Stepwise enhancement on optoelectronic performances of polyselenophene via electropolymerization of mono-, bi-, and tri-selenophene

Article abstract of DOI:10.1016/j.electacta.2020.135974

Although much progress have been made on polyselenophenes-based molecular systems, the poor optoelectronic performance of parent polyselenophene still hamper both the fundamental understanding and practical applications of such materials due to the monomer instability during the polymerization process and the lack of suitable monomeric precursors. In this work, we develop an effective method to improve the optoelectronic performances and stability of parent polyselenophene by stepwise increasing the initial monomeric chain length for electrochemical polymerization. We find that the chain length increment of the monomeric structures from selenophene to bi- and tri-selenophenes dramatically reduces the electropolymerization potential and thus enables the formation of high quality polyselenophene films with better conjugated structures and less structural defects. As-formed polyselenophene from tri-selenophene reveals lowered optical band gap (1.72 eV), better redox activity and stability, and significantly improved electrochromic nature of reversible and stable color changes between red and blue with high performance including superior optical contrast up to 75%, high coloration efficiency up to 450 cm² C^?1, and very fast switching time (0.7 s for oxidation and 0.4 s for reduction). These advantageous properties of as-prepared polyselenophene films afford the successful fabrication of patterned flexible electrochromic devices, which exhibit reversible and stable color changes upon both doping-dedoping and mechanical bending.

Full text of DOI:10.1016/j.electacta.2020.135974

Electrochimica Acta 340 (2020) 135974
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Stepwise enhancement on optoelectronic performances of
polyselenophene via electropolymerization of mono-, bi-, and tri-
selenophene
,
,
,
, *
Baoyang Lu a 1, Nannan Jian a 1, Kai Qu a, Faqi Hu a, Ximei Liu a **, Jingkun Xu a
,
Guoqun Zhao a
, b, ***
a Flexible Electronics Innovation Institute, Jiangxi Science & Technology Normal University, Nanchang, 330013, China
b Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, 250061, Shandong,
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Although much progress have been made on polyselenophenes-based molecular systems, the poor
optoelectronic performance of parent polyselenophene still hamper both the fundamental under-
standing and practical applications of such materials due to the monomer instability during the poly-
merization process and the lack of suitable monomeric precursors. In this work, we develop an effective
method to improve the optoelectronic performances and stability of parent polyselenophene by stepwise
increasing the initial monomeric chain length for electrochemical polymerization. We find that the chain
length increment of the monomeric structures from selenophene to bi- and tri-selenophenes dramati-
cally reduces the electropolymerization potential and thus enables the formation of high quality poly-
selenophene films with better conjugated structures and less structural defects. As-formed
polyselenophene from tri-selenophene reveals lowered optical band gap (1.72 eV), better redox activity
and stability, and significantly improved electrochromic nature of reversible and stable color changes
between red and blue with high performance including superior optical contrast up to 75%, high
Received 1 November 2019
Received in revised form
31 January 2020
Accepted 24 February 2020
Available online 27 February 2020
Keywords:
Conjugated polymers
Polyselenophene
Electrochemical polymerization
Flexible electronics
Electrochromics
coloration efficiency up to 450 cm2 ꢀ1, and very fast switching time (0.7 s for oxidation and 0.4 s for
C
reduction). These advantageous properties of as-prepared polyselenophene films afford the successful
fabrication of patterned flexible electrochromic devices, which exhibit reversible and stable color
changes upon both doping-dedoping and mechanical bending.
© 2020 Published by Elsevier Ltd.
1. Introduction
molecular systems, such as polythiophenes, polypyrroles, and their
derivatives, etc. Among them, polythiophene-based conjugated
Conducting polymers, as a fast-growing field in material science,
have attracted extensive scientific interests owing to their prom-
ising applications in diverse fields, such as organic field-effect
transistors[1], organic solar cells [2], and polymer electrochromics
[3,4], bioelectronics [5,6], soft actuators [7], etc. Numerous re-
searches have been reported on a wide variety of conjugated
polymers have received special attention due to their unique op-
toelectronic properties and ease of functionalization[8e11]. As a
close analogue of polythiophene, polyselenophene and its-based
conjugated molecular systems, show several advantages including
lower band gap, stronger intermolecular SeeSe interactions, more
rigid conjugated backbone, higher polarizability and lower redox
potentials[8,9]. These advantages make polyselenophenes-based
molecular systems promising for various applications like field-
effect transistors[1], solar cells [2], thermoelectric [12] and elec-
trochromic devices [13].
* Corresponding author.
** Corresponding author.
Although tremendous advancement has been made on the sci-
ence and technology for polyselenophenes in the last decade,
parent polyselenophene still displays poor optoelectronic perfor-
mances far from theoretical expectation due to the monomer
*** Corresponding author. Flexible Electronics Innovation Institute, Jiangxi Science
& Technology Normal University, Nanchang, 330013, China.
E-mail addresses: lxm5812@163.com (X. Liu), xujingkun1971@yeah.net (J. Xu),
zhaogq@sdu.edu.cn (G. Zhao).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.electacta.2020.135974
0013-4686/© 2020 Published by Elsevier Ltd.

2
B. Lu et al. / Electrochimica Acta 340 (2020) 135974
instability during the polymerization process and the lack of suit-
able monomeric precursors [8,9,14e16], which hinders both the
fundamental understanding and practical applications of such
materials. Apart from direct polymerization condition control in
previous reports[8], the chemical structure modification of initial
monomeric precursors is also expected to provide milder poly-
merization conditions if suitable monomeric precursors can be
found. Unfortunately, little has been known on the effect of initial
monomeric chain length on the polymerization behavior and the
properties of the resultant polyselenophene. Owing to longer
conjugated chain, oligoselenophenes like biselenophene and tri-
with the 6-31G(d,p) basis set (B3LYP/6-31G(d,p)). The energy gap
(Eg,DFT) was calculated from the HOMO-LUMO gap. Electrochemical
polymerization of all the precursors was performed by a Princeton
Versa Stat 3 electrochemical workstation, with Pt wires (diameter:
1 mm) as the working/counter electrodes and an Ag/AgCl electrode
as the reference electrode. The electrolytic solution was bubbled by
nitrogen before every experiment to eliminate the effect of oxygen.
In order to get sufficient polymer films for characterization, Pt sheet
(2 ꢃ 1.5 cm2; as the working electrode) and ITO-coated glass sheet
(2 ꢃ 2 cm2; as the counter electrode) were employed for the
electrosynthesis. A Bruke Vertex 70 Fourier-transform infrared (FT-
IR) spectrometer was used to record the infrared spectra of all the
monomeric precursors and corresponding polyselenophene.
Scanning electron microscope (SEM) images were taken by using a
VEGA II-LSU scanning electron microscope. By combining an elec-
trochemical workstation (Versa Stat 3) and a SPECORD 200 PLUS
UV-vis spectrophotometer, spectroelectrochemistry and electro-
chromic kinetic studies of the polyselenophene films were sys-
tematically investigated in a custom-made electrochemical cell
with an Ag/AgCl electrode as the reference electrode, indium tin
oxide (ITO)-coated glass slide (0.8 ꢃ 2.5 cm2) as the working
electrode, and a Pt wire (diameter: 1 mm) as the counter electrode,
respectively.
selenophene have larger
p delocalization than selenophene and
thus are much easier to oxidize while maintaining the conjugated
selenophene moieties[8,14e16]. Therefore, we propose that
increasing the initial monomeric precursor chain length is probably
an effective approach to enhance the optoelectronic performances
and stability of polyselenophene.
Herein, we carefully investigate the electropolymerization
behavior of selenophene (Se), biselenophene (2Se), and tri-
selenophene (3Se), and electropolymerize them to obtain the cor-
responding polyselenophene films. Multiple characterization
techniques including spectral, morphological, electrochemical, as
well as computational chemistry, have been used to systematically
investigate the effect of monomeric precursor chain length on the
electropolymerization behavior and the optoelectronic perfor-
mances of the resultant polyselenophene films. Moreover, high
quality polyselenophene film with excellent electrochromic per-
formance has been achieved, allowing us to successfully fabricate
patterned flexible electrochromic devices.
2.3. Synthesis
2,20-Biselenophene (2Se) and 2,2’:50,200-terselenophene (3Se)
were synthesized by Stille coupling (Fig. 1A). The detailed experi-
mental procedures are illustrated as follows.
2-Bromoselenophene. Selenophene (0.50 g, 3.80 mmol),
CH2Cl2 (4 mL), and CH3COOH (4 mL) were added into a flask and
kept at 0 ꢂC. Then N-bromosuccinimide (0.67 g, 3.80 mmol) was
slowly added and the mixture was stirred for 2 h at 0 ꢂC. After
washing with deionized water and removing the solvent, 2-
bromoselenophene (0.83 g, yield: 56%) was obtained as a colour-
less liquid by column chromatography. 1H NMR (400 MHz,
2. Experimental
2.1. Materials
Selenophene (99%; Shanghai Vita), dichloromethane (CH2Cl2,
99.8%; J&K), acetic acid (CH3COOH, 99.8%; Shanghai Vita), N,N-
dimethylformamide (DMF, 99.9%; Tianjin Bodi), N-bromosuccini-
mide (NBS, 98%; Tianjin Bodi), sodium bicarbonate (NaHCO3, 99%;
Sigma-Aldrich), chlorotributyltin (SnBu3Cl, J&K), n-butyllithium (n-
BuLi, 1.6 M in hexanes; J&K), lithium perchlorate (LiClO4, 99%; J&K),
acetonitrile (CH3CN, 99.9%; Superdry, water ꢁ 30 ppm, J&K), pro-
pylene carbonate (PC, 99.5%; Superdry, J&K), and poly(methyl
methacrylate) (PMMA, J&K) were used directly without any puri-
fication. Tetrahydrofuran (THF, 99%; Shanghai Vita) was distilled
from sodium benzophenone ketyl and used as the solvent for
stannylization. Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4,
99%; J&K) was used as the catalyst for Stille coupling. Tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6, 99%; Shanghai Vita)
was employed as the supporting electrolyte for electrochemical
polymerization and electrochromic tests after drying in vacuum for
24 h at 60 ꢂC.
DMSO‑d6,
d ppm): 8.14 (d, 1H), 7.36 (s, 1H), 7.09 (s, 1H).
2,5-Dibromoselenophene. Selenophene (1.00 g, 7.63 mmol)
and N-bromosuccinimide (3.00 g, 16.85 mmol) were added in
tetrahydrofuran (50 mL). The solution was stirred for 12 h at room
temperature. The solvent was removed and the crude product was
purified
dibromoselenophene as a yellow liquid (2.16 g, yield: 95%). 1H
NMR (400 MHz, CDCl3,
ppm): 7.00 (s, 2H); 13C NMR (400 MHz,
CDCl3, ppm): 132.99, 115.61.
2,2′-Biselenophene (2Se). 2-Bromoselenophene (0.80 g,
3.80 mmol), tributyl(2-selenophenyl)stannane (4.79 g,
by column
chromatography
to
obtain
2,5-
d
d
11.40 mmol), tetrakis(triphenylphosphine)palladium(0) (44 mg,
10 mol%) and DMF (20 mL) were added in the flask and the mixture
was refluxed for 8 h under argon atmosphere. Then the mixture
was poured into distilled water (80 mL) and extracted with diethyl
ether. The organic phase was washed with saturated NaHCO3 so-
lution and deionized water. 2,20-Biselenophene (colorless plates,
0.71 g, yield: 71%) was purified by column chromatography. 1H
2.2. Characterization
By using a Bruker AV 400 NMR spectrometer, 1H and 13C NMR
spectral measurements were taken with trimethylsilane (TMS) as
the internal standard and CDCl3 or DMSO‑d6 as the solvent. UV-vis
absorption spectra of all samples were studied by a SPECORD 200
PLUS UV-vis spectrophotometer. An F-4500 fluorescence spectro-
photometer was employed to determine the fluorescence spectra
with the excitation and emission slit set at 5 nm. All quantum
chemistry calculations were carried out by using the Gaussian 09
program package. Without symmetry constraints, the structures of
the selenophene precursors were optimized with a hybrid Becke’s-
three-parameter Lee-Yang-Parr functional (B3LYP) in combination
NMR (400 MHz, DMSO‑d6, d ppm): 8.08 (d, 2H), 7.34 (s, 2H), 7.25 (s,
2H); 13C NMR (400 MHz, DMSO‑d6,
d ppm): 144.17, 133.09, 130.41,
126.75.
2,2’:5′,200-Terselenophene (3Se). 2,5-Dibromoselenophene
(0.30 g, 1.00 mmol), tributyl(2-selenophenyl)stannane (2.10 g,
5.00 mmol), tetrakis(triphenylphosphine)palladium(0) (116 mg,
10 mol%) and DMF (20 mL) were added in the flask and the mixture
was refluxed for 8 h under argon atmosphere. Similarly, the
mixture was poured into deionized water (80 mL) after the reac-
tion, extracted with diethyl ether, and the organic phase was
washed with saturated NaHCO3 solution and deionized water. By

B. Lu et al. / Electrochimica Acta 340 (2020) 135974
3
Fig. 1. Synthetic route and photophysical properties. (A) Synthetic procedure of 2,20-biselenophene (2Se) and 2,2’:50,200-terselenophene (3Se). (B-C) UV-vis absorption spectra (B)
and fluorescence emission spectra (C) of Se, 2Se, and 3Se dissolved in CH2Cl2.
purifying with column chromatography, 2,2’:50,200-terselenophene
was obtained as a yellow leaflet (0.40 g, 78%). 1H NMR (400 MHz,
tributyl(selenophen-2-yl)stannane was obtained and directly used
for the next step without further purification [9,17,18]. Stille
DMSO‑d6,
d
ppm): 8.12 (d, 2H), 7.37 (d, 2H), 7.28 (s, 2H), 7.27 (s, 2H);
coupling
of
tributyl(selenophen-2-yl)stannane
with
2-
13C NMR (400 MHz, DMSO‑d6,
d
ppm): 143.52, 142.64, 131.58,
bromoselenophene and 2,5-dibromoselenophene gave us the
target compounds 2Se and 3Se in satisfactory yields (typically more
than 70%), respectively. Notably, the molecular structures of all the
intermediates and final products were confirmed by 1H NMR and
13C NMR spectra (Figs. S1eS7 in Supporting Information).
130.51, 127.62, 126.98.
2.4. Flexible electrochromic device fabrication
Flexible electrochromic devices (ECDs) were assembled into the
following configurations: ITO-PET/patterned electrochromic poly-
selenophene layer/gel electrolyte/ITO-PET. For the patterning, pol-
yselenophene electrochromic layer was electrosynthesized on ITO-
PET (working electrode) by using a stencil to obtain the patterned
ITO area and was then rinsed with CH2Cl2 to remove residual
monomeric precursors, oligomers and electrolyte. A Pt sheet
(2 ꢃ 2 cm2) was used as the counter electrode and an Ag/AgCl
electrode was employed as the reference electrode for the elec-
trosynthesis, respectively. For the ECD fabrication, the gel electro-
lyte with a composition of 3.5 wt%: 85.5 wt%: 5.5 wt%: 5.5 wt%
(ACN: PC: LiClO4: PMMA), which was prepared by mixing each
component and stirring for 12 h at 65 ꢂC, was coated on the
patterned electrochromic layer. After evaporation of ACN in the gel
electrolyte, another ITOePET sheet was covered as the top layer and
sealed with room temperature vulcanized (RTV) silicone rubber
(Ausbond). The thickness of the gel electrolyte layer was controlled
by a Scotch magic tape (3M). Copper strips (double conductive
copper foil tape, thickness: 0.1 mm) were used for wiring as the
electrodes of the ECDs.
3.2. Optical properties
To evaluate the optical properties of the selenophene pre-
cursors, UV-vis absorption spectra and fluorescence emission
spectra of Se, 2Se and 3Se were examined in CH2Cl2 (Fig. 1B and C),
and the relevant optical parameters are summarized in Table 1. The
absorption maxima for 2Se
lmax ¼ 381 nm) are bathochromically shifted compared to the
absorption maximum of Se (lmax ¼ 252 nm) (Fig. 1B), suggesting
higher extended -conjugation in 2Se and 3Se due to a certain
charge transfer character of transition of the
(
lmax
¼
326 nm) and 3Se
(
p
pep*
selenopheneeselenophene motif [19e22]. Stepwise red shifts are
also observed in the emission spectra of Se, 2Se and 3Se (Fig. 1C),
with the emission peaks at 291 nm, 444 nm and 465 nm (Table 1),
respectively. This phenomenon could also be attributed to stepwise
increase of the conjugated precursor chain length, which is in good
agreement with the results of UV-vis absorption spectra.
3.3. Electrochemical polymerization
3. Results and discussion
The electropolymerization behaviors of Se, 2Se, and 3Se have
been examined via cyclic voltammetry in CH2Cl2-Bu4NPF6
(0.1 mol Lꢀ1). From the anodic oxidation curves (Fig. 2A), the
oxidation onset potentials (Eonset) of 2Se (1.16 V vs Ag/AgCl) and 3Se
(0.93 V vs Ag/AgCl) were dramatically reduced than that of Se
(1.57 V vs Ag/AgCl) (Table 1). Stepwise increase in chain length of
3.1. Molecular design and synthesis
Oligoselenophenes like 2Se and 3Se show elongated conjugated
chain lengths and would possibly improve the stability and elec-
trochromic properties of polyselenophene. To synthesize oligose-
lenophenes, selenophene (Se) was used as the starting molecule
(Fig. 1A) and Pd(PPh3)4 was employed as the catalyst for Stille
coupling. Through the stannylization of selenophene at 2-position,
selenophene leads to futher delocalization of
p electrons along the
molecules, making the oxidation of 2Se and 3Se into corresponding
radical cations much easier. Notably, the significantly lowered Eonset
for the electrochemical polymerization of selenophenes are

4
B. Lu et al. / Electrochimica Acta 340 (2020) 135974
Table 1
Optical, electrochemical parameters, and DFT theoretical calculations of Se, 2Se, and 3Se.
Precursor
lmax,1 (nm)
lmax,2 (nm)
lem (nm)
EHOMO, DFT (eV)
ELUMO, DFT (eV)
Eg, DFT (eV)
Dihedral angles (ꢂ)
Eonset (V)
Se
2Se
3Se
252
272
275
e
291
444
465
ꢀ6.32
ꢀ5.45
ꢀ5.11
ꢀ0.35
ꢀ1.39
ꢀ1.83
5.96
4.06
3.28
e
1.57
1.16
0.93
326
381
0.13390
0.01957
Fig. 2. Electrochemical polymerization behavior. (A) Anodic oxidation curves of 0.01 mol Lꢀ1 Se, 2Se, and 3Se in CH2Cl2-Bu4NPF6 (0.10 mol Lꢀ1). (B-D) Cyclic voltammograms of
the electropolymerization of 0.01 mol Lꢀ1 Se (B), 2Se (C), and 3Se (D) on Pt working electrodes in CH2Cl2-Bu4NPF6. Potential scan rate: 50 mV sꢀ1
.
expected to effectively avoid the side reactions and overoxidation
[23e26], which is quite beneficial to the formation of high quality
polyselenophene films with much less structural defects.
potentials. After considering all the factors influencing the quality
of the deposited polyselenophene film, such as negligible over-
oxidation, less structural defects, suitable polymerization rate,
smooth and compact morphology, the selected applied potentials
for Se, 2Se, and 3Se are 1.65 V, 1.35 V, and 1.1 V vs. Ag/AgCl in
CH2Cl2-Bu4NPF6 (0.1 mol Lꢀ1).
Cyclic voltammetry (Fig. 2BeD) shows that the electro-
polymerization of 2Se and 3Se exhibits broad redox waves, which
are similar with those of polythiophenes, polypyrrole and other
classical conducting polymers previously reported[27]. Yet the CV
curves of Se are not satisfactory with poorly defined redox waves
and irregular increment (Fig. 2B), suggesting difficulty in the elec-
tropolymerization for Se. During the scanning process of 2Se and
3Se, dark polyselenophene films can be facilely obtained by visual
inspection and current densities in the cyclic votammogram curves
increase uniformly (Fig. 2C and D), indicating that the successful
formation and gradual accumulation of polymers on the working
electrode surface [28]. Meanwhile, the anodic and cathodic peak
potentials shift to higher potentials owing to the increased elec-
trical resistance required to balance the difference[29]. Overall, 2Se
and 3Se are much easier to electropolymerize on the working
electrode surface than Se due to the lowered onset oxidation po-
tentials from the extended conjugation chain length.
3.4. Structural characterization and surface morphology
3.4.1. DFT theoretical calculations
In order to interpret the polymer structure and polymerization
mechanism, the optimal molecular geometries and the frontier
molecular orbital distributions (HOMO and LUMO) of Se, 2Se and
3Se were performed by quantum chemical calculations (Fig. 3A).
Interestingly, both 2Se and 3Se show very planar structures with
optimized dihedral angles close to 0ꢂ, indicating the highest
possible
p-electron coupling. This phenomenon will reduce the
HOMO-LUMO gap of the corresponding polymers, and probably
allow the achievement of polyselenophene with enhanced main
chain planarity and higher performance. Notably, this finding also
demonstrates that the selenophene moiety can be employed as an
For the polymer structure, morphology, and property charac-
terization hereafter, potentiostatic electrolysis is employed to
electrodeposit all the polyselenophene films. To optimize the
applied potentials for electropolymerization, we recorded a set of
current transients (chronoamperometric curves, see Fig. S8) during
the electropolymerization of Se, 2Se, and 3Se at different applied
effective building block for the rational design of planar
p-conju-
gated polymers. From the frontier molecular orbital theory, the
electropolymerization mainly proceeds on the highest frontier
molecular orbitals [30,31]. For all these selenophene precursors, the
HOMO spreads dominantly over the a-positions of the selenophene

B. Lu et al. / Electrochimica Acta 340 (2020) 135974
5
Fig. 3. DFT theoretical calculations and structural characterization. (A) Optimized molecular geometries and frontier molecular orbital distributions (HOMO and LUMO) for the
selenophene precursors by DFT. (B-C) FT-IR spectra of the selenophene precursors (B) and their corresponding polyselenophenes (C).
ring, manifesting that the electropolymerization of Se, 2Se and 3Se
occurs preferentially at -positions. Furthermore, significant
3.4.3. SEM
Scanning electron microscopy (SEM) of the electrodeposited
a
decrease in the band gap (Eg, DFT) can be observed along with the
extension of initial conjugated chain lengths, namely, Se (5.96 eV),
2Se (4.06 eV), and 3Se (3.28 eV), in compliance with our previous
calculations for conjugated oligomers [47].
polyselenophene has been employed for the morphological study
(Fig. S8). At the magnification of ꢃ5000, PSe film obtained by direct
electropolymerization is thin and discontinuous, whereas P2Se and
P3Se films are uniformly continuous and compact. Interestingly,
typical coated polyselenophene nanowires could be found in P2Se
film, which is rarely observed for electrodeposited conducting
polymer films. P3Se film displays a compact surface with coral
reefs, a common morphology occurrence for electrosynthesized
conducting polymers[25,27,34,35]. Both P2Se and P3Se films reveal
the large specific surface area, which is beneficial for the immi-
gration/emigration of counterions during the doping-dedoping
process. Furthermore, doped and dedoped polymer films display
no obvious difference in morphology, demonstrating that the
morphology of these polyselenophene films is not damaged upon
doping-dedoping[19,22]. A few cracks in the SEM images for
dedoped P2Se and P3Se films probably result from the fast evap-
oration of low boiling solvents like dichloromethane.
To further investigate its deposition mechanism of the poly-
selenophene nanowires, we studied the morphology evolution of
P2Se films with varying deposition times (Fig. S9). For the nucle-
ation at the initial stage of electropolymerization, P2Se deposits
into a smooth morphology with random bulges on the ITO glass
electrode (the deposition time of 2 s, Fig. S9A). With increasing
polymerization time (10 s, Figs. S9BeE), the deposition rate on the
bulges is faster than other parts, affording the gradual growth of
polyselenophene into nanowires (Fig. S9E). Finally, the polymer
deposits into a unique mixed morphology with nanowires and
plates (Fig. S9F). We attribute this morphology to two-staged
deposition: polyselenophene deposits mainly on the bulges to
form nanowires after nucleation, and starts to deposit on the
nanowires when the nanowires grow too long and fall against the
surface. This unique deposition mode may be harnessed as a
3.4.2. FT-IR
In order to further clarify the electropolymerization mechanism
and interpret the chemical structures of polyselenophene, we
performed fourier transform infrared spectroscopy (FT-IR) of the
selenophenone precursors and corresponding neutral poly-
selenophene, as shown in Fig. 3B. Compared to selenophene pre-
cursors, the vibrational absorption peaks of all the polymers are
significantly broadened, probably due to the heterogeneous chain
lengths and complicated configuration [32,33]. The main absorp-
tion peaks of PSe, P2Se and P3Se are almost consistent with varying
peak shapes and intensities, indicating identical molecular struc-
tures of all these polyselenophene films. In the spectra of the
selenophene precursors, the peaks at around 3200e3000 cmꢀ1 can
be attributed to the ¼ CeH stretching vibration in the selenophene
ring, while these peaks are significantly weakened in the spectra of
polyselenophene films. Moreover, in clear contrast with the pre-
cursors, all three polyselenophenes display a new absorption peak
at 770e760 cmꢀ1, which is a characteristic out-of-plane bending
vibration of adjacent ¼ CeH bonding. These results confirm the
electropolymerization of selenophene precursors at
a-positions.
Notably, the peak intensities of P2Se and P3Se in the range of
1600e1300 cmꢀ1 (stretching vibration of C]C and CeSe bonds) are
obviously enhanced compared to those of PSe. This difference
clearly evidences better conjugated structures of P2Se and P3Se
with less structural defects than PSe, resulting from the elongated
precursor chain length and much lower polymerization potentials
as stated previously.

6
B. Lu et al. / Electrochimica Acta 340 (2020) 135974
template-free method to fabricate polyselenophene nanowires
with fine control of polymerization time.
to PSe, the redox current density values of both P2Se and P3Se are
almost identical, and the linearity fitting of the redox current
density versus potential scan rate for P2Se and P3Se are even better
with the R2 values closer to 1, indicating better electrochemical
activity induced by increasing the precursor chain lengths.
From the cyclic voltammogram curves, we can also observe an
obvious hysteresis in the redox peak potentials of polyselenophene,
that is, the oxidation and reduction peak potentials drift at varying
potential scan rates (Fig. 4AeC). Accompanying with the stepwise
decrease in potential scan rates, the oxidation potentials of poly-
selenophene drop down gradually yet the reduction potentials rise
step by step with the whole redox potential window of poly-
selenophene decreases correspondingly. This phenomenon is due
to the competition between the scan rates and the migration rate of
counter ions. Note that the peak potential drift of electrodeposited
conducting polymers is difficult to interpret by using conventional
kinetic diffusion theories (ion diffusion theory or interface charge
transfer process). The main reasons of this phenomenon are
generally as follows: different forms of electron transfer, local
rearrangement of conducting polymer chains, interconversion
among various conducting states, and mutual electron transfer
between interfaces such as metal/polymer and polymer/solution
interfaces.
3.5. Electrochemistry of polyselenophene
Cyclic voltammetry of conducting polymer-modified electrodes
is a simple yet effective method for detecting the electrochemical
activity and stability of conducting polymer films[36e38]. The
electrochemical properties of polyselenophene films are examined
in blank CH2Cl2-Bu4NPF6 (0.10 mol Lꢀ1), as illustrated in Fig. 4.
Similar to those results for typical conducting polymers, these
polyselenophene films exhibit broad anodic-cathodic waves
(Fig. 4AeC) with basically same redox potential windows in the
range of 0.2e1.4 V, which are similar with its analogue poly-
thiophene[39e43]. Such broad redox waves of these poly-
selenophene films could be attributed to the following reasons: (1)
the slow movement of counter ions in the polymer film; (2) the
capacitance changes of the polymer film during the potential
scanning; (3) wide chain length distribution of such poly-
selenophene; (4) inevitable structural defects such as a-b and b-b
coupling among selenophene moieties during the polymerization
process. Additionally, the redox peak current density shows a good
linearity correlation with the potential scan rate (Fig. 4DeF), indi-
cating that these polyselenophene films are well attached to the
working electrode and the doping-dedoping processes are non-
diffusionally controlled and reversible [19,20,22,25,26]. Compared
Long-term electrochemical stability of these polyselenophene
films is also recorded at the constant scan rate of 150 mV sꢀ1 in
blank CH2Cl2-Bu4NPF6 electrolyte. As shown in Fig. 4GeI, both P2Se
Fig. 4. Electrochemistry of polyselenophenes. (A-C) Cyclic voltammograms of PSe (A), P2Se (B), and P3Se (C) modified Pt electrodes in blank CH2Cl2-Bu4NPF6 (0.10 mol Lꢀ1) at
potential scan rates of 300 (a), 275 (b), 250 (c), 225 (d), 200 (e), 175 (f), 150 (g), 125 (h), 100 (i), 75 (j), 50 (k), and 25 (l) mV sꢀ1. (D-F) Plots of peak current densities vs. scan rates for
PSe (D), P2Se (E), and P3Se (F). jp denotes peak current density, and jp,a and jp,c are the anodic and cathodic peak current density, respectively. (G-I) Long-term cyclic voltammograms
of PSe (G), P2Se (H), and P3Se (I) in blank CH2Cl2-Bu4NPF6 (0.10 mol Lꢀ1) at the constant potential scan rate of 250 mV sꢀ1
.

B. Lu et al. / Electrochimica Acta 340 (2020) 135974
7
and P3Se display superior redox stability than PSe, and P3Se reveals
the best stability with retaining around 61% of its electroactivity
after 3000 cycling. Such performance probably results from their
better conjugated structures with less structural defects as
mentioned previously.
and P3Se with less structural defects as confirmed by FT-IR spectral
results and also the red-shifting in the maximum absorption peak
in UV-vis spectra.
In the neutral state, PSe, P2Se and P3Se all display brick-red or
red in color with typical coordinates of L* ¼ 62.72, a* ¼ 25.42,
b* ¼ 16.91 (Inset of Fig. 5AeC & Table 2). With stepwise increase of
the applied potentials, the single absorption band of poly-
selenophene in the visible region depletes gradually and a new
absorption arises in the near-infrared region (>800 nm) because of
the formation charge species upon doping. These evolution in the
absorption spectra also leads to obvious color changes towards
blue/baby blue (L* ¼ 87.07, a* ¼ ꢀ2.22, b* ¼ 0.01) (Fig. 5AeC &
Table 2). This electrochromism phenomenon of polyselenophene
are also observed by previous reports [8], but unfortunately the
electrochromic kinetics of parent polyselenophene has not been
carefully investigated due to the poor performance of the obtained
polyselenophene films.
3.6. Spectroelectrochemistry
The spectroelectrochemical method can realize the in-situ UV
scanning of conducting polymer films at varying potentials and
record the accompanying changes in chemical structure and
physiochemical properties during the doping/dedoping process
[19,20,22]. The electronic structure information can also monitor
the changes in optical properties of conducting polymers upon
doping/dedoping. Therefore, the spectroelectrochemistry of PSe,
P2Se and P3Se films from the fully reduced state to the oxidized
state is recorded in blank CH2Cl2-Bu4NPF6 (0.10 mol Lꢀ1
)
(Fig. 5AeC).
Compared to the selenophene precursors (Fig. 1B and C), poly-
selenophene films coated on ITO glass display a dominant single-
band absorption red-shifted to longer wavelengths (Fig. 5AeC) in
the neutral state, owing to the increased conjugated polymer chain
length. With increasing the precursor chain length from Se to 3Se,
the maximum absorption peak of polyselenophene shifts bath-
ochromically from 450 nm for PSe to 515 nm for P3Se, and the
optical band gap (Eg) of the corresponding polyselenophene is
gradually reduced from 1.92 eV (PSe) to 1.86 eV (P2Se) and
1.72 eV (P3Se), respectively. Notably, the band gap values of P2Se
and P3Se are also lower than polyselenophenes prepared either by
electropolymerizatin or by chemical oxidative polymerization
[8,15], mainly resulting from better conjugated structures of P2Se
3.7. Kinetic studies
To further evaluate the electrochromic performances of as-
formed polyselenophene films, the kinetic studies of PSe, P2Se,
and P3Se have been carried out by using a double-step chro-
noamperometry method. The transmittance-time profiles of these
polyselenophene films are illustrated in Fig. 5DeG, and the corre-
sponding electrochromic parameters including optical contrast,
coloration efficiency and response time, are summarized in Table 3.
Due to the poor film quality, the electrochromic performance
and stability of PSe is not good with the optical contrast of 28% at
1100 nm (Fig. 5D). With the improvement in the film quality, the
optical contrast of P2Se and P3Se are generally improved by 2e3
Fig. 5. Electrochromic performances of polyselenophenes. (A-C) Spectroelectrochemistry for PSe (A), P2Se (B) and P3Se (C) films on the ITO coated glass in blank CH2Cl2-Bu4NPF6
(0.10 mol Lꢀ1) solution. Applied potential ranges: 0.2e1.0 V for PSe,
D
E ¼ 0.1 V; 0.3e1.2 V for P2Se,
D
E ¼ 0.05 V; 0.05e1.2 V for P3Se, E ¼ 0.05 V. (D-F) Time-transmittance profiles
D
of PSe (D), P2Se (E) and P3Se (F) films recorded by double step chronoamperometry under the switching time of 10 s at different wavelengths. Applied potentials: PSe, ꢀ1.0 and
1.0 V; P2Se, ꢀ1.0 and 1.2 V; P3Se, ꢀ1 and 1.4 V. (G) Optical contrasts of P3Se (1800 nm, ꢀ1.0 and 1.0 V) recorded by double step under varying switching time intervals as indicated.

8
B. Lu et al. / Electrochimica Acta 340 (2020) 135974
Table 2
Spectroelectrochemical parameters and color coordinates of PSe, P2Se, and P3Se.
Polymer
lmax (nm)
lonset (nm)
Eg,opt (ev)
CIE color coordinates
Neutral state
Oxidized state
L
L
a
b
a
b
PSe
P2Se
P3Se
450
505
515
646
667
721
1.92
1.86
1.72
65.15
62.72
78.75
12.12
25.42
7.88
22.73
16.91
ꢀ1.73
67.64
69.37
87.07
ꢀ1.10
ꢀ8.96
ꢀ2.22
15.06
1.14
0.01
Table 3
Electrochromic parameters of different polyselenophene films at varying wavelengths.
Polymer
Wavelength (nm)
Tred
Tox
DT
Response time (s)
Coloration efficiency (cm2/C)
oOx
Red
PSe
495
800
18%
33%
70%
55%
74%
64%
25%
70%
75%
92%
97%
76%
29%
53%
88%
95%
96%
89%
30%
20%
42%
31%
52%
42%
52%
35%
22%
37%
39%
36%
65%
3%
12%
13%
28%
24%
22%
22%
27%
35%
53%
55%
58%
40%
36%
50%
60%
75%
64%
52%
7.2
2.9
5.6
3.8
8.8
6.4
2.7
3.8
4.6
1.4
1.2
0.8
6.6
3.5
2.7
1.0
0.7
0.7
1.7
8.4
7.4
5.6
8.7
7.3
0.5
2.9
2.6
1.3
1.0
0.7
3.9
4.6
5.2
0.9
0.5
0.4
41
15
40
57
89
115
96
1100
1200
1400
1800
505
P2Se
P3Se
740
90
1100
1200
1400
1600
510
122
230
241
227
122
169
224
450
370
245
850
1100
1200
1500
1700
28%
20%
32%
37%
times with significantly enhanced switching stability (Fig. 5E and F
& Table 3). Notably, P3Se film displays superior optical contrast over
50% in the whole near-infrared region (800e1800 nm) with the
highest value up to 75% at 1200 nm, comparable to excellent
organic electrochromic polymers reported so far [15]. Furthermore,
P2Se and P3Se also exhibit faster switching times to achieve 95%
optical contrast between the oxidation and reduction processes
with higher coloration efficiency (Table 3). Again, P3Se film reveals
an impressive switching time of 0.7 s for oxidation and 0.4 s for
reduction with the highest coloration efficiency of 450 cm2 Cꢀ1 at
1200 nm, better than most of conjugated polymers in the poly-
thiophene family like polythiophene and PEDOT[26]. By moni-
toring the percent transmittance change as a function of interval
time (from 15 s down to 0.5 s), P2Se and P3Se can preserve high
optical contrast over 44% while maintaining good stability even at a
very short switching time of 0.5 s, much better than PSe (Figs. 5G &
S10). Moreover, no obvious transmittance changes in the neutral
state can be observed from the open-circuit memory of P2Se and
P3Se films monitored at 1100 nm, while mild fluctuations of less
than 5% variation occur in their oxidized state (Fig. S11).
3.8. Flexible electrochromic devices
Flexible electrochromic devices have been extensively
employed in many applications, such as color-changing tattoos and
e-papers[45,46]. Enabled by the high performance of P2Se and P3Se
and the ease of patterning via electropolymerization, we can suc-
cessfully fabricate flexible electrochromic devices with the config-
uration of ITO-PET/patterned polyselenophene layer/gel
electrolyte/ITO-PET. As shown in Fig. 6, patterned flexible electro-
chromic devices of our university “JXSTNU” with P2Se or P3Se as
the functional layer display reversible color-changing phenomenon
upon doping/dedoping at low driving potentials of ꢀ1.0 and 1.0 V.
These devices are foldable, and show good stability against both
redox cycling and mechanical bending/relaxing, which are
appealing for wearable electronic technologies, and thus can serve
to revolutionize electrochromic applications towards deformable
electronics.
4. Conclusion
Overall, the electrochromic performance of polyselenophene
can be significantly enhanced by extending the monomeric pre-
cursor from Se to 3Se to yield high-quality polyselenophene film
with better conjugated structure and less defects via low-potential
electropolymerization. Additionally, P3Se film exhibit excellent
electrochromic performance with superior optical contrast, fast
switching time, high coloration efficiency, as well as good stability,
which are comparable to advanced electrochromic materials
including inorganic compounds, small organic molecules, and
other high-performance conjugated polymers [3,4], making it a
good material candidate for various electrochromic devices.
In this work, the effect of precursor chain length on the elec-
tropolymerization behavior and the optoelectronic performances
of the resultant polyselenophene films has been systematically
investigated by using various characterization methods including
electrochemical, spectral, morphological, together with quantum
calculations. Stepwise increase in chain length of the precursors
from Se to 3Se leads to much lowered onset oxidation potentials,
affording low-potential electropolymerization to achieve high
quality polyselenophene films with better conjugated structures
and less structural defects. The resultant P2Se and P3Se films
exhibit much enhanced optoelectronic properties such as better
redox activity and stability, lowered optical band gaps, higher

B. Lu et al. / Electrochimica Acta 340 (2020) 135974
9
Fig. 6. Polyselenophene-based flexible electrochromic devices. (A, B) Color changing digital images for P2Se (A) or P3Se (B)-based flexible electrochromic devices under relaxing
and bending conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
electrochromic performances. Enabled by these advantageous
properties, patterned flexible electrochromic devices have been
fabricated with electrodeposited polyselenophene films, and these
devices reveal reversible and stable color changes upon both
doping-dedoping and mechanical bending.
Ximei Liu: Supervision. Jingkun Xu: Supervision. Guoqun Zhao:
Supervision.
Declaration of competing interest
Acknowledgements
The authors declare that they have no conflicts interests. All the
authors listed and the responsible authorities where the work was
carried out have approved the manuscript that is enclosed.
This work was supported by the National Natural Science
Foundation of China (Grant No. 51763010 & 51963011), Techno-
logical Expertise and Academic Leaders Training Program of Jiangxi
Province (Grant No. 20194BCJ22013), Double Thousand Talents Plan
in Jiangxi Province - Youth Program, the Science and Technology
Research Program of Jiangxi Educational Committee (Grant
No. GJJ190622), and Research Project of the State Key Laboratory of
Mechanical System and Vibration (Grant No. MSV202013).
CRediT authorship contribution statement
Baoyang Lu: Conceptualization, Methodology, Writing - review
& editing. Nannan Jian: Data curation, Writing - original draft.

10
B. Lu et al. / Electrochimica Acta 340 (2020) 135974
[24] D. Yigit, S.O. Hacioglu, M. Gullu, L. Toppare, Synthesis and spectroelec-
trochemical characterization of multi-colored novel poly(3,6-
Appendix A. Supplementary data
dithienylcarbazole) derivatives containing azobenzene and coumarin chro-
mophore units, Electrochim. Acta 196 (2016) 140e152.
[25] Y.P. Wen, J.K. Xu, Scientific importance of water-processable PEDOT-PSS and
preparation, challenge and new application in sensors of its film electrode: A
review, J. Polym. Sci., Part A: Polym. Chem. 55 (2017) 1121e1150, https://
doi.org/10.1002/pola.27973.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.electacta.2020.135974.
References
[26] S.J. Zhen, J.K. Xu, B.Y. Lu, S.M. Zhang, L. Zhao, J. Li, Tuning the optoelectronic
properties of polyfuran by design of furan-EDOT monomers and free-standing
films with enhanced redox stability and electrochromic performances, Elec-
trochim. Acta 146 (2014) 666e678.
[27] Z.X. Xue, N. Gao, Y. Xue, B.Y. Lu, O. Waston, L. Zang, J.K. Xu, Structural design
and applications of stereoregular fused thiophenes and their oligomers and
[1] D. Khim, A. Luzio, G.E. Bonacchini, G. Pace, M.J. Lee, Y.Y. Noh, M. Caironi,
Uniaxial alignment of conjugated polymer films for high-performance organic
field-effect transistors, Adv. Mater. 30 (2018), 1705463.
[2] B.W. Xu, J.H. Hou, Solution-processable conjugated polymers as anode inter-
facial layer materials for organic solar cells, Adv. Energy Mater. 8 (2018),
1800022.
polymers,
Polym.
Rev.
(2019),
https://doi.org/10.1080/
[3] P.M. Beaujuge, J.R. Reynolds, Color control in pi-conjugated organic polymers
for use in electrochromic devices, Chem. Rev. 110 (2010) 268e320.
[4] K.W. Lin, S. Chen, B.Y. Lu, J.K. Xu, Hybrid p-conjugated polymers from dibenzo
pentacyclic centers: precursor design, electrosynthesis and electrochromics,
Sci. China B: Chemistry 60 (2017) 38e53.
[5] H. Yuk, B.Y. Lu, X.H. Zhao, Hydrogel bioelectronics, Chem. Soc. Rev. 48 (2019)
1642e1667.
15583724.2019.1673404.
[28] W.S. Li, Y.T. Guo, J.J. Shi, H.T. Yu, H. Meng, Solution-processable neutral green
electrochromic polymer containing thieno 3,2-b thiophene derivative as un-
conventional donor units, Macromolecules 49 (2016) 7211e7219.
[29] W. Chen, G. Xue, Low potential electrochemical syntheses of heteroaromatic
conducting polymers in a novel solvent system based on trifluroborate-ethyl
ether, Prog. Polym. Sci. 30 (2005) 783e811.
[6] B.Y. Lu, H. Yuk, S.T. Lin, N.N. Jian, K. Qu, J.K. Xu, X.H. Zhao, Pure PEDOT:PSS
hydrogels, Nat. Commun. 10 (2019) 1043.
[7] F.Q. Hu, Y. Xue, J.K. Xu, B.Y. Lu, PEDOT-based conducting polymer actuators,
Front. Robot. AI 6 (2019) 114.
[30] N.N. Jian, K. Qu, H. Gu, L. Zou, X.M. Liu, F.Q. Hu, J.K. Xu, Y. Yu, B.Y. Lu, Highly
fluorescent triazolopyridine-thiophene D-A-D oligomers for efficient pH
sensing both in solution and in the solid state, Phys. Chem. Chem. Phys. 21
(2019) 7174e7182.
[8] A. Patra, M. Bendikov, Polyselenophenes, J. Mater. Chem. 20 (2010) 422e433.
[9] A. Patra, M. Bendikov, S. Chand, Poly(3,4-ethylenedioxyselenophene) and its
derivatives: novel organic electronic materials, Acc. Chem. Res. 47 (2014)
1465e1474.
[31] N.N. Jian, K.W. Lin, B. Guo, G. Zhang, X.M. Liu, L. Zou, B.Y. Lu, J.K. Xu, A reusable
fluorescent sensor from electrosynthesized water-soluble oligo(1-
pyrenesulfonic acid) for effective detection of Fe3þ, New J. Chem. 1 (2018)
19450e19457.
[10] M.F.G. Klein, F.M. Pasker, S. Kowarik, D. Landerer, M. Pfaff, M. Isen,
D. Gerthsen, U. Lemmer, S. Hoeger, A. Colsmann, Carbazole-phenyl-
benzotriazole copolymers as absorber material in organic solar cells, Macro-
molecules 46 (2013) 3870e3878.
[11] K. Li, Z. Li, K. Feng, X. Xu, L. Wang, Q. Peng, Development of large band-gap
conjugated copolymers for efficient regular single and tandem organic solar
cells, J. Am. Chem. Soc. 135 (2013) 13549e13557.
[32] D. Bhattacharyya, K.K. Gleason, Low band gap conformal polyselenophene
thin films by oxidative chemical vapor deposition, J. Mater. Chem. 22 (2012)
405e410.
[33] A. Ghosh, R. Gonnade, M. Ravikanth, Rhenium(I) tricarbonyl complex of 5,20-
bis(p-tolyl)-10,15-bis(p-methoxyphenyl)-21-selenaporphyrin: first X-ray
structural characterization of metal complex of 21-selenaporphyrin, Dalton
Trans. 42 (2013) 10798e10806.
[12] B.Y. Lu, S. Chen, J.K. Xu, G.Q. Zhao, Thermoelectric performances of different
types of polyselenophene and its copolymers with 3-methylthiophene via
electropolymerization, Synth. Met. 183 (2013) 8e15.
[34] C.X. Chen, Z.Y. Gan, K. Zhou, Z. Ma, Y.Q. Liu, Y.H. Gao, Catalytic polymerization
of N-methylthionine at electrochemically reduced graphene oxide electrodes,
Electrochim. Acta 283 (2018) 1649e1659.
[13] B.Y. Lu, S.L. Ming, K.W. Lin, S.J. Zhen, H.T. Liu, H. Gu, S. Chen, Y.Z. Li, Z.Y. Zhu,
J.K. Xu, [1,2,5]Chalcogenodiazolo[3,4-c]pyridine and selenophene based
donoreacceptoredonor electrochromic polymers electrosynthesized from
high fluorescent precursors, New J. Chem. 40 (2016) 8316e8323, https://
doi.org/10.1039/C5NJ03432A.
[14] S.Z. Pu, J. Hou, J.K. Xu, G.M. Nie, S.S. Zhang, L. Shen, Q. Xiao, Electrosyntheses of
freestanding and conducting polyselenophene films, Mater. Lett. 59 (2005)
1061e1065.
[35] S.X. Xiong, J.P. Lan, S.Y. Yin, Y.Y. Wang, Z.Z. Kong, M. Gong, B.H. Wu, J. Chu,
X.Q. Wang, R.L. Zhang, Y.G. Li, Enhancing the electrochromic properties of
polyaniline via coordinate bond tethering the polyaniline with gold colloids,
Sol. Energy Mater. Sol. Cell. 177 (2018) 134e141.
[36] Y.E. Firat, A. Peksoz, Efficiency enhancement of electrochromic performance in
NiO thin film via Cu doping for energy-saving potential, Electrochim. Acta 295
(2019) 645e654.
[37] Z.J. Fu, Z.Y. Ma, T. Yu, L.H. Bi, A first blue fluorescence composite film based on
graphitic carbon nitride nanosheets/polyoxometalate for application in
[15] J.K. Xu, J. Hou, S.S. Zhang, G.M. Nie, S.Z. Pu, L. Shen, Q. Xiao, Electrosyntheses of
high quality freestanding polyselenophene films in boron trifluoride diethyl
etherate, J. Electroanal. Chem. 578 (2005) 345e355.
reversible electroluminescence switching, J. Mater. Chem.
C 7 (2019)
3253e3262.
[16] B. Dong, Y.H. Xing, J.K. Xu, L.Q. Zheng, J. Hou, F. Zhao, Electrosyntheses of free-
standing and highly conducting polyselenophene films in an ionic liquid,
Electrochim. Acta 53 (2008) 5745e5751.
[38] B.C. Ozkan, T. Soganci, H. Turhan, M. Ak, Investigation of rGO and chitosan
effects on optical and electrical properties of the conductive polymers for
advanced applications, Electrochim. Acta 295 (2019) 1044e1051.
[39] M. Abdulrazzaq, M.I. Ozkut, G. Gokce, S. Ertan, E. Tutuncu, A. Cihaner, A low
band gap polymer based on selenophene and benzobis (thiadiazole), Elec-
trochim. Acta 249 (2017) 189e197.
[40] H.Y. Chen, S.C. Yeh, C.T. Chen, C.T. Chen, Comparison of thiophene- and
selenophene-bridged donor-acceptor low band-gap copolymers used in bulk-
heterojunction organic photovoltaics, J. Mater. Chem. 22 (2012)
21549e21559.
[41] B. Ballarin, M. Lanzi, L. Paganin, G. Cesari, Electrochemical synthesis and
characterization of poly 3-(omega-bromoalkyl)thiophenes, Electrochim. Acta
52 (2007) 4087e4091.
[42] T. Ikeda, Y. Nagata, Y. Zheng, D. Liu, H.J. Butt, M. Shimoda, Electrochemically
durable thiophene alkanethiol self-assembled monolayers, Langmuir 30
(2014) 1536e1543.
[43] C. Moorlag, O. Clot, M.O. Wolf, B.O. Patrick, Switchable thiophene coordination
in Ru(II) bipyridyl phosphinoterthiophene complexes, Chem. Commun. 24
(2002) 3028e3029.
[45] M. Gratzel, Ultrafast colour displays, Nature 409 (2001) 575e576.
[46] X.X. Liu, A. Zhou, Y.B. Dou, T. Pan, M.F. Shao, J.B. Han, M. Wei, Ultrafast
switching of an electrochromic device based on layered double hydroxide/
prussian blue multilayered films, Nanoscale 7 (2015) 17088e17095.
[47] B.Y. Lu, Y.Z. Li, J.K. Xu, Electropolymerization study of benzothiophenes and
characterization of novel poly(dibenzothiophene-S,S-dioxide), J. Electroanal.
Chem. 643 (2010) 67e76, https://doi.org/10.1016/j.jelechem.2010.03.007.
[17] Y. Sun, S.C. Chien, H.L. Yip, Y. Zhang, K.S. Chen, D.F. Zeigler, F.C. Chen, B. Lin,
A.K.Y. Jen, High-mobility low-bandgap conjugated copolymers based on
indacenodithiophene and thiadiazolo 3,4-c pyridine units for thin film tran-
sistor and photovoltaic applications, J. Mater. Chem. 21 (2011) 13247e13255.
[18] S.G. Xu, Y.L. Liu, J.H. Li, Y.B. Wang, S.K. Cao, Synthesis and characterization of
low-band-gap conjugated polymers containing phenothiazine and benzo-
2,1,3-thia-/seleno-diazole, Polym. Adv. Technol. 21 (2010) 663e668.
[19] S.L. Ming, S.J. Zhen, K.W. Lin, L. Zhao, J.K. Xu, B.Y. Lu, Thiadiazolo 3,4-c pyridine
as an acceptor toward fast-switching green donor-acceptor-type electro-
chromic polymer with low bandgap, ACS Appl. Mater. Interfaces 7 (2015)
11089e11098.
[20] S.L. Ming, S.J. Zhen, X.M. Liu, K.W. Lin, H.T. Liu, Y. Zhao, B.Y. Lu, J.K. Xu,
Chalcogenodiazolo 3,4-c pyridine based donor-acceptor-donor polymers for
green and near-infrared electrochromics, Polym. Chem. 6 (2015) 8248e8258.
[21] H. Gu, S.L. Ming, K.W. Lin, S. Chen, X.M. Liu, B.Y. Lu, J.K. Xu, Isoindigo as an
electron-deficient unit for high-performance polymeric electrochromics,
Electrochim. Acta 260 (2018) 772e782.
[22] N.N. Jian, H. Gu, S.M. Zhang, H.T. Liu, K. Qu, S. Chen, X.M. Liu, Y.F. He, G.F. Niu,
S.Y. Tai, J. Wang, B.Y. Lu, J.K. Xu, Y. Yu, Synthesis and electrochromic perfor-
mances of donor-acceptor-type polymers from chalcogenodiazolo 3,4-c pyr-
idine and alkyl ProDOTs, Electrochim. Acta 266 (2018) 263e275.
[23] G.S. Collier, I. Pelse, J.R. Reynolds, Aqueous electrolyte compatible electro-
chromic polymers processed from an environmentally sustainable solvent,
ACS Macro Lett. 7 (2018) 1208e1214.