Synthesis and characterization of donor-acceptor type conducting polymers containing benzotriazole acceptor and benzodithiophene donor or: S -indacenodithiophene donor

Article abstract of DOI:10.1039/c6ra20480e

Two novel electrochromic copolymers, poly(benzodithiophene-benzotriazole) (PBDBT) and poly(s-indacenodithiophene-benzotriazole) (PIDBT) were synthesized successfully through Stille coupling reactions. Both polymers were characterized by cyclic voltammetry

Full text of DOI:10.1039/c6ra20480e

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PAPER
Synthesis and characterization of donor–acceptor
type conducting polymers containing
Cite this: RSC Adv., 2016, 6, 94014
benzotriazole acceptor and benzodithiophene
donor or s-indacenodithiophene donor
c
a
a
Di Zhang,ab Min Wang, Xiaoli Liu and Jinsheng Zhao
*
Two novel electrochromic copolymers, poly(benzodithiophene-benzotriazole) (PBDBT) and poly(s-
indacenodithiophene-benzotriazole) (PIDBT) were synthesized successfully through Stille coupling reactions.
Both polymers were characterized by cyclic voltammetry (CV), UV-Vis spectroscopy, density functional
theory (DFT), colorimetry and thermal gravimetric analysis. The band gaps of the two polymers calculated
according to spectroelectrochemistry data are 1.90 eV for PBDBT and 1.93 eV for PIDBT, respectively. The
PBDBT film turns rosybrown to transparent grey when it is oxidized and the PIDBT film turns pale violet-red
in the neutral state to transparent light grey in the oxidized state. PBDBT has an 55.0% optical contrast at
1500 nm, PIDBT has an 60.8% optical contrast at 1560 nm, and their corresponding response times are 1.49
s and 1.85 s, respectively. In addition, the coloration efficiency of PBDBT at 1500 nm is 144.70 cm2 Cꢀ1, and
Received 14th August 2016
Accepted 19th September 2016
DOI: 10.1039/c6ra20480e
the coloration efficiency of PIDBT at 1560 nm is 222.00 cm2
C
ꢀ1. Both copolymers demonstrate good
www.rsc.org/advances
properties, and can be promising electrochromic materials for commercial applications.
orbital (HOMO) energy, making oxidation easy to achieve, and
Introduction
the acceptor group can lower the lowest unoccupied molecular
orbital (LUMO) energy, which can promote the reduction
process.5,12 Vinylene, thiophene, 3,4-ethylenedioxythiophene
(EDOT) are electron-rich units, while quinoxaline, 2,1,3-benzo-
thiadiazole, 2,1,3-benzoselenadiazole, thienopyrazine13 are
usually electron-decient units.
Conducting polymers have received more and more attention
due to their wide range of applications, such as in organic light
emitting diodes (LEDs),1,2 lasers,2 image sensors3 and display
applications.4 The advantages manifested by them consist of
high optical contrast, fast response time, exible processability
and ne tuning.5 Electrochromic materials can display at least
two different colours when appropriate current density is
applied to the electrode,4 and among them, polythiophene and
its derivatives have been widely researched.6,7 Polarons and
bipolarons are two kinds of charge carrier in conducting poly-
mers, which is conrmed by theories and experiments.8,9
The band gap values are regarded as the vital parameters
resolving the electrochromic characteristics of polymers,2,10
which could be tuned by some feasible strategies including
bond-length alternation, inducing order, creating an planar
structure and adopting donor–acceptor (D–A) approach.7 D–A
approach is the most common and useful method among them.
It functions by the push–pull interaction in the molecular
backbone, which can facilitate intramolecular charge transfer.11
The electron donor group can raise highest occupied molecular
Taking benzotriazole as acceptor units, some D–A type
polymers have been prepared and be proven to be promising
candidates as electrochromic or solar energy generation mate-
rials, although the electron affinity ability of benzotriazole is
weaker than that of the majority of electron withdrawing units
mentioned above.14 Abidin Balan and his colleagues synthe-
sized one polymer taking EDOT as donor unit and benzotriazole
as acceptor unit, and the resulting copolymer showed superior
properties in many aspects.15 When changing the donor unit
from EDOT to thiophene, Abidin Balan synthesized another
copolymer monomer, 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo
[d][1,2,3]triazole (TBT), which could be polymerized to a poly-
mer by the electrodeposition method, the obtained polymer
could display three primary colours, changing from red to green
and nal blue when it is oxidized, while when it is n-doped, its
colour could change to light blue, and eventually transparent.16
When the alkyl group is substituted on the thiophene unit, the
steric repulsion effects could be anticipated between the
repeating units along the backbone of the polymer chains,
which could be avoided while placing the alkyl group on the
aShandong Key Laboratory of Chemical Energy Storage and Novel Cell Technology,
Liaocheng University, Liaocheng, 252059, P. R. China. E-mail: j.s.zhao@163.com;
Fax: +86-635-8539607; Tel: +86-635-8539607
bCollege of Chemical Engineering, China University of Petroleum (East China),
QingDao, 266580, P. R. China
cLiaocheng People's Hospital, Liaocheng, 252000, P.R.China
94014 | RSC Adv., 2016, 6, 94014–94023
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nitrogen atom of benzotriazole unit, and then making the directly when received from Laiyang Fine Chemical Factory.
resultant copolymer more planar.17
Sodium sulte (Na2SO3), magnesium sulfate (MgSO4, $99.0%)
Benzodithiophene has a high hole mobility and can form p– and other reagents were bought from Tianjin Chemreagent Co.,
p stacking easily due to its large planar structure,13,18 which was Ltd. and used directly. (4,8-Bis((2-ethylhexyl)oxy)benzo[1,2-
proved by the X-ray diffraction (XRD) analysis.18 Jianhui Hou b:4,5-b0]dithiophene-2,6-diyl)bis(trimethylstannane) (BT) (99%),
and his colleagues investigated the copolymers based on ben- (4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-
zodithiophene in details, and their band gaps can be tuned b0]dithiophene-2,7-diyl)bis(trimethylstannane) (IDT) (99%) were
between 1.0 eV to 2.49 eV by different combined manner.19 purchased from Zhengzhou Alfachem Co., Ltd (Zhengzhou,
Although the steric hindrance could affect the band gap, in case China). Indium-tin-oxide (ITO) coated glass (sheet resistance: <
of the benzodithiophene unit, the alkoxyl chains exist on the 10 U ,ꢀ1, purchased from Zhuhai kaivo Co., Ltd) were washed
side positions including 4 and 9 positions, which reduce the with ethanol before use. The electrolyte in the experiments was
steric hindrance effects to a very low degree.19 So, benzodi- 0.1 M TBAPF6/ACN.
thiophene and its derivatives are most popular donor units for
the fabrication of D–A type conjugated polymers.20 When ben- Instrumentation
zodithiophene unit is taken as the donor core, the introduction
1H NMR and 13C NMR spectra of the products were recorded
of the alternate electron decient units in the polymer chain
with a Varian AMX 400 spectrometer. The electrochemical
characteristics were fullled by cyclic voltammetry (CV) using
a Ivium-n-Stat Analyzer, where ITO glass, a Pt and an Ag wire
were employed as working, counter and pseudo reference
electrode, respectively. Spectroelectrochemical properties were
measured with a Varian Cary 5000 spectrophotometer, and the
could generate intra-molecular charge transfer, and conse-
quently the copolymer has a small bad gap.21
Polymers incorporating 4,9-dihydro-s-indaceno[1,2-b:5,6-b0]
dithiophene (IDT) as the donor units typically have excellent
solubilities.22 Planarity plays an important role in molecular
packing, and inuencing not only the conjugation length but
potential of the polymer lm (spray coated on ITO glass) to be
also the charge transfer speeds within the polymer lms stacked
tested was controlled by the above electrochemical analyzer
synchronously. Colorimetry was also investigated through the
Varian Cary 5000 spectrophotometer. Digital images of the
polymer were taken with a common digital camera. The poly-
by individual polymer molecules.23 As known to us, IDT
possesses ladder planar structure, which can extend the p
electron delocalization of the polymer backbone containing it,
making it a good electron donor.22 Taking IDT as the donor
mer lms were sprayed onto ITO conducting glasses with
unit, D–A type conjugated polymer could be achieved through
airbrush aer they were dissolved in chloroform (5 mg mLꢀ1).
a copolymerization reaction with appropriate acceptor unit, and
the resultant polymer has been applied to eld effect transis-
Synthesis of 4,7-bis(5-bromothiophen-2-yl)-2-dodecyl-2H-
tors24 and solar cells.23 Wei Teng Neo and his colleagues rstly
benzo[d][1,2,3]triazole
applied IDT-containing polymers to electrochromic area.22 The
The precursors of 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d]
reports about copolymers containing the above mentioned
[1,2,3]triazole (TBT) were synthesized as the method of litera-
units for electrochromic applications are still very limited,
ture.25 Then 4,7-bis(5-bromothiophen-2-yl)-2-dodecyl-2H-benzo
further efforts in this area should be strengthened.
[d][1,2,3]triazole was synthesized by the bromination of NBS as
In this study, two D–A type conducting polymers,
the previous literature25,26 and showed in Scheme 1. In a ask,
poly(benzodithiophene-benzotriazole) (PBDBT) and poly(s-
2.0 g of TBT (0.030 mol Lꢀ1) and 1.89 g of NBS (0.071 mol Lꢀ1
,
indacenodithiophene-benzotriazole) (PIDBT) were synthesized
by Stille coupling reaction, and then spray casting method was
used to pattern a copolymer lm on ITO conducting glass.
Finally, a series of characterization methods, including electro-
chemistry, spectroelectrochemistry, density functional theory
(DFT) calculation, colorimetry, kinetics and thermogravimetry,
were carried out for the detailed acquaintance of polymers, and
the results witnessed the excellent electrochromic properties of
the polymers. In this case, both of the polymers could be
potential candidates for the fabrication of electrochromic device.
2.4 equiv.) was dissolved in a mixture of 150 mL of CH2Cl2/glac
CH3COOH (1 : 1) solvent. The mixture was reuxed with stirring
for 24 hours in the dark and then poured into 200 mL of
deionized water. The organic phase was separated and poured
into 100 mL of saturated Na2SO3. Aer 4 times of extraction, the
solvent in organic phase was distilled and column chromato-
graph was nished with the eluent (DCM: n-hexane). Finally, we
can get yellow crystallized solid product. 1H NMR (400 MHz,
CDCl3, d ppm) 7.78 (d, 2H), 7.48 (s, 2H), 7.12 (d, 2H), 4.78 (t, 2H),
2.17 (m, 2H), 1.48–1.15 (m, 18H), 0.88 (t, 3H); 13C NMR (101
MHz, CDCl3, d ppm) 141.62, 141.15, 130.81, 126.88, 122.89,
122.11, 113.10, 56.87, 31.87, 29.99, 29.58, 29.51, 29.40, 29.30,
28.96, 26.52, 22.65, 14.10.
Experimental
Materials
Bis-(triphenylphosphine)palladium(II) chloride (Pd(PPh3)2Cl2,
99%), N-bromosuccinimide (NBS, 99.0%), acetonitrile (ACN),
Synthesis of PBDBT
tetra-n-butylammonium hexauorophosphate (TBAPF6, 98%) PBDBT and PIDBT were copolymerized as displayed in Scheme 1.
were all purchased from Aladdin Co., Ltd and used without 0.6245 g of DBTBT, 0.7723 g of BT (1 mmol), 0.02804 g of
further purication. Chloroform (99.0%), acetone (99.5%), Pd(PPh3)2Cl2 (0.04 equiv.) and 100 mL of toluene were mixed in
methanol (99.5%), ethanol (95%) and other solvents were used a ask and reuxed for 48 hours in argon atmosphere.25 Aer the
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Scheme 1 Synthetic routes of monomers and copolymers PBDBT and PIDBT.
toluene was distilled off, the crude product was dissolved in were dissolved in CHCl3, then spray coated on the ITO glasses
a small amount of chloroform, and the solution was added drop- until the solvent was evaporated thoroughly. As a result, some
wise into 100 mL of cold methanol, solid powder (crude polymer) copolymer thin lms coated on the glasses were achieved and
was precipitated on the bottom of the ask, and collected by placed in 0.1 M TBAPF6/ACN solution. As shown in Fig. 1, two
ltration. The resulting solid powder was extracted in the Soxhlet groups of redox peaks occurred in the positions of 1.08 V/1.28 V
extractor with methanol and acetone as the solvent, successively and ꢀ1.43 V/ꢀ1.35 V for PBDBT when the potentials were cycled
1
for 24 hours, until each of the solvent was transparent. H NMR from ꢀ2.0 V to 2.0 V. Similarly, the redox peaks located at 1.61
(400 MHz, CDCl3): 7.99 (d, 2H), 7.76 (s, 2H), 7.69 (s, 2H), 7.33 (d, V/0.87 V and ꢀ1.56 V/ꢀ1.25 V for PIDBT, respectively, when the
2H), 4.81 (t, 2H), 4.35 (d, 4H), 2.19 (m, 2H), 0.88–1.85 (m, 51H).
potentials were cycled between ꢀ2.0 V to 2.5 V.
The initial redox peaks at the positive potentials suggested
the existence of the p-type doping, which might be related with
the electron-donating property of benzodithiophene or indace-
nodithiophene.26 In D–A type polymer, the HOMO values of the
donor unit is approximately equal to the values of the polymer,
which is then determined by the initial oxidative potential
(Eonset) of the polymer. The stronger electron donating ability of
the donor unit is benecial to reduce the Eonset of the polymer,
and nally lead to the reduced Eg of the polymer. With the
potential increased, copolymers lose electrons, the rst peak
appearing. The Eonset value of PBDBT (0.64 V) is lower than that
of PIDBT (1.01 V), demonstrating the stronger electron-
donating ability of benzodithiophene than that of indacenodi-
thiophene unit (Fig. 1). And, when the potentials decrease from
the positive potentials to 0 V, the polymers restored to their
neutral states. Both copolymers have n-type doping abilities and
the initial negative potentials denote n-type doping. The n-type
Synthesis of PIDBT
0.3747 g of DBTBT (0.6 mmol), 0.7398 g of IDT (0.6 mmol, 1
equiv.) and 0.01685 g of Pd(PPh3)2Cl2 (0.04 equiv.) were dis-
solved in 100 mL of toluene in 250 mL of three-necked ask,
and then the mixtures were reuxed for 48 hours under argon
atmosphere. When the reaction was nished, it was treated as
the method of PBDBT. 1H NMR (400 MHz, CDCl3): d 8.00 (s, 2H),
7.57 (s, 4H), 7.41 (m, 4H), 7.24–7.03 (m, 16H), 4.83 (s, 2H), 2.58
(d, 8H), 2.20 (s, 2H), 1.54–1.01 (m, 50H), 0.87 (t, 15H).
Results and discussion
Electrochemistry
CVs were conducted for the determination of the redox prop-
erties of the polymers. The solid copolymers PBDBT and PIDBT
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Fig. 1 CV of spray coated PBDBT (a) film from ꢀ2.0 V to 2.0 V and
PIDBT (b) film from ꢀ2.0 V to 2.5 V.
Fig. 2 UV-Vis absorption spectra of PBDBT and PIDBT dissolved in
CHCl3 (a) and those of their films coated on ITO electrode in TBAPF6/
ACN (b) in neutral state.
doping voltage of PBDBT is larger (ꢀ1.43 V) than PIDBT (ꢀ1.56
V), signifying the PBDBT has the stronger electron-withdrawing
ability (Fig. 1).25
PIDBT lms (536 nm) in neutral state, manifesting that PBDBT
has a smaller energy absorbance. In other words, PBDBT need
a small energy to run p–p* transition and it has a more
conjugated structure, which is anticipated by researchers.
Optical properties of the copolymer solution and lms
As shown in Fig. 2, the UV-Vis absorption spectra of PBDBT in
solution was obtained through dissolving the product in CHCl3,
and that of PBDBT lm was obtained through placing the
polymer coated ITO electrode in ACN solution. The occurrence
of absorption peaks could be ascribed to the p–p* conjugation
system.27 The absorption peaks of the copolymers in solution
are located at 532 nm and 531 nm for PBDBT and PIDBT,
respectively, nearly located in the same position, except that the
width of the absorption band of the polymer PBDBT is some-
what wider than that of the polymer PIDBT. The reason for the
difference in the width of the absorption band may be that the
molecular weight distribution of the polymer PBDBT is wider
than that of the polymer PIDBT. They displayed two different
colours, medium violet red for PBDBT, and magenta for PIDBT.
The absorption peaks located at 532 nm in solution and 582
nm in lm solid state for PBDBT was observed, and 531 nm in
Morphology analysis
Atomic force microscopy (AFM) was used for the morphology
analysis of the polymers. As shown in Fig. 3, both polymer lms
presented homogenous and smooth morphology with the mean
roughness (Ra) of 18.3 nm and 27.9 nm, respectively, for PBDBT
and PIDBT. Although no direct evidence has been witnessed
that there is a denite relationships between the morphologies
of polymer lms and their electrochromic properties, the
smooth morphologies of two polymers at least suggested their
perfect lm-processing capabilities, which is conducive to the
preparation of large area electrochromic device and even for
commercial applications.
Spectroelectrochemistry
solution and 536 nm in lm state were also found for PIDBT, Spectroelectrochemistry is an effective and widely applied
respectively. Compared with the absorption in the solution, method to appraise the electrochromic properties of conduct-
there is an apparent red shi for the polymers existing in the ing polymers. When oxidation potential rises gradually, the
form of lm, which is ascribed to the better interchain inter- apparent colour changes could be observed. Fig. 4 displays the
action stemmed from p–p stacking of the copolymers.28 The spectroelectrochemistry property of PBDBT and PIDBT under
colour of PBDBT lm and PIDBT lm are rosybrown and pale stepwise oxidation, as well as the photos in neutral states and
violet red, respectively, at the neutral state. There is an obvious oxidized states. In neutral states, there is only one absorption
bathochromic shi for PBDBT lms (582 nm) compared with band for each polymer due to p–p* transition in the visible
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Fig. 4 Spectroelectrochemistry of PBDBT (a) and PIDBT (b) and their
photos in neutral states and oxidized states.
Fig. 3 AFM image of PBDBT (a) and PIDBT (b).
region. When the voltage is applied to the copolymer lms, the
charge carriers appear, including polarons and bipolarons.
For PBDBT, with potential increasing, the initial peak
located at 578 nm decreases, while another two new peaks
appear in near infrared region at 781 nm and 1388 nm,
respectively. The peak at 781 nm is because of the formation of
polaron (singly charged species), while the peak at 1388 nm
which located in a low energy region can be ascribed to the
bipolaron.29 The polaron peak reaches largest when the poten-
tial is up to 1.42 V, while the bipolaron peak is rising with the
increase of voltage, which is assigned to the repression of inter-
band transition.28,30 Same with PBDBT, the three peaks of PIDBT
are at 532 nm, 841 nm, 1390 nm corresponding to p–p* tran-
sition, formation of polaron and bipolaron, respectively. The
polaron peak reaches a maximum when the applied potential is
1.22 V, and the bipolaron peak is increasing when the potential
is turning large.
As shown in Fig. 4(a), with the applied potential increasing,
the colours of PBDBT lm change from rosybrown in neutral
states to transparent grey in oxidized states. White light is
a mixture of light that is formed by the superposition of various
wavelengths of light. When the potential changes, the absorp-
tion of light of different frequencies varied, and generated
different colours. In neutral states, there is an absorption peak
located mainly in green light region, so the lm demonstrates
rosybrown. With the potential increasing, the absorption of
green light deceases, while the absorption of red light increases.
Finally, the absorption of two peaks in visible region is nearly
equal, therefore, the lm is transparent grey in oxidized states.
Similarly, PIDBT also experiences an palpable colour change
from pale violet red to transparent light grey aer it is oxidized.
Some parameters of spectroelectrochemistry of PBDBT and
PIDBT were read from their graphs or calculated according to
previous studies27,31 and were summarized in Table 1, including
onset of the optical absorption spectra in neutral states (lonset),
maximum absorption wavelength of PBDBT and PIDBT in
solution (lmax, solution) and lm (lmax, lm) morphology, onset
oxidation potential (Eonset), optical band gap (Eg) and HOMO/
LUMO energy levels of both copolymers. Eg, EHOMO and
EHOMO were calculated with the formula Eg ¼ 1241/lonset, EHOMO
¼ ꢀe(Eonset + 4.4) and ELUMO ¼ EHOMO + Eg, respectively.27,31
The Eg values of the two polymers are 1.90 eV and 1.93 eV,
respectively, and the HOMO values exist a large diversity. The
electron donor group can raise HOMO energy, while the
acceptor group can lower LUMO energy.5,12 Since both polymers
have same acceptor unit, the difference of HOMO can be
assigned to the donor unit. In case of benzodithiophene unit,
benzene links with two thiophene directly, while in case of
s-indacenodithiophene unit, benzene is connected with thio-
phene through cyclopentene, so benzodithiophene has a better
planarity and more conjugated structure than s-indacenodi-
thiophene. Consequently, PBDBT has a smaller band gap and
a larger HOMO value than that of PIDBT. As a kind of homo-
polymer of benzodithiophenes, poly(4,8-bis(dodecyloxy)benzo
[1,2-b:4,5-b0]dithiophene) (H2, shown in Table 1) has a band gap
of 2.13 eV,19 which was higher than that of PBDBT (1.90 eV). The
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Table 1 Onset of the optical absorption spectra in neutral states (lonset), maximum absorption wavelength of PBDBT and PIDBT in solution
(lmax, solution) and film (lmax, film) morphology, onset oxidation potential (Eonset), optical band gap (Eg), HOMO/LUMO energy levels of both copolymers
lonset
lmax
lmax
Copolymers
(lm), nm
(solution), nm
(lm), nm
Eonset, V
Eg, eV
HOMO, eV
LUMO, eV
PBDBT
PIDBT
H2a
654
642
—
532
531
—
582
536
495
515
0.64
1.01
—
1.90
1.93
2.13
2.06
ꢀ5.04
ꢀ5.41
ꢀ5.16
ꢀ5.24
ꢀ3.14
ꢀ3.48
ꢀ2.67
ꢀ3.18
P3b
601
514
—
Referred from ref. 19. b Referred from ref. 22.
a
introduction of TBT unit is benecial to lower the band gap of BDBT monomer is smaller than IDIT, indicating that BDBT had
the resultant copolymers, and also demonstrates the advan- a more obvious planar structure. Since planar structure can
tages of donor–acceptor approach for this purpose. Compared extend p electron delocalization, which is benecial to reducing
with H2, the maximum absorption wavelength of PBDBT has the band gap, so the band gap of PBDBT is smaller than PIDBT,
a red shi no matter for polymer solution or polymer lm, which is consistent with the experiment.
which could be ascribed to the better conjugated structure of
PBDBT backbone than that of H2.
Electrochromic switching of PBDBT and PIDBT
Similarly, a recent reported copolymer of substituted s-
indacenodithiophene poly(2-([2,20-bithiophen]-5-yl)-4,4,9,9-tet-
It's important for conducting polymers to have a fast switching
property and to change colour quickly when the copolymer
rakis(4-(octyloxy)phenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b0]
dithiophene) (P3, Table 1), has a slightly higher band gap than
that of PIDBT due to the introduction of the bithiophene and
other than the TBT unit into the copolymer.22 Because benzo[d]
[1,2,3]triazole is a good acceptor unit, when it is introduced to
the polymer backbone, the push–pull interaction will promote
the carrier transfer and enhance the delocalized structure, and
then lower the band gaps of the copolymers.11 More specically,
the introduction of the benzo[d][1,2,3]triazole unit has
a pronounced impact on the LUMO energies of the copolymers,
as a resultant, the LUMO value of PIDBT is much lower than
that of P3.
lms change from neutral state to oxidized state. Electro-
chromic switching properties were studied through observing
the change of transmittance with time. Square wave potential
with an interval of 4 s was applied into the lm when multi-
potential steps technique was chosen and parameters were all
set up in CHI 760 C.32,34 The potential were interchanged
between 0 V and 1.45 V for PBDBT and ꢀ0.3 V and 1.4 V for
PIDBT. Fig. 5 demonstrates the change of transmittance with
time.
The optical contrast (DT%) is the change in transmission
rate of light between the neutral and doped state, which is
important to evaluate the property of polymers.32 As shown in
Fig. 6, the DT% of PBDBT are 16.8% at 580 nm, 15.7% at 850 nm
and 55.0% at 1500 nm, and the DT% of PIDBT are 32.0% at 570
nm, 26.2% at 850 nm and 60.8% at 1560 nm. Both polymers had
large DT%, demonstrating that the colour change is obvious.
Besides, the range of transmittance in near infrared region is
Density functional theory calculation
The frontier molecular orbital of the two monomers were
calculated by density functional theory (DFT) using Gaussian 09
soware at the parameter of 3-21G basic set. The charge
distribution is shown in Fig. 5. The values of HOMO and LUMO
are ꢀ5.20 eV, ꢀ2.53 eV for BDBT and ꢀ4.85 eV, ꢀ2.43 eV for
IDBT, respectively, and the theoretical band gaps of both
monomers are 2.66 eV for BDBT and 2.42 eV, for IDBT,
respectively. The band gap of BDBT is larger than IDBT, this
tendency is inconsistent with their copolymers in optical
experiments. This phenomenon may be explained by that BDBT
monomer is smaller than IDBT, and the conjugated scale is not
as large as later, as a result, the delocalization of molecule is not
so sufficient as IDBT. In addition, in DFT calculation, we neglect
the effect of solvent, and in the experiments, copolymers existed
in solid lm, which exists p–p stacking.32,33
In addition, we should notice that the dihedral angles in
both monomers have an obvious diversity. q1 and q2 are the
dihedral angles in benzotriazole unit, and q3 is the dihedral
angle between benzotriazole unit and benzodithiophene unit or
s-indacenodithiophene unit. q1, q2, q3 are 0.26ꢁ, 0.53ꢁ, 3.61ꢁ for
Fig. 5 Molecular orbital diagrams of HOMO and LUMO of PBDBT and
BDBT, and 0.92ꢁ, 3.15ꢁ, 5.51ꢁ for IDBT. The dihedral angle of PIDBT monomers.
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Table 2 The optical contrast (DT%), response time (t95%) and colora-
tion efficiency (h) of PBDBT and PIDBT at their corresponding
wavelengths
larger than that in visible region, indicating good potential NIR
applications.
Response time (t95%), reecting the speed switching between
neutral state and oxidized state, is dened as the time needed
for obtaining 95% of the entire optical contrast, because the
naked eye isn't very sensitive to the remaining 5% changes.35 As
shown in Table 2, the response time of PBDBT are 1.01 s at 580
nm, 0.75 s at 850 nm and 1.49 s at 1500 nm, and the response
time of PIDBT are 1.84 s at 570 nm, 0.66 s at 850 nm and 1.85 s
at 1560 nm, respectively. Both copolymers exhibited outgoing
switching times.
Response time Coloration efficiency
Copolymers l, nm DT%, % (t95%), s
(h), cm2 Cꢀ1
PBDBT
580
850
1500
570
850
16.8
15.7
55.0
32.0
26.2
60.8
1.01
0.75
1.49
1.84
0.66
1.85
69.16
66.93
144.70
143.22
85.75
PIDBT
1560
222.00
Coloration efficiency (h) is taken for evaluating the utiliza-
tion efficiency of the electronic power during switching, and
dened as the ratio of optical density change (DOD) and electric nondimensional, so the unit of coloration efficiency is cm2 Cꢀ1
.
charge per unit area (DQ) and illustrated by the following The coloration efficiency of PBDBT are 69.16 cm2 Cꢀ1 at 580 nm,
equations.32
66.93 cm2 Cꢀ1 at 850 nm and 144.70 cm2 Cꢀ1 at 1500 nm, while
the coloration efficiency of PIDBT are 143.22 cm2 Cꢀ1 at 570 nm,
85.75 cm2 Cꢀ1 at 850 nm and 222.00 cm2 Cꢀ1 at 1560 nm.
Polymers in near infrared region have a bigger coloration effi-
ciency because of the bigger optical density change.
Changing the interval in multi-potential steps from 10 s to 4
s, 2 s, and nally to 1 s, we can compare the decline degree of
optical contrast. As shown in Fig. 7, when the interval decreases
from 10 s to 1 s, PBDBT maintains an optical contrast of 16.5%
at 580 nm with a 2.6% decrease, an optical contrast of 15.5% at
850 nm with only a 1.1% drop and an optical contrast of 45.4%
at 1500 nm with a 8.2% decrease, while PIDBT maintains an
optical contrast of 24.0% at 570 nm with a 12.6% decrease, an
optical contrast of 21.7% at 850 nm with a 6.0% drop and an
optical contrast of 47.2% at 1560 nm with a 14.7% decrease.
Compared with PIDBT, PBDBT decreases inconspicuously,
which may be ascribed to its good planarity, promoting the
transfer speed of electron.
h ¼ DOD/DQ
DOD ¼ lg(Tb/Tc)
DQ ¼ Q/A
where Tb, Tc are transmittance in neutral states and oxidized
states, respectively. Q was calculated from the multi-potential
steps diagram, in which current is a function of time. We
chose a cycle of 4 s, and got the charge of one cycle through
integrating. The unit of area is cm2, and DOD is
Colorimetry
Colorimetry is one important method to evaluate the change of
colour. In this study, CIE 1976 L*a*b* colour space was used to
measure the colour at different potential, where L* represents
the lightness of the colour from 0 to 100 (black to white),
a* represents how much red versus green (positive value at x axis
means the colour inclines to be red and negative value at x axis
means the colour inclines to be green), and b* represents how
much yellow versus blue (positive value at y axis means the
colour inclines to be yellow and negative value at y axis means
the colour inclines to be blue).36
Fig. 8 demonstrates that the lightness (L*) varies with the
increase of voltage applied to the lm. Three lms with
different absorption values are measured for both polymers,
and the a* values and b* values in neutral states and oxidized
states are also marked in the graph. For PBDBT, when the
voltage rises, lightness (L*) rstly stays steady, then climbs to
a stable stage, meaning that the colour of lm turns to be light
and transparent. a* turns from a big positive value to a negative
value, in other words, red turns to be green, while b* shows
a contrary trend, increasing from a big negative value to a small
negative value, that means the color changes from blue to
Fig. 6 Electrochromic switching of PBDBT (a) between 0 V and 1.45 V
and PIDBT (b) between ꢀ0.3 V and 1.4 V.
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quadrant. As a result, the lm changes from rosybrown to
transparent grey.
In addition, we can get the result that the thinner the lm is,
the lighter it is. When the lm thickness of PBDBT changes
from 0.23 a.u. (532 nm) to 0.45 a.u. and to 0.73 a.u., the light-
ness is 83.7, 69.9, 54.4 in neutral state and 91.8, 83.2, 77.0 in
oxidized state, respectively (Fig. 8(a1)). There exist a signicant
increase, indicating that lm thickness is an important
parameter and should be considered carefully when people
want to tune the lm lightness.
Similar with PBDBT, the lightness of PIDBT lm rstly
keeps stable, and then has a dramatic increase when the
voltage is nearly 1.0 V. a* value gets smaller, while b* value gets
bigger (Fig. 8(b1)). When the lm thickness is 0.20 a.u. (531
nm), the L*, a*, b* changes from 87.1, 12.2, ꢀ4.9 to 96.3, ꢀ3.2,
0.6, respectively. When the lm turns thicker, the lightness
turns darker (Fig. 8(b1)), with the same tendency as that of
PBDBT.
Thermal gravimetric analysis of PBDBT and PIDBT
The molecular weight and the polydispersity of polymers are the
important parameters for electrochromic applications. A high
molecular weight favours the performance of electrochromic
device in several aspects, including the improvement of the
adhesion to the substrate, the resistant to the dissolution in the
supporting electrolyte, and the maintenance of a high optical
contrast aer repeatedly electrochromic switching.33 There are
three primary parameters characterizing the molecular weight
of the polymer, the weight-average molecular weight (Mw), the
number-average molecular weight (Mn), and the polydispersity
(Mw/Mn) of the polymers. The molecular weights of the polymers
against polystyrene standards were determined by gel perme-
ation chromatography (GPC) taking tetrahydrofuran as the
eluent phase. The GPC results showed that the Mw, Mn and the
Mw/Mn data were 16.5 kDa, 7.6 kDa and 2.17 for PBDBT,
respectively, and were 14.8 kDa, 7.8 kDa and 1.90 for PIDBT,
respectively.
To evaluate the thermal stability of the two copolymers,
thermal gravimetric (TG) analysis experiment was carried out.
The slight mass loss found in both polymers between 50 ꢁC and
100 ꢁC might be caused by the evaporation of the trace amounts
of water or organic solvents contained in the polymers (Fig. 9).
As shown in Fig. 9(a), PBDBT begins to decompose when the
temperature reaches to 320 ꢁC, and PIDBT begins to decompose
ꢁ
when the temperature is nearly 460 C. They both have a high
initial decomposition temperature, and PIDBT is more stable
than PBDBT with temperature rising. In order to more conve-
niently compare the thermal stabilities of two polymers,
differential thermal gravity (DTG) curves were also shown in
Fig. 9, from which the temperature points with the maximum
degradation rates are found to be 356.6 ꢁC and 489.6 ꢁC, for
PBDBT and PTDBT, respectively. The results also showed that
PTDBT has better stability than that of PBDBT, which might due
Fig. 7 Electrochromic switching of PBDBT (a1–a3) and PIDBT (b1–b3)
with an interval of 10 s, 4 s, 2 s and 1 s.
yellow. As shown in Fig. 8(a2) and (b2), it can be observed the to the multi benzene structure of PTDBT backbone. The data
trend how a* and b* change with the voltage. When the voltage revealed that both polymers had enough thermal stability for
increases, a* and b* go from the fourth quadrant to third electrochromic applications.35
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Fig. 9 TG and DTG curves of PBDBT (a) and PIDBT (b).
Conclusion
In summary, we have successfully synthesized two new copoly-
mers, PBDBT and PIDBT through Stille coupling reaction.
When oxidized, PBDBT turns from rosybrown to transparent
grey, while for PIDBT, it turns from pale violet red in neutral
state to transparent light grey in oxidized state. The band gap of
both copolymers are 1.90 eV and 1.93 eV for PBDBT and PIDBT,
respectively. They show fast response time in UV-Vis-NIR
region, and also have satisfactory optical contrast. The optical
contrasts in long wavelength region are very high. In addition,
they have a high coloration efficiency at longer wavelength, the
coloration efficiency of PBDBT at 1500 nm is 144.70 cm2 Cꢀ1
,
and the coloration efficiency of PIDBT at 1560 nm is 222.00 cm2
Cꢀ1. The challenges we will overcome are how to enhance the
solubility of the copolymers and trying to reduce the band gaps.
In summary, they both can be good candidates for electro-
chromic applications.
Acknowledgements
The work was nancially supported by the National Natural
Science Foundation of China (51473074, 31400044), the General
and Special Program of the postdoctoral science foundation
China (2013M530397, 2014T70861), and the Natural Science
Foundation of Shandong Province (ZR 2014JL009).
Fig. 8 Plot of lightness as a function of applied voltage of PBDBT (a1)
and PIDBT (b1) and the a*–b* graphs of PBDBT (a2) and PIDBT (b2).
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