silanes have previously been reported in improving the performance of OPV devices; a

siloxane precursor was successfully incorporated onto a fullerene derivative by

functionalising the solubilising chains with a trichlorosilane groups.279 The fullerenes self-

assembled onto the surface of TiOx when exposed to moisture in the air, without the need

of thermal treatment or UV exposure. This chemistry has also been applied to small

molecule systems, in which the conjugated molecules were terminated with triethoxysilane

groups and cross-linked, under mild conditions.280 Another use of silanes in OPVs has been

in the incorporation of trichlorosilane groups onto either end of a bithiophene, which can

then self-assemble on the surface of the indium titanium oxide (ITO) electrode, resulting in a

189

better match of interfacial energies between the electrode and poly(3-hexylthiophene)

(P3HT).281

The introduction of alkoxysilanes onto conjugated polymers has rarely been reported,

perhaps due to the synthetic challenges of incorporating such reactive groups onto

monomer units, which may then complicate the subsequent polymerisation process. As

discussed in a previous chapter, polyfluorenes with pendant triethoxysilanes have been

shown to cross-link in basic conditions with applications toward SPNs.282 In this case, the

modification required hydroxyl-terminated side-chains to incorporate the alkoxysilane post-

polymerisation. The development of cross-linking with silane groups on OPV polymers is

absent in the literature.

It has been shown in Chapter 3 how (3-mercaptopropyl)trimethoxysilane could be

incorporated onto the backbone of P(F8fBT), forming 3.7-silane, in one step. In this

chapter, the application of the post-polymerisation reaction to functionalise high-

performance OPV polymer poly-[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-

dithiophene)-alt-4,7-(5-fluoro-[2,1,3]-benzothiadiazole)] (P(CPDTfBT)) will be discussed.

This polymer has shown device efficiencies exceeding 6% in OPV devices148 and P(CPDTBT)

(the non-fluorinated analogue) has been used as the basis for a cross-linking material.

Albrecht et al. affixed alkenes to the end of the side-chains of P(CPDTBT), which could then

thermally cross-link and improve device stability.283

190

(P(CPDTfBT)) was synthesised and functionalised with trimethoxysilane-terminated side-

chains (5.2, Scheme 5.1a). The trimethoxysilane polymers formed were found to hydrolyse

and self-condense forming an insoluble cross-linked polymer without the need for additives,

external stimuli or exposure to air. The process was found to be greatly accelerated by the

presence of the common hole transporting layer poly(3,4-ethylenedioxythiophene)

polystyrene sulfonate (PEDOT:PSS) and heat (18 hours at 120 °C). Optimised OPV cells with

the modified material gave an impressive 22% increase in the power conversion efficiency

(PCE) after prolonged heating (336 h at 120 °C) compared to a 64% drop in PCE for the non-

modified analogue. Solar cell devices described in this chapter were fabricated by Dr.

Pabitra Shakya Tuladhar.

5.2 Synthesis of polymers

P(CPDTfBT)) (5.2) was synthesised, following literary procedure,148 by reacting equimolar

quantities of fluorinated benzothiadiazole monomer (2.1) and 4,4-bis(2-ethylhexyl)-2,6-

bis(trimethylstannanyl)-4H-cyclopenta-[2,1-b:3,4-b′]dithiophene (5.1) via Stille coupling

(Scheme 5.1). 5.1 was synthesised from 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-

b′]dithiophene following literary procedure.284 (3-Mercaptypropyl)trimethoxysilane was

substituted onto the polymer backbone of 5.2 using a similar method to that used for the

post-polymerisation modification of the polymers in Chapter 3. However, in this case the

reaction was found to work in THF as an alternative to chlorobenzene and DMF. Silane-

polymer (5.3-S1) was synthesised by dissolving 5.2 in a saturated solution of K2CO3 in THF,

191

to which a large excess of (3-mercaptypropyl)trimethoxysilane was added. The solution was

then heated overnight at 60 °C. As the presence of water was undesired, the reaction

solution was precipitated in solution of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-

6) in dry MeOH. 18-Crown-6 has a strong affinity for potassium ions285 so was used to

dissolve the excess K2CO3 into the organic phase, without the need for an aqueous work up.

Scheme 5.1: a) Synthesis of silane-functionalised P(CPDT-BT). i) o-xylene (with 10 vol% DMF), Pd2(dba)3, P(o-tol)3, 170 °C 40 min (Microwave). ii) (3-Mercaptopropyl)trimethoxysilane, THF. 5.3-S1 was synthesised by heating at 60 °C for 18 h, 5.3-S2 by heating at 100 °C for 30 min (microwave); b) Cross-linking mechanism of functionalised polymer with water.

This method yielded a fully modified polymer (5.3-S1), in which every fluorine atom on the

BT unit had been displaced by sulfur. Full substitution of the fluorine was evident from the

proton NMR (Figure 5.2). Integration of the new –SCH2- proton signals (δ 3.2 ppm) against

the aromatic proton signals (which should always integrate to 3H), gave the predicted value

192

of 2H. Also, absence of fluorine peak in the 19F NMR was observed (see Appendix). The

modification led to a large blue shift in the UV-Vis absorption spectrum (79 nm, Figure 5.4)

which is in-keeping with the changes observed in the polymers used in Chapter 3.

193

Figure 5.2: Stacked NMR spectra of 5.3-S1, 5.2 and 5.3-S2 with table of integration values. Integration of aromatic region (8.3 – 7.7 ppm) always set to 3 protons. And integration from 3.3 – 3.1 ppm measured relative to the aromatic region (labelled xH).

The next step was to investigate the cross-linking ability of 5.3-S1 (Scheme 5.1b). A pair of

thin films of the polymer were spun on glass substrates (5 mg/ml solution spin coated at

1000 rpm for 30 s) and the initial UV-Vis absorption spectra were measured. To one of the

films a drop of trifluoroacetic acid (TFA) was added, to accelerate cross-linking. The films

were then dipped and held in chloroform for 5 seconds and the UV-Vis absorption was

194

recorded again. As a control, the analogous experiment was then performed with the

unmodified polymer (5.2) with and without TFA (Figure 5.3). A fully cross-linked polymer

would be expected to be insoluble and show no change in UV-Vis absorption. Chloroform

washing of the fully modified polymer films (5.3-S1) showed full retention of film after the

addition of acid compared to approximately only a quarter of the intensity retained for 5.2.

This preliminary experiment showed that the silane-functionalised polymer (5.3-S1) can

successfully cross-link.

Figure 5.3: UV-Vis absorption of thin films of 5.3 and 5.3-S1, before and after dipping in chloroform, with and without a drop of TFA.

The full modification of P(CPDTfBT) (5.2) should not be necessary to achieve a cross-linkable

polymer. In addition, the widening of the band-gap upon thiol-substitution is not desirable

as this is found to reduce OPV performance, largely through reduced photocurrent.25 A test

reaction with 50 mol% of (3-mercaptypropyl)trimethoxysilane (relative to fluorine atoms on

the polymer backbone) and 5.2 (in the microwave for 1 hour at 150 ⁰C) resulted in no

195

substitution of fluorine atoms. This could be due to self-condensation of silane groups

under the reaction conditions.

It was found that the extent of modification could be more easily be controlled by changing

the reaction time, using excess (3-mercaptypropyl)trimethoxysilane instead (Figure 5.4). As

a small change in UV-Vis was desired, an alcohol-trimethoxysilane analogue could have

potentially been a good alternative as this has been shown to give a smaller change in the

UV-Vis spectrum (discussed in Chapter 3). However, the alcohol substitution reactions

require hydroxide as the base which would cause undesirable hydrolysis of the

trimethoxysilane groups.

196

Figure 5.4: Normalised UV-Vis spectrum of solutions of 5.2 and the products of the substitution of 5.2 with excess (3-mercaptypropyl)trimethoxysilane under the following reaction conditions: 5.3-S1 - 18 h at 100 °C; 5.3-S2 - 30 min at 100 °C (microwave); A - 10 min at 100 °C (microwave); B - 60 min at 100 °C (microwave); C - 60 min at 150 °C (microwave). All solutions are in THF.

Excess (3-mercaptypropyl)trimethoxysilane was reacted with 5.2 dissolved in THF with

excess K2CO3 in the microwave over a series of different reaction times and temperatures:

10 min at 100 ⁰C, 30 min at 100 ⁰C, 60 min at 100 ⁰C and 60 min at 150 ⁰C. A small aliquot

of each reaction mixture was added to THF (3 mL) and the UV-Vis of each reaction was

recorded (Figure 5.4). The ‘60 min at 150 ⁰C’ reaction (C) closely resembled the fully

modified polymer (5.3-S1), ‘60 min at 100 ⁰C’ resulted in a slightly less blue shifted polymer

(B) and a reaction time of 10 min at 100 ⁰C resulted in no change in the UV-Vis spectrum (A).

However, the reaction time of 30 min at 100 ⁰C resulted in a partially modified polymer with

a relatively small blue shift of 35 nm. The resulting polymer was selected as the appropriate

material to take further and is labelled 5.3-S2 (Scheme 5.1a). The extent of the modification

was then estimated, as for 5.3-S1, by integrating the new –SCH2- proton signal (δ 3.2)

197

against the aromatic protons (Figure 5.2). The integration value was found to be 0.64H,

therefore it could be estimated that approximately 32% of fluorine atoms had been

substituted.

It should be noted that the two peak in the NMR spectrum at 3.6-3.7 ppm are associated

with the trimethoxysilane protons. The peak at ~3.6 ppm integrates to the expected nine

protons, with respect to the aromatic protons. The additional peak at ~3.7 ppm is a result

of unreacted starting material. This could not be removed from the polymer by

precipitation and subsequent washing steps.

5.3 Cross-linking study

The cross-linking ability of the partially modified polymer (5.3-S2) was then investigated.

Thin-films of 5.3-S2 and 5.2 were treated with TFA as before. Images of thin films of 5.3-S2

and 5.2, before and after dipping in chloroform can be seen in Figure 5.5. It can clearly be

seen that films of 5.3-S2 are much more solvent resistant than 5.2 (UV-Vis data can be

found in Appendix Figure 40).

198

Figure 5.5: Images of thin films of 5.2 and 5.3-S2 before (left) and after (right) dipping in chloroform. Both films had been treated with TFA prior to chloroform wash.

This led to a study into whether cross-linking could occur with no additives (such as TFA,

from the previous test) which would most likely adversely affect device performance. An

experiment was set up in which the typical device fabrication process was imitated, with the

absence of fullerene and either electrode. The most common hole transporting layer in

conventional OPV devices is poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)

(PEDOT:PSS) which is hydroscopic in nature. It was hypothesised that this interlayer could

assist with the hydrolysis of the trimethoxysilane groups, leading to cross-linking.

PEDOT:PSS coated glass was prepared by spin-coating an aqueous PEDOT:PSS solution (3500

rpm for 40 s) onto glass slides and then heating for 20 min at 150 ⁰C (a standard procedure

in OPV device fabrication). The substrates were then transferred into a glove box and

solutions of 5.3-S2 (in anhydrous chlorobenzene) were spin-coated onto the PEDOT:PSS

coated glass (700 rpm for 60 s). Another pair of slides were prepared without PEDOT:PSS,

199

as a control. The films were left in the glove box for 18 hours. Each film was taken out of

the glove box and half of the film was immediately submerged in chloroform. The UV-Vis

absorption of the slides, on both the submerged and non-submerged half, was then

measured (Figure 5.6). Films of 5.3-S2 with a PEDOT:PSS interlayer showed a significant

enhancement in the retention of film compared to films without the interlayer (76 and 44%

film retention respectively, (Figure 5.6a). Control experiments with the unmodified polymer

(5.2) were also performed and exhibited very small film retentions (<20%) in both cases

(Figure 5.6b). The retention percentage was calculated by measuring the percentage

difference in the UV-Vis absorption before and after dipping in chloroform, at each

wavelength. This value was then averaged over wavelengths 20 nm either side of the λmax

to give an approximation of the percentage of the film retained after chloroform dipping.

200

Figure 5.6: PL spectra of a) silane-functionalised polymer (5.2-S2) and b) Unmodified polymer (5.2) with and without a PEDOT:PSS interlayer, before and after dipping into chloroform (CHCl3)

These experiments showed that the silane-functionalised polymer (5.3-S2) could cross-link

under the standard conditions of OPV device fabrication, without the need for any external

stimuli or additives. The next step was to apply these materials to OPV devices.

5.4 Stability of cross-linked polymer OPVs

As discussed in Chapter 2, the substitution of thioalkyl groups causes twisting in the polymer

backbone (and a blue shift in UV-Vis absorption as a result). Therefore, it was expected that

the optimised device performance would arise from different conditions for the unmodified

and modified polymers (5.2 and 5.3-S2 respectively). As a result, the performance of

devices was optimised for both P(CPDTfBT) (5.2) and silane functionalised polymer (5.3-S2)

independently. This work was done by Dr. Pabitra Tuladhar, details of the optimised

201

fabrication process can be found in section 5.7.2. It should be noted that the modified

polymer exhibited low stability in air due to cross-linking in ambient conditions. Therefore,

5.3-S2 was synthesised, purified and transferred into the glovebox for device fabrication in

the same day.

The optimised device parameters can be found in Table 5.1, averaged over the five best

performing pixels in a device. The power conversion efficiency of 5.3-S2 was found to be

approximately half of that found for 5.2. This was predominantly due to a dramatic drop in

the short circuit current (Jsc). This is most likely because of the increase in band-gap (blue

shift in UV-Vis) and drop in the absorption from 500 – 900 nm observed in the UV-Vis

spectra at a known concentration (Figure 5.7). It should be noted that the shape of the

polymer absorption spectra is different to Figure 5.4 due to the different solvents used.

Table 5.1: Performance parameters of 5.2 and 5.3-S2 in a device configuration of glass/ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al


5.2 0.71 ± 0.01 (0.71)b 15.90 ± 0.75 (16.70)b 0.42 ± 0.00 (0.42)b 4.75 ± 0.26a (4.98)b

5.3-S2 0.71 ± 0.00 (0.71)b 8.65 ± 0.38 (9.08)b 0.36 ± 0.00 (0.37)b 2.24 ± 0.11a (2.37)b

aAverage device efficiency over five devices. bBest device efficiency.

The initial performance for each device was measured, followed by the heating of

substrates at 120 ⁰C for two weeks. During this period, the short circuit current (Jsc), open

circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) were measured at

regular intervals. The data reported in Figure 5.8 is an average of the best three performing

202

pixels for each device, based on the final PCE value of each pixel. An average over more

pixels was not possible because repeated measurements began to degrade the metal

contacts resulting in fewer measureable pixels.

Figure 5.7: Solution UV-Vis of 5.2 and 5.3-S2 in chlorobenzene solution (1.67 x 10-2

g dm-3

)

After 24 hours of heating, 5.2 showed a large decrease in PCE from ∼4.7% to ∼2.6% due to

a large drop in Jsc and decline in the fill factor, whilst the Voc stayed steady at ∼0.6 V Figure

5.8). Upon further heating the fill factor and Voc decreased until reaching ∼0.37 and ∼0.53

V respectively, the Jsc initially increased to ∼11 mAcm-2 and then steadily decreased to ∼10

mAcm-2. The PCE slowly decreased to ∼2%, a drop to 44% of the original efficiency.

5.3-S2 devices showed a large increase in the short circuit current (Jsc) and PCE (to ∼13

mAcm-2 and ∼3.2 % respectively) after the first 24 hours of heating, possibly due to the

polymer cross-linking in this period (this will be discussed further), this was also

203

accompanied by a large drop in the open circuit voltage (Voc). After a further 24 hours of

heating the Jsc and Voc both decreased to near their initial values, whilst the fill factor

steadily increased to ∼0.40. The next 288 hours of heating resulted in no noteworthy

change in any parameters, ending with a PCE of 2.64 %, 22 % greater than the initial value.

Figure 5.8: Device parameters measured at regular intervals under heating at 120 ⁰C for 2 weeks: a) Short circuit current (Jsc), b) fill factor, c) open circuit voltage (Voc) and d) power conversion efficiency (PCE).

This experiment was repeated a further time and the jump in Voc and JSC (and subsequently

the PCE) in 5.3-S2 devices after 24 h of heating was observed in both cases, and therefore

not an anomalous result. The jump could potentially have been a result of further cross-

204

linking occurring after the initial measurement. To investigate this further, thin films of 5.3-

S2 were again spun onto PEDOT:PSS coated glass but substrates were heated for 18 hours at

100 ⁰C instead of being kept at room temperature. The chloroform dip test was performed

as before and the UV-Vis was recorded (Figure 5.9). In this case, the film showed greater

film retention than before, with or without the PEDOT-PSS interlayer (93 % and 80 %). The

heating allows for the cross-linking to occur to a greater extent, which could explain why the

initial device performance changes so rapidly after the first day of heating.

Figure 5.9: PL spectra of silane-functionalised polymer 5.3-S2 thin films with and without a PEDOT:PSS interlayer, before and after dipping into chloroform (CHCl3). All substrates were heated at 120 °C for 18 h.

OPVs have also shown to be unstable when exposed to air, which has led to the

development of additives and encapsulation techniques.286,287 This has been explored by

Krebs, where a fullerene-free device was made, with inert electrodes which showed

impressive air stability, however the efficiencies of these devices were initially very low.288

It was hypothesised that the cross-linking of the active layer could potentially act as a

205

barrier to the external environment. The electrodes used in the above devices consisted of

aluminium and calcium. Calcium is known to oxidise in air, which would most likely be

detrimental to device performance. As a result, OPVs were made using the same optimised

conditions, but prior to evaporation of the calcium/aluminium the devices were taken

outside of the glove box and left for 18 hours at room temperature. As a control, identical

devices were left inside the glovebox for the same amount of time. The calcium/aluminium

was then evaporated onto the active layer and the performance was measured.

Table 5.2: Performance parameters of 5.2 and 5.3-S2 in a device configuration of glass/ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al. Left inside (inert) and outside (air) the glovebox for 18 h before evaporation of calcium and aluminium electrodes.


5.2 Inert 0.77 ± 0.00 (0.77)b 15.98 ± 0.73 (17.1)b 0.42 ± 0.00 (0.42)b 5.20 ± 0.26a (5.56)b

Air 0.60 ± 0.00 (0.59)b 7.43 ± 0.45 (8.01)b 0.27 ± 0.00 (0.27)b 1.18 ± 0.07 a (1.28)b

5.3-S2

Inert 0.65 ± 0.01 (0.67)b 8.50 ± 0.29 (8.89)b 0.41 ± 0.02 (0.42)b 2.29 ± 0.20 a (2.52)b

Air 0.66 ± 0.00 (0.66)b 8.30 ± 0.34 (8.60)b 0.37 ± 0.01 (0.36)b 2.00 ± 0.07 a (2.08)b

aAverage device efficiency over four pixels. bBest device efficiency.

Table 5.2 shows the device parameters with and without exposure to air. The unmodified

polymer (5.2) showed a significant decline in Jsc, Voc and fill factor when exposed to air. This

resulted in a PCE of 1.2% compared to 5.2% for the substrate kept in the glovebox.

Gratifyingly, the silane-modified polymer (5.3-S2) showed impressive stability in all three

parameters, with the small decline in Jsc and fill factor resulting in a PCE only falling from

2.3% to 2.0% with air exposure.

206

5.5 Study of morphology

As discussed at the beginning of this chapter, cross-linking the components of the active

layer can prevent crystallisation of either component. Optical microscopy has been a

popular approach for studying the extent of crystallisation in polymer-fullerene blends.269

For example, Kim et al. showed how heated cross-linked blends showed no crystalline

domains under optical microscopy but the control blends presented the formation of

crystalline needles of approximately 20 μM in length.204

207

Figure 5.10: Optical microscopy images of 5.2 and 5.3-2/PCBM films after two weeks at room temperature (left) and 120°C (right) in an inert atmosphere.

Solutions of 5.2 and 5.3-S2 with PC70BM in chlorobenzene were spin-coated onto glass

slides, under the same conditions of OPV fabrication. The films were heated, in a glovebox,

for two weeks at 120°C whilst another pair of control films were kept at room temperature.

The optical microscopy images of the films after two weeks are shown in Figure 5.10.

Surprisingly, signs of crystallisation were only exhibited in the heated cross-linked film and

not in any of the controls, with crystalline regions approximately 2 μM in size observed. This

was unexpected as the cross-linked devices were more thermally stable so the

crystallisation of the components should have been supressed

208

Figure 5.11: Atomic force microscopy (AFM) images of 5.2 and 5.3-2/PCBM films after two weeks at room temperature (left) and 120°C (right) in an inert atmosphere.

The topology of the films was investigated using atomic force microscopy (AFM) (Figure

5.11). From the topology measurements it can be seen that the crystalline domains have

grown vertically from the surface of the film by approximately 300 nm. It was speculated

that crystallisation was only occurring at the surface of the film and the bulk morphology

was relatively unperturbed. To look into this further, the experiment described above was

repeated but the films were encapsulated with a glass slide before heating. The optical

microscopy images can be seen in Figure 5.12. Encapsulation appears to supress

crystallisation of the components, suggesting that the observed crystallisation was a result

of the surface being exposed and not a result of morphological instability.

209

Figure 5.12: Optical microscopy images of 5.2 and 5.3-2/PCBM films encapsulated under a glass slide, after two weeks at room temperature (left) and 120°C (right) in an inert atmosphere.

To investigate this further, optical microscopy images were then taken of the OPV devices

which were heated for two weeks at 120°C. Devices were illuminated from the aluminium

electrode side and images were taken through the glass side. Images of the active layer

were taken between and directly underneath the metal electrodes. Interestingly,

crystallisation only occurred in the areas where no aluminium was present (Figure 5.13).

This could explain why the devices showed excellent ambient stability. When left in air, a

layer of crystalline material forms on the surface of the film, which acts as a barrier for

further penetration of water and oxygen. However, this is only speculation and would

require further investigation.

210

Figure 5.13: Optical microscopy images of 5.3-S2 devices taken between and underneath the aluminium electrode. Devices had undergone heating at 120°C for two weeks.

5.6 Conclusion

The novel post-polymerisation reaction, developed in Chapter 3, has been applied to the

high-performance OPV polymer P(CPDTfBT) (5.2). The polymer was partially functionalised

with trimethoxysilane groups, in one step, yielding a cross-linkable polymer (5.3-S2). This

material was shown to cross-link under the mild conditions experienced in standard OPV

fabrication. OPV devices containing the modified polymer showed impressive thermal

stability compared to the unmodified analogue. However, the initial PCE of these modified

polymer devices was less than half that of the unmodified analogue. The air-stability of

these devices was also investigated and the cross-linked devices showed impressive stability

compared to the unmodified analogue. To take the work further, effort should be made to

synthesise a silane-functionalised polymer, which gives rise to similar OPV performance to

its unfunctionalised analogue. This could be achieved by synthesising a polymer with a

lower loading of trimethoxysilane, which would hopefully reduce the dramatic change in JSC

211

(and in turn the PCE) observed. Inverted devices with silver instead of calcium/aluminium

electrodes should also be fabricated. As silver does not oxidise in air, the ambient stability

of a whole device could then be investigated.

5.7 Experimental

5.7.1 Monomer and polymer synthesis

5.1 - 4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannanyl)-4H-cyclopenta-[2,1-b:3,4-

b′]dithiophene – based on literary procedure284

To a solution of 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (1.0 g, 2.49

mmol) in dry THF (50 mL) at -78 ⁰C was added n-butyllithium (4.96 mL of a 2.5M solution in

hexanes, 12.4 mmol) drop wise. The resulting mixture was stirred at this temperature for 30

min and allowed to warm to room temperature over 1 h. The reaction was then cooled to -

78 ⁰C, and trimethyltin chloride (9.93 mL of a 1M solution in THF, 9.93 mmol) was added

drop wise. The reaction was allowed to warm to room temperature and stirred for 18 h.

Water was added and the reaction was extracted with toluene. The organic layer was

washed with water, dried (MgSO4) magnesium sulfate, filtered and concentrated under

reduced pressure to afford a yellow viscous oil – which was used without further

212

purification (1.652 g, 91%): 1H NMR (CO(CD3)2): δ 6.89 - 6.79 (m, 2H), 2.04 – 1.9 (m, 4H),

1.05 – 0.85 (m, 18H), 0.77 (t, J = 6.9 Hz, 6H), 0.62 (t, J = 8.0 Hz, 6H), 0.48 – 0.31 (m, 18H).

5.2 P(CPDTfBT) – based on literary procedure289

Synthesis is based on method reported in the literature289. A 2.5ml microwave vial was

charged with 4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannanyl)-4H-cyclopenta-[2,1-b:3,4-

b′]dithiophene (5.1) (1.6516 g, 2.27 mmol) and 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole

(2.1) (0.7074 g, 2.27 mmol) in a mixture of o-xylene (9 mL) and DMF (0.9 mL). After being

purged with argon for 15 min Pd2(dba)3 (0.0997 g, 0.1089 mmol) and P(o-tol)3 (0.1325 g,

0.4354 mmol) were added consequently. The resulting mixture was heated in a microwave

reactor at 120 ⁰C for 5 min, 150 ⁰C for 5 min and 170 ⁰C for 40 min. After cooling to room

temperature, the resulting mixture was precipitated by adding to methanol (100 mL) and

filtered. The collected precipitate was subjected to Soxhlet extraction with methanol,

acetone, hexane and CHCl3. The CHCl3 fraction was collected and dried under vacuum. A

dark blue solid was obtained (1.007 g, 82%). The polymer (200 mg) was then fractionating

using a preparative GPC running in chlorobenzene to obtain 62 mgs of P(CPDTfBT) with Mn =

29 kDa, Mw = 43 kDa, Mw/Mn (Ð) = 1.5. 1H NMR (1,1,2,2-Tetrachloroethane-d2) at 403K: δ

8.30 (br, 1H), 8.18 (br, 1H), 7.84 (br, 1H), 2.27 – 2.07 (m, 4H), 1.26 – 1.11 (m, 18H), 0.82 –

0.74 (m, 12H). 19F NMR (1,1,2,2-Tetrachloroethane-d2) at 403K: δ -108.38, 108.21.

213

General Procedure - the modification of 5.2 with (3-mercaptopropyl)trimethoxysilane

5.2 and K2CO3 (10 eq compared to thiol) were added to a microwave vial equipped with a

stirrer bar. The vial was then sealed with a septum and flushed with argon, before THF (1

mL per 5 mg of polymer) and reaction was stirred at room temperature until the polymer

was fully dissolved. (3-Mercaptopropyl)trimethoxysilane (100 eq compared to moles of

repeat unit of polymer) was added. The reaction was refluxed overnight or reacted in the

microwave depending on the desired extent of modification required (see Figure 5.4 for

reaction times). The reaction was allowed to cool to room temperature and was added

drop wise to a stirring solution of dry methanol (10 mL for every 1 mL of THF) and 18-crown-

6 ether (1.2 eq compared to K2CO3). The precipitated polymer in solution was filtered and

dried overnight.

5.3-S1 – fully substituted P(CPDTfBT)

5.2 (20 mg, 0.036 mmol) reacted using general procedure. Reaction heated overnight at

60°C. Yielded black polymer 5.3-S1 (13 mg, 51%). 1H NMR (1,1,2,2-Tetrachloroethane-d2) at

403K: δ 8.23 (s, br, 1H), 8.09 (s, 1H), 7.85 – 7.70 (m, 1H), 3.65 – 3.59 (m, 9H), 3.21 (br, 2H),

2.27 – 2.06 (m, 4H), 2.04 – 1.93 (m, 2H), 1.30 – 1.07 (m, 20H), 0.88 – 0.74 (m, 12H); IR 1741

cm-1 (Si-OMe).

214

5.3-S2 – 32% substituted P(CPDTfBT)

5.2 (15 mg, 0.027 mmol) reacted using general procedure. Reaction heated at 100 °C in

microwave reactor for 30 min. Yielded black polymer 5.3-S2 (10 mg, 61%). 1H NMR (1,1,2,2-

Tetrachloroethane-d2) at 403K: δ 8.36 – 8.05 (m, 2H), 7.89 – 7.74 (m, 1H), 3.65 – 3.59 (m,

2.88H (0.32x 9H)), 3.26 – 3.15 (m, 0.64H (0.32 x 2H), 2.30 – 2.07 (m, 4H), 2.03 – 1.93 (m,

0.64H (0.32 x 2H), 1.28 – 0.90 (m, 20.64H (20H + 0.32 x 2H)), 0.88 – 0.76 (m, 12H); IR 1741

cm-1 (Si-OMe).

5.7.2 Device fabrication

ITO coated glass substrates (15 Ω per square from Psiotec) were routinely cleaned in soap

and DI water, acetone and isopropyl alcohol several times and then finally blow dried with

nitrogen. Cleaned ITO substrates are immediately transferred to oxygen plasma treatment

system and treated for 7 min at oxygen pressure of 0.25mbar. On the top of treated ITO,

poly(styrenesulfonate) (PEDOT:PSS) (Baytron P, VP AI 4083 grade, HC Stark) layer was spin-

coated at 3500rpm for 40secs, before being annealed at 150 0C for 20 min on the hot plate.

Before spin coating poly(styrenesulfonate) (PEDOT: PSS), it was filtered through 0.45 µm

filter. After annealing the ITO substrates were transferred in to the glove box to spin coat

215

the active layer. The efficiency of fabricated devices were and J-V characteristics were

generated by using Keithley 238 Source Measure Units. Illumination was provided using a

300 W xenon arc lamp solar simulator (Oriel Instruments) and calibrated using a silicon

photodiode in order to ensure the illumination intensity of 100 mW/cm2, at 1 sun AM 1.5.

During the measurements, the devices were kept in nitrogen environment in the sealed

device holder.

5.2:PC70BM devices

The blend solution of 1:2.5, 5.2:[70]PCBM was prepared with concentration of 30 mg/mL

from chlorobenzene and left stirring overnight in the glovebox at room temperature. The

active layer of 5.2:[70]PCBM blend films were spin coated on PEDOT:PSS layer at spin speed

of 3000 rpm. Finally calcium (25nm) and aluminium (100 nm) was evaporated under

vacuum 2.0 10-6 mbar defining active device area of 0.045 cm2. In order to optimise the

device thickness, the active layer was spun with different rpms ranging from 800 to 5000

rpm and found that 3000 rpm was optimum.

5.3-S2:PC[70]BM devices

5.3-S2 and PC[70]BM solutions were prepared in separate vials with concentration 20

mg/ml in Chlorobenzene. The solutions for the 5.3-S2 was left on the hot plate at 90 °C

under constant stirring and while the PC[70]BM solution was left under the constant stirring

at room temperature in the glove box. The blend solution was prepared with 1:2.5,

polymer:[70]PCBM. The blend solution was left in the glove box for at least an hour at 90 °C

216

before spinning on ITO substrates. The active layer of 5.3-S2:[70]PCBM blend films were

spin coated on PEDOT:PSS layer at spin speed of 800 rpm in the spin coater in the glove box.

In order to optimise the device thickness, the active layer for 5.3-S2 was spun with different

rpms ranging from 800 to 2500 rpm. The devices were annealed at 120 0C for 10mins The

active layer films were kept in the vacuum chamber at least for an hour and finally about Ca

(25 nm) and Aluminium (1000 nm) was evaporated under vacuum 2.0 10-6 mbar defining

active device area of 0.045 cm2.

217

References

1. L. H. Baekeland, Ind. Eng. Chem., 1909, 1, 149–161.

2. C. K. Chiang, C. R. Fincher, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, Phys. Rev. Lett., 1977, 39, 1098–1101.

3. The Nobel Prize in Chemistry 2000 - http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2000/ - Accessed September 2017, .

4. E. D. Williams, R. U. Ayres, and M. Heller, Environ. Sci. Technol., 2002, 36, 5504–5510.

5. A. Ghahremani and A. E. Fathy, Energy Sci. Eng., 2016, 4, 334–343.

6. R. Hill, J. Phys. C Solid State Phys., 1974, 7, 521–526.

7. S. E. Shaheen, D. S. Ginley, and G. E. Jabbour, MRS Bull., 2005, 30, 10–19.

8. C. Lungenschmied, G. Dennler, H. Neugebauer, S. N. Sariciftci, M. Glatthaar, T. Meyer, and A. Meyer, Sol. Energy Mater. Sol. Cells, 2007, 91, 379–384.

9. P. Chandrasekhar, Conducting Polymers, Fundamentals and Applications: A Practical Approach, 1999.

10. H. Meier, U. Stalmach, and H. Kolshorn, Acta Polym., 1997, 48, 379–384.

11. W. Brütting, Physics of Organic Semiconductors, 2012.

12. C. Shi, Y. Yao, Y. Yang, and Q. Pei, J. Am. Chem. Soc., 2006, 128, 8980–8986.

13. R. D. McCullough, S. P. Williams, R. D. Lowe, M. Jayaraman, and S. Tristram-Nagle, J. Am. Chem. Soc., 1993, 115, 4910–4911.

14. H. Mao and S. Holdcroft, Macromolecules, 1992, 25, 554–558.

15. N. E. Jackson, B. M. Savoie, K. L. Kohlstedt, M. Olvera De La Cruz, G. C. Schatz, L. X. Chen, and M. A. Ratner, J. Am. Chem. Soc., 2013, 135, 10475–10483.

16. P. Boufflet, Y. Han, Z. Fei, N. D. Treat, R. Li, D. M. Smilgies, N. Stingelin, T. D. Anthopoulos, and M. Heeney, Adv. Funct. Mater., 2015, 25, 7038–7048.

17. R. M. Souto Maior, K. Hinkelmann, H. Eckert, and F. Wudl, Macromolecules, 1990, 23, 1268–1279.

18. N. Kleinhenz, L. Yang, H. Zhou, S. C. Price, and W. You, Macromolecules, 2011, 44, 872–877.

19. K. L. Konkol, R. L. Schwiderski, and S. C. Rasmussen, Materials (Basel)., 2016, 9, 404–420.

218

20. M. Shahid, R. S. Ashraf, E. Klemm, and S. Sensfuss, Macromolecules, 2006, 39, 7844–7853.

21. R. S. Kularatne, H. D. Magurudeniya, P. Sista, M. C. Biewer, and M. C. Stefan, J. Polym. Sci. Part A Polym. Chem., 2013, 51, 743–768.

22. Q. T. Zhang and J. M. Tour, J. Am. Chem. Soc., 1998, 120, 5355–5362.

23. K. G. Jespersen, W. J. D. Beenken, Y. Zaushitsyn, A. Yartsev, M. Andersson, T. Pullerits, and V. Sundström, J. Chem. Phys., 2004, 121, 12613–12617.

24. T. Matsui, Y. Imamura, I. Osaka, K. Takimiya, and T. Nakajima, J. Phys. Chem. C, 2016, 120, 8305–8314.

25. A. Casey, R. S. Ashraf, Z. Fei, and M. Heeney, Macromolecules, 2014, 47, 2279–2288.

26. Y.-C. Hung, J.-C. Jiang, C.-Y. Chao, W.-F. Su, and S.-T. Lin, J. Phys. Chem. B, 2009, 113, 8268–8277.

27. A. Dkhissi, Synth. Met., 2011, 161, 1441–1443.

28. A. Armin, D. M. Stoltzfus, J. E. Donaghey, A. J. Clulow, R. C. R. Nagiri, P. L. Burn, I. R. Gentle, and P. Meredith, J. Mater. Chem. C, 2017, 5, 3736–3747.

29. K. F. Young and H. P. R. Frederikse, J. Phys. Chem. Ref. Data, 1973, 2, 313–410.

30. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 2006.

31. J.-L. Bredas, Mater. Horiz., 2014, 1, 17–19.

32. O. V. Mikhnenko, P. W. M. Blom, and T.-Q. Nguyen, Energy Environ. Sci., 2015, 8, 1867–1888.

33. B. Carlotti, R. Flamini, I. Kikaš, U. Mazzucato, and A. Spalletti, Chem. Phys., 2012, 407, 9–19.

34. B. J. Schwartz, Annu. Rev. Phys. Chem., 2003, 54, 141–172.

35. S. Cho, Y. Kim, Y. Park, M. Choi, J. Park, J. Lee, S. Park, M. Chang, J. Cho, I. In, and B. Park, J. Phys. Chem. C, 2016, 120, 26119−26128.

36. Z. Hu, A. P. Willard, R. J. Ono, C. W. Bielawski, P. J. Rossky, and D. A. Vanden Bout, Nat. Commun., 2015, 6, 8246.

37. L. Han, T. Hu, X. Bao, M. Qiu, W. Shen, M. Sun, W. Chen, and R. Yang, J. Mater. Chem. A, 2015, 3, 23587–23596.

38. F. C. Spano and C. Silva, Annu. Rev. Phys. Chem., 2014, 65, 477–500.

39. Y. Deng, W. Yuan, Z. Jia, and G. Liu, J. Phys. Chem. B, 2014, 118, 14536–14545.

40. A. Eisfeld and J. S. Briggs, Chem. Phys., 2006, 324, 376–384.

41. S. Holliday, Y. Li, and C. K. Luscombe, Prog. Polym. Sci., 2017, 70, 34–51.

42. http://www.belectric.co.uk/opv-for-african-union/, accessed September 2017, .

43. G. Yu and A. J. Heeger, J. Appl. Phys., 1995, 78, 4510–4515.

44. J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, and H. Yan, Nat. Energy, 2016, 1, 15027.

45. L. Lucera, F. Machui, H. D. Schmidt, T. Ahmad, P. Kubis, S. Strohm, J. Hepp, A. Vetter, H. J. Egelhaaf, and C. J. Brabec, Org. Electron. physics, Mater. Appl., 2017, 45, 41–45.

46. NREL, Best Research-Cell Efficiencies, http://www.nrel.gov, accessed: September 2017, .

219

47. A. Babayigit, A. Ethirajan, M. Muller, and B. Conings, Nat. Mater., 2016, 15, 247–251.

48. A. Fakharuddin, R. Jose, T. M. Brown, F. Fabregat-Santiago, and J. Bisquert, Energy Environ. Sci., 2014, 7, 3952–3981.

49. S. Castro-Hermosa, S. K. Yadav, L. Vesce, A. Guidobaldi, A. Reale, A. Di Carlo, and T. M. Brown, J. Phys. D. Appl. Phys., 2017, 50, 33001.

50. K. M. Tsoi, Q. Dai, B. A. Alman, and W. C. W. Chan, Acc. Chem. Res., 2013, 46, 662–671.

51. A. Maurano, R. Hamilton, C. G. Shuttle, A. M. Ballantyne, J. Nelson, B. O’Regan, W. Zhang, I. McCulloch, H. Azimi, M. Morana, C. J. Brabec, and J. R. Durrant, Adv. Mater., 2010, 22, 4987–4992.

52. S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, and A. J. Heeger, Nat. Photonics, 2009, 3, 297–302.

53. N. Zhou and A. Facchetti, Charge Transport and Recombination in Organic Solar Cells (OSCs), 2014.

54. A. Armin, M. Velusamy, P. Wolfer, Y. Zhang, P. L. Burn, P. Meredith, and A. Pivrikas, ACS Photonics, 2014, 1, 173–181.

55. K. Harigaya and S. Abe, Phys. Rev. B, 1994, 49, 16746–16752.

56. M. Causa’, J. De Jonghe-Risse, M. Scarongella, J. C. Brauer, E. Buchaca-Domingo, J.-E. Moser, N. Stingelin, and N. Banerji, Nat. Commun., 2016, 7, 12556.

57. M. Muntwiler, Q. Yang, W. A. Tisdale, and X. Y. Zhu, Phys. Rev. Lett., 2008, 101, 1–4.

58. S. K. Pal, T. Kesti, M. Maiti, F. Zhang, S. Hellstro, M. R. Andersson, F. Oswald, F. Langa, and O. Tomas, J. Am. Chem. Soc., 2010, 132, 12440–12451.

59. A. Foertig, J. Kniepert, M. Gluecker, T. Brenner, V. Dyakonov, D. Neher, and C. Deibel, Adv. Funct. Mater., 2014, 24, 1306–1311.

60. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science (80-. )., 1995, 270, 1789–1791.

61. C. J. Brabec, M. Heeney, I. McCulloch, and J. Nelson, Chem. Soc. Rev., 2011, 40, 1185–1199.

62. B. P. Lyons, N. Clarke, and C. Groves, Energy Environ. Sci., 2012, 5, 7657.

63. G. Wantz, L. Derue, O. Dautel, A. Rivaton, P. Hudhomme, and C. Dagron-Lartigau, Polym. Int., 2014, 63, 1346–1361.

64. S. Holliday, R. S. Ashraf, A. Wadsworth, D. Baran, S. A. Yousaf, C. B. Nielsen, C.-H. Tan, S. D. Dimitrov, Z. Shang, N. Gasparini, M. Alamoudi, F. Laquai, C. J. Brabec, A. Salleo, J. R. Durrant, and I. McCulloch, Nat. Commun., 2016, 7, 11585.

65. Y. Lin and X. Zhan, Mater. Horizons, 2014, 1, 470.

66. A. F. Eftaiha, J.-P. Sun, I. G. Hill, and G. C. Welch, J. Mater. Chem. A, 2014, 2, 1201–1213.

67. C. B. Nielsen, S. Holliday, H.-Y. Chen, S. J. Cryer, and I. McCulloch, Acc. Chem. Res., 2015, 48, 2803–2812.

68. T. H. Lai, S. W. Tsang, J. R. Manders, S. Chen, and F. So, Mater. Today, 2013, 16, 424–432.

69. K. Norrman, S. A. Gevorgyan, and F. C. Krebs, ACS Appl. Mater. Interfaces, 2009, 1, 102–112.

220

70. K. Feron, T. J. Nagle, L. J. Rozanski, B. B. Gong, and C. J. Fell, Sol. Energy Mater. Sol. Cells, 2013, 109, 169–177.

71. A. J. Parnell, A. J. Cadby, A. D. F. Dunbar, G. L. Roberts, A. Plumridge, R. M. Dalgliesh, M. W. A. Skoda, and R. A. L. Jones, J. Polym. Sci. Part B Polym. Phys., 2015, 54, 141–146.

72. S. Kundu, S. R. Gollu, R. Sharma, G. Srinivas, A. Ashok, A. R. Kulkarni, and D. Gupta, Org. Electron. physics, Mater. Appl., 2013, 14, 3083–3088.

73. K. Y. Pu and B. Liu, Adv. Funct. Mater., 2011, 21, 3408–3423.

74. Changfeng Wu, Barbara Bull, Craig Szymanski, Kenneth Christensen, and Jason McNeill, ACS Nano, 2008, 2, 2415–2423.

75. Y.-H. Chan and P.-J. Wu, Part. Part. Syst. Charact., 2015, 32, 11–28.

76. W. E. Moerner, PNAS, 2007, 104, 12596–12602.

77. T. Ha, a Y. Ting, J. Liang, W. B. Caldwell, a a Deniz, D. S. Chemla, P. G. Schultz, and S. Weiss, Proc. Nat. Acad. Sci. USA, 1999, 96, 893–898.

78. X. S. Xie, Science (80-. )., 2006, 312, 228–230.

79. Y. Wang, R. Hu, G. Lin, I. Roy, and K. T. Yong, ACS Appl. Mater. Interfaces, 2013, 5, 2786–2799.

80. P. Reineck, A. Francis, A. Orth, D. W. M. Lau, R. D. V. Nixon-Luke, I. Das Rastogi, W. A. W. Razali, N. M. Cordina, L. M. Parker, V. K. A. Sreenivasan, L. J. Brown, and B. C. Gibson, Adv. Opt. Mater., 2016, 4, 1549–1557.

81. J. P. Ryman-Rasmussen, J. E. Riviere, and N. A. Monteiro-Riviere, Nano Lett., 2007, 7, 1344–1348.

82. M. Zhang, Y. Ou, X. Du, X. Li, H. Huang, Y. Wen, and X. Zhang, J. Porous Mater., 2017, 24, 13–20.

83. A. Reisch and A. S. Klymchenko, Small, 2016, 12, 1968–1992.

84. H. S. Peng, J. A. Stolwijk, L. N. Sun, J. Wegener, and O. S. Wolfbeis, Angew. Chemie - Int. Ed., 2010, 49, 4246–4249.

85. W. Wu, S. Ye, G. Yu, Y. Liu, J. Qin, and Z. Li, Macromol. Rapid Commun., 2012, 33, 164–171.

86. D. Ding, K. Li, W. Qin, R. Zhan, Y. Hu, J. Liu, B. Z. Tang, and B. Liu, Adv. Healthc. Mater., 2013, 2, 500–507.

87. R. Hu, J. L. Maldonado, M. Rodriguez, C. Deng, C. K. W. Jim, J. W. Y. Lam, M. M. F. Yuen, G. Ramos-Ortiz, and B. Z. Tang, J. Mater. Chem., 2012, 22, 232–240.

88. C. Dai, D. Yang, X. Fu, Q. Chen, C. Zhu, Y. Cheng, and L. Wang, Polym. Chem., 2015, 6, 5070–5076.

89. Changfeng Wu, Barbara Bull, Craig Szymanski, Kenneth Christensen, and Jason McNeill, ACS Nano, 2008, 2, 2415–2423.

90. A. Kaeser and A. P. H. J. Schenning, Adv. Mater., 2010, 22, 2985–2997.

91. D. Tuncel and H. V. Demir, Nanoscale, 2010, 2, 484–494.

92. S. Y. Liou, C. S. Ke, J. H. Chen, Y. W. Luo, S. Y. Kuo, Y. H. Chen, C. C. Fang, C. Y. Wu, C. M. Chiang, and Y. H. Chan, ACS Macro Lett., 2016, 5, 154–157.

221

93. L. Feng, C. Zhu, H. Yuan, L. Liu, F. Lv, and S. Wang, Chem. Soc. Rev., 2013, 42, 6620.

94. T. Stahl, R. Bofinger, I. Lam, K. J. Fallon, P. Johnson, O. Ogunlade, V. Vassileva, R. B. Pedley, P. C. Beard, H. C. Hailes, H. Bronstein, and A. B. Tabor, Bioconjug. Chem., 2017, 28, 1734–1740.

95. P. Howes, M. Green, J. Levitt, K. Suhling, and M. Hughes, J. Am. Chem. Soc., 2010, 132, 3989–3996.

96. L. P. Fernando, P. K. Kandel, J. B. Yu, J. McNeill, P. C. Ackroyd, and K. a Christensen, Biomacromolecules, 2010, 11, 2675–2682.

97. K. Li, J. Pan, S. S. Feng, A. W. Wu, K. Y. Pu, Y. Liu, and B. Liu, Adv. Funct. Mater., 2009, 19, 3535–3542.

98. O. S. Wolfbeis, Chem. Soc. Rev., 2015, 44, 4743–4768.

99. Y. Rong, C. Wu, J. Yu, X. Zhang, F. Ye, M. Zeigler, M. E. Gallina, I. C. Wu, Y. Zhang, Y. H. Chan, W. Sun, K. Uvdal, and D. T. Chiu, ACS Nano, 2013, 7, 376–384.

100. D. Chen, I. Wu, Z. Liu, Y. Tang, H. Chen, J. Yu, C. Wu, and D. T. Chiu, Chem. Sci., 2017, 8, 3390–3398.

101. J. E. Millstone, D. F. J. Kavulak, C. H. Woo, T. W. Holcombe, E. J. Westling, A. L. Briseno, M. F. Toney, and J. M. J. Fréchet, Langmuir, 2010, 26, 13056–13061.

102. G. Zlateva, Z. Zhelev, R. Bakalova, and I. Kanno, Inorg. Chem., 2007, 46, 6212–6214.

103. H. Piwoński, T. Michinobu, and S. Habuchi, Nat. Commun., 2017, 8, 15256.

104. F. Schütze, M. Krumova, and S. Mecking, Macromolecules, 2015, 48, 3900–3906.

105. K. Landfester, R. Montenegro, U. Scherf, R. Güntner, U. Asawapirom, S. Patil, D. Neher, and T. Kietzke, Adv. Mater., 2002, 14, 651–655.

106. N. Kurokawa, H. Yoshikawa, N. Hirota, K. Hyodo, and H. Masuhara, ChemPhysChem, 2004, 5, 1609–1615.

107. A. Gupta, H. B. Eral, T. A. Hatton, and P. S. Doyle, Soft Matter, 2016, 12, 2826–2841.

108. N. T. K. Thanh, N. Maclean, and S. Mahiddine, Chem. Rev., 2014, 114, 7610–7630.

109. K. Landfester, Top. Curr. Chem., 2003, 227, 75–123.

110. O. Mert, S. K. Lai, L. Ensign, M. Yang, Y. Y. Wang, J. Wood, and J. Hanes, J. Control. Release, 2012, 157, 455–460.

111. K. Miladi, S. Sfar, H. Fessi, and A. Elaissari, Nanoprecipitation Process: From Particle Preparation to In Vivo Applications, 2017.

112. D. Tuncel and H. V. Demir, Nanoscale, 2010, 2, 484.

113. B. J. Berne and R. Pecora, Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics, 2000.

114. G. Mie, Ann. Phys., 1908, 330, 377–445.

115. S. Kano, T. Tada, and Y. Majima, Chem. Soc. Rev., 2015, 44, 970–987.

116. J. Gross, S. Sayle, A. R. Karow, U. Bakowsky, and P. Garidel, Eur. J. Pharm. Biopharm., 2016, 104, 30–41.

222

117. S. Habuchi, S. Onda, and M. Vacha, Phys. Chem. Chem. Phys., 2011, 13, 1743–1753.

118. J. Mei and Z. Bao, Chem. Mater., 2014, 26, 604–615.

119. M. Liu, Y. Liu, Z. Peng, C. Yang, Q. Mu, Z. Cao, J. Ma, and L. Xuan, Materials (Basel)., 2017, 10, 1–14.

120. A. L. Vahrmeijer, M. Hutteman, J. R. van der Vorst, C. J. H. van de Velde, and J. V. Frangioni, Nat. Rev. Clin. Oncol., 2013, 10, 507–518.

121. L. Yuan, W. Lin, K. Zheng, L. He, and W. Huang, Chem. Soc. Rev., 2013, 42, 622–661.

122. Z. Guo, S. Park, J. Yoon, and I. Shin, Chem. Soc. Rev., 2014, 43, 16–29.

123. C. P. Chen, Y. C. Huang, S. Y. Liou, P. J. Wu, S. Y. Kuo, and Y. H. Chan, ACS Appl. Mater. Interfaces, 2014, 6, 21585–21595.

124. Y. Jin, F. Ye, M. Zeigler, C. Wu, and D. T. Chiu, ACS Nano, 2011, 5, 1468–1475.

125. X. Zhang, J. Yu, Y. Rong, F. Ye, D. T. Chiu, and K. Uvdal, Chem. Sci., 2013, 4, 2143.

126. C. S. Ke, C. C. Fang, J. Y. Yan, P. J. Tseng, J. R. Pyle, C. P. Chen, S. Y. Lin, J. Chen, X. Zhang, and Y. H. Chan, ACS Nano, 2017, 11, 3166–3177.

127. D. L. Crossley, L. Urbano, R. Neumann, S. Bourke, J. Jones, L. A. Dailey, M. A. Green, M. Humphries, S. M. King, M. L. Turner, and M. J. Ingleson, ACS Appl. Mater. Interfaces, 2017, 9, 28243–28249.

128. Y. Arai and T. Nagai, J. Electron Microsc. (Tokyo)., 2013, 62, 419–428.

129. R. B. Sekar and A. Periasamy, J. Cell Biol., 2003, 160, 629–633.

130. D. Deng, Y. Zhang, J. Zhang, Z. Wang, L. Zhu, J. Fang, B. Xia, Z. Wang, K. Lu, W. Ma, and Z. Wei, Nat. Commun., 2016, 7, 13740.

131. C. B. Nielsen, A. J. P. White, and I. McCulloch, J. Org. Chem., 2015, 80, 5045–5048.

132. H. Bin, L. Gao, Z.-G. Zhang, Y. Yang, Y. Zhang, C. Zhang, S. Chen, L. Xue, C. Yang, M. Xiao, and Y. Li, Nat. Commun., 2016, 7, 13651.

133. Z. Zhao, Z. Yin, H. Chen, L. Zheng, C. Zhu, L. Zhang, S. Tan, H. Wang, Y. Guo, Q. Tang, and Y. Liu, Adv. Mater., 2017, 29, 1–6.

134. G. A. Artamkina, M. P. Egorov, and I. P. Beletskaya, Chem. Rev., 1982, 82, 427–459.

135. P. Sonar, S. P. Singh, Y. Li, M. S. Soh, and A. Dodabalapur, Adv. Mater., 2010, 22, 5409–13.

136. D. L. Crossley, I. Vitorica-Yrezabal, M. J. Humphries, M. L. Turner, and M. J. Ingleson, Chem. Eur. J, 2016, 22, 12439–48.

137. G. Tregnago, T. T. Steckler, O. Fenwick, M. R. Andersson, and F. Cacialli, J. Mater. Chem. C, 2015, 3, 2792–2797.

138. E. Perzon, F. Zhang, M. Andersson, W. Mammo, O. Inganäs, and M. R. Andersson, Adv. Mater., 2007, 19, 3308–3311.

139. J. Roncali, Macromol. Rapid Commun., 2007, 28, 1761–1775.

140. L. Dou, Y. Liu, Z. Hong, G. Li, and Y. Yang, Chem. Rev., 2015, 115, 12633–65.

141. J. W. Jung, J. W. Jo, E. H. Jung, and W. H. Jo, Org. Electron., 2016, 31, 149–170.

223

142. C. Liu, K. Wang, X. Gong, and A. J. Heeger, Chem. Soc. Rev., 2016, 45, 4825–46.

143. G. T. Verlag, Science of Synthesis: Houben-Weyl Methods of Molecular Transformations Vol. 13: Five-Membered Hetarenes with Three or More Heteroatoms, 2014.

144. T. C. Parker, D. G. (Dan) Patel, K. Moudgil, S. Barlow, C. Risko, J.-L. Brédas, J. R. Reynolds, and S. R. Marder, Mater. Horiz., 2015, 2, 22–36.

145. H. Zhou, L. Yang, A. C. Stuart, S. C. Price, S. Liu, and W. You, Angew. Chemie, 2011, 123, 3051–3054.

146. J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, and Y. Yang, Nat. Commun., 2013, 4, 1446.

147. T.-L. Wang, C.-H. Yang, and Y.-Y. Chuang, RSC Adv., 2016, 6, 47676–47686.

148. S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kurpiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch, and D. Neher, J. Am. Chem. Soc., 2012, 134, 14932–44.

149. A. C. Stuart, J. R. Tumbleston, H. Zhou, W. Li, S. Liu, H. Ade, and W. You, J. Am. Chem. Soc., 2013, 135, 1806–15.

150. A. Casey, Y. Han, Z. Fei, A. J. P. White, T. D. Anthopoulos, and M. Heeney, J. Mater. Chem. C, 2015, 3, 265–275.

151. A. Casey, S. D. Dimitrov, P. Shakya-Tuladhar, Z. Fei, M. Nguyen, Y. Han, T. D. Anthopoulos, J. R. Durrant, and M. Heeney, Chem. Mater., 2016, 28, 5110–5120.

152. D. G. Patel, F. Feng, Y. Ohnishi, K. a Abboud, S. Hirata, K. S. Schanze, and J. R. Reynolds, J. Am. Chem. Soc., 2012, 134, 2599–612.

153. H. Zhou, L. Yang, S. Xiao, S. Liu, and W. You, Macromolecules, 2010, 43, 811–820.

154. G. Li, C. Kang, X. Gong, J. Zhang, and C. Li, Macromolecules, 2014, 47, 4545–4652.

155. X. Gong, G. Li, C. Li, J. Zhang, and Z. Bo, J. Mater. Chem. A, 2015, 3, 20195–20200.

156. H. Yi, S. Al-Faifi, A. Iraqi, D. C. Watters, J. Kingsley, and D. G. Lidzey, J. Mater. Chem., 2011, 21, 13649.

157. R. Qin, W. Li, C. Li, C. Du, C. Veit, H.-F. Schleiermacher, M. Andersson, Z. Bo, Z. Liu, O. Inganäs, U. Wuerfel, and F. Zhang, J. Am. Chem. Soc., 2009, 131, 14612–3.

158. Q. Peng, X. Liu, D. Su, G. Fu, J. Xu, and L. Dai, Adv. Mater., 2011, 23, 4554–8.

159. A. Sharma, V. P. Mehta, and E. Van der Eycken, Tetrahedron, 2008, 64, 2605–2610.

160. K. T. Nielsen, K. Bechgaard, and F. C. Krebs, Macromolecules, 2005, 38, 658–659.

161. R. S. Ashraf, B. C. Schroeder, H. a. Bronstein, Z. Huang, S. Thomas, R. J. Kline, C. J. Brabec, P. Rannou, T. D. Anthopoulos, J. R. Durrant, and I. McCulloch, Adv. Mater., 2013, 25, 2029–2034.

162. S. A. Jenekhe, L. Lu, and M. M. Alam, Macromolecules, 2001, 34, 7315–7324.

163. P. M. Beaujuge, C. M. Amb, and J. R. Reynolds, Acc. Chem. Res., 2010, 43, 1396–1407.

164. U. Salzner and M. E. Köse, J. Phys. Chem. B, 2002, 106, 9221–9226.

165. J. Li and A. Grimsdale, Chem. Soc. Rev., 2010, 39, 2399–2410.

224

166. C. Hansch, A. Leo, and R. W. Taft, Chem. Rev., 1991, 91, 165–195.

167. G. L. Gibson, T. M. McCormick, and D. S. Seferos, J. Am. Chem. Soc., 2012, 134, 539–47.

168. A. D. Becke, J. Chem. Phys., 1993, 98, 5648.

169. L. Han, W. Chen, T. Hu, J. Ren, M. Qiu, Y. Zhou, D. Zhu, N. Wang, M. Sun, and R. Yang, ACS Macro Lett., 2015, 4, 0–5.

170. S. Wen, W. Chen, M. Fan, L. Duan, M. Qiu, M. Sun, L. Han, and R. Yang, J. Mater. Chem. A Mater. energy Sustain., 2016, 4, 18174–18180.

171. T. Yanai, D. P. Tew, and N. C. Handy, Chem. Phys. Lett., 2004, 393, 51–57.

172. D. Credgington, R. Hamilton, P. Atienzar, J. Nelson, and J. R. Durrant, Adv. Funct. Mater., 2011, 21, 2744–2753.

173. C. P. Yau, Z. Fei, R. S. Ashraf, M. Shahid, S. E. Watkins, P. Pattanasattayavong, T. D. Anthopoulos, V. G. Gregoriou, C. L. Chochos, and M. Heeney, Adv. Funct. Mater., 2014, 24, 678–687.

174. A. Garcia, G. C. Welch, E. L. Ratcliff, D. S. Ginley, G. C. Bazan, and D. C. Olson, Adv. Mater., 2012, 24, 5368–73.

175. E. L. Ratcliff, R. C. Bakus II, G. C. Welch, T. S. van der Poll, A. Garcia, S. R. Cowan, B. a. MacLeod, D. S. Ginley, G. C. Bazan, and D. C. Olson, J. Mater. Chem. C, 2013, 1, 6223.

176. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. Cioslowski, D. J. Fox, and W. Ct, Gaussian 09, Revis. D.01, 2009.

177. T. S. van der Poll, J. a Love, T.-Q. Nguyen, and G. C. Bazan, Adv. Mater., 2012, 24, 3646–9.

178. Y. Sun, S.-C. Chien, H.-L. Yip, K.-S. Chen, Y. Zhang, J. A. Davies, F.-C. Chen, B. Lin, and A. K.-Y. Jen, J. Mater. Chem., 2012, 22, 5587.

179. X. M. Hong and D. M. Collard, Macromolecules, 2000, 33, 6916–6917.

180. J. Lee, A. R. Han, J. Kim, Y. Kim, J. H. Oh, and C. Yang, J. Am. Chem. Soc., 2012, 134, 20713–20721.

181. C. Kanimozhi, N. Yaacobi-Gross, K. W. Chou, A. Amassian, T. D. Anthopoulos, and S. Patil, J. Am. Chem. Soc., 2012, 134, 16532–16535.

182. J. H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, U. Scherf, A. J. Heeger, and G. C. Bazan, J. Am. Chem. Soc., 2011, 133, 8416–8419.

183. A. Duarte, K. Y. Pu, B. Liu, and G. C. Bazan, Chem. Mater., 2011, 23, 501–515.

184. X. Chen, L. Chen, and Y. Chen, J. Polym. Sci. Part A Polym. Chem., 2013, 51, 4156–4166.

225

185. C.-Y. Nam, Y. Qin, Y. S. Park, H. Hlaing, X. Lu, B. M. Ocko, C. T. Black, and R. B. Grubbs, Macromolecules, 2012, 45, 2338–2347.

186. B. J. Kim, Y. Miyamoto, B. Ma, and J. M. J. Fréchet, Adv. Funct. Mater., 2009, 19, 2273–2281.

187. R. Png, P. Chia, J. Tang, B. Liu, S. Sivaramakrishnan, M. Zhou, S. Khong, H. S. O. Chan, J. H. Burroughes, L. Chua, R. H. Friend, and P. K. H. Ho, Nat. Mater., 2009, 9, 152–158.

188. G. Brotas, J. Farinhas, Q. Ferreira, J. Morgado, and A. Charas, Synth. Met., 2012, 162, 2052–2058.

189. T. Ube, T. Kosaka, H. Okazaki, K. Nakae, and T. Ikeda, J. Mater. Chem. C, 2017, 5, 1414–1419.

190. N. Guven, P. Camurlu, M. Desde, and B. Yucel, J. Electrochem. Soc., 2017, 164, H430–H436.

191. Y. Chan, F. Ye, M. E. Gallina, X. Zhang, Y. Jin, I. Wu, and D. T. Chiu, JACS, 2012, 134, 7309−7312.

192. C. Wu, Y. Jin, T. Schneider, D. R. Burnham, P. B. Smith, and D. T. Chiu, Angew. Chemie - Int. Ed., 2010, 9436–9440.

193. A. Casey, Thesis: ‘Optoelectronic properties of new conjugated materials’, 2016.

194. J. K. Stille, 862–866.

195. H. Zhou, Y. Zhang, C. K. Mai, S. D. Collins, G. C. Bazan, T. Q. Nguyen, and A. J. Heeger, Adv. Mater., 2015, 27, 1767–1773.

196. C. Wu, T. Schneider, M. Zeigler, J. Yu, P. G. Schiro, D. R. Burnham, J. D. McNeill, and D. T. Chiu, J. Am. Chem. Soc., 2010, 132, 15410–15417.

197. L. Chua, J. Zaumseil, J. Chang, E. C.-W. Ou, P. K.-H. Ho, H. Sirringhaus, and R. H. Friend, Nature, 2005, 434, 194–9.

198. K. Kwak, K. Cho, and S. Kim, Sci. Rep., 2013, 3, 2787.

199. J. S. Kim, P. K. H. Ho, C. E. Murphy, and R. H. Friend, Macromolecules, 2004, 37, 2861–2871.

200. J. C. de Mello, H. F. Wittman, and R. H. Friend, Adv. Mater., 1997, 9, 230–232.

201. C. L. Donley, J. Zaumseil, J. W. Andreasen, M. M. Nielsen, H. Sirringhaus, R. H. Friend, and J. Kim, J. Am. Chem. Soc., 2005, 127, 12890–9.

202. A. Perevedentsev, N. Chander, J. S. Kim, and D. D. C. Bradley, J. Polym. Sci. Part B Polym. Phys., 2016, 54, 1995–2006.

203. M. S. Vezie, S. Few, I. Meager, G. Pieridou, B. Dörling, R. S. Ashraf, A. R. Goñi, H. Bronstein, I. McCulloch, S. C. Hayes, M. Campoy-Quiles, and J. Nelson, Nat. Mater., 2016, 15, 746–753.

204. H. J. Kim, A. R. Han, C. H. Cho, H. Kang, H. H. Cho, M. Y. Lee, J. M. J. Frechet, J. H. Oh, and B. J. Kim, Chem. Mater., 2012, 24, 215–221.

205. D. Cui, C. Xie, Y. Lyu, X. Zhen, and K. Pu, J. Mater. Chem. B, 2017, 21, 4406–4409.

206. C. Xie, X. Zhen, Q. Lei, R. Ni, and K. Pu, Adv. Funct. Mater., 2017, 27, 1605397.

207. M. Castelaín, G. Martínez, P. Merino, J. Á. Martín-Gago, J. L. Segura, G. Ellis, and H. J. Salavagione, Chem. - A Eur. J., 2012, 18, 4965–4973.

208. E. Lieber, C. N. R. Rao, T. S. Chao, and C. W. W. Hoffman, Anal. Chem., 1957, 29, 916–918.

226

209. A. R. Davis and K. R. Carter, Macromolecules, 2015, 48, 1711–1722.

210. F. Ye, C. Wu, W. Sun, J. Yu, X. Zhang, Y. Rong, Y. Zhang, I.-C. Wu, Y.-H. Chan, and D. T. Chiu, Chem. Commun., 2014, 50, 5604–5607.

211. B. Wei, L. Ouyang, J. Liu, and D. C. Martin, J. Mater. Chem. B, 2015, 3, 5028–5034.

212. J. Wang, K. Lin, K. Zhang, X. F. Jiang, K. Mahmood, L. Ying, F. Huang, and Y. Cao, Adv. Energy Mater., 2016, 6, 1–9.

213. W. Huang, H. Janaka, T. Ogawa, and X. Z. You, Adv. Mater., 2010, 22, 2753–2758.

214. H.-P. Xu, B.-Y. Xie, W.-Z. Yuan, J.-Z. Sun, F. Yang, Y.-Q. Dong, A. Qin, S. Zhang, M. Wang, and B. Z. Tang, Chem. Commun. (Camb)., 2007, 13, 1322–1324.

215. M. Kraft, S. Adamczyk, A. Polywka, K. Zilberberg, C. Weijtens, J. Meyer, P. G??rrn, T. Riedl, and U. Scherf, ACS Appl. Mater. Interfaces, 2014, 6, 11758–11765.

216. M. Eliana, D. Lestard, M. E. Tuttolomondo, and A. Ben Altabef, Spectrochem. Acta Part A, 2015, 135, 907–914.

217. X. Zhang, J. Yu, C. Wu, Y. Jin, Y. Rong, F. Ye, and D. T. Chiu, ACS Nano, 2012, 6, 5429–5439.

218. D. Ding, K. Li, Z. Zhu, K.-Y. Pu, Y. Hu, X. Jiang, and B. Liu, Nanoscale, 2011, 3, 1997–2002.

219. J. M. Behrendt, A. B. Foster, M. C. McCairn, H. Willcock, R. K. O’Reilly, and M. L. Turner, J. Mater. Chem. C, 2013, 1, 3297.

220. C. D. Heinrich and M. Thelakkat, J. Mater. Chem. C, 2016, 4, 5370–5378.

221. S. K. Ahn, D. L. Pickel, W. M. Kochemba, J. Chen, D. Uhrig, J. P. Hinestrosa, J. M. Carrillo, M. Shao, C. Do, J. M. Messman, W. M. Brown, B. G. Sumpter, and S. M. Kilbey, ACS Macro Lett., 2013, 2, 761–765.

222. D. Van As, J. Subbiah, D. J. Jones, and W. W. H. Wong, Macromol. Chem. Phys., 2016, 217, 403–413.

223. J. M. R.Gref, A.Domp, P.Quellec, T.Blunk, R.H.Muller and and R. L. Verbavatz, Adv. Drug Del. Rev, 1995, 16, 215.

224. P. Howes and M. Green, Photochem. Photobiol. Sci., 2010, 9, 1159–1166.

225. C. B. Nielsen, A. Giovannitti, D. T. Sbircea, E. Bandiello, M. R. Niazi, D. A. Hanifi, M. Sessolo, A. Amassian, G. G. Malliaras, J. Rivnay, and I. McCulloch, J. Am. Chem. Soc., 2016, 138, 10252–10259.

226. F. He, F. Feng, S. Weng, Y. Li, and Z. Zhu, Daoben, J. Mater. Chem., 2007, 17, 3702–3707.

227. S. C. Price, A. C. Stuart, L. Yang, H. Zhou, and W. You, JACS, 2011, 133, 4625–4631.

228. A. Casey, J. P. Green, S. Tuladhar, M. Kirkus, Y. Han, D. Anthopoulos, and M. Heeney, J. Mater. Chem. A Mater. energy Sustain., 2017, 5, 6465–6470.

229. X. Wu, H. Liu, J. Liu, K. N. Haley, J. A. Treadway, J. P. Larson, N. Ge, F. Peale, and M. P. Bruchez, Nat. Biotechnol., 2002, 21, 41–46.

230. J. Geng, J. Liu, J. Liang, H. Shi, and B. Liu, Nanoscale, 2013, 5, 8593–601.

231. H. Tang, C. Xing, L. Liu, Q. Yang, and S. Wang, Small, 2011, 7, 1464–1470.

232. K. E. Sapsford, T. Pons, I. L. Medintz, and H. Mattoussi, Sensors, 2006, 6, 925–953.

227

233. J. Sun, S. Wang, and F. Gao, Langmuir, 2016, 32, 12725–12731.

234. W. Yang, G. Zhang, W. Weng, B. Qiu, L. Guo, Z. Lin, and G. Chen, RSC Adv., 2014, 4, 58852–58857.

235. X. Feng, F. Lv, L. Liu, H. Tang, C. Xing, Q. Yang, and S. Wang, ACS Appl. Mater. Interfaces, 2010, 2, 2429–2435.

236. C. Xing, L. Liu, H. Tang, X. Feng, Q. Yang, S. Wang, and G. C. Bazan, Adv. Funct. Mater., 2011, 21, 4058–4067.

237. Ö. Gezici, İ. Durmaz, E. Bilget Güven, Ö. Ünal, A. Özgün, R. Cetin-Atalay, and D. Tuncel, RSC Adv., 2014, 4, 1302–1309.

238. K. Chang, Y. Tang, X. Fang, S. Yin, H. Xu, and C. Wu, Biomacromolecules, 2016, 17, 2128–2136.

239. D. Zhang, M. Wu, Y. Zeng, N. Liao, Z. Cai, G. Liu, X. Liu, and J. Liu, J. Mater. Chem. B, 2016, 4, 589–599.

240. C. Wu, C. Szymanski, J. Mcneill, C. U. V, S. Carolina, and R. V January, Langmuir, 2006, 22, 2956–2960.

241. J. Yu, C. Wu, X. Zhang, F. Ye, M. E. Gallina, Y. Rong, I. Wu, W. Sun, Y. Chan, and D. T. Chiu, Adv. Mater., 2012, 24, 3498–3504.

242. M. B. Zeigler, W. Sun, Y. Rong, and D. T. Chiu, JACS, 2013, 135, 11453–11456.

243. K. Pu, A. J. Shuhendler, J. V Jokerst, J. Mei, S. S. Gambhir, Z. Bao, and J. Rao, Nat Nano, 2014, 9, 233–239.

244. S. Li, X. Wang, R. Hu, H. Chen, M. Li, J. Wang, Y. Wang, L. Liu, F. Lv, X. J. Liang, and S. Wang, Chem. Mater., 2016, 28, 8669–8675.

245. Z. Tian, J. Yu, C. Wu, C. Szymanski, and J. Mcneill, Nanoscale, 2010, 2, 1999–2011.

246. W. Anderson, D. Kozak, V. A. Coleman, Å. K. Jämting, and M. Trau, J. Colloid Interface Sci., 2013, 405, 322–330.

247. U. Nobbmann and A. Morfesis, Mater. Today, 2009, 12, 52–54.

248. V. Filipe, A. Hawe, and W. Jiskoot, Pharm. Res., 2010, 27, 796–810.

249. J. M. Baskin, J. A. Prescher, S. T. Laughlin, N. J. Agard, P. V Chang, I. A. Miller, A. Lo, J. A. Codelli, and C. R. Bertozzi, PNAS, 2007, 104, 16793–16797.

250. V. A. Online, J. Mater. Chem. B, 2014, 2, 4818–4825.

251. O. H. Laitinen, V. P. Hytönen, H. R. Nordlund, and M. S. Kulomaa, Cell. Mol. Life. Sci, 2006, 63, 2992–3017.

252. M. P. Nikiforov, B. Lai, W. Chen, S. Chen, R. D. Schaller, J. Strzalka, J. Maser, and S. B. Darling, Energy Environ. Sci., 2013, 6, 1513.

253. Y. Zhang, E. Bovill, J. Kingsley, A. R. Buckley, H. Yi, A. Iraqi, T. Wang, and D. G. Lidzey, Sci. Rep., 2016, 6, 21632.

254. M. Manceau, A. Rivaton, J.-L. Gardette, S. Guillerez, and N. Lemaître, Sol. Energy Mater. Sol. Cells, 2011, 95, 1315–1325.

255. B. C. Thompson and J. M. J. Fréchet, Angew. Chem. Int. Ed. Engl., 2008, 47, 58–77.

228

256. T. Wang, A. J. Pearson, D. G. Lidzey, and R. A. L. Jones, Adv. Funct. Mater., 2011, 21, 1383–1390.

257. M. Raïssi, H. Erothu, E. Ibarboure, H. Cramail, L. Vignau, E. Cloutet, and R. C. Hiorns, J. Mater. Chem. A, 2015, 3, 18207–18221.

258. J. U. Lee, J. W. Jung, T. Emrick, T. P. Russell, and W. H. Jo, Nanotechnology, 2010, 21, 105201.

259. C. Yang, J. K. Lee, A. J. Heeger, and F. Wudl, J. Mater. Chem., 2009, 19, 5416.

260. S. Kakogianni, A. K. Andreopoulou, and J. K. Kallitsis, Polymers (Basel)., 2016, 8, 24–26.

261. M. H. Yun, J. Kim, C. Yang, and J. Y. Kim, Sol. Energy Mater. Sol. Cells, 2012, 104, 7–12.

262. D. Kipp, R. Verduzco, and V. Ganesan, Mol. Syst. Des. Eng., 2016, 1, 353–369.

263. J. H. Tsai, Y. C. Lai, T. Higashihara, C. J. Lin, M. Ueda, and W. C. Chen, Macromolecules, 2010, 43, 6085–6091.

264. X. Guo, R. P. Ortiz, Y. Zheng, M. Kim, S. Zhang, Y. Hu, G. Lu, A. Facchetti, and T. J. Marks, JACS, 2011, 133, 13685–13697.

265. S. Zhu, Y. Liu, M. H. Rafailovich, J. Sokolov, D. Gersappe, D. A. Winesett, and H. Ade, Nature, 1999, 400, 49–51.

266. J. W. Rumer and I. McCulloch, Mater. Today, 2015, 18, 425–435.

267. C. P. Chen, C. Y. Huang, and S. C. Chuang, Adv. Funct. Mater., 2015, 25, 207–213.

268. G. Griffini, J. D. Douglas, C. Piliego, T. W. Holcombe, S. Turri, J. M. J. Fréchet, and J. L. Mynar, Adv. Mater., 2011, 23, 1660–4.

269. G. Wantz, L. Derue, O. Dautel, A. Rivaton, P. Hudhomme, and C. Dagron-lartigau, Polym. Int., 2014, 63, 1346–1351.

270. F. J. Kahle, C. Saller, A. Köhler, and P. Strohriegl, Adv. Energy Mater., 2017, 7, 1700306.

271. S. Miyanishi, K. Tajima, and K. Hashimoto, Macromolecules, 2009, 42, 1610–1618.

272. Z. Zhu, S. Hadjikyriacou, D. Waller, and R. Gaudiana, J. Macromol. Sci. Part A, 2004, 41, 1467–1487.

273. A. Charas, Q. Ferreira, J. Farinhas, M. Matos, L. Alcacer, and J. Morgado, Macromolecules, 2009, 42, 7903–7912.

274. A. Tournebize, A. Rivaton, J. L. Gardette, C. Lombard, B. Pépin-Donat, S. Beaupré, and M. Leclerc, Adv. Energy Mater., 2014, 4, 1–8.

275. a M. Dupraz, J. R. de Wijn, S. a v d Meer, and K. de Groot, J. Biomed. Mater. Res., 1996, 30, 231–8.

276. M. Abdelmouleh, S. Boufi, M. Belgacem, and a Dufresne, Compos. Sci. Technol., 2007, 67, 1627–1639.

277. C. Jiao, Z. Wang, Z. Gui, and Y. Hu, Eur. Polym. J., 2005, 41, 1204–1211.

278. D. a. Ramrus and J. C. Berg, J. Adhes. Sci. Technol., 2006, 20, 1615–1623.

279. W. Liang, C. Chang, Y. Lai, S. Cheng, H. Chang, Y. Lai, Y. Cheng, C. Wang, and C. Hsu, Macromoles, 2013, 46, 4781–4789.

229

280. O. Dautel, G. Wantz, and R. Almairac, JACS, 2006, 128, 4892–4901.

281. A. W. Hains, C. Ramanan, M. D. Irwin, J. Liu, M. R. Wasielewski, and T. J. Marks, ACS Appl. Mater. Interfaces, 2010, 2, 175–185.

282. J. M. Behrendt, A. B. Foster, M. C. McCairn, H. Willcock, R. K. O’Reilly, and M. L. Turner, J. Mater. Chem. C, 2013, 1, 3297–3304.

283. H. Waters, J. Kettle, S.-W. Chang, C.-J. Su, W.-R. Wu, U.-S. Jeng, Y.-C. Tsai, and M. Horie, J. Mater. Chem. A, 2013, 1, 7370.

284. Z. Zhu, D. Waller, R. Gaudiana, M. Morana, M. Scharber, C. Brabec, B. M. South, and J. Street, 2007, 40, 1981–1986.

285. M. B. More, D. Ray, and P. B. Armentrout, J. Am. Chem. Soc., 1999, 121, 417–423.

286. M. Jørgensen, K. Norrman, and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2008, 92, 686–714.

287. M. Jørgensen, K. Norrman, S. A. Gevorgyan, T. Tromholt, B. Andreasen, and F. C. Krebs, Adv. Mater., 2012, 24, 580–612.

288. F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2008, 92, 715–726.

289. Y. Li, J. Zou, H. Yip, C. Li, Y. Zhang, C. Chueh, J. Intemann, Y. Xu, P. Liang, Y. Chen, and A. K. Jen, Macromolecules, 2013, 46, 5497–5503.

231

General Experimental

All solvents and chemicals were purchased from either Sigma-Aldrich, VWR, Alfa Aesar or

TCI and used as received. All reactions were carried out under an inert argon atmosphere

using standard Schlenk line techniques, with dry solvents, purchased from Sigma-Aldrich.

A Biotage initiator V 2.3., in constant temperature mode, was used for all microwave

reactions. Nuclear magnetic resonance (NMR) spectra were recorded on Bruker AV-400

(400 MHz) spectrometer. Weight-average (Mw) and Number-average (Mn) molecular

weights were determined with an Agilent Technologies 1200 series GPC detected using the

refractive index signal in chlorobenzene at 80 °C using two PL mixed B columns in series, and

calibrated against narrow polydispersity polystyrene standards. A Shimadzu UV-1800 UV-

Vis Spectrophotometer was used to measure UV-Vis absorption. Preparative GPC was used

to fractionate polymers which consisted of a customer build Shimadzu recSEC system

comprising a DGU-20A3 degasser, an LC-20A pump, a CTO-20A column oven, an Agilent

PLgel 10 μm MIXED-D column and a SPD-20A UV detector. Photo-electron Spectroscopy in

Air (PESA) on a Riken Keiki AC-2 PESA spectrometer was used to measure ionization

potentials. Polymer thin films were prepared by spin-coating from 5 mg/mL polymer

solutions onto glass substrates. The PESA samples were run with a light intensity of 5 nW

232

and data processed with a power number of 0.5. Photoluminescence (PL) spectra were

acquired on a Fluorolog® FL3-31 Spectrofluorometer using a Hellma® Ultra Micro

fluorescence cuvette. Dynamic Light Scattering (DLS) was performed using a ZetaSizer Nano

ZS (Malvern). Nanoparticles Tracking Analysis (NTA) was acquired on a Malvern

Instruments NS3000. The capture settings of the NTA were kept consistent for all samples

at a screen gain of 1 and a camera level of 11 and the process settings were kept consistent

at a screen gain of 11 and a detection threshold of 5. Polystreptavidin test strips were

purchased from Mologic and photographs were acquired on the camera of a Samsung S8

mobile phone whilst the strips were illuminated by 2 6W black light bulbs (BLB-T5/6W) in a

dark room. Time-correlated single photo counting (TCSPC) setup was acquired using a

DeltaFlex (Horiba), pulsed 467 nm and fluorescence was detected at 535 nm (SPC-650

detector, Horiba) with a 515 nm filter. Atomic force microscopy (AFM) images were

obtained with a Picoscan PicoSPM LE scanning probe in tapping mode under ambient

conditions. Optical microscopy images were taken in transmission using a Nikon LV100.

Density functional theory (DFT) calculations were carried out using Gaussview 5.0.176

Samples were prepared for scanning transmission electron microscopy (STEM) by drop

casting onto TEM grids with holey carbon support films. The imaging and energy-dispersive

X-ray spectoscopy (EDX) was carried out in scanning TEM (STEM) mode using a JEM-2100F

microscope at 200kV.

233

Chapter 2 materials: precursor to 2.1, 5-fluorobenzo-[2,1,3]-thiadiazole, was purchased

from Fluorochem. 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester

comonomer was purchased from Sigma-Aldrich.

Chapter 3 materials: precursor to 3.3-M1, 4,7-Dibromo-5,6-difluoro-2-(2-butyloctyl)-2H-

benzotriazole and precursor to 3.3-F 2,6-bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophene-

2-yl)-benzo[1,2-b;4,5-b']dithiophene were purchased from SunaTec Inc. 11-

Mercaptoundecanoic acid was purchased from Sigma-Aldrich and was recrystallized from

heptane before use. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-

b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-

2-6-diyl)] (3.6-F) was purchased from Ossila. Precurser to 3.7-F, 9,9-dioctyl-9H-fluorene-2,7-

diboronic acid bis(pinacol) ester was purchased from Sigma-Aldrich.

Chapter 4 materials: DBCO-MB 594 was purchased from Kerafast.

Chapter 5 materials: precursor to 5.1, 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-

b′]dithiophene was purchased from Sigma-Aldrich.

235

Chapter 6 Appendix

Chapter 2

2.9: PC71BM (Inverted)

Ratio 1:3

Jsc (mA cm−2

) -4.75 ± 0.05 (-4.80)

Voc (V) 0.78 ± 0.01 (0.79)

FF 0.30 ± 0.01 (0.31)

PCE (%) 1.1 ± 0.1 (1.2)

Figure 1: Parameters of inverted OPV device of 2.9

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-10

-8

-6

-4

-2

0

2

4

Cu

rren

t D

ensi

ty (

mA

cm

-2)

Voltage (V)

\

\

236

Figure 2: 1H NMR of 2.9 in CDCl3


237


Chapter 3

Figure 5: Frontier molecular orbits of ground and excited states for the lowest energy transition (at ~400 nm)

with an oscillator strength > 1, for 3.7-F, 3.7-OR and 3.7-SR.

238

Figure 6: Frontier molecular orbits of ground and excited states for the high energy transition (at ~290 nm)

with an oscillator strength > 0.9, for 3.7-F, 3.7-OR and 3.7-SR.

500 550 600

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lis

ed

Flu

ore

sc

en

ce

Wavelength (nm)

3.7-SR

3.7-OR

3.7-F

SPN-N3

Figure 7: Normalised PL spectra for 3.7-F, 3.7-OR, 3.SR in THF solution and SPN-N3 in water. Pulsed at 450 nm

239

Figure 8 1H NMR of 2.11, 3.1 and 3.2 in CDCl3

240

Figure 9: 1H NMR of 3.3-F and 3.3-SR in TCE-d2 at 403K

Figure 10: 1H NMR of 3.4-F and 3.4-SR in CDCl3

241


Figure 12: 1H NMR of 3.6-F and 3.6-SR in TCE-d2 at 403K

242


Figure 14: 1H NMR of 3.7-N3 in CDCl3

243

Figure 15: 1H NMR of 3.7-Alkene in CDCl3

Figure 16: 1H NMR of 3.7-COOH in CDCl3

244

Figure 17: 1H NMR of 3.7-SAc in CDCl3

Figure 18: 1H NMR of 3.7-Silane in CDCl3

245

Figure 19: 1H NMR of 3.7-Copolymer in CDCl3

Figure 20: 1H NMR of 3.7-OR in CDCl3

246

Figure 21: 1H NMR of 3.7-PEG in CDCl3


247


Figure 24: 1H NMR of 3.9 (from a one-pot synthesis) in CDCl3

248

Figure 25: 19

F NMR of polymers 3.1 – 3.7 in CDCl3

249

Figure 26: 19F NMR of F8BT based polymer in CDCl3

250

Figure 27: Normalised UV-Vis of F8BT based polymers in chlorobenzene

251

Figure 28: IR spectra of 3.8

Chapter 4

Figure 29: Calculation of lifetime of excited state of free SPN-N3 (written by Dr. Robert Godin)

The fluorescence decay of the free SPN-N3 is complex in itself. The decay shows some

curvature in a log-lin plot, indicating that a single exponential decay is inappropriate. Good

fits (figure below) were obtained using a stretched exponential model1 of the form:

𝐼(𝑡) = 𝐼0 + 𝐴exp [− (𝑥

𝑡)

𝛽

]

252

Where I0 is the background level, x is the time constant, t is time, and β is the heterogeneity

factor. β takes values between 0 and 1, where values closer to 1 indicate a system with less

heterogeneity. The average lifetime is calculated as:

⟨𝜏⟩ = 𝛽𝑥Γ(𝛽)

Where Γ is the mathematical gamma function.

Figure of the time-resolved fluorescence decay of free nanoparticles. A mono exponential fit is compared to a stretched exponential fit. Fit residuals are shown below.

The heterogeneity factor (β = 0.87) has a value near 1, consistent with small degree of

heterogeneity possibly due to a distribution of NP size or polymer chain conformation. The

average lifetime obtained from the stretched exponential fit is 2.44 ns.

253

Figure 30: TCSPC decays of SPN-N3 with (left) reactive and (right) unreactive, normalized to the initial intensity.

Figure 31: Calculation of maximal FRET efficiencies (written by Dr. Robert Godin)

From the extracted FRET decays, we may extract <kFRET> from the average FRET lifetimes

obtained by a stretched exponential fit. For the reactive acceptors <kFRET> = 1.6 ± 0.2 x 109 s-

1 and for the unreactive <kFRET> = 8.7 ± 0.3 x 108 s-1. Considering the average lifetime of free

NPs, we calculate a maximal FRET efficiency of 0.80 ± 0.02 for clicked DA pairs and 0.68 ±

0.01 for free DA pairs.

254

Figure 32: STEM image and size distribution of 3.7-N3 after click with active dye

Figure 33: Raw NTA data for SPN-N3

255

Figure 34: Raw NTA data for SPN-COOH

256

Figure 35: Raw NTA data for SPN-Multi

257

Chapter 5

Figure 36: 1H NMR of 5.2 in TCE-d2 at 403K

Figure 37: 1H NMR of 5.3-S1 in TCE-d2 at 403K

258

Figure 38: 1H NMR of 5.3-S2 in TCE-d2 at 403K

Figure 39: 19

F NMR of 5.3-S1 and 5.2 in TCE-d2 at 403K

259

Figure 40: Normalised UV-Vis spectra of 5.2 (left) and 5.3-S2 (right), with and without a drop of TFA and before

and after dipping in chloroform.

260

Permissions

Chapter 2:

Development of One-Step Post-Polymerisation Methods for ...

Documents

Transcript of Development of One-Step Post-Polymerisation Methods for ...