Poly(3,4-ethylenedioxyselenophene): Effect of solvent and electrolyte on electrodeposition, optoelectronic and electrochromic properties

Article abstract of DOI:10.1039/d0ra01436b

In this article, we report the effect of electropolymerization conditions such as solvent and supporting electrolyte on the redox, optoelectronic and electrochromic properties of PEDOS. Monomer EDOS was synthesized by new and simple route and its electropolymerization was investigated by employing six different combinations of solvent-electrolyte namely TBAClO₄/MeCN, TBAPF₆/MeCN, TBABF₄/MeCN, TBAClO₄/PC, TBAPF₆/PC and TBABF₄/PC. Further, the electrochemical, spectroelectrochemistry, morphology and electrochromic properties of resultant PEDOS films were systematically studied. A pronounced effect of both solvent and supporting electrolyte on the electropolymerization, redox, optoelectronic and electrochromic properties on PEDOS film is noted. Among all solvent-electrolyte systems, MeCN and TBAClO₄ were found to be the most suitable medium for electropolymerization of EDOS. Further, PEDOS films prepared in PC showed red shifted absorption maxima, narrow absorption peaks in UV-vis-NIR spectra, slightly more smooth morphologies, and high optical contrasts ratio and coloration efficiency in comparison to MeCN. PEDOS films prepared in TBABF₄/PC exhibited longer λ_max (670 nm), smooth morphology, and the highest optical contrasts ratio (44.6%) and coloration efficiency (141.8 cm² C^-1) compared to the other solvent-electrolyte medium.

Full text of DOI:10.1039/d0ra01436b

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PAPER
Poly(3,4-ethylenedioxyselenophene): effect of
solvent and electrolyte on electrodeposition,
optoelectronic and electrochromic properties†‡
Cite this: RSC Adv., 2020, 10, 12395
Preeti Yadav, Sheerin Naqvi and Asit Patra *ab
ab
ab
In this article, we report the effect of electropolymerization conditions such as solvent and supporting
electrolyte on the redox, optoelectronic and electrochromic properties of PEDOS. Monomer EDOS was
synthesized by new and simple route and its electropolymerization was investigated by employing six
4 6 4
different combinations of solvent–electrolyte namely TBAClO /MeCN, TBAPF /MeCN, TBABF /MeCN,
TBAClO /PC, TBAPF /PC and TBABF /PC. Further, the electrochemical, spectroelectrochemistry,
4
6
4
morphology and electrochromic properties of resultant PEDOS films were systematically studied. A
pronounced effect of both solvent and supporting electrolyte on the electropolymerization, redox,
optoelectronic and electrochromic properties on PEDOS film is noted. Among all solvent–electrolyte
4
systems, MeCN and TBAClO were found to be the most suitable medium for electropolymerization of
EDOS. Further, PEDOS films prepared in PC showed red shifted absorption maxima, narrow absorption
Received 14th February 2020
Accepted 12th March 2020
peaks in UV-vis-NIR spectra, slightly more smooth morphologies, and high optical contrasts ratio and
coloration efficiency in comparison to MeCN. PEDOS films prepared in TBABF
4
/PC exhibited longer lmax
DOI: 10.1039/d0ra01436b
(670 nm), smooth morphology, and the highest optical contrasts ratio (44.6%) and coloration efficiency
2
ꢀ1
rsc.li/rsc-advances
(141.8 cm C ) compared to the other solvent–electrolyte medium.
for designing of multifunctional materials with better electronic
and optoelectronic properties.
1
. Introduction
19
Since the discovery of high electrical conductivity in doped
In contrast to polythiophene and PEDOT, their selenium
1
2
20,21
polyacetylene, many conjugated polymers have been devel- analogue have drawn attention only in recent years
despite
oped as active components for application in organic electronic possessing better properties such as narrow band gap, low
3
,4
devices such as organic eld effect transistors,
organic redox potential, better charge transport and higher doping. Yet,
photovoltaics, organic light emitting diodes, electrochromic very few groups have paid attention to the development and
5,6
7
8,9
10,11
12
22,23
devices and sensors,
polymers, polythiophene and its derivatives,
poly(3,4-ethylenedioxythiophene) (PEDOT),
etc. Among various conjugated study of selenophene derivatives for organic electronics.
In
namely particular, the extensive computational and experimental work
have proved to on oligo and polyselenophenes by Bendikov's group is quite
be the most promising organic electronic materials owing to signicant in understanding the chemistry of selenophene
1
3,14
15,16
24–28
their excellent physical and chemical properties. Indeed, the based conjugated systems.
Compared to the extensive
good electrochemical and thermal stability of PEDOT, moderate research focussed on EDOT relatively little is known about the
band gap (1.6 eV), high optical transparency in the doped state, electrochemical study of 3,4-ethylenedioxyselenophene (EDOS)
high electrical conductivity and low oxidation potential have based conjugated polymers. In particular, most of the efforts are
resulted in its widespread application in variety of electronic mainly devoted in achieving better chemical and optoelectronic
17,18
and optoelectronic devices.
Recently, two dimensional properties of poly(3,4-ethylenedioxyselenophene) (PEDOS) by
polymers and materials have emerged as promising candidate structural modication such as by introducing substituents on
the ethylenedioxy bridge or alkyl groups, incorporation of
29
heteroatom or by chemical copolymerization. Cihaner and
coworkers studied the electrochromic performance of electro-
a
Photovoltaic Metrology Section, Advanced Materials & Device Metrology Division, polymerized naphthalenylmethyl-substituted 3,4-propylene-
CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi-110012,
30
dioxyslenophene (ProDOS) derivatives. Xu and coworkers
India. E-mail: apatra@nplindia.org
b
reported the electrochemical synthesis of alkylated ProDOS in
Academy of Scientic and Innovative Research (AcSIR), Ghaziabad-201002, India
31
ionic liquid and EDOS in microemulsion system with water
†
‡
1
In memory of Prof. Michael Bendikov.
32
and studied their electrochromic properties. Most recently,
our group reported the electropolymerization of EDOS on
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra01436b
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
exible substrate and examined the effect of electrode surface report the electropolymerization of EDOS in conventional
on the electrochemical, spectroelectrochemical and electro- organic solvents acetonitrile (MeCN) and propylene carbonate
33
chromic performance of PEDOS lms.
(PC) in presence of three commonly used supporting electro-
It is known that electrochemical and optical properties of lytes, namely tetrabutylammonium perchlorate (TBAClO ), tet-
4
polymer lms are considerably inuenced by electro- rabutylammonium
hexauorophosphate
(TBAPF
6
)
and
polymerization conditions such as solvent and supporting tetrabutylammonium tetrauoroborate (TBABF
4
). Further,
electrolyte. However, few studies have focussed on the correla- a detailed study on the effect of solvent–electrolyte system on
tion of optical and electrochemical properties of polymer lms polymerization, electrochemical properties, spectroelec-
with electropolymerization conditions. Indeed, some groups trochemistry, morphology and electrochromic performance of
have studied the effect of polymerization conditions on the six resultant PEDOS lm is also presented followed by
nucleation and polymer growth and provided a correlation a throughout comparison. Monomer EDOS was synthesized by
between experimental parameters of electropolymerization with a simple and alternative route.
morphologies, optical and redox properties of conjugated
polymers. For example, Bendikov and coworkers systematically
studied the role of solvent and supporting electrolyte on the 2. Experimental section
morphology and electro-optical properties of PEDOT and
2
.1 Chemicals
demonstrated that solvent has a major effect on the properties
34
Selenophene, bromine (Br ), anhydrous methanol (MeOH),
copper(II) oxide (CuO) and anhydrous ethanol (EtOH) were
purchased from Sigma-Aldrich. Zinc powder (Zn), acetic acid
while electrolyte has only minor effect. Lu and coworkers re-
ported that poly(thieno[3,4-b]-1,4-oxathiane) showed better
electroactivity, compact morphology and better electrochromic
2
(AcOH), hydrochloric acid (HCl), potassium iodide (KI),
properties for lm polymerized in BmimPF
6
compared to
35
ethylene glycol and toluene were purchased from Sisco
Research Laboratories, Mumbai, India. For chromatography
separations, columns were prepared using silica gel (mesh size
2 2 6 2 2 6
CH Cl -BmimPF and CH Cl -TBAPF .
Xu and coworkers
studied the effect of polymerization solvent on the electro-
chemical, morphological and electrochromic properties of
nitrogen analogue of PEDOT, poly(N-methyl-3,4-dihydrothieno
4 6 4
60–230). TBAClO , TBAPF , TBABF were purchased from
36
Sigma-Aldrich and dried under vacuum before use. MeCN and
PC were obtained from Sigma-Aldrich. All the materials were
used as received without any purication unless otherwise
mentioned.
[
3,4-b][1,4]oxazine) (PMDTO). Arteaga and coworkers reported
the effect of supporting electrolyte on the conductivity,
37
morphology and electrochemical polymerization of PEDOT.
Zhao and coworkers reported the electropolymerization of 3-
methylselenophene in three different electrolytic medium and
studied the redox, morphology and optoelectronic properties of
38
2.2 Instrumentation
poly(3-methylselenophene).
Compared to numerous study on the effect of electro- UV absorption spectra were recorded using UV1800 Shimadzu
39
1
13
polymerization conditions on the properties of PEDOT,
spectrophotometer. H and C NMR spectra were recorded in
PEDOS has not been investigated in sufficient details consid- CDCl3 using tetramethylsilane as external standard. All elec-
ering its advantages over PEDOT. Indeed, a systematic and trochemical measurements were performed using Metrohm
detailed study on the effect of solvent and supporting electrolyte Autolab PGSTAT204 potentiostat in conventional three-
on the electrochemical and optical properties of PEDOS is still electrode cell using Pt disk or indium tin oxide (ITO) coated
limited to only one report till date. During preparation of this glass slide as working electrode, Au wire as counter electrode
+
manuscript, we came across the article describing the effect of and Ag/Ag as reference electrode. Prior to each electrochemical
electrolyte media on electropolymerization and optical proper- studies, solution was purged with nitrogen to remove any
40
ties of PEDOS lms by Xu and coworkers. Nevertheless, oxygen. PEDOS lms were obtained by electropolymerization of
experimental conditions in this study are limited to polymeri- 0.01 M EDOS in solvents (MeCN and PC) using supporting
zation in CH Cl -TBAPF , CH Cl -BmimPF and BmimPF6. electrolytes (TBAClO , TBPF and TBABF ) over 10–15 cycles at
2
2
6
2
2
6
4
6
4
ꢀ1
Moreover, this study uses ionic liquid as media for polymeri- 100 mV s . Aer polymerization, PEDOS lms were rinsed with
zation which has limitations of poor solubility and high MeCN to remove electrolyte and residual monomers before
viscosity. Yet, despite this effort, fundamental study on elec- their electrochemical, optical and morphology study. Spec-
tropolymerization of EDOS in conventional organic solvents troelectrochemical and electrochromic studies of PEDOS lms
and commonly used supporting electrolytes to provide (deposited on ITO coated glass slides, dimensions (8 mm ꢁ
ꢀ
1
a comprehensive understanding of the electrochemical behav- 50 mm ꢁ 1.1 mm, R < 10 U sq )) were performed in monomer
s
iour, morphology and optical properties of the resultant PEDOS free same solvent–electrolyte media as used for deposition of

lms is much needed for its various applications and has polymer lms. Surface morphologies of the polymer lms
a signicant scope for development.
deposited in different solvent–electrolyte media were investi-
Herein, we present a systematic study on the inuence of gated by scanning electron microscopy (SEM). SEM images of
solvents and supporting electrolytes on the electrochemical PEDOS lms were obtained using modal Zeiss EVO MA10. For
behaviour of EDOS in different media to obtain PEDOS lms morphology studies, PEDOS lms of similar polymerization
with better electrochemical and optoelectronic properties. We charges were prepared in different solvent–supporting
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electrolyte media by multisweep electropolymerization of washed once with brine (20 mL), dried over MgSO
4
and
0.01 M EDOS.
concentrated. Crude product was puried using column chro-
matography using a mixture of hexane and ethyl acetate (40 : 1
ratio) to yield colourless oil in 71% yield (0.47 g) that solidied
2
.3. Synthesis of 3,4-ethylenedioxyselenophene
1
aer storage at low temperature. H NMR (250 MHz, CDCl
3
)
2.3.1 Synthesis of 2,3,4,5-tetrabromoselenophene (2). To
1
3
d 6.56 (s, 2H), 3.86 (s, 6H) C NMR (62.5 MHz, CDCl
6.0, 57.0.
.3.4 Synthesis of 3,4-ethylenedioxyselenophene (5). To
3
) d 148.9,
a well stirred solution of selenophene (1) (1.40 g, 10.69 mmol) in
CHCl (20 mL) and AcOH (1.5 mL), Br (10.7 g, 3.40 mL) was
9
3
2
2
ꢂ
added dropwise at 0 C over a period of 1 h. The resulting
reaction mixture was stirred for 12 h at room temperature fol-
lowed by reux for another 6 h in oil bath. Aer completion of
the reaction, the resulting reaction mixture was allowed to cool
to room temperature and transferred to a large beaker. Excess
a solution of 3,4-dimethoxyselenophene (4) (250 mg, 1.32
mmol) in toluene (80 mL), ethylene glycol (0.5 mL, 7.93 mmol)
and p-toluenesulphonic acid (pTSA) (60 mg) were added and the
ꢂ
resulting reaction mixture was heated at 60 C for 6 h. Aer
completion of the reaction, residue was diluted with ethyl
acetate. The combined organic layer was washed with water (3
Br
2
was evaporated and the crude residue was diluted with
CHCl (30 mL). The organic phase was washed with water (3 ꢁ
3
ꢁ
4
25 mL), once with brine (20 mL), dried over MgSO and
25 mL), once with brine (20 mL), dried over MgSO and
4
concentrated. Crude product was puried by column chroma-
tography using a mixture of hexane and ethyl acetate in 20 : 1
ratio. EDOS was obtained as a colourless oil in 48% yield (0.12
concentrated under reduced pressure. The crude product tet-
rabromoselenophene (2) was puried by column chromatog-
raphy using hexane as eluent to yield white crystals in 75% yield
1
13
g). H NMR (250 MHz, CDCl ) d 6.78 (s, 2H), 4.16 (s, 4H),
C
1
3
3
(
3.57 g). C NMR (62.5 MHz, CDCl
.3.2 Synthesis of 3,4-dibromoselenophene (3). Zn powder
0.88 g, 13.45 mmol) was added in small portions to a reuxing
3
) d 117.9, 112.2.
NMR (62.5 MHz, CDCl ) d 142.7, 101.7, 64.3.
3
2
(
solution of tetrabromoselenophene (2.0 g, 4.48 mmol) in
a mixture of EtOH (10.2 mL), AcOH (2.5 mL) and 0.1 mL of HCl
3
. Results and discussion
3.1 Monomer synthesis
(11.97 N) over 5 min. Aer reuxing for 3 h, the reaction mixture
was ltered hot and upon cooling to room temperature The monomer EDOS was synthesized by a new and simple route
41
concentrated. The crude residue was diluted with CH
2 2
Cl . The for electropolymerization as presented in Scheme 1. EDOS was
combined organic layers were washed with water (3 ꢁ 25 mL), rstly prepared by Aqad and group using a ve-step synthetic
42
4
brine (20 mL) and dried over anhydrous MgSO and concen- methodology similar to the industrial synthetic route of EDOT.
trated under reduced pressure. Purication of crude product by We had previously reported, a much simpler synthetic strategy
column chromatography using hexane as eluent afforded oil 3 to prepare EDOS from 2,3-dimethoxy-1,3-butadiene and sele-
1
25
in 60% yield (0.77 g). H NMR (250 MHz, CDCl ) d 7.93 (s, 2H) nium dichloride in two steps. Zade and coworkers synthesized
3
1
3
C NMR (62.5 MHz, CDCl ) d 127.4, 114.3.
.3.3 Synthesis of 3,4-dimethoxyselenophene (4). To ide and selenium dichloride. However, these two synthetic
EDOS from trimethylsilylated diyne using zirconocene dichlor-
3
4
3
2
a solution of 3,4-dibromoselenophene (3) (1.0 g, 3.47 mmol) in schemes suffer from the usage of selenium dichloride which is
anhydrous MeOH (12 mL), sodium methoxide (16.3 mmol, toxic in nature and inhibited further study. Herein, we have
prepared by dissolving 376 mg Na in MeOH), CuO (0.69 g, 8.67 synthesized EDOS from selenophene (1) by a simple route.
mmol) and KI (115 mg, 0.69 mmol) were added successively. Compound 1 was subjected to bromination using liquid
The resulting mixture was heated to reux for 3 days under bromine to obtain 2,3,4,5-tetrabromoselenophene (2) which
nitrogen atmosphere. Aer completion of the reaction, residue was then selectively debrominated at 2 and 5-position using Zn-
was ltered and washed with CHCl (25 mL). Organic layer was AcOH to yield 3,4-dibromoselenophene (3). Compound 3 on
3
concentrated and crude residue was washed with water. treatment with freshly prepared sodium methoxide afforded
Aqueous layer was extracted with diethyl ether (3 ꢁ 25 mL), 3,4-dimethoxyselenophene (4) as a colorless oil. In the nal
Scheme 1 Synthesis route of 3,4-ethylenedioxyselenophene (5).
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and 1.13 V in TBABF
4
. However in PC, EDOS was oxidized at
1
.06 V in TBAClO , 1.11 V in TBAPF and 1.07 V in TBABF .
4 6 4
These results clearly indicate that the oxidation potential of
monomer in both solvents MeCN and PC are almost identical
whereas a noticeable difference is observed in the case of sup-
porting electrolytes.
Scheme
2 Electrochemical polymerization of EDOS in different
The multisweep elctropolymerization of EDOS in different
solvent–electrolyte media is shown in Fig. 2. As shown in Fig. 2a,
upon repetitive potential scanning of monomer EDOS in
solvent–electrolyte media.
TBAClO /MeCN between ꢀ1.0 V and 1.3 V, a broad redox wave
4
step, compound 4 was transethericated using ethylene glycol
in presence of acid catalyst to give 3,4-ethyl-
enedioxyselenophene (EDOS; 5). Compound 5 was stored at low
temperature and found to be stable for over years.
was generated in the region ꢀ0.7 to 0.9 V which corresponds to
redox behaviour of electrodeposited polymer PEDOS. This is
attributed to the formation of polymers with broad chain length
distribution and conversion of the conductive species from
neutral to oxidized states. An increase in current density upon
repetitive cycling suggests a gradual deposition of polymer lms
onto the electrode surface. A slight shi in cathodic and anodic
peak potentials indicated the increased electrical resistance of
the polymer lms and therefore requires slightly high potential
3.2 Electropolymerization in different solvent–electrolyte
media
In order to investigate the effect of polymerization media on the
electropolymerization, electrochemical and optical properties
of the resultant polymer PEDOS, monomer EDOS (0.01 M) was
electrodeposited by repetitive cycling on Pt electrode in
to overcome this. This behaviour is characteristic of conjugated
44–46
polymers during electropolymerization.
A similar result was
ꢀ
1
different solvent–electrolyte media at 100 mV s for 10 cycles
Scheme 2). We carried out the electrochemical polymerization
in the presence of three supporting electrolytes consisting of
different counter anions viz TBAClO , TBAPF and TBABF and
found during electropolymerization of EDOS in other solvent–
electrolyte media.
The polymerization behaviour of EDOS is found to be
different in different solvent–electrolyte media. Increment in
current density per cycle was found to be comparatively higher
in MeCN than PC which clearly suggests better polymerization
(
4
6
4
two solvent namely MeCN and PC. The rst anodic oxidation
curve of EDOS in different solvent–electrolyte systems is shown
in Fig. 1 and data is summarized in Table 1. Oxidation of EDOS
efficiency in MeCN. However,
a more uniform current
4 6
in MeCN was initiated at 1.09 V in TBAClO , 1.10 V in TBAPF
ꢀ1
Fig. 1 First anodic oxidation curve of EDOS on Pt electrode in different solvent–electrolyte media (a) MeCN (b) PC at scan rate of 100 mV s
.
Table 1 Oxidation potential of monomer EDOS and current increment during electropolymerization in different solvent–electrolyte media
ꢀ2
Current increment (mA cm
)
Solvent–electrolyte
media
Average current increment
ox
ꢀ2
st
nd
th
th
E
onset (V)
per cycle (mA cm
)
Between 1 and 2 cycle
Between 9 and 10 cycle
TBAClO
TBAPF /MeCN
/MeCN
/PC
/PC
/PC
4
/MeCN
1.09
1.10
1.13
1.06
1.11
1.07
0.060
0.049
0.042
0.016
0.015
0.014
0.033
0.038
0.023
0.010
0.011
0.010
0.067
0.051
0.056
0.017
0.015
0.015
6
TBABF
4
TBAClO
4
TBAPF
TBABF
6
4
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4 6 4
Fig. 2 Multisweep electropolymerization of 0.01 M EDOS on Pt electrode in (a) TBAClO /MeCN, (b) TBAPF /MeCN, (c) TBABF /MeCN, (d)
ꢀ1
4 6 4
TBAClO /PC, (e) TBAPF /PC and (f) TBABF /PC at scan rate of 100 mV s .
increment from cycle to cycle was observed in case of PC nature of solvents and counter anions as all other experimental
compared to MeCN. A signicant difference of counter anions parameters such as monomer concentration, number of cycles
on polymerization efficiency was also noted. When TBAClO
was used as supporting electrolyte, polymerization efficiency efficiency of EDOS observed in MeCN compared to PC is
was found to be better compared to TBAPF and TBABF as explained by difference of dielectric constant and viscosity (see
indicated by signicantly high current increment in case of ESI, Table S1‡). Due to high dielectric constant of PC, oligomers
4
and potential sweep rate were same. Better polymerization
6
4
th
4
TBAClO . We observed that current increment between 9 and of short-chain length formed during the initial stage of poly-
th
st
nd
1
0 cycle was higher than 1 and 2 cycle indicating a higher merization solubilize readily leading to slow deposition of
deposition rate of polymer on the electrode surface in nal polymer lms on the electrode surface. Moreover, high viscosity
cycle. As presented in Table 1, the current increment indicates of PC also slows down the speed of monomer and oligomers
3
4,36
the gradual increase in polymer deposition rate upon successive during electropolymerization.
cycling. more suitable medium for polymerization of EDOS as compared
The different electrochemical behaviour of EDOS during to PC similar to previously reported polymerization behaviour
This indicates that MeCN is
36
47
electropolymerization is mainly explained by difference in of MDTO and terthiophene.
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In addition to solvent, electropolymerization of EDOS is also clearly demonstrate that both solvent and supporting electrolyte
inuenced by supporting electrolyte. The different polymeriza- have signicant effect on the electropolymerization of EDOS,
tion efficiency observed in case of supporting electrolytes is but the effect of solvent is more pronounced as compared to the
attributed to difference in nature of anions, since cation (tet- electrolyte. We conclude that for electropolymerization of
4
rabutylammonium) is same in all the cases. Efficient polymer- EDOS, MeCN and TBAClO were found to be a more efficient
ization in case of TBAClO
due to high mobility of ClO
4
compared to TBAPF
6
and TBABF
4
is solvent and electrolyte respectively compared to other solvent–
ꢀ
ꢀ
ꢀ
4
as compared to PF
6
and BF
4
electrolyte media used in this study.
which results in high current increment indicating better
48
polymerization efficiency. During polymerization process,
formation of polaron–anion pair takes place which leads to the
immobalization of counter anions inside the polymer matrix.
Since an anion with high mobility will be replaced more rapidly
by monomers during polymerization and consequently better
3
.3 Electrochemistry of PEDOS lms
The redox behaviour of the PEDOS lms prepared from
different solvent–electrolyte media were investigated by cyclic
voltammetry (CV) in monomer-free solvent–electrolyte system
to evaluate their electrochemical properties. As shown in
48,49
polymerization efficiency in case of TBAClO
4
.
These results
Fig. 3 Cyclic voltammograms of PEDOS films prepared in different media recorded in monomer free solvent–electrolyte systems (a) TBAClO
4
/
MeCN, (b) TBAPF /MeCN, (c) TBABF /MeCN, (d) TBAClO /PC, (e) TBAPF /PC and (f) TBABF /PC at scan rates of 25, 50, 75, 100, 150, 200 and
6
4
4
6
4
ꢀ
1
2
50 mV s .
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Table 2 The redox properties of PEDOS polymerized in different solvent–electrolyte media
Solvent–electrolyte
ox
a
ꢀ2
b
ꢀ2
media
Epoly,onset (V)
Q
Oxid (mC cm
)
Q
Red (mC cm
)
QOxid/QRed
TBAClO
4
/MeCN
/MeCN
/MeCN
/PC
/PC
/PC
ꢀ0.68
ꢀ0.65
ꢀ0.52
ꢀ0.77
ꢀ0.74
ꢀ0.72
84.7
64.8
61.3
23.6
21.0
23.5
77.0
58.9
54.0
22.4
20.5
22.6
1.10
1.10
1.13
1.05
1.02
1.04
TBAPF
6
TBABF
4
TBAClO
4
TBAPF
6
TBABF
4
a
b
Oxidation charges. Reduction charges.
Fig. 3, CV of PEDOS lms in all the solvent–electrolyte media was noted that monomer EDOS shows similar absorption
at different scan rates showed broad redox peaks (cathodic maxima in MeCN (lmax 269 nm) and PC (lmax 271 nm) (Fig. S2‡).
and anodic), and signicant hysteresis in cathodic and anodic The spectroelectrochemical data of PEDOS lms recorded in
peak potentials. This may be attributed to slow, non- different media is shown in Fig. 4 and summarized in Table 3.
homogeneous electron transfer and slow interconversion As shown in Fig. 4a, the absorption spectra of PEDOS lm in
5
0
between redox species during the redox process. Further- neutral state at ꢀ0.9 V using TBAClO
4
/MeCN medium exhibits
more, a linear dependence between scan rates and peak absorption maxima at 648 nm originating from p–p* transi-
current in case of all solvent–electrolyte system showed that tions. Upon successive increase of potential from ꢀ0.9 to 0.5 V
the redox process is non-diffusion controlled with polymer (neutral to doped state), intensity of the p–p* transition grad-

lms being well adhered to the surface of working electrode ually decreases with shi of absorption maxima to NIR region
(
Fig. S1‡). In all solvent–electrolyte media, the ratio between owing to the formation of polarons and bipolarons in the doped
QOxid and QRed for PEDOS lm was found to be close to 1 or state along the polymer backbone. A similar trend was followed
slightly higher which indicates a good reversibility of redox in the absorption spectra of PEDOS lms prepared in other
processes.
solvent–electrolyte media as well. Consequently, changes in
It was observed that lms prepared in different solvent– absorption spectra were also accompanied with colour change
electrolyte media showed different electroactivity. PEDOS lms from deep blue in neutral state to colourless in doped (oxidized)
prepared in MeCN were oxidized at comparatively high poten- state in all solvent–electrolyte media. It is noteworthy to
tial and exhibited more well-dened oxidation and reduction mention that both solvent and supporting electrolyte used for
peaks than lms obtained in PC. This difference in the elec- polymerization has considerable effect on absorption spectra of
trochemical behaviour due to solvents is explained by slow and polymer lms. However, effect of solvent on absorption spectra
smooth growth of longer polymer chains in PC which results in of PEDOS is more predominant in comparison to supporting
36,51
faster electron transfer.
CV of PMDTO lms prepared in MeCN and PC. It is note-
worthy to mention that the redox behaviour of PEDOS lms red-shied as compared to MeCN (Fig. S3‡). In case of PEDOS
were also considerably inuenced by supporting electrolytes. lms prepared in PC using TBAClO , TBAPF and TBABF
A similar trend was noticed in the electrolytes.
36
The absorption spectra of PEDOS lms prepared in PC are
4
6
4
,
The oxidation potential of polymer lm prepared and charac- absorption maxima are red-shied by 4 nm, 10 nm, 47 nm,
terized in TBABF4 was slightly high in comparison to those respectively versus lms deposited in MeCN. Furthermore,
obtained in the case of TBAClO4 and TBAPF6 in both the absorption spectra in PC are slightly narrow in comparison to
solvents MeCN and PC (Table 2). This might be attributed to the MeCN as indicated by their full width half maximum (FWHM)
ꢀ ꢀ
high mobility of ClO and PF which results in easy migration values. This difference in absorption spectra of the lms
4 6
inside the polymer matrix for charge compensation during prepared in PC and MeCN is explained by high dielectric
ꢀ
oxidation process while for BF
4
incomplete doping occurs (as constant and viscosity of PC. It has been mentioned in literature
ꢀ
a result of high nucleophilcity of BF
solvent molecules which hinders its mobility into polymer due to its high dielectric constant than MeCN.
matrix) resulting in high oxidation level as compared to ClO4
4
it is more solvated by that oligomers of short-chain length have high solubility in PC
34,36
During the
52
ꢀ
polymerization process, short oligomers solubilize readily in PC
as compared to MeCN resulting in the formation of long-chain
length polymers and thereby producing a sharp UV spectrum
with longer absorption wavelength. Moreover, high dielectric
constant of PC also reduces the resistivity of the solution during
electropolymerization, leading to more smooth lm formation.
While deposition of oligomers of short-chain lengths in MeCN
onto the electrode surface leads to high nucleation centres and
results in the formation of polymer lms with broad distribu-
tion of chain length and high surface roughness. This results in
ꢀ
48
and PF6
.
3
.4 Spectroelectrochemical characterization of PEDOS lms
The optoelectronic properties of PEDOS lms prepared in
different solvent–electrolyte media were investigated using UV-
vis-NIR absorption spectroscopy. The UV-vis-NIR absorption
spectra of PEDOS lms in oxidized and neutral states were
recorded under different applied potential in the same elec-
trolyte–solvent systems as used for the preparation of lms. It
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Fig. 4 Spectroelectrochemistry of PEDOS films in monomer free solvent–electrolyte media on ITO coated glass substrate at different applied
potentials in (a) TBAClO /MeCN, (b) TBAPF /MeCN, (c) TBABF /MeCN, (d) TBAClO /PC, (e) TBAPF /PC and (f) TBABF /PC.
4
6
4
4
6
4
broad absorption spectra and shorter wavelength maxima for
Interestingly, supporting electrolyte has also substantial

lms prepared in MeCN as compared to PC similar to PEDOT effect on the absorption maxima of PEDOS lms (Fig. S4‡). It
34,36
and PMDTO.
was noted that absorption maxima in case of TBABF is blue-
4
Table 3 Spectroelectrochemical data of PEDOS films prepared in different solvent–electrolyte media
UV-vis absorption
Solvent–electrolyte
media
a
b
c
l
max (nm)
l
onset (nm)
FWHM (nm), [eV]
Band gap (eV)
TBAClO
4
/MeCN
/MeCN
/MeCN
/PC
/PC
/PC
648
644
623
652
654
670
893
885
890
890
887
886
259 [0.78]
255 [0.78]
250 [0.77]
251 [0.74]
245 [0.73]
237 [0.64]
1.38
1.40
1.39
1.39
1.39
1.39
TBAPF
6
TBABF
4
TBAClO
4
TBAPF
6
TBABF
4
a
b
Wavelength determined at absorption peak maximum of polymer in neutral (dedoped) state. Wavelength calculated from onset of the
c
absorption peak of polymer in neutral state. FWHM determined from absorption peak of polymer in neutral state.
12402 | RSC Adv., 2020, 10, 12395–12406
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shied in MeCN, as compared to TBAPF
6
and TBAClO
4
. In efficiency were investigated using chronoabsorptometry tech-
contrast to this, absorption maxima of PEDOS lm in TBABF is nique. Polymer lms were switched alternatively between
4
red shied in PC than TBAPF and TBAClO . Moreover, FWHM oxidized and neutral states upon switching the potential from
6
4
values are also signicantly affected by supporting electrolytes ꢀ0.9 V to 0.5 V at l
with a residence time of 5 s. For the study
in MeCN. It is noteworthy to mention that the absorption onset of electrochromic parameters, PEDOS lms were prepared in
of the PEDOS lms prepared in different media is unaffected different solvent–electrolyte system by repetitive cyclic voltam-
max
34
similar to that observed for PEDOT. Overall, absorption metry of EDOS. Switching studies were performed in the same
spectra of PEDOS lms is signicantly affected by both solvents solvent–electrolyte media as that used for the deposition of
and supporting electrolytes. However, the effect of solvent is polymer lms.
more predominant on optical properties as compared to sup-
Fig. 5 and Table 4 summarizes the optical switching data of
porting electrolyte, with TBABF being an exception.
polymers prepared in different solvent–electrolyte media at
4
l
max. It was noted that all polymers exhibit high transmittance
in the doped state irrespective of solvent–electrolyte media used
for the preparation of lms. A signicant change in optical
3.5 Electrochromic properties
Further, the electrochromic properties of PEDOS lms such as contrasts ratio of PEDOS lms was observed in both solvent and
optical contrasts ratio (DT%), response time and coloration
Fig. 5 Transmittance time profiles of PEDOS films obtained by switching the films between ꢀ0.9 to 0.5 V at time interval of 5 s in different
solvent–electrolyte system.
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Table 4 Electrochromic performance of PEDOS films prepared in different solvent–electrolyte media
Solvent–electrolyte
media
Wavelength
(nm)
Response
time (s) soxi
Injected charge
density (mC cm
a
b
2
ꢀ1
ꢀ2
DT%
CE (cm C
)
)
TBAClO
TBAPF /MeCN
/MeCN
/PC
/PC
/PC
4
/MeCN
648
644
623
652
654
670
29.8
30.1
34.2
33.1
38.8
44.6
0.8
1.5
2.0
2.0
2.6
2.5
81.6
67.1
90.7
97.4
104.5
141.8
3.51
3.55
3.25
3.28
3.08
3.26
6
TBABF
4
TBAClO
4
TBAPF
6
TBABF
4
a
b
Response time calculated at 95% of full switch from neutral to doped state. Coloration efficiency (CE) is calculated by CE ¼ log(TOxid/TRed)/
53
injected charge density, where Toxid refers to transmittance in doped state and TRed refers to transmittance in dedoped state.
electrolyte media. Nonetheless, the effect of solvent is more both solvent and supporting electrolyte has only a minor effect
prominent as compared to supporting electrolyte, but consid- on the surface morphologies of PEDOS lms. PEDOS lms
erable difference of supporting electrolytes on electrochromic prepared in PC have slightly smooth and uniform morphologies
properties were also observed in PC versus MeCN. PEDOS lms as compared to lms prepared in MeCN. This observation is
prepared in PC showed better optical contrasts ratio as observed similar to previous reports that polymerization in PC provides
in case of MeCN. It is also noted that though supporting elec- lm with smooth, homogeneous distribution of particles and
34,36
trolytes have only a small effect on DT% of PEDOS lms in at morphology than lms prepared in MeCN.
This is
MeCN, but considerable difference was observed in PC. Indeed, explained by high polarity of PC which favours the solubility of
DT% of PEDOS lms prepared in PC was signicantly higher short oligomers resulting in homogeneous growth of polymer
51
compared to those obtained in MeCN. PEDOS lm prepared in lms on the electrode surface. Moreover, high viscosity of PC
TBABF /PC shows the highest contrasts ratio of 44.6% and results in slow diffusion of species (monomer, oligomers and
4
2
ꢀ1
coloration efficiency 141.8 cm C compared to other solvent– counter anions) during polymerization limiting the polymeri-
electrolyte systems. These results are found to be in good zation rate as compared to MeCN. This indicates the formation
correlation with absorption spectra. High optical contrasts ratio of polymer lms with more ordered structure due to slow
observed in PC can be explained by its high dielectric constant process for lm growth resulting in smooth and compact
3
4,36,51
than MeCN. As mentioned previously that high solubility of morphology.
short oligomers in PC during the electropolymerization results TBABF showed more compact structure and smooth
in the formation of polymers of long-chain length and longer morphologies as compared to lms prepared using TBAPF and
absorption maxima. Whereas in case of MeCN, oligomers of TBAClO . It is also important to mention that PEDOS lm
short-chain lengths are present that cannot be switched under prepared using TBABF /PC has more smooth morphology as
It was noted that lms prepared using
4
6
4
4
the switching conditions used for PEDOS with long chains. In compared to other solvent–electrolyte media. This result seems
addition to this, deposition of short oligomers on electrode to correlate well with enhanced optical properties that is both
surface also leads to high nucleation centres resulting in lms longer absorption maxima and better electrochromic properties
with high roughness. This results in poor electrochromic of PEDOS prepared using TBABF
4
/PC. Consequently,
34,36
performance of lms in MeCN.
In contrary to optical morphology studies clearly indicate that properties of PEDOS
contrasts ratio, PEDOS lms in PC showed longer response time lms are clearly inuenced by polymerization conditions such
in comparison to those obtained in MeCN. This is mainly due to as solvent and supporting electrolyte.
high viscosity of PC which apparently reduces the mobility of
counter ions than in case of MeCN. In case of lms prepared
and analysed in MeCN, slightly longer response time was 4. Conclusions
noticed in case of TBABF
TBAClO
mobility of ClO
tates the easy migration of ClO
dedoping process and reduces soxi
4 6
(2.0 s) and TBAPF (1.4 s) than
In summary, we have electrodeposited PEDOS lms with
different redox and optoelectronic properties from EDOS
monomer by using six different combinations of solvent–elec-
trolyte media. EDOS was synthesized by a simple and alternative
route. It was noted that both solvent and supporting electrolyte
used for polymerization have signicant inuence on the elec-
tropolymerization, redox, optoelectronic and electrochromic
4
(0.8 s). This behaviour might be explained by the high
ꢀ
ꢀ
ꢀ
4
as compared to BF
4
and PF
6
which facili-
ꢀ
4
ions during the doping and
.
3.6 Morphology study
In order to further investigate the effect of polymerization properties of the resultant PEDOS lms. However compared to
conditions on the surface morphology and to obtain a correla- solvent, supporting electrolyte has only a moderate effect on the
tion between structure and optical properties of the resultant properties of PEDOS lms. We found that MeCN and TBAClO4
PEDOS lms, their surface morphologies were studied using are the most efficient solvent–electrolyte media for electro-
SEM. The surface morphologies of the polymer lms prepared polymerization of EDOS. Electrochemical study revealed that
in different medium is shown in Fig. S5.‡ It was observed that PEDOS lms obtained in PC were oxidized at comparatively
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lower potential than MeCN while for electrolytes the order 13 (a) V. Agrawal, Shahjad, D. Bhardwaj, R. Bhargav,
ETBABF4 > ETBAPF6 > ETBAClO4 is followed. Further investigation of
morphologies and optical properties revealed that appropriate
selection of solvent and supporting electrolyte can be used to
G. D. Sharma, R. K. Bjardwaj, A. Patra and S. Chand,
Electrochimica Acta, 2016, 192, 52–60; (b) A. Mishra,
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control optoelectronic and electrochromic properties. Optical 14 I. F. Perepichka and D. F. Perepichka, Handbook of
and SEM studies suggested that though MeCN was found to be
Thiophene-based Materials: Applications in Organic
more efficient solvent medium for polymerization, lms
Electronics and Photonics, John Wiley & Sons, 2009, vol. 2.
prepared in PC demonstrated more smooth morphologies, 15 J. Roncali, P. Blanchard and P. Frere, J. Mater. Chem., 2005,
better optoelectronic and electrochromic properties. This is 15, 1589–1610.
attributed to high dielectric constant and viscosity of PC which 16 L. B. Groenendaal, G. Zotti, P.-H. Aubert, S. M. Waybright
provides a better environment for polymer growth during and J. R. Reynolds, Adv. Mater., 2003, 15, 855–879.
polymerization resulting in deposition of more homogeneous 17 L. B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik and

lms as compared to MeCN. Among all solvent–electrolyte
J. R. Reynolds, Adv. Mater., 2000, 12, 481–494.
media, PEDOS lm prepared in TBABF
4
/PC medium exhibited 18 R. Vhargav, D. Bhardwaj, Shahjad, A. Patra and S. Chand,
longest absorption maxima (lmax 670 nm), highest optical
ChemistrySelect, 2016, 1, 1347–1352.
2
contrasts ratio (44.6%) and coloration efficiency (141.8 cm
19 (a) L.-M. Yang, I. A. Popov, T. Frauenheim, A. I. Boldyrev,
T. Heine, V. Ba ˇc i ´c and E. Ganz, Phys. Chem. Chem. Phys.,
2015, 17, 26043–26048; (b) J.-H. Liu, Y.-M. Yang and
E. Ganz, RSC Adv., 2019, 9, 27710–27719; (c) L.-M. Yang,
V. Ba ˇc i ´c , I. A. Popov, A. I. Boldyrev, T. Heine,
T. Frauenheim and E. Ganz, J. Am. Chem. Soc., 2015, 137,
ꢀ1
C
). Further study on electropolymerization of EDOS in the
aqueous system and behaviour of the resulting PEDOS lms are
54
under process.
Conflicts of interest
2757–2762; (d) J.-H. Liu, L.-M. Yang and E. Ganz, J. Mater.
There are no conicts to declare.
Chem. A, 2019, 7, 3805–3814; (e) Q. Zhang, H. Dong and
W. Hu, J. Mater. Chem. C, 2018, 6, 10672–10686; (f)
¨
P. Payamyar, B. T. King, H. C. Ottinger and A. D. Schl u¨ ter,
Acknowledgements
Chem. Commun., 2016, 52, 18–34.
P.Y. is thankful to CSIR-New Delhi for her fellowship.
20 (a) I. Kang, T. K. An, J.-A. Hong, H.-J. Yun, R. Kim,
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