NEW EDOT CONTAINING POLYMERS: EFFECT of RING SIZE on the BENZIMIDAZOLE ACCEPTOR

Article abstract of DOI:10.1016/j.electacta.2015.11.119

In this study, three different benzimidazole (BIm) based acceptor groups bearing cyclohexane, cycloheptane and cyclooctane rings at 2-C position of benzimidazole were synthesized and coupled with ethylenedioxythiophene (EDOT) donor units via Stille cross-

Full text of DOI:10.1016/j.electacta.2015.11.119

Electrochimica Acta 188 (2016) 165–174
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Electrochimica Acta
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NEW EDOT CONTAINING POLYMERS: EFFECT OF RING SIZE ON THE
BENZIMIDAZOLE ACCEPTOR
E.G. Cansu-Erguna, A.M. Önalb, , A. Cihaner
*
a,**
a
Atilim Optoelectronic Materials and Solar Energy Laboratory (ATOMSEL), Chemical Engineering and Applied Chemistry, Atilim University, TR-06836 Ankara,
Turkey
Department of Chemistry, Middle East Technical University, TR-06800 Ankara, Turkey
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 5 October 2015
Received in revised form 10 November 2015
Accepted 24 November 2015
Available online 28 November 2015
In this study, three different benzimidazole (BIm) based acceptor groups bearing cyclohexane,
cycloheptane and cyclooctane rings at 2-C position of benzimidazole were synthesized and coupled with
ethylenedioxythiophene (EDOT) donor units via Stille cross-coupling reaction. The resulting donor-
acceptor-donor (DAD) type monomers were examined in terms of their electrochemical and optical
behaviors. All three monomers exhibited dual absorption bands due to their donor-acceptor pattern and
exhibited one irreversible oxidation peak at about 0.80 V. The monomers were electrochemically
polymerized via both potentiostatic and potentiodynamic methods and electro-optical properties of
obtained conjugated polymer films were investigated in monomer free electrolytic solution. It was found
that the polymer films exhibited quasi-reversible redox behavior due to the doping/dedoping process of
the polymers, which was accompanied by a reversible electrochromic behavior. Electrochromic
investigations showed that all three polymer films showed similar multichromic behaviors: green in the
neutral state, gray in the oxidized state, and brick-red in the reduced state. Their band gap (Eg) values
were elucidated utilizing spectroelectrochemical data and it was found to be around 1.18 eV.
Furthermore, one of the polymer film was utilized for the construction of transmissive type
electrochromic device (ECD) with poly(3,4-ethylenedioxythiophene) (PEDOT).
Keywords:
Benzimidazole
donor-acceptor-donor systems
conjugated polymers
electrochromic device
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
conducting polymer applications. On the other hand, BIm has been
taken attention since it can have very unique properties due to the
The studies about conjugated systems were focused on DAD
approach since last three decades. In DAD systems, the hybridiza-
tion of molecular orbitals results in the narrower band gap as
compared to their original counterparts [1,2]. Also, by modifying
the molecular structures, not only the band gap value can be
altered, but also solubility, stability and electrochromic properties
can be enhanced [3,4]. Since those properties are under the same
roof, these systems are very popular in today's studies. For having
the desired properties, different types of donor and acceptor
groups have been designed and synthesized. Among the acceptor
groups, the mostly used type is benzazoles [5–8]. Benzothiadia-
zoles (BTD) [9,10], benzotriazoles (BTZ) [11,12], benzoselenadia-
zoles (BSe) [13–15], benzoxadiazoles (BOx) [16,17] and
benzimidazoles (BIm) are the most commonly used acceptors in
possibility for obtaining various number of BIm derivatives by
functionalizing on its 2-C position.
In 1998, Yamamoto group synthesized the homopolymers of
different BIm derivatives (including H, phenyl and heptyl groups
on its 2-C). The study revealed that the polymers are electro-
chromic and electrochemically active upon n-doping. Due to its
photoluminescence and solvatochromic properties, BIm was found
to be an interesting building block for p-conjugated polymers [18].
BIms were functionalized with alkyl chains to obtain soluble
conjugated polymers [19–24], or with EDOT, ferrocene, benzene
type aromatic structures to get different electrochromic properties
[25], or with tert-butylcyclohexane to compare the donor unit
effect [26,27]. Moreover, BIms were recently investigated in terms
of the effect of the ring size on BIm unit by Cihaner's group [28].
EDOT and thiophene donor units were coupled with BIms
including cyclopentane and cyclohexane rings on its 2-C position.
It was seen that the ring size on the BIm unit has no effect on the
optical properties of the resulting electroactive polymers which
contains the same donor unit, but the oxidation potentials of the
monomers and the polymers were affected. To the best of our
* Corresponding author. Tel.: +903122103188; fax: +903122103200.
** Corresponding author. Tel.: +903125868304; fax: +903125868091.
E-mail addresses: aonal@metu.edu.tr (A.M. Önal), atilla.cihaner@atilim.edu.tr
(A. Cihaner).
http://dx.doi.org/10.1016/j.electacta.2015.11.119
0013-4686/ã 2015 Elsevier Ltd. All rights reserved.

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E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
Scheme 1. General reaction scheme for the synthesis of acceptor units.
knowledge, there is no comparative investigation on the DAD type
monomers and their polymers with cycloalkane rings having 6,
7 and 8 members on BIm acceptor unit. Thus, to get more insight
on the ring size effect, we functionalized the BIms with cyclic units
with increasing ring size (i.e. 6, 7 and 8) and used as acceptor group
in the synthesis of a new series of monomers in DAD sequence.
EDOT was used as the donor unit. Electrochemical and optical
properties of the monomers; 4-(2,3-dihydrothieno[3,4-b][1,4]
dioxin-5-yl)-7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)-2-
cyclohexyl-2H-benzo[d]imidazole (E6E), 4-(2,3-dihydrothieno
[3,4-b][1,4]dioxin-5-yl)-7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-
yl)-2-cycloheptyl-2H-benzo[d]imidazole (E7E) and 4-(2,3-dihy-
drothieno[3,4-b][1,4]dioxin-5-yl)-7-(2,3-dihydrothieno[3,4-b]
[1,4]dioxin-7-yl)-2-cyclooctyl-2H-benzo[d]imidazole (E8E) were
Scheme 2. General reaction scheme for the synthesis of monomers.

E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
167
2.2. Synthesis of acceptors
For the synthesis of acceptor groups, the skeleton unit called
3,6-dibromobenzene-1,2-diamine (2) was synthesized first. Then,
the acceptor groups were obtained via the reaction of diamine with
cyclohexanone (3), cycloheptanone (4) and cyclooctanone (5),
separately (Scheme 1).
General procedure for the synthesis of acceptor units: [29]: 1 eq. of
diamine (2) and 3.7 eq. of cycloalkane (3, 4 or 5) were dissolved in
diethyl ether under Ar atmosphere with continuous stirring. After
addition of 0.1 eq. of acetic acid, the reaction mixture was refluxed
at 55 ꢁC and followed by thin layer chromatography (TLC). After the
reaction was stopped, the solvent was removed and the crude
product was diluted with ethyl acetate and washed with saturated
aqueous brine. The collected organic phase was dried over MgSO4
and purified with column chromatography by using a mixture of
hexane and ethyl acetate (20:1) as an eluent. The yellowish oil was
stirred under Ar atmosphere with 3 eq. of 85% activated MnO2 in
fresh tetrahydrofuran (THF) overnight. The reaction mixture was
filtered, washed with THF and solvent was evaporated under
vacuum. Finally, the product was purified with column chroma-
tography (15:1Hexane: Ethyl acetate) and the products (6, 7 or 8)
were collected as yellow needles.
Fig. 1. Normalized optical absorption spectra of E6E,E7E and E8E monomers in
DCM. Inset: Colors of the monomers under day light.
4,7-dibromo-2-cyclohexyl-2H-benzo[d]imidazole (6): Yield:
83%
investigated in terms of ring size effect and the monomers
polymerized electrochemically to obtain their corresponding
polymer films, poly(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-
7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)-2-cyclohexyl-2H-
benzo[d]imidazole) (P(E6E)), poly(4-(2,3-dihydrothieno[3,4-b]
[1,4]dioxin-5-yl)-7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)-2-
cycloheptyl-2H-benzo[d]imidazole) (P(E7E)) and poly(4-(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(2,3-dihydrothieno[3,4-
b][1,4]dioxin-7-yl)-2-cyclooctyl-2H-benzo[d]imidazole) (P(E8E)).
Electrochemical and optical properties of the polymers were
investigated using cyclic voltammetry and in situ spectroelec-
trochemical technique, respectively.
1H NMR (400 MHz, CDCl3)
d (ppm): 7.19 (s, 2H); 2.01-1.95 (m,
6H); 1.76 (s, br, 2H); 1.72 (s, br, 2H); 13C NMR (100 MHz, CDCl3)
(ppm): 156.9; 135.8; 118.8; 107.4; 32.6; 25.5; 24.5.
d
4,7-dibromo-2-cycloheptyl-2H-benzo[d]imidazole (7): Yield:
63%
1H NMR (400 MHz, CDCl3)
d
(ppm): 7.17 (s, 2H); 1.82-1.76 (m,
(ppm): 164.9; 135.8; 100; 79.9;
12H); 13C NMR (100 MHz, CDCl3)
33.8; 25.2; 23.4.
d
4,7-dibromo-2-cyclooctyl-2H-benzo[d]imidazole (8): Yield:
14%
1H NMR (400 MHz, CDCl3)
d
(ppm): 7.17 (s, 2H); 1.78-1.71 (m,
(ppm): 156.8; 135.7; 110.8; 77.2;
14H); 13C NMR (100 MHz, CDCl3)
41.9; 27.2; 22.7.
d
2. Experimental
2.1. Materials
2.3. Synthesis of monomers
All chemicals were purchased from Sigma Aldrich Chemicals
and used as received unless otherwise noted. Electrochemical
studies were performed by using platinum (Pt) disc electrode, Pt
plate and silver/silver chloride (Ag/AgCl) as working, counter and
reference electrodes, respectively. The electrochemical polymer-
izations were studied in acetonitrile-dichloromethane (ACN-DCM)
(90:10 v/v) containing 0.1 M lithium perchlorate (LiClO4) as
supporting electrolyte. The cyclic voltammograms were recorded
with Gamry Reference 600 model potentiostat-galvanostat. The
cyclic voltammetry measurements were calibrated with 10 mM
solution of ferrocene in DCM. Electrochemical and spectroelec-
trochemical properties of the polymer films were investigated in
ACN containing 0.1 M tetrabutylammonium tetrafluoroborate
(TBABF4) as supporting electrolyte. Spectroelectrochemical meas-
urements were performed using a Specord S600 model UV–vis
spectrometer combined with Gamry Reference 600 model poten-
For the synthesis of monomers via Stille coupling reaction
(Scheme 2), E6E, E7E and E8E,1 eq. of the related acceptor unit,
tiostat-galvanostat. Indium tin oxide (ITO, Delta Tech. 8-12
V,
0.7 cm ꢀ 5 cm) coated glass, Pt wire and Ag wire were used as
working, counter, and pseudo reference electrodes, respectively, in
spectroelectrochemical studies. 1H and 13C NMR spectra were
recorded on a Bruker Spectrospin Avance DPX-400 Spectrometer
and chemical shifts were given relative to tetramethylsilane as the
internal standard. All measurements were performed at room
temperature and ambient conditions.
Fig. 2. Normalized absorption and emission spectra of E6E recorded in DCM. Inset:
Wiev of E6E monomer under daylight and UV light (366 nm).

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E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
Fig. 3. Fluorescence spectra of (a) E6E; (b) E7E and (c) E8E in three different solvents.
2.1 eq. of tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl) stan-
nane and 2% eq. of Pd(II) were dissolved in freshly distilled toluene
under Ar atmosphere. The reactionwas followed by TLC during two
days. The solvent was evaporated; the crude product was dissolved
in DCM and washed with aqueous brine. After drying the organic
part over MgSO4, the product was purified with column
chromatography (3:1Hexane:DCM).
cathodically coloring polymer (dark blue in neutral state and
transmissive sky blue in oxidized state). After balancing the
charges, PEDOT was left in oxidized form and the polymer in this
study was left in its reduced form. Then, the polymer films were
sandwiched on their conducting sites with leaving thin layer gel
electrolyte between them. The ECD was left to dry for two days
before ready to use.
E6E: Purple like solid film. Yield: 47%
Gel electrolyte was prepared by mixing TBABF4 - PMMA - PC -
ACN with the ratio of 3: 7: 20: 70 (w/w) respectively.
1H NMR (400 MHz, CDCl3)
d (ppm): 7.83 (s, 2H); 6.47 (s, 2H);
4.35-4.25 (dt, 8H); 2.04-2.0 (t, 10H); HRMS: 467.1052 g/mol;
8.96 ppm.
3. Results and discussion
E7E: Purple like solid film.Yield: 22%
1HNMR(400 MHz,CDCl3)
4.25 (dt, 8H); 1.56 (t, br, 8H); 1.77 (t, 4H); HRMS: 481.1189 g/mol;
12.69 ppm.
d
(ppm):7.80 (s, 2H);6.48(s, 2H);4.35-
3.1. Synthesis of monomers and their electrochemical and
spectroscopic properties
E8E: Purple like solid film.Yield: 37%
In the present study three new conjugated hybrid monomers,
E6E, E7E and E8E, containing EDOT as heterocyclic side groups
were synthesized via Stille cross-coupling reactions. The mono-
mers were characterized by 1H NMR, UV–Vis and fluorescence
spectroscopies. The electronic absorption spectra of the monomers
were recorded in DCM and the results are depicted in Fig.1. As seen
from the figure, all three monomers exhibited two optical
absorption bands due to their donor-acceptor pattern. One of
the reasons for the existence of two absorption bands in DAD type
molecules might be intramolecular charge transfer (ICT) between
1HNMR(400 MHz,CDCl3)
d(ppm):7.81 (s, 2H);6.48(s,2H);4.36-
4.25 (dt, 8H); 1.72 (t, 4H); 1.25 (t, br 10H); HRMS: 495.1358 g/mol;
9.85 ppm.
2.4. Construction of Electrochromic Device
Transmissive type ECD was constructed with one of the CPs
containing cyclohexane ring on its benzimidazole unit (polymer of
E6E monomer) and PEDOT polymer film. PEDOT was chosen as
donor and acceptor units together with
p
!
p* transition [29]. The
wavelength maxima for E6E, E7E and E8E appeared at 314/539 nm,
312/532 nm and 312/530 nm, respectively. A close inspection of
these values indicates that the ring size on the acceptor unit has no
appreciable change in the absorption maxima except a slight blue
shift in the low lying band with increasing ring size. On the other
hand, intensity of the absorption peak at longer wavelength gives
an important clue for ICT efficiency between donor and acceptor
units. As seen from the normalized absorption spectra, the better
charge transfer feature between donor-acceptor occurred in
monomer E6E and it decreased with increasing ring size.
The fluorescence spectra of the monomers were recorded in
DCM and the fluorescence spectrum of E6E was given as a
representative example in Fig. 2. As seen from the figure, E6E
exhibited strong luminescence maxima at 650 nm corresponding
to red color, upon excitation at 525 nm. It is worth to be mentioned
that emission maxima for the three monomers appeared as very
close (650 nm, 648 nm and 645 nm for E6E, E7E and E8E,
respectively) with a small blue shift with increasing ring size
from 6 to 8.
In order to clarify the existence of ICT between D and A units,
solvatochromic experiments were also performed. The emission
spectra of monomers were monitored in three different solvents
with different dielectric constants: DCM (9.1), THF (7.5) and
Fig. 4. The cyclic voltammograms of E6E, E7E, E8E monomers in 0.1 M LiClO4 in
ACN/DCM (90:10 v/v) between -0.5 V and 0.90 V for E6E and E8E and between -0.5 V
and 1.00 V for E7E vs Ag/AgCl at a scan rate of 100 mV sꢂ1
.

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169
Table 1
Electrochemical and spectrochemical properties of E6E, E7E, E8E.
(UV)
Monomers
lmax (abs)
nm
lmax (ems)
nm
Eox
(V)
Eox(onset)
(V)
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
314
539
650
0.76
0.68
-5.18
-3.19
1.99
6
N
N
O
O
O
O
S
S
E6E
312
532
645
0.80
0.69
-5.19
-3.16
2.03
7
N
N
O
O
O
O
S
S
E7E
312
530
648
0.73
0.65
-5.15
-3.12
2.03
8
N
N
O
O
O
O
S
S
E8E
toluene (2.4) and the results were shown in Fig. 3. A slight red shift
was observed in the emisson maxima of all three monomers with
increasing solvent dielectric constant. In the case of E6E, the
maximum of the emission spectra shifted from 636 nm (toluene) to
648 nm (THF) and to 650 nm (DCM). Similar trend was also
observed in the case of E7E (lemission = 643 nm, 645 nm and 648 nm
in toluene, THF and DCM, respectively) and E8E (lemission = 641 nm,
644 nm and 645 nm in toluene, THF and DCM, respectively). This
red shift in the emission wavelength proves that the low-energy
band present in the absorption spectrum is originating most
probably from ICT [30]. Since the observed red shift with
increasing the polarity of solvents is the highest in E6E and the
lowest in E8E, it can be concluded that the extend of ICT decreases
with increasing ring size on benzimidazole unit for the monomers,
E6E, E7E and E8E. Furthermore, all three monomers exhibited large
Stokes shift (around 100 nm) indicating conformational changes of
the molecules upon excitation [31].
The electrochemical properties of all monomers synthesized in
this work were studied via cyclic voltammetry in ACN/DCM
(90:10 v/v) solvent mixture containing 0.1 M LiClO4 and the results
were given in Fig. 4. As seen from the figure, the monomers
exhibited an irreversible oxidation peak at about 0.80 V vs Ag/AgCl
(0.50 V vs Fc/Fc+) during the first anodic scan. Although all three
monomer behaviors seem similar, the lowest oxidation potential
was measured for E8E (0.73 V vs Ag/AgCl, 0.43 V vs vs Fc/Fc+) and
the highest oxidation potential value was found for E7E (0.80 V vs
Ag/AgCl, 0.50 V vs Fc/Fc+).
The optical band gap values of corresponding monomers were
also evaluated by using the onset of the longer wavelengths in the
optical absorption spectra. The band gap values of the monomers
were found as 1.99 eV for E6E and 2.03 eV for E7E and E8E. Table 1
summarizes the obtained electrochemical and optical properties of
the monomers.
Fig. 5. Electropolymerization of E6E between -0.5 V and 0.85 V (a) E7E between -0.4 V and 1.0 V (b) E8E between -0.5 V and 0.85 V (c) with 15 cycles in 0.1 M LiClO4 solution of
ACN/DCM at a scan rate of 100 mV sꢂ1 on Pt working electrode.

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E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
Fig. 6. (a) Cyclic voltammograms of the polymers (P(E6E), P(E7E), P(E8E)) obtained by constant potential electrolysis with the charge of 80 mC cmꢂ2 on Pt working electrode
between -0.5 V and 0.9 V and (b) Cyclic voltammograms of the polymers (P(E6E), P(E7E), P(E8E)) during n-doping process between 0.0 V and -1.25 V; in 0.1 M TBABF4/ACN
monomer free solution vs Ag/AgCl reference electrode.
well-defined reversible redox couples due to p-doping
(Eopx = +0.28 V and +0.61 V for P(E6E); Eopx = +0.25 V and +0.61 V for
P(E7E) and Eopx = +0.32 V together with two shoulders at about
3.2. Electrochemical polymerization of the monomers and
investigation of their properties
+0.11 V and 0.73 V for P(E8E) (Fig. 6a). The redox behavior of the
polymer films were also examined during n doping. As seen from
The electrochemical polymerizations of E6E, E7E and E8E were
achieved in electrolyte solution containing 0.1 M LiClO4 dissolved
in ACN/DCM mixture (90:10 v/v) via repetitive cycling. The solvent
mixture was used because of poor solubility of monomers in ACN.
Fig. 6b, the polymer films exhibited well-defined reversible n-
doping process (Enredꢂonset = -1.01 V for P(E6E); Enredꢂonset = -1.00 V for
P(E7E) and Ernedꢂonset = -1.05 V for P(E8E)).
Polymer films, P(E6E), P(E7E) and P(E8E), were deposited on the Pt
Scan rate dependence of the polymer films was also investigat-
ed, and a linear relation was observed between the peak current
and the scan rate indicating nondiffusional redox process and well-
adhered electroactive polymer films on the electrode surface (see
Supporting Information, Fig. S1).
Electrochemical stability of the polymer films obtained by
constant potential electrolysis (+0.90 V vs Ag/AgCl, 80 mC cmꢂ2) on
Pt working electrode disc was studied with a square wave potential
method under ambient conditions by applying the potentials of
-0.2 V and 0.6 V of 3 s for each. After 1000 cycles, cyclic
voltammogram was recorded and compared with the first cyclic
voltammogram of the polymer films in terms of amount of charge
lost.
working electrode surface via potential cycling from -0.5 V to
0.85 V for E6E and E8E; -0.4 V to 1.0 V for E7E. During potential
scanning, new oxidation and reduction peaks appeared in all three
polymerizations. Appearance of the new redox peak is the
fingerprint of the polymer formation. Also, the current intensities
were increased in every successive scan, which indicated the
increase in polymer film thickness on the electrode surface. Fig. 5
shows the cyclic voltammograms recorded during repetitive
cycling.
For elucidating the electrochemical and spectroelectrochemical
features of the polymers, electroactive polymer films were
obtained via constant potential electrolysis at applied potential
of 0.90 V vs Ag/AgCl with the same polymerization charge
(80 mC cmꢂ2). When P(E6E), P(E7E), and P(E8E) were scanned
anodically in monomer-free electrolyte solution, they exhibited
According to the voltammograms, P(E6E) showed about 9.5%
charge loss both in anodic and cathodic regions after
Fig. 7. Electronic absorption spectra of the polymer films ((a) P(E6E) of 30 mC cmꢂ2, (b) P(E7E) of 35 mC cmꢂ2, (c) P(E8E) of 20 mC cmꢂ2) during oxidation from -0.5 V to 1.0 V
via cyclic voltammetry with the scan rate of 20 mV sꢂ1. Inset: reduction from 0.0 V to -1.7 V with the scan rate of 100 mV sꢂ1
.

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171
Table 2
Electrochemical and optical properties of P(E6E), P(E7E), P(E8E).
P(E6E)
P(E7E)
P(E8E)
Eox(onset)
Ered(onset)
-0.02
-1.01
0.99
+0.05
-1.00
1.05
-0.02
-1.05
1.03
Eg (from CV) (V)
Eg(optical) (eV)
HOMO (eV)
LUMO (eV)
1.17
1.19
1.18
-4.48
-3.31
-4.55
-3.46
-4.48
-3.30
λmax1
λmax2
λmax1
λmax2
λmax1
λmax2
λmax (nm)
433
835
430
820
431
828
tox (s) **
(95 % of full contrast)
tred (s) **
1.7
0.6
1.2
0.6
1.7
0.6
1.6
0.8
0.7
0.8
0.7
0.5
(95 % of full contrast)
T %**
40
29
42.5
164
36
41
31.5
316
Ox **
CE at 95
162
304
266
135
% of full
contrast
Red **
160
166
182
215
191
247
Oxidized *
Neutral *
Reduced *
L: 79 a*: - 2 b*: 1.7
L: 72 a*: - 0.7 b*: 0.6
L: 81 a*: - 2 b*: 0.5
Colors at
different
redox
L: 68 a*: - 18 b*: 25
L: 76 a*: - 19 b*: 27
L: 65 a*: 7 b*: 26
L: 77 a*: - 24 b*: 34
L: 61 a*: 5 b*: 20
L: 60 a*: 6 b*: 25
states
*30 mC.cm-2
**25 mC.cm-2
*35 mC.cm-2
**25 mC.cm-2
*20 mC.cm-2
**20 mC.cm-2
Charge passed
during
polymerization
1000 switching. On the other hand, P(E7E) exhibited about 28%
charge loss upon 1000 switching. P(E8E) showed the best
electrochemical stability. Even after 5000 switching, there was
only 5% charge lost both in anodic and cathodic regions (see
Supporting Information, Fig. S2).
the changes in the electronic absorption spectra during cathodic
scan from 0.0 to -1.7 V vs Ag wire. During reduction the intensity of
dual band present in the neutral form of polymer films decreased
in intensity (insets of Fig. 7) which was accompanied by a color
change from green to brick-red indicating that the polymer films
exhibited electrochromic behavior upon reduction process as well.
The optical band gaps (Eg) of P(E6E), P(E7E), and P(E8E) were
For spectroelectrochemical investigations, the polymer films
were deposited on ITO working electrode via constant potential
electrolysis in ACN/DCM solvent mixture (90:10 v/v) containing
0.1 M LiClO4 as supporting electrolyte and results were depicted in
Fig. 7. As seen from the figure, P(E6E), P(E7E) and P(E8E) exhibited
two absorption maxima in their neutral form due to their DAD
characteristics. Absorption maxima of the polymer films appeared
at 433/835 nm, 430/820 nm and 431/828 nm for P(E6E), P(E7E) and
P(E8E), respectively. The evolution of the spectra upon electro-
chemical doping showed a simultaneous decrease in the absor-
bance bands, which was accompanied by the formation of a new
absorption band beyond 800 nm in the potential range from ꢂ0.5 V
to 1.0 V indicating the formation of charge carriers. The polymer
films also exhibited a color change from green (neutral) to gray
(oxidized). All spectra recorded during potential cycling between
ꢂ0.5 V and 1.0 V passes through an isosbestic point at about
965 nm for P(E6E), 950 nm for P(E7E) and 956 nm for P(E8E),
indicating that the polymer films were being interconverted
between their neutral and oxidized states (see Fig. 7). The n-doping
behaviors of the polymer films were also confirmed by recording
calculated from the onset of the low energy end of the
p
!
p*
transitions and found to be 1.17 eV, 1.19 eV, and 1.18 eV, respec-
tively, which were in well agreement with the band gaps calculated
from cyclic voltammogram data (see Table 2). An inspection of Eg
values obtained from two different methods indicates that P(E6E)
has the smallest band gap value indicating stronger ICT between D
and A units in accordance with the fluorescence measurements.
Some other EDOT containing polymers with different acceptor
groups (BIm (P1), BTD (P2), BSe (P3), BOx (P4), BTZ (P5)) in the
literature [26,32,33] were examined and the results were tabulated
in Table 3 together with EDOT containing polymers with BIm
acceptor unit bearing cyclopentane and cyclohexane rings (P6 and
P7) [28]. P1 to P5 polymers have at least two absorption bands and
the band gap values changes from 1.15 eV to 1.60 eV. The smallest
band gap value was reported for P1 and the highest one was for P5.
A comparison of these values with that of P(EXE)s synthesized in
this work indicates that BIm containing polymers have smaller Eg
values. This shows that BIm is a stronger acceptor unit causing

172
E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
Table 3
The electrochemical-optical properties of the EDOT containing polymers.
Polymers
lmax
(abs)
nm
EgUV
(eV)
Neut.Color
Green
Ox.
Color
Red. Color
Brick-red
440
1.15
Gray—blue
ꢃ740
N
N
O
O
O
O
S
S
n
P1
[26]
428
755
1.19
Green
Transparent
Blue
Not
Reported
S
N
N
O
O
O
O
S
S
n
P2
[32]
ꢃ420
1.21
Green
Transparent Blue
Transparent Blue
Transparent Blue
Not Reported
Not Reported
Not Reported
Se
N
N
O
O
O
830
O
S
S
n
P3
[33]
ꢃ400
1.26
Green
O
N
N
O
O
O
767
O
S
S
n
P4
[33]
624
684
1.60
Blue
C6H13
N
N
N
O
O
O
O
S
S
n
P5
[33]
430
813
1.21
Green
Transparent Blue
Not
Reported
5
N
N
O
O
O
O
S
S
n
P6
[28]
435
840
1.18
Green
Transparent Blue
Not Reported
6
N
N
O
O
O
O
S
S
n
P7
[28]

E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
173
Fig. 8. Transmittance change of P(E6E) (25 mC cmꢂ2); P(E7E) (25 mC cmꢂ2); and P(E8E) (20 mC cmꢂ2) films at their shorter wavelengths, in in their fully oxidized (1.1 V) and
reduced (-0.5 V) states in different switching times (10 s, 5 s, 3 s and 2 s).
better band gap compression and stronger ICT. Furthermore, all the
polymers listed in Table 3 have quite low oxidation potentials like
in P(EXE)s due to the electron rich EDOT donor unit.
Switching study provides information about switching time,
optical contrast and coloration efficiencies which are important
parameters for the application of conjugated polymers in electro-
chromic devices. Therefore, such properties of polymer films on
ITO were also investigated under
a square wave input of
predetermined potential ranges with the residence times of 10,
5, 3, and 2 s by monitoring the transmittance change. Percent
transmittance values were measured as 40%, 42.5% and 41% for P
(E6E), P(E7E) and P(E8E), respectively in 10 s residence times with
the potentials -0.5 V and 1.1 V at their shorter absorption wave-
lengths (Fig. 8). At longer wavelengths of polymer films, the
percent transmittance values were measured as 29%, 36% and
32.5% for P(E6E), P(E7E) and P(E8E), respectively. In order to test
the optical stability, the polymers were fully oxidized (by applying
1.1 V) and fully reduced (by applying -0.5 V) with 5 s time intervals.
The lost in percent transmittance after 50 switches for P(E6E), P
(E7E) and P(E8E) were found as 3%, 24% and 6%, respectively, at
their shorter wavelengths. P(E6E) and P(E8E) did not lose their
percent transmittance at longer absorption maxima after 50
switching. However, 12% loss was observed for P(E7E) at its longer
absorption wavelength upon 50 repetitive redox process. P(E6E)
and P(E8E) were found as similar optical behaviors and P(E6E)
exhibited better optical stability.
Coloration efficiency (CE) shows the relation between optical
absorbance change (at a specific wavelength) and charge/discharge
density required for a full switch [34]. CE (h) was calculated using
the following equation [35]:
D
Qd
OD
h
¼
ð1Þ
OD ¼ logTT
and Qd is the charge density (C cmꢂ2).
bleached
Where
D
colored
Tbleached and Tcolored, on the other hand, are the transmittance in the
oxidized and neutral states, respectively. CE values of the polymers
were calculated at 95% of the full optical contrast and reported in
Table 2.
3.3. Electrochromic Device Applications of P(E6E) and PEDOT
Among three polymer films, P(E6E) was utilized as anodically
coloring polymer for constructing transmissive type ECDs with
PEDOT, which is mostly used as cathodically coloring polymer. ECD
was tested by connecting the working electrode to P(E6E) and the
counter electrode (shortcut by the reference electrode) to PEDOT.
Fig. 9. (a) Spectrochemical behavior of P(E6E)–PEDOT ECD and between -0.6 V and
0.8 V. Inset: Cyclic voltammograms of ECD between -0.6 V and 0.8 V at scan rate of
100 mV sꢂ1; (b) Percent transmittance change of P(E6E)-PEDOT ECD switched
between -0.6 V and 0.8 V with the residence times of 10, 5 and 3 s.

174
E.G. Cansu-Ergun et al. / Electrochimica Acta 188 (2016) 165–174
During a successive anodic scan, the cyclic voltammogram and
UV–vis spectra were monitored. Working range between -0.6 V
and 0.8 V was found as suitable for the device. The ECD was deep
blue color at 0.8 V and green at -0.6 V (Fig. 9a). As seen from the
figure during the application of 0.8 V dual band of P(E6E) in its ECD
lost its intensity which was accompanied by the evolution of a new
band at about 620 nm. The newly formed band corresponded to the
electronic absorption band of PEDOT in its neutral form.
Switching time, optical contrast and coloration efficiency
features of ECD were also investigated under a square wave input
of predetermined potential ranges with the residence times of 10,
5, and 3 s by monitoring the transmittance and the kinetic
responses of the films at 417 and 816 nm. P(E6E)-PEDOT ECD
exhibited 6.5% transmittance in 10 s residence time between the
potentials -0.6 V and 0.8 V at both wavelengths. The response time
of ECD at 95% of full contrast was calculated as 1.4 s for oxidation
and 1.5 s for reduction at 417 nm. The response time at 816 nm was
calculated as 2.7 s for the oxidation and 1.0 s for the reduction. The
optical contrast values were found to be 6.5%, 6.3% and 6% for 10 s,
5 s and 3 s switching, respectively (Fig. 9b). Moreover, the
coloration efficiency of the ECD was calculated by using equation
1 (at 95% of the full contrast) and found as 150 cm2 Cꢂ1 at 816 nm.
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Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
electacta.2015.11.119.