Structure-property relationships of thiophene base derivatized polymers

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Structure-property relationships of thiophene base derivatized polymers
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Sankaran, Balasubramanian, 1963-
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Thesis (Ph. D.)--University of Florida, 1996.
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Includes bibliographical references (leaves 108-116).
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by Balasubramanian Sankaran.
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Vita.

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STRUCTURE-PROPERTY RELATIONSHIPS OF THIOPHENE BASE
DERIVATIZED POLYMERS













By


BALASUBRAMANIAN SANKARAN


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1996


























To my parents, brothers and sister.













ACKNOWLEDGMENTS

I am highly indebted to Dr. John R. Reynolds for his guidance and
support throughout my graduate stay at the University of Florida. He has been a
constant source of inspiration, and full of new ideas which made it exciting to do
research in his group. It was an honor to work with him.
The support and discussions of my colleagues and friends, especially
Peter Balanda, Dr. Seungho Kim, Dr. Myoungho Pyo, Dr. Michael diVerdi, Don
Cameron, Anthony Pullen, Fernando Larmat, David Irvin, Jennifer Irvin, Gregory
Sotzing, Jerry Reddinger and Shawn Sapp, are gratefully acknowledged.

I would also like to acknowledge the polymer floor people for making my
stay productive and enjoyable; special thanks goes to the tireless Lorraine
Williams for her help in getting all the paperwork done in time.
No words are enough to describe the four wonderful people, Srinagesh
Kaushik, Narasimhan, K, Venkatramani, C. J. and Dinesh Patwardhan, who
were my strength in United States.
Finally I would like to thank the Department of Chemistry for giving me

this opportunity to come here to do my graduate studies.
This work was supported by grants from the National Science
Foundation (CHE 9307732 ), Rockwell International, the Air Force Office of
Scientific Research and the Naval Air Warfare Center.














TABLE OF CONTENTS


ACKNOWLEDGEMENTS....................................................................................... iii

LIST OF TABLES .....................................................................................................vii

LIST OF FIGURES..................................................................................................viii

A BST RA C T .............................................................................................. ............... xii

1 INTRODUCTION ................................................................................................. 1
1-1 H isto ry .................................................................................................................. 1
1-2 Polyacetylene.............................................................................................. 3
1-3 Polyaniline .................................................................................................. 7
1-4 Polyphenylenes .......................................................................................... 8
1-5 Polypyrrole................................................................................................. 11
1-6 Polythiophene ........................................................................................... 12
1-7 M mechanism ........................................................................................................ 14
1-8 Band Theory .............................................................................................. 17
1-8-1 Redox Switching of Polymers and Theory of Conduction ............. 17
1-8-2 Degeneracy....................................................................................... 20
1-9 Electrochromism ......................................................................................... 22
1-9-1 Background...................................................................................... 22
1-9-2 Organic Electrochromic Materials.................................. ............. 23
1-9-3 Transition Metal complexes .................................................. ........24
1-9-4 Organic polymers ............................................................................ 25
1-10 Synopsis of my Work.................................................. ........................ 27

2 SYNTHESIS AND ELECTROCHEMISTRY OF POLY[1,4-BIS(2-
THIENYL)-2,5-DICYCLOHEXYLMETHYLOXYPHENYLENES].....................29
2-1 Background of Poly[1,4-bis(2-thienyl)-2-5-
disubstitutedphenylenes] .......................................................................... 29
2-2 Synthesis........................................................................................................ 31
2-2-1 Monomer, 1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxy
benzene (BTCMB).......................................................................................31
2-2-2 Chemically Prepared Poly[1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxyphenylenes] (PBTCMP) ...................................31
2-3 Electochemical Studies of BTCMB and PBTCMP...................................36
2-3-1 Electropolymerization of BTCMB .......................................... .... 36
2-3-2 Electrochemical Quartz Crystal Microbalance (EQCM)
Study of PBTCMP .................................................... .......................... 39
2-3-3 Cyclic Voltammetry of Poly[1,4-bis(2-thienyl)-2-5-
dicyclohexylmethyloxyphenylenes]........................................................ 41








2-3-4 Optoelectrochemistry of PBTCMP........................................ .....44
2-4 Experim ental.............................................................................................. 48
2-4-1 Materials and Methods................................................... .............48
2-4-2 Electrochemical and Spectroscopic Methods................................49
2-4-3 Monomer Synthesis
2-4-3-1 1,4-Dicyclohexylmethyloxybenzene....................................... 49
2-4-3-2 1,4-Dibromo-2,5-dicyclohexylmethyloxybenzene ...............50
2-4-3-4 1,4-bis(2-thienyl)-2-5-dicyclohexylmethyloxybenzene .........50
2-4-3-4 Poly[1,4-bis(2-thienyl)-2-5-dicyclohexylmethyloxy-
phenylenes ...................................................................................... 51

3 ELECTROCHEMISTRY, ELECTROCHROMISM AND LONG TERM
SWITCHING STABILITY OF POLY(3,4-ETHYLENEDIOXY-
THIOPHENE) AND ITS ALKYL DERIVATIVES............................................. 52
3-1 Introduction ................................................................................................. 52
3-2 Synthesis.................................................................................. .. ................. 53
3-2-1 Ethylenedioxythiophene (EDOT) and its Alkyl Derivatives.............53
3-2-2 Characterization of monomers, EDOT and its Octyl (EDOT-
C8) and Tetradecyl ( EDOT-C14) Derivatives.........................................55
3-3 Electrochemisry of EDOT and its Alkyl Derivatives..................................60
3-3-1 Electrochemical Polymerization ............................................. ..... 60
3-3-2 Solvent and Electrolyte Dependence of
Electropolymerization ..................................................... ................64
3-3-3 Optoelectrochemical Analyses.......................................................69
3-4 Long Term Switching Studies .................................................... .......... 74
3-4-1 Switching under ambient conditions..................................... ....74
3-4-2 Switching in a Completely Reversible Cell......................................84
3-5 An attempt to Find the Molar Absorptivity of PEDOT, PEDOT-C8
and PEDOT-C14 Films ........................................................................................... 88
3-6 Experim ental............................................................................................... 91
3-6-1 Electrochem istry ................................................................................... 91
3-6-2 Determination of Density of PEDOT and its Alkyl Derivatized
Polym ers................................................................................................. 94
3-6-3 Long Term Switching Studies............................................... ....94
3-6-4 Preparation of Reversible Cell for Long Term Switching ...............95
3-6-5 Materials and Methods for Synthesis............................................. 95
3-6-6 Synthesis of EDOT and EDOT-C8 and EDOT-C14,
M onom ers .............................................................................................. 97
3-6-6-1 Synthesis of diethylthioglycollate.............................................97
3-6-6-2 Disodium salt of 2,5-dicarboethoxy-thiophene-3,4-
dioxide ............................................................................................ 97
3-6-6-3 2,5-Dicarboethoxy-3,4-dihydroxythiophene.......................98
3-6-6-4 2,5-Dicarboethoxy-3,4-ethylenedioxythiophene.................... 98
3-6-6-6 2,5-Dicarboxy-3,4-ethylenedioxythiophene............................. 98
3-6-6-7 3,4-Ethylenedioxythiophene......................................... ....99
3-6-6-8 2,5-Dicarboethoxy-5-octyldioxeno[2,3-c]thiophene................99
3-6-6-9 5-Octyldioxeno[2,3-c]thiophene-2,5-dicarboxylicacid............99
3-6-6-10 2,5-Dicarboethoxy-5-tetradecyldioxeno[2,3-
c]thiophene .......................................................................................... 100








3-6-6-11 5-Tetradecyldioxeno[2,3-c]thiophene-2,5-
dicarboxylic-acid........................................................................... 100
3-6-6-12 5-Octyldioxeno[2,3-c]thiophene ...........................................101
3-6-6-13 5-Tetradecyldioxeno[2,3-c]thiophene .................................101

4 CONCLUSIONS
4-1 Poly[1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxyphenylenes].............103
4-2 Poly(3,4-ethylenedioxythiophen) and its octyl and tetradecyl
derivatives.... ............................................................... ......................................104

REFERENCES ........................................................................................108

BIOGRAPHICAL SKETCH .......................................................... ........................ 117













LIST OF TABLES


Table am

1 Electrochemical potentials for monomer oxidation

and polymer redox electroactivity........................................................ 67


2 Solvent/electrolyte dependence of polymer redox activity...................68


3 Molar absorptivity of the polymers in 0.1 M TBAP/CH3CN .................93














LIST OF FIGURES


Figure pae

1-2a Shirakawa's synthesis for polyacetylene............................. ..............3

1-2b Metathesis polymerization via soluble precursor to
synthesize polyacetylene............................................. ..................4

1-2c ROMP of benzvalene to yield polyacetylene....................... ............ 5

1-2d Synthesis of soluble polyacetylenes by ROMP of trimethylsilane
substituted cyclooctatetraene...............................................................5

1-2e Grafting polyacetylene onto polybutadiene backbone........................6

1-2f Anionic followed by Ziegler-Natta polymerization
of polystyrene with acetylene... ...............................................................7

1-3a The base form of polyaniline................................................................7

1-3b Protonated emeraldine hydrochloride salt.......................................... 8

1-4a Polymerization of benzene................................. ................................9

1-4b Polyphenylene via Wurtz-Fittig reaction................................ ............. 9

1-4c Yamamoto condensation reaction to synthesize
polyparaphenylene................................................... ...................... 10

1-4d Soluble precursor rout to synthesize high MW PPP.............................. 10

1-4e Synthesis of soluble acid PPP precursor to
obtain P P P ..................................................................................................... 11

1-4f Pd catalyzed boronic acid coupling reaction................................ ..11

1-6a Ni catalyzed Grignard coupling reaction to synthesize soluble
polythiophenes.............................................................................................12

1-6b Regioregular poly(3-alkylthiophenes) by Ni
catalyzed Grignard coupling reaction.................................................13








1-7a Oxidative polymerization mechanism for heterocycles.......................15

1-8a Redox switching behavior of poly[1,4-bis(2-thienyl)phenylenes].......18

1-8b Band diagram for the doping effect on conducting
polymer for the non-degenate ground states....................................19

1-8c Degeneracy of polyacetylene and non-degeneracy
of polyparaphenylene......................... .................. .......... ...........21

1-8d Doped polyacetylene and polyparaphenylene......................................22

1-9a Chemical reactions at both the electrodes of a
viologen based electrochromic device..................................................24

1-9b Transition metal complex and organic electro-optic materials............25

1-9c TTF based polystyrylester..................................... ............................ 26

1-9d Color contrast of thiophene based polymers in their
oxidized and reduced states....................... ......................................27

2-1 Synthetic scheme for the monomer 1,4-bis(2-thienyl)-
2,5-dicyclohexylmethyloxybenzene.......................................................32

2-2 1H NMR of 1,4-bis(2-thienyl)-2,5-dicyclohexyl
methyloxybenzene............................ ................... ..............................33

2-3 13C NMR of 1,4-bis(2-thienyl)-2,5-dicyclohexyl
methyloxybenzene............................ ................... ..............................34

2-4 Chemical polymerization of BTCMB...............................................35

2-5a IR spectra of BTCM B.................................. ................ ...............................35

2-5b IR spectra of PBTCMB............................... ...................................... 35

2-6 TGA of poly[1,4-bis(2-thienyl)-2,5-dicyclohexyl
methyloxybenzene]............................. ................. .............................. 37

2-7 (a) Slow film formation with scanning of BTCMB to E = +0.9 V
(b) Fast film formation with scanning to E = +1.1 V of BTCMB ............38

2-8 EQCM monitored frequency shifts during deposition of
poly[1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene]
from 0.1 M TBAP/CH3CN at 0.9, 1.1 and 1.2 V.....................................40

2-9 CV of poly[1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene]
electrosynthesized by scanning to 1.1 V and switched in








monomer-free 0.1M TBAP in (a) CH2CI2 and (b) CH3CN as a
function of scan rate................................................. .......................... 42

2-10a Potential dependence of the optical absorption for poly[1,4-bis(2-
thienyl)-2,5-dicyclohexylmethyloxybenzene] equilibrated in 0.1M
TBAP/CH3CN at (a) 0.0 V, (b) 0.50 V, (c) 0.60 V, (d) 0.65 V,
(e) 0.70 V, (f) 0.75 V and (g) 0.80 V........................................................ 45

2-10b Potential dependence of the optical absorption for poly[1,4-bis(2-
thienyl)-2,5-dicyclohexylmethyloxybenzene] euilibrated in 0.1M
TBAP/CH3CN at (a) 0.90 V, (b) 0.95 V, (c) 1.00 V, (d) 1.05 V,
and (e) 1.10 V .......................................................................................... 46

2-11 The evolution of band structure at different doping levels of the
poly[1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene]................47

3-1 Schematic diagram for the synthesis of EDOT monomers...................54

3-2 1H spectra of EDOT-C8. ........................................................................56

3-3 1H spectra of EDOT-C14........................................................................... 57

3-4 13C spectra of EDOT-C8............................. .............................................58

3-5 13C spectra of EDOT-C14.......................... ..............................................59

3-6 Oxidative polymerization and polymer redox for EDOT and its alkyl
derivatives........................................................................................... 61

3-7 Cyclic voltammograms of EDOT-Cs and EDOT-C14 in 0.1M
TBAP/CH3CN with Ag/Ag+.......................................................... .....62

3-8 Cyclic voltammogram of PEDOT-C14 at 100 mV/sec in
0.1 M TBAP/ CH2CI2 vs Ag/Ag+.............................................................63

3-9a Scan rate dependence of PEDOT-C14 in 0.1 M
TBAP/CH2Cl2 at 100, 150, 200 and 250 mV/sec................................65

3-9b Cyclic voltammogram of PEDOT-C8 in 0.1M
TBAP/CH3CN showing two redox processes during reduction..........66

3-10 Optoelectrochemical studies of PEDOT in 0.1M
LiCIO4/CH3CN at a= -1.0, b =-0.8, c =-0.6, d =-0.4, e =-0.2,
f =0.0, g =0.2, h =0.4, i=0.6 V vs Ag/Ag+.............................. .............70

3-11 Optoelectrochemical studies of PEDOT-C14 in 0.1M
LiClO4/CH3CN at a= -0.8, b=-0.6, c=-0.4, d=-0.3, e=-0.2,
f=-0.1, g=0.0, h=0.2, i=0.4, j=0.6, k=0.8, 1= 1.0........................................ 71








3-12 Optoelectrochemical studies of PEDOT-C8 in 0.1M
TBAP/CH3CN at a = -1.0, b = -0.8, c = -0.6, d = -0.4,
e = -0.2, f = -0.1, g = 0.0, h = 0.1, i = 0.3, j = 0.5, k = 0.8......................73

3-13a Long term switching studies of PEDOT in 0.1M TBAP/CH3CN.
(switches 1-3 show n)................................. ................ ...............................75

3-13b Optical transmittance of PEDOT in 0.1M
TBAP/CH3CN for the 3rd switch............................................... ......76

3-14a Long term switching studies of PEDOT-C14 in 0.1M
TBAP/CH3CN. (switches 16-21 shown).........................................77

3-14b Optical transmittance of PEDOT-C14 in 0.1 M
TBAP/CH3CN for the 19th switch.............................................. .........78

3-15a % Retention of charge in the long term switching
studies of PEDOT-C14 in 0.1 M TBAP/CH3CN......................................80

3-15b Optical transmittance of PEDOT-C14 in 0.1M
TBAP/CH3CN vs Ag/Ag+ (switches 1764-1769 shown).....................81

3-16a Long term switching studies of PEDOT-C8 in 0.1M
LiCIO4/CH3CN (switches 335-339 shown).............................................82

3-16b Optical absorbance of PEDOT-C8 in 0.1M
LiCIO4/CH3CN for the 335th switch..................................................... 83

3-17a Long term switching studies of PEDOT in 0.1M
LiCIO4/CH3CN (switches 18-21 shown)................................................ 85

3-17b Optical transmittance of PEDOT in 0.1 M
LiCIO4/CH3CN for the 19th switch....................................................86

3-18 Long term switching studies of PEDOT, PEDOT-C8 and
PEDOT-C14 in LiCIO4/PC with lithium as the counter electrode.........87

3-19 Sample calculations for molar absorptivity of PEDOT........................92

3-20 Electrochemical cell for long term switching studies
of PEDOT series with lithium as the counter electrode.......................96

4-1 Oxidized and reduced states of PEDOT-C14 films grown to similar
charge densities.........................................................................................106













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

STRUCTURE-PROPERTY RELATIONSHIPS OF THIOPHENE BASE
DERIVATIZED POLYMERS

By

Balasubramanian Sankaran

May 1996



Chairman: Dr. John R. Reynolds
Major Department: Chemistry

The synthesis of 1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene
(BTCMB), 3,4-ethylenedioxythiophene (EDOT), 5-octyldioxeno[2,3-c]thiophene
(EDOT-C8) and 5-tetradecyldioxeno[2,3-c]thiophene (EDOT-C14) and their
polymers are discussed in this dissertation. The chemical polymerization of
BTCMB yielded insoluble, brick red polymer with a decomposition temperature
of over 300 oC. Electropolymerization in CH3CN with tetrabutylammonium

perchlorate yields electroactive and conducting films. Cyclic voltammetry of the
monomer showed two redox processes during the polymerization. The first
redox process has an onset at 0.8 V vs Ag/Ag+ ( peak = 0.9 V ), while the
second has an onset at 1.0 V ( peak = 1.1 V ). When the potential is scanned to
0.9 V, very slow film formation is observed by cyclic voltammetry. Increasing the
switching potential of the scan up to 1.1 V leads to an increase in the rate of film
formation by nearly 25 times. This polymer is stable to electrochemical








switching up to 1.1 V. Optoelectrochemical studies showed that the highly

oxidized polymer has only one absorbance in the NIR suggesting a metallic-like
character.
Poly(3,4-ethylenedioxythiophene) and its alkyl derivatives show high
absorbance (dark blue, opaque) in the visible region in their reduced, insulating

state and significantly lower absorbance (light blue, relatively transparent) in
their oxidized, conducting state. The band gaps (Eg) of poly[5-octyldioxeno[2,3-

c]thiophene](PEDOT-C8) and poly[5-tetradecyldioxeno[2,3-c]thiophene]
(PEDOT-C14) are 1.77 and 1.75 eV respectively. When these polymers are
oxidized, at intermediate doping level, bipolarons are observed. In the highly
oxidized state, these polymers also show only a single absorbance in the NIR
suggesting metallic-like charge carriers. Long term switching studies of PEDOT
and its derivatives were conducted in 0.1 M TBAP/CH3CN, 0.1 M
LICIO4/CH3CN and 0.1 M LiCIO4/PC. The substituted PEDOT's show faster
switching times compared with unsubstituted polymer. When these polymers
are switched in a well-defined reversible cell with Li as the counter electrode,

they are stable to thousands of switches with over 45 % retention of their

electroactivity. For example, PEDOT-C14 retains over 60% of its electroactivity
after 16,000 double potential switches. PEDOT-C14 displays the maximum
optical contrast between its reduced and oxidized state among these polymers.
This high contrast suggests potential application of these materials in
electrochromic devices.













CHAPTER 1
INTRODUCTION

1-1 History

When shiny silver polyacetylene films were discovered to conduct

electricity after redox doping in 1977 by a group at The University of
Pennsylvania, a new field of conductive polymers was started.1 These
polyacetylene films were synthesized by Hideki Shirakawa and coworkers with
Zeigler-Natta polymerization on a glass surface,2 and they were found to exhibit
high conductivities. Naarmann and coworkers, in an improved synthesis, made

polyacetylene that had conductivity similar to copper metal.3 Unfortunately, the
polymer film was brittle and extremely sensitive to the presence of water and
oxygen.
One of the major structural criteria for a polymer to conduct is conjugation

providing an overlap of i orbital electrons; this allows any charge formed to be
delocalized along the polymer chain. Conductive polymers are insulators or
semiconductors in their neutral form. In order for a conducting state to be

formed, it is necessary to introduce charges by chemical or electrochemical
oxidation or reduction. In the presence of an electric field these charges are
mobile, and the polymer is an electrical conductor. The fascination of scientists
with these new conductors motivated a significant amount of research in various
aspects of conducting polymers. This work has been detailed in numerous
review articles published in the literature.4








A practical thrust behind the development of organic polymeric
conductors and semiconductors is the concept that polymers can be fabricated
into electrical wires, films or other shapes without loss of conductivity, unlike
conventional conductors and semiconductors. The light weight, relatively good
mechanical strength, processability and corrosion resistance of these organic
polymers are some of the factors that have created wide interest in such diverse
fields as chemistry, physics, material science and chemical engineering.5 This
interdisciplinary research has been directed towards a common goal of
developing organic conductors that have properties similar to, if not better than,
standard inorganic conductors and semiconductors.
One of the major differences between inorganic and organic polymeric
materials is the architectural designing ability of the molecule in organic
chemistry. In organic polymeric materials, structural design is varied at the
molecular level. This makes it possible to tailor a polymer according to the
needs of the potential technological applications by fine tuning the molecular
structure with changing of a functional group or the backbone of the polymer.
This has lead to a better understanding of the relationship between the

chemical structure of the repeat unit of the polymer to its mechanical and
electrical properties.
Even though polyacetylene had a very high conductivity, its intractability
and instability toward oxygen and water made it infeasible for

commercialization. In 1979, the first free standing film of polypyrrole was
electrochemically synthesized by oxidative polymerization of pyrrole. Though

polypyrrole is flexible and stable to water and oxygen, it is intractable and
insoluble. Then the electropolymerization was extended to other aromatic and
heteroaromatic compounds including thiophene, furan, indole, carbazole,
azulene, pyrene, benzene, fluorene.6,7








Polypyrrole,6 polythiophene,6 poly(phenylene sulfide),6 polyphenylene,8

poly(phenylene vinylene),9 poly(thienyl vinylene),10 and polyaniline6 were
chemically synthesized, and their electrical properties were studied. Among
these, polythiophene exhibits an adequate stability in both its doped and
undoped states making it potentially useful for the development of better
materials. Thiophene is also structurally versatile, making it possible to
derivatize to suit the needs of potential applications. Long alkyl chains
substituted in the 3-position of the thiophene made the once insoluble
polythiophene soluble without much compromise of its electrical properties.
Polyacetylene, polyphenylene, polypyrrole, polyaniline and
polythiophene and their deivatives are the most extensively studied
electroactive polymers. In the following paragraphs, a small snynopsis of these
polymer's evolution to obtain soluble material is outlined.

1.2 Polyacetylene

Shirakawa and coworkers were the first to synthesize polyacetylene
using a thin coating of a hetrogeneous Ziegler-Natta initiator system in a glass
reactor as shown in the Figurel.1.1.2 Polyacetylene sythesized in this manner

AI(C2H5)3
n H-CEC-H 0
Ti(OC4Hg)4
Figure 1.1: Shirakawa's synthesis for polyacetylene.


was highly crystalline, fibrillar and also very insoluble. When this synthesis is
carried out at -780C, the polyacetylene obtained is in the form of the cis-transoid
isomer. By heating the cis-transoid isomer to 1500C, it can be isomerized to
trans-transoid, which forms lustrous, silvery films that are thermodynamically








more stable than the cis-transoid isomer. Different types of initiator systems1
or annealing and aging12 of the catalyst led to improvements in the stability and
type of polyacetylene film formed. While this improvement decreases the
amount of defects in the film, it is still completely non-processable.

Since the polyacetylene formed was intractable, a soluble precursor
polymer route was developed. One of the most highly studied systems utilizing
the soluble precursor method is shown in Figure 1.2. The metathesis


F3C CF3 F CF3 F3C CF3
WCI6 heat

\n / (CH3)4Sn n

n

Figure 1.2: Metathesis polymerization via soluble precursor to synthesize
polyacetylene.


polymerization of 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0]deca-3,7,9-triene using

WC16 and CH3Sn was developed to provide a soluble precursor polymer that
can be subsequently converted to (CH)x thermally.13 This precursor is useful in
the production of thin films. The thermal conversion step is highly exothermic
and potential explosion hazard. The (CH)x films formed were much more
dense and less fibrillar than the material made from Shirakawa's technique.

The polymer obtained after elimination was amorphous, but stretching of the
film during the thermal elimination lead to highly oriented and crystalline
material.14
One of the deficits of this soluble precursor was that a large molecule had
to be thermally eliminated during the conversion. This leaves void spaces in
the polymer films and the elimination is not quantitative. To overcome this





5


Grubbs and coworkers utilized ring opening metathesis polymerization (ROMP)
to obtain a strained precursor, which will easily isomerize to yield (CH)x as
shown in Figure 1.3.15 ROMP of benzvalene results in polybenzvalene, which
has a bicyclobutane ring in the main chain. This ring can be thermally
isomerized to obtain (CH)x. The polymeric material obtained has fibrillar
morphology and saturated defects along the backbone.


ROMP



Benzavalene Polybenzvalene

Figure 1.3: ROMP of benzvalene to yield polyacetylene.


To improve the solubility of polyacetylene, substituent groups can be
attached.16 ROMP of substituted cyclooctatetraenes provides a convenient
route to a variety of substituted polyacetylenes whose properties vary with
substituent. ROMP of trimethylsilylcyclooctatetraene yielded a soluble polymer
as shown in Figure 1.4.17 The substituent groups break the conjugation, and




2. hu
Me3Si

Figure 1.4: Synthesis of soluble polyacetylenes
by ROMP of trimethylsilane substituted
cyclooctatetraene


hence the conductivity is lower than the Shirakawa polymer. The solubility of
this polymer allows the study of solution properties, which was impossible with
the earlier polyacetylenes. Many substituents were attached to this polymer,








and their molecular weight conductivity, spectroscopic and other properties
were investigated.18
Domains of polyacetylene can be introduced into soluble polymeric
materials to aid processing. This can be achieved either by grafting (CH)x
chains onto a carrier polymer or by making block copolymers with a soluble
segment. An example of the graft polymerization is shown in Figure 1.5.19
Polybutadiene has 1,2 and 1,4-diene linkages. The 1,2 linkage leads to a
vinylic group as a side chain. This vinylic group is utilized to graft polyacetylene
to the main polybutadiene chain using Ziegler-Natta conditions. This will lead

Ti(OBu)4 C-CC
J.^C-CA/WVVVV 'C- i .
/I / AIEt3 -C2H5 (C2Hs
Ti- Til



C2H5 C2H5 C2H2





A A

Figure 1.5: Grafting polyacetylene onto polybutadiene backbone.


to domains of polyactylene in the polymer backbone. Similarly, blocks of
polyacetylene can be incorporated into a soluble polymer backbone by
copolymerizing acetylene with another living polymer. The types of
copolymerization are increased by the ability to change the nature of the active
site after the synthesis of the first block by one mechanism and subsequently








use different propagation mechanism to attach the second block.20 For
example, polystyrene prepared by anionic polymerization to obtain a living site
can be utilized to copolymerize acetylene via Ziegler-Natta type polymerization
as shown in Figure 1.6. After forming the living polystyrene anionically, the
initiator Ti(OBu)4 is added to the reaction mixture. One of the butoxide groups
reacts with the lithium ion to form LiOBu, and Ti(OBu)3 attaches to the living site
of polystyrene. By losing one of the butoxide groups, Ti metal now has an
active site to receive an acetylene molecule to polymerize.

LI+

n-Bun TI(OBu)4n-Bu ) n-Bu

I C2H2


Figure 1.6: Anionic followed by Ziegler-Natta polymerization
of polystyrene with acetylene


1-3 Polyaniline

Polyanilines are a large class of conducting polymers which exist in three
different discrete oxidation levels both in their doped and undoped forms.21
Mixtures of these oxidation states are readily obtained. One of these discrete
oxidation states can be doped by a non-redox process without adding or
removing electrons from the polymer backbone. The reduced form of
polyaniline can also be doped by a conventional oxidation process.

H H


A B
Figurel.7: The base form of polyaniline










General composition of the base form of polyaniline is shown in Figure 1.7.
Polyanilines consist of alternating reduced (A) and oxidized (B) repeat units.22
The average oxidation state, 1-y, can be varied from 0 to give completely
reduced polymer, to 0.5 to give half oxidized polymer, and to 1.0 to give
completely oxidized polymer. When the oxidation is 0, 0.5 and 1.0, these states
are called as leuco-emeraldine, emeraldine and pernigraniline respectively.
The emeraldine base form (Figurel.8) can be protonated to obtain the

emeraldine hydrochloride salt.23 The imine nitrogen can be protonated in
whole or in part. Complete protonation of the emeraldine base results in the
formation of a radical cation, and in this form the polymer has the highest
conductivity. The number of electrons associated with the polymer after the
doping (protonation), is the same as in the undoped material, and the
processability of the polymer makes it an interesting polymer to study.


H H

Emeraldine Base
2x HCI



H H H H

Cl C'

Figure 1.8: Protonated emeraldine hydrochloride salt










1.4 Polyphenylenes

Polyphenylenes exhibit high heat resistance in the neutral form and
electrical conductivity on doping.24 The most important polyphenylene is the
polyparaphenylene, PPP. An ideal poly(1,4-phenylene) would possess high
molecular weight, all para-linked chains, which would have excellent thermal
stability and conductivity. However, direct synthesis of polyphenylenes leads to
totally insoluble, intractable solids. One of the first successful polyphenylenes
synthesized from benzene was reported by Kovacic and coworkers.25


n + 2n CuCI2 A---IC + 2n CuCI + 2n HCI

Figure 1.9: Polymerization of benzene


Benzene polymerization was carried out using CuCI2 as the oxidizing
agent to obtain the polyphenylene with a degree of polymerization (DP) ca. 10.
This polymer was slightly soluble in THF (Figure 1-9). The condensation of 1,4-

Dioxane
Cl- CI + Na -Cu + NaCl

Figure 1.10: Polyphenylene via Wurtz-Fittig reaction


dihalobenzene using alkali metals and copper powder (Figure 1.10) yielded
low molecular weight products with irregular structure.26 Yamamoto and
coworkers used a different catalyst and very mild conditions as shown in Figure
1.11.27 They made a mono Grignard of 1,4-dibromobenzene and condensed it
in the presence of NiCl2(bpy). The product was regular poly(1,4-phenylene)







with no defects. This method resulted in higher yields and higher DP's, but the
dibromobenzene monomer is much more expensive than benzene. A modified
Ni catalyst, Ni(cod)2 (cod: 1,5-cyclo-octadiene) was used by the same
researchers, resulting in higher yield and DP of the PPP polymer.28

NiCI2(bpy)
Br-- --Br+ Mg ON p + MgBr2

Figure 1.11: Yamamoto condensation reaction to synthesize
polyparaphenylene.


To overcome the insolubility of PPP, a soluble polymer precursor route
was employed. Benzene was subjected to bacterial oxidation using the
microorganism Pseudomonas putida to prepare the 5,6-cis-
dihydroxycyclohexa-1,3-diene. The dihydroxycyclohexadiene was esterified
and then radically polymerized to obtain a soluble PPP precursor, as shown in



n
HO OH ROCO OCOR ROCO OCOR
Figure 1.12: Soluble precursor route to synthesize high MW PPP.


Figure 1.12.29 Different types of ester groups were attached to obtain high
thermal conversion, resulting in higher DP. Another route to a obtain soluble
PPP precursor was, by using 2-carbomethoxy-1,4-dichlorobenzene as shown in
Figure 1.13.30 2-carbomethoxy-1,4-dichlorobenzene was polymerized using
Ni(0) as the catalyst in DMF. Saponification of the PPP-ester produced PPP-
carboxylic acid. This was soluble in pyridine, quinoline and NaOH. In the
presence of CuO and heat, the precursor decarboxylated to yield PPP. The DP







in this reaction was close to 100, and good PPP films were obtained using this
precursor route.
Unfortunately, none of the final polymers obtained in the above reaction
were soluble. Hence a new polymerization technique was developed where A-
B step-growth polymerization lead to soluble PPP as shown in Figure 1.14.31

CO2CH3 CO2CH3 CO2H
C NiBr2, Zn NaOH
P(Ph)3,
DMF CuO
CuO




Figure 1.13: Synthesis of soluble acid PPP precursor to obtain PPP.


The difunctional monomer is coupled in presence of heterogeneous Pd(0)
catalyst and aqueous Na2CO3 to yield the soluble PPP.

C6HI3 C6H13
6= 1 Pd [P(C6Hs)3]4
Br B(OH)2 ------ \
r B 2 C6H6, Na2CO3 n
H13C6 H13C6
Figure 1-14: Pd catalyzed boronic acid coupling reaction.



1-5 Polypyrrole

Polypyrrole is a heteroaromatic polymer having an oxidation potential
lower than PPP or polythiophene. Pyrroles can be chemically or








electrochemically polymerized to yield an insoluble black solid.32 Polypyrrole
is very stable in the oxidized, conducting state and highly reactive with air in the
reduced, insulating state. Extensive electrochemical studies of polypyrrole
have been carried out leading to many potential uses of polypyrrole in
batteries, sensors, as a biocompatible polymers, electromagnetic shielding, as
a coating on textiles etc.33 Limited work has been done on derivatization of
polypyrroles and property investigation of the deivatized pyrroles.

1-6 Polythiophene

Polythiophene is an insoluble, heteroaromatic macromolecule, having an
oxidation potential lower than that of PPP. Thiophene has been polymerized
using a number of routes to obtain a polymer which is blue black in its
conducting, oxidized form and brick red in its reduced insulating form. Attaching
a long alkyl chain to the backbone of an intractable polymer chain improves in
the dissolution of the polymer. On thiophenes, substituents can be attached at
the p-carbon atom before polymerizing to make the polymer soluble and fusible.
Soluble poly(3-alkylthiophenes) were first synthesized in 1986.34-37

R R R
SI Mg, THF Ni(dppp)Br2
rel -15:i MgI

Figure 1-15: Ni catalyzed Grignard coupling reaction to
synthesize soluble polythiophenes


Elsenbaumer et al. prepared the methyl, ethyl, butyl and octyl substituted
poly(3-alkylthiophenes) via a chemical route as in Figure 1-15. They prepared
in situ the monoiodo-Grignard reagent from 3-alkyl-2,5-diiodothiophene and
coupled using nickel catalyst. Among the poly(3-alkylthiophenes) obtained,








poly(3-octylthiophene) had the best solubility. Films of the polymer were easily
cast to study its properties. This chemically prepared polymer has a regular
linear structure with only a-a linkages and no cross linking. Both the

electrochemically polymerized and chemically polymerized (via oxidative
coupling with FeCl3) 3-alkylthiophenes resulted in cross-linking of the polymer
through the 3 carbon atoms. These give rise to defects in the polymer chains,
affecting the electrical and electronic properties of the polymer. Though the
poly(3-alkylthiophenes) synthesized as per Figure 1-16 have a regular linear
structure, they are still not regiospecific with respect to the alkyl chain attached.
The Grignard can form at the 2 or 5- position, making the eventual polymer
regio-irregular. The ratio of head-tail to head-head isomer is 63:37. The
removal of these defects was expected to increase the conductivity of the
polymer by at least two orders of magnitude.
To synthesize regio-regular poly(3-alkylthiophenes), McCullough and
coworkers developed a new synthetic method.38 They started with 2-bromo-3-
alkylthiophene as shown in Figure 1-16. The proton at the 5-position was

1. LDA/THF R
S 2. MgBr2'OEt2

S Br 3. Ni(dppp)CI2

Figure 1-16: Regioregular poly(3-methylthiophene)
by Ni catalyzed Grignard coupling reaction


specifically removed by using LDA, and that position was converted into a
Grignard site by adding MgBr2*dietherate. This was coupled with Ni catalyst to
yield a high degree (over 96 %) of regio-regularity in the back bone of the








polymer. Similar methodology to obtain regio-regular polymer was used to
synthesize poly(3-hexylthiophene-2,5-diyl).39


1.7 Mechanism


1-7-1 Mechanism of polymerization

The oxidative polymerization of a heterocyclic monomer leading to a

conjugated polymer can be performed chemically and electrochemically.
Chemically the polymerization is done via oxidative-coupling reactions. The
mechanism of the polymerization is not yet completely understood, but studies
relating to elucidation of the mechanism have recently been published.40
Electrochemistry has been a very important analytical tool used to study the
mechanism of heterocycle polymerization.

A simplified form of the elecrochemical sysnthesis mechanism is shown
in Figure 1.17.41 The initial step involves the anodic oxidation of the monomers
close to the surface of the electrode. The oxidation leads to the formation of a
radical cation.42 This radical cation can dimerize by adding to another radical

cation, as shown in Route A, or it can lose a proton to form a radical which can
either combine with another radical to form a dimer, as shown in Route B, or
add to another monomer, as shown in Route C. This initial step is very fast, and
there is not enough evidence to unequivocally point toward either route, but

there is evidence to suggest these steps.
An electron loss leading to the formation of a radical cation is now widely
accepted as the initial reaction. The presence of radical cations was confirmed
by ESR experiments on thiophene systems;43 the radical cation is stable for
several seconds. Addition of trifluoroacetic acid to the electrolytic medium









X = NH, S


Oxidation -e"

1q "+*


X


Route B


-2 Deprotonation
-2H+ 1 Deprotonation


3.
x


Oxidation



r1\4W>)+ .


1. -e 2. -H*



x \/


+| 1.Oxidative coupling
"X 2. Deproronation
| Repeat for
Spolymerization


Repeat for
polymerization


X%

1.-e- 2. -H








Repeat for
polymerization


Figure 1.7a: Polymerization mechanism for heterocycles.


Route A


Route C


H


*








increases the lifetime of the radical cation formed to several hours.44 Though
the disproportionation of the proton is very slow, for a neutral solution there may
be some disproportination, leading to the RouteB type of mechanism. To
disprove the proton disproportionation hypothesis, Saveant and coworkers
added a 100 mM base (2,6-di-tert-butylpyridine) to their 5 mM pyrrole system
during cyclic voltammetric experiments with scan rates ranging between 200
and 2000 Vs-1.45 The addition of base had no noticeable effect on the
voltammogram, giving credence to Route A as the plausible mechanism.
Wei et al. have proposed an alternate mechanism to Roures A and B
based on results from the following experiments.46 In the 0.2M 3-
methylthiophene electrolytic system, a small amount of 0.5 to 2 mM of 2,2'-
bithiophene dimerr) was added before polymerization. This addition led to an
increase in the rate of polymerization. This increase in rate was sustained until
the polymerization was stopped. The voltage required for the polymerization is
lower than the oxidation potential of 3-methylthiophene, suggesting that the
2,2'-bithiophene acts as an initiator. A similar increase in the rate of
polymerization was observed when 2,2':5'2"-terthiophene (trimer) was added.
This observation lead to the suggestion of Route C. In this proposed route, the
neutral monomer reacts with the radical cation in an electrophilic addition
manner. This adduct then loses an electron and two protons to form a neutral
dimer. This disproportion step is believed to produce an acidic environment in
the region around the anode.47 The dimer loses an electron to generate a new
radical cation, and the reaction is repeated.48 Though this experiment suggests
that there is an electrophilic addition during polymerization, it is still not clear
whether the same effect is dominant for the polymerization of the monomer
without the addition of any oligomers. In this polymerization, a constant








potential must be applied to form the polymer; this strongly suggests that the
mechanism followsRoute A.
The growth of the polymer on the surface of the electrode is proceeded

by nucleation.49 When the potential is applied in the electrochemical synthesis,
the monomer in the vicinity of the electrode is oxidized and combines to form
dimer. As the synthesis is continued, the oligomers precipitate onto the
electrode when they attain a certain length exceeding the solubility limit.50
Since these are conducting polymers, they serve as a nucleation site for more
polymer to grow on. The growth is normally one-dimensional, since growth
perpendicular to the electrode surface is more rapid than across the surface.51
In the electropolymerization of pyrrole, the polymer grows more in one direction,
leading to large number of nucleation sites on the electrode surface. This
increases the number of polymer chains, and the polymer film formed is more
dense and uniform.

1-8 Band Theory


1-8-1 Redox Switching of Polymers and Theory of Conduction

Conjugated polymers in their neutral form are insulators, and to make

them conduct, charge carriers must be created. Conducting polymers have the
ability to be switched reversibly between oxidized and reduced states as shown
in Figure 1-18. The switching is done in the presence of an electrolyte to

provide counter ions to stabilize the charges formed on the backbone of the
polymer.
Conjugated polymers have I orbitals which overlap throughout a chain if

the chain is planar. The conduction in these types of polymers is due to the
movement of the charge carriers along the backbone of the polymer and also























T +


Figure 1.8a: Redox switching behavior of poly[1,4-bis(2-thienyl)phenylenes]


Polaron










Bipolaron
BlIpolaron


























a
Neutral
Polymer


b
Polaron
States


C
Bipolaron
States


d
Bipolaron
Bands


Figure 1.8b: Band diagram for the doping effect on conducting polymer for non-degenerate ground states.








due to the hopping of these charges between chains. Since the n orbitals are

orthagonal to the plane of the molecule, the more planar the molecules in the
conduction state, the better the conductivity. More planar molecules also result
in better stacking of the polymer chains, leading to better overlap among chains
and higher conductivity by inter-chain hopping of the charges.
Figure 1-19 depicts a simplified version of the theory of conduction.52,53
The bonding and anti-bonding n molecular orbitals of the polymer consist of a

large number of energy levels, with a very small difference in energy between
any two consecutive levels. These energy levels are combined and depicted as
bands called valence and conduction bands. The difference between the
highest occupied molecular orbital (HOMO) in the valence band and the lowest
unoccupied molecular orbital (LUMO) of the conduction band is the energy gap
or band gap (Eg) of the polymer. In the neutral form, the energy gap is high,

and the polymer is an insulator. As the polymer is oxidized by removing an
electron from the polymeric system, a radical cation, called polaron, is
generated, resulting in a distortion of one of the energy levels, raising one of the
energy levels from the HOMO and lowering an energy level from the LUMO.
When another the electron from the polaron state is removed, the polymer forms
a dication, called bipolaron. As the oxidation is continued, the intermediate
levels in the bipolaron states continue to increase, eventually forming bands
called bipolaronic bands. The intermediate levels in the bipolaronic bands are
much closer to the valence band than to the conduction band. Hence, it is
easier to excite an electron into one of these levels, leading to conduction.

1-8-2 Degeneracy

Polyacetylene has a low band gap and high conductivity. One of the
reasons for this is its degeneracy,54 although the C-C double and single bonds







are of different lengths. The smaller this bond length alternation, the smaller the
band gap. If these bond lengths can be made equal, then the band gap will
vanish.55
Systems like polypyrrole, polythiophene and polyparaphenylen possess
a non-degenerate ground state (as shown for polyparaphenylene in Fig. 1-20).
The aromatic-type of geometrical structure is expected to dominate for these
systems. A quinoidal-type structure would have higher energy. Ab initio
calculations made by Bredas for a polythiophene system showed that a
quinoidal structure will have lower ionization potential and higher electron
affinity than an aromatic structure.56 With the help of Valence Effective
Hamiltonian (VEH) calculations, Bredas demonstrated that the linear decrease
in the band gap value is not a function of reduced bond length alternation but a
function of increasing quinoidal character of the geometry.






t I


Figure 1-20: Degeneracy of polyacetylene and non-degeneracy of
polyparaphenylene because of higher energy
content of b compared to a.


Polyacetylene is unique among all other conducting polymers. Like any
other conjugated polymers, polyacetylenes can also be redox doped to induce
charge carriers. Due to its degeneracy, the charge is highly delocalized. When
polyacetylene is doped, (reduced or oxidized), instead of generating two
intermediate states in the bipolaronic bands, it creates only one intermediate








level exactly mid way between the valence and conduction bands. This doped

state, called soliton, is shown in Figure 1-21.57,58 The energy on both sides of
the charge is the same except for the phase change in the orbitals. The charge
is free to move up and down the chain without needing to overcome any energy
barriers. This is not the case for all aromatic polymeric systems. When an
aromatic conjugated polymer is doped, the resulting charged polymer has to



Soliton






Figure. 1-21: Doped polyacetylene and polyparaphenylene


undergo a change in the structure from lower energy aromatic form to the higher
energy quinoidal form as shown in Figurel-21. The quinoidal form will try to
revert back to the aromatic form. In highly oxidized polyaromatic systems,
where there are a large number of charges, this collapse of quinoidal forms will
eventually bring two charges closer to each other. Since they are similar
charges, they repel each other. This repulsive force and energy difference
between aromatic and quinoidal forms controls the number of aromatic rings

between any two charges in a conjugated aromatic polymeric system.


1-9 Electrochromism


1-9-1 Background








Electrochromism is a phenomenon in which there is a persistent optical

absorbance change in the visible region of the spectrum with the application of
an external potential. An electrochromic device is an electrochemical cell in
which electrochromic material coated on an electrode is separated by an
electrolyte from the charge-balancing counter electrode. This phenomenon has
been known since 1969.59 Most of the electrochromic materials studied at that
time were inorganic oxides like tungsten and vanadium oxides.60 The
electrolytes used to study these materials or to make devices were inorganic
salts of Li and Fe. The introduction of organic polymers in the study of
electrochromic materials started by using polymer-based electrolytes like
poly(2-acrylamido-2-methylpropanesulfonic acid)[poly-AMPS],
polyvinylpyrrolidone[PVP] and polyethyleneimine [PEI]. These were pioneered
by Giglia and Haacke,61 and Randin and Viennet.62 The goal of this research
was to obtain an electrochromic display. As in any electrochemical cell, these
display devices had to have two electrodes and an electrolyte to carry the
charge. Both the electrodes were coated with thin films of inorganic oxides, and
a supporting electrolyte dissolved in a solvent was used to carry the charge.
The use of solvents led to sealing problems in the device, since the solvents
were free-flowing. To overcome this, polymer gels like polyethyleneoxide
(PEO), polymethylmethacrylate (PMMA), etc. were used instead of solvent.



1-9-2 Organic Electrochromic Materials

Viologen was one of the first organic materials used for the study of
potential application for electrochromic properties.63 Derivatives of viologen
have been made to vary electrochromic properties. To obtain different colors,
various derivatives of viologen were mixed together in solution, since they all








have different thresholds for coloration. Chung and Leventis from Molecular
Displays used viologen and ferrous salts to make an electrochromic device.64
The reactions leading to the change in coloration are shown in Figure 1-22.
Viologen was coated onto one of the electrodes, and Everitt's salt imbedded
polypyrrole was coated onto the other electrode with an aqueous polymeric gel
electrolyte between them. Viologen, which is a colorless salt, becomes deep
violet when reduced. At the same time at the other electrode, the colorless
Everitt's salt [Fe(ll)] changes to blue upon oxidation. Many derivatives of the
viologen have been studied to obtain different colors.

+ +2

2 '
HsC22H5 2 eeH" H5C2-N, NC2Hs
Viologen violet
colorless


K4Fe4[Fe(CN)]63 4e" + 4K+ + Fe4[Fe(CN)6]3

Everitt's salt [Fe(ll)] Prussian blue [Fe(lll)]
colorless blue

Figure 1-22 : Chemical reactions at both the electrodes of a viologen
based electrochromic device.

1-9-3 Transition Metal Complexes

Most of the materials studied for electrochromic properties undergo a
chemical change, like a change in oxidation state, during the phenomenon.
Abbott and coworkers at University of Leicester in England have made
semiconducting organic and organometallic complexes, like CuMNT and TTF
as shown in Figure 1-23, that change their absorbance wavelength in the
visible region when an electric potential is applied across them.65 The change








in absorbance wavelength for these materials was caused by a physical rather
than a chemical change. This work is unprecedented, since this is a physical
phenomenon, and there is a necessity for the ions to be non-centrosymmetric.
The application of potential seems to allign the ion-dipoles within the lattice,
changing the overlap of the molecular orbitals and therefore changing the
absorbance. When the potential is removed, the spectrum does not revert back
to its original form, but reversion does occur when the material is annealed in
an oven at 100 OC for 30 minutes.

+ NH3 3-

O } CH3 NC S 1 1
r jH 2Na I C Cu

Disodium copper maleonitrile dithiolate complex anion
with methylbenzylammonium cation (CuMNT)


SOH


Tetrathiafulvene complex cation
with mandelate anion (TTF)

Figure 1-23: Transition metal complex
and organic electro-optic material


1-9-4 Organic Polymers

The inorganic oxides and organic salts used are coated or electroplated
onto an ITO glass plate. After application of a potential, the individual
molecules contribute to the change in absorbances. The optical memories of
individual molecules are different, and the retention time of one of the states by
these molecules varies. Hence there is less uniformity across a film and more
chances of a defect in the film. To overcome this, electrochromic materials were








attached to polymer chains. One of the earlier examples of this was polystyryl
with TTF66 or pyrazoline67 as pendant groups as in figure 1-24.68






n

Figure 1-24: TTF based polystyrylester.


As the polymer is oxidized, the color of the film changes from pale yellow

to green to purple, as TTF goes first to TTF+1 and then to TTF+2. Different
derivatives of TTF pendant polymers were synthesized to study their
electrochromic properties. Similar to this type of polymer, Shirota and
coworkers synthesized polyethylene with terthiophene as a pendant group at

every other carbon atom.68 This polymer turns blue when oxidized, while in the
reduced state it is nearly colorless. In the reduced state the polymer is very
soluble like any other substituted polyethylene.
The color contrast in the oxidized and reduced states can also be

controlled by the effective conjugation length of the polymer chain. This color
contrast can be tailor made by using oligomers of a parent molecule for
polymerization as shown in Figure 1-25. Mastragostino and coworkers used
thiophene and methylthiophene to make dimers and tetramers, which were then

electrochemically polymerized and studied spectrochemically.69 Depending
upon the oligomers used for the electropolymerization, the effective conjugation
can be varied, due to steric demands of the methyl groups at different positions.
This leads to a change in absorbance of the eventual polymer in both the
states.








Polyaniline,70 polyisothianapthene,71 poly(N-methylisoindole),72 and
polycarbazole73 are some of the other polymeric materials that are used to
study the electrochromic behavior in conjugated systems. Copolymers of
conjugated systems like copolymers of aniline and N-alkylaniline are being
synthesized to tailor properties such as color, switching speed and switching
response.74


Color of the polymer
Monomer molecules
Doped Undoped


Orange Blue
S S0

/ Yellow Violet


SRed Blue
S S

/J Yellow Blue




Figure 1-25: Color contrast of thiophene based polymers in their oxidized
and reduced states



1-10 Synopsis of my Work

This dissertation entails the study of structure-property relationships of
thiophene based polymer derivatives. Polythiophene, being the most stable
among the conjugated systems in both oxidized and reduced states, is the








choice as the parent polymer. Two types of derivatives, with the thiophene
moiety within the back-bone, are synthesized to investigate these structure-

property relationships. These polymeric conjugated systems are electroactive
in nature.
The first polymer is obtained by derivatizing thiophene, to yield a
symmetrical monomer, 1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene.
Chemical and electrochemical syntheses was carried out on this monomer.

Electrochemical and optoelectrochemical studies were conducted on this
monomer and polymer.
The second type of derivatizaton was done by fusing a dioxane ring on to
the 3,4 position of the thiophene ring, thereby lowering the oxidation potential of
thiophene and also blocking the 3 and 4 positions from being a polymerization
site. Three different types of monomers, 3,4-ethylenedioxythiophene, octyl and
tetra-decylsubstituted 3,4-ethylenedioxythiophenes were synthesized. These
monomers were electrochemically polymerized in different electrolyte/solvent

systems and their properties studied. Optoelectrochemistry of these polymers
was investigated to determine the band gap and evolution of charge carriers as
a function of voltage. Long term switching studies were conducted and the
switching stability and rates of all three polymers are compared. An attempt is

made to determine the density and molar absorptivity of these polymer films to

determine the type of electronic transition that is occurring in these films when
irradiated with light.













CHAPTER 2
SYNTHESIS AND ELECTROCHEMISTRY OF POLY[1,4-BIS(2-THIENYL)-2,5-
DICYCLOHEXYLMETHYLOXYPHENYLENES]

2-1 Background of Polyll.4-bis(2-thienylv-2.5-disubstitutedDhenvlenes1

The structural modification of conjugated polymers by the attachment of
flexible substituents has proved effective in introducing solubility and fusibility to
the intrinsically insoluble and intractable backbones. As examples, soluble
derivatives of polythiophene,75 polypyrrole76 and poly(p-phenylene)77 have
been reported. Substitution with sufficiently long alkyl chains has led to the
production of true thermoplastic materials which can be processed using both
solution and melt methods. When the substituents are ionic in nature,
conjugated polyelectrolytes are formed which can be water soluble.78
In addition to the introduction of processability, substitution of conjugated
polymers has been shown to exert a strong influence on the optical, electrical,
and electrochemical properties of these polymers. The Reynolds group7 and
others80 have been probing these effects in our studies of the poly[1,4-bis(2-
heterocycle)-p-phenylenes] (PBHP's) by changing backbone and substituent
structures. The electron donating nature of alkyl and alkoxy substituents has
been shown to lower monomer and polymer oxidation potentials, and reduce
the optical band gap by increasing the electron density in x conjugated
systems.81 Pendant substituents have also been shown to have a strong steric
influence on the conformation of the conjugated backbone. Steric repulsions
between pendants on adjacent rings results in an increase in the energy barrier
to planarity of the ring system. Since a high degree of conjugation is necessary








for charge carrier transport, this torsional strain leads to an increase in oxidation
potentials and a decrease in electrical conductivity. This effect is demonstrated
by poly(1,4-bis(2-thienyl)-2,5-dimethylphenylene), where the thiophene-
phenylene torsional angle has been calculated to be significantly larger than
the unsubstituted, or dimethoxy, analog.79a More importantly, the energy
barrier to planarity increases from 3.1 kcal/mol to 15.8 kcal/mol with methyl
substitution. This results in an increase in both the monomer and polymer
oxidation potentials and decrease in conductivity of more than 5 orders of
magnitude.9a
The studies of the PBHP's were initially motivated by the potential for
improved solubility of the polymers relative to non-derivatized conjugated
polymers. The desirable electronic properties of the polyheterocycles are
combined with the ease of substitution of the phenylene ring. At the same time,
the monomers can be symmetrically derivatized yielding isoregic polymers with
enhanced order.80b It is now accepted that higher degrees of long range order
can lead to significant enhancement of electrical conductivity.7
In the PBHP's, it has been found that, although the parent
polyheterocycle properties are largely retained, some unique electrochemical
phenomenon are observed in both monomers and polymers. This chapter
reports synthesis and electrochemical studies of a new derivative to this series
containing cyclic alkoxy pendants, poly[1,4-bis(2-thienyl)-2,5-dicyclohexyl-
methyloxyphenylene] (PBTCMP), which displays two oxidation waves in the
cyclic voltammogram of its monomer. This behavior is absent in the other
substituted derivatives of poly[bis(2-thienyl)phenylene] series, and
microgravimetric analysis suggests that it is due to solubility of the deposited
oligomers. PBTCMP shows only one at low energy electronics transition at high
oxidation potential, displaying characteristics of metallic-like charge carriers.












2-2-1. Monomer. 1.4-bis(2-thienvl)-2.5-dicyclohexylmethyloxybenzene.
(BTCMB)

The monomer, 1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxy-benzene

was synthesized using methods developed to synthesize other bis(2-
heterocycle)benzenes as shown in Figure 2-1. Hydroquinone was

deprotonated with alcoholic KOH and dialkylated by drop-wise addition of 1-
bromomethyl-cyclohexane. This 1,4-dicyclohexylmethyloxybenzene was
brominated at the 2 and 5 positions by drop-wise addition of bromine in CHC3I
to yield 2,5-dibromo-l ,4-dicyclohexylmethyloxybenene, and subsequently
reacted with 2-thienyl-zinc chloride to yield 1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxybenzene.
Figure 2-2 illustrates the 1HNMR of the BTCMB. Resonances for the

aromatic hyrogens are all above 7.0 ppm, for the methylene and cyclohexyl part
are at 3.9 and between 1.0 to 2.0 ppm respectively. The integration of the
resonance peaks are proportional to the required number of protons. The
13CNMR for the monomer as seen in Figure 2-3, has four resonances between

25 and 39 ppm for cyclohexyl carbons, one resonance at 75.9 ppm for

methylene carbon and seven resonances for the aromatic carbons of the
thiophene and benzene moiety. The molecule is symmetrical across the plane
of benzene.

2-2-2. Chemically prepared Poly[1.4-bis(2-thienyl)-2.5-dicyclohexvl-
methvloxvphenylenel (PBTCMP)

The FeC13 induced oxidative polymerization of BTCMB, followed by
compensation with aqueous NH3 yielded a brick red neutral polymer that was











HO

+ 2

OH


78 %
1 nI


THF
Pd(PPh3)4


KOH, EtOH
O O.

64 %

Br2
CC14 1-1






-0 + 2 CIZn-

Br



THF
ZnCl2


LIU


89 %


1-111
Figure 2-1: Synthetic scheme for the monomer 1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxybenzene.






















5 ( A

3
1 2




I I I 3 2 1 0 ppm
7 6 5 4 3 2 1 Oppm


Figure. 2-2: 1HNMR of 1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene






10 12


5






6

I .-


a r U U U U
140 120 100 80 60 40 20

Figure. 2-3: 13CNMR of 1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxybenzene


0 ppm












S X
S 1. FeCI3, CHCI3 ,
2. NH40H o

0O




Figure. 2-4: Chemical polymerization of BTCMB


totally insoluble as shown in Figure 2-4. As illustrated in Figure 2-5, FTIR
studies indicated the disappearance of the a C-H stretching of the thiophene
moiety with the retention of the 0 C-H stretching as expected for the formation of
a conjugated thiophene derivatized polymer.

00 200 300 2500I
3000 2500 3000 2500


-1
cm


.1
cm


r


Figure. 2-5b: IR spectra of PBTCMB


Figure. 2-5a: IR spectra of BTCMB








TGA showed the polymer to be stable to ca. 300 oC (Figure. 2-6) and was
amorphous as determined by X-ray diffraction. Elemental analyses showed that
the product is relatively impure and difficult to purify due to its insolubility. While
it is evident that BTCMB polymerizes under these oxidative conditions, this
insolubility precluded proper structural analysis and study of electroactivity.
Electrochemical methods were subsequently employed to probe polymerization
and characterization of BTCMB.

2-3. Electrochemical Studies of BTCMB and PBTCMP


2-3-1. Electropolymerization of BTCMB

The monomer, BTCMB can be oxidatively polymerized using
electrochemical methods to yield electrically conducting polymers. The
electropolymerization and optoelectrochemistry was performed by Fernando
Larmat. Electropolymerization was carried out using 10 mM monomer solution
in 0.1 M TBAP/CH3CN with platinum button electrode as working, platinum
plate as counter and Ag/Ag+ as reference electrodes. The polymer was
deposited by scanning from 0.0 V to 1.2 V as shown in Figure 2.6. During
anodic scanning of a monomer solution, a large irreversible current response
indicates the formation and coupling of cation-radicals. The increased electron
donating ability of the alkoxy groups results in a reduction of the monomer
oxidation potential compared to the alkyl substituted monomers. Cyclic
voltammetric scanning electropolymerization of bis(2-heterocycle)benzene
(BHB) monomers is generally characterized by the observation of polymer
cathodic processes during the reverse scan. Multiple scans to, or slightly
beyond, Ep,m then yields CV's where both the anodic and cathodic polymer
redox processes grow in at potentials significantly lower than Ep,m. In contrast





100


314.42 oC, 89.98%


143.33 C
95.10%


80





60 428.5 C
70.2%




40





20
70 270 470 670

Temperature (C)


Figure. 2-6: TGA of poly[1,4-bis(2-thienyl)-2,5-dicyclohexylmethyloxyphenylene]









Volts vs AfAg4
0.8 0.6 0.4 0.2 0.0
0.8 0.6 0.4 0.2 0.0


I 10 iA


Volts vs Ag/Ag*
I I I I I I
1.0 03 :.5 0.4 0.2 0.0
Ec2. ~ .


I 10pA


/I


"t t
EIm EW


Figure. 2-7: (a) Slow flim formation with scanning of BTCMB to E +0.9V.
(b) Fast film formation with scanning to E = + 1.1V of BTCMB.








to all of the other substituted poly[1,4-bis(2-heterocycle)phenylenes] (PHBP's)
studied in our group, the cyclohexylmethyloxy substituted thiophene derivatives
(BTCMB) displays two monomer oxidation processes. Scanning to 0.9 V
(Figure 2-7a) results in an Ep,m at 0.8 V. Repeated scanning, however, shows a
very slow film growth under these conditions. Extending the anodic scan to 1.1
V (Figure 2-7b) reveals a second oxidation (E2,m) with an onset at 1.0 V.
repeated scanning under these conditions leads to the rapid development of
cathodic processes (Ec2,p and Ecl,p) at 0.55 V and 0.35 V along with an anodic
process (Ea,p) at 0.5 V. During this excursion to higher potential, rapid
electroactive film growth is observed. This behavior may be due to the solubility
of oxidized oligomers formed at low potentials in the electrolyte, whereby newly
formed coupling products diffuse away from the electrode surface preventing
further coupling. This phenomenon was observed in other, highly soluble,
derivatives such as 1,4-bis(2-thienyl)-2,5-dodecyloxybenzene where the
solubility of the polymer precluded electrochemical polymerization.79b The
second oxidation of BTCMB results in rapid electroactive film deposition. It is
possible that this second process is due to the reaction of the coupled product
to form a new cation radical. Since the result of this oxidation is rapid coupling
resulting in the deposition of an insoluble film on the electrode surface, cross-
linking of the polymer is likely.

2-3-2. Electrochemical Quartz Crystal Microbalance (EQCM) Study of PBTCMP

To examine the potential dependence of this deposition further, the
electrochemical quartz crystal microbalance (EQCM), was used to monitor the
mass change during electropolymerization of BTCMB as shown in Figure 2-8.
Scanning to 0.9 V results in a frequency decrease (mass increase) as expected
for electropolymerization. During the return scan, the frequency increases as







S0.9 V 1.1 V 1.2 V


-100


S --200


-300 -


-400


-500 -


-600 -


-700


-800
0.0 0.2 0.4 0.6 0.8 1.0 1.2

E (Volts vs. Ag/Ag*)


Figure. 2-8: EQCM monitored frequency shifts during deposition of poly[1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxyphenylene] from 0.1M TBAP/CH3CN at 0.9, 1.1 and 1.2 V.








the polymer formed dedopes and the anions are lost. A small net frequency
change is observed at 0.0 V indicating a small amount of polymer has
permanently adhered to the electrode surface during the initial scan. Scanning
with a switching potential change of 1.1 V, a plateau is observed in the

frequency change as the deposition occurs at a relatively slower rate on the
electrode surface. Again the reverse scan shows a small irreversible mass
change as a small amount of polymer deposition has occurred. Scanning
beyond the second oxidation to 1.2 V results in a significantly large frequency
decrease and rapid mass deposition as electroactive polymer forms on the
working electrode. The return shows a frequency increase, again indicative of
the expulsion of counterions during polymer reduction, but the large net mass
increase at 0.0 V indicated a significant amount of polymer deposited during
this scan.

2-3-3. Cyclic Voltammetry of Poly[1.4-bis(2-thienyl)-2.5-dicyclohexyl-
methyloxyphenylenel

Films of PBTCMB were synthesized onto an Indium-tinoxide (ITO) coated
glass electrode by repeated scanning of 10 mM BTCMB to +1.1 V in 0.1M
TBAP/CH3CN. The polymer was rinsed with CH3CN and was used to study

redox behavior in a monomer free 0.1M TBAP/CH3CN system. PBTCMB was

cycled at different scan rates between 0.0 and 0.8 V and the peak current
response was measured. The increase in peak current response in directly
proportional to the increase in the scan rate. The scan rate dependence is
nearly linear in the range examined and indicates that the electroactive species
is electrode bound. The decreased monomer oxidation potential of the alkoxy
substituted monomer, in comparison with other alkyl substitutes BTHB's, leads
to polymers with a lower degree of cross-linking and 3-coupling. For example,











Volts vs. A/Ag*


a







2 1-,0 A









0.8 0.6 0.4 0.2 0.0
I i I I i
Volts vs. Ag/Ag*










2 10 pA







Figure. 2-9: CV of poly[1,4-bis(2-thienyl)-2,5-dlcylohexylmethyloxyphenylene]
electroyntheslzed by scanning to +1.1 V and switched In monomer-ree
0.1 M TBAP In (a) CHaCIl and (b) CH3CN as a function of scan rate.








the cyclic voltammogram of polythiophene electropolymerized from bithiophene
displays two cathodic current responses, whereas the higher monomer
oxidation potential of thiophene leads to films with a single redox couple.84 The
monomer, BTCMB, was polymerized at a potential beyond its initial monomer
oxidation potential due to solubility concerns as described earlier. The cyclic
voltammogram of its polymer lacks the first, low potential couple when cycled in
CH3CN, but displays two couples in a 0.1 M TBAP/CH2CI2 system, as illustrated
in Figure 2-9. All other linear alkoxy substituted polybisthienylphenylenes are
soluble in CH2CI2 indicating that it is a better solvent for these systems. This
suggests that CH2CI2 effectively swells the insoluble PBTCMP sufficiently to
allow facile counterion insertion. These results suggest that the energy
required for counterion insertion is an important factor in the stability of
intermediate charge carriers.84 The potential necessary to insert counterions
into films of unsubstituted, alkyl substituted or crosslinked polymers is beyond
the potential for bipolaron formation; therefore, a single two-electron oxidation
results. The polymers which exhibit two couples allow facile counterion
movement due to low oxidation potentials (resulting in minimal cross-linking
during electropolymerization) and solvation effects induced by long side chains.
Two electrochemical properties support the suggestion that polarons are
formed at the first redox couple of these polymers. First the peak widths of the
first couple are quite small with respect to the second couple. Several
polythiophene derviatives containing ether groups at the 3 position have been
shown to display two distinct redox couples in their cyclic voltammograms, and
the presence of a relatively sharp initial couple is characteristic of these

systems.82 The width of current responses in the cyclic voltammograms of
surface bound species is ideally a constant value of 90.6/n mV where n is the
number of electrons transferred. Larger values are indicative of repulsive








interactions within the film.85 Because polarons possess only a single charge,
and bipolarons contain two positive charges, repulsive interactions are
expected to cause a larger peak broadening effect in the formation of
bipolarons. The narrow current responses in the low potential couple of these

system suggests that monovalent charge carriers rather that dicationic carriers
are present.
Secondly, the peak separation of the second couple is significantly
smaller in the polymers with two couples than in polymers which undergo a
single, two-electron process. The large peak separation traditionally observed
in conducting polymers has been ascribed to the large differences in
conductivity between the oxidized and reduced states.85 In those polymers
where intermediate charge carriers are formed at a lower potential (polarons)
the conductivity difference before and after the second oxidation is greatly
reduced resulting in a near zero separation between the anodic and cathodic
current responses.

2-3-4. Optoelectrochemistry of PBTCMP.

The electronic band structure of this polymer series has been

investigated using optoelectrochemistry. The electronic band gap is determined
from the onset of n to n* transition. Poly[1,4-bis(2-thienyl)-2,5-dicyclohexyl-

methyloxybenzene] was electrochemically synthesized on an ITO coated glass
electrode at a constant potential of 1.1 V in 0.1 M TBAP/CH2CI2. The polymer's
UV-Vis-NIR absorbance was measured in situ from 350 to 2500 nm as a
function of applied potential as shown in Figure 2-10a. At 0.0 V, in its fully
reduced state there is one strong absorbance with an onset at 2.4 eV and a
peak at 2.75 eV due to the n to n* transition of the neutral polymer. The

potential was sequentially stepped anodically to oxidatively dope the polymer













t t


1.0


1.5 2 2.5


Energy ( eV )


Figure. 2-10a:


Potential dependence of the optical absorption for poly[1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxyphenylene] equilibrated in 0.1 M TBAP/CH3CN at (a) 0.0 V,
(b) 0.50 V, (c) 0.60 V, (d) 0.65 V, (e) 0.70 V, (f) 0.75 V and (g) 0.80 V.


1.2

1.0

0.7

0.

0.2


u


0.5


3.0


3.5


- 1









t.

1.0

0.75


a
.0
0.25

0.0i I
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Energy (eV)


Figure. 2-10b: Potential dependence of the optical absorption for poly[1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxyphenylene] equilibrated in 0.1 M TBAP/CH3CN at (a) 0.90 V,
(b) 0.95 V, (c) 1.00 V, (d) 1.05 V and (e) 1.10 V.







a b


CONDUCTION
BAND


Neutral
Polymer


Polaron
States


c d



I I


(I I






Bipolaron Bipolaron
States Bands


Metallic-like
States at High
Doping Level


Figure. 2-11: The evolution of band structure at differentdoping levels of the poly[1,4-bis(2-thienyl)-2,5-
dicyclohexylmethyloxyphenylene]








and a spectrum taken at a number of different oxidation states. As the potential
is increased, two new absorbances at 1.45 eV and 0.55 eV are observed due to

formation of bipolaron states leading to three different transitions. At the same
time the intensity of the n to C* transition decreases. As the potential is

increased above 0.9 V, the intensity of absorption at 1.45 eV decreases while
the other two absorption at 0.55 eV and 2.75 eV continue with their previous
trend as illustrated in Figure 2-10b. At 1.1 V there is only one absorbance at
0.55 eV due to coalescence of bipolaron bands with the valence and
conduction band.

The reduced PBTCMP has one transition for the band gap of the
polymer. Upon electrochemical doping, there are new transitions observed due
to the formation of bipolarons as explained in 1-8, and intragap electronic levels
are created during charge carrier generation. As the doping levels is increased,
the peak attributed to the transition between the valence band and the higher
energy intragap state decreases in intensity, and the lowest energy transition
shifts to shorter wavelengths. This change in the optical spectra can be

attributed to the formation of metallic-like charge carriers.86 The intragap

bipolaron energy levels become sufficiently broad at high doping levels that
they intersect with the valance and conduction bands as illustrated in the band
structure diagram in Figure 2-11. This phenomenon has been observed
previously in polythiophene derivatives but is conspicuously absent in other

polyheterocylces.86a In our PBTP system, this behavior is observed in the
alkoxy substituted derivatives.

2-4. EXPERIMENTAL


2-4-1. Materials and Methods.








Cycohexylmethylbromide, hydroquinone, bromine, KOH, thienyllithium,
zinc chloride, palladium tetrakistriphenylphosphine were used as purchased
from Aldrich without any further purification. All solvents were freshly dry
distilled before use. All reactions were carried out under an argon atmosphere.
Characterization was carried out using 1H and 13C NMR spectroscopy on
Varian XL-200 and Varian XL-300 MHz spectrometers, FTIR on a Biorad/Digital
FTS-40 spectrophotometer, and mass spectrometry on a Finnigan MAT 95 Q
spectrometer. Elemental analysis was performed by Atlantic Microlab Inc.



2-4-2. Electrochemical and Spectroscopic Methods.

Cyclic voltammetry was conducted utilizing platinum working and counter
electrodes and an Ag/Ag+ reference electrode. Potentials were controlled using
a PAR EG&G model 273 potentiostat/galvanostat. Polymer films for
optoelectrochemical measurements were prepared on ITO coated glass slides
(Delta Technologies) as the working electrode. Electrolytes employed were 0.1
M tetrabutylammonium perchlorate (TBAP) in acetonitrile or methylene chloride
(distilled over P20s prior to use). Electronic spectra were obtained using a
Varian Cary 5E UV-Vis-NIR spectrophotometer. Experimental details have
been presented previously.79d Mass changes at gold working electrodes
(Valpey-Fisher) were monitored using the electrochemical quartz crystal
microbalance (EQCM) as described previously.83

2-4-3. Monomer Synthesis

2-4-3-1. 1.4-Dicyclohexylmethyloxybenzene (1-1)
Alcoholic KOH (14.5 g, 258 mmol) was added slowly to hydroquinone
(13.5 g, 123 mmol) in 100 ml of ethanol. The reaction mixture was stirred for








two hours and 50 g, (282 mmol) of cyclohexylmethylbromide in 50 ml of ethanol
was added drop-wise over a period of 30 minutes. The mixture was refluxed for
48 hours and the ethanol was distilled. The residue was extracted with CCl4,
dried over CaCI2 and filtered. The solution was evaporated to dryness and
white flaky crystals were obtained after recrystallization from ethanol. 1H NMR

(ppm): 6.85(s), 3.7(d, J = 7.7 hz) 1-2(m). 13C NMR (ppm): 154.9,116.9, 75.9,
39.2, 31.4, 28.2, 27.1. (mp = 93-95 C, (23 g) 64 %). HRMS: molecular ion
peak, 302.2239 amu (theoretical 302.4558); base peak, 206.1242 amu.
2-4-3-2. 1.4-Dibromo-2.5-dicyclohexylmethyloxybenzene.(1-Il)

Bromine (12 g; 76 mmol), in 100 ml of CCl4 was added drop-wise to a
150 ml CCl4 solution of 1-1 (7.6 g; 25 mmol). The reaction mixture was stirred
for 72 hours, poured in 150 ml of 1.0 M KOH and stirred for 2 additional hours.
The aqueous layer was discarded and the organic layer washed with dilute HCI
until the pH became 7.0. The solution was concentrated by evaporation and
poured slowly into cold methanol. The precipitate was collected by filtration,
dried and recrystallized from ethyl acetate to give 10.2 g of white solid. (mp =
130-132 C, 89% ). 1H NMR (ppm): 7.05(s) 3.73(d, J = 7.5 hz) 1-2(m). 13C
NMR (ppm): 150.2, 111.4, 111.1,75.6, 37.7, 29.8, 26.5, 25.8. Anal. Calcd. for

C20H2802Br2: C, 52.19; H, 6.13. Found: C, 51.95; H, 6.13.
2-4-3-3. 1.4-bis(2-thienyl)-2.5-dicyclohexylmethyloxybenzene (1-IIl).
2-Thienyl lithium (36 mmol) was added slowly with stirring at 0C to a
flask containing zinc chloride (8.7g; 64 mmol) in 100 ml of THF. In a separate
flask, the dibromide (1-11) (5.5g; 12 mmol), in 100 ml of THF was added to 50
mg of palladium tetrakistriphenylphosphine via cannula at 0 C over a period of
30 min. The 2-thienylzinc chloride was cannulated into the flask containing 1-11
and the mixture was stirred for 72 hours. The mixture was then poured into 100
ml of dilute HCI and the organic layer was separated. The aqueous layer was








extracted with ether and the ether fractions were combined. The product was
precipitated in cold methanol and recrystallized from acetone to yield 4.6g of
greenish solid. (mp = 162-164 C, 82%). 'H NMR (ppm): 7.52(dd, J= 3.6, 1.2
hz), 7.33(dd, J = 5.2, 1.2 hz), 7.11 (dd, J = 3.8, 1.6 hz), 3.8(d, J = 5.8 hz), 1-2(m).
13C NMR (ppm): 149.3, 139.3, 126.6, 125.6, 125.1, 122.9, 112.8, 75.3, 37.9,
30.1, 26.5, 25.9. Anal. Calcd. for C28H3402S2 C, 72.06; H,7.34; S,13.74.
Found: C, 71.98; H,7.42; S,13.56. HRMS: molecular ion peak, 466.2019 amu;
base peak, 274.0139 amu.

2-4-4. Poly[1.4-bis(2-thienyl)-2.5-dicyclohexylmethyloxyphenylene.
Ferric chloride (140 mg, 0.851 mmol) and 200 ml of CHC13 were refluxed
for 2 hours and cooled. The monomer, 1-111, (100 mg, 0.214 mmol) was added
with 75 ml of CHCI3. The mixture was refluxed for 72 hours. The solvent was
removed by rotovap and the product re-precipitated in cold methanol. The
black-red powder was compensated by the addition of 30 ml of concentrated
ammonium hydroxide to yield the brick red neutral polymer. Anal. Calcd. for

C28H3202S2: C, 72.06; H, 7.34; S, 13.74. Found: C, 71.98; H, 7.4; S, 13.56.














CHAPTER 3
ELECTROCHEMISTRY, ELECTROCHROMISM AND LONG TERM SWITCHING
STABILITY OF POLY(3,4-ETHYLENEDIOXYTHOPHENE) AND ITS ALKYL
DERIVATIVES

3-1 Introduction

It is important to synthesize electroactive and conducting polymers
having low oxidation potentials which yield materials of high ambient stability in
the conducting state.87 Of the conducting polymers studied, polypyrrole88 and
polyaniline89 stand out for their ease of oxidation and stability making them the
materials of choice for applications and commercial consideration. Among the
conjugated heterocyclic polymers that have been studied extensively,
polythiophenes have attracted attention due to their ease of derivatization and
polymerizability by chemical and electrochemical methods.90
Unsubstituted polythiophenes are insoluble and infusible. Alkyl chain
derivatization at the 3-position induces solubility and fusibility into the polymers,
making them true thermoplastics.91 Attachment of an electron donating alkoxy
group to the 3-position of the thiophene reduces the polymer's oxidation
potential yielding better electrical and optoelectronic properties.92 This electron
donating group raises the highest occupied molecular orbitals (HOMO), thereby
lowering the oxidation potential, while having less effect on the lowest
unoccupied molecular orbitals (LUMO), which decreases the polymer's
electronic band gap.93 Disubstitution at the 3 and 4 positions, which would
eliminate the possibility of P coupling and crosslinking, leads to strong steric








interactions along the chains reducing conjugation.94 One method of
overcoming these steric interactions is to fuse a cyclic ring on to the thiophene.
Poly(ethylenedioxythiophene) (PEDOT, R=H) has a dioxane ring fused
on to the c face of the thiophene. This cyclic substituent presents less steric
demands than those encountered with a disubstituted polythiophene. PEDOT
has a lower oxidation potential than any of the polythiophenes reported,95
making it quite stable under ambient conditions and potentially useful in many
applications as an anti-static material96 and as a solid electrolyte in
capacitors.97 Thin films of the polymer are light blue-gray in their oxidized state
and dark blue-violet in their reduced state.98 This optical contrast between the
oxidized and reduced states forebodes the use of PEDOT as an electrochromic
material.99
In order to develop new electrochromic polymers, and to understand
substituent effects, octyl and tetradecyl substituted poly(ethylenedioxy-
thiophenes) (PEDOT-C8 and PEDOT-C14) were synthesized. This chapter
outlines the structure-property relationships in these polymers and compares
them to the parent PEDOT.

3-2 Synthesis


3-2-1 Ethylenedioxythiophene and its alkyl derivatives

The EDOT monomers were synthesized by modifying methods first
reported by Fager.100 Thiodiglycollic acid was esterified with ethanol in the
presence of an acid catalyst to yield the ester, diethylthioglycollate (3-1) as
shown in Figure 3-1. This diester was reacted with diethyloxalate in the
presence of sodium ethoxide to obtain the ring closed thiophene moiety,
disodium salt of 2,5-dicarboethoxythiophene-3,4-dioxide (3-2). The disodium










0 O0
HO> S OH


C2HsOH, H
0 0
Reflux C2HsO S OC2H5
3-1


O O
HsC20 OC2H5
J


NaOC2H5


H2S04


HO OH


HsC2OOC'S COOC2Hs
3-3


K2C03
DMF, 90


Br Br


Na -O ONa+


HsC200C S COOC2Hs
3-2


R

O O
DC


HsC2OOC S COOC2H5


KOH, Reflux


Quinoline, 150 C

Copper chromite


R = H, 3-6a
CsH, 3-6b
C14 H2,, 3-6c


R = H, 3-4a
CsH 7 3-4b
14 29, 3-4c
C "


R

0 O



HOOC2 S COOH
R = H, 3-5a
CsH17, 3-5b
C14H29, 3-5c


Figure 3-1: Schematic diagram for synthesis of EDOT monomers








salt was acidified to yield 3,4-dihydroxy-2,5-dicarboethoxythiophene (3-3).
The diol was reacted with 1,2-dibromoalkane (prepared from bromine and the
required alkene) in anhydrous DMF to yield the alkyl substituted (octyl and
tetradecyl) 2,5-dicarboethoxy-3,4-dioxyethylenethiophenes (3-4b and 3-4c).
This ring closed diester thiophene moiety was hydrolyzed with KOH resulting in
the formation of diacids (3-5b and 3-5c) which were subsequently
decarboxylated in quinoline with copper chromite as a catalyst to yield the
monomers, 5-octyldioxeno[2,3-c]thiophene (3-6b, EDOT-C8) and 5-
tetradecyldioxeno[2,3-c]thiophene (3-6c, EDOT-C14). The unsubstituted
derivative, EDOT is now commercially available at Bayer.

3-2-2 Characterization of monomers. EDOT and its octyl (EDOT-C_) and
tetradecyl ( EDOT-C14) derivatives

1H and 13C NMR, mass spectroscopy and elemental analysis were used
to characterize the monomers. The proton NMR spectra for both the EDOT-C8
and EDOT-C14 shows four different sets of peaks shown in Figures 3-2 and 3-3.
The methyl and methylene proton peaks of the alkyl chains can be observed at
0.9 and between 1.2 to 1.7 ppm; the methine and methylene peaks of the
ethylenedioxy segment between 3.8 and 4.2 ppm as an ABX pattern; and the
aromatic protons of the thiophene rings can be seen at 6.3 ppm. The area under
these peaks integrate to the required number of protons of the molecules within
the specific regions. The 13C NMR spectra for the monomers are shown in
Figures 3-4 and 3-5. Resonances are observed between 15 and 35 ppm for the
alkyl cabon atoms; 75 and 68 ppm for the methine and methylene carbons of
the ethylenedioxy segment; and 112 and 142 ppm for the aromatic carbon
atoms of the thiophene moiety of the monomers. The mass spectrometry and




















S



C8H17


2,3


2I I 1 ,
7 6 5 4 3 2 1 ppm 0
Figure 3-2: 1H spectra of EDOT-C8











































2, 3


0 ppm


Figure 3-3: 'H spectra of EDOT-C14


_ _1


''
























1,2


-C8H17


140 120 100 80 60 40 20 ppm

Figure 3-4: 3C spectra of EDOT-C8


3,4
























V14' '28 Vy
(0




34 32 30

3,14


1,2


SI I I UIm

140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

Figure 3-5: 3C spectra of EDOT-C14








elemental analysis results of these monomers show the expected values further
confirming the structure of the molecule.

3-3 Electrochemisry of EDOT and its alkvl derivatives


3-3-1 Electrochemical Polymerization

EDOT-C8 and EDOT-C14 were electrochemically polymerized in a
number of solvent/electrolyte combinations. A general electropolymerization
path is outlined in Figure 3-6. The parent monomer, EDOT, exhibits a
monomer oxidation peak (Ep,m) at 1.0 V while EDOT-C8 and EDOT-C14 exhibit

Ep,m's at 0.89 V and 0.92 V respectively in 0.1 M TBAP/CH3CN as illustrated in
Figure 3-7. During this oxidation, the monomers polymerize and electroactive
polymer films grow on the working electrode surface. On the reverse scan of
the CV's, EDOT-C8 exhibits two cathodic peaks at -0.14 (Ec2,p) and -0.52 V

(Eci,p), while EDOT-C14 exhibits two similar peaks at -0.14 V (Ecl,p) and -0.32
V (Ec2,p), which can be attributed to reduction of the oxidized polymer. The
PEDOT-Cs formed re-oxidizes at -0.22V (Ea,p), while the PEDOT-C14 formed re-
oxidizes at -0.19 V (Ea,p). As the electropolymerization of these monomers is
continued by repeated scanning, the oxidation and reduction peak currents
increase and shift slightly in potential. This increased current is due to build-up
of the electroactive polymer on the electrode.
The polymer deposited onto the electrode is then cycled in a monomer
free electrolyte to study its stability and reversibility of its redox behavior. All
three polymers are well behaved and reversibly redox switch. This is illustrated
in Figure 3-8 for PEDOT-C14 grown and switched in 0.1M TBAP/CH2CI2.
Analysis of the cyclic voltammetric behavior of the polymers in monomer free
electrolyte show all of the peak currents scale linearly with the scan rate,













R =H
= Ca H17
= C14H29


Red. Ox.


Figure 3-6: Oxidative polymerization and polymer redox reaction for EDOT
and its alkyl derivatives




Cyclic Voltammetric Growth of PEDOT-C8


Cyclic Voltammetric Growth of PEDOT-C14


I
I
Eam= 0.89 ,


05
0.5


I I
0


-0.


I
-0.9


1.0,


I I
J Volts vs Ag/Ag+ I


iI
I
I
I

Ea,p-0.22


Ea.m=0.93


SE,p=-0.36
I EC2.=-0.36


II 5 pA



SEa,p=-0.19
Ecl,p=-0.16


Figure 3-7: Cyclic Voltammogram of EDOT-C8 and EDOT-C14 in 0.1M TBAP/CH3CN with Ag/Ag+


0.5


I ~ I
I t
I'


-0.9


0.5


-- --


50 pA






63




I I
).1 -1.0 V













2 2A/cm


Figure 3-8: Cyclic voltammogram of PEDOT-C14 at 100mV/sec in 0.1 MTBAP/CH2Cl2 vs Ag/Ag+








indicating that the polymer (and thus all electroactive sites) is adhered to the
surface of the electrode as seen in Figure 3-9a.

3-3-2 Solvent and Electrolyte Dependence of Electropolymerization.

EDOT-C8 and EDOT-C14 were polymerized by the repeated scanning
methodology onto ITO coated glass in a series of solvent/electrolyte systems
and their electrochemical properties during film growth compared as shown in
Table 1. Both PEDOT-C8 and PEDOT-C14 show two redox processes upon
reduction in 0.1 M LiCIO4/CH3CN, and 0.1 M TBAP/CH3CN. Figure 3.9b
illustrates the two redox processes during the reduction of PEDOT-C8 in 0.1 M
TBAP/CH3CN. The polymers deposited in CH3CN show a well behaved redox
behavior. The two redox processes during reduction are due to the
transformation of the oxidized polymer to radical cation and then to final
reduced form. In 0.1 M LiCIO4/PC and 0.1 M TBAP/CH2CI2 there is only one
broad redox response during reduction. The radical cation formed in PC and
CH2CI2 is not discernible. 0.1 M LiCIO4/Tetraglyme was utilized as a potential
electrochemical medium, but did not allow for the deposition of electroactive
polymer. The polymers formed are soluble in tetraglyme, making it impossible
to grow a film.
The amount of electroactive polymer deposited with repeated scanning
was compared for EDOT, EDOT-C8 and EDOT-C14 as a function of
electrodeposition medium. The anodic current response for the polymer
oxidation on the tenth scan during deposition was monitored as a means of
determining the relative amounts of polymer deposited as shown in Table 2. It
can be seen that the electrodeposition rate of EDOT proceeds more rapidly than
EDOT-C8, which is more rapid than EDOT-C14 in all of the solvent/electrolyte
systems studied. This can be attributed to solubilization of the oligomers










I I
1 -1.0oV


I5 LA/cm


Figure 3-9a: Scan rate dependence of PEDOT-C14 in 0.1 M TBAP/CH2CI2 at
100, 150, 200 and 250 mV/sec.











I I I I I I
-1.0 V


vs Ag/Ag+


I20 A


Figure 3-9b: Cyclic voltammogram of PEDOT-C8 in 0.1 M TBAP/CH3CN showing
two redox processes during reduction.





Table 1. Electrochemical Potentialsa for Monomer Oxidation and Polymer Redox Electroactivity


Polymerization EDOT-C8 EDOT-C14

Electrolyte System Ep,m Eap Ecl,p Ec2,p Ep,m Ea,p Eclp Ec2,p

0.1 M TBAP / CH3CN 0.89 -0.22 -0.14 -0.52 0.92 -0.19 -0.14 -0.32
0.1 M TBAP / CH2C12 0.96 -0.24 -0.56 0.96 -0.22 -0.54

0.1M LiCIO4/PC 1.01 0.05 -0.3 0.78 -0.15 -0.2
0.1 M LiC0O4 / Tetraglyme No polymer was deposited on the electrode surface, as the oligomers dissolved

0.1 M LiC4 / CH3CN 1.0 -0.2 -0.2 -0.5 0.96 -0.1 -0.2 -0.44
a Volts versus Ag / Ag+ ( 0.01M ) reference electrode.







Table 2. Solvent / Electrolyte Dependence of Polymer Redox Electroactivity


Redox Switching Peak Anodic Current Response (tA)a

Electrolyte System PEDOT PEDOT-C8 PEDOT-C14

0.1 M LiCIO4 / CH3CN 235 162 64
0.1 M TBAP/CH3CN 115 84.6 33.8

0.1 M LiC104 / PC 46 42 22

0.1 M TBAP / CH2C12 38 20 14.4
a Cyclic Voltammograms run at 100 mV/s on polymer films deposited during 10 cyclic scans between 0.8 V to 1.0 V in 10 mM
of monomer solution of the same solvent/electrolyte combination.








forming at the electrode surface by the long alkyl chains. Comparing solvents, it
can be seen that the deposition is most efficient in CH3CN based electrolytes
and least efficient in CH2CI2. This is due to the fact that CH2CI2 is a better
solvent for thiophene based polymers. In a practical sense, 0.1 M LiCIO4 /
CH3CN serves as the best electrodeposition medium for EDOT and alkyl
substituted EDOT polymers.

3-3-3 Optoelectrochemical Analyses

In order to probe the electronic structure of the polymers, and to examine
the optical changes that occur during redox switching, which are important for
electrochromic applications, optoelectrochemical analyses were carried out. A
film of the unsubstituted parent polymer, PEDOT, was potentiostatically
synthesized from 10mM EDOT and 0.1 M LiCIO4/CH3CN solution on an ITO
coated glass electrode at 1.1 V. After rinsing the film with monomer free
electrolyte, the polymer coated ITO glass electrode was used as the working
electrode, along with Ag/Ag+ (0.01 M) as reference and platinum as counter
electrodes in 0.1 M LiCIO4/CH3CN. The optoelectrochemical spectral series
was monitored while scanning from 300 nm (4.1 eV) to 1600 nm (0.78 eV) as
the polymer was sequentially stepped between -1.0 V and 0.6 V as shown in
Figure 3-10. When held at a reducing potential of -1.0 V, PEDOT exhibits an Eg

of approximately 1.6-1.7 eV and has a strong absorbance peak at 2.2 eV
causing the films to be deep blue and absorbing. As the potential was
sequentially increased, this peak reduces in intensity with the concommitant
growth of a low energy absorption at 1.4 eV. In the highly oxidized state, a
contiuous absorption band through the NIR is observed which tails into the
visible region of the spectrum, leaving the film in a sky blue and transmissive
state. Figure 3-11 shows the optoelectrochemical series for a PEDOT-C14 film



















0.6


0.2


0.5 1 1.5 2 2.5 3 3.5 4
Eg = 1.6

Energy (eV)



Figure 3-10: Optoelectrochemical studies of PEDOT in 0.1M LiCIO4/CH3CN at a = -1.0, b = -0.8, c = -0.6,
d = -0.4, e = -0.2, f = 0.0, g = 0.2, h = 0.4, i = 0.6 V vs Ag/Ag*






























1 a 1


SI I I I I I


1.5 2
Eg = 1.75


2.5
Energy (eV)


Figure 3-11: Optoelectrochemical studies of PEDOT-C11 in 0.1M UCIO4/CH3CN at a = -0.8, b = -0.6,
c = -0.4, d = -0.3, e = -0.2, f =- 0.1, g = 0.0, h = 0.2, i = 0.4, j = 0.6, k = 0.8, = 1.0.


2 L.


2.5


0 1




0.5-


1


0


3.5


4.
4.5


0.5








synthesized using the methodology described for PEDOT at a constant potential
of 1.0 V. The absorbance was monitored in situ as a function of potential
between -0.8 V and 1.0 V. In the reduced, neutral, form the polymer exhibits a
high absorbance throughout the visible region (1.75 eV to 3.0 eV) and is a deep
purple in color. As the potential is increased, the visible absorbance decreases
to the point that the film is light gray and quite transmissive when oxidized.
Similar behavior is found for PEDOT-C8 as seen in Figure 3-12. The Eg values

of both PEDOT-C8 and PEDOT-C14 are approximately 1.75 eV. The Eg values
for the polymer does not vary as the elelctrolyte or solvent is changed.
Comparison of Figure 3-10 with Figures 3-11 and 3-12 bring out the important
similarities and differences of the substituted and non-substituted polymers. In
the neutral state, PEDOT exhibits a single broad absorption, while the interband
transition for PEDOT-C14 (and PEDOT-Cs) is split into two peaks at 1.9 eV and
2.1 eV along with a high energy shoulder at 2.2 eV. This splitting can be
attributed to the vibronic coupling with the electronic absorption.101
Both PEDOT, PEDOT-C8 and PEDOT-C14 lose absorption throughout
the visible region due to depletion of the interband transition, while concurrently
increasing absorption in the NIR region with doping due to formation of
bipolaronic bands. PEDOT exhibits an increase of NIR absorption with a peak
evolving at 1.4 eV along with a low energy absorbance below 1 eV. In the case
of PEDOT-Cs and PEDOT-C14, a peak evolves at 1.25 eV. Above 0.0 V the
peak at 1.25 eV starts to decrease in intensity as absorption continues to
increase at 0.8 eV. At the highest oxidation levels the inter-band transition and
the transition at 1.25 eV reach a minimum, while the absorbance at 0.8 eV
continues to grow leading to a second isobestic point at 1.1 eV. This causes
PEDOT-Cs and PEDOT-C14 to have a lower absorption throughout the visible
region compared to unsubstituted PEDOT. This high optical contrast in the






0.7 0


0.6 Eg


0.5


0.4 h
0

S0.3 d


0.2


0.1 c Eg = 1.75 eV
b
a
0 -I I -I t I II i
0.5 1 1.5 2 2.5 3 3.5 4
Energy (eV)

Figure 3-12: Optoelectrochemical studies of PEDOT-Cs in 0.1M TBAP/CH3CN at a = -1.0, b = -0.8,
c = -0.6, d =-0.4, e = -0.2, f =- 0.1, g = 0.0, h = 0.1, i = 0.3, j = 0.5, k = 0.8.








visible region for the substituted polymers, make them better materials for
electrochromic devices. At high potentials the bipolaronic bands coalesce with
the valence and conduction bands giving rise to only a single absorption at low
energy suggesting metallic-like state at this potentials.86
The optoelectrochemical spectral series for PEDOT-C14 was examined
using two further electrolyte/solvent systems for growth and switching (0.1 M
LiCIO4/PC and 0.1 M TBAP/CH3CN). In both cases, essentially identical band

gaps, energies of transitions, existence and energies of isosbestic points were
found. The major difference was the overall absorbance values, though each
film was deposited with an identical amount of charge (100 mC/cm2). The
absorbance at Xmax (ca. 1.9-2.0 eV) for the neutral polymer was found to 2.4, 1.7
and 0.8 for films synthesized and switched in LiCIO4/CH3CN, TBAP/CH3CN,
and LiCIO4/PC respectively. This scales with the polymer's anodic current
response results reported in Table 2 and serves as a confirmation of the
solvent/electrolyte dependence for the efficiency of electropolymerization.

3-4 Long Term Switching Studies


3-4-1 Switching under ambient conditions

Long term switching studies were carried out to monitor absorbance

changes with time during repeated potential stepping between reduced and
oxidized states to obtain an insight into changes in the optical contrast. As
shown in Figure 3-13a and 3-13b, unsubstituted PEDOT was switched by
stepping the potential between -1.1 V to + 1.1 V with a switching interval of 20
seconds in a 0.1 M TBAP/CH3CN electrolyte. The polymer film changed from a
dark blue absorptive reduced state at -1.1 V to a light gray transmissive oxidized
state at +1.1 V with a high contrast. This switching could be carried out a






40-

35-

30-

25

20-

15-

10-

5-


0O


a -


S 20
20


S












a a a


40


60


* A A A A


80 100

Time (sec.)


I 2
120


a a a a a


140
140


160


Figure 3-13a: Long term switching studies of PEDOT in 0.1M TBAP/CH3CN. (Switches 1-3 shown).


a-iI U


I













40 ----


35 -





0 25 *0 2

S *
20 \
0 gI

15 5'


10 *

o II




30 35 40 45 50 55 60 65
Time (sec.)


Figure 3-13b: Optical transittance PEDOT in 0.1M TBAP/CH3CN for the 3 rd switch




IV

60

, 50

a
S40
0
C
3 30
I-
20


10



0 10 20 30 40 50 60

Time (sec.)


Figure 3-14a: Long term switching studies of PEDOT-C14 in 0.1M TBAP/CH3CN.
(Switches 16-21 shown).

















I I 8
I I
1.5 2 sec.' 3sec.
= B



I B
I I
I B










* B
I I


I I
B I I I





*
*I B I B
B I I I
I I
II B B
*


B A

B I B B
*I B
B I I

I B B
B I

*/ B

a a a a a a a a a a a a a


I I


a a I I I I I I I 1 I a 0 I1


Time (sec.)





Figure 3-14b: Optical transmittane of PEDOT -C14 in 0.1 M TBAP/CH3CN for the 19th switch


70 T


60




I 50




| 40



30


20




10


IIC


I I








number of times with little loss of optical contrast. It can be seen that the
reductive process proceeds significantly more rapidly than the oxidation
process. While the reduction is complete in ca. 1.5 seconds, reoxidation
requires greater than 10 seconds. As the film is oxidized, the polymer layer in
contact with the electrode oxidizes initially and a conductive front propagates
through the polymer. The rapidity of the oxidation can be attributed to the ease
of transport through the conducting film from the electrode contact. The
reduction of the oxidized film forms an insulator at the polymer-electrode
interface, leading to longer time for complete reduction of the film.
It was found that the alkyl substituted polymers are significantly faster in
their redox switching rates than the unsubstituted derivative. For example,
Figure 3-14a and 3-14b shows the switching of PEDOT-C14 between -0.8 V and
0.6V with a switching interval of 5 seconds in 0.1 M TBAP/CH3CN. PEDOT-C14
switches from reduced to oxidized state at nearly the same speed as PEDOT in
0.1 M TBAP/CH3CN, but takes about 3 seconds to revert back to reduced state
while PEDOT takes about 10 seconds. A multiple switching study of ca a 250
nm thick film of PEDOT-C14 in 0.1 M TBAP/CH3CN showed it to retain 40 % of
its electroactivity after 1800 switches while PEDOT looses most of its
electroactivity in 150 switches. In the oxidized state, PEDOT-C14 film was
initially 60% transmissive and less than 5 % transmissive in its reduced state.
After 1800 switches, the charge retention is over 50% as can be seen in Figure
3-15a and the contrast exhibited a slight drop to 55 % transmission in its
oxidized form and 10 % transmission in its reduced form as can be seen from
Figure 3-15b.
When the electrolyte is changed to 0.1 M LiCIO4/CH3CN PEDOT-C8 and
PEDOT-C14 were found to switch more rapidly. Figures 3-16a and 3-16b
shows that PEDOT-C8 reduces in less than 0.8 seconds and reoxidizes in about







100

90

80

70

60
50

40

30

20


600


U 0


800


1000


Number of double switches


Figure 3-15a: % Retention of Charge in the long term switching studies of PEDOT-C14
in 0.1 M TBAP/ CH3CN.


. a
0
0
a a


EP
0
d"rf~a" u


10

0


&# &R


P0o


%o gSo


200


400


1200


1400
1400


1600


1800


t........



























0 10 20 30 40 50
Time (sec.)



Figure 3-15b: Optical transmittance of PEDOT-C14 in 0.1M TBAP/CH3CN vs Ag/Ag+.
(switches 1764-1769 shown).












"
0
o









0 10 20 30 40 50 60
Time (sec.)

Figure 3-16a: Long term switching studies of PEDOT-Cs in 0.1M LiCIO4/CH3CN.
(Switches 335-339 shown).













0.6-0.8 sec. 2-2.5 sec.

r I I






o J






o S

















S I I I
* I I





















Time (sec.)
e
|
o
|














Figure 3-16b: Optical absorbance of PEDOT-CB in 0.1M LiCIO4/CH3CN for the 335th switch








2-3 seconds. PEDOT-C14 shows a similar speed of switching. PEDOT also
switches faster in 0.1 M LiCIO4/CH3CN as shown in Figures 3-17a and 3-17b.
PEDOT-C14 retains more than 70 % of its electroactivity after 1100 switches in
0.1 M LiCIO4 /CH3CN, while PEDOT retains less than 50 % of its electroactivity
after 140 switches.

3-4-2 Switching in a Completely Reversible Cell

In the previous experiments, a platinum plate was used as a counter
electrode and the electrochemistry occurring at this electrode was ignored.
Electrochemical reactions at this junction is not known as platinum does not
undergo redox chemistry under the experimental conditions used. At the redox
potential of the electrochemical cell for the polymeric system, the reaction at the
counter electrode can lead to formation of ions, radicals or decomposition
products that can be detrimental to the life of the cell, and to the life of polymer
under investigation. Hence, to have a well defined electrochemical reaction at
the counter electrode, a Li electrode was chosen.102
The two half reactions that make up the electrochemical cell can be
denoted as
PEDOT PEDOT+ + e

LU+ + e Li

PEDOT + Li +~ PEDOT+ + Li


Due to the reactivity of lithium, this experiment was carried out in the dry-
box under argon. It should be noted that the use of Li in commercial batteries is
quite common, making these studies practically important, in addition to
providing two reversible redox couples. Propylene carbonate was the solvent













60

50

c40


E30 c1
C






0 I I I II I
0 (D M CO 0 to N CD
I M O vI
Time (sec.)



Figure 3-17a: Long term switching studies of PEDOT in 0.1M LiCIO4/CH3CN.
(Switches 18-24 shown).















60


50





40



8

I 30

E
20


0 20





10




0
O 5


1.5-2.0 sec.












II
*





15
St
*







II
*
I
a I
a I
II
I









a
so



S
a I a
I a
S, S _


I I


ca 4 sec.


S I
a\ S
a I
I
*\ a
*\ a







I
S I
a I

* a


A a A A


I I I *


a a a a S S S ra


I I I I 1 I


Time (sec.)



Figure 3-17b: Optical transmittance of PEDOT in 0.1M LiCIO4/CH3CN for the 19th switch


- -


a. a 2. .






120
pEDOT-C14

100 A pEDOT-C8

L : pEDOT
* so ..
. t A *AAA


9 40


20
0


0 2000 4000 6000 8000 10000 12000 14000 16000
Number of Double Switches

Figure 3-18: Long term switching studies of PEDOT, PEDOT-C8 and PEDOT-C14 in 0.1M LiCIO4/PC
with Lithium as counter electrode.




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