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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)
________________________________________________
________________________________________________
________________________________________________
________________________________________________
________________________________________________
(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
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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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4.2 Experimental Section 72
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
5.2 Experimental Section 83
5.3
Two Alternative, Convenient Routes to Bis(diphenylacetylene)Pt0
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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
6.2 Experimental Section 107
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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7.2 Experimental Section 139
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 electro- polymerization.
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.
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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).
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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
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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
34
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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
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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
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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.
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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.
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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
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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
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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