Antti T. Myller

DissertationsDepartment of ChemistryUniversity of Eastern Finland

No. 138 (2016)

108/2011 KALLIO Juha: Structural studies of Ascomycete laccases – Insights into the reaction pathways109/2011 KINNUNEN Niko: Methane combustion activity of Al2O3-supported Pd, Pt, and Pd-Pt catalysts: Experimental and theoretical studies110/2011 TORVINEN Mika: Mass spectrometric studies of host-guest complexes of glucosylcalixarenes111/2012 KONTKANEN Maija-Liisa: Catalyst carrier studies for 1-hexene hydroformulation: cross-linked poly(4-vinylpyridine), nano zinc oxide and one-dimensional ruthenium polymer112/2012KORHONENTuulia:Thewettabilitypropertiesofnano-andmicromodifiedpaintsurfaces113/2012JOKI-KORPELAFatima:Functionalpolyurethane-basedfilmsandcoatings114/2012 LAURILA Elina: Non-covalent interactions in Rh, Ru, Os, and Ag complexes115/2012 MAKSIMAINEN Mirko: Structural studies of Trichoderma reesei, Aspergillus oryzae and Bacillus circulans sp. alkalophilus beta-galactosidases – Novel insights into a structure-function relationship116/2012 PÖLLÄNEN Maija: Morphological, thermal, mechanical, and tribological studies of polyethylene compositesreinforcedwithmicro–andnanofillers117/2013LAINEAnniina:Elementaryreactionsinmetallocene/methylaluminoxanecatalyzedpolyolefin synthesis118/2013TIMONENJuri:Synthesis,characterizationandanti-inflammatoryeffectsofsubstitutedcoumarin derivatives119/2013 TAKKUNEN Laura: Three-dimensional roughness analysis for multiscale textured surfaces: Quantitative characterization and simulation of micro- and nanoscale structures120/2014 STENBERG Henna: Studies of self-organizing layered coatings121/2014 KEKÄLÄINEN Timo: Characterization of petroleum and bio-oil samples by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium dichloride and its performance as a support in the Ziegler-Natta catalytic system123/2014 PIRINEN Sami: Studies on MgCl2/ether supports in Ziegler–Natta catalysts for ethylene polymerization124/2014 KORPELA Tarmo: Friction and wear of micro-structured polymer surfaces125/2014 HUOVINEN Eero: Fabrication of hierarchically structured polymer surfaces126/2014 EROLA Markus: Synthesis of colloidal gold and polymer particles and use of the particles in preparation of hierarchical structures with self-assembly127/2015 KOSKINEN Laura: Structural and computational studies on the coordinative nature of halogen bonding128/2015 TUIKKA Matti: Crystal engineering studies of barium bisphosphonates, iodine bridged ruthenium complexes, and copper chlorides129/2015JIANGYu:Modificationandapplicationsofmicro-structuredpolymersurfaces130/2015 TABERMAN Helena: Structure and function of carbohydrate-modifying enzymes 131/2015KUKLINMikhailS.:Towardsoptimizationofmetaloceneolefinpolymerizationcatalystsvia structuralmodifications:acomputationalapproach132/2015SALSTELAJanne:Influenceofsurfacestructuringonphysicalandmechanicalpropertiesof polymer-cellulosefibercompositesandmetal-polymercompositejoints133/2015 CHAUDRI Adil Maqsood: Tribological behavior of the polymers used in drug delivery devices134/2015 HILLI Yulia: The structure-activity relationship of Pd-Ni three-way catalysts for H2S suppression135/2016 SUN Linlin: The effects of structural and environmental factors on the swelling behavior of Montmorillonite-Beidellite smectics: a molecular dynamics approach136/2016 OFORI Albert: Inter- and intramolecular interactions in the stabilization and coordination of palladium and silver complexes: DFT and QTAIM studies137/2016 LAVIKAINEN Lasse: The structure and surfaces of 2:1 phyllosilicate clay minerals

The Effect of a Coupling Agent on the Formation of Area-selective Monolayers of Iron α-Octabutoxy Phthalocyanine on a Nano-patterned Titanium Dioxide Carrier

Antti T. M

yller: The Effect of a C

oupling Agent on the Form

ation of Area-selective M

onolayers of Iron a-Octabutoxy P

hthalocyanine on a Nano-patterned Titanium

Dioxide C

arrier 138

O

O O

O

O

OO

O

N

N

N

N

N

N

N

NFe

CH3

CH3

CH3

CH3 CH3

CH3

CH3

CH3

NH2

O

P

OH

OHO

NH3

+

O

P

OH

O

O-

TiO2

Glass

The Effect of a Coupling Agent on the Formationof Area-selective Monolayers of Iron -Octabutoxy

Phthalocyanine on a Nano-patterned TitaniumDioxide Carrier

Antti T. Myller

Department of ChemistryUniversity of Eastern Finland

Finland

Joensuu 2016

Antti MyllerDepartment of Chemistry, University of Eastern FinlandP.O. Box 111, FI-80101 Joensuu Finland

SupervisorProf. Tuula T. Pakkanen, University of Eastern Finland

RefereesProf. Risto Laitinen, University of OuluProf. Timo Repo, University of Helsinki

OpponentD. Sc. (Tech.) Riikka Puurunen, VTT, Technical Research Center of Finland.

To be presented with the permission of the Faculty of Science and Forestry of theUniversity of Eastern Finland for public criticism in Auditorium N100, Yliopistokatu7, Joensuu, on October 21st, 2016, at 12 o’clock noon.

Copyright © 2016 Antti Myller

ISBN: 978-952-61-2306-6 (pdf)ISBN: 978-952-61-2255-7 (print)ISSNL: 2242-1033ISSN: 2242-1033

Grano Oy JoensuuJyväskylä 2016

3

ABSTRACT

Metal phthalocyanines are a class of complexes that possess various useful thin filmapplications in the field of photovoltaic cells, catalytic degradation of organicmolecules and high sensitivity gas sensors.

All of the metal phthalocyanine applications have a common problem: a poorlycontrolled structure of the complex film. An optimal thickness of the film is crucial inphotovoltaic applications, a high surface area is needed for catalytic applications, andgas sensors can deteriorate due to the migration of metal phthalocyanine thin film. Theproblems with thin film metal phthalocyanine applications can be addressed by bindingthe complexes to a solid carrier by using a coupling agent. The carrier offers a highersurface area than a traditional bulk film. The coupling agent allows better control overthe thin film thickness and can strengthen the bonding of the complexes to the carriersurfaces, thus reducing the migration tendencies.

Titanium dioxide (TiO2) was used as a carrier for metal phthalocyanine thin filmswhen treated with a suitable coupling agent. A nano-patterned thin film of TiO2 on aglass substrate was area-selectively amino-functionalized with a simple 2-aminoethyldihydrogen phosphate (AEPH2) solution immersion. AEPH2 is a small bifunctionalzwitterion that is capable of bonding strongly to TiO2 surfaces, thus creating amino-functionalized TiO2 surfaces. A thin film of iron(II) -octabutoxy phthalocyaninecomplex (Fe(OBu)8Pc) was deposited on the nano-patterned TiO2 surfaces. Thethickness and area-selectivity of the deposited thin film of the metal phthalocyaninecomplex depended on both the functionality of the target surface and on the polarity ofthe solvent.

By using tetrahydrofuran (THF) as a deposition solvent on amino-functionalized nano-patterned TiO2 thin films, a monomolecular layer of Fe(OBu)8Pc was prepared. TheFe(OBu)8Pc monolayer area-selectively replicates the nano-pattern of the TiO2 thinfilm while leaving surface sites of the glass substrate unmodified. The monolayer wasstrongly bonded to the surface via the coupling agent, thereby eliminating themigration problems of gas sensor application. Surfaces sites of the glass substrateremained free for further functionalization, offering an approach that creates aselectively functionalized multifunctional thin film surface that can simultaneouslyhave multiple different gas sensing or catalytic applications.

4

LIST OF ORIGINAL PUBLICATIONSThis dissertation is a summary of the following original publications I-III.

I The pH behavior of a 2-aminoethyl dihydrogen phosphate zwitterionstudied with NMR-titrations, A. T. Myller, J. J. Karhe, M. Haukka, T. T.Pakkanen, Journal of Molecular Structure, 1033 (2013) 171–175.

II Preparation of Aminofunctionalized TiO2 Surfaces by Binding ofOrganophosphates, A. T. Myller, J. J. Karhe, T. T. Pakkanen, AppliedSurface Science, 257 (2010) 1616-1622.

III Area-selective Monolayer Deposition of Iron(II) -Octabutoxy-phthalocyanine Complex on Nano-patterned TiO2 Thin Films on a GlassSubstrate, A.T. Myller, I. Koshevoy, M. Järn, Q. Xu, M. Linden, T.T.Pakkanen, Thin Solid Films, 616 (2016) 579-586.

The author has carried out all of the experimental work in publications I-III, except theHCN elemental analysis in publications I and III, part of the 1H NMR couplingmeasurements in publications I and II, the single crystal structure determinations by X-ray diffraction in publication II and III, and the XPS measurements in publication III.The author has written the manuscripts for publications I-III.

5

CONTENTS

ABSTRACT ................................................................................................................... 3

LIST OF ORIGINAL PUBLICATIONS .................................................................... 4

CONTENTS ................................................................................................................... 5

ABBREVIATIONS........................................................................................................ 6

1. INTRODUCTION ..................................................................................................... 71.1. CARRIER ..................................................................................................................................... 11

1.2. COUPLING AGENT .................................................................................................................... 12

1.3. ACTIVE MOLECULE .................................................................................................................. 13

1.4. APPLICATIONS .......................................................................................................................... 15

1.5. AIMS OF THE STUDY ................................................................................................................ 17

2. EXPERIMENTAL .................................................................................................. 182.1. MATERIALS ................................................................................................................................ 18

2.2. INSTRUMENTS AND METHODS ............................................................................................. 18

2.3. TITRATION OF NMR SAMPLES ............................................................................................... 20

2.4. SYNTHESIS OF PHTHALOCYANINE COMPLEXES .............................................................. 20

2.5. FUNCTIONALIZATION OF TIO2 POWDERS........................................................................... 21

2.6. IMMOBILIZATION OF METAL COMPLEXES TO TIO2 THIN FILMS .................................. 21

3. RESULTS AND DISCUSSION .............................................................................. 223.1. TIO2 CARRIERS II, III .................................................................................................................... 22

3.2. COUPLING AGENT I,II ................................................................................................................ 23

3.3. IMMOBILIZATION OF COUPLING AGENTS ON TIO2 CARRIERS I, II, III ............................. 25

3.4. SURFACE STRUCTURE OF COUPLING AGENT MODIFIED CARRIERS II ........................ 28

3.5. ACTIVE MOLECULE III .............................................................................................................. 29

3.6. BONDING BETWEEN AMINO GROUP AND METAL COMPLEXES III................................ 31

3.7. METAL PHTHALOCYANINE LAYER ON NANO-PATTERNED TIO2 THIN FILM III ......... 33

4. CONCLUSIONS ...................................................................................................... 35

ACKNOWLEDGMENTS ........................................................................................... 36

REFERENCES ............................................................................................................ 37

6

ABBREVIATIONS

AEPH2 2-Aminoethyl dihydrogen phosphateAEPHNH4 2-Aminoethyl hydrogen ammonium phosphateALD Atomic layer depositionBET Brunauer–Emmett–Teller theoryBuNH2 n-Butyl amineCVD Chemical vapor depositionDMF N,N-dimethylformamideDSC Differential scanning calorimetryEISA Evaporation-induced self-assemblyFe(OBu)8Pc Iron(II) -octabutoxy phthalocyanine complexFWHM Full width at half maximumHCN Hydrogen, carbon, and nitrogenHOMO Highest occupied molecular orbitalIR InfraredLUMO Lowest unoccupied molecular orbitalMCD Magnetic circular dichroismMPc Metal phthalocyanineNIR Near infraredNMR Nuclear magnetic resonancePc PhthalocyaninePET Polyethylene terephthalateSAM Self-assembled monolayerSiO2 Silicon dioxide, silicaTGA Thermogravimetric analysisTHF TetrahydrofuranTiO2 Titanium dioxide, titaniaUV UltravioletVis Visible light

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1. INTRODUCTION

Nano-patterned surfaces of inorganic oxides with a monolayer of a functional metalphthalocyanine complex (MPc) have numerous interesting applications 1. Metalphthalocyanine complexes are compounds capable of a multitude of usefulapplications, such as sensing gases 2, 3, 4 from air, vehicle exhausts, and liquids 5 fromindustrial wastes 6. A MPc can function in flexible, cheap and high efficiencyphotovoltaic cells 7, 8, thin film transistors 9, and light emitting diodes 10, 11. In addition,catalytic applications 12, even catalytic degradation of dangerous industrial wastes 13, 14,

15, can be achieved by using MPcs.

Since the discovery of a phthalocyanine ligand in the early 20th century, 16 MPccomplexes have been used in various applications from fabric dyes to quantumcomputing. The intense blue-green color combined with exceptional thermal, UV-ray,and chemical resistance have made metal phthalocyanines a notable source of syntheticdyes that are still used today in, e.g., recordable CDs and oil paints (phthalo blue).

Gas sensing applications were originally based on a simple bulk material withinterlaced electrodes for monitoring changes in electrical resistance 17. Unfortunately,gas sensors constructed using bulk MPc complex films without any chemical bondingto the solid carrier have several problems. At elevated temperatures, gas sensorsensitivity may decrease due to structural re-organizations that cause grain formation(Figure 1.) and agglomeration (Figure 2.) 18, which lead to a reduced surface area 19, 20.Thicker bulk film sensors also have a poor recovery rate and recovery ratio, even atroom temperature 21, 22. Also, interactions with NOx molecules are known 23, 24 to causesurface migration of phthalocyanine molecules which are not chemically bound to thecarrier surface.

Grain formation

Carrier

Bulk film

Figure 1. Grain formation in a bulk film on a solid carrier.

Agglomeration

Carrier

Bulk film

Figure 2. Agglomeration of a bulk film on a solid carrier.

8

Metal phthalocyanine (MPc) has a similar structure to the light harvesting section ofthe chlorophyll molecule (Figure 3.), making it an effective photosensitizer in dye-sensitized photovoltaic cells 25. With photovoltaic cells the thickness of the active dyecomponent layer is a crucial factor for both the energy efficiency of the cell and cost.Thicker layers have higher production costs, but also the incoming light may not beable to penetrate deeply into the active component, thus causing a lower rate ofharvested electrons per active dye molecules. The chemical bond between the activedye component and charge carrier support increases in the overall efficiency of thecharge transfer. Dye films that are not chemically bound to the supporting layer areprone to mechanical and chemical erosion.

NN

NN

Mg

CH3

CH2

CH3

CH3CH3

O

O

O

CH3

CH3O

O

CH3

CH3

CH3

CH3

CH3

N

N

N

N

N

N

M

Figure 3. MPc structure compared to chlorophyll-a found in photoautotrophic plants,algae and cyanobacteria.

Catalytic applications of MPcs have problems with agglomeration, leaching from thecarrier, and remaining in the end product (Figure 4.). An active catalyst anchored to asolid carrier is commonly known as a heterogeneous catalyst and is largely andsuccessfully used in the chemical industry. Performance of such catalysts isconsiderably affected by the surface area of the active catalyst. Large surface areas canbe achieved by using a carrier with a porous-, micro-, or nanostructure instead of asolid bulk film catalyst.

Leaching

Carrier

Thin film

Figure 4. Leaching of catalyst thin film from a solid carrier.

9

In order to address the various problems associated with thin films of metalphthalocyanine complexes, a new preparation pathway for the nanostructured carrier-bound phthalocyanine films was designed.

The system is based on three main components (Figure 5.):

1. The carrier with a nanostructure offers high surface area and solid structure foractive molecules to rest on. This increases the surface area available for gas sensingand catalytic applications, reduces the amount of active species needed, and shields theactive component from mechanical stress.

2. A coupling agent immobilizes the active molecules to the solid carrier. Thiscomponent enables the carrier to be coated uniformly. On a carrier with a nano-pattern,it is possible to replicate the nano-pattern with the active molecule. The coupling agentcreates a chemical bond with both the carrier and axial vacancy of the active MPccomplex. Such a structure anchors the active species tightly to the surface of the carrierand prevents its migration and aggregation.

3. An active molecule, a metal phthalocyanine complex, has photovoltaic, gas sensingand/or catalytic abilities at very low concentrations. Notable changes inelectromagnetic properties or electric conductivity can be detected when gas binding orcatalytic reaction occurs.

Carrier

Coupling agent Active molecule

Figure 5. Bonding active molecules to a carrier by using coupling agent bonding.

Similar three layer systems have been built on a flexible polyethylene terephthalate(PET) substrate using a TiO2 carrier and phosphate coupling agent with methylene blueas the active photosensitive molecule 26, on a TiO2 carrier using pyridine basedcoupling agent and titanium phthalocyanine complex 27, and on gold and SiO2 surfacesusing long pyridine based coupling agents and ruthenium phthalocyanine complexes 28.The described thin films are not nanostructured, but plain solid surfaces with an evencoverage of an active component.

Preparation of nano-patterned thin films of carrier-supported MPc molecules usuallyrequire the use of time consuming methods, such as atomic layer deposition (ALD) 17,

29, or chemical vapor deposition (CVD) 30, 31. CVD does have the risk of forming threedimensional structures like granules, clusters, wires, and needles 30, 31.

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The carrier used in this research is ultrathin TiO2 film with hexagonally ordered cratersof 30 nm in diameter on a glass substrate. The nano-craters allow adjustment of thetopology of the active molecule layer. By tuning the coupling agent and polarity of thesolvent used in the active molecule deposition, it is possible to area-selectively coatonly the TiO2 portion between the nano-craters, only the nano-craters, or bothstructures of the film with a MPc monolayer (Figure 6.). By using two differentcoupling agents, it is possible to area-selectively functionalize TiO2 sites and SiO2 siteswith different coupling agents.

MPC thin filmTiO2 carrier

SiO2 substrate

Coupling agent

TiO2 binding only SiO2 binding only Both SiO2 and TiO2 binding

Figure 6. Effect of area-selectivity by selection of coupling agent. TiO2 binding only,SiO2 binding only, and binding to both TiO2 and SiO2.

Nano-cratering of the thin film will offer a high active surface area available forapplications and allow control of the wetting properties of the platform 32 for liquidstate catalytic application. It also allows modulation of the surface properties betweenconducting / semiconducting and insulating areas in photovoltaic applications and thetuning of the surface energy to adjust the molecular affinity and recognition for sensingapplications 33.

In summary, the presented pathway allows preparation of nano-patterned TiO2 thinfilms with area-selective surface coverage of active molecules. The method is fastbecause self-assembly requires no specialized equipment, and the active molecule isbonded to the surfaces via the coupling agent, and offers good thermal and mechanicalwear resistance. The nano-craters of the carrier also enable a selective multi-functionalization by subsequent functionalizing of the glass substrate surface sites,resulting in a surface with two different active surface sites.

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1.1. CARRIER

The first component is the solid carrier with a high surface area. Titanium dioxide(TiO2), which has a number of interesting applications as a photocatalyst and as acarrier for photocatalysts 34, heterogeneous catalysts 35, and gas sensors 3. TiO2 alsoworks as a component in photovoltaic cells 7 and in biocompatible cell cultivationscaffolds 36. It is also a good electron acceptor and transport material 37.

Titanium dioxide with an anatase crystal structure has strong Lewis acid sites, weakBrønsted acid sites, and O2- sites (Figure 7.) 38, 39. The surface acid sites are thermallystable and can withstand temperatures of up to 500 oC 40. The chemical surfaceproperties of titanium dioxide also depend on the presence of adsorbates, commonlyphosphates.

Lewis acidic Ti(IV) surface sites of TiO2 act as anchoring points for phosphates, whilethe less acidic hydroxyl Brønsted acid sites remain free. Phosphate modificationeliminates any Lewis acidity on anatase 41, 42, whereas Brønsted acid sites remain freeon the phosphate-loaded surface 43. Particularly in its anatase crystalline form, TiO2

forms strong and stable bonds with phosphonic acids (organic phosphonates) andorganic esters of phosphoric acid (organic phosphates).

Ti-

Ti Ti Ti Ti-

O O O O

O-

OH

Figure 7. Surface sites of anatase, a Lewis acid site, a Brønsted acid site, and an O2-

site.

TiO2 powders in the form of microporous bulk films 44 or nanoparticle thin films 25

have the ability to absorb UV 45, 46, visible light 44 and NIR 47 with high efficiencywhen sensitized with a photon absorbing charge-transfer dye, like phthalocyanine orporphyrin complexes 44. This is due the surface contact between the electron injectingdye molecules and semiconducting TiO2 which acts as the electron acceptor and chargecarrier. The contact surface area between the dye and TiO2 carrier seems to have anotable effect on the photon absorbing efficiency, in which the high-surface areananostructured TiO2 works better than flat bulk films or films containing multiplecoatings of dye molecules 25.

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1.2. COUPLING AGENT

The second component is a bifunctional organic amino phosphate with a short alkylchain: 2-aminoethyl dihydrogen phosphate (NH2(CH2)2OPO3H2) (AEPH2) and the 2-aminoethyl hydrogen ammonium phosphate (NH2(CH2)2OPO3HNH4) (AEPHNH4)synthesized especially for this study.

2-Aminoethyl dihydrogen phosphate (AEPH2) is a bifunctional short-chained organicmolecule that has both a phosphate group and an amino group. Its synthesis wasoriginally patented in 1953 48, forgotten, and reinvented in 1972 49. AEPH2 is made bysimple esterification of 2-aminoethanol and orthophosphoric acid. AEPH2 is a fairlyunknown compound even it is a potential surface modification agent. The only use ofAEPH2 with TiO2 has been in anti-corrosion paints as an additive for TiO2.

The two functional groups of AEPH2, the amino group and the phosphate group canboth work as a proton donating group. The amino group is amphoteric, so it can alsoact as a proton acceptor. Since AEPH2 has both proton donating and acceptingfunctional groups, it is possible for it to exist in a form known as a zwitterion (Figure8.). A zwitterion has an overall neutral charge, but is a molecule that has bothpositively and negatively charged functional groups formed by either intramolecularproton transfer or by extramolecular proton exchange via a protic solvent. Zwitterionstructures exist extensively in biological systems such as amino acids and peptides 50.

NH2

OP

OH

OH

O

NH3+

OP

OH

O

O

-

Figure 8. Formation of an AEPH2 zwitterion.

The AEPH2 molecule is a neutral zwitterion in the solid state and in water solutions ata pH range of between 1 and 5. This improves its solubility in aqueous media, but alsolimits pH range where the functionalization of TiO2 can be achieved.

The coupling agent attaches to the TiO2 surface by its phosphate group leaving theamino group available for bonding with the active MPc complex. This amino-functionalized TiO2 surface could potentially work as a catalyst, since amino groupsimmobilized on a carrier are known to catalyze Knoevenagel condensation 51. Aminogroups can also work as anchoring points for the immobilization of biomolecules 52.

13

Selection of AEPH2 was based on its small size, which would allow it to occupy TiO2

surface sites on nano-crater walls and leave room for the bonding of the activecomponent. The short hydrocarbon chain also provides a short path for electrontransfer between the MPc complex and TiO2 charge carrier in semiconductorapplications. The small size of the AEPH2 molecule also increases its solubility inprotic solvents 53.

The NH4 salt of AEPH2 (AEPHNH4) was synthesized and used with the TiO2 due toconflicting results found in the literature suggesting that ammonium salts of organicphosphates would bind to the TiO2 by P-O-Ti complexing bonds 33 instead ofcovalently bonding with Lewis acid sites 43.

1.3. ACTIVE MOLECULE

The third component, iron(II) octabutoxy phthalocyanine (Fe(OBu)8Pc), is a metalcomplex where octabutoxy phthalocyanine acts as a ligand with four coordinationvacancies. The central metal atom is surrounded by the phthalocyanine macrocyclewhich bonds to the central metal atom by four nitrogen atoms while maintaining inmany complexes the overall flat shape of the aromatic structure. The bonding is verystrong and slightly specific depending on the atomic radius 54, 55. Very large ions areunable to reside inside the cavity and can form cup-shaped or saddle-shapedcomplexes, where the ion resides axially off-center of the deformed macrocycle, orsandwiched structures, where the metal atom resides between two phthalocyanineligands 3, 56. This method of bonding is similar to hemeproteins containing a porphyrinring, 57 where the central metal atom is also bound by four nitrogen atoms, but alsosimilar to the 16-crown-4 ethers, where the metal atom binds to oxygen instead ofnitrogen 58.

The metal phthalocyanine complex Fe(OBu)8Pc has been synthesized especially forthis study in order to attach it to the amino-functionalized TiO2 surface. Ironphthalocyanine (FePc) is one of the oldest and most stable metal phthalocyaninecomplexes known. FePc is effective as a gas 59 and liquid 60 phase sensor, but it hasvery poor solubility with any solvents. Radial butoxy substituents of the octabutoxyphthalocyanine ligand provide enhanced solubility in organic polar solvents, thusallowing the Fe(OBu)8Pc complex to be deposited on surfaces at room temperature andwithout any special equipment or highly toxic solvents.

The phthalocyanine (Pc) ligand is similar to benzene, but its delocalized electron cloudis much larger due to the aromatic nature of all its constituent rings (Figure 9.).According to the molecular orbital theory, the benzene rings and pyrrole analoguerings are all aromatic with 6 electrons. The large central ring is aromatic with 16 electrons, but the entire macrocycle is also aromatic with 42 electrons. All of the electrons are located in delocalized bonding orbitals that cover both sides of the

14

macrocycle plane instead of forming localized double bonds, making the macrocycleextremely strong. The electronic structure causes intermolecular interaction betweenthe Pc molecules, known as -stacking, which contributes to its poor solubility. Theradial octabutoxy substituents reduce intermolecular interactions by producing a sterichindrance that reduces the -stacking of macrocycles and provide short aliphatic chainswhich increase solubility in organic solvents.

N

N

NH

N

NH

N

N

N

N-

N-

N-

N-

N-

N-

N-

N-

H H

N-

N-

N-

N-

N-

N-

N-

N-

H H

Figure 9. Delocalized electrons in a phthalocyanine ligand.

Phthalocyanine complexes usually have semiconducting properties, sometimes evenmetallic conductivity at decreased temperatures due to overlapping electron clouds ofstacked phthalocyanine molecules. Electric conductivity allows a MPc to be used inelectronic applications, such as chemical sensors and photovoltaic cells. Some MPccomplexes also have magnetic and electrical properties that can be easily controlledmaking it a good alternative for a short term quantum bit memory 61, with ananosecond scale bit switching and microsecond scale decay. The semi-conductivity isalso very easy to notice by means of spectrophotometry. The strong charge transferabsorption band is the origin of the vibrant blue/green color for which MPc complexesare known 62. The electronic structure of phthalocyanine type complexes, mainly theHOMO-LUMO gap, electron transfer paths 63, 64, 65, and the spin state of metal 66 areinfluenced by changes in the oxidation state of the central metal atom 67, 68, 69 and anysubstituents either at ring structure or at axial sites. The substitution results in changesin spectral 70, 71, 72 and conductive properties 73. Aggregation is also known to causechanges in the electronic transition which thus alters the color of the complex 74, 75, 76, 77.

The addition of an axial ligand to phthalocyanine can cause similar changes in theunpaired electrons of the d orbital of the metal, as binding with gas molecules does 78.Some phthalocyanine complexes with transition metal central atoms may even changetheir electron configuration between n-type and p-type semi-conductivity uponintroduction of axial ligands, depending on the ligand ability to act as a donor oracceptor 79, 80.

Radial ligands can be used to enhance the solubility of the MPc complex, but they canintroduce deformation to the macrocycle 56, 81. Radial ligands on the phthalocyaninemacrocycle also contribute to the activity of the central metal atom 82, but also theresponse rate, sensitivity and reversibility rate 83. Generally, it seems that highly

15

electronegative radial substituents decrease the electron density on themetal/macrocycle ring system, making further reactions with oxidizing agents harderand easier with reducing agents.

1.4. APPLICATIONS

Phthalocyanine complexes, originally used as dyes with high thermal stability, haveraised interest because of their wide variety of applications in important chemical andphysical processes.

Phthalocyanine complexes have been successfully used for enhancing the catalyticproperties of metal oxides, e.g. photocatalytic degradation 12, 84 and photo oxidation 85,

86 of industrial wastes 13, 14 and chemical warfare agents 15. Phthalocyanines canimprove delocalization, separation, and the transfer rate of charge on the metal oxidesurface during the photoreaction. The performance of such a catalyst is heavilyaffected by the amount of the phthalocyanine complex and the surface area of thecatalyst 84, and the pH in the case of aqueous solutions 85.

Phthalocyanine complexes have semi conductive and even metallic properties 87 due toa overlap of electron clouds of adjacent molecules 30, which allows them to be usedin various electronic thin film applications, such as transistors 9, light emitting devices10, photovoltaic cells 7, 8, and chemical sensors for liquids 5 and gases 3, 4.Phthalocyanine complexes can be modified by changing the substituents or the centralmetal atom in order to achieve p-type 88, 89, 90 and n-type 30, 91 semiconductors, or eveninsulators 92.

Dye sensitized photovoltaic cells made with MPc complexes as an absorber layer onthe metal oxide carrier, like TiO2 or ZnO2 are much cheaper to produce than traditionalcrystalline n-and p-type doped silicon semiconductor ones 93. Good photo-stability andphoton absorption capability, even in very thin films, allows the manufacture of lightand flexible photovoltaic cells 93, which are impossible to make with crystalline silicon.

The surface contact interface between the electron injecting dye molecules andsemiconducting electron accepting TiO2 acts as the charge carrier in photovoltaicapplications. The contact surface area between the dye and TiO2 carrier seems to havea notable effect on the photon absorption efficiency. The high-surface area nano-structured TiO2 works better than flat bulk films or multiple coatings of dye molecules25.

Sensors based on metal phthalocyanine complexes are usually characterized as fast andsensitive, in both gaseous 94 and liquid mediums 95. A potentiometric sensor based onMPc complexes can have sub-micromolar detection rates in aqueous solutions 96.Phthalocyanine complexes coordinated with axial ligands and deposited as a thin film

16

on a solid carrier show impressive results for optical gas sensing that can reach a 20ppb detection limit and high selectivity 97. Metal phthalocyanine films also have verygood long-term stability in gas sensing applications 83, 98. Metal phthalocyaninecomplexes can be used to detect a large variety of gaseous molecules 94, like NOx

3, 23,NO2

6, 24, methanol 99, CO 100, O278, 101, NH3

102, and water vapor 92.

Gas sensing applications with CuPc can have a low detection limit of 100 – 50 ppb 22,

92. The behavior of the gas sensor can be tuned with the addition of ligands. With ironporphyrin, a species with a single axial imidazole ligand has stronger bonding with O2

and CO gas molecules compared to unsubstituted iron porphyrin. NO gas has nosignificant difference in bond energy between substituted and unsubstituted ironporphyrin 78. When comparing ZnPc and hexadecafluorinated ZnF16Pc, the fluorinesubstituted species has a better response rate, sensitivity and reversibility rate whenusing NH3 gas 83. When comparing Fe(PhBu2)8Pc and FeCl16Pc, the latter has lowersensitivity when measuring CO 82.

Iron(II) porphyrin complexes, closely related to MPc, are found in a number ofbiologically important metalloproteins 78, 101 known as hemeproteins (Figure 10.). Ironporphyrin complexes are known to function as gas sensors for CO, O2 and NO. Ironporphyrin complexes bound to a SiO2 surface using coupling agents have been used formaking a highly sensitive thin film NO detector 103. The high sensitivity is based on back-bonds between the iron atom and the NO+ ion 104. The working principle of suchthin film detectors is similar to a biological system found in mammalneurotransmission involving nitrosylation of hemeproteins, like cytochromes 105.

OOH

N N

N N

Fe

CH3

CH3

CH2

CH3CH2

CH3

O OH

N

N

N

N

N

N

M

Figure 10. MPc structure compared to Heme B, a hemeprotein found in hemoglobinand myoglobin.

The sensitivity and tuneability of MPc complexes is based on the interactions betweenthe d-shell valence electrons and the delocalized electrons of the phthalocyaninemacrocycle ring. Even a weak intermolecular interaction to the transition metal centercan cause a change in the oxidation state of the central metal atom and in the

17

hybridization of d orbitals. Any changes in the electron configuration of the centralmetal atom can cause a notable change in the UV-Vis 75, 81, 106, 107, Raman 108, IR 3, 6, 23,and XPS spectra 24, as well as in the electric conductivity of the MPc complex 3, 23, 73.

1.5. AIMS OF THE STUDY

The particular target of this work was to study how a coupling agent can control theformation, binding and area-selectivity of monolayers on a nano-patterned titaniumdioxide surface. The main focus was on a bifunctional coupling agent that can beapplied to the surface using a simple solution immersion method. The approach of thiswork offers solutions to problems associated with formation and binding of thin filmsof metal phthalocyanine complexes. The preparation pathway for the nanostructuredcarrier-bound phthalocyanine can be utilized in industrial processes for organicphotovoltaic cells, gas sensors and flexible electronics. The essential aims of the studywere:

1. The study on pH behavior of 2-aminoethyl dihydrogen phosphate (AEPH2) couplingagent in order to optimized the conditions for functionalizing the TiO2 surfaces.

2. Binding and area-selectivity of the coupling agent on the TiO2 substrate. The aimwas to bind the AEPH2 strongly to the TiO2 surface only.

3. Creating a hybrid material that replicates the nano-pattern of the carrier with anactive metal phthalocyanine complex. Various conditions were studied in order tocreate a method that allows a uniform layer of the active molecule and a strong bindingvia the coupling agent.

18

2. EXPERIMENTAL

2.1. MATERIALS

2-Aminoethyl dihydrogen phosphate powder 98% was recrystallized from a 1:3water/ethanol solution before use.

Ninhydrin, ethanol 99.5%, ethyl acetate, acetic acid 99.8%, and heptane were used tomake the ninhydrin test solution for detecting the amino groups on the surface of theAEPH2 treated TiO2 powder samples.

All water used in the research, in titration, washing, elution, and solutions, was 18 Mcm-1 Milli-Q water purified on site. Organic solvents, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were dried over 4Å molecular sieves for 24 h and distilledover sodium in an argon atmosphere prior to use.

The rest of the reagents and solvents were used as they were received from thesuppliers.

2.2. INSTRUMENTS AND METHODS

HCN elemental analyses of AEPH2-treated TiO2 powder samples and phthalocyaninecomplexes were conducted with an Elementar Vario Micro series GC elemental micro-analyzer with a thermal conductivity detector. Sample pyrolysis was done at 950 oCwith the use of oxygen as the burning gas. Helium was the GC carrier gas.

UV-Vis analyses of the (OBu)8Pc ligand, Fe(OBu)8Pc complex and a-(BuNH2)2Fe(OBu)8Pc complex were conducted with a Perkin Elmer Lambda 900 UV-Vis-NIR spectrophotometer. The measuring range was 260 – 860 nm, resolution 1 nm,and the measurement was done in absorption mode. Samples were dissolved in argonbubbled DMF and placed in airtight 10 mm quartz cuvettes inside a glovebox. Argonbubbled DMF was used as a sample reference.

Liquid-phase NMR measurements were used to characterize the reaction between theFe(OBu)8Pc complex and n-butyl amine, and NMR titrations in order to determine thebehavior of AEPH2 as a function of pH. The spectrometer used was a Bruker Avance400 NMR equipped with a 5 mm liquid state probe. For 1H and 13C measurementsTMS and CDCl3 were the internal standards (0.00/0.00 ppm and 7.26/77.16 ppm for 1Hand 13C respectively) 109. For 31P measurements, 85% orthophosphoric acid was theexternal standard (0.00 ppm). Solid-state 13C and 31P CP/MAS measurements were

19

used for AEPH2 treated TiO2 powder samples in order to determine the bonding typeformed between the phosphate group and the TiO2 surface. The spectrometer used wasa Bruker AMX 400 NMR equipped with a 4 mm MAS unit using contact times of 2 msfor 13C measurements and 5 ms for 31P measurements. The spinning rate was 10 kHzand chemical shift measurements were calibrated with the glycine carbonyl 13C signal(176.1 ppm) and hydroxyapatite 31P signal (2.8 ppm) 110 before each samplemeasurement.

Single crystal X-ray diffraction studies of AEPH2 and 4,7-dibutoxy-1H-isoindole-1,3(2H)-dione were conducted with a Bruker Smart ApexII diffractometer. A singlecrystal was immersed in cryo-oil, mounted in a nylon loop and measured at 150 Kusing a Mo K X-ray radiation source ( = 0.71073 Å). The structures were solved bydirect methods using the SHELXS-2013 program with the WinGX 111 graphical userinterface. A semi-empirical absorption correction (SADABS) 112 was applied to alldata.

The X-ray photoelectron spectroscopy (XPS) analyses of nano-patterned TiO2 thinfilms on a glass substrate with AEPH2 and Fe(OBu)8Pc treatments were conductedwith a Thermo Fisher Scientific ESCALAB 250Xi spectrometer using amonochromatic Al K radiation source (1486.7 eV) operated at 20 mA and 15 kV.

SEM images of nano-patterned TiO2 thin films on a glass substrate were taken with aHitachi S4800 electron microscope using 1.0 to 5.0 kV acceleration voltage and 1 to 5µA emission current. Sample thin films were placed on an aluminum holder usingcarbon fiber tape. No sputtering of any kind was administered.

TGA measurements of AEPH2-treated TiO2 powder samples were conducted with aMettler Toledo TGA/STDA851e instrument. Samples were placed in 70 µl aluminacrucibles that had been flash-heated for 15 minutes at 1000 oC in an electric furnace.Samples were stored in a desiccator until use. Differential scanning calorimetry (DSC)measurements of AEPH2 treated TiO2 powder samples, and Fe(OBu)8Pc and a-(BuNH2)2Fe(OBu)8Pc complexes were conducted with a Mettler Toledo DSC821eseries calorimeter. Samples were placed in 40 µl aluminum crucibles and pressedtightly against the bottom of the crucible.

BET surface area measurements for TiO2 powder samples were taken with aMicrometrics ASAP 2010 physisorption apparatus using nitrogen as the adsorption gasand five-point correlation for calculations.Titrations for determination of the pKa values of AEPH2 were performed using aMetrohm 775 Dosimat for titrant dosing and an Orion Model 420A pH meter with acombination electrode for pH measurements.

Ninhydrin tests 113 of AEPH2-treated TiO2 powder samples were conducted by soakingthe powder samples in ninhydrin solution, filtering off the excess, and drying overnight

20

at ambient temperature. Samples treated with coupling agent turned light violet,whereas pure TiO2 remained white.

2.3. TITRATION OF NMR SAMPLES

Two 0.1 M AEPH2 solutions were prepared in D2O, with an initial pH of 3.5. First, theD2O solution of AEPH2 was titrated using 5 M, 1 M, and 0.1 M H2SO4 solutions.Samples for 1H and 31P NMR titrations were taken at intervals of 0.5 pH in the pHrange of 1 to 3.5. The second D2O solution was titrated using 1 M and 0.1 M NaOHsolutions. Samples for 1H and 31P NMR titrations were taken at intervals of 0.5 pH inthe pH range of 3.5 to 12.

In another experiment, a 0.1 M AEPH2 solution in D2O was used and titrations weredone with fully deuterated chemicals. The solution was divided into two separatesolutions with an initial pH of 3.5. This time the titrations were conducted using 35 w-% DCl in D2O for a pH range of 1 to 3.5 and 40 w-% NaOD in D2O for a pH range of3.5 to 12 and samples for 1H and 31P NMR titrations were taken at intervals of 1.0 pH.

2.4. SYNTHESIS OF PHTHALOCYANINE COMPLEXES

Fe(OBu)8Pc was synthesized using a commercial (OBu)8Pc ligand, and a methodadapted from the article by Ding et al 114. (OBu)8Pc ligand, potassium carbonate,ferrous acetate (Fe(COO)2), and dried and argon bubble DMF were packed in a threenecked 100 ml round bottomed flask equipped with an Allihn condenser and needlefiltration unit. Assembly of the reaction apparatus was conducted inside a gloveboxunder a nitrogen atmosphere. The solution was refluxed on an oil bath at 150 oC for 24hours resulting in a dark green solution. The product was cooled using an ice bath andprecipitated with water. The solvent was removed with the filter needle and the productwas washed twice with 200 ml of argon-bubbled water prior to vacuum drying,resulting in a dark green powder. The product was separated by columnchromatography using a glass fritted column with 20 g of silica as an adsorbent. THFwas used as an eluent, the product was collected from the first fraction. The excesseluent was vacuum distilled away before precipitating, filtering, and extracting thepurified product. The product was characterized by NMR, DSC, UV-Vis, and HCNelemental analysis III.

The a-(BuNH2)2Fe(OBu)8Pc complex was synthesized, by initially making a batch ofFe(OBu)8Pc, but after refluxing at 150 oC for 24 hours the temperature was decreasedto 75 oC and a solution of n-butyl amine dissolved in dried and argon bubbled DMFwas injected into the flask under an argon atmosphere. The solution was mixed at 75oC for 1 hour under the argon atmosphere. The product solution was cooled in an ice

21

bath and the product was precipitated with 150 ml of cooled argon bubbled water. Thesolvent was removed with a filter needle and the product was washed twice with 200ml of argon-bubbled water prior to vacuum drying, resulting in a black powder. Thedissolved product was separated by column chromatography under an argonatmosphere using THF as the eluent and using the same method as in the case ofFe(OBu)8Pc. The product separated in the first fraction. Excess eluent was removed invacuum before precipitation from the leftover THF inside the glovebox under anitrogen atmosphere, resulting in a dark olive green powder. The product wascharacterized by NMR, DSC, UV-vis, and HCN elemental analysis III.

2.5. FUNCTIONALIZATION OF TiO2 POWDERS

Initially, 0.01 M AEPH2 solution was prepared with a pH value of 3.48. The TiO2

powder was added to a round-bottomed flask containing the freshly made 0.01 MAEPH2 solution and mixed with magnetic stirrer for 24 hours at ambient temperature.Afterwards, the powder was filtered and washed with water and ethanol using aBüchner funnel and eluted in a Soxhlet extractor with water for 24 hours. The productswere then dried in an oven at 120 oC for 24 hours.

2.6. IMMOBILIZATION OF METAL COMPLEXES TO TiO2 THIN FILMS

TiO2 thin films with 30 nm nano-craters obtained from the Nanofused group (ÅboAkademi University) had been prepared using the EISA method described by Fisher etal. 33 using glass as the substrate material 115. Films were calcined in a furnace for 5minutes at 400 oC and cooled inside a nitrogen filled desiccator prior tofunctionalization.

Half of the films were AEPH2-treated using the method used previously for TiO2

powders. Films were immersed in a 0.01 M AEPH2 water solution for 6 hours in a lightshielded container and then washed with water and ethanol before drying at 110 oC for24 hours. Treated films are then stored in a light shielded desiccator.

Each of the TiO2 thin films, both AEPH2 treated and untreated, were immersed in a0.0001 M Fe(OBu)8Pc solution in dried THF or DMF for 1 hour and then washed withthe solvent and dried at 160 oC for 2 hours. Treated films were then stored in adesiccator that was shielded from light.

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3. RESULTS AND DISCUSSION

Before amino-functionalizing the TiO2 thin films, the suitability of 2-aminoethyldihydrogen phosphate (AEPH2) as a coupling agent was tested by using the TiO2

powder as a model system. The TiO2 powder had the same anatase structure as theprepared thin films 33. The powder model was used to resolve the surface coverage,orientation, and bonding type of the AEPH2 on the TiO2 surface.

Before depositing the Fe(OBu)8Pc complex on the amino-functionalized surface of theTiO2 thin films the bonding between Fe(OBu)8Pc and an amino group was examined.This was done by using a solution state model system where a n-butyl amine wasreacted with the Fe(OBu)8Pc complex.

Finally, thin films of the Fe(OBu)8Pc complex were deposited on nano-patterned TiO2

carriers on a glass substrates. Conditions, including solvents and amino-functionalitysuitable for an area-selective deposition of Fe(OBu)8Pc monolayers on nano-patternedTiO2 carrier were investigated.

3.1. TiO2 CARRIERS II, III

Two kinds of TiO2 carriers were used: a bulk powder for a model system to examinethe binding of AEPH2 on the TiO2 surface, and a nano-patterned TiO2 thin film on aglass substrate to study chemisorption of the active Fe(OBu)8Pc complex on the TiO2

surface.

Bulk TiO2 powder with a BET surface area of 9.454 ± 0.027 m2/g was used to examinethe coupling agent binding. The TiO2 thin films were not suitable for analyzation of theformed bonds between AEPH2 and the TiO2 surface, as the coupling agent treated filmswere too thin for most of the spectroscopic instruments to reliably detect the boundAEPH2.

Ultrathin (2 nm) TiO2 films with hexagonally ordered craters of 30 nm in diameter on aglass substrate were used for active complex deposition and the surface selectivitystudy. Thin films were manufactured by Järn et al. using the EISA dip-coating method33, 115 on a glass substrate. SEM imaging of an untreated plain TiO2 thin film (Figure11.) revealed an even distribution of nano-crater features with an approximately 30 nmdiameter, a few larger craters, and some solid particles embedded in the film.

The TiO2 thin films, if untreated, are superhydrophilic, most likely due to TiO2 ‘sability to adsorb water, and hydrophobic if treated with hydrocarbons andfluorocarbons 32. The nano-craters and hydrophilicity of the TiO2 thin film can

23

contribute to the area-selectivity of the coupling agent binding. After amino-functionalized, the thin films are no longer superhydrophilic, which can enhance thearea-selectivity of the active complex deposition, and may allow control of the wettingproperties of the thin film in solution applications.

Figure 11. Untreated nano-cratered TiO2 thin film.

The nano-crater structure of the TiO2 carrier thin film enables selectivemultifunctionalization through the use of separate treatments of the TiO2 carrier ridgesand glass substrate on crater floors e.g. with phosphates and silanes, respectively.Differences in the chemical properties of substrate and carrier surfaces results in twotypes of surface sites having different functionalities which allow modulation of thesurface between conducting and insulating areas, and tuning of the surface energy forthe molecular affinity and recognition in sensing applications 33.

3.2. COUPLING AGENT I,II

Selection of 2-aminoethyl dihydrogen phosphate (AEPH2) as a coupling agent wasbased on its small size along with the functional groups. The short alkyl chain length ofphosphate would increase the solubility in protic solvents 53. The phosphate group ofAEPH2 can bind to TiO2 surfaces, but not to surface sites of a glass substrate 116,allowing area-selective amino-functionalization of nano-patterned TiO2 thin film. Theshort alkyl chain enables AEPH2 to occupy TiO2 sites on crater surfaces and to provide

24

a short path for electron transfer in semiconductor applications. A longer hydrocarbonchain in the axially linked coupling agent would decrease the efficiency of energytransfer between the donor-acceptor pair of the MPc complex and a TiO2 carrier 117.

AEPH2 was expected to be in zwitterionic form in a solid state (Figure 12.), based onthe high melting point (246.6 oC) and its good solubility in water. A single crystal X-ray diffraction analysis confirmed this hypothesis. The X-ray diffraction result ofAEPH2 agrees well with the crystallographic data previously measured with the X-rayphotographic method 118 and neutron diffraction 119. AEPH2 has a zwitterionic NH3+-CH2-CH2-OPO3H- configuration, resulting in a three dimensional hydrogen bondednetwork of cross-linked sheets, similar to -aminopropanephosphonic acid 120.Phosphorus – oxygen bond lengths in AEPH2 are between the reference bond lengthsof single (P-O) and double (P=O) bonds (P1-O3 and P1-04 bonds in Figure 12.). Thus,both of these bonds have a bond order of 1.5 with a delocalized negative chargecapable of hydrogen bonding. AEPH2 was previously assumed to have one (P=O)double bond with the remaining being (P-O) single bonds 118.

Figure 12. Structure and hydrogen bonding network of a crystalline AEPH2 zwitterion.

The different forms of AEPH2 in aqueous solution were studied by means of NMRtitration, a method particularly useful for zwitterions like amino acids 121. Apotentiometric titration on AEPH2 shows two deprotonation steps. Both of the protonsare normally part of the phosphate group, but in the zwitterion form one of the labileprotons is in the OPO3H- group and one is in the NH3+ group. Transition from theOPO3H2 to the OPO3H- group occurs at a pH below the measurable range of water-based potentiometric titrations. A NMR titration method was implemented in order todetect all of the deprotonation steps and to identify the specific functional groupsinvolved in these steps.

25

The AEPH2 molecule is in a neutral zwitterionic form in water solutions at pH rangebetween 1 and 5 (Figure 13.). The maximum zwitterion concentration can be found atpH 2. The amino group undergoes a proton release at pH 11 and the phosphate groupreleases protons around pH 1 and 6. The calculated pKa values from NMR titrationexperiments closely match those measured earlier by potentiometric titration 122.

0

20

40

60

80

100

0 2 4 6 8 10 12 14pH

Rel

.Con

cent

ratio

n%

NH2NH3+OPO3 2-OPO3H-OPO3H2

Figure 13. Relative concentrations of AEP ions in water solutions as a function of pH.

3.3. IMMOBILIZATION OF COUPLING AGENTS ON TiO2 CARRIERSI, II, III

A powder model system of titanium dioxide (TiO2) was chemically modified with 2-aminoethyl dihydrogen phosphate (AEPH2) in order to achieve an amino-functionalized TiO2 carrier suitable for detailed characterization of the binding ofAEPH2 to a TiO2 surface. Data obtained from the powder model system was used foramino-functionalizing nano-patterned TiO2 thin films in order to further functionalizethe thin films with metal phthalocyanine complexes.

AEPH2 is able to form complexes in aqueous solutions with some transition metals andit tends to favor binding to a metal by using its phosphate group 123. The NH3+ group ofthe AEPH2 zwitterion does not absorb on the TiO2 surface 116. So it is crucial toimmobilize the coupling agent on the carrier prior to complexing with the activecomponent, in order to amino-functionalize the TiO2 surface. If the coupling agent isfree when the metal phthalocyanine in deposited there is a possibility for the binding ofthe phosphate group to the metal center, leaving the amino group of AEPH2 unbound.

26

In this case, the metal complex would not be linked to the TiO2 because the aminogroup is unable to make strong bonds with the TiO2 surface.

Phosphates chemisorb on the Lewis acidic Ti4+ groups on the TiO2 surface, leaving theless acidic hydroxyl Brønsted acid sites free 41, 42, 43. Amino-functionalized TiO2

surfaces have not been prepared earlier even though the amino group has a broadabsorption spectrum 124 and known potential as a ligand for various metal complexespossessing catalytic properties 125, 126. Amino groups can also work as anchoring pointsfor the immobilization of biomolecules 52.

For treated TiO2 powder samples, the AEPH2 ion content was found to be limited to amaximum of 1.5 wt-%, at which, according to a surface density calculations, all theactive surface sites are occupied. According to XPS 42, 127 and ellipsometry studies 128,alkyl phosphates have been found to form highly organized self-assembled monolayers(SAM) on TiO2 surfaces due to van der Waals interactions between the alkyl chains 53,

116. With organic phosphate concentrations exceeding monolayer capacity,physisorption and formation of polyphosphates begin, but these are usually muchslower processes than the initial formation of the chemisorbed monolayer. Thechemisorbed monolayer limits the organophosphate contents on a TiO2 surface toabout 1.5 wt-% when using short chained (one to four –CH2- units) organophosphatesand to about 5.0 wt-% when using long chained (more than 10 –CH2- units) 129. Thismatches well the result of 1.5 wt-% achieved in this study with the AEPH2 bound tothe TiO2 powder model system.

Phosphates similar to carboxylic acids 130 can theoretically bind to the TiO2 surface viathree different bonding modes: monodentate, bidentate, and tridentate 129. XPSmeasurements done on AEPH2-treated TiO2 thin films III and 31P MAS NMRmeasurements done on AEPH2-treated TiO2 powders I indicated that all of the AEPH2

is bonded to the surface and only by using one bonding mode, not a combination ofdifferent modes.

The monodentate binding via P=O coordination to the Ti4+ Lewis-acid site would leadto a mobile structure resulting in a narrow signal in the 31P MAS NMR spectra, but theobserved 31P signal was wide (FWHM 360 Hz), suggesting a more rigid structure. P=Ocoordination would also cause a significant downfield shift in the 31P MAS NMRsignal of AEPH2 due to reduced shielding of the P=O bond compared to the zwitterionstructure of AEPH2

131. Lack of P=O coordination also excludes tridentate bindingsince this also involves P=O coordination to the Ti4+ site along with bidentate covalentbonding. Tridentate bonding also requires the release of the last proton of thezwitterionic phosphate group, but according to the NMR titration results it does stayattached to the phosphate in the pH region used. In addition, the zwitterion form ofAEPH2 does not have a P=O double bond, but instead two P-O bonds with a bondorder of 1.5.

27

Solution attenuated total reflectance (ATR) FTIR studies on linear polyphosphateshave revealed that the phosphate species interact bidentately through the P-O groups,forming Ti-O-P bonds 41 with the exposed Ti4+ ions on the TiO2 surface. Weakinteraction of P=O with other surface groups, such as Brønsted acid sites of TiO2,while theoretically possible, would be unlikely according to selective poisoning studies43. Therefore, Brønsted acid sites on phosphate-loaded surfaces of TiO2 remain free.

It is theoretically possible for the zwitterion form of AEPH2 to bind to the TiO2 surfaceby its NH3+ group instead of the OPOH2- group. Since there is only one clear 31P NMRsignal in treated TiO2 powder samples, the majority of the phosphate groups arebonded to the TiO2 surface. Also, the AEPH2 molecule is so short that binding fromboth functional groups is inhibited due to steric reasons. In addition, a ninhydrin testgives positive results only if the amine groups are free to react.

According to the 31P NMR and elemental analysis results, AEPH2 forms bidentatebonds between the phosphate group and TiO2. As shown earlier 129, the binding of thephosphate species to TiO2 is predominantly bidentate in neutral and acidic aqueoussolutions. The AEPH2 solution (pH 3.48) used for functionalizing the anatase powderwas in a pH range (pH < 11.0) where only the bidentate chelate bonding is possible.

O

P

O

O O

Ti Ti

OH OH

R

O

P

OH

O O

Ti Ti

OH OH

3+

Ra b

TiO2

Figure 14. Bonding models of phosphates on TiO2, a = bridged bidentate, b = chelatedbidentate.

In conclusion, phosphate can form two types of bidentate bonds with the TiO2 surfaceto give a bridge (Figure 14a) or a chelate complex (Figure 14b). A bridge complex isformed by releasing two protons of the hydroxyl groups and a chelate complex byreleasing only one proton. The type of bidentate bond formed depends heavily on thepH, and the two types are usually difficult to distinguish on the basis of IR alone 129.According to the NMR titration results for AEPH2, the last proton of the zwitterionform is not released at the applied pH, hence the bidentate chelate species, presented inFigure 14b., is the most probable bonding mode. According to single crystal X-raymeasurements, delocalized electrons in the AEPH2 zwitterion lead to a structure withtwo P-O bonds with a bond order of 1.5 and no P=O double bonds, which is similar to

28

the bidentate chelated bonds to TiO2 in the proposed bonding model in Figure 14b.Moreover, according to the XPS measurements III of the AEPH2-treated nano-patternedTiO2 thin films, there is only one type of phosphate on a TiO2 surface, indicating thatall of the phosphates are bound to the TiO2 surface via chelated bidentate bonds.

3.4. SURFACE STRUCTURE OF COUPLING AGENT MODIFIEDCARRIERS II

The AEPH2 coupling agent bound on the surface of the TiO2 powder samples exhibitsproperties expected for a self-assembled monolayer (SAM). The complete lack ofendothermic DSC peaks and the positive ninhydrin dye tests for AEPH2-treated TiO2

samples indicate that all AEP groups are chemically bonded to the surface with Ti-O-Pbonds, while amino groups remain accessible. 13C MAS NMR measurements revealthat the carbon chain stays attached to the phosphate.

Figure 15. AEPH2 treated nano-cratered TiO2 thin film.

In the SEM image, the AEPH2 treated nano-patterened TiO2 thin film (Figure 15.) hasthe 30 nm craters visible, as with the untreated sample. There is a very small amount ofvisible agglomerates inside the craters and on crater edges, but mostly AEPH2 is amonolayer only on the surface of the TiO2, so there should be no hindrance for furtherfunctionalizing of the glass substrate on the bottom of the craters with, e.g., silanes.

29

The amino end of the surface bound AEPH2 species is incapable of bonding with theTiO2 surface, according to the 13C and 31P CP/MAS studies.

A steric hindrance caused by the shortness of the hydrocarbon chain limit thearticulation of the hydrocarbon chain. Chain packing caused by cohesion between theadjacent surface bound AEPH2 species 129 forces the carbon chains of the AEPH2 to bevertically aligned and leave the amino groups accessible.

Surface densities of AEPH2, based on the average number of coupling molecules pernm2 on the TiO2 powder surface and the size of the surface active headgroup(phosphate) on the planar surface, were calculated from the carbon contents of theHCN elemental analyses and the BET surface area of the TiO2 powder. The AEPH2-treated TiO2 samples had a surface density of 6.87 nm-2 with a headgroup size of 15 Å2.The surface density was only slightly higher than the values reported forphenylphosphonic acid (4.8 nm-2) and diphenyl phosphonic acid (3.0 nm-2) 132, but theheadgroup size is a little smaller than that of octadecylphosphonic acid (24 Å2) 133,which is expected to have a similar chain packing than that of the surface-bondedAEPH2 group.

This type of vertical packing creates an uniform amino-functionalized TiO2 surfacefully occupied by AEPH2 groups while leaving the glass surface sites on the bottoms ofthe craters free. This amino-functionalized surface can be used as a base for furtherfunctionalization using molecules that are normally unable to bind to the TiO2 surface,but can interact with amino groups. The SiO2 sites on the glass surface can befunctionalized with another coupling agent to create a selectively multifunctionalizedsurface.

3.5. ACTIVE MOLECULE III

Iron(II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine or -octabutoxy-phthalocyanine complex (Fe(OBu)8Pc) (Figure 16.) is a typical non-periphericallysubstituted phthalocyanine where the radial butoxy groups enable dissolution of themetal phthalocyanine complex in common organic solvents. It has a high sublimationpoint (227.1 oC) and its solutions have a very intense color.

Iron is a versatile central metal for phthalocyanine ligand. It fits inside thephthalocyanine ring without causing severe distortion on the macrocycle ring. Iron canhave an oxidation state +2 or +3 when bound to phthalocyanine ligand. Iron(II) wasused as it is capable of octahedral coordination. Four of the coordination sites are takenby the octabutoxyphthalocyanine ligand, and two sites are free for coordination of twoaxial ligands. Iron (II) also has a proper number of valence electrons to have thephthalocyanine ring ligand, two axial ligands, like an amino group of a coupling agentand a possible gas molecule all at once, in its coordination sphere.

30

O

OO

O

O

O O

O CH3

N

N

N

N

N

N

N

N Fe

CH3

CH3

CH3CH3

CH3

CH3

CH3

Figure 16. Iron(II) 1,4,8,11,15,18,22,25-Octabutoxyphthalocyanine complex.

Iron phthalocyanine complexes are usually between green and blue in color as isFe(OBu)8Pc, but if the solution is allowed to react with atmospheric oxygen the colorbecomes reddish brown which is a more typical color for the closely related ironporphyrin complexes, like iron(III) cytochrome C hemeprotein 134. Typically, the greencolor of phthalocyanine solutions is a result of multiple different absorption bands inthe visible light region, letting only blue, green, and yellow light pass through thesolution.

The typical absorption bands of metal phthalocyanine complexes are: the N absorptionband at 250-300 nm ultra-violet range is associated to a charge transfer between the Pcligand and the metal atom. The B band around violet wavelengths at 300-400 nm (alsocalled Soret absorption 135) is caused by intramolecular electron * transitions of themacrocycle 136, 137. The charge transfer band (CT band) at blue wavelengths of 400-450nm is caused by charge transfer between the metal atom and an axial ligand 81, 136. TheQ band around red wavelengths at 500-790 nm also arises from an electron *transition in the macrocycle structure 71, 81, 138.

In case of the Fe(OBu)8Pc, the Q-band is significantly less intensive than that observedfor similar complexes. The low intensity of the Q-band is usually caused by theformation of dispersed aggregates of the phthalocyanine complex 75, 76, 77 and/or bydeformation in the macrocycle structure due to strain caused by the radial substituentsof the Pc macrocycle 56, 81.

31

3.6. BONDING BETWEEN AMINO GROUP AND METALCOMPLEXES III

Iron phthalocyanine complexes (FePc) can form bonds with various nitrogencontaining functional groups, including amines. The binding occurs between the axialvacancy of the iron metal center and the lone pair of the nitrogen atom of primaryamines and cyclic amines. Several amines are known to bond this way, e.g.,butylamine (BuNH2), propylamine (PrNH2), n-pentylamine (C5H11NH2) 139, 140,piperidine 141, imidazole 142, pyridine, and substituted pyridines 143, 144. Secondaryaliphatic amines do not bind, due to steric hindrance caused by the wide angle betweenthe substituents 139.

Similar to the FePc, Fe(OBu)8Pc seems to have interactions with n-butyl amine(BuNH2). Compared to FePc, the Fe(OBu)8Pc complex has eight electrodonating radialbutoxy groups that increase the electron density of the macrocycle 145. The increasedelectron density does not prevent coordination of amines to the axial vacancies of theFe(OBu)8Pc complex.

In comparison to hemeproteins, a class of porphyrin macrocycles with an iron(II)center is similar to Fe(OBu)8Pc with 4 of the 6 coordination sites occupied by theporphyrin ring and two available vacancies. Hemeproteins accept one axial amineligand via low energetic dipole-dipole interaction. If a second amine ligand is binding,the central iron(II) atom of hemeproteins is oxidized to iron(III) and both of the axialamine ligands are covalently bound. Reaction is similar to the oxidation of thehemoglobin iron center 57. In the case of the formed a-(BuNH2)2Fe(OBu)8Pc complex,the oxidation of the central metal atom to iron(III) does not occur according to the UV-Vis results. From 1H NMR results it can be seen that the axial n-butyl amine liganddoes not lose any hydrogens from its amine group. Thus, the axial amine ligands bindto a central iron atom via coordination bonds.

An a-(BuNH2)2Fe(OBu)8Pc complex (Figure 17.) forms when BuNH2 moleculesattaches to the axial sites of the phthalocyanine complex. The phthalocyaninemacrocycle ring deforms to an asymmetrical saddle-shape instead of flat shape due toelectrostatic repulsion from the radial substituents 56 and binding of the amino ligands146. The deformation causes the radial butoxy groups, above and below the previouslyflat macrocycle ring, to have asymmetrical positions with respect to the macrocyclicring that can be detected as the signal broadening and resonance splitting in a 1H NMRspectrum, and as a blueshift of the Q band in an UV-Vis spectrum. The samephenomenon has also been observed in the case of a FePc complex with no radialsubstituents 146. The Fe-N bonds between the iron atom and phthalocyanine macrocyclewere all of the same length, but the Fe-N bonds of axial amine ligands had differentlengths.

32

NH2

CH3

NH2

R

O

OO

O

O

O O

OR

N

N

N

N

N

N

N

N Fe

R

RR

R

R

RCH3

Figure 17. Iron(II) axial-dibutylamine 1,4,8,11,15,18,22,25-octabutoxyphthalocyaninecomplex, R = butyl.

According to XPS measurements (Figure 18.), the central metal atoms of Fe(OBu)8Pccomplexes are in iron(II) configuration when bound to amino groups of AEPH2-functionalized TiO2 surfaces. The central iron atom of Fe(OBu)8Pc has one axialvacancy left free for further functionalization, e.g., catalytic applications or bondingwith gaseous molecules.

23700

24200

24700

25200

25700

26200

705710715720725730

CP

S[a

rb.u

nits

]

Binding energy [eV]

Fe 2p3/2

Fe 2p1/2

THF

DMF

Iron 2p

2 4 0 0

2 9 0 0

3 4 0 0

128130132134136138Binding energy [eV]

CP

S[a

rb.u

nits

]

DMF

THF

Phosphorus 2p

Figure 18. XPS spectra of iron 2p and phosphorus 2p peaks of the Fe(OBu)8Pccomplex bound on the AEPH2 treated nano-patterned TiO2 surface, using THF andDMF solutions.

33

3.7. METAL PHTHALOCYANINE LAYER ON NANO-PATTERNEDTiO2 THIN FILM III

Iron octabutoxyphthalocyanine complexes were deposited on a nano-patterned TiO2

thin film of a glass substrate via a liquid immersion method. Both unmodified TiO2

surfaces and surfaces with amino-functionalization achieved with an AEPH2 couplingagent were treated with the Fe(OBu)8Pc complex.

Metal phthalocyanine based SAMs have been previously prepared on gold surfacesusing HS-groups as linkers 147 and on TiO2 surfaces using carboxylic acid as linkers 148.‘Umbrella’ shaped Pc complexes containing a single coupling agent as axial ligandform a highly organized monolayer, whereas radially substituted ‘octopus’ shaped oneswith coupling functionality at the end of the radial substitute form irregular and tiltedlayers where a large portion of coupling agent linker-groups were left unbound to thecarrier.

Nano-patterned TiO2 thin films were first treated with AEPH2 in order to create anamino-functional surface rather than treat Fe(OBu)8Pc with an AEPH2 coupling agent.AEPH2 could bind with a MPc by using the phosphate group instead of the aminegroup and could favor bonding on both sides of the Fe(OBu)8Pc macrocycle leaving noaxial vacancies left for further reactions in possible catalytic or gas sensingapplications.

SEM analysis of Fe(OBu)8Pc treated nano-cratered thin films revealed that depositionof phthalocyanine complex depends on both the functionality of the target surface andon the polarity of the solvent. All surfaces seem to be able to accept some activecomplexes despite the used solvent, even those surfaces without any kind of amino-functionality.

Figure 19. Nano-cratered TiO2 thin film with amino-functionalization and Fe(OBu)8Pccomplex deposited from a DMF solution (A) and a THF solution (B).

34

When using a highly polar organic solvent, like DMF capable of complexing with theaxial sites of the active complex, the active Fe(OBu)8Pc complex forms a uniformlayer on a nano-cratered amino-functionalized TiO2 thin film. The active complex fillsevery nano-crater with a thick layer that has a rough texture (Figure 19A.). The surfacebound layer is partially composed of a DMF solvent that coordinates with the axialsites of the Fe(OBu)8Pc complexes, and with the TiO2 surface itself. The axiallycoordinated DMF solvent molecules seem to limit the aggregation and bulk filmformation of active complexes. The amino groups of the surface bound coupling agentare probably able to replace a labile DMF ligand 107 and the initial layer of theFe(OBu)8Pc complex is formed on the TiO2 thin film, but due to weak van der Waalsinteractions between the DMF ligands a multilayer structure forms. Without thecoupling agent an even thicker aggregated bulk film was formed.

On the amino-functionalized nano-patterned TiO2 thin film, a less polar THF solutionof the Fe(OBu)8Pc seems to produce a rather homogeneous layer with open craters.According to XPS results, the active complex macrocycle stays intact in contact withthe TiO2 surface. SEM imaging indicates that the self-assembled monolayer (SAM)structure forms on the amino-functional TiO2 surface of the nano-patterned thin filmwith the glass substrate on the bottom of the craters left free. When binding from theaxial vacancy, the active metal complex and coupling agent form an ‘umbrella’ shapedstructure on the TiO2 surface (Figure 20.). The active complex lies parallel to the TiO2

surface, while leaving some of the coupling agent amino sites free, but still bound tothe TiO2 surface.

OP

O+O

OH

+

CH3 CH3

NH2

O CH3O

CH3

OCH3

O

CH3

O

CH3

O

CH3

OP

O O

OH

+

CH3 CH3

NH2

Fe

nanocratered TiO2 thin film

Figure 20. Surface arrangement of Fe(OBu)8Pc bonded to the TiO2 surface via AEPH2

coupling agents (The circle is the phthalocyanine macro ring. Two of the butoxygroups are omitted).

35

4. CONCLUSIONS

A simple method for preparing an area-selective monolayer of phthalocyanine complexon a nano-patterned titanium dioxide (TiO2) thin film was developed with the use of a2-aminoethyl dihydrogen phosphate (AEPH2) as a coupling agent.

The pH behavior of the AEPH2 was characterized by NMR titration, which is a verypowerful technique for evaluating the acid base equilibrium of the functional groups ofzwitterions. The AEPH2 was found to be in neutral zwitterionic form in water solutionsat a pH range between 1 and 5. AEPH2 was used to modify TiO2 powder and nano-patterned TiO2 thin films on a glass substrate. The modifications were conducted viathe immersion method in water solutions at pH 3.5 in order to maximize the number ofamino-functional surface sites on the TiO2 thin film.

From the analysis of the modified TiO2 powder it was found that about 1.5 wt-% of thecoupling agent is present on the surface of TiO2 powders. All the AEPH2 groups werechemically bound to the surface with Ti-O-P bonds via bidentate chelate bonding. Themethod can be utilized in creating amino-functionalized TiO2 surfaces having manydifferent kinds of morphologies.

Deposition of an active Fe(OBu)8Pc complex on nano-patterned TiO2 thin filmsdepends on both the functionality of the target surface and on the polarity of thesolvent. Amino-functionalized surface accepts the complexes more readily by offeringan amino anchoring site for the axial vacancy of the complexes. The binding betweenFe(OBu)8Pc and amino sites are based on coordination bonds and all bound complexeshave iron at an oxidation state of +2.

In DMF solutions, dispersed crystal forming promotes agglomeration of Fe(OBu)8Pccomplexes on the TiO2 thin film. Agglomerates form on the surface a thick bulk filmconsisting of DMF coordinated to a Fe(OBu)8Pc complex and bound to the TiO2

surface. When using less polar THF as the solvent, the deposition rate decreases to alevel where formation of area-selective monofilms is possible on the nano-patternedTiO2 surfaces that have been treated with AEPH2 coupling agents. The glass substrateat the bottom of the nano-craters is left unmodified.

Functional thin film material generated using the present approach has a high surfacearea of active complexes, but glass sites on the bottoms of the craters are also availableto other functional coupling agents that can enable a preparation of multifunctional thinfilm surfaces for possible catalytic, sensor and photovoltaic applications. The area-selectivity and binding to the carrier surface addresses most of the problems associatedwith thin films of metal phthalocyanine complexes: migration, agglomeration, andleaching. Also the nano-pattering allows completely new ways to control the gassensing and wetting properties of metal phthalocyanine thin films.

36

ACKNOWLEDGMENTS

This work was carried out during 2008 – 2016 at the Department of Chemistry,University of Eastern Finland. Funding from the Academy of Finland (project 118168),and the Faculty of Science and Forestry (SCITECO grant) is gratefully acknowledged.

I am deeply grateful to my supervisor Prof. Tuula Pakkanen, who gave me theopportunity to work in field of surface chemistry and nanostructures, and for hersupport and invaluable advice during the years. I am likewise grateful to the staff at theDepartment of Chemistry for their help and guidance in the laboratory, especially,Päivi Inkinen, Dr. Tapani Venäläinen, and Dr. Sari Suvanto. I would also like tosincerely thank Kathleen Ahonen and Dr. Greg Watson for revising the language of theoriginal publications and the manuscript of this dissertation.

I am deeply indebted to my friends for their outstanding support and especially to myfamily for their vital care and encouragement throughout my studies during all theseyears. Finally, I want to express my most loving thanks to my fiancée Henna-Riikkafor all her love and inspiration.

Joensuu, August 2016

37

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