Yellow-to-blue switching of indole[3,2-b]carbazole-based electrochromic polymers and the corresponding electrochromic devices with outstanding photopic contrast, fast switching speed, and satisfactory cycling stability

Article abstract of DOI:10.1016/j.electacta.2019.02.054

Three novel π-conjugated indolo[3,2-b]carbazole derivatives, 2,8-di(thiophen-2-yl)-5,11-dioctyl-indolo[3,2-b] carbazole (TICZ), 2,8-bis(4-ethylthiophen-2-yl)-5,11-dioctyl-indolo[3,2-b]carbazole (ETICZ) and 2,8-bis(2-(3,4-ethylenedioxythiophene))-5,11-dioctyl-indolo[3,2-b]carbazole (EDTICZ), were synthesized and electrochemically polymerized to afford the corresponding polymers. The effects of different substituent groups on thiophen units in terms of optical, electrochemical, and electrochromic properties and the performance of electrochromic devices (ECDs) were investigated in detail. Electrochemical and optical analysis demonstrated that incorporation of strong auxochromic groups resulted in polymer PEDTICZ with low oxidation potentials, red-shifted absorption peaks and corresponding narrow band gap values but is unfavorable for incorporating ethyl groups. Spectroelectrochemical studies indicated an evident variation in the absorption spectra as well as significant color changes for all of the polymer films upon application of an external potential. PTICZ had high optical contrast with visible light (58.6%), fast switching speed (0.92 s), and satisfactory coloration efficiency (167 cm² C^?1). Of these three systems, the results for PEDTICZ were next best, and those for PETICZ were third. Also, there was a larger contrast (82.7% and 57.8%) in the NIR region (1500 nm) for the PTICZ and PEDTICZ films, respectively. Along with high coloring efficiencies, these results indicate that these systems are favorable for many NIR applications.

Full text of DOI:10.1016/j.electacta.2019.02.054

Accepted Manuscript
Yellow-to-blue switching of indole[3,2-b]carbazole-based electrochromic polymers
and the corresponding electrochromic devices with outstanding photopic contrast, fast
switching speed, and satisfactory cycling stability
Zhen Xu, Yuling Zhang, Bozhen Wang, Liu Zhi, Jinsheng Zhao, Yu Xie
PII:
S0013-4686(19)30297-X
https://doi.org/10.1016/j.electacta.2019.02.054
EA 33645
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 28 October 2018
Revised Date: 12 February 2019
Accepted Date: 12 February 2019
Please cite this article as: Z. Xu, Y. Zhang, B. Wang, L. Zhi, J. Zhao, Y. Xie, Yellow-to-blue switching of
indole[3,2-b]carbazole-based electrochromic polymers and the corresponding electrochromic devices
with outstanding photopic contrast, fast switching speed, and satisfactory cycling stability, Electrochimica
Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.02.054.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Yellow-to-blue switching of indole[3,2-b]carbazole-based electrochromic
polymers and the corresponding electrochromic devices with outstanding
photopic contrast, fast switching speed, and satisfactory cycling stability
1
2
1
2
2
3
Zhen Xu , Yuling Zhang , Bozhen Wang , Liu Zhi , Jinsheng Zhao *, Yu Xie *
1
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo
2
55049, China
2
Shandong Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng
University, Liaocheng, 252059, P.R. China
3
Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang
Hangkong University, Nanchang, 330063, P. R. China
*
Corresponding author: Prof Jinsheng Zhao; Yu Xie
E-mail Address: j.s.zhao@163.com (J.S. Zhao); xieyu_121@163.com (Yu Xie)
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Abstract
Three novel π-conjugated indolo[3,2-b]carbazole derivatives, 2,8-di(thiophen-2-yl)-5,11-dioctyl-indolo[3,2-b]
carbazole (TICZ), 2,8-bis(4-ethylthiophen-2-yl)-5,11-dioctyl-indolo[3,2-b]carbazole (ETICZ) and
,8-bis(2-(3,4-ethylenedioxythiophene))-5,11-dioctyl-indolo[3,2-b]carbazole (EDTICZ), were synthesized and
2
electrochemically polymerized to afford the corresponding polymers. The effects of different substituent groups
on thiophen units in terms of optical, electrochemical, and electrochromic properties and the performance of
electrochromic devices (ECDs) were investigated in detail. Electrochemical and optical analysis demonstrated
that incorporation of strong auxochromic groups resulted in polymer PEDTICZ with low oxidation potentials,
red-shifted absorption peaks and corresponding narrow band gap values but is unfavorable for incorporating
ethyl groups. Spectroelectrochemical studies indicated an evident variation in the absorption spectra as well as
significant color changes for all of the polymer films upon application of an external potential. PTICZ had high
optical contrast with visible light (58.6%), fast switching speed (0.92 s), and satisfactory coloration efficiency
2
-1
(
167 cm ·C ). Of these three systems, the results for PEDTICZ were next best, and those for PETICZ were third.
Also, there was a larger contrast (82.7% and 57.8%) in the NIR region (1500 nm) for the PTICZ and PEDTICZ
films, respectively. Along with high coloring efficiencies, these results indicate that these systems are favorable
for many NIR applications.
Keywords: π-conjugated polymers; indolo[3,2-b]carbazole; electrochemical polymerization;
electrochromic;
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1
. Introduction
Over recent decades, great attention has been focused on π-conjugated polymers because of their
promising applications in electrochromic devices (ECDs) [1-4], smart windows, organic light-emitting
diodes (OLEDs) [5,6], photovoltaic cells (PCs) [7], organic field-effect transistors (OFETs) [8], and
supercapacitors (SCs) [9,10]. Such polymers are promising because of their great versatility, such as
solution processability, ease of fabrication, low cost, and flexibility [11,12]. Electrochromic polymers
(ECPs) are subjected to oxidation-reduction reaction under applied voltages, resulting in a stable and
reversible change in optical properties (transmittance, reflectance, and absorptivity, etc.), as well as a
reversible color change in appearance. Compared with other electrochromic materials that involve
transition metal oxides (WO , NiO, MoO , and V O ), inorganic counterparts (prussian blue), and small
3
3
2
5
organic molecules (viologen), research on ECPs is a newer area. However, ECPs attracted extensive
attention because of their fast speed switching, high contrast ratio, superior coloration efficiency,
long-term redox stability, and more importantly the easy accessibility to a variety of colors with low
band-gaps through structure modification [13-15].
The neutral color display of ECPs is crucial for their application in electrochromic devices. To
achieve multipurpose colors, substituents of polymer side chains with functional groups can be modified
or the conjugation plane in the molecular chains can be improved by combining the often-used thiophene
or thiophene derivatives with other aromatic units (e.g., benzene ring, naphthalene ring, pyrene,
triphenylamine, furan, and pyridine). Also, a D-A approach has been used to achieve multiple colors. A
series of methods have been used for structural modification to obtain desired ECPs in multiple colors
with improved electronic properties, tunable band gaps, and broad absorptions, but some of these
ECPs had poor stability and solubility. In more recent research, indolo[3,2-b]carbazole-based materials
were shown to have certain structural and electronic properties, like great stability and solubility [16],
2
-1 -1
high crystallinity [17], high charge carrier mobilities (up to 0.20 cm ·V ·S ) [18], and favorable
performance in OFETs [19-27] and PCs [16,24,28-37], because of the rigid, linear conjugate planes of the
molecular chain framework that are similar to pentacene and flexible side chain modification on the -NH
groups. Also, some results demonstrated that indolo[3,2-b]carbazole-based compounds usually have
highly efficient luminescence quantum yield, indicating that they have potential applications as promising
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optoelectronic materials for high-performance organic light emitting devices, such as electroluminescent
devices [38-41], electrophosphorescent devices [42,43], and organic light-emitting diodes [21,42-46].
However, until now, research on indolo[3,2-b]carbazole (ICZ) molecules in electrochromic
applications has been rare and should be further expanded [40,47-49]. Suzuki et al. [47] reported the
electron-donating dihydrobenzindolocarbazole (BICs) with a reversible electrochromic response; the
BICs exhibited clean conversion in UV/Vis/NIR spectra upon constant-current electrochemical oxidation.
Another indolo[3,2-b]carbazole-based compound was reported by Zhang et al.[40]. The compound had
distinct electrochromism properties that were achieved by linking diphenylamine moieties to the rigid
π-conjugated indolo[3,2-b]carbazole core, and the resulting material had two terminal redox-active amine
centers, which allowed access to reversible oxidation steps. It is worth noting that the above two
electrochromism systems were each detected in a solution of the monomers. Similar to diphenylamine,
the electron-rich thiophene units are considered to be ideal redox-active species for investigating basic
electron-transfer. They can be easily oxidized and stably reduced, accompanied by reversible color
changes. More recently, soluble ECPs [48] have been constructed on the basis of the alternating
distribution of bi(didecyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine)(ProDOT-didecyl) and the ICZ
unit. The electrochromic properties of the resulting polymer were studied, and a reversible change from
yellow-to-cyan color and high optical contrasts were found. Synthesis of ProDOT-didecyl is costly and
time consuming, and this restrict its use in practical applications. Alkyl substitution to the ICZ unit can
also increase the solubility of the monomers or polymers and make it possible to use other thiophene
derivatives, such as thiophene or 3,4-ethylenedioxythiophene (EDOT) as the counterpart units. In addition,
recent developments in our group have provided access to new neutral-state green polymers with
favorable electrochromic properties via the incorporation of ICZ units as donors [49]. Therefore, it could
be interesting to develop such ICZ derivatives and to evaluate their performance in ECPs.
It has been shown that extended conjugation in the main chain can be induced by incorporating
electron-withdrawing (e.g., benzothiadiazole, quinoxaline, and diketopyrrolopyrrole) [50,51] or
electron-donating moieties (e.g., phenyl or thiophene derivatives) [18,52] at the 2,8- or 3,9-positions of
the ICZ unit at the expense of the solubility. Fortunately, the alkyl side chain substituents at the N-5 and
N-11 positions can not only compensate for the loss of solubility but also enhance the self-organization of
the obtained materials. The molecular packing is then improved from three or four dimensional branches
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(
e.g., in the compound DDDBICZ [41]) to linear π-π stacking, which thus leads to improved charge
transport properties.
In this contribution, the conjugation length of indolocarbazole was extended by incorporating
thiophene, ethylthiophen, and EDOT units at the 2,8-positions of 5,11-dioctyl-indolo[3,2-b] carbazole,
and the resulting monomers are named TICZ, ETICZ, and EDTICZ, respectively. This
thiophene-terminated structure gives the monomer good electropolymerization performance and excellent
electrochemical activity, which makes it possible to obtain electrochromic polymers with distinct
properties in terms of color transitions, optical contrasts, and response speed driven by voltage changes
[
53]. Moreover, different molecular organizations of these one-dimensional linear ICZ derivatives in
thin-films have significant influences on the molecular packing motifs, charge transport, and spectral
absorption. This provides more information on the relationship between molecular structures and
photoelectric performances. The electrochromic devices based on ETICZ and EDTICZ exhibit superior
electro-, photo-, and switching-properties, and have outstanding permanent optical memory properties
because there was almost no transmittance change under open circuit conditions. Other excellent features
of the fabricated devices include their high optical contrast (as high as 55%) in the visible region and fast
switching speed (as low as 0.5 s). These features indicate that ICZ-based copolymers have bright
prospects in the application of electrochromic windows, e-papers, or eyewear devices.
2
. Experimental
2
.1. Materials and instrumentation
All of the chemicals used for monomer synthesis, including 4-bromophenylhydrazine hydrochloride
(
Sigma Aldrich, 99%), 1,4-cyclohexanedione (Sigma Aldrich, 98%), anhydrous ethanol (Sinopharm,
9%), glacial acetic acid (Sinopharm, 99%), concentrated sulfuric acid (Sinopharm, 98 %),1-bromooctane
Sigma Aldrich, 99%), tetrabutylammonium bromide (Sigma Aldrich, 99%), NaOH (Aladdin, 96%), N,
N-dimethylformamide (DMF) (Aladdin, 99.5%), thiophene (Sigma Aldrich, 99%), 3-ethylthiophene
9
(
(
Sigma Aldrich, 99%), 3,4-ethylenedioxythiophene (Sigma Aldrich, 99%), n-butyllithium (n-BuLi)
Aladdin, 2.2 in hexane), tributyltinchloride (Sn(Bu) Cl) (Aladdin, 96%),
(
M
3
palladium(II)bis(triphenylphosphine) dichloride (Pd(PPh ) Cl ,Sigma Aldrich, 95%) and toluene (Aladdin,
3
2
2
9
9%), were from commercial sources and were used without further purification except for
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tetrahydrofuran (THF). THF was refluxed with sodium and benzophenone until the solution turned blue
to provide absolute anhydrous THF.
For electrochemical polymerization and analysis, a mixture of chromatographic grade
dichloromethane (DCM) (Tedia, 99.9 %) and acetonitrile (ACN) (Tedia, 99.9 %) in a ratio of 1:1 and
combined with tetrabutylammonium hexafluorophosphate (TBAPF ) (TCI, 98%) of 0.2 M was directly
6
used as the electrolyte.
1
13
H NMR and C NMR of the monomeric materials were measured using a Varian AMX 400 NMR
Spectrometer (CDCl , 400 MHz) with tetramethylsilane (TMS) as an internal reference. Electrochemical
3
synthesis and analysis were carried out on a CHI 760C electrochemical workstation (China Shanghai
Chen Hua Instrument Co., Ltd.) using a standard three-electrode electrolytic cell. The three electrodes
were a platinum wire (0.5 mm diameter, the working electrode), a platinum ring (the counter electrode),
and a silver wire (the reference electrode). The thickness and surface roughness of the polymer films were
measured using a KLA-Tencor Alpha-Step D-100 profilometer. Microstructures of the polymer films
were observed using a Hitachi SU-70 thermal field emission scanning electron microscope. UV-Vis-NIR
absorption spectra were recorded on a Varian Cary 5000 spectrophotometer. Spectroelectrochemical,
kinetics, and colorimetry analysis of the films were measured using a Varian Cary 5000
spectrophotometer connected to the electrochemical testing system. However, an ITO coated glass slide
2
(
electrode effective area: 1.0 × 2.0 cm ) was used instead of a platinum wire as the working electrode. A
Canon Power Shot A3000 IS digital camera was used to collect the photographs with different colors of
the polymer films.
2
.2. Synthesis of monomers (TICZ, ETICZ, and EDTICZ)
2
,8-Dibromo-5,11-dioctyl-indolo[3,2-b]carbazole (5) was prepared according to the synthetic
methodologies [16,41,54] published in the literatures (Scheme 1).
: 69.8% yield. (400 MHz, CDCl , ppm) δ=8.31(d, 2H, ArH), 7.59(s, 2H, ArH), 7.56(dd, 2H, ArH),
5
3
7
.28(m, 2H, ArH), 4.36(t, 4H), 1.96(dd, 4H), 1.35(m, 20H), 0.86(t, 6H). (See Supplementary Materials
Figure S1)
Tributyl (thiophen-2-yl) stannane, tributyl (4-ethylthiophen-2-yl) stannane and tributyl
3,4-ethylenedioxythiophene-2-yl) stannane were also synthesized according to the literature methods
55].
(
[
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2
,8-Dibromo-5,11-dioctyl-indolo[3,2-b]carbazole (5) (1.96 mmol) and excess tributylstannane
compounds (6 mmol) were dissolved in 100 mL of anhydrous toluene. After purging with argon gas more
than five times and heating to 100 °C, Pd(PPh ) Cl (0.20 mmol) was added to one portion. The mixture
3
2
2
was left to react under reflux conditions for 36 h under an inert atmosphere. It was then cooled to room
temperature. Toluene was removed via distillation using a vacuum rotatory evaporator to obtain a crude
product, which was then purified using column chromatography; the eluent was a certain proportion of
the solution of n-hexane and dichloromethane.
1
TICZ (Yellow, Yield= 46.2%). H NMR(400 MHz, CDCl , ppm) δ=8.41(d, 2H ,ArH), 8.02(d, 2H,
3
ArH), 7.75(dd, 2H, ArH), 7.38(m, 4H, ArH), 7.26(s, 2H, ArH), 7.12(dd, 2H, ArH), 4.41(t, 4H), 1.96(m,
1
3
4
1
2
H), 1.30(d, 20H), 0.84(t, 6H); C NMR (101 MHz, CDCl , δ ppm) δ=146.03, 141.24, 136.47, 127.99,
3
24.82, 124.39, 123.40, 123.14, 122.89, 121.82, 117.79, 108.66, 99.14, 43.47, 31.85, 29.47, 29.24, 28.89,
7.42, 22.64, 14.10. (See Supplementary Materials Figure S2)
1
ETICZ (Pale yellow, Yield= 51.1%). H NMR(400 MHz, CDCl , δ ppm) δ=8.40(d, 2H, ArH), 8.03(s,
3
2
4
1
2
H, ArH), 7.73(dd, 2H, ArH), 7.39(d, 2H, ArH), 7.24(d, 2H, ArH), 6.86(s, 2H, ArH), 4.41(t, 4H), 2.71(m,
1
3
H), 1.96(dd, 4H), 1.45(d, 4H), 1.32(m, 22H), 0.85(t, 6H); C NMR (101 MHz, CDCl , δ ppm) δ=145.64,
3
41.20, 136.46, 125.12, 124.20, 123.01, 117.51, 108.57, 99.10, 43.45, 31.84, 29.46, 29.22, 28.89, 27.41,
3.90, 22.62, 14.66, 14.07. (See Supplementary Materials Figure S3)
1
EDTICZ (Dark yellow, Yield= 57.9%). H NMR(400 MHz, CDCl , δ ppm) δ=8.41(d, 2H, ArH),
3
8
1
1
2
.01(s, 2H, ArH), 7.86(d, 2H, ArH), 7.38(d, 2H, ArH), 6.24(d, 2H, ArH), 4.31(m, 12H), 1.94(m, 4H),
1
3
.34(m, 20H), 0.78(m, 6H); C NMR (101 MHz, CDCl , δ ppm) δ=142.27, 140.67, 136.75, 136.35,
3
24.65, 123.25, 122.96, 119.10, 118.05, 108.45, 99.12, 96.03, 64.83, 64.62, 43.38, 31.90, 29.60, 29.24,
8.90, 27.40, 22.63, 14.10. (See Supplementary Materials Figure S4)
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Scheme 1. Synthetic route for producing the monomers
3
. Results and discussion
3
.1 Electrochemical polymerization and characterization
Using cyclic voltammetry (CV) scans, it was determined that the three monomers were successfully
electropolymerized and attached to the surface of the working electrode (platinum wire). CV
measurements were carried out in 0.2 M TBAPF ACN / DCM (1:1 volume ratio) electrolyte containing
6
-1
0
.005 M monomer at a scan rate of 100 mV·s . The potentials were swept between 0 V and 1.2 V vs. Ag
wire for TICZ and EDTICZ, and 0 V to 1.5 V for ETICZ to obtain the CV curves (Figure 1). Also, the
CV curves during the electropolymerization make it easy to understand the process of monomer oxidation
and polymer deposition. From the first lap, the onset oxidation potentials (Eonset) of the monomers were
calculated to be 0.65 V for TICZ, 0.74 V for ETICZ, and 0.59 V for EDTICZ. After irreversible oxidation,
peaks were also observed at 0.79 V, 0.98 V, and 0.68 V, respectively. Generally, the oxidation potential is
negatively correlated with the electron donating strength of the donor unit within the D-A type polymer,
and this can be adjusted by incorporating additional auxochromic group on thiophen units. As expected,
when EDTICZ had EDOT, it exhibited the lowest monomer oxidation potential. In contrast, when ETICZ
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had the stronger electron donor ethylthiophen unit, the monomer had a more positive oxidation potential
than that of TICZ, and this can be ascribe to the steric-hindrance effect of the ethyl group.
Current density increased with the number of scans, and simultaneously, a new redox peak was
generated from the second cycle onward, indicating that electroactive films of corresponding polymers
were deposited on the platinum working electrode and that the thickness increased during the process [56].
The resulting polymers exhibited reversible p-type doping/dedoping properties with broad and evident
redox peaks at positive potentials. The CV curves of PTICZ revealed a redox couple at 0.60 V and 0.52 V
and also a should-like oxidation peak that is not conspicuous at about 1.06 V. Combined with strong
EDOT donor units, the resulting PEDTICZ has lower redox potentials located at 0.37 V and 0.32 V, and
after the complete oxidation and reduction process, there are peaks at 0.68 V and 0.66 V. The highest
redox potentials were observed for PETICZ (0.93 V and 0.85 V), and this is consistent with the previous
high oxidation potential of ETICZ. It can be seen from the CV results that the organization of thiophen
donor segments exerts important influence on the electro-oxidation and elctropolymerization properties of
the monomers or the corresponding polymers.
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Figure 1. Repeated potential scans for electropolymerization of (a) TICZ, (b) ETICZ and (c) EDTICZ in 0.2 M TBAPF
6
-1
ACN / DCM solution at a scan rate of 100 mV·s .
The sweeping potentials for six cycles were in accordance with the above method of
electropolymerization to afford PTICZ, PETICZ, and PEDTICZ polymer films coated on Pt wire. After
dedoping for five minutes, the electrochemical behaviors of the films were characterized using cyclic
-1
-1
voltammetry at differential scanning rates from 300 mV·s to 25 mV·s in the blank electrolyte solution
(Figure 2). Similar to the CV curves during electropolymerization, the CV curves of the three polymers
revealed pseudo-reversible redox process but with slight shifting of peak potentials at different sweeping
speeds. The oxidation peak for PTICZ continuously shifted in the negative direction (minor in magnitude)
with an increase in the scanning rate. For PTICZ, the polymer electrode was polarized at the neutral state
potential in the dedoping process, and the potential shifts might due to the "first scan" or "memory" effect,
which may result from structural reorganization of the polymer chains. The polymer PETICZ also
exhibited satisfactory redox reversibility, as indicated by the nearly intact redox potential within the
whole range of scanning rates. The polymer PEDTICZ exhibited two pairs of redox peaks with the first
pair of redox peaks slightly shifted in the positive direction. (The potential difference remained
unchanged.) Meanwhile, the second pair of redox peaks had almost no change in potential as the scanning
rates increased from 300 mV/s to 25 mV/s. The different redox behaviors of the three polymers indicate
that the structures of the polymers play a crucial role in the regulation of their electrochemical properties.
The current densities of the redox peaks have a linear correlation with the scan rates of cyclic
voltammetry (Figure S5). This further confirms that electroactive polymer films can be well attached to
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the working electrode surface and that the entire electrochemical redox process is not controlled by
diffusion [57]. Corresponding to the monomers, PEDTICZ showed the lowest initial oxidation potential
(Eonset) with a value of 0.19 V, compared to 0.42 V for PTICZ and 0.54 V for PETICZ. This is because
the stronger electron-donating ability of EDOT lead to the extended conjugation, which facilitated
electron delocalization and charge transport in PEDTICZ-conjugated backbone.
Figure 2. CVs plots as a function of the scanning speed for (a) PTICZ, (b) PETICZ, and (c) PEDTICZ films at 25
-1
-1
-1
-1
-1
-1
-1
mV·s , 50 mV·s , 100 mV·s , 150 mV·s , 200 mV·s , 250 mV·s , 300 mV·s .
By measuring the electroactivity retention after thousands of cycles between the neutral and oxidized
-1
states in the blank electrolyte solution at a scan rate of 200 mV·s , we also quantitatively characterized
the long-term switching cyclability of the films that were deposited on Pt wires via the same cyclic
voltammetry methods. The amount of electric charge in every scanning cycle can be obtained by
1
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integrating the CV curves. Figure 3 shows the CV curves of the three polymeric films at the first, 500th,
and 1000th cycles. From calculating the remaining charge in the repeated process of electrochemical
redox, PTICZ exhibited favorable cyclability with 94.8% of its electroactivity still retained after 500
cycles and 80.7% retained after 1000 cycles. The retained electroactivity was observed to be 89.8% and
8
5.6% for the PETICZ film after 500 and 1000 cycles, respectively, and was 93.7% and 88.7% for the
PEDTICZ film after 500 and 1000 cycles, respectively. PEDTICZ exhibited the highest cycling stability
58], followed by PETICZ and then PTICZ. But on the whole, all three polymers showed reasonable
[
robustness and redox stability even under ambient conditions, and this indicates that there is the
remarkable application potential in sealing ECDs [59].
Figure 3. First, 500th and 1000th CV curves of (a) PTICZ, (b) PETICZ, and (c) PEDTICZ films in 0.2 M TBAPF6
ACN/DCM (1:1) electrolyte. Scan rate of 200 mV/s.
3
.2 Spectroscopy of monomers and thin films
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The absorption spectra of the monomers and also of the neutral states for the conjugated copolymer
films are shown in Figure 4. All of the monomers were dissolved in dichloromethane for spectral analysis,
and the polymer films were attached to ITO-coated transparent conductive glass via potentiostatic
deposition at 1.2 V for TICZ and DETICZ and at 1.5 V for ETICZ with film formation electric charge of
-2
3
.0 × 10 C. Similarly, the films should be dedoped at the voltage of the neutral state in blank electrolyte
solution before absorption measurement.
As seen in Figure 4, the results of UV-Vis-NIR absorption analysis indicate that all three of the
monomers featured similar double absorption peaks that are shoulder to shoulder and correspond to π-π*
transition. The maximum absorption wavelength (λmax) were located at about 335 nm and 410 nm with a
slight wavelength shift from one another. Meanwhile, the optical band gaps (E ) that are based on the
g
absorption onset (λonset) of characteristic absorption peaks were calculated to be 2.79 eV for TICZ, 2.76
eV for ETICZ, and 2.75 eV for EDTICZ with little variation and no systematic regularity. Using density
functional theory (DFT) calculations, the HOMO and LUMO energy levels and corresponding theoretical
band gaps were determined and are detailed in Table 1. It should be noted that the data of E and the
g
absorption maximum of TICZ that were determined in this paper are comparable to those of the same
monomer (2TOIND) obtained from the literature [18]. Also, there are very small differences in the
HOMO and LUMO levels when the same calculation formula for HOMO is used according to oxidation
onset. From the molecular orbital profiles of monomers shown in Figure 5, the electronic clouds of the
HOMO are delocalized on the main chain aromatic conjugate construction, and the density distribution of
the electron cloud of the LUMO is localized on the indolo[3,2-b] carbazole units. However, none of the
electron density is on the side chains for both HOMO and LUMO, and this can facilitate intramolecular
charge transport [60, 61]. From the simulated data, the electron donating effects of the side chains on the
thiophene ring cause a simultaneous decrease in the HOMO and LUMO energy levels of the obtained
monomers, and the amplitude is approximately the same. Nevertheless, the optical band gaps that were
determined experimentally were somewhat lower than the theoretical data; the latter also have important
reference values.
There is a sharp contrast with the ultraviolet absorption spectra of the neutral state induced by the
polymers in film, and with the incorporation of different donor groups, there are obvious differences in
the characteristic peaks that correspond to the π-π* transition in terms of shape, number, and intensity.
The UV spectrum of PTICZ showed two separate absorption peaks at 347 nm and 415 nm, whereas the
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UV spectrum of PEDTICZ showed maximum absorption peaks at 334 nm, 438 nm, and 472 nm with an
apparent red shift observed for the long wavelength absorption peak. This was because of the stronger
electron donating ability of EDOT compared to that of thiophene. Spectral analysis indicates that PETICZ
had no other characteristic adsorption peak except at 361 nm. However, there was a blue shift, as
predicted, because of the steric effect caused by the presence of ethyl, and its electron donating effect
cannot compensate for this. Results indicated that, the kind and position of the substituents on the
thiophene ring greatly affected the conjugation length of the products. Also, EDOT has strong electron
donating effects, which facilitate electron transition and result in a red shift in the optical absorption
spectra. However, the steric hindrance effect of the ethyl substituent is more pronounced than its
electron-donating ability, and this is considered to be detrimental to the extension of the conjugation
length. The optical band gaps (E ) for the polymers were also calculated using the formula: E =
g
g
1
241/λonset [62-65], and these values were lower than those for the corresponding monomers because of
the extended conjugation effect that accompanies the increase in the length of molecular chains. As
expected, PEDTICZ has the lowest optical band gap value of 2.42 eV followed by PTICZ (2.49 eV), and
PETICZ has the largest value of 2.58 eV. In addition, the HOMO and LUMO energy levels of the
polymers were also obtained using the corresponding calculation methods, and the results are shown in
Table 1.
Figure 4. UV-Vis absorption spectra of the monomers. Inset: UV-Vis absorption spectra of the conjugated copolymer
films in neutral states.
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4

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Figure 5. Optimized geometric structures and molecular orbital profiles of monomers.
Table 1. Initial oxidation potential (Eonset), maximum absorption wavelength (λmax), peak absorption onset (λonset), optical
band gap (E ), and HOMO and LUMO energy levels for monomers and polymers.
g
a
b
c
d
d
d
Eonset,vs.
λmax(nm)/
λonset (nm)
Eg
HOMO
(eV)
LUMO
(eV)
E
HOMO
(eV)
LUMO
(eV)
Compounds
(
Ag-wire)(V)
(eV)
(eV)
4
08,333/
TICZ
0.65
2.79
-4.90
-2.11
-
3.54
-4.93
-1.39
445
334, 353,
2.70
e
e
e
2
TOIND
ETICZ
0.69 (vs SCE)
0.74
-5.39
-
-
-
e
e
411, 435
(film)
4
4
3
12,335/
49
13,337/
52
47,415/
98
61/
81
38,472/
12
2.76
2.75
2.49
2.58
2.42
-4.99
-4.84
-4.67
-4.79
-4.44
-2.23
-2.09
-2.18
-2.21
-2.02
3.53
-4.88
-1.35
4
EDTICZ
PTICZ
0.59
3.54
-4.73
-1.19
4
0.42
-
-
-
-
-
-
-
-
-
4
3
PETICZ
PEDTICZ
0.54
4
4
0.19
5
1
5

a
b
0/+
ACCEPTED MANUSCRIPT
Eg,op (eV) = 1241/λonset. Fe potential was also measured to calibrate the pseudo Ag wire electrode (0.55 V vs. Ag
c
wire electrode). HOMO positions were determined as follows: EHOMO(eV) = -4.8-(Eonset-0.55). LUMO = MOMO+Eg,op
.
d
e
Calculated from DFT calculations. Data in the literature (ref [18]).
3
.3 Morphology and thickness of polymer films
Three polymer films were synthesized on the surface of the ITO/glass electrode via potentiostatic
electropolymerization with a film charge of 0.1 C. The surface profiles, micromorphologies and structures
of the cleaned and dedoped films were then investigated using the step profiler and scanning electron
microscopy (SEM). In all three of the polymers, PTICZ exhibited the lowest surface roughness with an
average of 931 nm layer thickness (Figure 6). The average film thickness that was based on the scanning
curves supported by the step profiler were calculated to be 714 nm for PETICZ and 848 nm for PEDTICZ.
From the SEM results, on a microscopic view, the surface of PTICZ was relatively flat but still had cracks
and bulges of different sizes distributed on the surface, and the other two films presented coarse surface
with large pores and notches. These observations were consistent with the results of the stepping method.
In addition, the polymer layer coatings on conducting glass substrates (using the electrochemical
polymerization treatment) had some serious roughness and surface defects, which improved the migration
rate of doping anions for oxidation reduction reactions.
1
6

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Figure 6. Step profiler results and SEM (inset) images of (a) PTICZ, (b) PETICZ, and (c) PEDTICZ films spray-coated
on ITO/glass.
3
.4 Spectroelectrochemistry of polymer films
To observe the fully detailed optical behavior changes, the polymer films deposited on ITO-coated
transparent conductive glass were subjected to UV-Vis-NIR spectroscopy measurements by stepping up
the potential from the neutral to oxidized state. Along with the increase in the applied potential, oxidation
of the polymer molecules occurred and caused obvious changes in the absorption bands as well as the
colors. Figure 7 illustrates the spectroelectrochemical responses in both the UV-Vis-NIR absorption
spectra and the colors of the three films with a sweeping potential range from 0 V to 1.2 V vs. Ag wire for
PTICZ and PEDTICZ and from 0 V to 1.5 V for PETICZ. A quantitative measure for evaluating the color
changes during the oxidation process was also used; specifically, colorimetric assessment was conducted
according to the CIE 1976 L*a*b* system. The colorimetry results are shown in Figure 8 and are also
detailed in Table 2.
In the neutral form, the PTICZ film featured double absorption peaks at 347 nm and 415 nm,
corresponding to π-π* transition, resulting in willow yellow color, as indicated by its color coordinates a*
=
-4.07 and b* = 16.47 (Figure 8). Upon electrochemical oxidization, the intensity of the ground-state
absorption was progressively depleted, whereas the concomitant low-energy transitions at 585 nm and
over 1000 nm arose due to the formation of positively charged polarons and bipolarons. A further increase
in the applied potential to 1.2 V depressed the polaronic transition to some extent [66]. Also, there was a
dominant bipolaronic transition around 1400 nm in the NIR region [67], and this occurred in conjunction
with the color changing to navy blue. PETICZ and PEDTICZ had maximum absorption peaks at 361 nm
1
7

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and 438 nm, respectively, in their neutral states, which were pine and light olive colors, respectively. Also,
the spectroelectrochemical curves of both films show that the characteristic absorption peaks are
bathochromic-shifted with potential. For example, PTICZ is shifted because of its molecular oxidation
and the lattice distortion effect. In the fully oxidized states, the PETICZ and PEDTICZ films were indigo
(a* = -4.07, b* = 5.01) and deep blue (a* = -11.31, b* = -10.74), respectively, and more or less increased
in transparency according to the heightened L*color coordinates. As a whole, the color photographs and
the L*a*b* values were consistent with the UV-Vis-NIR absorption spectra at different applied potentials.
Also, the above detected reversible optical behavior changes prove that the chosen synthetic ICZ-based
molecular structure are feasible for electrochromic applications.
Figure 7. Spectroelectrochemical curves of (a) PTICZ, (b) PETICZ, and (c) PEDTICZ films at various applied potentials.
Inset: Color photographs of the corresponding polymer films in their neutral and oxidized states.
1
8

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Figure 8. The a* b* chromaticity coordinates and the colors of PTICZ, PETICZ and PEDTICZ films in the neutral and
oxidized states. (Triangles represent the neutral state; squares represent the oxidized state; blue represents PTICZ; red
represents PETICZ; green represents PEDTICZ).
Table 2. Chromaticity coordinates of PTICZ, PETICZ, and PEDTICZ polymer films in the neutral and oxidized states.
E, vs.
CIELAB 1976 (L*a*b*)
CIE 1931
Polymer
PDBTP
(
Ag-wire)(V)
L*
a*
b*
Color coordinates
0
96.18
92.36
88.07
81.38
79.00
70.17
-4.07
-1.36
-4.98
-4.07
-3.17
-11.31
16.47
-4.06
18.38
5.01
x=0.34; y=0.36
x=0.30; y=0.32
x=0.34; y=0.37
x=0.32; y=0.34
x=0.40; y=0.44
x=0.27; y=0.31
1.2
0
PDBETP
PDBDTP
1
1
.5
0
49.88
-10.74
.2
3
.5 Electrochromic switching of polymer films
To further understand the switching properties, transmittance changes (EC contrast, ∆% T), response
times, and coloring efficiencies (η) at different wavelengths (both in the visible and near-infrared region)
were examined using square-wave potential steps between the neutral and fully oxidized state with
potential residence time of 4 s, and the results are shown in Figure 9 and Table 3.
The kinetic results showed that of the evaluated polymers, PTICZ exhibited the largest contrast
(46% at 430 nm, 59% at 610 nm, and 83% at 1270 nm), and the highest coloring efficiencies were
calculated using the formula: η = ꢀOD/ꢀQ [68]. A decrease to the integrated contrast was observed by
incorporating the EDOT unit, and this was especially true for the peak wavelength in the near infrared
region, dropping from 83% in the neat polymer to 58% in the blend. Also, at the short wavelength peak
(430 nm), PEDTICZ had fully oxidized states with a transmittance change (ꢀ% T) of 42%, which is
slightly greater than the ꢀ% T at 640 nm, which is 40%. PEDTICZ also had lower coloration efficiencies
1
9

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2
-1
compared to PTICZ. The coloration efficiencies for PEDTICZ were calculated as 149 cm ·C , 141
2
-1
2
-1
cm ·C , and 97 cm ·C respectively. Still, there is an improvement in the switching speed to visible and
infrared light with response times for oxidation that are less than 1s. PETICZ revealed the lowest
integrated contrast due to its high degree of external steric strain from ethylthiophene, and this prevents
enhanced delocalization of charged states. Also, it has the poorest coloring capability and the most
unsatisfactory conversion speed from the neutral to oxidized state. Longer switching and degradation
experiments were performed for 2000s were investigated for the three polymer films at wavelength of 610
nm, 660 nm, and 640 nm, respectively. PEDTICZ exhibited relatively good switching cyclability with
7
2.6% of its optical contrast still retained after 2000 s. The retained optical contrast was observed to be
0.0% for PTICZ, but there was almost no residue for PETICZ film. This phase can then be followed by
3
the manufacture of an electrochromic device (ECD) based on PTICZ and PEDTICZ due to their superior
switching abilities to better understand the potential application.
Table 3. Optical contrasts (ꢀ% T), response times, and coloring efficiencies (η) of the polymer films at different
wavelengths.
λ
Optical contrast
Response time
(s)
Coloration efficiency
Polymer
PTICZ
2
-1
(
nm)
(ꢀ% T)
(η
, cm ·C )
4
30
610
270
46
59
83
24
30
38
42
40
58
1.2
0.9
1.3
2.8
1.8
2.2
0.9
0.8
1.0
250
167
270
137
53
1
3
90
PETICZ
660
1
280
73
4
30
141
97
PEDTICZ
640
1
300
149
2
0

ACCEPTED MANUSCRIPT
Figure 9. Square-wave potential step absorptiometry of (a) PTICZ, (b) PETICZ, and (c) PEDTICZ films monitored at
different wavelengths with a time interval of 4 s.
3
.6 Electrochromic properties of ECDs
As previously mentioned, because of the relatively outstanding switching properties of PTICZ and
PEDTICZ, we constructed typical sandwich-type ECDs (Figure 10) based on the research of anodically
colored polymers and complementary cathodic coloring PEDOT. In doing so, we achieved the highest
level of accessibility for high-performance with large photopic contrast, in which PTICZ or PEDTICZ
film were used as the working electrode (eletrochromic layer), a PEDOT film was used as the counter
electrode (charge storage layer), and a mixture of ACN: PC: PMMA: LiClO in a ratio of 70:20:7:3 was
4
processed into gel with high transmittance and conductivity for bonding together.
2
1

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Figure. 10. Electrochromic device structure and schematic diagram of color changes for the reduced and oxidized state.
Note that the given typical example is PTICZ as the electrochromic polymer
.
The PTICZ/PEDOT ECD was switched between -1.4 V and 1.4 V, and the working range for
PEDTICZ/PEDOT ECD was from -1.4 V to 1.0 V. The spectroelectrochemical results were plotted and
are shown in Figure 11a. PTICZ/PEDOT ECD was yellow in its reduced form at -1.4 V and had a similar
but slight bathochromic-shift absorption maxima compared to PTICZ centered at 353 nm and 428 nm.
This was a result of the neutral state willow yellow color of the PTICZ film and the transmissive light
blue color of PEDOT in the oxidized state (Figure 10: Left). Upon electrooxidation, obvious spectral
changes occurred in the visible region, in which the absorbances located at 300 nm to 500 nm decreased,
whereas the absorbance of the long wavelength increased. At 1.4 V, PTICZ was oxidized and PEDOT
was reduced, resulting in deep blue color for PTICZ/PEDOT ECD (Figure 10: Right) with one single new
broad absorption around 620 nm. Interestingly, the device also exhibited high contrast values that
exceeded 55% and had excellent long-term switching stability and superior switching speed 0.7 s for full
oxidation and 0.3 s for reduction at 620 nm (Figure 11b and Figure 11c). The optical memory of an
electrochromic device is an important parameter for applications because of its direct relevance to power
consumption [58, 69]. The transmittance changes over time for PTICZ/PEDOT ECD were recorded using
an impulse voltage of 1 s per 100 s interval at 620 nm under the driving voltage of –1.4 V or 1.4 V. As
seen in Figure 11d, the device exhibited outstanding permanent memory for the yellow reduced state
because there was almost no transmittance change even under open circuit condition for 100 s. For the
blue state, the transmittance increased from 23.5% to 24.5%, indicating the loss for full oxidation in the
absence of a supply voltage. Fortunately, there was complete recovery when current pulses were exerted.
As shown in Figure S6, the other device that contained PEDTICZ also switched from yellow to blue and
also showed similar variations in the absorption spectra. In addition, PEDTICZ/PEDOT ECD exhibited a
transmittance change of 38% when fully oxidized in short switching time (on the order of 0.5 s) at a long
2
2

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wavelength peak of 631 nm and simultaneously had improved optical memory for the blue oxidation
state.
Figure. 11. (a)Spectroelectrochemical curves of PTICZ/PEDOT ECD at various applied potentials. Inset: Color
photographs of the electrochromic devices in their reduced and oxidized states. (b) Square-wave potential step
absorptiometry of PTICZ/PEDOT ECD monitored at 620 nm for 1100 s. (c) Square-wave potential step
absorptiometry of PTICZ/PEDOT ECD monitored at 620 nm with time interval of 4 s. (d) Transmittance
changes over time for PTICZ/PEDOT ECD recorded at 620 nm under an impulse voltage of –1.4 V and
1
.4 V, respectively.
4
. Conclusion
Three novel π-conjugated monomers containing 5,11-dioctyl-indolo[3,2-b] carbazole and different
thiophene derivatives (thiophene, ethylthiophen and EDOT) were successfully synthesized. The
corresponding polymer films were obtained via electrochemical oxidation and showed reversible optical
behavior changes in color and absorption spectra by applying different driving potentials. Also, long-term
2
3

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redox stability and reversibility and favorable p-doping properties for the three polymers were observed
using detailed CV methods. Within this series, we identified that PEDTICZ (the copolymer that contains
a strong EDOT donor unit) combines a low oxidation potential, longer wavelength peak, and an extended
conjugation effect. However, the steric hindrance effect of the ethyl substituent on the thiophene ring is
more pronounced than its electron-donating ability, and this results in relatively poor optoelectronic
properties. In addition, the kinetic results indicated that PTICZ and PEDTICZ had superior switching
abilities that are suitable for the manufacture of electrochromic devices (ECDs). As expected, both
PTICZ/PEDOT ECD and PEDTICZ/PEDOT ECD exhibited high electrochromic contrasts, fast switching
speed, and outstanding permanent optical memory, and these observations indicate the remarkable
application potential of these materials in electrochromic windows, e-papers, or eyewear applications.
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China (51473074 and
2
1601079), and the Doctor Start-up Fund of Shandong University of Technology (4041/416024).
2
4

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