Synthesis and multi-electrochromic properties of asymmetric structure polymers based on carbazole-EDOT and 2, 5–dithienylpyrrole derivatives

Article abstract of DOI:10.1016/j.electacta.2019.03.050

Two novel asymmetric structure monomers, 3,6-Bis-(2,3-dihydro-thieno[3,4- b][1,4]dioxin-5-yl)-9-[4-(2,5-di-thiophen-2-yl-pyrrol-1-yl)-phenyl]-9H-carbazole (BDTC) and 2,7-Bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-9-[4-(2,5-di-thio phen-2-yl-pyrrol-1-yl)-phenyl]-9H-carbazole (BDDC), were designed and synthesized. Both the monomers could be electropolymerized to form the related polymer films. The electrochemical and electrochromic properties of polymer films were different from that of their homopolymers, which could be attributed to the similar copolymerization effect of asymmetric structure. Different linked positions also led to different features, PBDDC film showed more colors (orange, yellow green, green and blue) than that of PBDTC film (yellow, green and purple blue). However, after the introduction of 2, 2′-Bi(3,4-ethoxylene dioxy thiophene)(BIEDOT), the resulting copolymers exhibited similar electrochromic features(red, brown yellow, green and blue) due to the leading effect of EDOT units. Moreover, all the polymer films displayed fast switching time, reasonable optical contrast and good coloration efficiency, which made them have potential application in electrochromic devices.

Full text of DOI:10.1016/j.electacta.2019.03.050

Electrochimica Acta 305 (2019) 1e10
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and multi-electrochromic properties of asymmetric
structure polymers based on carbazole-EDOT and 2,
5edithienylpyrrole derivatives
Bin Hu*, Chunyang Li, Zengchen Liu, Xinlei Zhang, Wen Luo, Lin Jin**
Henan Key Laboratory of Rare Earth Functional Materials, The Key Laboratory of Rare Earth Functional Materials and Applications, Zhoukou Normal
University, Zhoukou, 466001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel asymmetric structure monomers, 3,6-Bis-(2,3-dihydro-thieno[3,4- b][1,4]dioxin-5-yl)-9-[4-
(2,5-di-thiophen-2-yl-pyrrol-1-yl)-phenyl]-9H-carbazole (BDTC) and 2,7-Bis-(2,3-dihydro-thieno[3,4-b]
[1,4]dioxin-5-yl)-9-[4-(2,5-di-thio phen-2-yl-pyrrol-1-yl)-phenyl]-9H-carbazole (BDDC), were designed
and synthesized. Both the monomers could be electropolymerized to form the related polymer films. The
electrochemical and electrochromic properties of polymer films were different from that of their ho-
mopolymers, which could be attributed to the similar copolymerization effect of asymmetric structure.
Different linked positions also led to different features, PBDDC film showed more colors (orange, yellow
green, green and blue) than that of PBDTC film (yellow, green and purple blue). However, after the
introduction of 2, 20-Bi(3,4-ethoxylene dioxy thiophene)(BIEDOT), the resulting copolymers exhibited
similar electrochromic features(red, brown yellow, green and blue) due to the leading effect of EDOT
units. Moreover, all the polymer films displayed fast switching time, reasonable optical contrast and good
coloration efficiency, which made them have potential application in electrochromic devices.
© 2019 Elsevier Ltd. All rights reserved.
Received 22 December 2018
Received in revised form
26 February 2019
Accepted 7 March 2019
Available online 9 March 2019
Keywords:
Asymmetric structure
Electropolymerization
Multi-electrochromism
Spectroelectrochemistry
Coloration efficiency
1. Introduction
ability comparing with the linear molecules. Ma group designed
and synthesized a novel poly(3,4-dioxythiophene) derivative (PM-
For the past decades, conjugated polymers have gained many
attentions owing to their potential applications in the fields of
organic light emitting diodes (OLEDs) [1,2], electrochromic devices
(ECDs) [3,4], organic solar cells (OSCs) [5,6] and Li-polymer batte-
ries [7,8]. Especially, as the active layers of ECDs, conjugated poly-
mers possess the advantages of easily turned color-showing via
modifying the structure, fast switching time and high coloration
efficiency [9e11]. Up to now, many methods including donor-
acceptor (D-A) effect [12,13], the influence of suspended groups
at side chain [14,15] and copolymerization [16,17] have been
employed to explore the relationship between structure and per-
formance of the polymer. However, design and synthesis of high
performance electrochromic polymer materials are still a challenge.
Recently, cross-linked conjugated polymers have attracted re-
searcher's interest due to their better stability and charge transport
BTE) with soft nano-network structure, which displayed low
switching potentials and excellent stability [18]. Zhang group
synthesized a novel cross linked polymer (PTPHSNS) connected by
conjugated terphenyl groups, which showed fast switching time
and excellent color memory behavior [19]. Moreover, Toppare
group designed and synthesized a asymmetric structure polymer
containing benzotriazole and 2, 5-di(2-thienyl)-1H-pyrrole units,
study results indicated that similar copolymerization effect could
be achieved [20]. However, the connected groups between the
different units of asymmetric structure polymer were hexyl, which
ignored the effect of intramolecular charge transfer. With the
consideration of the above research results, it is anticipated that
cross linked asymmetric structure polymer connected by conju-
gated groups may hold some unexpected electrochemical and
electrochromic properties.
2, 5-dithienylpyrrole derivatives are famous for their low
oxidized potential, easy modified structure via N-position of pyr-
role units and good electrochemical film-forming ability [21e23].
However, electrochromic properties of resulting polymer film
needed to be enhanced by copolymer [24e26]. Fortunately, such
* Corresponding author.
** Corresponding author.
E-mail addresses: hubin30003@163.com (B. Hu), Jinlin_1982@126.com (L. Jin).
https://doi.org/10.1016/j.electacta.2019.03.050
0013-4686/© 2019 Elsevier Ltd. All rights reserved.

2
B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
the deficiency can be made up by similar copolymerization effect of
asymmetric structure, so 2, 5-dithienylpyrrole units can be
considered as the better choice to construct the asymmetric
structure monomer. Moreover, carbazole derivatives possess multi
functioned positions, such as 3.6-position, 2.7- position and N-
functioned position. And our recent studies indicated that 3.6-
position linked carbazole-EDOT derivatives showed good electro-
chemical and electrochromic properties via modifying the N-po-
sition of carbazole units [27e29]. Other related researches showed
that the polymers based on 2.7-position linked carbazole could
exhibit lower band gap, which was also benefit for electrochemical
and electrochromic properties [30e33]. So it is interesting to
integrate 2, 5-dithienylpyrrole units into carbazole-EDOT deriva-
tive to gain the asymmetric structure compound and explore its
electrochemical and electrochromic properties.
further study the effect of increasing EDOT units of the polymer
main chain, copolymers with 2, 20-Bi(3,4-ethoxylene dioxy thio-
phene)(BIEDOT) were also prepared, whose electrochemical and
electrochromic characteristics were studied and compared with
those of asymmetric structure polymers.
2. Experimental
2.1. Materials and measurements
3,6-Dibromocarbazole and 2,7-Dibromocarbazole were pur-
chased from Energy Chemical (98%), Tributyl-(3,4-ethoxylene-
dioxythiophen-2-yl)-stannane were purchased from Suna Tech Inc
(95%), PdCl2 and Pd(OAc)2 were purchased from Shanghai Macklin
Biochemical Co., Ltd, BIEDOT was purchased from Energy Chemical
(98%), PtBu (1.0 M in toluene) were purchased from Aladdin, com-
mercial HPLC grade acetonitrile (ACN, Aladdin, 99.9%) was used
without further purification. Toluene was processed with CaH2 for
three days and collected with Molecular sieves (4 Å). Tetrabutyl
ammonium perchlorate (TBAP, Zhengzhou Xipaike Chemical Co.,
Ltd, 98%) was dried at 80 ꢀC in vacuum for 12 h. Anhydrous
In this manuscript, asymmetric structure monomers based on
carbazole-EDOT and 2, 5-dithienylpyrrole connected via p-phe-
nylenediamine structure were designed and synthesized (Scheme
1). Both the monomers can be electropolymerized to form result-
ing polymer films, the electrochemical and electrochromic prop-
erties of polymer films were investigated in detail. Moreover, to
Scheme 1. Synthesis route of DTP-Br, DTC, DDC, BDTC and BDDC.

B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
3
dichloromethane (DCM, 99.9%) was purchased from Aladdin. Other
regents were commercial products used as received.
0.2 g) and Pd(OAc)2 (0.02 g) were dissolved in anhydrous toluene
(20 mL) in a three-neck round-bottomed flask, then the mixture
was degassed and placed in a nitrogen atmosphere, PtBu (2 mL)
was injected with syringe. The mixture was refluxed for 12 h. After
cooled to room temperature, the mixture was extracted with DCM
and a lot of water, the organic phase was collected and removed
under column pressure, and the crude product was processed by
column chromatography on silica gel with petroleum: ethyl acetate
Bruker AVANCE III (Germany) instrument was used to record the
1H and 13C NMR spectra of prepared monomers in CDCl3. The Mass
spectrometry (MS) analysis of the obtained monomers was
measured on a GCT Premier spectrometer (Waters, USA). UVevis
spectrophotometer (Lambda 25, Perkin Elmer, USA) was used to
investigate the UVevis spectra of the polymer films. The electro-
chemical properties were tested on an Autolab PGSTAT302 N
electrochemical analyzer (Metrohm, Switzerland). The color im-
ages of polymer films were taken by Nikon D90.
(10:1) to give
a
yellow-green power(0.95 g, 63%). 1H NMR
: 8.45 (s, 2H), 7.81 (d, 2H), 7.61 (d, 2H), 7.54 (d,
(500 MHz, CDCl3)
d
2H), 7.43 (d, 2H), 7.15 (t, 2H), 6.92 (d, 2H), 6.72 (d, 2H), 6.60 (s, 2H),
6.31 (d, 2H), 4.37 (t, 4H), 4.30 (t, 4H). 13C NMR (125 MHz, CDCl3)
d:
142.32, 139.73, 137.32, 134.75, 131.65, 130.24, 127.37, 125.91, 124.52,
124.00, 118.43, 118.30, 110.18, 109.85, 96.73, 64.86, 64.59. MS (EI):
calculated for C42H28N2O4S4 m/z: 752.09, found m/z: 752.51.
2.2. Monomer synthesis
2.2.1. Synthesis of 1-(4-Bromo-phenyl)-2,5-di-thiophen-2-yl-1H-
pyrrole(DTP-Br)
1,4-di(20-thienyl)-1,4-butadione
(2 mmol,
0.50 g),
4-
2.2.5. Synthesis of 2,7-Bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-
yl)-9-[4-(2,5-di- thiophen-2-yl-pyrrol-1-yl)-phenyl]-9H-carbazole
(BDDC)
Bromoaniline (6 mmol, 1.01 g) and p-toluenesulfonic acid
(2 mmol, 0.39 g) were dissolved in anhydrous toluene (50 mL) in a
three-neck round-bottomed flask. The reaction mixture was stirred
at 90 ꢀC for 48 h under N2 atmosphere. After being cooled to room
temperature, toluene was removed under column pressure, then
the mixture was extracted with DCM and saturated brine. The
organic phase was collected and removed under column pressure,
followed processed by column chromatography on silica gel with
petroleum: ethyl acetate (20:1) to give a light yellow power (0.52 g,
The monomer BDDC was prepared according to the above
synthesized method.
1H NMR (500 MHz, CDCl3)
d
: 8.07 (d, 2H), 7.82 (s, 2H), 7.67 (m,
4H), 7.58 (d, 2H), 7.10 (d, 2H), 6.90 (t, 2H), 6.75 (d, 2H), 6.62 (d, 2H),
6.32 (s, 2H), 4.24 (m, 8H). 13C NMR (125 MHz, CDCl3)
: 142.33,
d
141.54, 138.09, 137.14, 134.56, 131.65, 128.01, 136.97, 124.34, 120.43,
118.98, 110.04, 107.01, 97.59, 64.81, 64.47. MS (EI): calculated for
68%). 1H NMR (500 MHz, CDCl3)
2H), 6.84 (dd, 2H), 6.54 (d, 2H). 6.52 (s, 2H). 13C NMR (125 MHz,
CDCl3) : 137.51, 134.53, 132.38, 131.49, 129.97, 127.02, 124.76,
d: 7.53 (d, 2H), 7.16 (d, 2H), 7.09 (dd,
C42H28N2O4S4 m/z: 752.09, found m/z: 752.87.
d
3. Results and discussion
124.22, 122.95, 110.31. MS (EI) (m/z): calcd for C18H12BrNS2, 384.96;
found, 385.51.
3.1. Synthesis and characterization
2.2.2. Synthesis of 3,6-Bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-
yl)-9H-carbazole (DTC)
The synthesis route of the monomers was displayed in Scheme
1, firstly 1,4-di(20-thienyl)-1,4-butadione was used to prepare DTP-
Br by reacting with 4-Bromoaniline through Knorr-Paal reaction
[20], and tributyl-(3,4-ethoxylene -dioxythiophen-2-yl)-stannane
was used to react with 3,6-Dibromocarbazole and 2,7-
Dibromocarbazole by Still coupling reaction to obtain the in-
termediates DTC and DDC, respectively. Then DTC and DDC reacted
with DTP-Br to prepare the desired monomers of BDTC and BDDC
by Friedel-Crafts reaction.
Tributyl-(3,4-ethoxylene-dioxythiophen-2-yl)-stannane
(5 mmol, 2.2 g), 3,6-Dibromocarbazole (2.5 mmol, 0.8 g) and PdCl2
(0.171 mmol, 0.12 g) were dissolved in anhydrous toluene (50 mL)
in a 250 mL round bottom flask, then the mixture was degassed and
refluxed for 48 h under a nitrogen atmosphere. After being cooled
to room temperature, the mixture was washed with saturated salt
water and extracted with trichloromethane (TCM). The organic
phase was collected and dried with anhydrous MgSO4, solvent was
removed under column pressure and the product was processed by
column chromatography on silica gel with petroleum: TCM (10 : 1,
by volume) to give a light-yellow power (0.65 g, 58% yield). 1H NMR
1H NMR, 13C NMR, mass spectroscopy (MS) and FT-IR analyses
were used to characterize the purity of intermediates and mono-
mers. The NMR and MS spectra of compounds can be seen in
Figs. S1eS5 of the supporting information. Fig. 1 shows the FT-IR
spectra of DTP-Br, DTC, DDC, BDTC and BDDC. For DTP-Br, the ab-
sorption peaks at 698 and 830 cmꢁ1 can be ascribed to the
(500 MHz, CDCl3)
d
: 8.09 (s, 1H), 7.82 (d, 2H), 7.42 (d, 2H), 6.33 (s,
: 147.39,141.05,138.26,
2H), 4.37 (t, 8H). 13C NMR (125 MHz, CDCl3)
d
134.19, 130.66, 125.60, 124.18, 123.80, 117.46, 63.57, 63.28, MS (EI):
calculated for C24H17NO4S2 m/z: 447.06, found m/z: 447.56.
stretching mode of
a
-H and
b-H of thiophene rings [26], the peak at
760 cmꢁ1 are for
b
-H of pyrrole unit [34], the peaks at 1410 and
1490 cmꢁ1 can be attributed to the bending of C-C of aromatic rings.
Comparing the spectrum of DTC and DDC, both the spectra exhibit
the characteristic peaks of EDOT groups as follows: 1363 cmꢁ1 (C-C
stretching), 1166 and 1070 cmꢁ1(C-O-C stretching) [35]. However,
some different absorption peaks (1296 cmꢁ1 for DTC, 1328 cmꢁ1 for
DDC) still exist due to the different substituted positions of carba-
zole. The spectra of BDTC and BDDC are almost consisted of that of
both the asymmetrical sections. According to results of NMR, MS
and FT-IR analyses, the pure asymmetrical monomers are synthe-
sized successfully.
2.2.3. Synthesis of 2,7-Bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-
yl)-9H-carbazole (DDC)
The monomer DDC was prepared according to the above syn-
thesized method.
1H NMR (500 MHz, CDCl3)
d
: 7.98 (s, 2H), 7.80 (d, 2H), 7.56 (d,
2H), 6.32 (s, 2H), 4.31 (t, 8H). 13C NMR (125 MHz, CDCl3)
d: 142.33,
140.37, 137.91, 130.85, 122.05, 120.30, 118.50, 118.28, 107.96, 97.44,
64.84, 64.52, MS (EI): calculated for C24H17NO4S2 m/z: 447.06,
found m/z: 447.68.
2.2.4. Synthesis of 3,6-Bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-
yl)-9-[4-(2,5-di- thiophen-2-yl-pyrrol-1-yl)-phenyl]-9H-carbazole
(BDTC)
3.2. Electrochemical polymerization of the monomers
Cyclic voltammetry (CV) was used to investigate the polymeri-
zation process of DTC, DDC, BDTC, BDDC, BDTC/BIEDOT and BDDC/
DTP-Br (2 mmol, 0.77 g), DTC (2 mmol, 0.90 g), NaOtBu (2 mmol,

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B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
BIEDOT in 0.1 M TBAP/ACN-DCM solution at the scan rate of
100 mV/s, and the results were depicted in Fig. 2. As seen from
Fig. 2, all the first cycle of CV curves show irreversible redox process
(Inset of Fig. 2), which can be attributed to the oxidation of the
monomers to form radical-cations [36,37]. For DTC and DDC, the
obvious oxidation peaks located at 0.78 and 0.95 V can be observed,
the differences can be attributed to the better donor ability of 3, 6-
positon than 2,7-positon of carbazole [38,39]. After the introduc-
tion of DTP unit, BDTC and BDDC exhibit obvious redox peaks at
0.91/0.60 V and 1.01/0.63 V, respectively, which are different from
that of DTC, DDC and DTP, indicating that interactions exist be-
tween DTP and carbazole-EDOT units. For BDTC/BIEDOT and BDDC/
BIEDOT, the existence of BIEDOT leads to the broad reduction wave
between ꢁ0.2 and 0.6 V. With the scan cycle increasing, new
reversible redox peaks (0.48/0.46 V and 0.89/0.76 V for DTC; 0.43/
0.29 V and 1.01/0.84 V for DDC; 0.89/0.59 V for BDTC; 0.92/0.63 V
for BDDC; 0.58/0.49 V for BDTC/BIEDOT; 0.51/0.36 V for BDDC/
BIEDOT) can be observed thanks to further polymerization of all the
Fig. 1. The FT-IR spectra of DTP-Br, DTC, DDC, BDTC and BDDC.
Fig. 2. Continuous CV curves of 2 mM DTC, and 2 mM DDC, 2 mM BDTC, 2 mM BDDC, 2 mM BDTC/2 mM BIEDOT and 2 mM BDDC/2 mM BIEDOT) in the 0.1 M TBAP/ACN:DCM
(v:v ¼ 1:1) solution at a scan rate of 100 mV/s (Inset picture: the first cycle of CVs of the compounds).

B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
5
monomers. Meanwhile, the current intensity of new redox peaks
become bigger gradually, indicating that resulting polymer films
form on the surface of ITO/glass electrode successfully and the color
change of formed polymer films upon the different applied po-
tential can be seen by the naked eye.
according to the formula [40]: EHOMO ¼ -(Eoxonset vs Ag/AgCl þ 4.43)
eV, the HOMO of PDTC, PDDC, PBDTC, PBDDC, P(BDTC/BIEDOT) and
P(BDDC/BIEDOT)
are
calculated
to
be ꢁ4.56 eV, ꢁ4.53 eV, ꢁ4.84 eV, ꢁ4.86 eV, ꢁ4.49 eV and ꢁ4.35 eV,
respectively, and the other optical and electrochemical parameters
can be seen in Table 1. Fig. S6 shows CV of PBDTC, PBDDC, P(BDTC/
BIEDOT) and P(BDDC/BIEDOT) films at different scan rate ranging
from 50 to 300 mV/s. As seen from Fig. S6, all the peak current
intensity increase with the scan rate increasing, and a linear rela-
tionship between them can be observed in the inset of Fig. S6,
implying that all the polymer films show good electroactive and
well-adhered, and the oxidation-reduction processes are non-
diffusional controlled [29].
Long time stability was an important parameter for applications,
Fig. 4 exhibits the multiple cycle CV curve of PBDTC, PBDDC,
P(BDTC/BIEDOT) and P(BDDC/BIEDOT) films in monomer-free 0.1 M
TBAP/ACN solution at the scan rate of 100 mV/s for 500 times cycle.
As seen from Figs. 4 and 30% and 28% loss can be observed for
PBDTC and PBDDC films, which may be ascribed to the effect of
doped and dedoped ion into the polymer chain on the adhered
ability of polymer film on the electrode. P(BDTC/BIEDOT) and
P(BDDC/BIEDOT) films show higher stability(only 11% and 8% loss,
3.3. Electrochemical properties of the polymer films
The polymer films were deposited on the surface of ITO/glass
work electrode via continued CV sweeps for 16 cycles, then, the
films were washed with clear ACN to remove the TBAP and the
unreacted monomers. Fig. 3 displays the CV of resulting polymer
films in monomer-free 0.1 M TBAP/ACN solution at the scan rate of
100 mV/s. PDTC film shows an oxidation peak at 0.79 V with a
reduction peak at 0.59 V, PDDC film exhibits redox peak at 0.75/
0.58 V, the different CV curves of PDTC and PDDC film indicate that
the distinct electrochemical properties can be obtained via
changing linkage positions. After the introduction of DTP and BIE-
DOT units, the redox peaks (0.85/0.60 V for PBDTC; 0.86/0.64 V for
PBDDC; 0.82/0.47 V for BDTC/BIEDOT; 0.87/0.41 V for BDDC/BIE-
DOT) can be observed. The CV curves become similar due to the
increase of same units of the conjugated polymer chain. Moreover,
Fig. 3. CV curves of the PDTC, PDDC, PBDTC, PBDDC, P(BDTC/BIEDOT) and P(BDDC/BIEDOT) films in monomer-free solution of 0.1 M TBAP/ACN a scan rate of 100 mV/s.
Table 1
Electrochemical and optical properties of PDTC, PDDC, PBDTC, PBDDC, P(BDTC/BIEDOT) and P(BDDC/BIEDOT) films.
Polymers
Eox
V
EHOMO eV
ELUMO eV
lmax nm
lonset nm
Eg eV
onset
PDTC
PDDC
PBDTC
PBDDC
P(BDTC/BIEDOT)
P(BDDC/BIEDOT)
0.13
0.10
0.41
0.43
0.06
ꢁ0.08
ꢁ4.56
ꢁ4.53
ꢁ4.84
ꢁ4.86
ꢁ4.49
ꢁ4.35
ꢁ2.07
ꢁ2.27
ꢁ2.52
ꢁ2.65
ꢁ2.60
ꢁ2.52
410
469
420
448
478
496
498
549
534
561
656
678
2.49
2.26
2.32
2.21
1.89
1.83

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B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
Fig. 4. Long time electrochemical stability of PBDTC, PBDDC, P(BDTC/BIEDOT) and P(BDDC/BIEDOT) film in the 0.1 M TBAP/ACN solution at the scanning rate of 100 mV/s.
respectively), which can be attributed to the increase of EDOT units
in the conjugated chains [19].
intensity of peaks change, for P(BDTC/BIEDOT) and P(BDDC/BIE-
DOT) films, the characteristic peaks of PBDTC and PBDDC films can
still be observed. However, the characteristic peaks of EDOT units at
1070 and 1360 cmꢁ1 become stronger, which can be ascribed to the
increasing content of EDOT groups of the polymer chains. The study
results indicate that P(BDTC/BIEDOT) and P(BDDC/BIEDOT) films
may be the mixture of their homopolymer or copolymer.
3.4. FT-IR of the polymers films
The chemical structure of PBDTC, PBDDC, P(BDTC/BIEDOT) and
P(BDDC/BIEDOT) films were characterized by FT-IR spectra, and the
results are shown in Fig. S7. The PBDTC and PBDDC films display
almost all the characteristic peaks of BDTC and BDDC, the broader
peak at 1080 cmꢁ1 can be attributed to the existence of ClOꢁ4 of
residual TBAP [26]. After the introduction of BIEDOT units, the
3.5. Surface morphology
SEM was used to investigate the surface morphology of the
Fig. 5. SEM images of the PDTC, PDDC, PBDTC, PBDDC, P(BDTC/BIEDOT) and P(BDDC/BIEDOT) films.

B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
7
polymer films, which was related with doped ion or electron
transfer speed, and the results were shown in Fig. 5. PDTC and
PDDC exhibit unsmooth surface with many particles and some
aggregates, PBDTC and PBDDC films display smoother surface, but
many grains also can be observed, especially for PBDDC films. After
the introduction of BIEDOT units, P(BDTC/BIEDOT) exhibits cross-
linking network surface, which can be considered to be benefit
for the doped ion or electron transfer speed [17]. While the uniform
particles on the surface can be found for P(BDDC/BIEDOT) films.
Moreover, the thickness of polymer films was test by cross-section
SEM, as seen from Fig. S8, the thickness of PBDTC and PBDDC films
are found to be 176 nm and 164 nm, respectively, and 198 nm and
204 nm are observed for P(BDTC/BIEDOT) and P(BDDC/BIEDOT)
films, the thicker films of copolymer films can be attributed to the
effect of BIEDOT units.
3.6. Spectroelectrochemical properities
Spectroelectrochemistry experiments were performed to
investigate the doped and undoped changes of polymer film upon
different applied potentials via using both electrochemical station
and UVevis spectrophotometer [41]. Fig. 6 exhibits the spec-
troelectrochemical properities of PDTC, PDDC, PBDTC, PBDDC,
P(BDTC/BIEDOT) and P(BDDC/BIEDOT) film. For PDTC and PDDC,
Fig. 6. Spectroelectrochemical spectra changing process for PDTC (a), PDDC (b), PBDTC (c), PBDDC (d), P(BDTC/BIEDOT)(e), P(BDDC/BIEDOT) (f) films upon the different applied
potentials in 0.1 M TBAP/ACN solution (Inset picture: photos of the polymer films upon different applied potentials).

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B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
the polymer films show the maximum absorption peaks at 410 and
469 nm in the neutral state due to the transitions, according to
purple and blue color, respectively.
p-
p
The existence of BIEDOT units improves the conjugated ability of
polymer, which makes the absorption peaks of P(BDTC/BIEDOT)
and P(BDDC/BIEDOT) red shift. As seen from Fig. 6e and f, the
maximum absorption peaks at 478 and 496 nm can be observed for
P(BDTC/BIEDOT) and P(BDDC/BIEDOT) film at the neutral state,
according to our previous study [43], the only one max absorption
peak indicates that P(BDTC/BIEDOT) and P(BDDC/BIEDOT) film are
the copolymer and not mixture of their homopolymers. And their
band gaps are found to be 1.89 and 1.83 eV, respectively, both of
them reveal deep red color for the neutral state. Both the polymer
films show similar absorption change with the applied potential
increasing, the new absorption waves around 800 and 1100 nm
appear thanks to the formation of polaron and bipolaron. The color
of brown green (0.4 V), green (0.7 V) and blue (0.9 V) are observed
for both the polymer films (Inset of Fig. 6e and f), the only differ-
ence is that P(BDTC/BIEDOT) shows deep blue at 1.1 V due to the
stronger absorption around 600 nm. The rich color showing ability
may enlarge the application of the polymer film.
the onset of their maximum absorption, their band gap are found to
be 2.49 and 2.26 eV, respectively. And the absorption features make
PDTC and PDDC films reveal yellow-green and reddish orange color
at 0.0 V (Inset of Fig. 6a and b). With the applied potential
increasing gradually, the intensity of both the maximum absorption
peaks at 410 and 469 nm decreases, and new absorption waves
around 600 and 1100 nm grow up gradually owing to the formation
of polaron and bipolaron [42], the color of PDTC film turns to brown
green (0.4 V) and blue purple (0.8 V) (Inset of Fig. 6a), while the
color of gray(0.5 V) and deep blue (0.8 V) can be observed for PDDC
films(Inset of Fig. 6b).
After the introduction of DTP-Br group, PBDTC and PBDDC films
exhibit the maximum absorption peaks at 420 and 448 nm for the
neutral state (Fig. 6c and d), their band gaps were calculated to be
2.32 and 2.21 eV, respectively, the differences from these of PDTC
and PDDC films can be ascribed to the existence of DTP units in the
conjugated chain, and the color of polymer at 0.0 V is found to be
light yellow (PBDTC) and orange (PBDDC) (Inset of Fig. 6c and d). As
the potentials increase, both the PBDTC and PBDDC films show an
absorption wave around 800 nm (0.4e0.9 V), which can be attrib-
uted to oxidation of carbazole-phenyl-pyrrole units of polymer, and
the color of PBDTC changes to light green (0.8 V) (Inset of Fig. 6c),
while yellow green (0.6 V) and green (0.9 V) can be seen for PBDDC
film (Inset of Fig. 6d). With the potentials further increasing, the
maximum absorption wave move to around 600 nm gradually,
meanwhile the absorption wave in near infrared region (around
1100 nm) increase obviously, PBDTC and PBDDC films reveal light
3.7. Electrochromic switching
Optical contrast, switching time and coloration efficiency (CE) of
polymer films are three important parameters for their potential
applications. Double potential chronoamperometry was performed
to explore such properties between the neutral and oxidized state
using electrochemical station and UVevis spectroscopy simulta-
neously. Fig. 7 shows the switching properties of PBDTC and
P(BDTC/BIEDOT) films at different wavelengths with a residence
Fig. 7. Kinetic study performed of PBDTC (a) and P(BDTC/BIEDOT)(c) films monitored at different absorption maxima with the switching time of 10 s; Calculated switching time and
applied potential step changes of PBDTC at 617 nm(b) and P(BDTC/BIEDOT) at 1100 nm (d).

B. Hu et al. / Electrochimica Acta 305 (2019) 1e10
9
Table 2
Electrochromic parameters of PBDTC, PBDDC, P(BDTC/BIEDOT), P(BDDC/BIEDOT) films and other reported polymer films.
Polymers
l
(nm)
△T(%)
33%
tc/tb(s)
△
Q(mC/cm2)
h
(cm2/C)
oD
PBDTC
PBDDC
P(BDTC/BIEDOT)
P(BDDC/BIEDOT)
P2 [18]
617
620
1100
1100
400
630
1100 nm
420
830
800
1.20/2.29
0.73/0.76
1.83/2.67
1.79/2.71
1.5
7.35/5.66
0.84/0.58
4.3
0.32
0.16
0.89
0.87
e
2.13
1.51
1.60
1.96
e
150
106
556
444
e
27%
38%
39%
22%
15%
32%
22%
24%
17%
45.61%
PTPHSNS [19]
e
e
e
PTPHSNS-EDOT (feed ratio 1:4) [19]
Poly(TCT-N) [32]
P(DTP-DPA) [40]
P(DTP-CPA-DTP) [40]
PBIEDOT [46]
e
e
e
e
e
264
90.06
94.69
159.56
2.9/2.8
2.3/4.3
1.07/0.33
0.145
0.164
1.61
1.73
1050
time of 10 s. As seen in Fig. 7a, PBDTC film exhibits 12%, 33% and 32%
transmittance change at the wavelengths of 426, 617 and 1100 nm
switching between 0.0 V and 1.4 V, however, the transmittance
decrease with the cycle time increasing, which is related to the
stability of PBDTC film. P(BDTC/BIEDOT) film displays the optical
contrast of 24% and 38% at 480 and 1100 nm switching between
0.0 V and 1.2 V(Fig. 7c), and after 300 s cycle, the copolymer films
reveal better stability due to the increase of EDOT units of the
polymer chain. The transmittance changes of PBDDC and P(BDDC/
BIEDOT) films were calculated to be 27% (620 nm) and 39%
(1100 nm), which have been summarized in Table 2. According to
the reported works [44], 90% of full-transmittance change is suit-
able to calculate the response time due to the limitation of our
naked eye. The response of PBDTC film at 617 nm is found to be
1.20 s from the bleached state to the colored state (tc) and 2.29 s
from the colored state to the bleached state (tb) (Fig. 7b). P(BDTC/
BIEDOT) film shows the tc and tb as 1.83 and 2.67 s at 1100 nm
(Fig. 7d). The response of PBDDC and P(BDDC/BIEDOT) can be seen
in Table 2.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.electacta.2019.03.050.
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Acknowledgments
The authors gratefully thank the financial support of the Pro-
gram of National Natural Science Foundation of China (21701203),
Natural Science Foundation of Henan Province of China
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University of Henan Province (17HASTIT007).

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