Electrochromic properties of polymers/copolymers via electrochemical polymerization based on star-shaped thiophene derivatives with different central cores

Article abstract of DOI:10.1149/2.0161706jes

Three star-shaped thiophene derivatives, namely TPAT, PHT and TPTT with different central cores of triphenylamine, phenyl and 2,4,6-triphenyl-1,3,5-triazine respectively, were synthesized and characterized. They were further successfully prepared into the

Full text of DOI:10.1149/2.0161706jes

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Journal of The Electrochemical Society, 164 (4) E84-E89 (2017)
0013-4651/2017/164(4)/E84/6/$37.00 © The Electrochemical Society
Electrochromic Properties of Polymers/Copolymers via
Electrochemical Polymerization Based on Star-Shaped Thiophene
Derivatives with Different Central Cores
Weijun Li,a Lan Chen,a Yuyu Pan,b Shuanma Yan,a Yuyu Dai,a Jin Liu,a Yue Yu,a
Xingxing Qu,a Qingbao Song,a,z Mi Ouyang,a and Cheng Zhanga,z
aState Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, College of Chemical Engineering,
Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China
bSchool of Petrochemical Engineering, Shenyang University of Technology, Liaoyang, 111003, People’s Republic of
China
Three star-shaped thiophene derivatives, namely TPAT, PHT and TPTT with different central cores of triphenylamine, phenyl and
2,4,6-triphenyl-1,3,5-triazine respectively, were synthesized and characterized. They were further successfully prepared into the
corresponding cross-linked polymers pTPAT, pPHT and pTPTT via electrochemical polymerization. The cyclic voltammetry curves
showed that pPHT and pTPTT displayed the relatively higher onset oxidative potentials than pTPAT. After being applied with the
positive voltages in an electrochemical cell, pTPAT exhibited the obvious electrochromic behaviors, while such phenomenons were
not observed in the case of pPHT and pTPTT. The theory calculation results together with the electrochemistry properties revealed
that the electrochromism of pTPAT may mainly depend on its relatively lower oxidative potential because of the introduction of
triphenylamine with low ionization energy. Through the copolymerization with 3,4-ethylenedioxythiophene (EDOT) which is also of
relatively low ionization energy, pPHT-EDOT and pTPTT-EDOT both exhibited excellent electrochromic properties. The oxidative
potential or ionization energy of polymer is thought to be one of the key factors determining the electrochromism.
© 2017 The Electrochemical Society. [DOI: 10.1149/2.0161706jes] All rights reserved.
Manuscript submitted December 19, 2016; revised manuscript received January 30, 2017. Published March 8, 2017.
In the past decades, electrochromic (EC) materials have attracted
corresponding cross-linked polymers pTPAT, pPHT and pTPTT via
electrochemical polymerization. After being applied with the posi-
tive voltages in an electrochemical cell, pTPAT exhibited the obvious
electrochromic behaviors, while no electrochromism was observed in
the cases of pPHT and pTPTT. Theory calculation and electrochem-
istry results revealed that the electrochromism of pTPAT should been
mainly ascribed to the introduction of triphenylamine with low ion-
ization energy. Obtained by electrochemical copolymerization20 with
3,4-ethylenedioxythiophene (EDOT) being of low ionization energy,
both pPHT-EDOT and pTPTT-EDOT copolymers exhibited excellent
electrochromic properties. These results demonstrate that the oxida-
tive potential or ionization energy of polymer should be one of the
key factors determining the electrochromism.21 The experimental data
and results will be showed as followed in detail.
more and more attentions due to their potential applications in smart
windows, displays and switchable mirrors.1–3 Among them, organic
conjugated polymers (CPs) are viewed as one kind of the most promis-
ing materials because of their abundant structure modification, facile
processability, fast switching time, outstanding coloration efficiency
and color adjustment4–7 and so on. However, for commercial appli-
cation equipment, novel CPs with superior electrochromic properties
are still rare in current day. The essential factor should be attributed
to the lack of clearly understanding of the relationship between the
molecular structure and electrochromic properties.
Among electrochromic CPs, polythiophenes8–10 have been inves-
tigated widely by researchers owing to their rich structural modifica-
tion induced color richness in neutral or oxidative state as well as the
ease of synthesis. The cross-linked polymer network configuration,
which is beneficial to the robust stability of the obtained polymer
films,11 are attractive to more and more researchers in the design
of electrochromic materials.12 Recently, dithienothiophene derivative
possessing four thiophene unites at the peripherals was reported to
generate cross-linked polymers with excellent electrochromic prop-
erties by Arne Thomas’ group.13,14 Besides, the cross-linked network
may possess some microporous structures in polymer films and thus
benefit the electrolyte ions’ inserting/deserting behavior during the
electrochromic process, which is considered to contribute to the fast
switching time.15–17
Results and Discussion
Synthesis.—The terminal compounds TPAT, PHT and TPTT were
synthesized according to the synthesis route in Figure 1, and charac-
terized by NMR and Mass spectra. All the reagents or chemicals were
In this paper, we introduced the triphenylamine, phenyl, and
2,4,6-triphenyl-1,3,5-triazine groups with different electron donating
or withdrawing abilities into the central core and three thiophenes
for polymerization into their peripheral part to form the star-shaped
monomers TPAT, PHT and TPTT respectively (Figure 1). These star-
shaped monomers can generate the cross-linked polymer structure
via electrochemical polymerization18,19 (Figure 2) and maybe possess
some microporous structures in the polymer films, which are ex-
pected to help us achieve the excellent electrochromic materials with
fast color switching and good film stability. Different central cores
with electron donating or withdrawing abilities in the star-shaped
monomers may bring us better understanding of the relationship be-
tween the structure and electrochromic properties for EC materials.
Three star-shaped thiophene monomers TPAT, PHT and TPTT
were synthesized successfully, and then further prepared into the
Figure 1. The molecular structures and synthesis routes of three star-shaped
monomers TPAT, PHT and TPTT.
zE-mail: czhang@zjut.edu.cn; qbsong@zjut.edu.cn
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Journal of The Electrochemical Society, 164 (4) E84-E89 (2017)
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Figure 2. The scheme of electropolymeriza-
tion reaction and polymeric structure for pT-
PAT, pPHT and pTPTT with triphenylamine,
phenyl and 2,4,6-triphenyl-1,3,5-triazine as
central cores respectively.
added dropwise at 0◦C and stirred for 0.5 h, after that removed the
flask to room temperature and stirred overnight. Added 70 mL deion-
ized water to the flask and stirred for half an hour, then NaOH solution
was dropwise added to neutralize the mixture until pH = 7. Then ex-
tracted the mixture with CH2Cl2 and brine, dried the organic layer with
MgSO4 and purified on a silica gel column (petroleum ether-CH2Cl2
5:3 as eluent) to obtain the final product as a white powder (0.17 g,
15%). MALDI-TOF-MS (M) (m/z):555.7 [M + H]+. 1H NMR (500
MHz, CDCl3) δ 8.81 (d, J = 8.4 Hz, 2H), 7.91-7.81 (m, 2H), 7.52
(dd, J = 3.6, 1.0 Hz, 1H), 7.41 (dd, J = 5.0, 0.8 Hz, 1H), 7.18 (dd,
J = 5.0, 3.6 Hz, 1H).
commercial products without further purification. 1H (500 MHz)
NMR spectra of the synthesized compounds were recorded on
Bruker AVANCE III instrument (Bruker, Switzerland). Mass spectra
(MALDI-TOF-MS) analysis was recorded using an AXIMA-CFRTM
plus instrument.
Tris[4-(thiophen-2-yl)phenyl]amine
(TPAT).—Thiophene-
2-boronic acid (1.02 g, 8.0 mmol) was mixed with Tris(4-
bromophenyl)amine (0.63 g, 1.3 mmol) and K2CO3 (1.21 g, 8.8
mmol) in ethoxyethanol/deionized water (9:1, 15 mL) in a 100 mL
two necked round bottom flask. Pd(PPh3)4 (94.0 mg, 0.08 mmol)
was added to the stirred suspension, which was then heated rapidly
to 130◦C and maintained under reflux conditions for 4 hours under a
nitrogen atmosphere.22 After cooling to room temperature, deionized
water (50 mL) was added to precipitate the main part of the product
and then the mixture was wished by water and extracted with
dichloromethane consecutively. The product was then dried with
MgSO4 and purified on a silica gel column (petroleum ether-CH2Cl2
5:1 as eluent) to obtain the final product as a light yellow powder
(0.54 g, yield 84%). MALDI-TOF-MS (M) (m/z):493.1 [M + H]+.
1H NMR (500 MHz, CDCl3) δ 7.53 (d, J = 8.6 Hz, 6H), 7.27-7.25
(m, 6H), 7.17-7.14 (m, 6H), 7.10-7.08 (m, 3H).
Electrochemical polymerization.—The polymers of pTPAT,
pPHT and pTPTT were fabricated to show as films by electrochemical
polymerization from the monomers TPAT, PHT and TPTT respec-
tively. Due to the three thiophenes in the peripheral part of monomers,
the obtained polymers would obviously show as the cross-linked net-
work structure as shown in Figure 2. And owing to the relatively rigid
structural configuration, these cross-linked network polymers may
possess the possible microporous structure in them. The cross-linked
network and possible microporous structure can make these polymers
exhibit good electrochromic properties.
The electrochemical polymerization was carried out on ITO-coated
glasses (9 mm × 20 mm) by the cyclic voltammetry polymeriza-
tion method via applying a continuous scanning voltage at a po-
tential scan rate of 400 mV/s in a conventional three-electrode cell
with 0.1 M Tetrabutyl-ammonium hexafluorophosphate (TPAPF6) in
Acetonitrile(ACN)/CH2Cl2 (1:1, by volume) as the electrolyte solu-
tion. Electrochemical polymerization of the monomers, and electro-
chemical properties of the corresponding films were performed on
CHI660E electrochemical analyzer (Chenhua, China). Different from
each other for TPAT, PHT and TPTT during the cyclic voltammetry
polymerization process were the applied voltage ranges of −0.3 V
∼1.5 V, −0.3 V∼1.6 V and −0.3 V∼1.5 V on them respectively,
which were tightly related to the corresponding potential of their
first coupled oxidative-reduction peaks. Figure 3 showed the cyclic
voltammetry curves during the electrochemical polymerization pro-
cess of TPAT, PHT and TPTT. The current intensity in their cyclic
voltammetry curves rose up gradually as the scanned cycle number
increased, which clearly demonstrated that these monomers under-
went the effective polymerization reaction with their corresponding
polymers formed and successfully deposited on ITO.23
1,3,5-Tri(thiophen-2-yl)benzene (PHT).—The synthesis process
of PHT was the same as TPAT only used 1,3,5-Tribromobenzene
(0.41 g, 1.3 mmol) to replace Tris(4-bromophenyl)amine and PHT
was obtained as a white powder (0.38 g, yield 91%). MALDI-TOF-
MS (M) (m/z):325.7 [M + H]+. 1H NMR (500 MHz, CDCl3) δ 7.76
(s, 1H), 7.43 (dd, J = 3.6, 1.2 Hz, 1H), 7.36 (dd, J = 5.1, 1.1 Hz, 1H),
7.15 (dd, J = 5.0, 3.6 Hz, 1H).
4-(Thiophen-2-yl)benzonitrile (M1).—4-Bromobenzonitrile (1.46
g, 8.0 mmol) was mixed with (PPh3)4PdCl2 (0.11 g, 0.16 mmol) in
30mL dried tetrahydrofuran(THF) in a 100 mL two necked round
bottom flask, then 2-(Tributylstannyl)thiophene (3.58 g, 9.6 mmol)
was added, the stirred mixture was then heated to 65◦C and maintained
under reflux conditions for 16 hours under a nitrogen atmosphere.22
After cooling to room temperature, the mixture was extracted with
CH2Cl2 then the organic layer was washed with brine and dried with
MgSO4, and purified on a silica gel column (petroleum ether-CH2Cl2
1:1 as eluent) to obtain the product as a white powder (1.39 g, yield
1
94%). H NMR (500 MHz, CDCl3) δ 7.75-7.70 (m, 2H), 7.69-7.65
(m, 2H), 7.44 (dd, J = 3.7, 0.9 Hz, 1H), 7.42 (dd, J = 5.1, 1.0 Hz,
1H), 7.15 (dd, J = 5.0, 3.7 Hz, 1H).
Electronic energy.—The electronic energy levels of the oxidative
states for the obtained polymers pTPAT, pPHT and pTPTT were mea-
sured by the cyclic voltammetry electrochemistry method in film state
as shown in Figure 4. The onset oxidative potential of three polymers
pTPAT, pPHT and pTPTT were obtained to be 0.76 V, 1.18 V and
2,4,6-Tri[(5-thiophen-2-yl)-phenyl]-1,3,5-triazine (TPTT).—Af-
ter added 40 mL dried CHCl3 to dissolve M1 (1.11 g, 6 mmol) in
a 250 mL eggplant-shaped flask, CF3SO3H (3.60 g, 24 mmol) was
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Journal of The Electrochemical Society, 164 (4) E84-E89 (2017)
Figure 5. The calculated molecular energy levels and corresponding elec-
tronic orbitals pictures of HOMOs and LUMOs for monomers TPAT, PHT and
TPTT.
was mainly distributed at the triphenylamine part, while those of PHT
and TPTT were mainly delocalized at the phenyl thiophene part,27
resulting in their much lower energy level lying at −5.7∼5.8 eV. Thus
the introduction of donor triphenylamine with strong electron donat-
ing ability or low ionization energy into the central core of star-shaped
molecule may effectively enhance the molecular electronic cloud den-
sity and thus decrease the molecular oxidative potential. Meanwhile,
from the lowest unoccupied molecular orbital (LUMO) of TPAP, PHT
and TPTT, another conclusion can be summarized that the introduc-
tion of strong acceptor 2,4,6-triphenyl-1,3,5-triazine into the central
core of star-shaped molecule may effectively decrease the molecu-
lar LUMO level or reductive potential with relatively much weaker
effect on the HOMO level or oxidative potential. The theoretical cal-
culation results further demonstrated that the key reason for pTPAT
being easier oxidized under relatively lower applied voltage should be
mainly attributed to the introduction of triphenylamine, which were
consistent with the measured electrochemistry energy results.
Figure 3. The cyclic voltammetry curves of the electrochemical polymeriza-
tion process for TPAT, PHT and TPTT.
1.28 V respectively. Obviously, the onset oxidative potential of pT-
PAT was much lower than those of pPHT and pTPTT. Moreover, the
current intensity of pTPAT was much higher than those of pPHT and
pTPTT at the first-couple oxidative-reduction peak, which indicated
the more injected charge in the pTPAT polymer. All these results
demonstrated the easier being oxidized under relatively lower applied
voltage for pTPAT polymer film. The key reason may be attributed
to the introduction of triphenylamine with low ionization energy into
the central core.24
In order to further investigate the electronic character, the elec-
tronic cloud distribution and energy level of the electronic transition
orbital for three monomers TPAP, PHT and TPTT were calculated
by B3LYP method25,26 as shown in Figure 5. The highest occupied
molecular orbital (HOMO) of TPAT with an energy level of −4.8 eV
Electrochromic properties.—The electrochromic properties of
pTPAT, pPHT and pTPTT polymer films were measured by carry-
ing out the spectroelectrochemical experiment in ACN solution with
0.1 M TPAPF6 as electrolyte. Spectroelectrochemistry test were inves-
tigated by Shimadzu UV-1800 spectrophotometer (Shimadzu, Japan)
integrated with the CHI660E electrochemical analyzer. As shown in
Figure 6, pTPAT exhibited a maximum absorption peak at ∼425 nm at
the neutral state (under 0 V), and the polymer film appeared to be light
yellow color. With the increase of the applied voltage, the peak inten-
sity at ∼425 nm decreased obviously and two new absorption peaks
rose up gradually at ∼650 nm and ∼1100 nm, which may be attributed
to the electron transition of ‘polaron’ and ‘bipolaron’ respectively.28
The ‘polaron’ and ‘bipolaron’ derived from the electron extractions
or hole injections of polymer pTPAT. In this progress, the color of
pTPAT film turned to gray eventually at the voltage of 1.5 V as shown
in the inset photos of Figure 6a. The UV spectra of pPHT and pTPTT
polymers exhibited the nearly same unfeatured absorption character
in the range of 380∼500 nm at the neutral state (0 V) with the films
showing as the weak yellowish color. In contrast to pTPAT, the obvi-
ous absorption peak at ∼425 nm was not observed in the UV spectra
of pPHT and pTPTT polymers. It indicated that this special absorp-
tion peak at ∼425 nm in UV spectra of pTPAT should be attributed
to the introduction of donor triphenylamine which resulted in the en-
hance of molecular conjugation in pTPAT polymer. Upon increase
of the applied voltage, it is surprising that electrochromism effects
were not obviously observed with nearly none changes of the UV
Figure 4. The film cyclic voltammetry curves of the obtained polymers pT-
PAT, pPHT and pTPTT.
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Journal of The Electrochemical Society, 164 (4) E84-E89 (2017)
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Figure 6. The UV absorption spectra and film photos under different voltages
between 0 V and 1.6 V for the polymers a) pTPAT, b) pPHT and c) pTPTT in
the electrochemical cell with 0.1 M TPAPF6 in ACN solution as electrolyte;
inset curves of b) and c) are the UV spectra under different voltages for pPHT
and pTPTT in the electrochemical cell with 0.1 M TPAPF6 in ACN/CH2Cl2
(1:1, by volume) solution as electrolyte respectively.
spectra and exterior colors for both pPHT and pTPTT polymer films.
Actually, the current-voltage curves in Figure 3 clearly demonstrated
that they underwent the obvious reverse oxidative and reductive reac-
tion during the cyclic voltammetry polymerization process,29 which
meant that the electrochromism phenomenon of their polymer films
should also appear during the electrochemical polymerization pro-
cess. In fact, clear electrochromism between light yellow and gray
color appeared during the electrochemical polymerization process
of pTPAT, while nearly none obvious electrochromism phenomenon
was observed for the polymer films of pPHT and pTPTT during their
electrochemical polymerization process. In order to further verify the
electrochromism of pPHT and pTPTT, the same experimental condi-
tions as their electrochemical polymerization process were applied to
measure their UV spectra under different voltages with 0.1M TPAPF6
in ACN/CH2Cl2 (1:1, by volume) mixture solution as electrolyte. As
shown in the inset curves of Figures 6b and 6c, pPHT almost exhibited
the same UV spectra to the previous one, while pTPTT showed little
difference with UV spectra intensity around ∼630 nm and ∼1000 nm
enhanced as the voltage increased, though the change of exterior color
of pTPTT film was still not very obvious. The electrochromism color
and discolor switching time of pTPTT film at 630 nm were roughly
estimated to be 1.2 s and 4.7 s respectively30 (Figure 7a), and the
electrochromism optical contrasts of pTPTT film were obtained to be
only around 10% at both 630 nm and 995 nm, which also decreased
gradually as time delayed (Figure 7b). These results reflected that the
observed electrochromism effect of pTPTT film in the UV spectra
was unstable. From the UV spectra character, the peaks at 630nm
should be attributed to the absorption spectra of pTPTT ‘polaron’ (or
oxidative state), which remained in the spectra of pTPTT under 0V as
shown in the inset curves of Figure 6c. It indicated that pTPTT film in
ACN/CH2Cl2 (1:1, by volume) mixture solution with 0.1M TPAPF6
as electrolyte possessed the relatively high level doping of oxidative
state (accompanied by the inserted electrolyte anions) in its film under
Figure 7. a) Electrochromic switching response of pTPTT films at 630 nm
and b) the optical contrasts of pTPTT polymer film at 630 nm and 995 nm (in
ACN/CH2Cl2 (1:1, by volume) solution containing 0.1 M TPAPF6 between
0 V to 1.5 V with a residence time of 10 s).
0 V. This might benefit the injection of holes into the pTPTT film and
contributed to the observed unstable electrochromism in UV spectra.
Therefore, the nearly none or weak electrochromism effect for pPHT
and pTPTT polymers may be ascribed to their relatively high oxidative
potentials as well as the possible insufficient injected holes in films,
which could also be seen from their relatively lower current levels in
their common cyclic voltammetry electrochemistry curves (Figure 4).
These results also demonstrated that the electrochromism effects of
pTPAT polymer should mainly derive from the oxidation behavior of
triphenylamine cores under the applied voltages. This is consistent
with the previous conclusion that pTPAT being easier oxidized un-
der relatively lower applied voltage in comparison with pPHT and
pTPTT polymers should be mainly attributed to the introduction of
triphenylamine.
By introducing 3,4-ethylenedioxythiophene (EDOT) which is also
of relatively low ionization energy31,32 into the monomers to form
the mixture (ratio of 1:1) solutions of PHT-EDOT and TPTT-EDOT,
copolymers of pPHT-EDOT and pTPTT-EDOT were fabricated by
electrochemical polymerization method under the same experimen-
tal conditions to their corresponding homopolymers of pPHT and
pTPTT. And since EDOT was used as the comonomer during the
copolymerization, we also measured the electrochromic properties
of its polymer PEDOT under the same experimental conditions. As
shown in Figures 8a, 8b and 8c, the obtained copolymers of pPHT-
EDOT and pTPTT-EDOT both exhibited obviously different clec-
trochemical redox behaviors to their corresponding homopolymers
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Journal of The Electrochemical Society, 164 (4) E84-E89 (2017)
Figure 8. The cyclic voltammetry curves of the electrochemical copolymerization process for a) PHT-EDOT, b) TPTT-EDOT and the electrochemical homopoly-
merizaition process for c) EDOT in the electrochemical cell with 0.1 M TPAPF6 in ACN/CH2Cl2 (1:1, by volume) solution as electrolyte; the electrochromic UV
absorption spectra under different voltages between −0.2 V and 1.2 V for copolymers d) pPHT-EDOT, e) pTPTT-EDOT and for homopolymer f) PEDOT, and the
optical contrasts at 1100 nm between 0 V to 1.5 V with a residence time of 10 s for copolymers g) pPHT-EDOT and h) pTPTT-EDOT and between −0.8 V to
1.2 V with a residence time of 10 s for homopolymer i) PEDOT (in ACN solution containing 0.1 M TPAPF6).
and PEDOT. The oxidative potentials of the copolymers were lower
than their corresponding homopolymers and higher than PEDOT.
After being applied with the voltages from −0.2 V to 1.2 V, two
copolymers pPHT-EDOT and pTPTT-EDOT both showed the obvi-
ous electrochromic effects, with the original peaks around 550 nm
decreasing and two new peaks appearing at 700 nm and 1100 nm
in their UV spectra as shown in Figures 8d and 8e. And as shown
in Figure 8f, the homopolymer PEDOT showed the original peak
around 620 nm decreasing and one new peak appearing at 1100 nm.
The differences of the cyclic voltammetry curves and UV spetra be-
tween the copolymers and the homopolymers proved the successful
copolymerization of the copolymers pPHT-EDOT and pTPTT-EDOT.
Meanwhile, the copolymers’ electrochromism optical contrasts were
measured to be 23% with the color (discolor) switching time of 0.7 s
(0.8 s) at 1100 nm under 1.5 V for pPHT-EDOT and 29% with the
color (discolor) switching time of 0.8 s (0.5 s) at 1100 nm under
1.5 V for pTPTT-EDOT as shown in Figures 8g and 8h. The elec-
trochromism optical contrasts of PEDOT were measured to be 43%
with the color (discolor) switching time of 0.3 s (0.7 s) at 1100 nm
under 1.2 V as shown in Figure 8i. The better performance of elec-
trochromism for copolymers pPHT-EDOT and pTPTT-EDOT than
their corresponding homopolymers pPHT and pTPTT should mainly
be attributed to the introduction of EDOT unit, further indicating the
importance of polymers’ oxidative potentials to their electrochromic
properties.
Conclusions
Three star-shaped thiophene derivatives, namely TPAT, PHT and
TPTT with different central cores of triphenylamine, phenyl and 2,4,6-
triphenyl-1,3,5-triazine respectively, were synthesized and characte-
riaed. They were further successfully prepared into the corresponding
cross-linked polymers pTPAT, pPHT and pTPTT via electrochemi-
cal polymerization. The polymers pPHT and pTPTT displayed the
relatively higher onset oxidative potentials than pTPAT. And pTPAT
exhibited the obvious electrochromic behaviors, while no such phe-
nomenons were observed in the case of pPHT and pTPTT. Theory
calculation results together with the electrochemistry properties re-
vealed that the electrochromism of pTPAT may mainly depend on
its relatively lower oxidative potential because of the introduction of
triphenylamine with low ionization energy. Through the copolymer-
ization with EDOT being of relatively low ionization energy, copoly-
mers pPHT-EDOT and pTPTT-EDOT both exhibited excellent elec-
trochromic properties. The oxidative potential or ionization energy
of polymer is thought to be one of the key factors determining the
electrochromism.
Acknowledgments
The authors gratefully appreciate for the support from the Na-
tional Natural Science Foundation of China (51603185, 51673174,
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Journal of The Electrochemical Society, 164 (4) E84-E89 (2017)
E89
51573165, 51273179), Zhejiang Provincial Natural Science Foun-
dation of China (LY17E030001, LY15E030006), and the special
fund for scientific research from Zhejiang University of Technology
(3817101101T).
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