Electroactive (A3+B2)-type hyperbranched polyimides with highly stable and multistage electrochromic behaviors

Article abstract of DOI:10.1016/j.electacta.2017.10.036

Multicolored electrochromic materials have attracted intense attentions due to their diverse smart applications. In this work, we demonstrate a facile strategy to realize multistage visible/near-infrared (vis/NIR) electrochromism by designing and synthesi

Full text of DOI:10.1016/j.electacta.2017.10.036

Accepted Manuscript
Title: Electroactive (A3+B2)-type hyperbranched polyimides
with highly stable and multistage electrochromic behaviors
Authors: Ningwei Sun, Shiyao Meng, Ziwei Zhou, Danming
Chao, Ye Yu, Kaixin Su, Daming Wang, Xiaogang Zhao,
Hongwei Zhou, Chunhai Chen
PII:
S0013-4686(17)32095-9
DOI:
Reference:
https://doi.org/10.1016/j.electacta.2017.10.036
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4-10-2017
4-10-2017
Please cite this article as: Ningwei Sun, Shiyao Meng, Ziwei Zhou, Danming
Chao, Ye Yu, Kaixin Su, Daming Wang, Xiaogang Zhao, Hongwei Zhou,
Chunhai Chen, Electroactive (A3+B2)-type hyperbranched polyimides with
highly stable and multistage electrochromic behaviors, Electrochimica
Acta https://doi.org/10.1016/j.electacta.2017.10.036
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Electroactive (A3+B2)-type hyperbranched polyimides with highly stable and multistage
electrochromic behaviors
Ningwei Sun,a1 Shiyao Meng,a1 Ziwei Zhou,b1 Danming Chao,a Ye Yu,c Kaixin Su,a Daming Wang,a
Xiaogang Zhao,a Hongwei Zhoua *and Chunhai Chena
Corresponding author: Hongwei Zhou
Email address: zhw@jlu.edu.cn
a Alan G. MacDiarmid Institute of Jilin University, Changchun 130012, P. R. China
b
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun
130012, P.R. China
c
Leibniz Institut für Polymerforschung Dresden e.V, Institute of Physical Chemistry and Polymer
Physics, Hohe Str. 6, D-01005, Dresden, Germany
Graphical Abstract
Novel hyperbranched polyimides with star-shaped triphenylamine were synthesized, which exhibited
multistage visible/near-infrared electrochromic and supercapacitive properties.
1

1 The authors contributed equally to this work.
Abstract
Multicolored electrochromic materials have attracted intense attentions due to their diverse smart
applications. In this work, we demonstrate a facile strategy to realize multistage visible/near-infrared
(vis/NIR) electrochromism by designing and synthesizing two kinds of novel hyperbranched
polyimides (HBPIs) from a novel triamine as an A3 monomer and two dianhydrides as B2 monomers.
The resulting HBPIs exhibit excellent solubility and thermostability. Cyclic voltammograms of the
HBPIs films reveal four reversible redox couples along with five color conversions (colorless-yellow-
red-indigo-black). Furthermore, the HBPIs exhibit superb electrochromic performances including
continuous modulation over the entire visible region, short response time, high optical contrast, high
coloration efficiency, excellent switching stability and long-term optical memory. Accordingly, these
HBPIs are promising materials in the applications of electrochromism and even endowing
difunctionality combining with supercapacitance.
Keywords: HBPIs; Triphenylamine; Multi-electrochromism
1. Introduction
Electrochromic materials with redox-active species achieve reversible changes in
absorption/transmittance upon electrical stimulus. The controllable optical properties of
electrochromic materials have garnered increasing interest spanning the field of smart windows,
displays, sunglasses, anti-glare rear-view mirrors and camouflage materials [1-7]. Compared with
traditional inorganic oxides, the organic electrochromic materials possess numerous advantages,
including facile processability, fast switching time, high coloration efficiency and fine-tuning of the
band gap with structure modification [8-14]. Early studies of electrochromic materials mainly focused
2

on the change of visible light. Recently, the field of modulation of near-infrared light has also received
great attention, and the NIR electrochromism is desirable for thermal control in buildings or spacecraft,
data storage and optic-fiber communication [15-17]. Various materials including conjugated polymers,
cyclometalated complexes, composites of transition metal oxide (TMO)-nanocrystal and derivatives
with p-phenylenediamine structures have been exploited as NIR electrochromic materials [18-26].
Additionally, electrochromo-supercapacitors bridging the gap between the electrochromism and
energy storage are attracting scientists’ interest due to their smartness and integration [27-29]. Yet to
meet the requirements of commercial applications, there are still some crucial factors need to be
improved including high optical contrast, long-term stability, high specific capacitance and facile
processing technology, which will greatly promote the development of versatile materials.
Triphenylamine derivatives with fascinating photo- and electro-active properties have been
widely applied in organic solar cells, hole transporters, organic light emitting diodes (OLEDs)
and memory devices [30-35]. Moreover, triphenylamine-containing polymers have been
recently developed as attractive electrochromic materials due to their facial oxidation to
corresponding radical cations; the redox process is highly stable as long as the para-position
of triphenylamine is protected by electron-donating substituent such as methoxy group [36-
43]. Generally, the more electroactive species of triphenylamine were introduced, the more
stages of electrochromism occurred during the redox process [44-46]. However, it is difficult
to introduce more than two electroactive species for traditional linear materials because of the
intricate multi-step synthetic procedures. To overcome this limitation, synthesis of
hyperbranched polymers containing star-shaped triphenylamine will be an effective and facile
method. The trifunctional monomer could be imparted with three electroactive triphenylamine
3

through a one-step synthesis, which will greatly simplify the synthesis and purification
technique. Moreover, the unique loose packing of hyperbranched architecture could enable a
fast redox process and enhance the optical response stability by facilitating the ion
insert/extraction [47-49].
In this work, we describe two kinds of novel (A3+B2)-type hyperbranched polyimides (HBPIs)
with a new triamine containing star-shaped triphenylamine, “4, 4’, 4’’-tris[4-aminophenyl(4-
methoxyphenyl)amino]triphenylamine”, as an A3 monomer and two dianhydrides as B2
monomers. Introducing star-shaped triphenylamine with para-substituted methoxy group
along the HBPI backbone is expected to obtain superior electrochromic materials with excellent
integrated performances, such as low response time, multistage modulations in the vis/NIR
regions, low trigger voltage and high stability. Moreover, the capacitive properties of the HBPIs
were also initially investigated to explore their potential application as smart supercapacitors.
Figure 1
2. Experimental section
2.1 Materials
Tris(4-iodophenyl)amine (TCI), 4-fluoronitrobenzene (Acros), 4-methoxyaniline (TCI),
potassium carbonate (TCI), copper powder (Acros), 18-crown-6-ether (TCI), triethylamine
(Acros), hydrazine monohydrate (TCI), 10% palladium on charcoal (Pd/C, TCI), acetic
anhydride (TCI) were used as received. N-methyl-2-pyrrolidinone (NMP), dimethyl sulfoxide
(DMSO), o-dichlorobenzene (TCI) and pyridine (Acros) were dried using 4 Å molecular sieves
prior to use. Commercially available tetracarboxylic dianhydrides including 3,3’,4,4’-
4

oxydiphthalic anhydride (ODPA, TCI) and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane
dianhydride (6FDA, TCI) were used after being dried under vacuum at 140 °C for 12 h.
Tetrabutylammonium perchlorate (TBAP, Acros) was recrystallized twice from ethanol under
nitrogen atmosphere and then dried under vacuum before use. All other commercially available
reagents and chemicals were used without further purification.
2.2 Synthesis of 4-Methoxy-4’-nitrodiphenylamine
4-Methoxy-4’-nitrodiphenylamine was synthesized according to the reported procedure [38]
with simple modifications. In a 1000 mL three-neck round-bottom flask, a mixture of 60.0 g
(487.5 mmol) 4-methoxyaniline, 52.9 g (375.0 mmol) 4-fluoronitrobenzene, 50.0 g
(487.5 mmol) triethylamine and 350 mL dried DMSO was heated at 90 °C for 36 h under
nitrogen atmosphere. After cooling to ambient temperature, the mixture was poured into ice
water to precipitate orange powder. Then the crude product was recrystallized from
DMSO/ethanol (1:3) to give orange needle crystals (82 % of yield, mp=156 °C, literature [38]
153 °C by DSC at 10 °C/min). 1H NMR (300 MHz, DMSO-d6, δ, ppm): 9.12 (s, 1H), 8.05 (d,
J = 9.3 Hz, 2H), 7.18 (d, J = 8.9 Hz, 2H), 6.98 (d, J = 8.9 Hz, 2H), 6.88 (d, J = 9.3 Hz, 2H),
3.76 (s, 3H).
2.3 Synthesis of “4, 4’, 4’’-tris[4-nitrophenyl(4-methoxyphenyl) amino]triphenylamine
To a 250 mL round-bottom flask equipped with a stirring bar, 10.0 g (16.0 mmol) of tris(4-
iodophenyl)amine, 17.6 g (72.0 mmol) of 4-methoxy-4’-nitrodiphenylamine, 6.1 g
(96.0 mmol) of copper powder, 13.3 g (96.0 mmol) of K2CO3 and 2.5 g (9.6 mmol) of 18-
crown-6-ether were added along with 50 mL dried o-dichlorobenzene, which were then stirred
at 180 °C for 24 h under nitrogen atmosphere. Then, the reaction mixture was immediately
5

filtered, and the filtrate was cooled down to room temperature to precipitate red powder. The
crude product was recrystallized from DMF/ethanol (2:1) to obtain 13.0 g of trinitro compound
1
(83% yield, mp=271 °C). FT-IR (KBr): 1310 cm-1, 1582 cm-1(-NO2 stretch). H NMR (300
MHz, DMSO-d6, δ, ppm): 8.05 (d, J = 9.3 Hz, 6H, Ha), 7.37–7.20 (m, 12H, He+Hc), 7.13 (d, J
= 8.9 Hz, 6H, Hf), 7.05 (d, J = 8.8 Hz, 6H, Hd), 6.75 (d, J = 9.2 Hz, 6H, Hb), 3.79 (s, 9H, -
13
OCH3). C NMR (75 MHz, DMSO-d6, δ, ppm): 157.78, 153.78, 144.48, 139.86, 138.28,
137.34, 128.82, 127.76, 125.58, 125.05, 115.57, 115.39, 55.35.
2.4 Synthesis of “4, 4’, 4’’-tris[4-aminophenyl(4-methoxyphenyl) amino]triphenylamine
In a 500 mL round-bottom flask equipped with a stirring bar, 6.0 g (6.2 mmol) of trinitro
compound and 1.0 g of 10% Pd/C were dispersed in 150 mL of 1,4-dioxane. After heating the
suspension solution to reflux, 15.5 g of hydrazine monohydrate was added to the mixture
slowly. Then the mixture was stirred at the reflux temperature for further 48 h. Pd/C was
removed by immediately filtered, and the filtrate was cooled under a nitrogen flow to grow
faint yellow crystals. The crystals were collected by filtration and dried under vacuum at 80
°C, yielding 3.9 g of the desired triamine compound in 71 % with a melting point of 265 °C.
1
FT-IR (KBr): 3369 cm-1, 3446 cm-1(-NH2 stretch). H NMR (300 MHz, DMSO-d6, δ, ppm):
7.04–6.71 (m, 24H, Hc+Hb+Hd+he), 6.67 (d, J = 8.9 Hz, 6H, Hf), 6.52 (d, J = 8.6 Hz, 6H, Ha),
13
4.99 (s, 6H, -NH2), 3.70 (s, 9H, -OCH3). C NMR (75 MHz, DMSO-d6, δ, ppm): 155.01,
145.96, 143.99, 141.65, 140.83, 136.46, 127.57, 125.10, 124.44, 121.16, 115.35, 115.06, 55.65.
Scheme 1
2.5 Synthesis of HBPIs
6

Two kinds of HBPIs were prepared by two-step condensation polymerizations (Scheme 2), and
the resulting polymers were abbreviated to ODPA-HBPI and 6FDA-HBPI, respectively. In a
typical procedure, 0.2221 g of 6FDA (0.50 mmol) was dissolved in 5.5 g of DMAc under
nitrogen atmosphere, to which, the triamine (0.2203 g, 0.25 mmol) in 4.0 g DMAc was added
dropwise over 1 h. The solid content of the reaction solution was about 4.5 %. After 12 h, 2 mL
of acetic anhydride and 1 mL of pyridine were added to the resulting polyamic acid solution,
which was heated to 120 °C and stirred for further 12 h. The resulting polymer solution was
then precipitated from 200 mL of stirred methanol, and the orange precipitate was collected by
filtration, washed thoroughly with ethanol, and dried under vacuum at 100 °C for 10 h and 250
°C for 2 h. FT-IR (KBr): 1378, 1720 and 1785 cm-1 (imide group) 1H NMR (300 MHz, DMSO-
d6, δ, ppm): 8.09 (d, 6H), 7.89 (s, 6H), 7.70 (d, 6H), 6.95 (s, 36H), 3.71 (s, 9H).
Scheme 2
2.6 Measurements
Fourier transform infrared (FT-IR) spectra were recorded through a Bruker Vector 22
spectrometer at a resolution of 4 cm-1 in the range of 400-4000 cm-1. Nuclear magnetic
resonance (NMR) spectra were run on a BRUKER-300 spectrometer in deuterated DMSO.
Weight-average molecular weights (Mw) and number-average molecular weights (Mn) were
obtained via gel permeation chromatographic (GPC) analysis on the basis of polystyrene
calibration on a PL-GPV 220 instrument with DMF as an eluent at a flow rate of 1.0 mL/min.
Differential scanning calorimetric (DSC) analysis was performed on a TA instrument DSC
Q100 at a scanning rate of 10 °C/min in a nitrogen flow of 50 mL/min. Thermo gravimetric
analysis (TGA) in the temperature of 100-800 °C was studied on the TA 2050, with a heating
7

rate of 10 °C/min under nitrogen atmosphere. Cyclic voltammetry (CV), galvanostatic charge-
discharge (GCD) tests and electrochemical impedance spectroscopy (ESI) were conducted in
a three-electrode cell with a CHI 660e electrochemical workstation. The working electrode was
prepared by drop-coating 0.1 mL of polymer solutions (3mg/mL in DMAc) onto an ITO glass
substrate and then dried in vacuum (the area of polymer films is about 1 cm2 and the thickness
is about 800-900 nm). A platinum wire was used as an auxiliary electrode and an Ag/AgCl,
KCl (sat.) was used as a reference electrode. 0.1 M TBAP in dried CH3CN was used as
supporting electrolyte. Ultraviolet-visible (UV-vis) spectra were measured using a Shimadzu
UV 3101-PC spectrophotometer. CIE Yxy chromaticity coordinates were obtained from the
software of UVPC Color Analysis. The molecular simulation of the model compound was
performed at the time-dependent density functional theory (TD-DFT) with a method of the
B3LYP/6-31G (d, p).
3. Results and discussion
3.1 Synthesis of monomers and HBPIs
The star-shaped triamine with four electroactive triphenylamine units was selected to prepare
the HBPIs due to the simplified synthetic route of introducing multiple triphenylamine [41]
and the advantages are depicted in Figure 1. The triamine was synthesized via a three-step route
(Scheme 1): first, 4-methoxy-4’-nitrodiphenylamine was synthesized by the reaction of 4-
methoxyaniline and 4-fluoronitrobenzene under triethylamine assistance; second, Ullmann
reaction was employed to obtain trinitro monomer from 4-methoxy-4’-nitrodiphenylamine and
tris(4-iodophenyl)amine; third, the target triamine monomer was successfully synthesized by
the Pd/C-catalyzed hydrazine reduction of the trinitro monomer. The structures of the resulting
8

monomers were systematically analyzed and identified by FT-IR and NMR spectroscopic
techniques. From the FT-IR spectra (Figure S1), we could see that the characteristic bands of
nitro groups at 1582 and 1310 cm-1 (-NO2 asymmetric and symmetric stretching) disappeared
after reduction, and N-H stretching absorptions pairs appeared in the region of 3300-3500 cm-
1, meaning the formation of the target triamine. Figure 2 illustrates the 1H NMR spectra of the
trinitro and triamine monomers with the assignments for proton signals, which agree well with
their target molecular structures. Furthermore, the complete transformations from nitro groups
to amino groups were also supported by the high field shift of the aromatic protons and the
resonance signals of amino protons at around 4.99 ppm [44]. Thus, all the spectral analyses
verified that the new star-shaped triamine was successfully synthesized.
Next, as shown in Scheme 2, two kinds of HBPIs were prepared from the as-prepared star-
shaped triamine and two dianhydrides (ODPA and 6FDA) through a two-step polymerization.
To avoid gelation in the polymerization process [50], the triamine was added to the dianhydride
slowly with a monomer molar ratio of 1:2 (the molar ration between the amino and anhydride
groups was 3:4), and the resulting anhydride-terminated polyamic acids were subsequently
converted into polyimides by the chemical imidization. The molecular weights of these HBPIs
were determined by GPC, showing the number-average molecular weights (Mns) in the range
of 26100-30800. The FT-IR spectra of these HBPIs exhibited characteristic absorption bands
at 1785 (C=O asymmetrical stretching), 1720 (C=O symmetrical stretching) and 1378 cm-1 (C-
N stretching) without the absorption band at around 1680 cm-1 (polyamic acid), which indicated
1
that these HBPIs were fully imidized (Figure S2). Furthermore, typical H NMR spectrum of
9

6FDA-HBPI was illustrated in Figure S3 and all the proton signals were well assigned to the
hydrogen atoms in the repeating unit.
Figure 2
3.2 Solubility and thermal properties
To generally reveal the inherent physical properties of the HBPIs, the solubility and thermal
tests were systematically studied. The former was conducted by dissolving 10 mg of HBPI
powders in 1 mL of various organic solvents, with the corresponding solubility data listed in
Table 1, from which we could see that the as-prepared HBPIs exhibited excellent solubility in
polar solvents such as NMP, DMAc, DMF and DMSO, bringing great benefit to the fabrication
of large-scale thin film devices in practical optoelectronic applications. Notably, the 6FDA-
HBPI with hexafluoroisopropylidene structure revealed even more enhanced solubility in less
polar solvents like THF and CHCl3. We attributed the outstanding solubility to the introduction
of bulky star-shaped triphenylamine groups and the resulting loose packing of hyperbranched
structure [51]. On the other hand, the thermal behaviors of the HBPIs were evaluated by DSC
and TGA (Figure S4). Their glass transition temperatures were observed at 252-286 °C in the
DSC curves depending on the stiffness of the polymer structures. TGA curves of the two kinds
of HBPIs by heating under nitrogen atmosphere both showed excellent thermal stability with
10% weight loss in the range of 490-504 °C. Their carbonized residue at 800 °C accounted for
over 53%, which could be ascribed to the high aromatic content.
Table 2
3.3 Optical and electrochemical properties
10

We further investigated the basic optical properties of polymer solutions and drop-cast films
by UV-vis spectroscopy with the detailed data summarized in Table 2. Their absorption
maxima located at ultraviolet region (315, 345 and 303, 339 nm) are attributed to the π–π*
transition of star-shaped triphenylamine [42]. The optical energy band gaps (Eg) lay between
2.86 and 2.88 eV, which were estimated from λonset of the HBPI films (Figure S5). As to the
electrochemical properties, CV measurements were performed to investigate the
electrochemical behaviors of the trinitro monomer, triamine monomer, and HBPI films in an
electrolyte solution of 0.1 M TBAP/ CH3CN with an Ag/AgCl reference electrode and a Pt
wire counter electrode. As shown in Figure 3 (a, b), the trinitro monomer exhibited four pairs
of reversible redox current peak at 0.75/0.64 V, 0.94/0.84 V, 1.41/1.05 V and 1.48/1.21 V,
respectively; while the triamine monomer exhibited five redox couples at 0.26/0.14 V,
0.54/0.44 V, 0.69/0.58 V, 0.93/0.71 V and 1.05/0.81 V, respectively. Given that the redox
reactions are sensitive to the electronic effect of different substituents [52], the triamine
monomer with electron-donating amine group was more easily oxidized in comparison with
the trinitro monomer containing electron-withdraw nitro group. In the case of the HBPIs, the
electronic effect of imide groups locates between those of the nitro and amine groups and thus
the oxidation potentials of the HBPIs occurred moderately (Figure 3c). In the quest to identify
the oxidization order of the four electroactive species in the HBPIs, TD-DFT calculation was
carried out to simulate the molecular orbitals of the model compound. As depicted in Figure
3f, the highest occupied molecular orbital (HOMO) was mainly located at the center
triphenylamine, while the HOMO-1 and HOMO-2 were spread over the side triphenylamines,
which indicated that the electrochemical redox reaction initially occurred in the center
11

triphenylamine and subsequently in the side triphenylamines (Figure 3e). Accordingly, in the
first oxidation stage of the CV measurement of 6FDA-HBPI (Figure 3d), corresponding to the
redox process of center triphenylamine, the 6FDA-HBPI thin film exhibited a low onset
potential of 0.33 V and highly reversible CV behaviors for 5000 cycles, which could be
ascribed to the strong electron-donating effect of the side triphenylamine [52]. Besides, the
HOMO levels of these HBPIs were estimated to be 4.77-4.78 eV on the basis of their onset
oxidation potentials and ferrocene/ferrocenium reference. The corresponding lowest occupied
molecular orbital (LUMO) levels were estimated by HOMO levels and Egs [53], which were
calculated to be 1.90-1.91 eV (LUMO = HOMO-Eg). Notice, however, the optical band gap
may be quite different from the electrochemical band gap [54], and thus the LUMO levels were
roughly estimated. All the details of electrochemical data as well as HOMO and LUMO levels
were gathered in Table 2.
Figure 3
Table 2
3.4 Spectroelectrochemical analysis
Considering the interesting phenomenon that the HBPI films switched between five colors
reversibly during the electrochemical oxidation, in situ UV-vis-NIR spectroelectrochemical
measurements were performed to compare the optical properties of the HBPIs at different
applied potentials. These two kinds of HBPI films exhibited similar electrochromic properties
with the typical absorption optical changes of 6FDA-HBPI shown in Figure 4 (ODPA-HBPI
in Figure S6). Upon the electro-oxidation steadily arising from 0.0 V to 0.6 V, the intensity of
the characteristic absorption peak at 339 nm for the neutral form of 6FDA-HBPI decreased
12

gradually, while two new well-defined absorption bands at 443 and 1250 nm grew up,
respectively. The change in visible light range was attributed to the formation of monocation
+
radical 6FDA-HBPI in the center star-shaped triphenylamine unit and the broad NIR
absorption resulted from the IV-CT excitation between states in which the positive charge is
centered at different N atoms [55]. As the applied potential was further increased to 0.8 V, the
absorption peak at 443 nm decreased with a broad band at 446 nm and a new NIR band centered
at 1080 nm emerged, corresponding to the formation of 6FDA-HBPI 2+. For the third oxidation
stage from 6FDA-HBPI 2+ to 6FDA-HBPI 3+, the absorption band in the visible region shifted
to 520 nm and the strong IV-CT band at 1050 nm could be still observed. When the potential
was applied up to 1.3 V, the 6FDA-HBPI 4+ revealed a broad absorption in the whole visible
region and the NIR absorption band decreased significantly due to the fully oxidation of four
triphenylamines in 6FDA-HBPI [56]. Accordingly, the multiple electro-switchings in both
visible and NIR regions were realized, and the continuous modulation covered the entire visible
region. The series of spectra changes revealed a good consistence with the distinguished color
changes of the 6FDA-HBPI film which switched from near colorless neutral state (CIE Yxy:
112.12, 0.3273, 0.3440) to yellow first-oxidized state (CIE Yxy: 74.64, 0.4249, 0.4793), red
second-oxidized state (CIE Yxy: 35.57, 0.3957, 0.3664), indigo third-oxidized state (CIE Yxy:
26.89, 0.3382, 0.2813) and black fourth-oxidized state (CIE Yxy: 12.31, 0.3256, 0.3317)
(Figure 4). Notably, the achievement of “transmissive to black” electrochromism is promising
in the application of energy-efficient displays [57].
Figure 4
3.5 Electrochromic performance
13

To evaluate the prospects of the as-prepared HBPIs in electrochromic applications, some key
parameters including stability, switching time, optical contrast, coloration efficiencies and
optical memory were systematically analyzed and disclosed in detail. The electrochromic
switching studies were investigated by chronoamperometric-transmittance coupling analysis
with the transmittance at the given wavelengths monitored as a function of time. As shown in
Figure 5, four electrochromic switching processes were investigated in the visible region. For
the first two redox stages (0.00-0.60 V and 0.00-0.80 V), the 6FDA-HBPI film exhibited
excellent stability for 100 continuous cycles. With the switching potential applied higher (1.10
V), the electrochromic switching stability of the polymer film started to reduce gradually. By
further applying increased positive potential value up to 1.30 V, the switching stability got
weaker with the optical contrast decreasing obviously. The lower switching stability may be
ascribed to the electroactivity loss of the polymers at high potentials [6]. Optical contrasts
during the switching processes were found to be 63%, 42%, 44% and 49%, respectively. While
attaining 90% of the total transmittance change, the response time for coloring/blenching were
2.6/1.9 s, 1.8/4.3 s, 2.3/1.8 s and 4.9/3.1 s as summarized in Table 3. Compared with some liner
polymers [58], the fast switching speed of 6FDA-HBPI is partly attributed to its hyperbranched
architecture, which results in the favourable loose packing of the polymer chains. Moreover,
the coloration efficiency (CE) is also an essential parameter for electrochromic materials,
which is estimated through the following equation: CE = log (Tblenched/Tcolored)/Qd
(Tblenched/Tcolored: transmittance in the blenched/colored state; Qd: charge injected/ejected into
the polymer film per unit area during a redox step). The CE values of 6FDA-HBPI was
calculated to be 196 cm2 C-1 at 430 nm, 92 cm2 C-1 at 430 nm, 86 cm2 C-1 at 520 nm, and 89 cm2
14

C-1 at 600 nm, respectively. Additional, the optical switching of 6FDA-HBPI film in NIR
region was also studied, which excellent stability for the first two redox stages consistent with
the results in the visible region (Figure S7). Figure S8a depicted the long-term stability test by
keeping 1000 s for the coloring/blenching processes, which also exhibited highly stable
electrochromic behaviors. On the other hand, optical memory (open-circuit memory) describes
the ability of a material to retain its colored state upon removing the external bias, and here,
the long-term memory of 6FDA-HBPI electrochromic film was measured by monitoring the
transmittance changes of open-circuit states at 1250 nm and 1080 nm after applying 0.6 V and
0.8 V for 20 s, respectively. Very slight dropping of transmittance contrast could be observed
in the absence of potential for 10000 s with only 3% decay at 1250 nm and 7% decay at 1080
nm (Figure S8b). Hence, the HBPIs exhibited excellent electrochromic properties and their
NIR optical modulations may endow them with a promising application in the buildings for
environment control of heat gain or loss [59].
Figure 5
Table 3
3.6 Capacitive properties
The highly stable and reversible electrochemical redox properties of the HBPIs will enable them as
promising pseudocapacitors [60]. Therefore, the galvanostatic charge-discharge (GCD) curves at
different current densities and electrochemical impendence spectra (EIS) of the 6FDA-HBPI film were
performed to investigate their potential capacitive properties. As shown in Figure 6a, the GCD tests
showed two plateaus during the charging/discharging processes which are in consistence with the
peaks in the CV experiments. The specific capacitance calculated from the discharged part of GCD
15

experiment is 69 F g-1 at a current density of 0.5 A g-1. Compared with other similar materials reported
in the literature [61-65], the capacitance value of 6FDA-HBPI is moderate. However, the capacitance
of the polymers could be further enhanced by increasing the pore density of polymer films by physical
or chemical modifications [66], which is currently underway in our lab. The EIS (frequency, 0.01-
100000 Hz) was simulated using an equivalent circuit (Figure 6b). The Nyquist plots include a straight
line in the low frequency region related to the ion diffusion process and a semicircle in the high
frequency region derived from the interfacial charge-discharge resistance (Rct). The impedance
parameters of Rct and double layer capacitance (Cdl) by fitting the experiment spectra with the
equivalent circuit were found to be 86 Ω and 5.6×10-8 F. In view of the fact that the charge-discharge
process accompanied with the color changes, these novel HBPIs may find an application as smart
electrochromo-supercapacitors in the future.
Figure 6
4. Conclusion
In summary, we have readily synthesized two kinds of HBPIs with star-shaped triphenylamine
via a facile route to introduce multiple redox species. In addition to the excellent solubility and
thermal stability, the resulting HBPIs also exhibited multistage light modulations in vis/NIR
regions. The electrochromic performances of these HBPIs revealed continuous modulation
over the entire visible region, high electroactive reversibility, high optical contrast (63 % at
443 nm), low response time (2.6 s/1.9 s at 443 nm), high coloration efficiency (196 cm2 C-1 at
443 nm), outstanding stability and optical memory properties at once. On the other hand, the
preliminary capacitive tests of the HBPI films demonstrated an acceptable specific capacitive
value of 69 F g-1. Considering these crucial factors mentioned above, the bifunctional HBPIs
16

with star-shaped triphenylamine are promising materials in the applications of electrochromic
displays and electrochromo-supercapacitors. Overall, the comprehensive investigations of
these HBPIs provide a deep understanding of their electro-optic effect, meanwhile paving the
way for future intelligence applications.
Acknowledgments
Y. Yu thanks the Alexander von Humboldt Foundation for support in the form of the AvH-
fellowship.
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26

Scheme 1 Synthesis of the triamine with star-shaped triphenylamine.
Scheme 2 Synthesis of HBPIs.
Figure 1 Design concept of HBPIs with star-shaped triphenylamine.
1
Figure 2 H NMR spectra of trinitro (a) and triamine (b) monomers.
Figure 3 CV diagrams of trinitro (a), triamine (b) and HBPI thin films coated on ITO glass
slide (c) in 0.1 M TBAP/CH3CN at a scan rate of 50 mV/s. (d) CV diagrams of 6FDA-HBPI
thin film coated on ITO glass slide over 5000 cyclic scans for the first two redox processes in
0.1 M TBAP/CH3CN at a scan rate of 50 mV/s. (e) Presumptive order of oxidation reactions
in the star-shaped triphenylamine segment of the model compound. (f) Calculated molecular
orbitals of the model compound (TD-DFT method at the B3LYP/6-31G (d, p)).
Figure 4 (a) Colors of 6FDA-HBPI film at different potentials. (b) UV-vis-NIR
spectroelectrochemistry of 6FDA-HBPI thin film coated on an ITO glass slide in 0.1M
TBAP/CH3CN at different applied potentials from 0.0 to 1.3 V. (c) CIE 1931chromaticity
diagram of 6FDA-HBPI at different potentials.
Figure 5 Electrochromic switching of 6FDA-HBPI thin film coated on the ITO glass slide in
0.1 M TBAP/CH3CN with a cycle time of 20 s. (a) Optical switching between 0.00 V and 0.60
V with the wavelength monitored at 443 nm; (b) Optical switching between 0.00 V and 0.80 V
with the wavelength monitored at 446 nm; (c) Optical switching between 0.00 V and 1.10 V
with the wavelength monitored at 520 nm; (d) Optical switching between 0.00 V and 1.30 V
with the wavelength monitored at 600 nm.
Figure 6 (a) Galvanostatic charge-discharge curves of 6FDA-HBPI film in a three-electrode
configuration at different current densities. (b) Impendence spectra of 6FDA-HBPI film (inset: the
equivalent circuit diagram).
27

Table 1 Molecular Weights and Solubilities of the HBPIs.
GPCa
Solventsb
Sample
Mw
Mn
PDI
NMP DMAc DMF DMSO THF CHCl3 CH3CN
ODPA-
6FDA-
53000 26100 2.03
63700 30800 2.07
++
++
++
++
++
++
++
+-
+-
--
--
++
++
++
a
b
HBPI
Relative to polystyrene standard, using DMF as the eluent; Qualitative solubilities were tested with 10
mg of polymers in 1mL of solvent. ++, soluble at room temperature; +-, partially soluble; --, insoluble
even on heating.
28

Table 2 Optical and electrochemical properties of the HBPIs.
Solution(nm
Film(nm)
Oxidation Potional(V) b
a
)
c
polyme λmax
λonse λmax
Eonse E1/2
Eg
HOMO LUMO
d
e
r
t
t
1st 2nd 3rd 4th
ODPA 315,345
-HBPI
430 314,35
2
0.34 0.4 0.7 0.9 1.2 2.8 4.78
1.90
6
2
8
3
8
6FDA- 303,339
HBPI
433 305,34
3
0.33 0.4 0.7 0.9 1.2 2.8 4.77
1.91
6
2
9
4
6
a The polymer concentration was 1×10-5 mol/L in NMP. b From CV vs. Ag/AgCl; E1/2 (Average potential
of the redox couple peaks). c The data were calculated by the equation: Eg = 1240/λonset (energy gap between
d
HOMO and LUMO). The HOMO energy levels were calculated from CV and were referenced to
ferrocene (4.8 eV; Eonset = 0.36 V). e LUMO = HOMO- Eg.
29