Kokil Akshay

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Conjugated Polymer Networks: Synthesis and Properties

by

Akshay Kokil

Submitted in partial fulfillment of the requirements

For the degree of Doctor of Philosophy

Thesis Advisor: Dr. Christoph Weder

Department of Macromolecular Science and Engineering

CASE WESTERN RESERVE UNIVERSITY

August 2005


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CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the dissertation of

______________________________________________________

candidate for the Ph.D. degree *.

(signed)_______________________________________________(chair of the committee)

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________________________________________________

________________________________________________

________________________________________________

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(date) _______________________

*We also certify that written approval has been obtained for anyproprietary material contained therein.

Akshay Kokil

Dr. Christoph Weder

Dr. Kenneth D. Singer

Dr. David Schiraldi

Dr. Stuart Rowan

05/15/2005


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Table of Contents 1

List of Tables 4

List of Schemes 5

List of Figure 7

List of Acronyms and Abbreviations 13

Abstract 16

Chapter 1 Introduction

1.1 (Semi)conducting Conjugated Polymers - An Emerging

Class of Organic Materials 19

1.2 Charge Transport in Conjugated Polymers 20

1.3 Conjugated Polymer Networks 30

1.4 Poly( p -phenylene ethynylene)s: An Important Class of Conjugated

Polymers 39

1.5 Energetics of Conjugated Polymer Networks 42

1.6 References 46

Chapter 2 Scope and Objectives 53

Chapter 3 Charge Carrier Transport in Poly( p -phenylene ethynylene)s

3.1 Introduction 56

3.2 Experimental Section 57

3.3 Charge Carrier Transport in EHO-OPPE 58

3.4 Conclusions 69

3.5 References 70

Chapter 4 Synthesis and Properties of Poly(2,5-dialkoxy- p -phenyleneethynylene)-Pt II Networks

4.1 Introduction 71

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4.3 Charge Transport in PPE-Pt II Networks 72

4.4 Conclusions 77

4.5 References

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Chapter 5 Synthesis and Properties of Poly(2,5-dialkoxy- p -phenyleneethynylene)-Pt 0 Networks

5.1 Introduction 80


5.3

Two Alternative, Convenient Routes to Bis(diphenylacetylene)Pt0

86

5.4 Synthesis of Organometallic -Conjugated PPE-Pt 0 Networks 89

5.5 Charge Carrier Mobility in Organometallic -Conjugated PPE-Pt 0

Network 97

5.6 Conclusions 102

5.7 References 104

Chapter 6 Synthesis and Properties of Organometallic Networks Based on 2,2-Bipyridine-Containing Poly( p -phenylene ethynylene)s

6.1 Introduction 106


6.3 Synthesis and Characterization of BipyPPEs and Their

Organometallic Networks 114

6.4 Conclusions 134

6.5 References 135

Chapter 7 Synthesis and Characterization of Cross-linked Poly( p -phenyleneethynylene)s

7.1 Introduction 138

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7.3 Bulk Synthesis and Characterization of Crosslinked Poly( p -phenylene

ethynylenes)s 146

7.4 Synthesis of Crosslinked Conjugated Polymer Milli-, Micro- and Nanoparticles 153

7.5 Conclusions 157

7.6 References 159

Chapter 8 Conclusions and Outlook 161

Acknowledgements 167

List of Publications Based on this Thesis 169

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List of Tables

Table 3.1 Results of data fits to the Gaussian disorder model of charge 68

transport.

Table 6.1 Optical absorption and PL emission data of BipyPPE 1 (13 ) and 116

BipyPPE 2 (14 ) after complexation with different metal ions.

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List of Schemes

Scheme 1.1

Scheme 1.2

Scheme 1.3

Scheme 1.4

Scheme 1.5

Scheme 4.1

Scheme 5.1

Scheme 5.2

Scheme 6.1

Scheme 6.2

Scheme 6.3

Simplified schematic representation of cross-linked conjugated polymer networks with organometallic cross-links (top) and

covalent cross-links (bottom).

Schematic representation of the cross-linking reaction reported tooccur in zinc phorphyrin-linked poly( p-phenylene ethynylene)s.

Schematic representation of the cross-linking reaction proposed tooccur in poly[(4-ethynyl)phenylacetylene] upon thermal treatment.

Schematic representation of the proposed cross-linking reactionoccurring upon heat treatment of Cr(CO) 3-benzene containing

poly(arylene ethynylene)s.

Simplified representation of the ligand-exchange reaction betweenMEH-OPPE ( 1) and [Pt-(-Cl)Cl(PhCH=CH 2)]2 (2), leading tocross-linked organometallic hybrid materials 3.

Simplified representation of the ligand-exchange reaction betweenEHO-OPPE ( 4) and [Pt-(-Cl)Cl(PhCH=CH 2)]2 (2), leading tocross-linked organometallic conjugated polymer network.

Synthesis of Pt( 2-Ph-C C-Ph) 2 (8) through (i) ligand exchangeexchange between Pt(PhCH=CH 2)3 (7) and Ph-C C-Ph, and (ii)

reduction of cis -PtCl 2(PhCH=CH 2)2 (6) with triphenylsilane in the presence of Ph-C C-Ph.

Ligand-exchange reaction between EHO-OPPE ( 4) andPt(PhCH=CH 2)3 (7) leading to the target EHO-OPPE-Pt 0 (9)networks.

Reaction scheme for the synthesis of 5,5-bis((trimethylsilyl)-ethynyl)-2,2-bipyridine.

Reaction scheme for the synthesis of 5,5-bis(ethynyl)-2,2-

bipyridine ( 9).

Synthesis and molecular structure of the 2,2-bipypridine-containing poly(2,5-dialkyloxy- p-phenylene ethynylene)sBipyPPE 1 (13 ) and BipyPPE 2 (14 ).

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Scheme 6.4

Scheme 7.1

Schematic representation of the formation of metallo-supramolecular networks via the formation of metal-bis-ligandcomplexes with 2,2-bipypridine-containing poly(2,5-dialkyloxy- p-

phenylene ethynylene)s BipyPPE 1 (13 ) and BipyPPE 2 (14 ).

Synthesis of cross-linked PPEs ( 18 ) by the palladium-catalyzedcross-coupling reaction of 2,5-diiodo-4-[(2-ethylhexyl)oxy]methoxybenzene ( 16 ) or 1,4-bis[(2-ethylhexyl)oxy]-2,5-diiodo

benzene ( 10 ), 1,4-diethynyl-2,5-bis-(octyloxy)benzene ( 11 ) and thetri-functional cross-linker 1,2,4-tribromobenzene ( 17 ). R 1=2-ethylhexyl, R 2=n-octyl, R 3=methyl ( 16 ) or 2-ethylhexyl ( 10 ).

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List of Figures

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

Figure 1.7

Figure 1.8

Figure 1.9

Figure 1.10

Figure 1.11

Photographs of applications of conjugated polymers in (a)anticorrosion coatings, (b) field-effect transistors (FETs) and (c)

light-emitting diodes (LEDs).

Schematic representation of the doping of poly( p-phenyleneethynylene) (PPE) under formation of polarons (radical cations,radical anions) and bipolarons (dications, dianions). Note thatmultiple carriers can coexist on each macromolecule.

Schematic representation of intra chain charge diffusion (left) andinter chain charge diffusion (hopping, right) in polyacetylene.

Chemical structures of (a) polyacetylene, (b) polyaniline, (c) poly (p-

phenylene vinylene), (d) poly(N-phenylimino-1,4-phenylene-1,2-ethenylene-1,4-(2,5-dioctoxy)-phenylene-1,2-ethenylene-1,4- phenylene), (e) poly-9,9dioctyl-fluorene-co-bithiophene, (f) poly(benzobisimidazobenzophenanthroline) and (g) poly(benzo-

bisimidazole).

Two different preferential orientations of the ordered domains of PAT with respect to the employed substrate. (a) in-plane orientationand (b) out-of-plane orientation.

Schematic chemical structure of conjugated cross-links in electro-chemically synthesized polypyrrole.

Chemical structure of poly( o-toluidine) cross-linked with palladium II.

Bridged Pd terthiophene complex employed for electropolymerization.

General schematic structures of poly(arylene ethynylene)s (PAEs), poly(arylene)s (PAs), poly(arylene vinylene)s (PAVs), and poly(diacetylene)s (PDAs).

Chemical structure of poly[2,5-dioctyloxy-1,4-diethynyl-phenylene-alt -2,5-bis(2'-ethylhexyloxy)-1,4-phenylene] (EHO-OPPE, 4).

Simplified schematic of electron (top) and hole (bottom) transfer inconjugated polymers with matched energy levels (a and c) and un-matched energy levels (b and d).

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Figure 1.12

Figure 1.13

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 4.1

Schematic structure of metallopolymer demonstrating chargetransport by outer sphere electronic coupling.

Schematic structure of metallopolymer demonstrating chargetransport by inner sphere electronic coupling.

Chemical structure of poly[2,5-dioctyloxy-1,4-diethynyl-phenylene-alt -2,5-bis(2'-ethylhexyloxy)-1,4-phenylene] (EHO-OPPE, 4), the

poly( p-phenylene ethynylene) (PPE) derivative investigated here.

Schematic representation of sample preparation (left) and thearchitecture of the sample used for TOF measurements (right).

Simplified representation of the conventional time-of-flight setup.The transients displayed are schematic representations of the signal

observed for non-dispersive transport (top) and dispersive transport(bottom).

Electron time-of-flight photocurrent transients of a solution-cast filmof EHO- OPPE ( 4, L=8 m), measured at 295 K and an electric fieldof 2.5 10 5 Vcm -1 in a ( a ) linear and ( b ) double logarithmicrepresentation.

Hole time-of-flight photocurrent transients of a solution-cast film of EHO-OPPE ( 4, L=8 m), measured at 295 K and an electric field of 2.5 10 5 Vcm -1 in ( a ) linear and ( b ) double logarithmic representation.

Electron ( a ) and hole ( b ) mobilities of solution cast films of EHO-OPPE ( 4) as function of electric field at various film thicknesses(open triangles: L=6.5 m, filled circles: L=8 m, open squares: L=12m).

Temperature dependence of ( a ) electron and ( b ) hole mobilities of anEHO-OPPE ( 4) film ( L=8 m) measured at E = 2 105 (squares), 3 105 (circles) and 4 105 Vcm -1 (triangles).

Temperature dependence of the diagonal disorder parameter for

holes (squares) and electrons (circles). Parameters have been obtained by fitting the experimental data (Figure 3.7) to Equations 3.2 and 3.6.

Hole TOF photocurrent transients of EHO-OPPE-Pt II (5),

[Pt II]/[PE]=0.17 with film thickness L=9 m (black solid line) and L=13 m (red solid line), linear (top) and logarithmic (bottom) plots,measured at 295 K and F =910 4 Vcm -1.

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Figure 4.2

Figure 4.3

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

Figure 5.7

Figure 5.8

Electron TOF photocurrent transients of EHO-OPPE-Pt II (5),

[Pt II]/[PE]=0.17 with film thickness L=9 m (black solid line) and L=13 m (red solid line), linear (top) and logarithmic (bottom) plots,measured at 295 K and F =910 4 Vcm -1.

Electron (top) and hole (bottom) mobility of EHO-OPPE-PtII

(5) asfunction of [Pt II]/[PE] ([Pt II]/[PE]: filled symbols =0.35, opensymbols =0.17), electric field F and thickness ( =18.4 m, =10.1m, =11.3 m, =9 m, =13 m, =6.5 m) . Mobilities for EHO-OPPE ( ) are displayed for reference.

195Pt NMR spectra for Pt(PhCH=CH 2)3 (7, a), 7 with 1.47 eq. of Ph-CC-Ph (b) and 7 with 3.42 eq. of Ph-C C-Ph (c).

Photoluminescence spectra of a solution of EHO-OPPE ( 4) in styrene(solid line, 5 mg / mL) and an EHO-OPPE-Pt 0 (9) gel (broken line,

[Pt0

] / [PE] = 0.68, 5 mg PPE / mL styrene). The spectra wereacquired under excitation at 380 nm.

Left: Styrene solution of EHO-OPPE ( 4, 6.7 mg/mL). Right: EHO-OPPE-Pt 0 (9) / styrene gel ([Pt 0] / [PE] = 0.68, 6.7 mg PPE / mLstyrene), carrying a steel ball. The picture on the left was taken under illumination with UV light (365 nm).

Pictures of an EHO-OPPE-Pt 0 (9) film in styrene (left) and toluene(right) under illumination with UV-light (365 nm). The dissimilar

dissolution characteristics of the photoluminescent polymer demonstrate the reversibility of the ligand-exchange reaction and thecross-linked nature of the material.

Raman spectra of Ph-C C-Ph ( a , top left), EHO-OPPE ( 4, b , topright), Pt( 2-Ph-C C-Ph) 2 (8, c, bottom left) EHO-OPPE-Pt 0 (9, d ,[Pt 0] / [PE] ~ 0.17, bottom right).

UV-Vis absorption spectra of films of EHO-OPPE ( 4) (solid line) andEHO-OPPE-Pt 0 (9) with [Pt 0] / [PE] = 0.086 (filled circles), 0.17(open circles), 0.25 (filled triangles), and 0.34 (open triangles).

Crystal structure for Pt( 2-Ph-C C-Ph) 2 (8).

Photoluminescence spectra of films of EHO-OPPE ( 4) (solid line)and EHO-OPPE-Pt 0 (9) with [Pt 0] / [PE] = 0.086 (filled circles), 0.17(open circles), 0.25 (filled triangles), and 0.34 (open triangles).

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Figure 5.9

Figure 5.10

Figure 5.11

Figure 5.12

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Electron TOF photocurrent transients of EHO-OPPE ( 4, solid, L=8m) and EHO-OPPE-Pt 0 (9, dotted, L=30 m, [Pt 0]/[PE]=0.17) filmsin linear (top) and logarithmic (bottom) plots, measured at 295 K and

F = 1.510 5 Vcm -1.

Electron (top) and hole (bottom) mobility of EHO-OPPE-Pt0

(9) asfunction of [Pt 0]/[PE] and electric field F ([Pt 0]/[PE]: =0, =0.016, =0.086, =0.17, =0.25, =0.34).

Electron (top) and hole (bottom) mobility at F = 1.110 5 ( ) and1.510 5 Vcm -1 ( ) as function of [Pt 0]/[PE].

Electron (a) and hole (b) mobility of films of EHO-OPPE-Pt 0 (9) with[Pt 0] / [PE] = 0.25 having thickness 25 m (filled squares) and 20 m(open squares).

UV-Vis absorption (a) and PL emission (b) spectra acquired uponaddition of tetrakis(acetonitrile)Cu(I)-hexafluorophosphate toBipyPPE 1 (13 ) (concentration of polymer-bound Bipy = 1.93 10 -5 M)in CHCl 3:CH 3CN (15:1 v/v). Shown are spectra at selected[Cu +]:[Bipy] ratios of 0 (solid line), 0.09 (filled squares), 0.19 (filledcircles), 0.28 (filled triangles), 0.38 (filled inverted triangles), 0.48(filled rhombus), 0.57 (empty squares), 0.76 (empty circles), 0.96(empty triangles) and 1.92 (empty inverted triangles). The insetsshow the absorption at 452 nm (a) and the emission at 459 nm (b) asa function of [Cu +]:[Bipy] ratio.

UV-Vis absorption (a) and PL emission (b) spectra acquired uponaddition of tetrakis(acetonitrile)Cu(I)-hexafluorophosphate toBipyPPE 2 (14 ) (concentration of polymer-bound Bipy = 3.23 10 -5 M)in CHCl 3:CH 3CN (15:1 v/v). Shown are spectra at selected[Cu +]:[Bipy] ratios of 0 (solid line), 0.1 (filled squares), 0.2 (filledcircles), 0.3 (filled triangles), 0.4 (filled inverted triangles), 0.5 (filledrhombus), 0.6 (empty squares), 0.8 (empty circles), 1.0 (emptytriangles) and 2.0 (empty inverted triangles).The insets show theabsorption at 440 nm (a) and the emission at 482 nm (b) as a functionof [Cu +]:[Bipy] ratio.

UV-Vis absorption spectra of model compound 15 in CHCl 3:CH 3CN(15:1 v/v). Solid line: preparation in-situ . Dashed line: isolatedcompound.

Representation of the UV-Vis absorption data (a) and PL emissiondata (b) shown in the inset of Figures 6.1a and 6.1b respectively in aScatchard plot.

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Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 6.10

Representation of the UV-Vis absorption data (a) and PL emissiondata (b) shown in the inset of Figures 6.2a and 6.2b respectively in aScatchard plot.

UV-Vis absorption (a) and PL emission (b) spectra acquired uponaddition of free 2,2-bipyridine to a mixture of BipyPPE 1 (13 ) (concentration of polymer-bound Bipy = 1.93 10 -5 M) andtetrakis(acetonitrile)Cu(I)-hexafluorophosphate (ratio of [Cu +]:[Bipycomprised in the polymer] = 1:0.96) in CHCl 3:CH 3CN (15:1 v/v).Shown are spectra for ratios of [free Bipy]:[polymer-bound Bipy] of 0:1 (dashed line), 1:1 (filled squares), 2:1 (filled circles), 5:1 (filledtriangles) and 10:1 (filled rhombus). The spectra of neat BipyPPE 1 (13 , solid line) are included for reference.

UV-Vis absorption (a) and PL emission (b) spectra acquired upon

addition of cobalt(II)tetrafluoroborate hexahydrate (filled squares),nickel(II)perchlorate hexahydrate (empty circles), zinc(II)perchloratehexahydrate (filled triangles), and cadmium(II)perchlorate hydrate(filled circles) to BipyPPE 1 (13 ) (concentration of Bipy comprised inthe polymer in solution = 1.93 10 -5 M) in CHCl 3:CH 3CN (15:1 v/v).Shown are spectra for [M 2+]:[Bipy] ratios of 0.5:1. The spectra of neat BipyPPE 1 (13 . solid line) are included for reference.

UV-Vis absorption (a) and PL emission (b) spectra acquired uponaddition of cobalt(II)tetrafluoroborate hexahydrate (filled squares),nickel(II)perchlorate hexahydrate (empty circles), zinc(II)perchlorate

hexahydrate (filled triangles), and cadmium(II)perchlorate hydrate(filled circles) to BipyPPE 2 (14 ) (concentration of polymer-boundBipy =3.23 10 -5 M) in CHCl 3:CH 3CN (15:1 v/v). Shown are spectrafor [M 2+]:[Bipy] ratios of 0.5:1. The spectra of neat BipyPPE 2 (14 ,solid line) are included for reference.

(a) UV-Vis absorption and (b) PL emission spectra of spin-coatedfilms of complexes produced by ligand-exchange reactions betweenBipyPPE 1 (13 ) and zinc perchlorate hexahydrate (filled rhombus), or cadmium perchlorate hydrate (filled squares). Shown are spectra for [metal]:[Bipy] ratios of 0.5:1. The spectra of neat BipyPPE 1 (13 ,

solid line) are included for reference.

(a) UV-Vis absorption and (b) PL emission spectra of spin-coatedfilms of complexes produced by ligand-exchange reactions betweenBipyPPE 2 (14 ) and zinc perchlorate hexahydrate (filled rhombus), or cadmium perchlorate hydrate (filled squares). Shown are spectra for [metal]:[Bipy] ratios of 0.5:1. The spectra of neat BipyPPE 2 (14 ,solid line) are included for reference.

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Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

Figure 8.1

Photograph of a gel of covalently cross-linked PPE network.

Residual content of bromine detected by elemental analysis in thecross-linked PPEs, as function of the ratio of monomers 16:17 ( =18a , = 18b , = 18c , = 18d , = 18e ).

Normalized equilibrium weight increase for polymers 18a 18eswollen in toluene as function of the ratio of monomers 16:17 ( =18a , = 18b , = 18c , = 18d , = 18e ).

Photoluminescence spectra of the cross-linked PPEs investigated here(18a , open triangles, a; 18b , open circles, b; 18c , open squares, c;18d , filled triangles, d; 18e , filled circles, e; all swollen in toluene)and a (dry) MEH-OPPE reference film (filled squares, f). Spectraare scaled to optimally fit the graph.

Photograph of an image through a cross-linked film of MEHO-OPPE-X 3.4 18f .

Photographs (a), optical micrographs (b) and scanning electronmicrographs (c) of cross-linked conjugated milli- (a) micro- (b), andnanoparticles (prepared in reaction nanometer-sized particles a) (c).Photographs and optical micrographs were taken in fluorescencemode under excitation at 366 nm and transmission/reflection mode,with the polymer particles dispersed in toluene.

Size-distribution of cross-linked PPE micro- (a) and nanoparticles(prepared in reaction nanometer-sized particles a) (b). The size of theindividual particles was determined from optical transmissionmicroscopy (a) and scanning electron microscopy (b) images. The

particles evaluated in (a) were swollen with toluene.

SEM image of cross-linked EHO-OPPE nanoparticles.

Photoluminescence spectra of PPE milli (Method B, dashed line),micro- (Method C, dotted line) and nanoparticles (Method D, dash-dotted line), suspended in toluene, and as a reference an MEH-OPPEsolution in toluene (solid line).

Electron (circles) and hole (squares) mobilities for covalently cross-linked nano-particles (9 % mol cross-linker, filled symbols) andEHO-OPPE (open symbols).

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List of Acronyms, Abbreviations and Chemical Compounds

Bipy 2,2-bipyridine

E g band gap

k boltzman constant

carrier mobility

t tr carrier transit time

e charge on the charge carrier

c critical extent of reaction

diagonal (energetic) disorder parameter

c electrical conductivity

E electric field

EDTA ethylenediamine tetraacetate

FET field-effect transistor

HOMO highest occupied molecular orbital

ITO indium-tin-oxide

LED light-emitting diode

LUMO lowest unoccupied molecular orbital

MLCT metal to ligand charge transfer NMR nuclear magnetic resonance

M n number-average molecular weight

Xn number average degree of polymerization

n number of charge carriers

off-diagonal (positional) disorder parameter

1 D one dimensional

PE phenylene ethynylene

PA polyacetylene

PAT poly(3-alkyl thiophene)

PAE poly(arylene ethynylene)

PAV poly(arlene vinylene)

BBL poly(benzobisimidazobenzo phenanthroline)

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PDA poly(diacetylene)

EHO-OPPE poly[2,5-dioctyloxy-1,4-diethynyl-phenylene- alt -2,5,-bis(2'-ethylhexyl-

oxy)-1,4-phenylene]

MEH-OPPE poly[2,5-dioctyloxy-1,4-diethynyl-phenylene- alt -2-methoxy,5-2-ethyl-

hexyloxy)-1,4-phenylene]

BipyPPE 1 poly[2,2-bipyridine-5,5diylethynylene[2,5-bis(2-ethylhexyl)oxy-1,4-

phenylene]ethynylene]

BipyPPE 2 poly[(2,2-bipyridine-5,5diylethynylene[2,5-bis(2-ethylhexyl)oxy-1,4-

phenylene]ethynylene)-co-(2,5-dioctyloxy-1,4-diethynyl-phenylene-alt-

2,5-bis(2'-ethylhexyloxy)-1,4-phenylene)]

PPE poly( p-phenylene ethynylene)

PPV poly( p-phenylene vinylene)LED polymer light-emitting diode

PL photoluminescence

0 prefactor mobility

L sample thickness

SDS sodium dodecyl sulphate

T temperature

TOF time-of-flightt tr transient time

UV-Vis ultraviolet-visible

V voltage

max wavelength of maximum absorption or emission

1 MEHO-OPPE

2 [Pt-(-Cl)Cl(PhCH=CH 2)]2

3 MEHO-OPPE-Pt II networks

4 EHO-OPPE

5 EHO-OPPE-Pt II networks

6 PtCl 2(PhCH=CH 2)2

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7 Pt(PhCH=CH 2)3

8 Pt( 2-Ph-C C-Ph) 2

9 EHO-OPPE-Pt 0

10 1,4-Bis[(2-ethylhexyl)oxy]-2,5-diiodobenzene

11 1,4-diethynyl-2,5-bis-(octyloxy) benzene

12 5,5-diethynyl-2,2-bipyridine

13 BipyPPE 1

14 BipyPPE 2

15 Bis(2,2-bipyridine)Cu(I)-hexafluorophosphate

16 2,5-Diiodo-4-[(2-ethylhexyl)oxy]methoxybenzene

17 1,2,4-tribromobenzene

18a O-OPPE-X

18b MEHO-OPPE-X 0.5

18c MEHO-OPPE-X 1.3

18d MEHO-OPPE-X 4.4

18e MEHO-OPPE-X 10.5

18f MEHO-OPPE-X 3.4

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Abstract

The experimental research program that forms the basis of this thesis has been

directed towards the design, synthesis, processing and physical characterization of well-

defined conjugated polymer networks. It attempts to provide answers to the questions

how such materials can be synthesized and processed and how the introduction of cross-

links can be exploited for the creation of polymeric materials with optimized optic and

electronic characteristics. Interestingly, this family of materials has received little

attention in the past, at least as far as systematic studies of well-defined systems are

concerned. This situation may be a direct consequence of the challenge to introduceconjugated cross-links into conjugated polymers and retain adequate processibility.

We have shown that organometallic polymer networks based on linear conjugated

polymers are readily accessible through ligand-exchange reactions. This approach was

exemplified by exploiting the ethynyl moieties comprised in poly( p-phenylene

ethynylene) (PPE) derivatives as ligand sites, which allow for complexation with selected

metals and cross-linking via the resulting PPE-Metal complexes. Focusing on the

dinuclear complex [Pt-(-Cl)Cl-PPE] 2 and PPE-Pt 0 as crosslinks, we have conducted an

in-depth investigation on how the nature of the metal cross-links influences the materials

characteristics, in particular the charge transport properties. We first investigated the

charge carrier mobility of poly[2,5-dioctyloxy-1,4-diethynyl-phenylene- alt -2,5-bis(2'-

ethylhexyloxy)-1,4-phenylene] (EHO-OPPE), as a classic representative of poly( p-

phenylene ethynylene) (PPE) derivatives, which represent an important class of

conjugated polymers. In what appears to be the first study ever conducted on the mobility

of any PPE, we found that EHO-OPPE displays ambipolar charge transport

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characteristics with very high electron (1.9 10 -3 cm2V -1s-1) and hole (1.6 10 -3 cm 2V-1s-1)

mobilities. Most importantly, the introduction of Pt 0 cross-links was found to enhance the

charge carrier mobility of the investigated systems by up to an order of magnitude.

Electron and hole mobilities of the order of 1.5 10 -2 cm2V -1s-1 were measured for these

materials, representing the highest mobilities ever reported for disordered conjugated

polymers. More importantly, the study unequivocally proves that the introduction of

conjugated cross-links indeed leads to a significant improvement of the carrier mobility

of conjugated polymers. Work involving [Pt-(-Cl)Cl-PPE] 2 cross-links suggests that the

nature of the metallic cross-links is exceedingly important. We have shown that the PtII

centers act as traps for both electrons and holes and impede efficient charge transport.

With the aim to expand the materials basis and exploit other chemical platforms for the

cross-linking process, we integrated 2,2-bipyridine (bipy) moieties into the conjugated

polymer backbone. Bipy is a most versatile ligand, which can bind to a broad variety of

metals. We have synthesized a library of PPEs in which the bipyridine content is

systematically varied. Using a variety of transition metal complexes, we have

systematically investigated the ligand-exchange reactions of these polymers.

Complexation studies suggest that ligand exchange indeed leads to three-dimensional

networks, which feature BipyPPE-metal-BipyPPE cross-links and display interesting

optoelectronic properties. We also showed that the ligand exchange is a cooperative

process, which in the presence of a competing ligand is fully reversible. Complexes with

group 12 d 10 ions (Zn 2+ and Cd 2+) are emissive, while other transition metals such as Cu +,

Ni 2+ form non-radiative metal-to-ligand charge-transfer complexes with the polymers.

17


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In a second approach we have demonstrated that the processing issues associated

with covalently cross-linked conjugated polymer networks can be readily overcome if

these materials are synthesized in the form of cross-linked nanoparticles and processed as

suspensions. In a series of ground-breaking experiments, we have shown that covalently

cross-linked conjugated polymer particles can be produced by conducting cross-coupling

reactions in aqueous emulsions instead of homogeneous solutions and using

multifunctional cross-linkers. The size of the spherical polymer particles can easily be

tuned over a wide range (mm to nm) by modification of the reaction conditions. The

resulting materials can be processed in the form of suspension and promise interestingelectronic properties.

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Chapter 1: Introduction

1.1 (Semi)conducting Conjugated Polymers an Emerging Class of OrganicMaterials

The role of polymers in the electronics industry has traditionally been associated

with electrically insulating properties and macromolecular materials have found

widespread use in packaging applications as passive dielectrics. However, the discovery

of electrically (semi)conducting conjugated polymers has radically changed this

situation.1 These materials are attracting significant interest not just academically but also

industrially, since they may combine the processability and mechanical properties of polymers with the readily-tailored optoelectronic properties of organic molecules.2

Especially the potential use of these "synthetic metals" as electrical conductors,3 in light-

emitting diodes (LEDs),4, ,5 6 field-effect transistors (FETs),7 photorefractive devices8, and

Figure 1.1 Photographs of applications of conjugated polymers in (a) anticorrosion

coatings, (b) field-effect transistors (FETs) and (c) light-emitting diodes

(LEDs).

19


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photovoltaic cells9 have motivated the development of synthesis and processing methods

of conjugated polymer materials with unique field-responsive properties (Figure

1.1).3, ,10 11 Breathtaking progress has been made in the last two decades, and the field has

matured to the onset of commercial exploitation of conjugated polymers as polymer

conductions in corrosion control,12 in light-emitting diodes,4,5,6 sensors13 and a number of

other applications.14 One key problem for the full commercial exploitation of the field is

that the charge carrier mobility of state-of-the-art polymer semiconductors is much lower

than required for many applications.14b This thesis is focused on the investigation of a

new concept of improving this important property. It is based on the hypothesis that thecharge transport in these polymers can be significantly improved through the introduction

of conjugated cross-links between the individual polymer chains.

1.2 Charge Transport in Conjugated Polymers

Conjugated polymers are characterized by a molecular structure that features

alternating single and multiple bonds. The electronic and physico-chemical

characteristics of conjugated macromolecules are not only governed by the nature of the

polymer backbone, but also by the intermolecular interactions. Electrical conductivity is a

direct consequence of delocalized chemical bonding.15 The absence of sp3 hybridized

carbon atoms leads to a situation where overlap of p orbitals on successive carbon atoms

enables the delocalization of -electrons along the polymer backbones. The large number

of atomic orbitals in the macromolecules translates into a large number of molecular

orbitals, which form a band of energies. In the case of a metal this energy band is a

continuum; due to the high density of electronic states with electrons of relatively low

20


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binding energy, "free electrons" can easily redistribute and under an applied electric field

move easily from atom to atom, thus rendering the material electrically conducting.

Hckel's theory predicts that in conjugated macromolecules-electrons are delocalized in

similar fashion over the entire chain, so that one would expect that the electronic

properties of a polymer material composed of sufficiently long conjugated chains are also

described well by a continuous energy band. According to this model, an individual chain

of the conjugated polymer would be a one-dimensional (1D) metallic conductor.

However, as a result of the Peierls instability, the density of -electrons in conjugated

organic molecules is not the same between all atoms;16

there is a distinct alternation between single and multiple bond character, as is for example evident from the chemical

structure of poly( p-phenylene ethynylene) (PPE) shown in Figure 1.2. Thus, the

electronic properties of conjugated polymers in their neutral oxidation state are usually

better described by a filled valence band ( -band, bonding) formed by the highest

occupied molecular orbitals (HOMOs) and an empty conduction band ( *-band,

antibonding) formed by the lowest unoccupied molecular orbitals (LUMOs). Because the

energy difference between the highest occupied and the lowest unoccupied band, referred

to as band gap ( E g ), is usually not near zero, and because there are no partially filled

bands, conjugated polymers are typically semi conductors in their neutral,undoped state.

E g depends on the molecular structure of the polymer's repeat unit and can be controlled

via modification of the latter.In their pioneering work on polyacetylene, MacDiarmid, Shirakawa and Heeger

demonstrated thatdoping allows one to increase the electrical conductivity of conjugated

polymers by many orders of magnitude so that metallic properties are achieved. Doping

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refers to either removing (oxidation, p-doping) or adding electrons (reduction, n-doping)

to the polymer, as shown schematically in Figure 1.2 at the example of PPE. This can be

accomplished by either conventional chemical or electrochemical means. A number of,

defect types can be formed upon doping, including radical cations or anions (referred to

as polarons) and dications or dianions (referred to as bipolarons). As a result of the

doping process, the electrochemical potential (Fermi level) of the polymer is moved into

an energy regime with a high density of electronic states. Charge neutrality is maintained

through counter-ions, and the doped polymer effectively becomes a salt. The extra

electrons or vacancies (holes) introduced through doping act as charge carriers. The

delocalization of these carriers can be quite limited, partly because of Coulomb attractionto their counter-ions, and partly because of a local change in the equilibrium geometry of

the doped relative to the neutral molecule. However, since every repeat unit is a potential

redox site, the doping level of conjugated polymers can usually be rather well controlled.

nn n

+ -- e - + e -

(p-doping) (n-doping)

PPE- e -

n

2+

+ e -

n

2-

Figure 1.2 Schematic representation of the doping of poly( p -phenylene ethynylene)

(PPE) under formation of polarons (radical cations, radical anions) and

bipolarons (dications, dianions). Note that multiple carriers can coexist on

each macromolecule.

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For many materials platforms high levels of electrical conductivity can be achieved at

high doping levels, which are concomitant with a high density of charge-carriers,

especially in the case of trans -polyacetylene, conductivities upto 108 S/cm (similar to that

of Cu) have been observed.17 The electrical conductivity ( c) in conjugated polymer

systems is expressed by:

c = n e Equation 1.1

where n is the number of charge carriers (which can be adjusted by the level of doping),e is their charge and signifies the charge carrier mobility. The investigation of new

concepts for the design of materials in which this important figure of merit is maximized

is the focus of the present work.

The charge transport characteristics of conjugated polymers are, of course,

+ ..

+

.+

+ .

+

.

+

+

Figure 1.3 Schematic representation of intra chain charge diffusion (left) andinter chain

charge diffusion (hopping, right) in polyacetylene.

intra chain charge diffusion inter chain charge diffusion

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24

primarily governed by the nature of the polymer backbone itself, but intermolecular

interactions also exert an important influence on the macroscopic materials properties.

The charge carrier mobility of conjugated polymers is a function of intra chain charge

diffusion and inter chain interactions, i.e., hopping (Figure 1.3).3,7,9,10,11 Both factors

depend on a number of variables; the former is mostly based on the polymer's chemical

structure, the number and nature of defect sites, conformation of the polymer backbone,

and the molecular weight, while the latter strongly depends on the supramolecular

architecture, i.e., the degree of contact, order and orientation.18 The fact that conducting

polymers usually show a different variation of conductivity with temperature than metalss

and, thus, tly, much work has been

focused on maximizing the supramolecular order in conjugated polymers, in order to

optimiz

has been related to disorder effects, which may dramatically limitinter chain interaction

charge transport in these systems.18b,d,h,k Consequen

e their charge transport characteristics and electrical conductivity, and also a

variety of other properties.18 As a result, exciting progress has been documented for the

charge carrier mobility (and also electrical conductivity) in semiconducting polymer

systems with high molecular order and/or orientation. Amorphous, semiconducting

polymers exhibit a field-effect mobility (for holes) of the order of 10-5 cm2/Vs (about five

orders of magnitude lower than the mobility required for the potential application in

plastic thin film transistors, 1 cm2/Vs). The original work on the development of polymer

field-effect transistors based on conjugated polymers relied on polyacetylene (Figure

1.4a)19a and the carrier mobility of this material was reported to be in the range of 10-4

cm2/Vs.19b However, due to the lack of processablitiy of this polymer, the

semiconducting film was prepared via direct polymerization on a substrate, which might


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have lead to the presence of various defects which act as traps and limit the mobility.

Field-effect mobilities of the order of 10-6 - 10-5 cm2/Vs for undoped polyaniline and 10-5

- 10-3 for polyaniline doped with 2,4,5-trichlorobenzene sulphonic acid have also been

reported in the literature (Figure 1.4b).19f In an interesting recent study it has been

reported that electrospun camphor-sulphonic-acid-doped polyaniline/polyethylene oxide

nanofibers display FET behaviour with field-effect mobilities around 10-4 cm2/Vs.19g In

the case of poly( p-phenylene vinylene)s (PPV), widely used as the electroluminescent

layer in LEDs, field-effect mobilities (holes) of the order of 10-8 - 10-7 cm2/Vs were

reported upon doping with iodine (Figure 1.4c).19c

Also in this case, the PPV was polymerized directly on to the substrate using a precursor route, which might lead to

various structural defects and impurities in the semiconducting layer. A time-of-flight

(TOF) study carried out with a PPV synthesized using a different protocol exhibited a

room-temperature charge carrier mobility of the order of 10-5 cm2/Vs at a field of 105

V/cm.19d A subsequent TOF study on phenyl amino substituted PPVs reported charge

carrier mobilities in the range of 10-4 - 10-3 cm2/Vs (Figure 1.4d).19e

More recent studies have shown that the charge carrier mobility is greatly

enhanced in ordered structures due to improved interchain hopping. For example, charge

carrier mobilities of the order of 0.1 cm2/Vs can be achieved at room temperature in self-

organized semicrystalline films of poly(3-alkylthiophene)s (PATs).18e, a20 In pioneering

studies by Lovinger et al.18e and Sirringhaus et al.20a the charge carrier mobility in PATs

was related to the degree of self ordering observed in these materials. It was reported that

in case of polymers with high regioregularities that the interdigitated side chains tend to

orient perpendicular to a substrate and that -stacked conjugated polymer lamellae

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deposit parallel to the substrate, leading to highly efficient in-plane charge transport

(Figure 1.5).20a It has also been reported that the on-off ratio of field-effect transistors

depends on the orientation of the polymer on the substrate and increases with better in-

plain orientation.18e

Recently it was reported that the increase of the charge carrier mobility withincreasing molecular weight coincided with the amount of overlap between the ordered

regions of the film. It was demonstrated for low-molecular-weight PATs that the induced

orientational order (thus the overlap) between aggregates is low when processing them

N

H

n n n

H17 C8 C8H17

S

S

N

NN

N

O O

N

N

H17C8O

OC H8 17

n

NN

N

O O

n

nn

Figure 1.4 Chemical structures of (a) polyacetylene, (b) polyaniline, (c) poly(p- phenylene vinylene), (d) poly(N-phenylimino-1,4-phenylene-1,2-ethenylene-1,4-(2,5-dioctoxy)-phenylene-1,2-ethenylene-1,4-phenylene),(e) poly-9,9dioctyl-fluorene-co-bithiophene, (f) poly(benzobisimidazo

benzo-phenanthroline) and (g) poly(benzobisimidazole).

f g

a b c

d e

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from a low-boiling-point solvent such as chloroform. However, the charge carrier

27

mobility was observed to increase if these films were annealed or processed from a high-

boiling-point solvent such as xylene or 1,2,4-trichlorobenzene, this was attributed to

better ordering and overlap between the aggregates.20c,d

Using a rigid-rod nematic conjugated polymer, poly9,9dioctyl-fluorene-co-

bithiophene (Figure 1.4e), it has also been demonstrated that the charge carrier mobility

is enhanced upon macroscopic alignment of the conjugated polymer.20e While field-effect

S

SSSS

S

SS

SS

SS

a

b

Figure 1.5

respect to the employed substrate. (a) In-plain orientation and (b) out-of-Two different preferential orientations of the ordered domains of PAT with

lain orientation.


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transistors based on isotropic layer of this polymer displayed a charge carrier mobility of

0.003 - 0.005 cm2/Vs, the carrier mobility was increased to 0.009 - 0.02 cm2/Vs for

devices in which the polymer chains had been aligned parallel to the source-drain gap by

using an alignment layer and quenching the polymer (after annealing in a thermotropic

liquid crystalline state) into an ordered glass. In the case of aligned samples a significant

anisotropy for field-effect mobilities (/ ) was also noted.20e

In an interesting study by Moses and coworkers it was reported that charge carrier

mobilities of up to 0.2 cm2/Vs could be obtained in the case of field-effect transistors

based on PATs, if an ultrathin film (20 40 ) of the semiconducting polymer was processesed using a dip-coating technique.20f This significant enhancement of the charge

conjugated m

mobility is to our best knowledge, the highest that has been reported to date for

conjugated polymers.

p y(benzobisimidazobenzo

carrier mobility was explained with a significantly improved structural order of the

acromolecules near the polymer - insulator interface.20f This charge carrier

All the above examples are p-type materials and the quoted mobilities are related

to the transport of holes. Rather interestingly, none of these conjugated polymers

displayed significant n-type transport, a behaviour that was attributed to the existence of

strong traps for electrons but not for holes.21a Although a number of n-type organic

molecules and oligomers with high electron mobility have been reported,21a,b,c the

realization of high electron mobilities in conjugated polymers remains an important goalfor the implementation of high performance plastic electronics.

High electron mobilites up to 0.1 cm2V-1s-1 were recently reported by Babel et al.

for a high-electron-affinity conjugated ladder polymer, ol

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phenan

variety of conjugated polymers that earlier were reported to be

good h

throline) (BBL Figure 1.4f).21d These values are significantly higher than the

mobility reported for the non-ladder type analogue poly(benzobisimidazole) (Figure 1.4g)

which was of the order of 10-6 cm2/Vs. This significant difference was explained with the

ordered semicrystalline morphology observed in the ladder-type polymers which is in

contrast with the amorphous nature of films obtained from the non-ladder

poly(benzobisimidazole).

In a very recent study, n-type behaviour was demonstrated in field-effect

transistors based on a

ole conductors only. The key for efficient electron transport was reportedly the useof a hydroxyl-free gate dielectric based on divinyltetramethysiloxane-(bis benzyl

cyclobutene). The reported electron mobilities were in the range of 10-3 - 10-2 cm2/Vs for

unaligned poly(fluorene) copolymers and dialkyl substituted PPVs.21e It was also reported

that the lack of n-type transport in the FETs studied earlier was due to the electro-

chemical electron trapping by the hydroxyl group, occurring in the SiO2, poly(vinyl

alcohol) or polyimide gate dielectrics.

In summary, good (in-plain) charge transport is usually desired in FETs and other

applications. While the above discussed approach of ordering/orienting conjugated

polymers indeed leads to materials with significantly improved charge carrier mobilities,

the required processing protocols are usually intricate and incompatible with preferred

processes for plastic electronic manufacturing which include, inkjet printing,22,23 screen

printing,24,25 roll to roll processes26 and others. Thus an alternative general approach for

the improvement of the charge transport in these materials is desirable.

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1.3 Con

allow the creation of polymeric materials with most interesting characteristics, including

jugated Polymer Networks

It can be expected that the charge transport characteristics (and other properties)

of conjugated polymers can also be improved through the introduction of - conjugated

cross-links between the conjugated macromolecules.18c One may surmise that in an ideal

-conjugated macromolecular network, which featuresconjugated cross-links (Scheme

1.1), intrachain diffusion is the predominant mechanism for charge transport, while

interchain processes, if at all, only play a subordinate role. Thus, this architecture might

Schem

networks with organometallic cross-links (top) and covalent cross-links

e 1.1 Simplified schematic representation of cross-linked conjugated polymer

(bottom).

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high charge carrier mobility, good electrical conductivity, and also large nonlinear optic

response.18c,27 The approach may represent an attractive technological alternative to the

aforementioned systems with high molecular order and/or orientation,18,20 which often

require rather demanding processing conditions. As shown in Scheme 1.1 the networks

can be designed to rely on either non-covalent or covalent interactions. In the first case a

moiety is integrated in the conjugated polymer backbone that acts as a binding site for a

metal and the network formation occurs through coordination bonds that provide

adequate electronic interactions, i.e. conjugation. The second case takes inspiration from

the framework used for the synthesis of known thermosetting polymers and is based onthe introduction of a conjugated tri-functional monomer along with the conjugated bi-

functional monomers. Obviously, this approach leads to a (non-processable) three-

dimensional cross-linked conjugated polymer network with covalent cross-links which

Interes pite the diverse research activities focused on the chemistry,

material science and physics of conjugated polymers, the most interesting feature of

conjugated cross-links has received little attention, at least as far as systematic studies

or

has to be processed prior to or during network formation.

tingly, des

and well-defined systems are concerned. This situation may be a direct consequence of

the challenge to introduce such cross-links and retain adequate processability. While

conjugated-polymer-based networks featuringnon-conjugated cross-links based on

covalent28 non-covalent bonds29,30 have been deliberately prepared and studied by a

number of research groups, examples of conjugated cross-links involving either covalent

bonds31,32 or metal-complexes between chains33,34 are rare, and in many cases have been

obtained serendipitously and lack unambiguous characterization. The knowledge base

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generated through these studies however is important for the present thesis, as it

represents the starting point for this research endeavor.

In an important study, Joo et al. have compared the electronic characteristics of

polypyrrole samples, which feature different degrees of conjugated side chains and/or

cross-links (Figure 1.6). Unfortunately, the analytic techniques employed in this study do

not allo

in the polymer after workup led to cross-linking of zinc phorphyrin-linked poly( p-

w an unambiguous discrimination between originally unintentionally introduced

side chains and covalent cross-links. However, highest conductivities were found for the

material for which the highest cross-link density was assumed.32b

In another study by Krebs et al. it was found that the residual Pd0 catalyst trapped

igure 1.6 Schematic representation of the chemical structure of conjugated cross-links

in electrochemically synthesized polypyrrole.

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

NH

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phenylene ethynylene).35 It was suggested that the cross-linking reaction caused the

formation of substituted benzenes, which were formed from enynes and activated alkynes

in a [4+2] cycloaddition reaction and from terminal alkynes in a [2+2+2] cycloaddition

reaction in the presence of Pd0 without an inert atmosphere (Scheme 1.2). Unfortunately,

the electronic properties of these materials have not been reported.

I H

C8H17

H17 C8

C8H17

H17C8H17 C8

C8H17

C8H17

H17C8

C8H17

H17C8

C8H17

H C

C H8 17H17C8

17 8

n2

Pd

Pd

Scheme 1.2 Schematic representationof the cross-linking reaction reported to occur in

zinc phorphyrin-linked poly( p-phenylene ethynylene)s.

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Scheme 1.3 Schematic representation of the cross-linking reaction proposed to occur in

poly[(4-ethynyl)phenylacetylene] upon thermal treatment.

H

n

n

300 oC

An interesting study by Lavastre and co-workers reported the formation of

conjugated polymer networks through heat treatment of poly[(4-ethynyl)phenylacetylene]

(Scheme 1.3).36 The cross-linking reactions were studied via thermo-gravimetric

techniques, and the resulting product obtained was reported to be insoluble. Poor

solubility of poly[(4-ethynyl)phenylacetylene] in common organic solvents was also

mentioned. However, a thorough chemical characterization elucidating the role of

inter molecular reactions as compared to theintra molecular reactions, determination of

the cross-link density as well as the electronic and photophysical properties of the cross-

linked products have not been reported.

n

The introduction of transition metals into conjugated polymers has also recently

received considerable attention in particular due to the potential to manipulate the

electronic properties of these materials.37,38 Conventional concepts for the design of -

conjugated organometallic polymers include the incorportation of metal centers into

polymers either integrated directly in the polymer chain, or bound coordinatively to the

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conjugated backbone,34,38, ,3940 or alternatively, they can be attached via conjugated or

nonconjugated spacer units in the form of side groups.40

35

poly(arylene

with the loss of CO and the formation of phenylene-Cr(CO)2-ethynyl cross-links (Scheme

However the linear polymers synthesized by Wright exhibited low solubility and

prevented the in-depth characterization of the product afforded by this process. Thus

A study by Wright claimed that upon thermal treatment or UV irradiation of

ethynylene)s containing the Cr(CO)3 benzene moiety, cross-linking occurred

Scheme 1.4 Schematic representation of the proposed cross-linking reaction occurring

upon heat treatment of Cr(CO)3-benzene containing poly(arylene

ethynylene)s.

Cr(CO) 3

Cr(CO) 2

- CO

m n m n

1.4).

exact chemical structure and the degree of cross-linking remains somewhat questionable

and also in this case no investigation of the materials charge transport characteristics was

undertaken.

With the objective to exploit the redox properties after coordination of transition

Figure 1.7 Chemical structure of poly(o-toluidine) cross-linked with palladiumII.

N N NH

PdX 2

N N NH


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metal having a relevant redox function to a conjugated polymer, Hirao et al. synthesized a

organometallic network of poly(o-toluidine) with PdII coordinating to the imine moieties

in the polymer (Figure 1.7). The linear polymers synthesized were reported not to be

completely soluble and only dissolved fractions were used for the study. These species

having numb

during exper d to study the electrochemical behaviour of the systems.

In another study conducted in the Weder group, which nucleated the present

41

ene moieties per repeat unit as potential

ligand s 42

as the cross-linker in the initial experiments. It was demonstrated that the ethynylene

er average molecular weight of ~3000 were further observed to polymerize

iments conducte

thesis, Huber et al. demonstrated that the unsaturated (acyclic) carbon-carbon bonds in

the poly( p-phenylene ethynylene) (PPE) backbone can be utilized as binding motif. Theconjugated polymer employed in the reported work was poly[2,5-dioctyloxy-1,4-

diethynyl-phenylene-alt -2-methoxy,5-2'-ethylhexyloxy-1,4-phenylene] (MEH-OPPE,1),

a highly soluble PPE and offers two ethynyl

ites (Scheme 1.5). The dinuclear [Pt-( -Cl)Cl(PhCH=CH2)]2 (2) was employed

resulted in films of good optical quality, which were unequivocally cross-linked. While

the latter were completely insoluble in solvents that would readily dissolve the

uncomplexed polymer, they readily dissolved upon addition of an excess of an olefinic

moieties comprised in the PPE can readily coordinate to PtII, in exchange with weakly-

bound styrene ligands. The bifunctional [Pt-( -Cl)Cl(PhCH=CH2)]2 employed under

appropriate conditions indeed allowed the formation of PPE-Pt networks (Scheme1.5,

3, z>0), as demonstrated in an extensive NMR study. In dilute solutions, the equilibrium

of the investigated PPE-Pt systems dictated non-cross-linked structures (Scheme 1.5,3,

z=0), and the system remained homogeneous and therewith processable. Spin-coating

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ligand (e.g. styrene), demonstrating the reversibility of the complexation. As expected,

the coordination of PtII markedly influences the photophysical characteristics of the PPE;

its PL is efficiently quenched, and at high Pt-contents, the absorption maximum

experienced a hypsochromic shift. Thus, it was successfully demonstrated that well-

defined organometallic polymer networks can be easily synthesized and processed by

employing carefully designed ligand-exchange reactions, but again no charge transport

studies were conducted for the materials employed. It should be noted that while this

study has an important character for the present thesis, the Cl-bridged dinuclear joints41

1 2

3

Scheme 1.5 Simplified representation of the ligand-exchange reaction between MEH-

OPPE (1) and [Pt-(-Cl)Cl(PhCH=CH2)]2 (2), leading to cross-linked

organometallic hybrid materials3.

O O

OO

ClPt Pt

ClCl

Cl

OR

RO

OR

RO

PtCl Cl

Pt ClCl

OR

RO

PtCl Cl

Pt ClCl OR

RO

+n

nx y z

37


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employed in the above studies, may not provide significant -conjugation between

chains.

In parallel to the experiments summarized above, Wolf et al.30b recently

tween

chains.

In parallel to the experiments summarized above, Wolf et al.30b recently

synthesized polythiophenes, which are cross-linked via a dinuclear Pd complex (Figure

1.8). Their approach relies on the electropolymerization of a dimeric metal-terthiophene

complex (which, remarkably, was rather similar to the Pt-complex employed) and,

conceptually, provides an alternative access path to these target-structures; however, it

lacks the freedom which the ligand-exchange approach imparts to the processing of these

materials.

Thus in summary, the knowledge base regarding the possibility to synthesize and

process -conjugated materials which feature conjugated cross-links is very limited.

Concomitantly, the question of how this structural motif can be employed to design polymeric materials with optimized optic and electronic characteristics remains

essentially unanswered.

synthesized polythiophenes, which are cross-linked via a dinuclear Pd complex (Figure

1.8). Their approach relies on the electropolymerization of a dimeric metal-terthiophene

complex (which, remarkably, was rather similar to the Pt-complex employed) and,

conceptually, provides an alternative access path to these target-structures; however, it

lacks the freedom which the ligand-exchange approach imparts to the processing of these

materials.

Thus in summary, the knowledge base regarding the possibility to synthesize and

process -conjugated materials which feature conjugated cross-links is very limited.

Concomitantly, the question of how this structural motif can be employed to design polymeric materials with optimized optic and electronic characteristics remains

essentially unanswered.

S

S

SS

S

S

PdCl

ClPd

P

P

Ph

PhPh

Ph

Figure 1.8 Bridged Pd terthiophene complex employed for electropolymerization.

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1.4 Poly( p -phenylene ethynylene)s: An Important Class of Conjugated Polymers

Among a plethora of materials platforms, poly(arylene ethynylene) (PAE)

derivatives have attracted the attention of numerous research groups and we elected to

exemplarily employ this family of conjugated polymers as the basis for the present study.

As the name implies, PAEs feature aromatic rings and ethynylene groups in the polymer

backbone. The connection of these moieties results in an alternating

r (e.g. meta vs. para substitution), the introduction of hetero-

atoms or metals, the nature of solubilizing side chains, and non-covalent interactions with

and

ethynyl

Ar

Ar

Ar n nn

PAE PAVPA

n

PDA

Figure 1.9 General schematic structures of poly(arylene ethynylene)s (PAEs),

and

ol (diacet lene)s (PDAs).

poly(arylene)s (PAs), poly(arylene vinylene)s (PAVs),

sequence of single- and multiple bonds and gives rise to-conjugation along the

macromolecules. PAEs are closely related to poly(arylene)s (PAs),43 poly(arylene

vinylene)s (PAVs) and poly(diacetylene)s,44 which all represent important classes of

conjugated polymers (Figure 1.9). The chemical structure of PAEs can readily and

significantly be manipulated, for example via the choice of the aromatic moiety, the

connectivity of the latte

metals. The possibility to integrate conjugated moieties other than arylenes

enes (for example vinylene groups, non-conjugated aliphatic spacers, etc.)

represents another synthetic tool, which leads to PAE copolymers. These structural

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changes allow one to tailor the property matrix of these polymers over a wide range.

Hundreds of different PAEs and PAE copolymers have been reported to date, and during

the last ten years this family of materials has established itself as an important class of

number of excellent texts that discuss the application of PAEs and other polymers in

sensors. The intriguing nanoarchitectures that can be created with PAEs have

been addressed by Moore. The most comprehensive review on the synthesis,

properties, structures, and application of PAEs was edited by Weder et al. in 2005. The

electrically (semi)conducting nature of PAEs has traditionally received comparably little

attention. However, during the last decade more and more research efforts have been

devoted to this subject, and PAEs have eventually been recognized as a potentially very

well as the emitter in light emitting devices were negated by Weder et al. who clearly

demonstrated the usefulness of PPEs in LEDs. Ofer et al. have reported electrical

conductivities for doped PPEs in the range of ~0.18 and ~4.5 Scm at a potential of ~1.6

V vs. SCE. The doping was performed under rather severe conditions at -70C in liquid

SO

conjugated polymers with interesting optical and electronic properties. A number of

excellent texts have reviewed the synthesis, physico-chemical characteristics, and optical

properties of these materials. The early work on PAEs has been summarized in 1996 by

Giesa.45 A review covering various aspects regarding PAEs was published by Bunz in

2000 and s

since.47,48,49

Y important subject of heteroaromatic PAEs.50,51

PAE electrolytes are part of an outstanding article by Schanze.52 Swager has published a

13,53,54,55

56,57

58

powerful class of polymeric semiconductors.59 Previous claims that PPEs do not function

60

-1

n

46 everal complements and updates to this compilation have appeared

amamoto has reviewed the

2 comprising [( -Bu)4 N]AsF6 as electrolyte, due to the high oxidation and reduction

40


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O O

OOn

Figure 1.10 Chemical structure of poly[2,5-dioctyloxy-1,4-diethynyl-phenylene-alt -2,5-

bis(2'-ethylhexyloxy)-1,4-phenylene] (EHO-OPPE,4).

4

potentials observed for the PPEs.61 The conductivities reported in this study are the

highest reported to date for poly( p-phenylene ethynylene)s.

Owing to the linear rigid rod backbone and- stacking, unsubstituted PPEs are

intractable i.e. they are insoluble and as well cannot be thermally processed.62 To increase

the solubility of PPEs one of the strategies adopted is to introduce solubilizing side chains

on the aromatic rings in the polymer backbone. Poly[2,5-dioctyloxy-1,4-diethynyl-

phenylene-alt -2,5-bis(2'-ethylhexyloxy)-1,4-phenylene] (EHO-OPPE,4), a PPE

derivative consisting of alternating units substituted with sterically hindered and linear

alkoxy side chains (Figure 1.10) displays an interesting array of characteristics.

Employing facile synthetic protocols EHO-OPPE can be directly synthesized in high

purity without the need for precursor polymers.63 It has been reported by Weder et al. that

this derivative of PPE has a high PL quantum efficiency of around 85% in solution,

around 35% in the solid state and can be used in LEDs as the electroluminescent layer.

41


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42

Owing to the rigidity and the linearity of the conjugated backbone, EHO-OPPE displays

outstanding orientability of the molecules. In a clever study by Weder et al this concept

was exploited for fabrication of devices in which polarized absorption and / or emission

of light are desired.64. Interesting NLO properties have also been reported with extremely

efficient two photon absorption for EHO-OPPE.65 Owing to this interesting property

matrix EHO-OPPE was elected to be employed as the central materials platform for the

present experimental study that forms the basis of this thesis.

1.5 Energetics of ConjuAs discussed already in Chapter 1.3 the introduction of conjugated cross-links

ay pl

gated Polymer Networks

m ay a significant role in the improvement of inter-chain charge transfer processes

and thus effectively improve charge transport in conjugated macromolecules. Matched

energy levels (viz. HOMO and LUMO or ionization potential or electron affinity) of both

the conjugated polymer and the employed cross-linker are required for efficient transport

to occur. In the case of conjugated polymer with no chemical defects due to the presence

of isoenergetic electronic states the charge transport is not hindered, however the

presence of a chemical defect in the conjugated backbone may introduce energy levels

with energies differing from the polymer main chain that can act as traps and hinder the

charge transport through the backbone (Figure 1.11a,c). Low lying energy levels

compared to the HOMO can act as electron traps (Figure 1.11b) and those compared to

LUMO can act as hole traps (Figure 1.11d). In the case of covalently cross-linked

conjugated polymers by careful selection of cross-linking moieties with similar chemical

structure as the repeating unit this possible energy mismatch can be avoided.


45/171

Recently, Swager and Holliday discussed the different electron transport

mechanisms in conducting metallopolymers by outer and inner sphere electronic coupling

between the metal center and conjugated polymer.66

Charge transport by outer sphereelectronic coupling is predominantly observed in the case of redox active metal centers

tethered to conjugated polymer backbone via a conjugated or non-conjugated linker

(Figure 1.12). Thus in such materials there is a lack of electronic interaction between the

Figure 1.11 Simplified schematic of electron (top) and hole (bottom) transfer in

conjugated polymers with matched energy levels (a and c) and un-matched

energy levels (b and d).

a b

c d

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metals d-orbitals and the conjugated polymers -system. In the case of organometallic

complexes an isoenergic series of electronic states exits, which facilitates the charge

transport through the system. When these organometallic complexes are tethered to the

conjugated polymer (possessing its own band of electronic states) these isoenergic

electronic states remain un-affected and distinct, thus as a result these metal complexes

may not be significantly involved in the macroscopic charge transport through the

material. The redox and the charge transport properties of these hybrid materials depend

on the length and the nature of the tether employed for appending the metal center to the

conjuga

67

electronic coupling. Intimate mixing of the metals d orbitals and the polymers system

M

M

M

Figure 1.12 Schematic structure of metallopolymer demonstrating charge transport by

outer sphere electronic coupling.

ted polymer. It has been reported before that as the length of the tether is

decreased the conductivity observed in these systems switches from the self-exchange

mechanism between the attached organometallic moieties to one involving the conjugated

polymer backbone.

Metallopolymers with the transition metal centers incorporated in the polymer

backbone (Figure 1.13) predominantly display charge transport via inner sphere

44


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is observed in this case and the metal centers can from the part of theintra chain charge

transport. In such metallopolymers a va of ligands can be e loyed for the

coordination of the metal center to the polymer backbone. Importantly, while there may

be a direct overlap between the orbitals of the polymer backbone and the transition metal

in these materials, the electron are still highly dependent on the

electronic levels of the involved polymer and metal. To obtain optimum electronic

properties in these systems matched energy levels of the conjugated polymer and the

metal center are an important pre-requisite.

M

M

M

Figure 1.13 Schematic structure of metallopolymer demonstrating charge transport by

inner sphere electronic coupling.

riety mp

ic properties of the system

45


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1.6 References

1 Shirakawa, H.; Louis, E. J.; MacDiarm J.

Chem. Soc. Chem. Comm. 1977 , 578.2 (a) Heeger, A. J. J. Phys. Chem. 2001 , 105 , 8475. (b) MacDiarmid, A. G. Rev. Mod .

3 Skotheim, T. J.; Elsenbaumer, R. L.; Reynolds, J. R. Eds. Handbook of conducting

polymers 2nd edn. Dekker, New York,1998 .

en, J. C. Adv. Funct , Mater . 2001 , 11 , 15.

10 Nalwa, H. S. Ed. Handbook of organic conductive molecules and polymers , John

W 1996.

11 Mark, J. A. Ed. Physical properties of polymers handbook , American Institute of

Physics, New York,1996.

12 Tallman, D. E.; Spinks, G.; Dominis, A.; Wallace, G. G. J . Solid State Electrochem .

2002 , 6 , 73.

13 Swager, T. M.Chem . Res. Toxicol . 2002 , 15, 125.

id, A. G.; Chiang, C. K.; Heeger, A. J.

Phys . 2001 , 73 , 701. (c) Shirakawa, H. Rev. Mod . Phys . 2001 , 73, 713.

4 Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998 , 37 , 403.

5 Mitschke, U.; Buerle, P. J . Mater . Chem . 2000 , 10, 1471.

6 Greiner, A.; Weder, C. Light-emitting diodes, In: Kroschwitz J. I. Ed. Encyclopedia

of polymer science and technology 3rd edn. Wiley-Interscience, New York, 2003 ,

Vol. 3 p 87.

7 Horowitz, G. Adv. Mater . 1998 , 10, 365.

8 Moerner, W. E.; Silence, S. M.Chem . Rev. 1994 , 94, 127.

9 Brabec, C. J.; Sariciftci, N. S.; Hummel

iley & Sons, New York,

46


49/171

14 (a) Karg, S.; Riess, W.; Meier, M.; Schwoerer, M.Synth. Met . 1993 , 55-57 , 4186.

17 1, 40, 2581.

)

stein, A. G. Phys. Rev.

et, J. P.; Oh, E. J.; Weisinger, J. M.; Min, Y.; MacDiarmid, A. G.;

7. (e) Bao, Z.; Dodabalapur, A.; Lovinger,

iebooms, R.; Menon, R.;

.;

. 1997 , 84 , 71.

. A.; Friend, R. H. Nature 1988 , 335 , 137. (c) Pichler,

edev, E.; Dittrich, T.; Petrova-Koch, V.; Karg, S.; Brtting, W. Appl .

H.; Scherf, U.; Hrhold, H. H. J .

Chem . Phys . 1999 , 110 , 9214. (f) Kho, C. T.; Chiou, W. H.Synth. Met . 1997 , 88,

(b) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.Chem .

Rev. 2005 , 105 , 1491.15 Roth, S; Caroll, D. Eds.One-dimensional metals , 2nd edn. Wiley-VCH, Weinheim,

2004.

16 Peierls, R. E.Quantum Theory of Solids , Clarendon Press, Oxford,1955 .

MacDiarmid, A. G. Angew. Chem. Int. Ed. 200

18 (a) Cao, Y.; Andreatta, A.; Heeger, A. J.; Smith, P. Polymer 1989 , 30 , 2305. (b

Wang, Z. H.; Li, C.; Scherr, E. M.; MacDiarmid, A. G.; Ep

Lett . 1991 , 66 , 1745. (c) MacDiarmid, A. G.; Min, Y.; Wiesinger, J. M.; Oh, E. J.;

Scherr, E. M.; Epstein, A. J.Synth. Met . 1993 , 55, 753. (d) Joo, J.; Oblakowski, Z.;

Du, G.; Poug

Epstein, A. J. Phys. Rev. B 1994 , 49, 297

A. J. Appl. Phys. Lett . 1996 , 69, 4108. (f) Aleshin, A.; K

Wudl, F.; Heeger, A. J. Phys. Rev. B 1997 , 56 , 3659. (g) Aleshin, A.; Kiebooms, R

Reghu, M.; Heeger, A. J.Synth. Met . 1997 , 90, 61. (h) Kim, J. H.; Sung, H. K.;

Kim, H. J.; Yoon, C. O.; Lee, H.Synth. Met

19 (a) Ebisawa, F.; Kurokawa, T.; Nara, S. J . App . Phys . 1983 , 54 , 3255. (b)

Burroughes, J. H.; Jones, C

K.; Jarett, C. P.; Friend, R. H.; Ratier, B.; Moliton, A. J . Appl . Phys . 1995 , 77 ,

3523. (d) Leb

Phys . Lett . 1997 , 71 , 2686. (e) Hertel, D.; Bssler,

47


50/171

.;

4.

20 own, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.;

, P.; de Leeuw, D. M. Nature 1999 , 401 , 685. (b) sterbacka, R.; An, C. P.;

ehee, M.

. Macromolecules 2005 ,

21

. Soc. 2003 , 125 , 13656.

22

23

23. (g) Pinto, N. J.; Johnson, A. T. Jr.; MacDiarmid, A. G.; Mueller, C. H

Theofylaktos, N.; Robinson, D. C.; Miranda, F. A. Appl . Phys . Lett . 2003 , 83, 424

(a) Sirringhaus, H.; Br Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W;

Herwig

Jiang, X. M.; Vardey, Z. V.Science 2000 , 287 , 839. (c) Kline, R. J.; McG

D.; Kadnikova E. N.; Liu, J.; Frchet, J. M. J.; Toney, M. F

38 , 3312. (d) Chang, J. F.; Sun, B.; Breiby, D. W.; Nielsen, M. M.; Slling, T. I.;

Giles, M.; McCulloch, I.; Sirringhaus, H.Chem . Mater . 2004 , 16 , 4772. (e)

Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.;

Grell, M.; Bradley, D. D. C. Appl . Phys . Lett . 2000 , 77 , 406. (f) Wang, G.;

Swensen, J.; Moses, D.; Heeger, A. J. J . Appl . Phys . 2003 , 93 , 6137.

(a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater . 2002 , 14, 99. (b) Katz,

H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; Lin, Y. Y.;

Dodabalapur, A. Nature 2000 , 404 , 478. (c) Pappenfus, T. M.; Chesterfield, R. J.;

Frisbie, C. D.; Mann, K. R.; Casado, J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc.

2002 , 124 , 4184. (d) Babel, A.; Jenekhe S. A. J . Am. Chem

(e) Chua, L. L.; Zaumsell, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus,

H.; Friend, R. H. Nature , 2005 , 434 , 194.

Kawase, T.; Shimoda, T.; Newsome, C.; Sirringhaus, H.; Friend, R. H.Thin Solid

Films , 2003 , 438 - 439 , 279.

Wang, J. Z.; Zheng, Z. H.; Li, H. W.; Huck, W. T. S.; Sirringhaus, H. Nature Mat .

2004 , 3, 171.

48


51/171

24

26

27

28 (

ourmizaie, F.;

29 . G.; MacDiarmid, A. G.; Epstein, A. J.

30 C. Coord. Chem. Rev. 1996 , 147 , 339. (b) Clot, O.;

31 ynolds, J. R.Macromolecules 1996 , 29 , 7629.

Bao, Z.; Feng, Y.; Dodabalpur, A; Raju, V. R.; Lovinger, A. J.Chem . Mater . 1997 ,

9, 1299.

25 Bao, Z.; Rogers, J. A.; Katz, H. E. J . Mater . Chem . 1999 , 9, 1895.Bcklund, T. G.; Sandbert, H. G. O.; sterbacka, R.; Stubb, H.; Mkel, T.; Jussila,

S. Synth. Met . 2005 , 148 , 87.

Brdas, J. L.; Meyers, F.; Heeger, A. J. Conjugated Polymers: On the Parallel

Between the Electrical Conduction Mechanism and the Nonlinear Optical

Response. InOrganic Molecules for Nonlinear Optics and Photonics Messier, J.;

Prasad, P.N.; Kajzar, F. Eds. Kluwer Academic Publishers: Dordrecht,1991 , 25.

a) Li, X. C.; Yong, T. M.; Gruner, J.; Holmes, A. B.; Moratti, S. C.; Cacialli, F.;

Friend; R. H.Synth. Met . 1997 , 84 , 437. (b) Liu, G.; Freund, M. S.Macromolecules

1997 , 30 , 5660. (c) Roitman, D. B.; Antoniadis, H,; Helbing, R.; P

Sheats, J. R. Proc. SPIE 1998 , 3476 , 232. (d) Muller, D.; Gross, M.; Meerholz, K.;

Braig, T.; Bayerl, M. S.; Bielefeldt, F.; Nuyken, O.Synth. Met . 2000 , 111 , 34.

Schanze, K. S.; Bergstedt, T. S.; Hauser, B. T.; Cavalaheiro, C. S. P. Langmuir

2000 , 16 , 795. (e) Inaoka, S.; Advincula, R.Macromolecules 2002 , 35, 2426. (f)

Taranekar, P.; Baba, A.; Fulghum, T. M.; Advincula, R.Macromolecules 2005 , 38,

3679.

Angelopoulos, M.; Dipietro, R.; Zheng, W

Synth. Met . 1997 , 84, 35.

(a) Deronzier, A.; Moutet, J.

Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2000 , 122 , 10456.

Kumar, A.; Re

49


52/171

lecules 2000 , 33, 5151. (b) Joo, J.; Lee, J. K.; Baeck, J. S.; Kim, K. H.;

34 ara, S.Tetetrahedron Lett. 1999 , 40, 3009.

36

37

39

40

41

42

43

44

46 . 2000 , 100 , 1605.

32 (a) Joo, J.; Lee, J. K.; Lee, S. Y.; Jang, K. S.; Oh, E. J.; Epstein, A. J.

Macromo

Oh, E. J.; Epstein, A. J.Synth. Met . 2001 , 117 , 45. 33 Wright, M. E.Macromolecules , 1989 , 22, 3256.

Hirao, T.; Yamaguchi, S. Fukuh

35 Nielsen, K. T.; Spanggaard, H.; Krebs, F. C.Macromolecules 2005 , 38, 1180.

Lavastre, O.; Cabioch, S.; Dixneuf, P. H.; Sedlacek, J.; Vohlidal, J.

Macromolecules 1999 , 32, 4477.

Manners I. Angew. Chem. Int. Ed. Engl. 1996 , 35 , 1062.

38 Rehahn M. Acta Polymer 1998 , 49 , 201.

Ley, K. D.; Whittle, C. E.; Bartberger, M. D.; Schanze, K. S. J. Am. Chem. Soc.

1997 , 119 , 3423.

Wong C. T.; Chan W. K. Adv. Mater. 1999 , 11 , 455.

Huber, C.; Bangerter, F.; Caseri, W.; Weder, C. J. Am. Chem. Soc. 2001 , 123 , 3857.

Albinati, A.; Caseri, W. R.; Pregosin, P. S.Organometallics 1987 , 6 , 788.

Scherf, U.Top. Curr. Chem. 1999, 201 , 163.

Cantow, H. J. Ed. Advances in Polymer Science , Vol 63, Springer-Verlag, Berlin,

1998.

45 Giesa, R. J. M. S. Rev. Macromol. Chem. Phys. 1996, C36 , 631.

Bunz, U. H. F.Chem. Rev

47 Bunz, U. H. F. Acc. Chem. Res. 2001 , 34, 998.

50


53/171

s utilizing in situ transition metal

H. Ed.,

51

55 ager, T. M.Curr. Opin. Chem. Biol. 2002 , 4, 715.

om Synthesis to Applications ;

59 rician, G.; Weder, C. Electronic Properties of PAEs. In: Weder, C. Ed.

ances in Polymer

ol. 177: 209.

98 , 97 , 123.

61 Ofer, D.; Swager, T. M.; Wrighton, M. S.Chem. Mater. 1995 , 7 , 418.

62 Trumbo, T. R.; Marvel, C. S. J . Polym . Sci. Part A, 1987 , 25, 1027.

48 Bunz, U. H. F. 2002 , The ADIMET reaction: synthesis and properties of

poly(dialkylparaphenyleneethynylene)s. In: Astruc, D. Ed.,Modern arene

chemistry . VCH-Wiley, 217.49 Bunz, U. H. F. 2003 , Acyclic diyne metathesi

catalysts: an efficient access to alkyne-bridged polymers. In: Grubbs, R.

Handbook of metathesis . VCH-Wiley, Vol. 3: 345.

50 Yamamoto, T. Bull. Chem. Soc. Jpn. 1999 , 72, 621.

Yamamoto, T. Synlett . 2003 , 425.

52 Pinto, M. R.; Schanze, K. S.Synthesis 2002, 1293.

53 Swager, T. M. Acc. Chem. Res. 1998 , 31 , 201.

54 McQuade, D. T.; Pullen, A. E.; Swager, T. M.Chem. Rev. 2000 , 100 , 2537.

Wosnick, J. H.; Sw

56 Moore, J. S. Acc. Chem. Res. 1997 , 30, 402.

57 Hill D. J.; Mio M. J.; Prince R. B.; Hughes T. S.; Moore J. S.Chem. Rev. 2001 ,

101 , 3893.

58 Weder, C. Ed. Poly(arylene ethynylene)s Fr

Advances in Polymer Science Series Vol. 177; Springer, Heidelberg,2005 .

Voske

Poly(arylene ethynylene)s From Synthesis to Applications ; Adv

Science Series Springer, Heidelberg,2005 , V

60 Montali, A.; Smith, P.; Weder, C.Synth. Met. 19

51


54/171

64

66 5, 23.

63 Weder, C.; Wrighton, M. S.Macromolecules 1996 , 29, 5157.

Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P.Science 1998 , 279 ,

835.65 Weder, C.; Wrighton, M. S.; Spreiter, R.; Bosshard, C.; Gnter, P. J. Phys. Chem.

1996 , 100 , 18931.

Holliday, B. J.; Swager, T. M.Chem . Commun . 200

67 Wolf, M. O. Adv. Mater . 2001 , 13 , 545.

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Chapter 2: Scope and Objectives

Motivated by the significant interest in conjugated polymers with high charge

carrier mobility, this thesis is focused on the design, synthesis, processing and

characterization of cross-linked conjugated polymer systems or conjugated polymer

networks . As summarized in Chapter 1.2, the charge carrier mobility in conjugated

polymers is typically governed by the disorder effects. While exciting progress has been

documented in the case of semiconducting polymer systems with high degree of

molecular order and/or orientation, the high carrier mobility in such systems comes at the

expense of elaborate processing protocols that are rather incompatible with low-cost

manufacturing processes such as inkjet printing, screen printing or roll-to-roll processing.

In an orthogonal approach we postulated that the charge carrier mobility (and other

properties) in conjugated polymers can also be improved by the incorporation of

conjugated crosslinks. It was the objective of the present thesis to prove this hypothesis.

With this overall goal in mind, an experimental research program was designed that

sought to (a) develop synthetic routes for the prepartation of well-defined conjugated

polymer networks with covalent as well as non-covalent conjugated cross-links; (b)

model adequate processing protocols for these materials; and (c) conduct in-depth studies

of the charge transport in these new materials.

Among the various families of conjugated polymers, poly( p-phenylene

ethynylene)s (PPE) encompass an interesting array of materials properties as summarized

earlier in Chapter 1.4, and therefore PPEs were elected as the central materials platform

for the present study. Rather surprisingly, the charge transport properties of

representatives of this family have hitherto not been studied. Thus, in order to develop a

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thorough understanding of this most important aspect, Chapter 3 presents an in-depth

study on the charge transport of poly[2,5-dioctyloxy-1,4-diethynyl-phenylene- alt -2,5-

bis(2'-ethylhexyloxy)-1,4-phenylene] (EHO-OPPE), on the basis of time-of-flight (TOF)

experiments.

In order to investigate the influence of presumably non-conjugated cross-links on

charge transport characteristics of EHO-OPPE, Chapter 4 presents a study of the charge

transport in EHO-OPPE-Pt II networks that were synthesized according to previously

reported protocols via ligand-exchange reactions between the polymer and a low

molecular PtII

complex.The synthesis of organometallic PPE networks that incorporate Pt 0, as a

presumably conjugated cross-linker is reported in Chapter 5, together with a model study

on low-molecular-weight Pt 0 model compounds and an in-depth characterization of the

optic and electronic properties of these polymer systems. Particular attention was, of

course, given to the investigation of the charge transport characterization, of the EHO-

OPPE-Pt 0-EHO-OPPE networks produced.

Incorporation of auxiliary ligands in the conjugated polymer backbone may allow

an access to a plethora of metals centers for obtaining the polymer-metal networks. With

this approach in mind 2,2-bipyridine, was incorporated in the PPE backbone. Chapter 6

discusses the synthesis of PPEs having varying amounts of incorporated 2,2-bipyridine

as well as their organometallic networks with transition metals. Systematic in-depth

characterization of optic properties of these polymers and their organometallic networks

was performed and is reported in Chapter 6.

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In an alternate approach, PPE networks containing covalent cross-links were also

synthesized. Importantly to facilitate the processing, an approach was developed to

synthesize millimeter to nanometer sized particles of these networks structures. The

detailed synthetic protocols and the optical characterization of these materials in bulk and

in the form of particles is further discussed in Chapter 7.

The results obtained during the present research initiative and the insights gained

in the respective Chapters are summarized in Chapter 8. In addition, possible follow-up

research initiatives are also presented in the outlook.

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Chapter 3: Charge Carrier Transport in Poly( p -phenylene ethynylene)s *

3.1 Introduction

As summarized in Chapter 1.4, poly( p-phenylene ethynylene) (PPE) derivatives

have attracted the attention of a number of research groups. 1 Rather surprisingly,

however, the charge carrier mobilities of PAEs have hitherto been completely

unexplored. In this chapter, we present an in-depth study of the charge transport

properties of poly[2,5-dioctyloxy-1,4-diethynyl-phenylene- alt -2,5,-bis(2'-ethylhexyloxy)-

1,4-phenylene] (EHO-OPPE, 4, Figure 3.1), 2 a soluble poly( p- phenylene ethynylene)

derivative, which i

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