A low band gap benzimidazole derivative and its copolymer with 3,4-ethylenedioxythiophene for electrochemical studies

Article abstract of DOI:10.1149/2.0311501jes

A novel monomer; 2-(3-nitrophenyl)-4, 7-di(thiophen-2-yl)-1H-benzo[d]imidazole) (BIMN) was used for electrochemical copoly-merization with 3, 4-ethylenedioxythiophene (EDOT). Polymerization was achieved in acetonitrile (ACN)/dichloromethane (DCM) (1:1, 1:

Full text of DOI:10.1149/2.0311501jes

H6
Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
A Low Band Gap Benzimidazole Derivative and Its Copolymer
with 3,4-Ethylenedioxythiophene for Electrochemical Studies
a
a
b
a,b,c,d,z
Saniye Soylemez, Serife O. Hacioglu, Sema Demirci Uzun, and Levent Toppare
a
Department of Chemistry, Middle East Technical University, Ankara 06800, Turkey
Department of Polymer Science and Technology, Middle East Technical University, Ankara 06800, Turkey
Department of Biotechnology, Middle East Technical University, Ankara 06800, Turkey
The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University,
b
c
d
Ankara 06800, Turkey
A novel monomer; 2-(3-nitrophenyl)-4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole) (BIMN) was used for electrochemical copoly-
merization with 3,4-ethylenedioxythiophene (EDOT). Polymerization was achieved in acetonitrile (ACN)/dichloromethane (DCM)
(
1:1, 1:3 and 1:5, molar ratios) solution containing sodium perchlorate (NaClO4) and lithium perchlorate (LiClO4) mixture as the
supporting electrolyte on an ITO electrode. The chemical structures of monomer and copolymers were characterized by nuclear mag-
1
netic resonance ( H NMR) and scanning electron microscopy (SEM). The optical and electrochromic properties were investigated
by UV-vis spectrophotometer and cyclic voltammetry, respectively. Combination of BIMN and EDOT provides lower oxidation
potential, lower bandgap and higher optical contrast.
©
The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0311501jes] All rights reserved.
Manuscript submitted September 18, 2014; revised manuscript received October 14, 2014. Published November 7, 2014.
been also widely studied previously.16–18 However, to the best of our
knowledge, use of a novel BIMN for that matter has not yet been
reported.
Great efforts have been focused on the design and synthesis of
conducting polymers to achieve desirable properties for applica-
1
,2
tions such as organic solar cells, biosensors and electrochromic
3
–5
devices.
Over the last two decades conducting polymers (CPs)
In this work, 2-(3-nitrophenyl)-4,7-di(thiophen-2-yl)-1H-
benzo[d]imidazole) (BIMN) (Scheme 1) was synthesized. The
structure of conducting polymer was analyzed by spectral methods
have received considerable attention due to their electrochemical and
6
–8
3–5
optical properties, low cost, tunable band gaps and fast switching
9
1
13
times. Modification of bandgap of conducting polymer is a crucial
( H, C NMR) and the morphology was characterized using scanning
electron microscopy (SEM). In addition, electrochemical copoly-
merization was undertaken by cyclic voltammetry with a mixture of
BIMN and EDOT monomers in acetonitrile (ACN)/dichloromethane
(DCM) solution. Electrocopolymerization was achieved in different
ratios of comonomers such as 1:1, 1:0, 1:3 and 1:5 BIMN:EDOT.
The copolymers were characterized by UV-visible absorption
spectroscopy (UV-Vis), scanning electron microscope (SEM),
spectroelectrochemistry and kinetic studies.
strategy to achieve desirable electrochromic properties. In order to
design a low bandgap material, donor–acceptor approach is one of
the most plausible ways. In accordance with this approach, regular
alternation of donor unit with a high HOMO level and an acceptor
unit with a low LUMO level along a conjugated chain is proposed.
Moreover, combination of donor and acceptor moieties in the poly-
mer backbone not only enhances D-A interactions but also helps to
10
improve optical, mechanical and electronic properties.
Recently, different kind of molecules like benzothiadiazole (BTd),
quinoxaline and benzotriazole (BTz) are widely used as the elec-
tron deficient units. Especially, benzimidazole (BIm) has some at-
tractive properties since the presence of both imine and amine nitro-
gen in its structure provides desired bandgap and optical properties.
These DAD type conjugated molecules can be synthesized either by
Experimental
Materials.— Bromine,
2,1,3-benzothiadiazole,
ammo-
nium cerium (IV) nitrate, ethyl acetate, chloroform,
dichloromethane, bis(triphenylphosphine)palladium (II) dichlo-
ride, 3-nitrobenzaldehyde, thiophene, n-butyllithium solution (2.5
11,12
chemical or electrochemical polymerizations.
Electrochemically
4
M in hexane), tributyltinchloride sodium perchlorate (NaClO ) and
generated copolymerization offers efficient modification of the struc-
tures of conjugated polymers, enable control the bandgap of conduct-
13
ing polymer. Homogeneous copolymerization can be achieved at
the electrode surface through which the film thickness is controlled.
Copolymerization is a very promising and efficient method to im-
3
–5
prove electrochemical properties of the conducting polymers. In
addition to these promising properties BIm unit has some drawbacks
11,12
such as low stability and poor electrochromic contrasts.
To over-
come these, copolymerization is an easy and effortless method to
combine electrochromic properties of the comonomers and acquiring
14
low oxidation potentials. 3,4- Ethylenedioxythiophene (EDOT) is
a good candidate as a co-monomer since it leads to a low bandgap
polymer than thiophene due to the presence of two electron donating
oxygen atoms. Moreover, it contributes to fast switching times, high
15
conductivity and good stability in ambient conditions.
Furthermore, copolymerization is a facile method to combine and
11,12
enhance optical and kinetic properties.
Low oxidation potential
and high stability of EDOT makes it a possible candidate for copoly-
merizations to enhance the electrochemical properties of resulting
copolymers. Electrochemical properties of copolymers of EDOT have
Scheme 1. Synthetic route of monomer synthesis. i: Br2/HBr, ii:
NaBH4/ethanol, iii: (NH4)2Ce(NO3)6/H2O2/acetonitrile, iv: Pd(Ph3)2Cl, dry
THF.
zE-mail: toppare@metu.edu.tr
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
H7
Scheme 2. Schematic illustration of the copolymerization.
lithium perchlorate (LiClO
Hydrobromic acid was purchased from Acros. Tetrahydrofuran was
purchased from Fisher and purified over benzophenone and sodium.
4
) were purchased from Sigma-Aldrich.
133.7, 132.0, 130.6, 128.6, 127.1, 126.5, 125.3, 123.8, 123.0, 121.8.
+
+
HRMS: Calculated [M] = 404.0527, Measured [M] = 404.0515.
The preparation of BIMN and copolymer film by
electropolymerization.—Electrochemical polymerization of BIMN
was performed on indium tin oxide (ITO)-coated glass slides
Equipment.— Electropolymerization was performed with a Gamry
instruments Reference 600 potentiostat/galvanostat/ZRA in a three-
electrode cell consisting of indium tin oxide (ITO) coated glass slide
as the working electrode. A platinum wire as the counter electrode,
and a Ag wire as the pseudo reference electrode were employed. For
the spectroelectrochemical studies of the polymer films Varian Cary
using equimolar 0.1 M sodium perchlorate (NaClO
4
) and lithium
perchlorate (LiClO ) as the supporting electrolyte in a mixture
4
of dichloromethane (DCM) and ACN (5:95, v/v) with repeated
scan intervals between 0 and 1.40 V versus Ag wire pseudo
reference electrode. Polymerization of EDOT was also achieved on
ITO-coated glass slides using 0.1 M sodium perchlorate (NaClO
4
)
5
000 UV-Vis spectrophotometer was used in order to determine the
and lithium perchlorate (LiClO ) containing 5:95 (DCM : ACN)
4
absorption bands of both neutral and oxidized states of the polymers.
For surface imaging of the electrodes, scanning electron microscope
solution with repeated scan intervals between −1.0 and 1.5 V. It was
seen that PBIMN forms an orange polymer film on the electrode
surface.
(
SEM) (JEOL JSM-6400 model) was used. To confirm the structures
1
13
of the monomers, H NMR and C NMR spectra were recorded in
CDCl on Bruker Spectrospin Avance DPX-400 spectrometer and the
chemical shifts were expressed in ppm relative CDCl as the internal
Electropolymerizations (BIMN:EDOT) were performed in 1:1
0.86: 0.86 mM); 1:3 (0.86: 2.58 mM) and 1:5 (0.86: 4.3 mM) con-
centration ratios of BIMN and EDOT using 0.1 M NaClO , LiClO
3
(
3
4
4
standard. HRMS data were recorded on Waters SYNAPT MS System
to confirm the monomer synthesis.
supporting electrolyte in 3.0 mL 5:95 (v/v) DCM:ACN solution with
repeated scan intervals between −1.0 and 1.5 V versus Ag wire
pseudo-reference electrode (Figure 2). Schematic illustration of the
copolymerization was illustrated in Scheme 2. The use of perchlorates
enhances good polymer film formation on the electrode surface. The
size difference between the anion and the cation of the supporting
electrolyte facilitates doping-dedoping processes.
Synthesis of the monomer (BIMN).—Synthesis of 4,7-dibromo-
2
(3-nitrophenyl)-1H-benzo[d]imidazole (4).— Synthesis of the ac-
19,20
ceptor group was achieved in accordance with the literature.
Firstly, 3,6-dibromobenzene-1,2-diamine (3) was synthesized as a re-
sult of bromination and further reduction of 2,1,3-benzothiadiazole
(
5
1) in ethanol was carried out by NaBH
.6 mmol) was dissolved in acetonitrile (ACN) (9.0 mL) at room tem-
perature. After dropwise addition of H (0.54 mL, 23 mmol) and
-nitrobenzaldehyde (0.85 g, 5.6 mmol), ammonium cerium (IV) ni-
4
. Then, compound 3(1.50 g,
Results and Discussion
2
O
2
Electrochemical properties of PBIMN and copolymers.— Electro-
chemical and spectral studies were conducted as further characteriza-
tion techniques to get a deeper knowledge on the properties of both
homopolymers and copolymers. BIMN is a novel monomer and its
electrochemical properties were not discussed before, hence in each
section these results will be discussed together with (BIMN: EDOT)
copolymers. For electrochemical polymerization and copolymeriza-
tions 10 cycles of CV were run to make a comparison with those of
related polymers.
After the electrochemical synthesis of both BIMN and (BIMN:
EDOT) copolymers, electroactivity of the polymers and the oxidation–
reduction potentials were also determined via cyclic voltammetry
technique. The cyclic voltammogram (CV) was performed on indium
tin oxide (ITO) coated glass slides with 0.1 M (DCM : ACN) (5/95,
3
trate (0.3 g, 0.6 mmol) was added to the solution. The reaction was left
to stirring overnight and ended with TLC monitoring. Subsequently,
the reaction mixture was poured into the ice/water bath and filtered
for the solids. The residue was washed with the ACN and the acceptor
21
group (4) was yielded as light pink solid (0.40 g, 1.08 mmol, 20%).
1
H NMR (400 MHz, CDCl
3
): δ 12.78 (s, 1H), 8.84 (s, 1H), 8.50
(
d, 1H, j = 7.82 Hz), 8.30 (d, 1H, j = 8.21 Hz), 7.67 (t, 1H, j = 8.01
13
Hz),7.31 (s, 2H). C NMR (CDCl
3
): δ158.5, 148.4, 143.6, 134.7,
1
33.9, 131.9, 127.3, 124.2, 121.3, 104.8.
Synthesis
of
2-(3-nitrophenyl)-4,7-di(thiophen-2-yl)1H-
benzo[d]imidazole (BIMN).—To synthesize the corresponding
monomer, 4,7-dibromo-2(3-nitrophenyl)-1H-benzo[d]imidazole
4) (0.40 g, 1.08 mmol) was dissolved in dry THF. Subse-
(
4 4
v/v) solution in the presence of NaClO and LiClO as the supporting
22
quently, tributyl(thiophene-2-yl)stannane (1.99 g, 5.40 mmol)
electrolytes. In Figure 1a, after formation of the irreversible monomer
oxidation in the first cycle, a new reversible redox couple with an
increasing current intensity appeared which proves the formation of
electroactive polymer film of BIMN (PBIMN). Then to investigate the
p-type and n-type doping properties of the polymer, CV was recorded
was added to stirred solution under argon atmosphere. The mixture
was heated at reflux temperature for 45 minutes. Afterwards
bis(triphenylphosphine)palladium (II) dichloride was added as the
catalyst and the reaction proceeded in reflux under argon atmosphere
for 16 hours with TLC monitoring. The solvent was evaporated and
product (BIMN) was purified by column chromatography as pale
yellow solid (eluent: Chloroform:Hexane, 3:1) (0.74 g, 0.18 mmol,
4 4
in a monomer free solution (0.1 M LiClO /NaClO /ACN).
PBIMN has only p-type doping character with a reversible redox
couple at 1.12 V. HOMO energy level was calculated from the onset
potential of oxidation peak as −5.63 eV for PBIMN. The results
1
7%).
1H NMR (400 MHz, CDCl
.29 Hz), 8.08-8.04 (m, 3H), 7.89 (s, 1H), 7.66–7.45 (m, 4H), 7.23
+
3
): δ 11.96 (s, 1H), 8.34 (d, 2H, j =
): δ152.2, 149.8, 141.4, 137.8,
were calibrated with respect to Fc/Fc and the band energies were
8
calculated relative to the vacuum level considering that the value of
SHE is −4.75 eV vs vacuum. Figure 1 shows the CV of BIMN (a)
13
(d, 2H, j = 7.0 Hz). C NMR(CDCl
3
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
Figure 1. a) Repeated potential scan polymerization of BIMN in 0.1 M LiClO4/NaClO4, CH2Cl2/ACN (5:95, v:v) solution b) Single scan cyclic voltammogram
of PBIMN in a monomer free 0.1 M LiClO4/NaClO4ACN solution on an ITO electrode.
and PBIMN (b) which were run between 0 V and 1.4 V at a scan rate
of 100 mV/s.
Colors and electrochemical properties of the CPs can be con-
trolled via copolymerization technique. It is not only a simple strat-
egy but also has significant effects on the properties of these mate-
scan rate was observed which shows that the electroactive polymer
films were well adhered on the ITO surface and the redox processes
were non-diffusion controlled for all copolymers ((a,b) 1:1, (c,d) 1:3,
(e,f) 1:5 (BIMN:EDOT) copolymers.
Table I represents electrochemical, spectroscopic and electro-
optical parameters measured for these polymers.
24
rials such as oxidation potential, HOMO energy levels, E
g
and λmax
values. In literature, this strategy was applied for different CPs to im-
These references from the literature were chosen according to
general structure of acceptor unit namely benzimidazole unit. As seen
in Scheme 3 all polymers are benzimidazole derivatives (Scheme 3).
These molecules have the same acceptor unit which affects elec-
trochemical and optical properties of the polymers. Also all these
derivatives were polymerized electrochemically and for each polymer
10 cycle polymerization was performed. This similar conditions of
preparation make easy and reliable comparison of PBIMN with these
molecules. Electrochemical polymerization potential ranges were also
reported as 0.5 V/ 1.8 V for PBIBA, −0.3 V/ 1.5 Vfor PBIED and
−0.5 V/ 1.8 V for PTTBI.
3
–8
prove the mentioned properties. Due to outstanding electrochemi-
cal properties such as; low oxidation potential, good stability and low
bandgap, EDOT is one of the excellent candidates for electrochemical
23
copolymerization.
As a result, in this study copolymerization technique was applied
to improve the electrochemical properties of this novel monomer
(
BIMN) by insertion of EDOT unit. Three novel BIMN and EDOT
based copolymers with (1:1), (1:3) and (1:5) (BIMN: EDOT) ratios
were synthesized electrochemically to observe the effect of increasing
EDOT ratio on the mentioned properties of resulting copolymers.
Figure 2 illustrates the cyclic voltammogram for electrochemical
polymerization of BIMN(d), EDOT (e) and 1:1 (a) 1:3 (b) 1:5 (c)
Benzimidazole based studies in literature were discussed in
Tables I and II. Electrochemical, spectroelectrochemical and ki-
netic properties of both PBIMN and EDOT bearing copoly-
mers were compared with three different donor-acceptor-donor
type conjugated polymers. In the literature EDOT bearing D-A-
D type polymers/copolymers generally have lower oxidation po-
tentials compared to other derivatives because of the electron
rich character of EDOT unit. The significant decrease in the ox-
idation potential of (BIMN:EDOT) copolymers with respect to
PBIMN is clearly shown in Table I. In addition, when both
PBIMN and copolymers are compared with those from literature
(Refs. 26–28) both have very low oxidation potentials compared
(
BIMN:EDOT) copolymers to observe the effect of EDOT unit with
different ratios to the polymer backbone. In addition, to investigate
the oxidation potentials and HOMO energy levels, single scan cyclic
voltammograms for three different copolymers were recorded in 0.1
M LiClO
in Figure 3. Both Figure 3 and Table I show that 1:1 (a) 1:3 (b) 1:5
c) (BIMN:EDOT) copolymer films revealed reversible redox couple
4 4
/NaClO - ACN solution on an ITO electrode and reported
(
at positive potentials pointed at 0.44 V, 0.45 V and 0.51 V versus Ag
wire pseudo-reference electrode, respectively.
It is generally known that EDOT bearing D-A-D type poly-
mers/copolymers have lower oxidation potentials compared to thio-
phene bearing ones due to the electron rich character of this group
which makes easy to undergo any redox process. In the light of
this knowledge, the significant decrease on the oxidation potential
of (BIMN:EDOT) copolymers with respect to pristine PBIMN could
be explained by the electron rich character of EDOT (Figure 2).
In this study, similar to the PBIMN, all BIMN and EDOT bearing
copolymers are p-type dopable as illustrated in Figure 3. HOMO
energy levels were calculated from the onset potential of oxidation
peaks as −4.44 eV, −4.58 eV and −4.63 eV for (BIMN:EDOT)
copolymers, respectively.
to other benzimidazole derivatives. Moreover, similar trend was ob-
op
served for λmax and E
g
values. Insertion of EDOT into the polymer
op
backbone causes red shift with higher λmax and lower E
g
values. Tai-
loring bandgap is important for donor-acceptor-donor type molecules
since one can change and enlarge the area of application for these
materials via this technique.
Spectroelectrochemical Studies of PBIMN and copolymers.—
Electro-optical properties of the polymer films are important to de-
termine the proper application fields of these materials and also to
improve these structures via modifications. The changes in electronic
absorption spectra upon doping/dedoping processes lead to investigate
the optical and structural responses of the polymer films. Spectroelec-
trochemical changes of electrochemically synthesized polymers and
copolymers were observed by monitoring in situ UV–Vis–NIR spectra
in a 0.1 M NaClO –LiClO /ACN monomer free medium.
In order to designate the scan rate dependence of doping- dedop-
ing processes, single scan cyclic voltammograms of (BIMN:EDOT)
copolymers were recorded at different scan rates (50, 100, 150,
2
00 mV/s). This experiment could be used to prove whether these
processes (doping-dedoping) are diffusion controlled or not. As seen
in Figure 4, linear relationship between the current density and the
4
4
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
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Figure 2. Cyclic voltammogram of 1:1 (a) 1:3 (b) 1:5 (c) (BIMN:EDOT) copolymers, BIMN(d) and EDOT (e).
Polymer/copolymer films were reduced to their neutral states
by applying constant potential before oxidation. After removal of
any trapped charge and dopant ion, the neutral film absorption and
the changes in the absorption spectra upon stepwise oxidation were
recorded by using UV–Vis–NIR spectrophotometer.
Spectroelectrochemistry studies combine electrochemical and
spectroscopic techniques that can be operated at the same time. Hence
it can provide information on both electrochemical and optical char-
acteristics of all states of the polymers. In addition via this technique,
Figure 5 illustrates the λmax values for the lowest energy π –
π∗ transitions for PBIMN centered at 474 nm. In addition optical
bandgap of the corresponding polymer was also calculated from the
neutral state absorption onset as 1.97 eV.
PBIMN revealed multichromic behavior with an orange color in
its neutral state (λmax = 474 nm) and blue color during stepwise
oxidation, while intensity of absorption bands in the visible region
decrease, new absorption bands (polaron and bipolaron bands) ap-
pear steadily in the NIR region (Figure 5). Then, fully oxidized
state results in blue colored polymer. Spectroelectrochemical proper-
ties of 1:1(BIMN:EDOT), 1:3(BIMN:EDOT) and 1:5 (BIMN:EDOT)
copolymers were also performed similar to those of homopolymer
and results were summarized in Figure 6.
one can get and calculate some important parameters for conduct-
op
ing polymers such as; λmax values, optical bandgap (E
g
) and po-
laron/bipolaron bands. λmax can be defined as the wavelength at which
light is strongly absorbed by the absorbing species. This also gives
crucial information on the colors of the polymers. From onset of the
When electro-optical properties of homopolymer and copolymers
were compared, insertion of electron rich EDOT unit into poly-
mer backbone results in a significant red shift on the neutral state
op
λ
max, E
g
could be calculated which can be defined as an energy
difference (eV) between the valence band and the conduction band.
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
Figure 3. Single scan cyclic voltammogram for 1:1 (a) 1:3 (b) 1:5 (c) (BIMN:EDOT) copolymers in 0.1 M LiClO4/NaClO4 ACN monomer free solution on an
ITO electrode (d) Single scan cyclic voltammograms for all (BIMN:EDOT) copolymers at different feed ratios.
Table I. Electrochemical (Eox (V), HOMO (eV), LUMO (eV)) and optical properties (λmax (nm), Egop (eV)) of PBIMN, 1:1 (BIMN:EDOT)
copolymer, 1:3 (BIMN:EDOT) copolymer and 1:5 (BIMN:EDOT) copolymer with respect to earlier studies.
∗
op∗
Eg
(eV)
Eox
HOMO
(eV)
LUMO
(eV)
λmax
(nm)
∗
∗
(V)
PBIMN
:1
BIMN:EDOT)
copolymer
1.12
0.44
−5.63
−4.44
−3.66
−3.3
474
592
1.97
1.14
1
(
(
(
1
:3
0.45
0.51
−4.58
−4.63
−3.46
−3.58
605
555
1.12
1.05
BIMN:EDOT)
copolymer
1
:5
BIMN:EDOT)
copolymer
Ref
PBIBA)
Ref
PBIED)
2
6
1.57
1.26
1.43
−6.1
−5.31
−5.73
−4.18
−4.16
−4.13
347/478
430/ 837
−
1.92
1.15
1.60
(
(
(
2
7
2
8
Ref
PTTBI)
∗
∗
calculated with respect to reference 25. (Eox were calculated from Fig. 1 and Fig. 3; λmax and Egop were calculated from Fig. 5 and Fig. 6).
LUMO energy levels were calculated using HOMO energy levels and optical bandgap values.
∗
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
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Figure 4. Scan rate dependence of (a,b) 1:1 (c,d) 1:3 (e,f) 1:5 (BIMN:EDOT) copolymers in a 0.1 M NaClO4–LiClO4/ACN solution on an ITO electrode.
absorptions and 117 nm, 130 nm, 80 nm λmax shifts with respect to
PBIMN. As can be seen in Figure 6 absorption range in the visible re-
gion for copolymers were also enlarged with respect to that of PBIMN
Electrochromic contrast and switching studies of PBIMN and
copolymers.— Kinetic studies can be used to test the electrochem-
ical stability of these types of polymers which is very crucial for
the electrochromic device applications. In addition, electrochromic
contrast and switching times can be monitored by potential stepping
between two extreme states (neutral and oxidized) in a monomer-
free solution The wavelengths at which kinetic studies recorded were
determined from maximum absorbance values obtained in the spec-
troelectrochemical studies.
(
Figures 5 and 6) and λmax values for 1:1, 1:3, 1:5 (BIMN:EDOT)
copolymers were reported as 592 nm, 605 nm and 555 nm
Table I).
Optical band gaps for the three different copolymers were also
(
calculated from the neutral state absorption onsets similar to the ho-
mopolymer and reported as 1.14 eV, 1.12 eV and 1.05 eV, respectively
(
Table I). Finally this tremendous decrease in the band gaps of copoly-
During kinetic studies, potential was set at an initial potential
for a period of time, and was stepped to a second potential for the
same period of time. Potentials were determined from single scan CV
mers compare to PBIMN could be due to the electron rich character
of EDOT unit.
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
Table II. Summary of kinetic and optic studies of PBIMN and 1:1
BIMN:EDOT) copolymer.
(
Wavelength
nm)
Switching
Times(s)
(
T%
PBIMN
:1 (BIMN:EDOT) copolymer
1190
1370
710
1820
555
22
53
29
46
0.3
0.3
1.4
0.6
3.0
−
1
26
Ref (PBIBA)
2
7
Ref (PBIED)
28
Ref (PTTBI)
copolymer has better kinetic parameters then those of PBIMN in
terms of optical contrasts in NIR region.
In the light of the points mentioned in results and discussion
part, it can be clearly pointed out that insertion of EDOT yields
(BIMN:EDOT) copolymers with lower oxidation potential, lower
bandgap and higher optical contrast. These improved properties make
the copolymers desirable for electrochromic applications.
Finally, effect of EDOT unit on the kinetic properties were investi-
gated and compared with other benzimidazole derivatives in Table II.
I:1 (BIMN:EDOT) copolymer showed the highest optical contrast as
53%. Compared to PBIED which contains EDOT as the donor unit,
insertion of EDOT into copolymer backbone yields an increase of 7%
in optical contrast.
Scheme 3. Structures of the polymers chosen from the literature for
comparison.
and each potential was applied for 5 s. From Fig. 7 optical contrasts
and switching times were calculated. Optical contrast is the percent
transmittance change at a given wavelength. The switching time is
defined as the time required for one full switch between the two
extreme states.
In this study, the stabilities, percent transmittance and switch-
ing times of the polymer films between their neutral and oxidized
states were monitored in the near NIR region at 1190 nm for PBIMN
Characterization.—Scanning electron microscopy (SEM)
studies.— The morphologies of polymer films were investigated by
scanning electron microscopy (SEM). All the figures were taken
after 10 cycles of electrocopolymerization on ITO surface. Figure
9 shows the SEM images of the conducting polymer coated ITO
electrode surfaces PBIMN, poly EDOT (PEDOT) and (BIMN:EDOT)
copolymer. For the pure polymer, Figure 9A depicts the porous and
cauliflower structure of a typical conducting polymer. In Figure 9B,
it can be observed that, the PEDOT film reveal a morphology com-
posed of uniform surface. Also, it was clearly shown that polymer
was spread over the electrode surface homogeneously. The surface
of the (BIMN:EDOT) copolymer film changed drastically after the
combination of the PBIMN or PEDOT and shows a porosity and a
network. This change can be due to compatibility of the polymers.
Compared with the PBIMN and PEDOT, the network surface of
(BIMN:EDOT) copolymer film can be seen in Figure 9C–9D, which
can be attributed to the effect of copolymerization. The results clearly
indicate that copolymerization can lead to a different surface morphol-
ogy, which may have an effect on the electrochromic performance.
21
and 1370 nm for 1:1 (BIMN:EDOT) copolymer in 0.1M LiClO
NaClO /ACN electrolyte/solvent couple.
4
-
4
PBIMN showed 22% transmittance change with a switching time
of 0.3 s at 1190 nm. After copolymerization of BIMN and EDOT with
1
:1 ratio, resulting copolymer displayed 53% transmittance change
and 1.4 s switching time at 1370 nm (Figures 7 and 8). The 31%
increase in the percent transmittance results may lead to possible
application areas for such electrochromic materials.
This results illustrate that insertion of EDOT unit into the polymer
backbone via copolymerization technique affects the kinetic param-
eters significantly. As summarized in Table II, I:1 (BIMN:EDOT)
Figure 5. Electronic absorption spectra for (a) PBIMN film in 0.1 M NaClO4–LiClO4/ACN solution between 0.0 V and 1.15 V. (b) Colors of PBIMN at the
neutral and oxidized states.
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Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
H13
Figure 6. Electrochemical p-type doping electronic absorption spectra of 1:1 (a) 1:3 (b) 1:5 (c) (BIMN:EDOT) copolymers in 0.1 M LiClO4/NaClO4/ACN
solution.
Figure 8. Percent transmittance change and switching times for 1:1
Figure 7. Switching times and % transmittance changes recorded at 1190 nm
for PBIMN in 0.1M LiClO4-NaClO4/ACN solution between 0.0 V and 1.4 V.
(BIMN:EDOT) copolymer monitored at 1370 nm in 0.1 M NaClO4–
LiClO4/ACN electrolyte-solvent couple between −1.0 V and 1.4 V.
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H14
Journal of The Electrochemical Society, 162 (1) H6-H14 (2015)
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