Electrochromic polymers electrochemically polymerized from 2, 5–dithienylpyrrole (DTP) with different triarylamine units: Synthesis, characterization and optoelectrochemical properties

Article abstract of DOI:10.1016/j.electacta.2017.01.071

A series of polymers, consisting both 2, 5-dithienylpyrrole (DTP) and different triarylamine units were synthesized by electrochemical oxidative polymerization. The structures of the monomers, including N-(4-(2,5-di(thiophen-2- yl) ?1H-pyrrol-1-yl)phenyl)

Full text of DOI:10.1016/j.electacta.2017.01.071

Electrochimica Acta 228 (2017) 332–342
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochromic polymers electrochemically polymerized from 2, 5–
dithienylpyrrole (DTP) with different triarylamine units: Synthesis,
characterization and optoelectrochemical properties
a
a
a
a,
a
a
Shuwei Cai , Hailin Wen , Shuzhong Wang , Haijun Niu *, Cheng Wang , Xiankai Jiang ,
a
b
Xuduo Bai , Wen Wang
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Department of Macromolecular Science and Engineering, School of
Chemical, Chemical Engineering and Materials, Heilongjiang University, Harbin 150086, PR China
a
b
School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 28 November 2016
Received in revised form 2 January 2017
Accepted 12 January 2017
Available online 14 January 2017
A series of polymers, consisting both 2, 5-dithienylpyrrole (DTP) and different triarylamine units were
synthesized by electrochemical oxidative polymerization. The structures of the monomers, including N-
(4-(2,5-di(thiophen-2- yl) ꢀ1H-pyrrol-1-yl)phenyl)-N-Phenyl Naphthalene-2-amine (DTP-PNA), 4-(2,5-
di(thiophen-2-yl)-1H-pyrrol-1-yl)-N,N-diphenylaniline (DTP-DPA), 4-(tert-butyl)-N-(4-(tert-butyl)phe-
nyl)-N-(4-(2,5-di(thiophen-2-yl)-1H ꢀpyrrol-1-yl)phenyl) aniline (DTP-TBPA), 4-(2,5-di(thiophen-2-yl)-
Keywords:
1
H-pyrrol-1-yl)-N-(4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)phenyl)-N-(p-tolyl)aniline (DTP-PTA-DTP),
2
, 5-dithienylpyrrole
and N-(4-(9H-carbazol-9-yl)phenyl)-4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)-N-(4-(2,5-di(thiophen-
triarylamine
1
13
2
-yl)-1H-pyrrol-1-yl)phenyl)aniline (DTP-CPA-DTP) were elucidated by FT-IR, H NMR, C NMR, Mass
spectra (MS) and the performance and morphology of resulting polymers (P(DTP-PNA), P(DTP-DPA), P
DTP-TBPA), P(DTP-PTA-DTP), and P(DTP-CPA-DTP)) were determined by cyclic voltammetry (CV), ultra-
electrochemical oxidative polymerization
optoelectronic functional material
electrochromic
(
visible-near infrared (UV–Vis–NIR) spectrophotometer, and scanning electron microscopy (SEM),
respectively. The polymers exhibit good reversible multicolor in the visible and NIR region (original
yellowish to dusty blue until violet occurred), with better coloration efficiency (CE), fast response time.
©
2017 Elsevier Ltd. All rights reserved.
1
. Introduction
As has been stated earlier, polythiophene and polypyrrole
derivatives have the great optoelectrochemical performance
[11,12]. The advantages of them are the lower energy gap, better
conductivity, good stability, photochemical properties, electro-
chromic properties and film-forming properties, which make them
suitable to design the electrochromic devices, sensors and solar
cells [13]. The polythiophene and polypyrrole can be obtained
chemically or electrochemically, however, the potential which is
used to oxidize the thiophene monomer, meanwhile, often leads to
overoxidation of the polymer itself, because the oxidation
potential is a little higher and reaches the potentials of the
polythiophene selfoxidation. This is known as the “polythiophene
paradox” [14]. Similar problem occurred with polypyrrole.
Although we can prevent the paradox from reducing the oxidation
potentials, the performances will degrade at the same time. In
order to avoid this phenomenon and improve the performances of
the polymer, the dimers and trimers with two or more hetero-
aromatic rings, such as dithiophene, trithiophene and thienylpyr-
role, having lower oxidized potential without destroying their
At present, electrochromic (EC) polymers, which exhibit
reversible absorption spectral changes by changing the applied
potentials, have attained much prominence [1,2–8]. They are used
widely in our daily life, such as smart windows, optical displays
and e-paper due to their high optical contrast, stable oxidation
states, excellent switching reproducibility and flexibility [9].
Organic conjugated polymers have been generally considered as
one of the most prospective intelligent materials, such as
polythiophene, polypyrrole, polyaniline and polycarbazole, owing
to their good conductivity, photochemical and electrochromic
properties compared with inorganic and small organic compounds
[10].
*
Corresponding author.
E-mail addresses: haijunniu@hotmail.com, niuhaijun@hlju.edu.cn (H. Niu).
http://dx.doi.org/10.1016/j.electacta.2017.01.071
013-4686/© 2017 Elsevier Ltd. All rights reserved.
0

S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
333
optical and mechanical performance, have been selected as
monomers for electropolymerization [15].
From previous article, Ferraris and Skiles [16] obtained the 2,5-
bis(2-thienyl)-1H-pyrrole polymer (PSNS) by the direct polymer-
ization of thiophene and pyrrole derivative (SNS) in 1987 firstly.
From that moment on, many researchers have synthesized a
series of different polymer of 2,5-bis(2-thienyl)-1H-pyrrole, and
discussed their physical and chemical properties [17,18–20]. And
furthermore, from beginning of 2000, Enric Brillas has begun to
study their electrochemical performance. Later on, Chane Ching
Scheme 1. Synthesis route of 1, 4-di(thiophen-2-yl)butane-1,4-dione.
starting material, 1, 4-di(thiophen-2-yl)butane-1,4-dione was
prepared from Paal-Knorr reaction by the method proposed in
the literature [39,40] (Scheme 1), and N-(naphthalen-2-yl)-N-
phenylbenzene-1,4-diamine (TPA-1), N,N-diphenylbenzene-1,4-
diamine (TPA-2), N,N-bis(4-(tert-butyl)phenyl)benzene-1,4-di-
amine (TPA-3), N-(4-aminophenyl)-N-
(p-tolyl)benzene-1,4-diamine (TPA-4) and N-(4-(9H-carbazol-
9-yl)phenyl)-N- (4-aminophenyl)benzene-1,4-diamine (TPA-5)
were synthesized according to the method similarly described
in the literature [41]. Dichloromethane, toluene, and dimethyl
sulfoxide (DMSO) were purified by distillation over calcium
hydride. All the other reagents and solvents were analytical grade
and without further purification. Flash column chromatography
purification was carried out on silica gel (200–300 mesh).
[
21], L. Toppare [22,23] and A. Cihaner [24–27] synthesized a large
number of polymers and studied their photoelectrochemical
properties in detail. For example, Toppare proved that the new
conducting polymers of 1-(4-nitrophenyl)-2,5-di (2-thienyl)
ꢀ
1H-pyrrole can be used as the immobilization matrices for
invertase. In A. Cihaner’s research, a series of polymers of 1-(1-
naphthyl)-2,5-di(thiophen-2-yl)-1H-pyrrole,1-(2-naphthyl)-2,5-
di(thiophen-2-yl)-1H-pyrrole,1-(9H-fluoren-2-yl )ꢀ2,5-di(thio-
phen-2-yl)-1H-pyrrole and 1-(benzo-15-crown-5)-2,5-di(thio-
phen-2-yl)- 1H-pyrrole were synthesized by electrochemical
polymerization, which exhibited multielectrochromic behavior
(
yellow, green, blue and violet).
On the other hand, Liou and Hsiao's groups have been making
2.2. Synthesis of N-(4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)phenyl)-
N-Phenyl Naphthalene-2-amine (DTP-PNA)
efforts on the triarylamine compounds [28], which attracted much
interest as EC materials because of their relative high mobilities
and low ionization potentials [29]. Especially, Hsiao's group has
done some electropolymerization of poly(amide-triarylamine)s
containing triptycene units, which exhibit reversible electrochem-
ical oxidation process, higher coloration efficiency and stronger
color changes [30]. In addition, You's group synthesized poly-DTP-
Ph-TPA by electropolymerization, which proved the polymer has
the high contrast and a very short response time [31]. In J. S. Zhao's
group, they obtained a kind of polymer based on 4,4'-di(N-
carbazoyl)biphenyl and 2, 2'-bithiophene units by electropolyme-
rization in 2011 [32]. What’s more, J. K. Xu's et. al designed and
synthesized the various length polyether chains bridged thiophene
A three-neck round-bottomed flask equipped with a Dean-stark
trap, a nitrogen inlet and a reflux condenser was charged with
0.25 g (1.00 mmol) compound 1, 0.3105 g (1.00 mmol) TPA-1,
0.0264 g (0.14 mmol) PTSA, and 80 ml toluene. The reaction
mixture was stirred and refluxed for 3 days under N
After cooling to room temperature, evaporation of the toluene,
adding enough CH Cl to dissolve solid, then undissolved solid was
separated under reduced pressure, evaporation of the filtrate,
followed by flash column chromatography (SiO column, elution
2
atmosphere.
2
2
2
with dichloromethane) afforded the crude compound. Finally, the
desired product was obtained as yellow solid followed by flash
(
T-E-T) and 2,2-bithiophene (BT-E-BT) groups [33].
2
column chromatography (SiO column, elution with dichloro-
Compared with the traditional method, named chemical routes
methane and petroleum ether). Other compounds 4-(2,5-di
(thiophen-2-yl)-1H-pyrrol-1-yl)-N,N-diphenylaniline (DTP-DPA),
4-(tert- butyl)-N-(4-(tert-butyl)phenyl)-N-(4-(2,5-di(thiophen-2-
yl)-1Hꢀpyrrol-1-yl)phenyl) aniline (DTP-TBPA), 4-(2,5-di(thio-
phen-2-yl)-1H-pyrrol-1-yl)-N-(4-(2,5-di(thiophen-2-yl)-1H-pyr-
rol-1-yl)phenyl)-N-(p-tolyl)aniline (DTP-PTA-DTP), N-(4-(9H-
carbazol-9-yl)phenyl)-4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)-
N-(4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)phenyl)aniline (DTP-
CPA-DTP) were prepared by an analogous procedure. The
synthetic route of the monomers and polymers are shown in
Scheme 2.
to obtain the electrochromic (EC) polymer, electrochemical
polymerization has several advantages, such as it saves the time
for coating the film [34], requires only a small quantity of monomer
to synthesize the polymer [35], can control the thickness of the
films [36] and provides mild experimental conditions at room
temperature [37].
However, up to now, there is not intensive and systematical
research on the effect of different triarylamine substituted group
on the photoelectrochemical properties. Thus, in this study, we
synthesized successfully five new monomers, which containing 2,
5
-dithienylpyrrole (DTP) and different aromatic amines groups.
Then, the polymer films were prepared on the conductive
substrates directly by electrochemical oxidative polymerization.
Finally, we researched the properties of the different triarylamine
substituted group on 2,5-bis(2-thienyl)-1H-pyrrole based
polymer and discussed the performance effect of different
triarylamine units. These polymers have the both advantages of
the monomers. Meanwhile, 2, 5-dithienylpyrrole derivatives
could be widely used in the optoelectronic field due to their
good environmental stability and electrical chemical properties
2.3. DTP-PNA
ꢀ1
1
FTIR: (KBr,
v/cm ): 3044, 844, 820, 760, 694; H NMR: (DMSO-
d
6
, 400 MHz): 7.90 (d, 1 H), 7.85 (d, 1 H), 7.71 (d, 1 H), 7.49 (m, 2 H),
7.45 (m, 1 H), 7.43 (m, 1 H), 7.40 (m, 2 H),7.36 (s, 2 H), 7.30 (d, 1 H),
7.23 (d,1 H), 7.16(d,1 H), 7.08 (d, 2 H), 6.96(m,1 H), 6.82 (d, 2 H), 6.17
13
6
(d, 2 H); C-NMR (DMSO-d , 100 MHz): 145.7, 143.3, 142.7, 140.5,
139.9, 138.6, 138.1, 135.9, 134.8, 134.6, 133.7, 132.1, 130.8, 129.8,
129.3, 128.4, 127.8, 127.1, 126.7, 126.6, 125.3, 125.2, 124.7; MS (EI)
+
[
38].
(m/z): [M] calcd for C34
24
H N
2
S
2
, 524.0; found, 524.07.
2. Experimental
2.4. DTP-DPA
FTIR: (KBr, v/cm 1): 3050, 841, 826, 760, 732, 694; 1H NMR:
ꢀ
2.1. Materials
(
DMSO-d
(m, 2 H), 7.01 (t, 2 H), 6.93 (d, 2 H), 6.80 (d, 4 H), 6.56 (d, 2 H); C-
NMR (DMSO-d , 100 MHz): 148.7, 147.6, 135.4, 131.9, 131.1, 130.5,
6
, 400 MHz): 7.34 (d, 2 H), 7.21 (d, 2 H), 7.11 (m, 2 H), 7.09
13
Thiophene, succinyl chloride, aluminum chloride and
p-toluenesulfonic acid (PTSA) were supplied from TCI Co. The
6

3
34
S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
Scheme 2. Synthesis routes of DTP-PNA, DTP-DPA, DTP-TBPA, DTP-PTA-DTP, DTP-CPA-DTP and P(DTP-PNA), P(DTP-DPA), P(DTP-TBPA), P(DTP-PTA-DTP), P(DTP-CPA-DTP).
1
29.7, 127.1, 125.3, 124.3, 124.0, 123.7, 122.9; MS (EI) (m/z): [M]+
calcd for C30 , 474.0; found, 474.08.
2.8. Electrochemical polymerization
22 2 2
H N S
Electrochemical polymerization of the DTP-TPAs were accom-
plished via potential dynamic method in a compartment cell
furnished with Pt electrode, ITO working electrode, reference
electrode, in the presence of 0.10 mol/L TBAP (tetrabutylammo-
2
.5. DTP-TBPA
ꢀ
1
1
FTIR: (KBr, v/cm ): 3051, 844, 830, 755, 730; H NMR: (DMSO-
ꢀ
3
d
6
6
, 400 MHz): 7.38 (d, 2 H), 7.33 (d, 2 H), 7.18 (d, 2 H), 7.04 (m, 2 H),
nium perchlorate) of the CH
2
Cl
. The electrodes coated with the polymer were rinsed with
in order to get rid of the salts and unreacted monomer.
2
, 2 ꢁ10 mol/L monomer of the
.95 (d, 4 H), 6.93 (d, 2H), 6.80 (m, 4H), 6.56 (d, 2H), 1.27(s, 18H);
CH
CH
2
Cl
Cl
2
1
3
C-NMR (DMSO-d
30.5, 127.2, 126.3, 124.5, 123.9, 121.9, 109.2, 34.4, 31.5; MS (EI)
6
, 100 MHz): 148.9, 146.5, 144.5, 135.3, 131.1,
2
2
1
+
(
m/z): [M] calcd for C38
H
38
N
2
S
2
, 586.0; found, 586.2.
2.9. Instrument
2.6. DTP-PTA-DTP
The obtained compounds were characterized by the following
techniques: FT-IR spectra were recorded on
Spectrum 100 Model FT-IR spectrometer. H NMR and C NMR
a PerkinElmer
FTIR: (KBr, v/cm 1): 3101, 859, 787, 631; 1H NMR: (DMSO-
ꢀ
1
13
d
6
,400 MHz): 7.33(d, 4H), 7.30(q, 4H), 7.28(d, 4H), 7.25(d, 4H), 7.12
spectra were measured on a Bruker AC–400 MHz and AC–100 MHz
spectrometer using DMSO-d as solvent. Mass Spectra (MS) was
6
13
(
(
1
d, 4H), 6.93(q, 2H), 6.80(d, 4H), 6.57 (d, 4H), 2.30(s, 3H); C-NMR
DMSO-d , 100 MHz): 148.1, 143.7, 134.5, 132.6, 131.8, 131.1, 130.4,
27.7, 126.9, 125.2, 124.4, 123.6, 122.5, 109.5, 20.9; MS (EI) (m/z):
6
performed on a Finnigan LC–MS/MS system (Thermo Electron, San
Jose, CA, USA).
The cyclic voltammogram (CV) was conducted on a CH
Instruments 660A electrochemical analyzer with the use of a
three-electrode cell at the different scan rate with a 0.1 mol/L
+
[
M] calcd for C43
.7. DTP-CPA-DTP
FTIR: (KBr, v/cmꢀ1): 2963, 1505, 802, 750, 688; 1H NMR:
31
H N
3
S
4
, 717.0; found, 717.2.
2
LiClO
4
as the supporting electrolyte under nitrogen atmosphere in
CN). Geometry optimizations were carried
dried acetonitrile (CH
3
(
4
(
1
DMSO-d , 400 MHz): 8.27(d, 4H), 7.76(d, 2H), 7.64(d, 4H), 7.48(d,
H) 7.42(d,2H), 7.40(s,2H), 7.38(d, 4H), 7.32(s, 2H), 6.96(t, 2H), 6.86
6
out using the B3LYP functional as implemented in Gaussian 98
program.
For the electrochromic investigations, the films were deposited
on an ITO-coated glass substrate by electrochemical polymeriza-
tion, and a home-made electrochemical cell was built from a
commercial UV–vis cuvette. The cell was placed in the optical path
13
d, 4H), 6.64 (d, 4H); C-NMR (DMSO-d
37.3, 135.0, 131.7, 130.4, 127.6, 127.0, 126.2, 124.7, 124.4, 123.8,
6
, 100 MHz):140.6, 138.4,
+
1
20.4, 110.3, 109.6; MS (EI) (m/z): [M] calcd for C54
36 4 4
H N S , 868.0,
found, 868.1.

S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
335
of the sample light beam in a UV–vis spectrophotometer, which
allowed us to acquire electronic absorption spectra under potential
control in a 0.1 mol/L LiClO /CH CN.
4 3
Colorimetry measurements were performed by using Conica
Minolta CS-200 chromameter with viewing geometry as recom-
mended by Commision Internationale de I’Eclairage (CIE).
According to the CIE system, the color is made up of three
aspects: luminance (L), hue (a), and saturation (b). These values
the protons (H-4) on the pyrrole unit of these DTP-TPAs monomers.
Taking DTP-DPA for example, the integrals of the proton (H-4) and
the other protons are 0.10 and 1.00, respectively, which are
consistent with the theoretical values. In conclusion, these results
indicate that the presence of thiophene and pyrrole rings in the
polymer structure.
3.2. Electrochemical polymerization of the DTP-TPAs
(
L; a; b) were measured at the neural, intermediate and fully
oxidized state of the electrochromic polymer on the ITO/glass
surface.
The P(DTP-TPA)s coated on ITO electrodes were obtained by
the electrochemical polymerization of DTP-TPA in 0.1 mol/L TBAP
SEM measurement was carried out on a S4800 instrument with
accelerating potentials of 2 kV, and the samples were sputtered
with Pt prior to observation.
2 2
dissolved in CH Cl solutions at the room temperature. The scan
potentials were added between 0.0 V and 1.5 V at the rate of
50 mV/s. As seen in the first cycle (Fig. 1 Scheme 1 and S6), the
monomer oxidation peak appeared at 1.10 V, 1.12 V, 1.05 V, 0.86 V
and 0.94 V, respectively, which resulted from 2, 5-dithienylpyr-
role (DTP) group[43]. However, it was a little different from the
previous report, perhaps due to the TPA unit inserting the
monomer, which influencing the potential. During the second
scan, a couple of new reversible redoxidation peaks were
observed for different monomers at 0.72V/0.82V, 0.87 V/0.70V,
0.90 V/0.84V, 1.01V/0.62V, and 0.77 V/0.46V respectively, which
contribute to the oxidation of the monomers gradually. Mean-
while, the current intensity of this reversible peak became higher
and higher after each successive cycle, which clearly indicated the
electroactive polymer was deposited on the ITO/glass surface
successfully. After the polymerization, the deposited films were
3. Results and Discussion
3.1. Synthesis and Characterization
In this study, we devoted our efforts to synthesize a series of 2,
5-dithienylpyrrole derivative monomers and obtain polymer films
by electrochemical polymerization. The monomers were prepared
in two steps mainly (Scheme 1). The starting reagent 1, 4-di(2-
thienyl)-1, 4-butanedione and five kinds of TPAs compounds were
synthesized according to the previously reported work. Then the
monomers were prepared by the condensation reaction, Knorr-
Paal reaction, between compound 1 (1, 4-di(2-thienyl)-1, 4-
butanedione) and TPAs in presence of PTSA enough as the catalyst
in toluene, and yellow crude products were obtained. Finally, the
polymer films were fabricated via electrochemical polymerization.
And structural identification of these compounds was elucidated
2 2
washed with CH Cl for several times to wash off the inorganic
salts and the unreacted monomers.
3.3. Electrochemical properties of the P(DTP-TPA)s
1
13
by using FT-IR, H NMR, C NMR, MS techniques after each
reaction.
Electrochemical properties of P(DTP-TPA)s were investigated
via cyclic voltammetry (CV) (Fig. 2). In the cyclic voltammetry of
P(DTP-PNA), peaks at 0.91 V and 1.36 V were observed as the
oxidation process while 0.85 V and 1.22 V were the reduction
peaks. As shown in the inset of Fig. 2, a linear relationship was
showed between the peak current and the scan rate, signifying this
polymerization was a reversible and non-diffusional redox process
and the electroactive polymer films were well-adhered, which was
proving that the process of the oxidation-reduction reaction was
due to the electronic gain and loss. Other polymers’ cyclic
voltammetry at different scan rate were given in Supporting
Information Fig. S7. The cyclic voltammetrys curves of the
monomers were given in Supporting Information Fig. S8 and
After condensation reaction, molecular structures of the
1
monomers (DTP-TPAs) were also identified by FT-IR, H NMR
13
spectra, C NMR spectra and MS (Seen in Supporting Information
Figs. S1–S5). On the one hand, in the FT-IR of DTP-PNA, we can
ꢀ
1
ꢀ1
ꢀ1
observe three peaks at 820 cm , 760 cm and 621 cm , which
attribute to ’-hydrogens of thiophene rings, -hydrogens of
pyrrole ring and -hydrogens of thiophene rings respectively [42].
Compared with DTP-PNA, the P(DTP-TPA)’s peak at 621 cm
b/
b
a
b
ꢀ
1
disappears but the other peaks still exist. On the other hand, in the
1
H NMR spectra, the characteristic amine band which was
attributed to TPA disappeared by the accomplishment of the
synthesis of DTP-TPAs. The tall singlet signal at 6.58 ppm belongs to
2 2
Fig. 1. Electrochemical polymerization of DTP-PNA and DTP-PTA-DTP in 0.1 mol/L TBAP/CH Cl via repetitive cycling at a scan rate 50 mV/s.

3
36
S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
(
a)
(b)
Fig. 2. (a) Electrochemical oxidation process behavior of P(DTP-PNA) and P(DTP-PTA-DTP) film at different scan rates (b) anodic and cathodic current peaks as a function of
scan rate in 0.1 mol/L LiClO /CH CN.
4
3
Fig. 3 showed the CV of the polymers. During the scan, we can see
the polymers exhibit different oxidation peaks compared with the
monomers. The first oxidation peaks of the polymer P(DTP-TPA)s
are at 0.91 V, 0.85 V, 0.65 V, 0.67 V, 0.62 V respectively, which are
the reported TPA derivatives [44]. The peak appeared at 1.36 V,
1.34 V, 1.20, 0.97 V, 1.00 V respectively, maybe occur from DTP unit
[31]. All these observation are proved that the polymers are
synthesized successfully.
+
ascribed to the radical cation (TPA ) formation which are similar to
3
.4. Geometry and electronic structure of P(DTP-TPA)s
P(DTP-PNA)
In order to obtain deeper insight into the polymer structure-
1.36
property relationships, the ground-state geometry optimizations
and the energy levels of P(DTP-TPA)s were carried out by using
density functional theory at the B3LYP/6-31G level.
0.91
The average C N bond lengths between the pyrrole ring and
phenylene ring in DTP-PNA, DTP-DPA, DTP-TBPA, DTP-PTA-DTP
and DTP-CPA-DTP are 1.412 nm, 1.414 nm, 1.415 nm, 1.414 nm and
¼
0
.85
1
.22
P(DTP-DPA)
1.34
1
.413 nm (1.414 nm), respectively, and the torsional angles (
F)
0.85
between the C N linkage and adjacent phenylene or pyrrole ring
¼
ꢂ
ꢂ
are in the range of 178 ꢀ180 (Fig. 4 and Table S1), which suggest
that they are nearly in one planar conformation.
0
.70
To better understand the evolution of the oxidation and
reduction potentials, we studied the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels of the polymers via cyclic voltammetry (CV) method.
1.06
1.20
P(DTP-TBPA)
0
.65
The EHOMO and the optical band gap (E
g
) were calculated from Eonset
and onset (Table 1) [45]. In order to obtain the accurate redox
l
potential, we use EFc/Fc+ = ꢀ4.80 eV as reference electrode, which
0.60
1.03
has been calibrated already. We know the E1/2(Fc/Fc+vsAg/AgCl) = 0.37
+
P(DTP-PTA-DTP)
P(DTP-CPA-DTP)
V via cyclic voltammetry of (F /F
). Then the EHOMO, ELUMO and E
c
c
g
were summarized in Table 1. The results showed that the TPA
derivatives had an important influence on regulating the electronic
structure of the 2, 5 ꢀdithienylpyrrole derivatives. The lower
potentials they are, the easier electron goes into the polymer films
from ITO.
On the other hand, EHOMO represents the supply electronic
capability while ELUMO is relating to acceptance electronic power.
In HOMO state, the electron cloud is completely distributed in 2, 5
0
.97
0
.67
1
.35
0
.96
1
.00
0.62
ꢀ
dithienylpyrrole part, which in favor of higher hole mobility
Fig. 5). However, in LUMO state, electron cloud is mainly
distributed in TPA unit which can reduce E effectively. The lone
pair electron of N in the TPA core unit would couple with electron
(
1
.32
g
0
.84
p
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
strongly, resulting effective density distribution in the molecule
during the charge transfer. From Table 1, we can know the
theoretical data are basically consistent with the experimental
Potential (V) vs Ag/AgCl
4 3
Fig. 3. Cyclic voltammetry curves for P(DTP-TPA)s in 0.1 mol/L LiClO /CH CN at a
scan rate of 50 mV/ s.
data. In experimental data, E(P(DTP-DPA))> E(P(DTP-PNA))> E(P(DTP-TBPA))>

S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
337
Fig. 4. Optimized geometries of the DTP-TPAs.
Table 1
Optical and electrochemical properties of the polymers.
a
ox
onset
b
c
d
e
quntum
f
Polymers
lmax nm
l
onset
E
E
HOMO
E
LOMO
E
g
E
g
nm
eV
eV
eV
eV
eV
P(DTP-PNA)
P(DTP-DPA)
P(DTP-TBAP)
P(DTP-PTA-DTP)
P(DTP-CPA-DTP)
328
326
325
453
448
477
476
484
545
560
0.67
0.62
0.45
0.50
0.46
ꢀ6.10
ꢀ5.05
ꢀ4.88
ꢀ4.93
ꢀ4.89
ꢀ3.50
ꢀ2.44
ꢀ2.32
ꢀ2.66
ꢀ2.68
2.60
2.61
2.56
2.27
2.21
3.64
3.67
3.96
2.41
2.67
a
l
onset: the absorption starting wavelength.
b
c
d
e
f
E
ox/onset(Fc/Fc+vsAg/AgCl) = 0.37 V.
ox
onset
based on formula (1) EHOMO = ꢀ(E
vs Ag/AgCl + 4.43) eV.
based on formula (2) ELUMO =
based on formula (3) Eg = 1240/
quntum: values of theoretical calculation.
E
HOMO+
g
E .
l
onset
.
E

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38
S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
Fig. 5. Pictorial representations of the electron density in the frontier molecular orbitals of repetition units.
E
E
(P(DTP-PTA-DTP))> E(P(DTP-CPA-DTP)). However, in theoretical data,
(P(DTP-TBPA))> E(P(DTP-DPA))> E(P(DTP-PNA))> E(P(DTP-CPA-DTP))> E(P(DTP-
b:-16) to violet (L:65; a:-4; b:-9), which is according with the view
that conjugated polymers are promising materials for electro-
chromic applications. However, the absorption of P(DTP-PTA-DTP)
at 453 nm decreased gradually when the applied potentials
increased, and two new bands which appeared at 755 nm and
1181 nm increased. Other electronic absorption spectra changing
process are given in Supporting Information Fig. S9. From Fig. 7,
we can know the mechanism of the electron transfer of the
P(DTP-TPA)s.
PTA-DTP)). The slight deviation may be owing to the influence of the
adjacent benzene ring. Moreover, theoretical data only resulted
from the monomer, not long polymer chain. In fact, the solvent and
electrolyte also play an important effect on the result.
3
.5. Spectroelectrochemical properities
Spectroelectrochemical measurements were conducted by
Colorimetric analysis is used to evaluate the color changes of
electrochromic material. The measurements were performed by
Commission Internationale de L'Eclairage (CIE) system. According
to CIE system, the color is made up of three aspects: luminance (L),
hue (a), saturation (b) [47], which representing the brightness of
the materials, how red versus green and how yellow versus blue,
respectively. In other words, this means the higher of the L, a, b, the
brighter of the materials, the redder of the materials and the more
yellow of the materials. These color and corresponding L, a, b
values are shown in Table S2 in Supporting Information.
UV–Vis–NIR spectrophotometer upon incrementally changing
the applied potential (from 0.0 V to 1.5 V) of the polymer films.
The optical behavior as well as electronic structure of the polymers
can be explained. Taking P(DTP-PNA) for example (Fig. 6), the
polymer film is yellow at neutral state in the visible region and
transparent in the NIR regions, which has a strong band at 321 nm
*
and 397 nm. The absorptions were accounted for the
p
-p
and n-
*
p
transitions of the DTP backbones and the TPA subunits [46]. But
the absorption of 321 nm intensity decreased sharply when the
potentials grew up, meanwhile, the absorption at 397 nm gradually
increased. Also, the absorption at 816 nm and 1313 nm gradually
increased with the potentials growing up, and the color of the film
became from yellow (L:65; a:-5; b:23) to dusty blue (L:61; a:-22;
Response time, one of the most important characteristics of
electrochromic materials, is the time needed to perform a
switching between the neutral and oxidized states of the materials
[48], which was measured by UV–Vis–NIR spectroscopy at fixed
4 3
Fig. 6. Electronic absorption spectra changing process for P(DTP-PNA) and P(DTP-PTA-DTP) in 0.1 mol/L LiClO /CH CN solution (insets are the photos of changing color).

S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
339
Fig. 7. The proposed electrochromism mechanization of P(DTP-PNA) and P(DTP-PTA-DTP).
wavelength from 0.0 V to 1.0 V. The response is found to be 2.9 s
from the neutral to the oxidized state and 2.8 s from the oxidized to
the reduced state of P(DTP-PNA) in 816 nm at Fig. 8(a). The
coloration time is different from fading time, which may be owing
to dispersion dynamics of ionic resulting from the different surface
images of the oxidation-reduction process. At Fig. 8(b), we find that
the current density of PDTP-PNA keeps stable in 100 s from 0.0 V to
speed, stability and so on. So the morphologies of film surface of
the P(DTP-TPA)s were investigated specially by SEM. The surface
morphologies of the polymer films were shown in Fig. 9, which
were deposited on ITO electrodes via electrochemical polymeriza-
tion. The closer the distance of movement of ion is, the faster of the
ion transfers, and the more efficiency of electrochromic gets. From
Fig. 9, we can see the polymer films are composed of many well-
distributed microspheres. When the images are enlarged, as the
inset in Fig. 9, the particles diameter is estimated as approximately
300–1000 nm. The larger surface area of the particles is in favor of
increasing the area of the electrolyte solution contacting to the
polymers surface, then facilitating the ion carriers transferring
freely passing in out the films.
1.0 V, which indicating that P(DTP-PNA) has stable electrochromic
properties. P(DTP-PNA), P(DTP-DPA), P(DTP-TBPA), P(DTP-PTA-
DTP) and P(DTP-CPA-DTP) films have a high optical contrast, which
are attributed to the introduction of triarylamine units into the
polymer backbone, but the P(DTP-PNA), P(DTP-DPA) and P(DTP-
TBPA) are more faster than P(DTP-PTA-DTP) and P(DTP-CPA-DTP),
which may be attribute to their more smaller steric hindrance.
Other P(DTP-TPA)s have a similar explanation.
4. Conclusions
The coloration efficiency (CE) is another important character-
istic for the electrochromic materials. CE can be calculated using
the equations given below [49]
In summary, a series of polymers based on 2,5–dithienylpyrrole
(DTP) and triarylamine units (TAA) with excellent optical
transmittance change were successfully designed and synthesized.
Spectroelectrochemical as well as electrochemical properties were
used for the structural characterication. The polymers P(DTP-PNA),
P(DTP-DPA) and P(DTP-TBPA) with single branched DTP unit have a
better performance than P(DTP-PTA-DTP), P(DTP-CPA-DTP) con-
taining double branched DTP unit, which may be due to the steric
hindrance of the latter two is larger than the first three, so the
properties of the P(DTP-PTA-DTP) and P(DTP-CPA-DTP) need
further improvement. Meanwhile by SEM, we can see all films
surface are regular, which are in favor of the carrier’s input and
output in polymer structure, and have a potential application on
the device. Also, an obvious color change was observed from a
yellow neutral to dusty blue and violet. These prominent features
make P(DTP-TPA)s have many applications in the feature, such as
light-emitting, hole-transporting materials, electrochromic mate-
rials and memory device application.
d
OD = lg(T
b
/T
c
)
(1)
h
=
dOD/Q
(2)
where T
b
and T
c
denote the transmittances of the film before and
after colorations, respectively.
dOD is the change of the optical
density, which is proportional to the amount of created color
2
centers.
amount of injected charge per unit sample area.
percentage transmittance change. As a result, the calculating data
h
denotes the coloration efficiency (CE). Q (mC/cm ) is the
D
T(%) is the
2
2
shows the CEs of the P(DTP-TPA)s are 75.72 cm /C, 90.06 cm /C,
1
2
2
2
02.4 cm /C, 83.55 cm /C, 94.69 cm /C, respectively (Table 2). At
the same time, the T(%) of the polymers are 23%, 24%, 13%, 19%
D
and 17%, respectively. It indicates that the materials have not
relative high CEs but have a relative high color changing velocity.
3.6. Surface Morphology
Acknowledgements
The morphology and stacking mode of the polymer have a very
important influence on their properties, such as electron transfer
The authors are grateful to National Science Foundation of
China (Grant No.51373049, 51372055, 51303045, 51273056,

3
40
S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
2
5
52
6
4
2
0
6
5
4
3
2
1
0
0
0
0
0
0
0
(
a)
(
a)
P(DTP-PNA)
P(DTP-DPA)
5
4
4
0
8
6
20
15
1
5
0
0
44
42
(
b)
830 nm color switching for 2.9 s bleaching for 2.8 s
(b)
8
16 nm color switching for 2.9 s bleaching for 2.8 s
4
3
0
8
-
5
36
0
20
40
60
80
100
0
20
40
60
80
100
Time (s)
Time (s)
0
.004
8
4
P(DTP-PTA-DTP)
(
a)
2
2
8
4
80
(
a)
P(DTP-TBAP)
4
4
3
3
2
2
4
0
6
2
8
4
7
7
6
6
6
6
2
8
4
0
0.003
20
0
.002
1
1
8
4
0
6
2
(b)
765 nm color switching for 3.1 s bleaching for 3.7 s
(
b)
828 nm color switching for 2.1s bleaching for 1.9 s
56
0.001
0.000
5
2
48
44
40
-
4
-
0.001
3
6
0
20
40
60
80
100
0
40
80
120
160
200
Time(s)
Time (s)
(a)
P(DTP-CPA-DTP) 1.6
80
70
60
50
40
1.2
0.8
0.4
0.0
(b) 800 nm color switching for 2.3 s bleaching for 4.3 s
-0.4
0
20
40
60
80
100
120
140
Time(s)
Fig. 8. Optical switching procedures: (a) potential step transmittance of P(DTP-TPA)s by applying a potential step (0.0-1.0 V); (b) Current consumption of P(DTP-TPA)s.
Table 2
Optical and Electrochemical Data Collected for Coloration Efficiency Measurements of P(DTP-TPA)s.
a
b
Q(mC/cm2)c
(cm /C)d
2
D
T(%)e
Polymers
l
(nm)
d
OD
h
P(DTP-PNA)
P(DTP-DPA)
P(DTP-TBAP)
P(DTP-PTA-DTP)
P(DTP-CPA-DTP)
816
830
828
765
800
0.1310
0.1450
0.1280
0.0715
0.1641
1.730
1.610
1.250
0.856
1.733
75.72
90.06
102.4
83.55
94.69
23
24
13
19
17
a
The given wavelength where the date were determined.
Optical density change at the given wavelength.
Injected charge, determined from the in situ experiments.
Coloration efficiency is derived from the Eq. (2).
The percentage transmittance changes.
b
c
d
e

S. Cai et al. / Electrochimica Acta 228 (2017) 332–342
341
Fig. 9. SEM image of P(DTP-TPA)s deposited on ITO electrodes via electrochemical polymerization method.
2
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