Blue to light gray electrochromic polymers from dodecyl-derivatized thiophene Bis-substituted dibenzothiophene/dibenzofuran

Article abstract of DOI:10.1002/pola.27973

3-Dodecylthiophene end-capped two monomers: 2,8-bis-(4-dodecyl-thiophen-2-yl)-dibenzothiophene (DBT-3DTh) and 2,8-bis-(4-dodecyl-thiophen-2-yl)-dibenzofuran (DBF-3DTh) were synthesized via Stille coupling reaction. Both monomers exhibited emission peaks at about 400 nm with fluorescence quantum yields ranging from 0.16 to 0.21. The corresponding electroactive polymers poly(2,8-bis-(4-dodecyl-thiophen-2-yl)-dibenzothiophene) (PDBT-3DTh) and poly(2,8-bis-(4-dodecyl-thiophen-2-yl)-dibenzofuran) (PDBF-3DTh) were obtained by electropolymerization method and displayed good electrochemical stability. Both polymers switched between light gray in the neutral state and blue in the oxidized state. Kinetic investigations showed that PDBT-3DTh exhibited a maximum optical contrast (ΔT %) of 25.23% at 575 nm with the coloration efficiency (CE) of 196 cm² C^-1. However, the electrochromic properties of PDBF-3DTh were inferior to PDBT-3DTh. Further detailed discussions with EDOT and 3-alkylthiophenes end-capped DBT/DBF hybrid electrochromic polymers were comparatively studied.

Full text of DOI:10.1002/pola.27973

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Blue to Light Gray Electrochromic Polymers from Dodecyl-Derivatized
Thiophene Bis-Substituted Dibenzothiophene/Dibenzofuran
Kaiwen Lin, Yao Zhao, Shouli Ming, Hongtao Liu, Shijie Zhen, Jingkun Xu, Baoyang Lu
School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330013, China
Correspondence to: J. Xu (E-mail: xujingkun@tsinghua.org.cn) or B. Lu (E-mail: lby1258@163.com)
Received 14 September 2015; accepted 28 October 2015; published online 12 November 2015
DOI: 10.1002/pola.27973
ABSTRACT: 3-Dodecylthiophene end-capped two monomers:
2,8-bis-(4-dodecyl-thiophen-2-yl)-dibenzothiophene (DBT-3DTh)
and 2,8-bis-(4-dodecyl-thiophen-2-yl)-dibenzofuran (DBF-3DTh)
were synthesized via Stille coupling reaction. Both monomers
exhibited emission peaks at about 400 nm with fluorescence
quantum yields ranging from 0.16 to 0.21. The corresponding
electroactive polymers poly(2,8-bis-(4-dodecyl-thiophen-2-yl)-
dibenzothiophene) (PDBT-3DTh) and poly(2,8-bis-(4-dodecyl-
thiophen-2-yl)-dibenzofuran) (PDBF-3DTh) were obtained by
electropolymerization method and displayed good electro-
chemical stability. Both polymers switched between light gray
in the neutral state and blue in the oxidized state. Kinetic inves-
tigations showed that PDBT-3DTh exhibited a maximum optical
contrast (DT %) of 25.23% at 575 nm with the coloration effi-
ciency (CE) of 196 cm2 C21. However, the electrochromic prop-
erties of PDBF-3DTh were inferior to PDBT-3DTh. Further
detailed discussions with EDOT and 3-alkylthiophenes end-
capped DBT/DBF hybrid electrochromic polymers were compa-
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2015 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2016, 54, 1468–1478
KEYWORDS: conjugated polymers; electrochemistry; macromo-
nomers; dibenzofuran; dibenzothiophene; electrochromism
As high triplet energy host materials of blue PHOLEDs, DBT,
and DBF,7,8 with a planar p-conjugated structure, have been
typically used to improve the quantum efficiency in PHO-
LEDs. Moreover, Zhou et al. have studied PDBF electropoly-
merized in boron trifluoride diethyl etherate which revealed
good redox activity and excellent fluorescence properties, as
demonstrated from our previous reports.9 PDBT, as new
materials applied in electrochromic devices, shows good
hole-transporting ability, emission properties, and rigid pla-
narity, which makes it a good candidate for constructing
host materials.10 Given that, a series of EDOT and 3-
alkylthiophenes end-capped DBT/DBF hybrid electrochromic
polymers (Scheme 1; PDBT-EDOT, PDBF-EDOT, PDBT-Th,
PDBF-Th, PDBT-3MeTh, PDBF-3MeTh, PDBT-3HexTh, PDBF-
3HexTh) have been researched deeply in our lab on their
electrochemical and electrochromic performance.11,12 How-
ever, their solubility cannot meet the demands in real life.
Moreover, because of their outstanding electro-optical prop-
erties of 3-alkylthiophenes end-capped DBT/DBF hybrid
electrochromic polymers, it is necessary to detect the rule
of performance with the increase in the length of alkyl
chain attached to the thiophene units. Meanwhile, 3-
dodecylthiophene, as one kind of 3-alkylthiophene, whose
INTRODUCTION Electrochromism is commonly defined as the
visible and reversible change in the transmittance of a mate-
rial caused by a definite applied voltage.1 Conjugated poly-
mers as electrochromic materials have been extensively
investigated since they show excellent electrochemical and
optical properties including processability, fast switching
time, high optical contrast, and ability to modify their struc-
ture to achieve multicolored polymers.2 Therefore, extensive
investigations on the design and synthesis of electrochromic
polymers using of new, different inner and end units with
excellent electrochemical and electrochromic performance
have been the major attractive area. In recent years, fluo-
renes/carbazoles as inner units to synthesize novel electro-
chromic polymers have exhibited many excellent advantages
such as excellent chemical stability, ease of synthesis and
modification, good electrical conductivity, and plentiful color
changes switching between the reduced and oxidized
states.3–5 However, excimer formation in the solid state, and
high-energy barrier for hole injection limit their application.6
To circumvent these obstacles, a promising way to improve
charge injection and optical properties is replacing the inner
units with their close analogs dibenzothiophene (DBT) and
dibenzofuran (DBF).
Kaiwen Lin and Yao Zhao contributed equally to this work.
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Designed and previously synthesized DBT/DBF-heterocycle hybrid electrochromic polymers in our lab. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
flexible alkyl side chains attached to the stiff backbones
endow them high solubility in common organic solvents, and
it has been utilized as the end unit in many systems5.
Instrumentation
NMR spectra were recorded on a Bruker AV 400 NMR spec-
trometer with CDCl3 or DMSO-d6 as the solvent and tetrame-
thylsilane as an internal standard (TMS, singlet, chemical
shift: 0.0 ppm). UV-Vis spectra of the monomers dissolved in
MeCN were taken by using Perkin-Elmer Lambda 900
Ultraviolet-Visible Near-Infrared spectrophotometer. With an
F-4500 fluorescence spectrophotometer (Hitachi), the fluo-
rescence spectra of the monomers and polymers were deter-
mined. Infrared spectra were determined with a Bruker
Vertex 70 Fourier-transform infrared (FT-IR) spectrometer
with samples in KBr pellets. Thermogravimetric analysis
(TGA) was performed with a Pyris Diamond TG/DTA thermal
analyzer (PerkinElmer) under a nitrogen stream from 290 to
1100 K at a heating rate of 10 K min21. Scanning electron
microscopy (SEM) measurements were made by using a
VEGA II-LSU scanning electron microscope (Tescan) with the
polymer films deposited on the ITO-coated glass.
Keeping all these in mind and combine the both merits of
inner units DBT/DBF and end unit 3-dodecylthiophene, DBT-
3DTh and DBF-3DTh were herein synthesized via Stille cou-
pling reaction. Both two neutral state light gray electrochromic
polymers (PDBT-3DTh and PDBF-3DTh, Scheme 2) based on
DBT-3DTh and DBF-3DTh were electropolymerized. Further-
more, their electrochemical and thermal properties, spectroe-
lectrochemical properties, and electrochromic properties were
investigated in detail. Further discussions with EDOT and
3-alkylthiophenes end-capped DBT/DBF hybrid electrochromic
polymers were comparatively studied.
EXPERIMENTAL
Chemicals
3-Dodecylthiophene (98%; J&K Scientific Ltd), bromine, gla-
cial acetic acid, methanol, n-butyllithium (2.5 mol L21 in hex-
ane; Energy Chemical) and chlorotributyltin (98%; Energy
Chemical) were all used as received without further treat-
ment. Dibenzothiophene (DBT, 99%; Energy Chemical),
dibenzofuran (DBF, 99%; Energy Chemical) and tetrakis(tri-
phenylphosphine)palladium(0) (Pd(PPh3)4, 99%; Energy
Chemical) were stored at 4 8C and used as received. Tetrabu-
tylammonium hexafluorophosphate (Bu4NPF6, 98%; Energy
Chemical) was dried under vacuum at 60 8C for 24 h before
use. Tetrahydrofuran (THF), dichloromethane (DCM), and
acetonitrile (MeCN) were all analytical grade and purchased
from Beijing East Longshun Chemical Plant and used after
reflux distillation.
Synthesis of Monomers
2,8-Dibromodibenzothiophene
Bromine (3.1 mL, 60.5 mmol) was added dropwise to a mix-
ture of DBT (5.0 g, 27.1 mmol) and chloroform (30 mL) at
0 8C.13 Under nitrogen atmosphere the reaction mixture was
vigorous stirred 12 h at room temperature. The crude prod-
uct was filtered off and washed with methanol to isolate 2,8-
dibromodibenzothiophene. The product was obtained as a
white powder in 85% yield.
1H NMR (400 MHz, CDCl3, ppm): 7.38 (d, J 5 1.6 Hz, H), 7.41
(d, J 5 2 Hz, H), 7.52 (s, H), 7.54 (s, H), 8.05 (d, J 5 2 Hz,
2H); 13C NMR (400 MHz, CDCl3, ppm): 138.74, 136.43,
130.71, 125.78, 125.48, 118.72.
SCHEME 2 Synthetic routes of monomers and corresponding polymers. Reagents and conditions: (i) X 5 S; CHCl3, Br2 (2.2 eq), N2;
(ii) X 5 O; CH3COOH, Br2 (2.2 eq), N2; (iii) Pd(PPh3)4, THF, 90 8C. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
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2,8-Dibromodibenzofuran
described in Scheme 2, and the product was obtained as a
white powder in 60% yield.
A 250 mL round bottom flask containing 8.4 g (50 mmol)
dibenzofuran dissolved in 100 mL glacial acetic acid
equipped with an addition funnel.14 Bromine 5.1 mL (100
mmol) in 30 mL glacial acetic acid was added dropwise via
the addition funnel to dibenzofuran under vigorous stirring.
This reaction mixture was stirred at room temperature for
4 h. After refluxed for 6 h, the reaction was cooled to room
temperature. The solid was collected by filtration and
washed for three times with 100 mL portions of water.
Recrystallized from 100 mL acetic anhydride and obtained
12.2 g pure white solid 2,8-dibromodibenzofuran in 75%
yield.
1H NMR (400 MHz, CDCl3, ppm): d 8.19 (s, 2H), 7.72 (t,
J 5 11.8 Hz, 2H), 7.55 (d, J 5 8.2 Hz, 2H), 7.24 (s, 2H), 6.91
(s, 2H), 2.66 (t, J 5 15.8 Hz, 4H), 1.70 (m, 4H), 1.31 (m,
36H), 0.92 (m, 6H). 13C NMR (400 MHz, CDCl3, ppm):
156.15, 144.43, 143.93, 130.12, 125.69, 124.65, 124.53,
119.27, 117.87, 111.99, 31.93, 30.72, 30.50, 29.69, 29.63,
29.51, 29.39, 29.36, 22.69, 17.53, 14.11, 13.59.
Electrochemistry
All the electrochemical experiments and polymerization of
monomers were performed in
a one-compartment cell
1H NMR (400 MHz, CDCl3, ppm): d 7.44 (d, J 5 8.0 Hz, 2H),
7.57 (d, J 5 8.0 Hz, 2H), 8.03 (d, J 5 4.0 Hz, 2H).
with the use of Model 263A potentiostat-galvanostat (EG&G
Princeton Applied Research) under computer control. For
electrochemical tests, the working and counter electrodes
were both Pt wires with a diameter of 1 mm, while the
reference electrode (RE) was Ag/AgCl. The Ag/AgCl RE
was prepared electrochemically by chronoamperometry
method at potential of 1.5 V for 100 s in hydrochloric acid
(6 mol L21) and calibrated against the SCE system. Bu4NPF6
(0.1 mol L21) was used as electrolyte, dissolved in DCM/
MeCN (50/50, v/v) which was freshly distilled before its
use. All the solutions were deaerated by a dry nitrogen
stream and maintained under a slight overpressure through
all the experiments. Polymer films were obtained electro-
chemically in potentiodynamic regime. After polymerization,
the films were washed repeatedly with anhydrous MeCN to
remove the electrolyte and monomer.
Synthesis of the Stannylation of 3-Dodecylthiophene
A solution of 3-dodecylthiophene (2.52 g) in dry THF
(30 mL) was cooled to 278 8C and blanked by nitrogen for
three times.15 n-BuLi (2.5 mol L21 in hexane, 4.8 mL) was
added dropwise to the solution within 30 min. The mixture
was stirred for 1.5 h at 278 8C and then warmed to 240 8C.
Chlorotributyltin (1.73 g) was added slowly to the solution,
and after that, the temperature was slowly warmed to room
temperature. And the mixture was stirred at room tempera-
ture for 12 h under argon atmosphere. The residue was fil-
tered and the solvent removed under reduced pressure by
rotary evaporation. The tributylstannane compound was
used directly for the Stille coupling reaction without further
purification.
The cyclic voltammetry (CV) and linear sweep voltammetry
(LSV) were employed to evaluate the HOMO, LUMO levels
and Eg values of monomers according to the empirical equa-
tions as follow:16
2,8-Bis-(4-Dodecyl-Thiophen-2-yl)-Dibenzothiophene
(DBT-3DTh)
DBT-3DTh was synthesized via Stille coupling reaction as
described in Scheme 2. To a mixture solution of 2,8-dibromo-
dibenzothiophene (1.0 g, 2.9 mmol) and tributyl(3-dodecylth-
iophene-2-yl) stannane (7.8 g, 14.5 mmol) in dry THF
(50 mL), Pd(PPh3)4 (0.165 g) was added as the catalyst. The
mixture was stirred magnetically at room temperature under
nitrogen atmosphere. After half an hour, the mixture was
heated to reflux with vigorous stirring for another 24 h, and
then concentrated under reduced pressure. Finally, column
chromatography was used to purify the reaction mixture.
The product was obtained as a white powder in 65% yield.
HOMO 5 2ðEox14:80Þ eV
(1)
(2)
À
Á
LUMO 5 HOMO1Eg;opt eV
where Eox and Eg,opt are the onset oxidation potential and
the optical bandgap of compounds, respectively.
Electrochromic Experiments
Spectroelectrochemistry and kinetic studies were recorded
on a Specord 200 plus (Analytik Jena) spectrophotometer
and the potentials were cont rolled using Versa STAT 3
(Princeton Applied Research). The spectroelectrochemical
cell consisted of a quartz cell, an Ag/AgCl electrode as refer-
ence electrode, a Pt wire as counter electrode, and an indium
tin oxide (ITO) coated glass as the transparent working elec-
trode. All measurements were carried out in MeCN contain-
ing Bu4NPF6 (0.1 mol L21).
1H NMR (400 MHz, CDCl3, ppm): d 8.38 (s, 2H), 7.83 (d,
J 5 7.4 Hz, 2H), 7.72 (dd, J 5 12.2 Hz, 2H), 7.28 (d, J 5 12.4
Hz, 2H), 6.94 (s, 2H), 2.66 (t, J 5 15.8 Hz, 4H), 1.71 (m, 4H),
1.31 (m, 36H), 0.92 (m, 6H). 13C NMR (400 MHz, CDCl3,
ppm): 143.97, 143.34, 138.36, 135.40, 131.07, 124.63,
124.28, 122.61, 118.98, 118.08, 31.40, 30.21, 30.00, 29.16,
29.13, 28.99, 28.88, 28.83, 22.16, 17.01, 13.56, 13.04.
2,8-Bis-(4-Dodecyl-Thiophen-2-Yl)-Dibenzofuran
(DBF-3DTh)
The synthetic method of DBF-3DTh is similar to DBT-3DTh,
which was synthesized via Stille coupling reaction as
The optical density (DOD) at the specific wavelength (k) was
determined by DT% values of electrochemically oxidized and
reduced polymer films, using the following equation:17
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TABLE 1 The Onset Oxidation Potentials of DBT/DBF-Heterocycle Hybrid Monomers
Samples DBT-EDOT DBF-EDOT DBT-Th DBF-Th DBT-3MeTh DBF-3MeTh DBT-3HexTh DBF-3HexTh DBT-3DTh DBF-3DTh
Eox,onset (V) 0.91 0.95 1.12 1.21 1.11 1.23 1.03 1.11 1.05 1.12
DOD 5 logðTox=Tred
Þ
(3)
high oxidation onset potentials than 3-HexTh end-capped
monomers. The phenomenon was mainly attributable to sig-
nificant steric hindrance in the large alkyl chain. In addition,
DBF-based monomers were higher than DBT-based mono-
mers when the end units were the same. Lower electronega-
tivity, larger atomic size, and better polarizability of sulfur
atom than oxygen atom resulted in DBT-based systems elec-
tron richer than DBF-based systems.
The coloration efficiency (CE) is defined as the relation
between the injected/ejected charge as a function of elec-
trode area (Qd) and the change in optical density (DOD) at
the specific wavelength (k) of the sample as illustrated by
the following equation:18
CE5 DOD=Qd
(4)
Figure 1 shows cyclic voltammograms (CVs) corresponding
to the potentiodynamic electropolymerization of two mono-
mers. During repetitive cycling of electrolytes containing the
corresponding monomers, new reversible redox couples
emerged at 0.8 to 1.0 V versus Ag/AgCl. As the cyclic scan
continued, the increase of anodic and cathodic peak current
densities in CVs implied that the amount of the polymer
films increased on the surface of working electrode. In par-
ticular, electrochemical polymerization of DBT-3DTh revealed
RESULTS AND DISCUSSION
Monomer Synthesis
The synthetic routes of DBT-3DTh and DBF-3DTh are
revealed in Scheme 2. These two monomers were synthe-
sized via Stille cross coupling reaction. Importantly, these
two monomers were synthesized and their properties were
investigated for the first time as far as we know. NMR and
FT-IR spectra of some intermediates and target compounds
are displayed in Supporting Information Figures S1 to S6
and Table S1, respectively.
Electrochemical Polymerization of DBT-3DTh
and DBF-3DTh
Supporting Information Figure S7 shows anodic polarization
curves of 0.01 mol L21 monomers in DCM/MeCN-Bu4NPF6
(0.1 mol L21). The monomers can easily dissolve in DCM,
and partially dissolve in MeCN (formed flocculus). And the
electrochemical polymers became rough, discontinuous, and
heterogeneous when electropolymerized in DCM-Bu4NPF6 or
in MeCN-Bu4NPF6 rather than their mixture. Therefore, a
mixture of DCM and MeCN (50/50, v/v) was chosen as the
solvent system to obtain the polymer films on an ITO coated
glass slide. The onset oxidation potential (Eox,onset) of DBT-
3DTh was 1.05 V versus Ag/AgCl, lower than DBF-3DTh
(1.12 V vs. Ag/AgCl), which was caused by smaller overlap
integral of oxygen atom compared with sulfur atom in the
dibenzo-five-membered ring. Deeply, to find the relationship
between their structures and properties, all the Eox,onset val-
ues of DBT/DBF-heterocycle hybrid monomers11,12 were
offered in Table 1, and all values were detected in the same
experimental conditions. Among them, EDOT end-capped
monomers, DBT-EDOT and DBF-EDOT, were 0.91 V and 0.95
V versus Ag/AgCl, lower than those of thiophene and alkylth-
iophene end-capped monomers. This is an expected result
since EDOT as the strong electron-donating group increases
the electron density of DBT/DBF system and makes their
oxidation relatively easy. Further, the Eox,onset values of 3-
DTh and 3-HexTh end-capped monomers were less than Th
and 3-MeTh end-capped monomers, which can be ascribed
to the electron-donating ability of the alkyl chain. Meanwhile,
the dodecyl chain was electron richer relative to the hexyl
group, but 3-DTh end-capped monomers exhibited relatively
FIGURE 1 Cyclic voltammograms of 0.01 mol L21 DBT-3DTh
(A) and DBF-3DTh (B) in DCM/MeCN-Bu4NPF6 (0.1 mol L21).
Potential scan rate: 100 mV s21
.
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TABLE 2 The Calculated and Experimental Parameters of DBT-3DTH and DBF-3DTH
Compounds
HOMO (eV)
LUMO (eV)
Eox,onset (V)
Eg,opt (eV)
kabs,max (nm)
kem,max (nm)
DBT-3DTh
DBF-3DTh
25.85
25.90
22.35
22.37
1.05
1.10
3.50
3.53
311
308
399
400
an obvious redox couples without potential shift. To compare
the redox peak potentials with structural analog, DBT-EDOT
and DBF-EDOT exhibited obvious redox peaks at 0.55 to 0.80
V versus Ag/AgCl,11 lower than those of thiophene and
alkylthiophene end-capped polymers owing to the presence
of the strong electron-donating group of EDOT. Thiophene
end-capped polymers, DBT-Th and DBF-Th, displayed reversi-
ble peaks with oxidation potentials of 1.35 V and reduction
potentials of about 1.10 V versus Ag/AgCl,12 both values
were higher than those of polymers with alkylthiophene
as end units and DBT/DBF as inner units. The steric hin-
drance and electron-donating effect of alkyl chain may
both play role on this process. Moreover, there are differences
in the oxidation peak current density (from low to
high), DBT-3MeTh (3.1 mA cm22, the 10th cycle, the same
below) < DBT-3DTh (4.6 mA cm22) < DBT-Th (5.2 mA cm22) <
DBT-3HexTh (9.7 mA cm22); DBF-3MeTh (1.2 mA cm22) <
DBF-3DTh (1.8 mA cm22) < DBF-3MeTh (2.3 mA cm22) < DBF-
Th (5.1 mA cm22). The phenomenon could be ascribed to the
interdigitation of alkyl chains attached on the thiophene ring,
which is beneficial to the formation of high quality films.
Also, the details of the band assignments for monomers and
polymers are given in Supporting Information Table S1.
FT-IR spectra of the doped polymers PDBT-3DTh and PDBF-
3DTh showed the characteristic peaks of corresponding
monomers, which confirmed that the structure of DBT-3DTh
and DBF-3DTh was not destroyed during the electrochemical
polymerization process. The obviously broadening of poly-
mers, in comparison with those of monomers, were similar
to those of other conjugated polymers reported previously.19
More specifically, the absorption peaks of polymers at
around 839 cm21 and 837 cm21 are due to the C-H out of
plane bending vibrations of 2,3,5-trisubstituted thiophene
ring.20 Compared with the spectra of monomers, the disap-
pearance of peaks at 723 and 725 cm21 which attributed to
C-H bending vibrations of 2,4-substituted thiophene ring,
The HOMO energy levels of monomers were calculated from
linear sweep voltammetry from the intersection between
base line and tangent drawn to the increasing current line.
HOMO levels were 25.85 eV for DBT-3DTh and 25.90 for
DBF-3DTh, respectively. The LUMO levels, estimated from the
HOMO and the onsets of the absorption peaks, were found
to be 22.35, and 22.37 eV for DBT-3DTh and DBF-3DTh,
respectively (Table 2).
Optimisation of Electrical Conditions and Preparation
of the Hybrid Polymers
To optimize the applied potential for polymerization, a set of
current transients during the electropolymerization at differ-
ent applied potentials were recorded, as shown in Support-
ing Information Figure S8. According to the experiment,
PDBT-3DTh and PDBF-3DTh films used for the characterisa-
tion mentioned below were all prepared by the chronoam-
perometry method at constant potentials of 1.25 V and 1.30
V (equal to that of PDBF-3HexTh) vs. Ag/AgCl, respectively.
Also, visual inspection demonstrated that PDBT-3DTh and
PDBF-3DTh films with smooth, homogeneous, and continu-
ous surfaces were formed at the optimal oxidation potentials,
as predicted from their I-t curves, and they showed good
adherence against the electrode.
Structural Characterization
FT-IR spectra of DBT-3DTh, DBF-3DTh, doped PDBT-3DTh,
and doped PDBF-3DTh were recorded, as shown in Figure 2.
FIGURE 2 FT-IR spectra of DBT-3DTh, DBF-3DTh, doped PDBT-
3DTh, and doped PDBF-3DTh. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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ger wavelength is the absorption, the higher conjugation
length is the polymer.21 Therefore, the spectral results con-
firmed the occurrence of electrochemical polymerization.
The fluorescence spectra of the monomers and dedoped
polymers dissolved in MeCN are illustrated in Figure 4. The
emission peak of DBT-3DTh emerged at 399 nm when
excited at 310 nm whereas a maximum emission peak at
475 nm characterised the spectrum of doped PDBT-3DTh
when excited at 420 nm. A dominant maximum emission at
400 nm of DBF-3DTh was observed when excited at 305 nm,
while doped PDBF-3DTh exhibited a maximum emission
peak at 452 nm under excitation at 400 nm. The relatively
small red shift observed for the monomers suggests that
there is little geometric change between the ground and
excited states.22 Meanwhile, obvious red shifts could be
observed between the monomers and soluble polymers
(76 nm between DBT-3DTh and PDBT-3DTh and 52 nm
between DBF-3DTh and PDBF-3DTh), which is primarily
attributed to the elongation of the their delocalised
p-electron chain sequence, called Stokes shift.23 For all the
3-alkylthiophenes end-capped DBT/DBF hybrid electrochro-
mic polymers, with the increase in the length of alkyl chain
(Scheme 1, R 5 CH3, C6H13, C12H25) attached to the thio-
phene unit at third position, the main chain emission
FIGURE 3 UV-Vis spectra of two monomers in MeCN and cor-
responding doped polymers electrodeposited on ITO. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
implied that the electrochemical polymerization occurred at
a positions of two thiophene rings (Scheme 2), in good
agreement with previous reports.11,12 In addition, the bands
due to Ca–H stretching at approximately 3106 cm2l and
Cb–H stretching at 3027 cm2l in the monomer spectra are
nearly absent or weak, also indicating the occurrence of elec-
tropolymerization at the thiophene rings. All these results
confirmed the occurrence of electrochemical polymerization
among the monomers to form conducting polymers.
UV-Vis and Fluorescence Spectra of Monomers and their
Corresponding Polymers
The UV-Vis spectra of both monomers dissolved in CH3CN
and corresponding doped polymers electrodeposited on ITO
were examined, as shown in Figure 3. Both monomers exhib-
ited similar characteristic p-p* transition peaks in the region
of 256 to 311 nm, which coincided with their structural ana-
log.11,12 On the contrary, the spectrum of both polymers
showed a much broader absorption, which revealed an red-
shifted peaks at 353 nm for PDBT-3DTh, and at 329 nm for
PDBF-3DTh. The red shifts for these two polymers were
attributed to their high conjugation length. Generally, the lon-
FIGURE 4 Emission spectra of monomers and corresponding
polymers in MeCN. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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FIGURE 5 Cyclic voltammograms of PDBT-3DTh (A) and PDBF-3DTh (B) modified Pt electrodes in monomer-free DCM/MeCN-
Bu4NPF6 (0.10 mol L21) at different potential scan rates. Right: plots of redox peak current densities versus potential scan rates for
PDBT-3DTh (C) and PDBF-3DTh (D). jp is the peak current density, and jp,a and jp,c denote the anodic and cathodic peak current
densities, respectively. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
appears at short wavelength, which can be ascribed to the
large interchain p–p stacking distance and weak intermolec-
ular interaction of polymers that make the polymers main
chains decreased orbital overlap and conjugation.
electrochemical behaviors of PDBT-3DTh and PDBF-3DTh by
cyclic voltammetry in monomer-free DCM/MeCN-Bu4NPF6
(0.1 mol L21) were investigated, as shown in Figure 5. A
redox process was observed between 0.75 V and 1.06 V for
PDBT-3DTh; between 0.65 V and 1.30 V for PDBF-3DTh.
PDBT-3DTh exhibited a lower oxidation potential (0.98 V)
than PDBF-3DTh (1.15 V), which can be attributed to the
presence of more polarizable sulfur atom. In particular, CVs
of PDBF-3DTh showed an obvious hysteresis, i.e., an obvious
potential shift of redox peaks. This phenomenon is hardly
explained by conventional kinetic limitations such as ion dif-
fusion or interfacial charge transfer processes.26 The main
reasons are usually slow heterogeneous electron transfer,
local rearrangement effect of polymer chains, and electronic
charging of interfacial exchange corresponding to the metal/
polymer and polymer/solution interfaces.27 On the other
hand, the peak current densities was proportional to poten-
tial scanning rates for both films, indicating that the redox
process was nondiffusional and the electroactive polymers
were well adhered to the working electrode surface.28
The fluorescence quantum yields of DBT-3DTh and DBF-3DTh
were measured with anthrancene in MeCN (standard, /
ref 5 0.27)24 as a reference and were calculated according to
the well-known method given in eq 3:
n2Aref
n2refAIref
I
u
overall5
3uref
(3)
where n, A, and I denote the refractive index of the solvent,
the absorbance at the excitation wave length, and the intensity
of the emission spectrum, respectively. The subscript ref
denotes the reference, and no subscript denotes the sample.
The absorbance of the samples and the standard should be
similar.25 By calculation, the fluorescence quantum yields
were 0.21 for DBT-3DTh and 0.16 for DBF-3DTh, respectively.
Electrochemistry of the Polymers
For the sake of getting a deeper insight into the electrochem-
ical activity and stability of as-formed polymer films, the
As far as we can know from CVs of all the 3-alkylthiophenes
end-capped DBT/DBF hybrid polymers, with the elongation
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switching stability, both polymers displayed better stability
than those of thiophene-substituted polymers. Meanwhile,
DBF as inner unit polymers exhibited better stability than
DBT-based polymers, attributing to better conjugation of the
polymer backbone.
Thermal Analysis
To investigate the thermal stability of PDBT-3DTh and PDBF-
3DTh films, thermogravimetric analytical experiment (TGA)
was performed under a nitrogen stream at a heating rate of
10 K min21, as shown in Supporting Information Figure S9.
PDBF-3DTh shows slightly higher thermal stability over PDBT-
3DTh. Both polymers initially underwent weight decrease
about 3.6% for PDBT-3DTh and 6.5% for PDBF-3DTh at rela-
tively low temperature (from 293 K to 350 K), which may be
attributed to water evaporation according to many authors.29
It means that the onset decomposition temperatures of PDBT-
3DTh and PDBF-3DTh are at 324 K and 350 K, respectively.
When the temperature increased up to about 1100 K, the res-
idue of PDBT-3DTh and PDBF-3DTh were about 45% and
48%, respectively. The similar thermogravimetric degradation
curves were exhibited for PDBT-3HexTh and PDBF-3HexTh.12
However, under similar conditions, PDBT-Th and PDBF-Th12
exhibited better thermal stability. To be specific, because of
alkyl chain, PDBT-3DTh and PDBF-3DTh were easily to crack
while thiophene-substituted polymers need higher tempera-
ture to decompose the skeletal backbone chain structure.
Morphology
FIGURE 6 Long-term cyclic voltammograms of PDBT-3DTh (A)
and PDBF-3DTh (B) in monomer-free DCM/MeCN-Bu4NPF6 (0.1
mol L21). Potential scan rate: 150 mV s21. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
Scanning electron micrographs (SEM) of polymers provide
their clear surface and bulk morphologies, which are two
important aspects closely related to their optical and electri-
cal properties, such as charge transport and counterions
storage capability.30 The surface morphology of PDBT-3DTh
and PDBF-3DTh deposited on the ITO electrode was investi-
gated by SEM, as shown in Supporting Information Figure
S10. PDBT-3DTh (A) and PDBF-3DTh (B) films exhibited
smooth and compact morphology at high magnifications,
which was beneficial to increase the electron transfer capa-
bility of conjugated polymer.26 Also, the smooth and compact
morphology of the polymers films were especially significant
in the fabrication of electrochromic devices. The smooth sur-
face of both polymer films were similar to P(DBT-EDOT) and
P(DBF-EDOT),11 while thiophene end-capped polymers12
showed a lot of holes with different diameters. These differ-
ences were due to the different end units.
of the alkyl chain length (Scheme 1, R 5 CH3, C6H13, C12H25),
the corresponding polymer exhibited sharper redox peaks in
comparison with that of PDBT-Th and PDBF-Th, implying
faster doping/dedoping processes of alkyl thiophene based
polymers. This phenomenon probably results from the steric
hindrance from the alkyl chains, which causes the polymer
chains to separate from each other. The separation generally
promotes a more expanded morphology and facilitates the
transport of counter balancing ions across the conjugated
block, and thus usually favors fast switching of conjugated
polymers.5
The long-term stability of the redox activity for PDBT-3DTh
and PDBF-3DTh was investigated in DCM/MeCN-Bu4NPF6 (0.1
mol L21) at the potential scan rate of 150 mV s21, as shown in
Figure 6. The polymer films exhibited good stability, for the
amount of exchange charge of PDBT-3DTh remained 78.3%
after 100 cycles and 63.8% after 300 cycles, while PDBF-3DTh
remained about 88.8% and 68.5% of electrochemical activity
after 100 and 300 cycles (Table 3). In contrast, the stability of
PDBT-EDOT and PDBF-EDOT remained 83.76% and 88.47%
after 100 cycles,11 better than PDBT-3DTh and PDBF-3DTh,
which can be ascribed to the good stability of EDOT. Similar to
3-HexTh/3-MeTh end-capped polymers12 in the long-term
Spectroelectrochemistry
Spectroelectrochemical properties of the polymers (Fig. 7)
were investigated in CH3CN-Bu4NPF6 (0.1 mol L21) solvent
and UV-Vis-NIR spectra were recorded upon external bias.
Electrochemically produced polymer films on ITO were
reduced to their neutral states at 20.5 V potential to remove
any trapped charge and dopant ion before spectroelectro-
chemical analysis.
The electronic absorption spectra of neutral form of the
films illustrate an absorption band at around 325 nm, which
are attributed to the transitions from the thiophene-based
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TABLE 3 Optical and Electrochemical Properties of Polymers
Electrochemistry and Linear Fitting
Redox Stability
Absorption
Polymers
Eox,peak (V)
Ered,peak (V)
Ra2n
Rc2at
100th
300th
konset (nm)
kmax (nm)
Eg,opt (eV)
PDBT-3DTh
PDBF-3DTh
0.92
1.10
0.88
0.90
0.997
0.998
0.999
0.997
78.3%
88.8%
63.8%
68.8%
440
430
353
329
2.82
2.72
valence band to its antibongding band, are mainly absorbing
in the UV-region, with minor tailing into the visible region.31
The band gaps of PDBT-3DTh and PDBF-3DTh were calcu-
lated to be 2.82 eV and 2.72 eV through the onset of the
p-p* transition. Compared with its analogs such as PDBT-Th
(Eg 52.53 eV), PDBF-Th (Eg 52.43 eV), PDBT-3MeTh (Eg 5
2.63 eV), PDBF-3MeTh (Eg 52.46 eV), PDBT-3HexTh (Eg 5
2.89 eV), and PDBF-3HexTh (Eg 52.75 eV), we found that
due to enhanced interchain packing of polymer chains
caused by alkyl substituents on thiophene rings and raising
their subtle electronic changes on the conjugated block, the
entire absorption spectra of DBT-3HexTh and DBF-3HexTh
are redshifted compared with all the 3-HexTh/3-MeTh end-
capped polymers.12 In agreement with their results on elec-
tropolymerization behavior, the absorption spectra of 3-DTh
end-capped polymers stayed slightly blueshift compared
with those of 3HexTh end-capped polymers due to the
increased strain of the longer dodecyl group (decreased
orbital overlap and conjugation). Furthermore, the entire
absorption bands of DBF-based polymers except for PDBF-
3DTh shifted to the lower energy region compared with
DBT-based polymers. As a consequence, a band gap reduc-
tion was observed for DBF-based polymers owing to the sub-
stitution of oxygen atom compared with DBT-based polymers
(sulfur atom).
Upon the oxidation process of the polymer films, the absorp-
tions in the UV-region started to decrease and new bands
were intensified due to the formation of lower energy charge
carriers such as polarons and bipolarons on the polymer
backbone in the visible-region and near-IR-region. In the
case of PDBT-3DTh, as the valence-conduction band at
350 nm diminished, a new band started to intensify at about
575 nm, accompanied with a new broad band around
1050 nm, which were due to the formation of lower energy
charge carriers on the polymer backbone, along with a color
change. Furthermore, both spectra recorded during potential
cycling at 1.00 V to 1.55 V for PDBT-3DTh and 1.0 V to 1.4 V
for PDBF-3DTh passed through clear isosbestic points at
TABLE 4 Colorimetric Data for Synthesized Polymers. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
CIE Color Coordinates
Colors of Polymers
Neutral Oxidized
Polymers
Neutral
79.0822
Oxidized
48.2419
PDBT-3DTh
L*
a*
b*
L*
2.9496
2.5041
83.2266
7.6619
217.2970
71.3795
FIGURE 7 Spectroelectrochemistry of PDBT-3DTh (between
1.00 V and 1.55 V, DE 5 0.05 V) and PDBF-3DTh (between 1.0 V
and 1.4 V, DE 5 0.05 V) on the ITO coated glass in MeCN-
Bu4NPF6(0.1 mol L21). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
PDBF-3DTh
a*
b*
1.5351
7.0708
2.3060
21.7633
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TABLE 5 Electrochromic Parameters of the Polymers at Differ-
sion in thin films), magnitude of the applied potential, film
thickness, and morphology of the thin film.26
ent Wavelengths
Deeply, for all the 3-alkylthiophenes end-capped DBT/DBF
hybrid electrochromic polymers, DBT as inner unit based elec-
trochromic polymers displayed higher optical contrast ratio,
faster response time, and higher CE compared with DBF-based
polymers. We ascribed this phenomenon to the reason that
low onset oxidation potential for DBT based monomers make
it reality to obtain better polymer films at relatively low poten-
tials. Further, 3-MeTh/3-HexTh end-capped polymers exhibited
better electrochromic properties, mainly attributed to the
electron-donating effect of longer alkyl chain. Howeve, due to
the introduction of dodecyl group on the thiophene ring, 3-DTh
end-capped DBT/DBF polymers displayed commonly electro-
chromic properties becauese of decreasing the co-planarity and
conjugation of aromatic systems.
Response time (s)
Wavelength
Sample (nm)
CE
DT
Oxidation Reduction (cm2 C21
)
PDBT-
3DTh
350
10.53% 8.6
9.6
171
575
25.23% 7.4
16.11% 8.6
4.59% 8.2
8.8
9.2
8.4
197
136
87
1050
325
PDBF-
3DTh
550
950
8.69% 6.6
5.94% 6.6
9.0
8.8
93
78
around 410 nm, indicating both polymer films being inter-
converted between their neutral and oxidized states.
Optical memory, one of the most important characteristics of
electrochromic materials was monitored at 1050 nm as a
function of time at 21.0 V and 1.4 V by applying the poten-
tial for 2 s for each 100 s time interval. As depicted in Sup-
porting Information Figure S12, both polymer films in yellow
colored state showed a true permanent memory effect since
there was a little transmittance change under the applied
potential (21.0 V). In purple colored state, the polymers
were rather less stable in terms of color persistence, which
is probably due to the dopants reinclusion into the films and
make the films partly doped.
To determine the color change, the CIE 1976 (L*, a*, b*) color
space and photographs of the polymers were determined, in
which L* is the parameter of the lightness, a* is the red-green
balance and b* is yellow-blue balance (2a* and 1a* corre-
spond to green and red, while 2b* and 1b* correspond to
blue and yellow, respectively).32 The colorimetric data with a
D65 illuminant as detailed for synthesized polymers are given
in Table 4. Globally, fully oxidized state polymer films on one
piece of ITO-coated glass were easy to obtain, and both poly-
mers illustrated similar colors (blue in the oxidized state and
light gray in the neutral state), which were similar to PDBT-
3HexTh and PDBF-3HexTh.12 The color would relate to the
alkyl chain of thiophene, since the EDOT and thiophene end-
capped polymers exhibited yellow in the neutral state.11,12
CONCLUSIONS
In summary, the synthesis and electrochemical polymeriza-
tion of DBT-3DTh and DBF-3DTh were comprehensively
reported. As-formed polymers PDBT-3DTh and PDBF-3DTh
were studied by cyclic voltammetry, UV-Vis, fluorescence,
morphology, and thermogravimetric analysis. Further, their
spectroelectrochemistry and electrochromism were also sys-
tematically investigated. PDBF-3DTh exhibited better redox
stability and higher decomposition temperature than PDBT-
3DTh. Reversely, PDBT-3DTh possessed lower optical band
gap of 2.82 eV compared with PDBF-3DTh (2.72 eV). Spec-
troelectrochemistry displayed that both polymers were blue
to light gray electrochromic polymers. The maximal contrast
for PDBT-3DTh was measured to be 25.23% at 575 nm with
the highest CE of 196 cm2 C21. Due to the introduction of
dodecyl group on the thiophene ring, 3-dodecylthiophene-
linked DBT/DBF polymers displayed commonly electrochro-
mic properties becauese of decreasing the planarity and
conjugation of aromatic systems.
Electrochromic Switching Studies
Optical contrast (or percent transmittance, refers to the per-
centage transmittance change (DT%) at a specified wave-
length), switching time (or response time, defined as the nec-
essary time for 95% of the full optical switch), and coloration
efficiency (CE) are very important parameters to evaluate
electrochromic materials. They were analyzed by changes that
occurred in the transmittance while switching the potential
step wisely between the neutral state and oxidized state with
a residence time of 10 s. Electrochromic parameters of the
polymer films were summarized in Table 5.
It is clearly seen from the transmittance curves in Support-
ing Information Figure S11 that PDBF-3DTh displayed sub-
standard optical contrast ratios. However, the obtained
polymer PDBT-3DTh showed superior optical contrast com-
pared with PDBF-3DTh, the optical contrast of which at
575 nm was found to be 25.23% with the highest CE value
of 196.32 cm2 C21. However, these obtained polymers exhib-
ited slow response time in the oxidation and reduction proc-
esses, owing to the ionic conductivity of the electrolyte,
accessibility of the ions to the electroactive sites (ion diffu-
DBT/DBF-heterocycle hybrid electrochromic polymers have
been reported continuously in our lab. The heteroatom substi-
tution in similar systems can affect the electrochemical and
optoelectronic properties of conjugated structures. DBT-based
systems are electron richer than DBF-based systems, as a
result, DBT-based conjugated polymers showed better electro-
chemical and electrochromic performances than DBF-based
conjugated polymers due to the lower electronegativity, larger
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10 B. Wex, B. R. Kaafarani, E. O. Danilov, D. C. Neckers, J.
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atom. Moreover, substituents on the end units affect the perform-
ance not only through modulating the electronic nature but also
by changing steric hindrance. Therefore, there is still an ongoing
dynamic and further research on electrochromism in respect of
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691–695.
We are grateful to the National Natural Science Foundation of
China (grant numbers: 51303073, 51463008, 51572117),
Ganpo Outstanding Talents 555 Projects (2013), the Training
Plan for the Main Subject of Academic Leaders of Jiangxi Prov-
ince (2011), the Science and Technology Landing Plan of Uni-
versities in Jiangxi province (KJLD12081), Provincial Projects
for Postgraduate Innovation in Jiangxi (YC2014-S431, YC2014-
S441) and Scientific Research Projects of Jiangxi Science &
Technology Normal University (2014QNBJRC003) for their
financial support for their financial support of this work.
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