Electrochromic enhancement of poly(3,4-ethylenedioxythiophene) films functionalized with hydroxymethyl and ethylene oxide

Article abstract of DOI:10.1002/pola.27206

2-((2,3-Dihydrothieno[3,4-b]dioxin-2-yl)methoxy)methyl oxirane (EDOT-MO) was successfully synthesized by the reaction of epichlorohydrin with hydroxymethylated-3,4-ethylenedioxylthiophene (EDOT-MeOH), which was synthesized via a simple four-step sequence. Poly(hydroxymethylated-3,4- ethylenedioxylthiophene) (PEDOT-MeOH) and poly(2-((2,3-dihydrothieno[3,4-b] dioxin-2-yl)methoxy)methyl oxirane) (PEDOT-MO) were electrosynthesized through electropolymerization of EDOT-MeOH and EDOT-MO, respectively. Structural, electrochemical, optical, and thermal properties of as-formed polymers were investigated by FTIR, cyclic voltammetry, UV-vis, and thermogravimetry. Spectroelectrochemistry studies demonstrated that PEDOT-MeOH and PEDOT-MO could be reversibly oxidized and reduced accompany with obvious color changes. Further kinetic studies demonstrated that the introduction of hydroxymethyl or ethylene oxide group significantly improved electrochromic properties of 3,4-ethylenedioxythiophene (PEDOT) and resulted in high contrast ratios (57.3% at 585 nm) and coloration efficiencies (338.5 cm² C^-1), low switching voltages, and fast response time.

Full text of DOI:10.1002/pola.27206

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Electrochromic Enhancement of Poly(3,4-ethylenedioxythiophene) Films
Functionalized with Hydroxymethyl and Ethylene Oxide
Shimin Zhang, Jingkun Xu, Baoyang Lu, Leiqiang Qin, Long Zhang, Shijie Zhen, Daize Mo
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013,
People’s Republic of China
Correspondence to: J. K. Xu (E-mail: xujingkun@tsinghua.org.cn) or B. Y. Lu (E-mail: lby1258@163.com)
Received 14 November 2013; accepted 22 March 2014; published online 22 April 2014
DOI: 10.1002/pola.27206
ABSTRACT: 2-((2,3-Dihydrothieno[3,4-b]dioxin-2-yl)methoxy)
methyl oxirane (EDOT-MO) was successfully synthesized by
the reaction of epichlorohydrin with hydroxymethylated-3,4-
ethylenedioxylthiophene (EDOT-MeOH), which was synthesized
via a simple four-step sequence. Poly(hydroxymethylated-3,4-
ethylenedioxylthiophene) (PEDOT-MeOH) and poly(2-((2,3-dihy-
drothieno[3,4-b]dioxin-2-yl)methoxy)methyl oxirane) (PEDOT-MO)
were electrosynthesized through electropolymerization of
EDOT-MeOH and EDOT-MO, respectively. Structural, electro-
chemical, optical, and thermal properties of as-formed poly-
mers were investigated by FTIR, cyclic voltammetry, UV–vis,
and thermogravimetry. Spectroelectrochemistry studies dem-
onstrated that PEDOT-MeOH and PEDOT-MO could be reversi-
bly oxidized and reduced accompany with obvious color
changes. Further kinetic studies demonstrated that the intro-
duction of hydroxymethyl or ethylene oxide group significantly
improved electrochromic properties of 3,4-ethylenedioxythio-
phene (PEDOT) and resulted in high contrast ratios (57.3% at
2
21
585 nm) and coloration efficiencies (338.5 cm
switching voltages, and fast response time.
C
), low
V
C
2014 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52,
1989–1999
KEYWORDS: conducting polymers; electrochemical polymeriza-
tion; electrochemistry; electrochromics; ethylene oxide; func-
tionalization of polymers; PEDOT derivatives
INTRODUCTION Conducting polymers (CPs) have been exten-
sively studied in both fundamental and practical perspectives
due to their applications in various fields, such as solid elec-
trolyte capacitors, printed wiring boards, indium tin oxide
electrochromic effect has attracted significant attention and
has been the focus of numerous publications.
2
5–29
3
0
31–35
Havinga’s and Reynolds’s
groups have reported the
(
ITO) substitution, organic light-emitting diodes, organic
first neutral-state solution processable PEDOT derivatives by
introducing solubilizing side chains to the ethylenedioxy
bridge of PEDOT. Previously, acrylate- and methacrylate-
functionalized 3,4-ethylenedioxythiophene (EDOT) and their
solar cells, electrochromics, biosensors, and organic field-
effect transistors.
attracted many scientists’ attention because of their out-
standing advantages over transition metal oxides, such as
long-term stability, high contrast ratio and coloration effi-
ciency (CE), fast response time, low switching potential, and
1
–8
CPs as eletrochromic materials have
3
6
electrochromic properties were investigated by Xu et al.
3
7
and Kim et al. These studies have demonstrated that the
presence of substituents on the polymer main chain, or
increasing the size of the alkylenedioxy bridge, significantly
change the oxidation potentials of the monomers, band gap
and solubility, and optical properties of corresponding CPs.
9
–23
the ease of color control through structure modification.
Among numerous CPs materials that have been studied,
poly(3,4-ethylenedioxythiophene) (PEDOT) and its deriva-
tives are the most extensively explored CPs in basic and
applied research because of their low oxidation potentials,
high CEs, and their excellent stability. These outstanding
properties are derived from its innovative electro-rich poly-
meric backbone possessing high oxygen content and inhibi-
ting a-, b-, and b, b-crosslinks on polymerization. Within the
field of electrochromism, PEDOT film is dark blue in its
reduced state, showing a band-gap of about 1.6 eV, and
Substitution of side groups on the polymer main chain not
only change the optical properties of materials but also can be
used as the active site to introduce chemically bonded func-
3
8
tional groups.
Hydroxymethylated-3,4-ethylenedioxylthio-
phene (EDOT-MeOH), which was first synthesized in 1992 by
3
9
Blohm et al., has been used as the versatile intermediate for
the synthesis of different EDOT derivatives bearing various
functional groups via organic reactions of hydroxyl group with
2
4
40–46
nearly transmissive sky blue in its oxidized state. This
substitutions.
Subsequently, the electropolymerization of
V
C
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SCHEME 1 Efficient synthesis routes for EDOT-MeOH and EDOT-MO, and their electrochemical polymerization.
EDOT-MeOH and spectroelectrochemical properties of the
corresponding polymer, poly(hydroxymethylated-3,4-ethylene-
dioxylthiophene) (PEDOT-MeOH), were briefly reported by
Roncali and Reynolds. In this work, we synthesized EDOT-
MeOH by an efficient four-step sequence with a high global
yield (ca. 42%).
Synthesis
2-Chloromethyl-2,3-dihydrothieno[3,4-b][1,4]dioxine
To a three-necked flask equipped with a nitrogen purge,
4
0
44
CH ONa (20.8 g, 30% mass fraction, 107.8 mmol), 3,4-dibro-
3
mothiophene (5.0 g, 20.7 mmol), CuO (16.5 g, 207.4 mmol),
KI (1.4 g, 8.3 mmol), and CH OH (20 mL) were added. The
3
reaction was refluxed for 72 h. Then, another CH ONa (20.8
g, 107.8 mmol) was added, and the solution was refluxed for
3
Ethylene oxide (EO) is
a well-known sterilizing and
crosslinking agent. Here, we introduced the EO group into 3,4-
ethylenedioxythiophe to form a monomer of 2-((2,3-dihydro-
thieno[3,4-b]dioxin-2-yl)methoxy)methyl oxirane (EDOT-MO),
and the corresponding polymer (PEDOT-MO) was electro-
chemically synthesized via potentiostatic polymerization
2
4 h and was then cooled to room temperature and filtered
through a sintered glass fritted funnel. The resulting soiled
was then extracted with ether. The organic fractions were
combined and dried with anhydrous MgSO . After removal of
the solvent, the remaining crude product was isolated by
flash chromatography (silica gel, petroleum ether) to give
4
(
Scheme 1).
3
,4-dimethoxythiophene (2.6 g, 18.0 mmol) as colorless oil
The electrochemical polymerization of EDOT-MeOH and
EDOT-MO was researched in detail. Furthermore, the proper-
ties of resulting polymers, such as electrochemical behavior,
structural information and electrochromic properties were
investigated, and their electrochromic performance was
emphasized in particular.
in 86.9% yield.
To a three-necked flask equipped with a nitrogen purge, 3,4-
dimethoxythiophene (1.4 g, 10.0 mmol), 3-chloro-1,2-
propanediol (2.5 g, 22.7 mmol), p-toluene sulfonic acid (0.15
g, 0.88 mmol), and dry toluene (27 mL) were added. The
ꢀ
solution was heated at 90 C for 24 h. Then, another (2.5 g,
2
2.7 mmol) of diol was added, and the solution was heated
EXPERIMENTAL
ꢀ
at 90 C for another 3 h and was then allowed to cool to
room temperature. After removal of the solvent, the remain-
ing crude product was isolated by flash chromatography
(silica gel, hexane/DCM, 8/2, V/V) to give (1.2 g, 6.3 mmol)
a white solid in 62.9% yield.
Materials
3
,4-Dibromothiophene (98%, Shanghai Bangcheng Chemi-
cal); sodium methoxide (CH ONa, 30%, Tianjin Fuchen
Chemical Reagent Factor); 3-chloro-1,2-propanediol (Wuhan
Remote Technology Development); tetrabutylammonium
3
l
H NMR (400 MHz, CDCl , ppm): d 6.37 (s, 2H), 4.35–4.40
3
hexafluorophosphate (Bu
4
NPF
ꢀ
6
, 98%; Acrocs Organics) was
(m, 1H), 4.27–4.35 (m, 1H), 4.14–4.18 (m, 1H), 3.65–3.75
dried under vacuum at 60 C for 24 h before use and epi-
chlorohydrin (99%; Aladdin) was used without further
treatment. Dichloromethane (DCM) was purified by distilla-
tion over calcium hydride before use, sodium acetate
(m, 2H).
(2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl Acetate
(
CH COONa, 99%), (Tianjin Bodi Chemicals). Other reagents
To a 100-mL three-necked flask, 2-chloromethyl-2,3-dihydro-
3
were all analytical grade and used as received without fur-
ther treatment.
thieno[3,4-b][1,4]dioxine (1.9 g, 10.0 mmol), CH COONa
(1.25 g, 15.2 mmol), and DMSO (30 mL) were added. The
3
1
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ꢀ
solution was stirred for 1 h at 120 C. The mixture was
poured into water and extracted with DCM. After removing
DCM under reduced pressure, light yellow oil was obtained
and used in the next step without further purification.
electrode, and it revealed sufficient stability during the
experiments.
The typical electrolytic solution was DCM-Bu NPF (0.1 mol
4
6
2
1
21
L
) containing 0.01 mol L EDOT-MeOH or EDOT-MO. All
the solutions were de-aerated by a dry nitrogen stream
more than 20 min) and maintained under a slight overpres-
(
2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methanol
(
To a 100-mL round-bottom flask equipped with a reflux con-
denser, (2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl ace-
tate (EDOT-MeOAc; 1.91 g, 10.02 mmol) was added to a
solution of NaOH (1.40 g, 35.0 mmol) in water (40 mL). The
mixture was refluxed for 1 h and then cooled to room tem-
perature. Then, 20 mL of water was added. The mixture was
acidified, and then extracted with DCM. The solvent was
removed under reduced pressure, and column chromatogra-
phy (hexanes/DCM, 8/2, V/V) was performed to give (1.30
g, 7.55 mmol) product in 85.0% yield.
sure through all the experiments. The amount of polymer
deposited on the electrode was controlled by the integrated
charge passed through the cell. The polymer films were
grown potentiostatically, and their thickness was controlled
by the total charge passed through the cell, which was read
directly from the current–time (I–t) curves by computer.
After polymerization, the polymer films were washed repeat-
edly with anhydrous diethyl ether in order to remove the
electrolyte, monomer, and oligomers. For spectral analyses,
the polymer films were dedoped chemically with 25%
ammonia for 3 days and washed repeatedly with pure water.
l
3
H NMR (400 MHz, CDCl , ppm): d 6.34 (s, 2H), 4.24 (d, 2H),
ꢀ
Finally, the polymer films were dried at 60 C under vacuum
4.07–4.12 (m, 1H), 3.84–3.87 (m, 2H).
for 24 h.
2
-((2,3-dihydrothieno[3,4-b]dioxin-2-yl)methoxy)methyl
Characterization
Oxirane
Infrared spectra were determined with a Bruker Vertex 70
Fourier-transform infrared (FTIR) spectrometer with samples
in KBr pellets. Electrochemical, spectroelectrochemical, and
The solution of EDOT-MeOH (1.0 g, 5.8 mmol), epichlorohy-
drin (1.3 g, 14 mmol), tetrabutylammonium bromide (TBAB,
0.05 g, 0.16 mmol), and 5 mL n-hexane were added to a
kinetic studies were carried out on
a
Model 263
flask combined with sodium hydroxide solution (7 mL, 2 M).
potentiostat-galvanostat (EG&G Princeton Applied Research)
and a Cary 50 UV–vis-NIR spectrophotometer under com-
puter control. Thermogravimetric analysis (TGA) was per-
formed with a Pyris Diamond TG/DTA thermal analyzer
ꢀ
The solution was heated and stirred at 70 C for 5 h under a
nitrogen atmosphere. Then, thin layer chromatography indi-
cated almost complete consumption of the starting material.
The resulting mixture was poured into water and extracted
with DCM. The organic layer was washed with water, and
(
PerkinElmer) under a nitrogen stream from 290 to 1100 K
2
1
at a heating rate of 10 K min
.
then dried with MgSO . After the removal of solvent, the
4
remaining crude product was isolated by column chromatog-
raphy with silica (ethyl acetate/petroleum ether, 1/8, V/V)
to give EDOT-MO as a colorless liquid (yield 80%).
Spectroelectrochemical measurements were carried out to
consider absorption spectra of these polymer films under
applied potentials. The spectroelectrochemical cell consists
of a quartz cell, an Ag wire, a Pt wire, and an ITO/glass as
transparent working electrode. All measurements were car-
1
H NMR (400 MHz, CDCl , ppm): d 6.33 (d, 2H), 4.33 (m,
3
2
H), 4.06–4.10 (m,1H), 3.68–3.74 (m, 2H), 3.42–3.45 (m,
H), 3.79–3.88 (m, 1H), 3.15 (s, 1H), 2.8 (t, 1H) 2.60 (t, 1H).
ried out in 0.1 M Bu NPF6 as the supporting electrolyte in
4
1
1
3
DCM.
C NMR (100 MHz, CDCl
3
, ppm): d 141.1, 140.9, 99.2, 99.1,
72.1, 71.8, 69.2, 65.5, 50.1, 43.5.
The optical density (DOD) at the specific wavelength (kmax
)
was determined by using optical contrast values (DT%) of
Electrochemical Polymerization of EDOT-MeOH and
EDOT-MO
electrochemically oxidized and reduced films, using the fol-
lowing equation:
4
7
Electrochemical tests and polymerization of monomers were
performed in a one-compartment cell with the use of a
Model 263A potentiostat-galvanostat (EG&G Princeton
Applied Research) under a computer control. For electro-
chemical tests, the working and counter electrodes were
both Pt wires with a diameter of 1 mm. They were placed 5
mm apart during the tests. To obtain a sufficient amount of
the polymer films for characterization, Two Pt sheets with a
DOD5logðTox=TredÞ
(1)
CE is defined as the relation between the injected/ejected
charge as a function of electrode area (Q ) and the change in
optical density (DOD) at the specific dominant wavelength
d
4
8
(
kmax) as illustrated by the following equation:
CE5DOD=Qd
(2)
2
surface area of 3 3 2 cm were used as the working and
counter electrodes, respectively. These aforementioned elec-
trodes were polished carefully with abrasive paper (1500
mesh), cleaned by water and acetone successively, and then
dried in air before each experiment. An Ag/AgCl electrode
directly immersed in the solution served as the reference
RESULTS AND DISCUSSION
Cyclic Voltammety
The electrochemical polymerization of EDOT-MeOH and
2
1
EDOT-MO in DCM-Bu NPF (0.1 mol L ) were investigated
4
6
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1
21
FIGURE 1 CVs of EDOT-MeOH (A) and EDOT-MO (B) in DCM-Bu
4
NPF
6
(0.1 mol L ). Monomer concentration: 0.01 mol L , Poten-
21
tial scan rate: 100 mV s . [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
by cyclic voltammetry (CV), and the resulting CVs were
shown in Figure 1. The representative electrochemical
growth reveals the electrochemical performance of mono-
mers and the formation of corresponding electroactive poly-
mers on the Pt electrode surface. As can be seen from Figure
tion, regular morphology, and good adherence against the
working electrode, the optimized applied potentials were 1.2
V versus Ag/AgCl for EDOT-MeOH and 1.3 V versus Ag/AgCl
for EDOT-MO. Therefore, PEDOT-MeOH and PEDOT-MO films
used for the characterization mentioned below were all pre-
pared by the chronoamperometry method at the constant
potential of 1.2 and 1.3 V versus Ag/AgCl in DCM-Bu NPF
1(A), the onset oxidation potential of EDOT-MeOH was found
at about 0.98 V, a little higher than that of EDOT-MeOH in
4
6
4
1
21
acetonitrile. Meanwhile, the onset oxidation potential of
EDOT-MO was about 1.23 V, approximately the same as
EDOT (1.2 V) and acrylate modified 3,4-ethylenedioxythio-
(0.1 mol L ), respectively.
Structural Characterizations
3
6
49,50
phene, but slightly higher than alkylsubstituted EDOTs.
FTIR can provide much structural information for newly
obtained polymers, especially for insoluble and infusible
polymers. A comparison of the evolution of the vibrational
modes appearing in polymers and in some simple molecules
acting as references usually facilitates the interpretation of
the experimental absorption spectra. In order to elucidate
the structure of polymers and interpret the polymerization
mechanism, FTIR spectra of monomers [Fig. 3(A,C)] and the
dedoped polymers [Fig. 3(B,D)] were recorded. As can be
seen from Figure 3, the absorption bands of the polymers
were obviously broadened in comparison with the mono-
mers. This mainly due to the wide chain dispersity of the
resulting product composed of oligomers/polymers. In Fig-
Also, the anodic current began to increase at about 1.15 V,
and there was a current loop between 1.15 and 1.40 V. The
formation of this loop is characteristic for nucleation
processes.
The EDOT-MO monomer presents two irreversible cathodic
peaks at 20.26 and 0.6 V in CV measurements [Fig. 1(B)].
This can be attributed to the reduction of electroactive poly-
mer films on the electrode surface. At the same time, the
two cathodic peaks may be due to sequential reduction of
dicationic bipolaronic charge carriers initially to cation radi-
cal polaronic charge carriers and then to neutral polymer or
5
0
may be due to the ion transport properties of the polymer.
21
ure 3, peaks located in the range of 2860–2930 cm corre-
spond to the CH vibrations and were present in all simples.
2
Moreover, the increase of anodic and cathodic peak current
densities in CVs implied that the amount of the polymer ele-
trodeposited increased on the electrode surface. The broad
redox waves of the polymer may be ascribed to the wide dis-
tribution of polymer chain length. The potential shift of the
wave current maximum provided information about the
increase in the electrical resistance of the polymer film and
the over potential needed to overcome this resistance.
21
Vibrations at 1500, 1450, and 1339 cm are attributed to
the stretching modes of C5C and C-C in the thiophene ring.
In the spectra of EDOT-MO [Fig. 3(C)], the peaks at 3055
21
and 860 cm were closely related to the stretching vibra-
21
tion of alkylene oxide. The bands at 3115 cm in the spec-
tra of EDOT-MeOH [Fig. 3(A)] and EDOT-MO [Fig. 3(C)] was
produced by C-H vibration of the 2,5-positions in the thio-
phene ring, while it disappeared in the spectra of PEDOT-
MeOH [Fig. 3(B)] and PEDOT-MO [Fig. 3(D)], indicating the
electropolymerization occurred at 2,5-positions of the thio-
Optimization of Electrical Conditions and Preparation of
PEDOT-MeOH and PEDOT-MO
3
6
phene ring.
To optimize the applied potential used for polymerization, a
set of current transients during the electropolymerization of
EDOT-MeOH and EDOT-MO at different applied potentials
were recorded, as shown in Figure 2. Considering the overall
factors affecting the quality of the as-formed polymer film,
such as moderate polymerization rate, negligible overoxida-
Electrochemistry of PEDOT-MeOH and PEDOT-MO Films
In order to get a deeper insight into the electroactivity and
stability of PEDOT-MeOH and PEDOT-MO films, the electro-
chemical behavior were determined carefully by CV in
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4 6
FIGURE 2 Chronoamperograms of EDOT-MeOH (A) and EDOT-MO (B) in DCM-Bu NPF (0.1 mol L ) solutions on Pt electrode at
21
different applied potentials. Monomer concentration: 0.01 mol L . [Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
2
1
monomer free DCM-Bu NPF (0.1 mol L ) (Fig. 4). Similar
to previous results,
group has little influence on the fluorescence properties of
EDOT-MeOH and PEDOT-MeOH. The red shift between the
monomer and the polymer (ca. 63 nm) could be clearly seen
from Figure 6, which was mainly attributable to the elonga-
tion of the polymer delocalized p-electron chain sequence.
Based on above discussion, PEDOT-MeOH and PEDOT-MO
were typically blue-light-emitting polymers.
4
1–55
6
5
steady-state CVs represented broad
anodic and cathodic peaks and the peak current densities
were proportional to the potential scan rates (inset of Fig.
4
), indicating that the redox processes were not controlled
by diffusion limits and the formation of well-adhered electro-
active polymer films on the surface of the electrode. With
the increase of the scan rate, the potentials of anodic and
cathodic peak current densities show positive and negative
shift, which means a quasi-reversible redox processes at
high scan rates. At the potential scan rate of 25 mV s , the
PEDOT-MeOH film could be oxidized and reduced from 0.17
anodic peak potential, E ) to 20.04 V (cathodic peak poten-
a
Spectroelectrochemical Properties
Spectroelectrochemical studies of PEDOT-MeOH and PEDOT-
MO, as shown in Figure 7, were performed to evaluate their
electronic properties and band gaps and, consequently, to
elucidate the effect of the hydroxymethyl and EO substitution
on the electronic properties of PEDOT. The electro-optical
properties of the polymer films were investigated using the
changes in electronic absorption spectra at various applied
potentials. Polymer films were eletrodeposited on ITO-coated
glass slides, and a home-made electrochemical cell was built
from a commercial UV–visible cuvette. The cell was placed in
2
1
(
tial, E ), and PEDOT-MO film from 0.10 (E ) to 20.23 V (E ),
c
a
c
respectively.
The long-time CV of the polymers was also investigated in
2
1
monomer-free DCM-Bu
4 6
NPF (0.1 mol L ), as shown in Fig-
ure 5. The number of scanning cycles was 6000 between
0.3 and 0.45 V at the potential scan rate of 150 mV s
2
1
2
.
From Figure 5, both the PEDOT-MeOH and PEDOT-MO films
could be cycled repeatedly between the conducting (oxi-
dized) and insulating (neutral) states without significant
decomposition. After completion of 1000 cycles, PEDOT-MO
had 94.0% of its original electroactivity, while PEDOT-MeOH
displayed inferior stability of 91.9%. After completion 6000
cycles, the stability of PEDOT-MeOH exhibited a decrease to
65.8%, while PEDOT-MO still had about 79.2% of electroac-
tivity, and maintained good redox behavior. This indicated
that the EO group could improve the stability of PEDOT-
MeOH.
Fluorescence Spectra
Figure 6 shows the fluorescence emission spectra of the
monomers dissolved in DMSO and depoded polymer films
on ITO-coated glass. It was observed that the emission peaks
for the monomers emerged at 354 nm whereas the maxi-
mum emission at 417 nm characterized the spectra of the
dedoped polymer films. This indicates that the pendant EO
FIGURE 3 FTIR spectra of EDOT-MeOH (A), dedoped PEDOT-
MeOH (B), EDOT-MO (C), and dedoped PEDOT-MO (D).
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1
FIGURE 4 CVs of PEDOT-MeOH (A) and PEDOT-MO (B) in monomer free DCM-Bu
4
NPF
00, (b) 250, (c) 200, (d) 150, (e) 100, (f) 50, and (g) 25 mV s . Inset: plots of redox peak current densities versus potential scan
rates. j is the peak current density: and jp,a and jp,c denote the anodic and cathodic peak current densities, respectively. [Color fig-
ure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
6
(0.1 mol L ) at potential scan rates of (a)
21
3
p
the optical path of the sample light beam in a UV–vis-NIR
spectrophotometer, which allowed us to acquire electronic
absorption spectra under potential control in DCM-Bu NPF
PEDOT-MO and PEDOT-MeOH, the oxirane group has little
influence on the electronic properties of PEDOT-MeOH. It is
well known that the oxidation of an electrochromic material
will produce radical cations (polarons) and further oxidation
produce dications (bipolarons), allowing new electronic tran-
sition thereby changing absorption spectra. As can be seen
in Figure 7, upon stepwise increase of the applied potentials,
formation of charge carriers leads to new absorption bands
at longer wavelength. When the oxidation potential reaches
4
6
21
(
0.1 mol L ) solution.
Figure 7 shows the spectroelectrochemistry of PEDOT-MeOH
and PEDOT-MO films revealing p–p* transition in the
reduced state, which fades with the increase of the applied
potential. In the reduced state, the films exhibited strong
absorption at the wavelength around 590 nm and tailed to
around 750 nm due to the p–p* transition. Compared with
0.5 V, the intensity of the p–p* transition reduces and the
simultaneous increase of absorbance at 900 nm can be
observed. This change represents the formation of polaron
and bipolaron bands.
4
0
5
80 nm obtained for PEDOT, the optical maximum (kmax
of neutral PEDOT-MeOH [Fig. 7(A)] and PEDOT-MO [Fig.
(B)] are slightly red-shifted to 592 and 590 nm, respec-
tively. The spectroeletrochemically measured band gaps
extracted from the onset of p–p* transition) is 1.68 eV (738
)
7
The film colors of PEDOT-MeOH and PEDOT-MO at different
potentials were presented in Figure 8. The PEDOT-MeOH
film exhibited blue and transmissive sky blue at neutral
(21.0 V) and oxidized state (0.5 V), respectively. While the
PEDOT-MO film displayed four different color under various
(
nm) for PEDOT-MeOH and PEDOT-MO is 1.70 eV (730 nm),
which are slightly higher than PEDOT (1.6 eV). Considering
the small difference (0.02 eV) between the band gap of
2
1
FIGURE 5 CVs of PEDOT-MeOH (A) and PEDOT-MO (B) upon repeated cycling at the scan rate of 150 mV s . [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 6 Emission spectra of the monomers dissolved in
FIGURE 8 Optical images of PEDOT-MeOH (A) and PEDOT-MO
(B) under various applied potentials. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
DMSO and polymer films on ITO-coated glass: EDOT-MeOH
(
A), EDOT-MO (B), dedoped PEDOT-MeOH (C) and dedoped
PEDOT-MO (D). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
(
DT%), response time and CE of the polymer films were
applied potentials. The neutral PEDOT-MO film was dark
blue (21.2 V), and it changed to purple (0 V), transparent
gray (0.2 V), and transmissive sky blue (0.6 V) upon oxida-
tion. PEDOT-MeOH and PEDOT-MO films showed different
multicolor electrochromism, which can be ascribed to the
different substitutions on the polymer main chain. The multi-
color properties have significant potential applications in
smart windows or displays, and so on.
investigated by increment and decrement in the transmit-
tance with respect to time at different wavelength, as shown
in Figure 9. The potentials switched stepwise between neu-
tral and oxidized states with a residence time of 10 s. The
applied potentials for PEDOT-MeOH were interchanged
between 21.0 and 0.5 V at 590 nm and 21.0 and 20.1 V at
8
2
60 nm. While the applied potentials for PEDOT-MO was
1.2 and 0.4 V at 585 and 900 nm. The optical contrast
ratio, response time, and CE of PEDOT-MeOH and PEDOT-MO
films were summarized in Table 1.
Electrochromic Switching of PEDOT-MeOH and
PEDOT-MO Films
The DT% of PEDOT-MeOH was found to be 41.1% at 590 nm
and 17.9% at 860 nm. For PEDOT-MO, DT% was found to be
In electrochromic applications, it is important that a polymer
can switch rapidly between its reduced and oxidized state
and exhibit striking color changes. The electrochromic
switching studies were investigated using a square wave
potential step method coupling with optical spectroscopy
known as chronoabsorptometry in a monomer free solution.
The electrochromic parameters, such as optical contrast ratio
29.7% at 900 nm and 57.3% at 585 nm, which indicating
the EO group significant improves the contrast ratios of
PEDOT-MeOH. The DT% value of PEDOT-MO at 585 nm
(
(
57.3%) is higher than that of analogous polymer PEDOT-AA
50.9% at 620 nm) and PEDOT-MA (40.8% at 578 nm).
3
6
37
FIGURE 7 Spectroelectrochemistry of PEDOT-MeOH (A) and PEDOT-MO (B) on ITO-coated glass in monomer free DCM-Bu
4
NPF
6
21
(
0.1 mol L ) solution. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 1 Electrochromic Parameters for PEDOT-MeOH and PEDOT-MO
Response Time (s)
Oxidation Reduction
Wavelength
Coloration Efficiency
2
21
Polymer
(nm)
T
neut (%)
T
oxi (%)
DT (%)
(cm C
)
PEDOT-MeOH
590
18.5
54.2
3.9
59.6
36.4
61.2
24.2
41.1
17.8
57.3
29.7
1.0
1.2
2.1
1.1
3.2
3.5
1.8
3.7
222.7
125.9
338.5
160.7
8
60
585
00
PEDOT-MO
9
53.9
The CE is another important parameter for electrochromic
material and obtained for a certain amount of the charge
injected in the polymer as a function of the change in optical
density. In this work, CE of PEDOT-MO was measured as
of ions moving into the polymer chains during the doping
process. As illustrated in Figure 9, the response time
required to attain 95% of the total transmittance difference
was found to be 1.0 s at 590 nm for PEDOT-MeOH and 1.1 s
at 900 nm for PEDOT-MO from the reduced state to the oxi-
dized state, while from oxidized state to reduced state, the
response time was 3.2 s at 590 nm for PEDOT-MeOH and
1.8 s at 585 nm for PEDOT-MO, respectively. Thus, PEDOT-
MeOH and PEDOT-MO can be rapidly switched to the oxi-
dized state, which can be attributed to the ease of charge
transport in the conducting film when it is oxidized. The fast
response time can be attributed to a good affinity to the
2
21
3
38.5 cm
C
(at 585 nm), approximately the same as
2
21 36
PEDOT-AA (339 cm
MeOH (222.7 cm C ), PEDOT-MA (175 cm C ) , PEDOT
C
), but much higher than PEDOT-
2
21
2
21 37
2
21 47
(137 cm
C
)
and many inorganic electrochromic
materials.
As for electrochromic applications, response time of polymer
is another important parameter since it indicates the speed
FIGURE 9 Electrochromic switching and optical absorbance monitored at 590 nm (A) and 860 nm (B) for PEDOT-MeOH and at 585
nm (C) and 900 nm (D) for PEDOT-MO recorded during double step spectrochronoamperometry.
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FIGURE 10 Open circuit memory of PEDOT-MeOH (A) and PEDOT-MO (B) monitored at 590 nm and 585 nm, respectively. [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
electrolyte with the polymer, the small solution resistance,
and little interference of substitutions with ion transport.
Based on the discussion, the high contrast ratio, high CE and
the fast switching property make PEDOT-MeOH and PEDOT-
MO promising electrochromic materials.
contrast, which is probably due to the dopants’ reinclusion
into the films and make the films partly doped.
Optical Stability of PEDOT-MeOH and PEDOT-MO Films
It is well known that the good stability of CPs is very impor-
tant for their applications in electronic devices. The optical
stability of polymer films was measured by using a square
wave potential step method together with measuring its per-
centage transmittance. Figure 11 displays the switching of
PEDOT-MeOH and PEDOT-MO at an interval of 10 s in DCM-
Open Circuit Stability
The color persistence of electrochromic devices is an impor-
tant feature because it is directly related to its application
and energy consumption during that utilization. This property
was studied by polarizing the material between reduced and
oxidized states together with measuring its spectra under
open circuit conditions at regular time intervals. As shown in
Figure 10, initially, the polymer film was oxidized or reduced
for 2 s, then the circuit was held disconnected for 100 s and
the transmittance was recorded at the time. As can be seen in
Figure 10, the oxidized PEDOT-MeOH and PEDOT-MO are
highly stable and the films keep their color without significant
loss declared by the insignificant variations in optical contrast.
The reduced polymer films show slightly variations in optical
21
Bu NPF (0.1 mol L ). After 100 double switched in poten-
4 6
tial in the electrolyte, the optical activity of PEDOT-MeOH
was preserved up to 91.3%, while that of PEDOT-MO film
was preserved up to 94.7%. The decrease in contrast ratio
after each redox cycle resulted from the trapping of ions
among the polymer main chains, and the irreversibility of
ionic transport resulted in a decay in electroactivity. So, the
loss in electroactivity resulted from the polymer chains los-
ing their original optical switching capability after the trap-
5
6
ping of ion pairs.
FIGURE 11 Optical activity of PEDOT-MeOH (A) at 590 nm and PEDOT-MO (B) at 585 nm upon repeated cycling with the switching
time of 10 s. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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2
21
ratios (57.3% at 585 nm), higher CEs (338.5 cm
C
for
PEDOT-MO), lower switching potentials (-1.0 V to 0.5 V for
PEDOT-MeOH and -1.2 V to 0.4 V for PEDOT-MO), fast
response time (1.0 s for PEDOT-MeOH and 1.1 s for PEDOT-
MO), good electrochemical, and optical stability. The PEDOT-
MO film showed better electrochemical and optical stability
than PEDOT-MeOH, which could be ascribed to the incorpo-
ration of EO group. Considering all these results, the poly-
mers, especially PEDOT-MO, are promising optoelectronic
materials and hold promise for electrochromic devices and
display applications.
ACKNOWLEDGMENTS
The authors are grateful to the National Natural Science Foun-
dation of China (grant number: 51303073), Ganpo Outstanding
Talents 555 projects (2013), the Natural Science Foundation of
Jiangxi Province (grant number: 20122BAB216011), the Train-
ing Plan for the Main Subject of Academic Leaders of Jiangxi
Province (2011), the Science and Technology Landing Plan of
Universities in Jiangxi province (KJLD12081), and Jiangxi Sci-
ence and Technology Normal University (YC1225, YC1233, and
YC1234) for their financial support of this work. Thanks are
also due to Guangming Nie in Qingdao Science and Technology
University for his kind assistance on electrochromic tests.
FIGURE 12 TG curves of the dedoped PEDOT-MeOH (A) and
PEDOT-MO (B) films. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
By considering all the above data concerning electrochromic
properties, it can be seen clearly that the electrochromic per-
formance of PEDOT-MO is superior to that of PEDOT-MeOH.
Further more, in comparison with the previously reported
acrylate and methacrylate modified PEDOT, PEDOT-MO
showed higher optical contrast ratio, higher CE, lower
switching potential, together with fast response time and
favorable stability.
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