Poly(thieno[3,4- b ]-1,4-oxathiane): Medium effect on electropolymerization and electrochromic performance

Article abstract of DOI:10.1021/la503948f

The asymmetrical sulfur analog of 3,4-ethylenedioxythiophene (EDOT), thieno[3,4-b]-1,4-oxathiane (EOTT), was synthesized, and its electropolymerization was comparatively investigated by employing different solvent-electrolyte systems (room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF₆), CH₂Cl₂-Bu₄NPF₆, and CH₂Cl₂-BmimPF₆). Further, the effect of solvents and supporting electrolytes on the structure, morphology, electrochemical, electronic, and optical properties and electrochromic performance of the obtained poly(thieno[3,4-b]-1,4-oxathiane) (PEOTT) films were minutely studied. PEOTT film with a band gap (E_g) of about 1.6 eV could be facilely electrodeposited in all the solvent-electrolytes and displayed excellent electroactivity, outstanding redox stability in a wide potential window, and improved thermal stability. Cyclic voltammetry showed that EOTT could be electropolymerized at a lower oxidation potential in BmimPF₆ (～1.0 V vs Ag/AgCl) due to several advantanges of RTIL BmimPF₆ itself, such as high intrinsic conductivity and mild chemical conditions, etc., and the resulting PEOTT film exhibited compact morphology with better electroactivity and stability and higher electrical conductivity. On the other hand, PEOTT films from all the sovent-electrolytes also showed the electrochromic nature by color changing from gray blue to green, and further kinetic studies revealed that PEOTT had decent contrast ratios (36%), higher coloration efficiencies (212 cm²/C in BmimPF₆), low switching voltages, moderate response time (1.2 s), excellent stability, and color persistence. From these results, PEOTT provides more plentiful electrochromic colors and holds promise for display applications.

Full text of DOI:10.1021/la503948f

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Article
Poly(thieno[3,4-b]-1,4-oxathiane) : Medium Effect on
Electropolymerization and Electrochromic Performance
Zhipeng Wang, Jingkun Xu, Baoyang Lu, Shimin Zhang, Leiqiang Qin, Daize Mo, and Shijie Zhen
Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503948f • Publication Date (Web): 03 Dec 2014
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Poly(thieno[3,4-b]-1,4-oxathiane): Medium Effect on
Electropolymerization and Electrochromic Performance
a
a,
b,
a
a
Zhipeng Wang , Jingkun Xu *, Baoyang Lu *, Shimin Zhang , Leiqiang Qin , Daize
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a
a
Mo , Shijie Zhen
a
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology
Normal University, Nanchang 330013, PR China
b
School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang
3
30013, China
ABSTRACT: The asymmetrical sulfur analog of 3,4ꢀethylenedioxythiophene (EDOT),
thieno[3,4ꢀb]ꢀ1,4ꢀoxathiane (EOTT), was synthesized and its electropolymerization
was comparatively investigated by employing different solventꢀelectrolyte systems
(
room temperature ionic liquid 1ꢀbutylꢀ3ꢀmethylimidazolium hexafluorophosphate
(BmimPF ), CH Cl ꢀBu NPF and CH Cl ꢀBmimPF ). Further, the effect of solvents
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and supporting electrolytes on the structure, morphology, electrochemical, electronic
and optical properties and electrochromic performance of the obtained
poly(thieno[3,4ꢀb]ꢀ1,4ꢀoxathiane) (PEOTT) films were minutely studied. PEOTT film
g
with a band gap (E ) of about 1.6 eV could be facilely electrodeposited in all the
solventꢀelectrolytes and displayed excellent electroactivity, outstanding redox stability
in a wide potential window, and improved thermal stability. Cyclic voltammetry
showed that EOTT could be electropolymerized at a lower oxidation potential in
6 6
BmimPF (~1.0 V vs. Ag/AgCl) due to several advantanges of RTIL BmimPF itself,
such as high intrinsic conductivity and mild chemical conditions, etc., and the resulting
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PEOTT film exhibited compact morphology with better electroactivity and stability
and higher electrical conductivity. On the other hand, PEOTT films from all the
soventꢀelectrolytes also showed the electrochromic nature by color changing from grey
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blue to green, and further kinetic studies revealed that PEOTT had decent contrast
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ratios (36%), higher coloration efficiencies (212 cm /C in BmimPF
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), low switching
voltages, moderate response time (1.2 s), excellent stability and color persistence. From
these results, PEOTT provides more plentiful electrochromic colors and holds promise
for display applications.
ꢀ
INTRODUCTION
Since the epochꢀmaking discovery made by Shirakawa et al. in the late 1970s that
polyacetylene could attain extremely high electrical conductivities, the field of
1
conducting polymers (CPs) has progressed rapidly. Among the numerous polymers
that have been grown and studied over the past four decades,
poly(3,4ꢀethylenedioxythiophene), often referred to as PEDOT, has been developed
into one of the most successful materials from both academic and industrial point of
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view. The undesirable presence of
α,βꢀcouplings is prevented by blocking the
β
ꢀpositions of the thiophene ring, resulting in a more regioregular material with a
coplanar heterocyclic backbone and high oxygenic pendant group. This renders
PEDOT a unique combination of environmental properties—low oxidation potential,
g
moderate bandgap (E ca. 1.6 eV), electrically conductive, optically transparent in its
5
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thin, oxidized film at ambient and elevated temperature. As a result, PEDOT has
generated a considerable current interest in relation to numerous technological
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applications, e.g., lightꢀemitting diodes (OLEDs/PLEDs), supercapacitors, sensors,
electrochromic devices.
The tempting properties and extensive applications of organic electronic materials
and devices demand the ability to tune the rigidity and planarity of conjugated
molecules in order to control parameters such as conjugation length, electrical
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conductivity, and mobility. By changing the heteroatom in the conjugated backbone or
peripheral ring, the delicate tuning and moderate control of electronic properties (take
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g
E for example) could be realized. Therefore, many research groups have paid
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attention to the development of EDOT analogs. Up to now, ringꢀsize variations,
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sulfur analogs,
selenium analogs and oxygenic analogs have been developed.
We also summarized the sulfur analog poly(3,4ꢀethylenedithiathiophene) (PEDTT) in
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our previous research work. The allꢀsulfur analog has much larger band gap and
hypsochromic shift maximum absorption in the dedoped film than PEDOT, which was
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reported by some research groups.
The reasons accounting for this phenomenon
are as follows: S…S steric repulsion among adjacent EDTT monomer units leads to the
distortion of the πꢀconjugated backbone and this disparity is owed more to the
conformation of the polymers than the inductive and resonance effects of the chalcogen
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substituents. This results in the shortening of the effective conjugation length and
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probably poor solid state ordering.
Among them, thieno[3,4ꢀb]ꢀ1,4ꢀoxathiane
(
EOTT), an intermediate structure between EDOT and EDTT, was firstly synthesized
1
9
by Roncali’s group in 2002. In this structure only one oxygen atom of EDOT in the
ethylenedioxy bridge is replaced by sulfur. As expected, the electrochemical and
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optical properties of the monomer and the resulting polymer were intermediate between
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those of their parent systems, PEDOT and PEDTT. However, systematic research on
the synthesis and optoelectronic properties of this sulfur analog PEOTT was quite
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limited.
In contrast to traditional molecular solvent/supporting electrolyte system
propylene carbonate (PC)ꢀBu NPF , e.g.), room temperature ionic liquids (RTILs)
(
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have been attracting increasing interest because of their combined properties of high
thermal stability, inflammability and low volatility, good intrinsic conductivity, wide
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potential window (over 4 V) and extremely redoxꢀrobust.
Because RTILs are
composed entirely of ions, the nature of which will determine their physical and
electrochemical properties themselves and also influence the polymer’s
electrochemical activity. Pringle’ group once reported that the use of
pyrrolidiniumꢀbased TFSA RTIL produced smoother and denser polythiophene films
than those from the imidazolium analog, demonstrating different RTIL influenced the
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nature on the morphology and electrochemical activity of the polymers. Moreover, it
was shown that polypyrrole films prepared in PCꢀBu NPF but measured in BmimPF
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showed improved electrochemical performance and increased lifetime up to a million
cycles revealing the use of RTILs as the electrolyte could be beneficial to the
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electromechanical efficiency and lifetime of conducting polymers. Also, the use of
the RTILs as both the growing medium and an electrolyte for the electroꢀsynthesis and
chemical cycling of polypyrrole films resulted in significantly smoother surface
morphology and considerably improved electrochemical activity than films grown in
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4 6 3 3
PCꢀBu NPF system. Fuchigami et al. reported that use of neat RTIL EMICF SO
revealed improved polymerization rate for the electroꢀsynthesis of pyrrole and
enhanced electrochemical capacity, doping level, conductivity and smoother
morphological structure of polypyrrole film than those from EMICF SO ꢀH O and
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EMICF
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SO ꢀMeCN. Our group reported the electrosynthesis of polyselenophene,
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polyfluorene, and PEDTT in BmimPF
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ꢀbased electrolytes. The use of BmimPF
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to the formation of smoother and more compact films and the electrical conductivity
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of asꢀformed polymer films was much higher than that from the conventional medium
(
molecular solvent/supporting electrolyte system, MeCNꢀBu NPF ).
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Owing to quite limited research on this sulfur analog PEOTT, we herein
synthesized EOTT (the asymmetrical analog of EDOT) and systematically investigated
the medium (different solvents and supporting electrolytes (RTIL BmimPF
CH Cl ꢀBu NPF , CH Cl ꢀBmimPF )) effect on the structure, morphology,
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electropolymerization behavior, electrochemical and spectroelectrochemical properties
and electrochromic performance of the asꢀprepared PEOTT films.
ꢀ
EXPERIMENTAL SECTION
Materials. 3,4ꢀDibromothiophene was purchased from Vita Chemical Reagent Co.
Ltd (Shanghai, China). BmimPF
Physics, Chinese Academy of Sciences and dried under vacuum before use.
Dichloromethane (CH Cl , analytical grade; Beijing East Longshun Chemical plant)
was purified by distillation with calcium hydride and stored with 4 Å molecular sieve
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was purchased from Lanzhou Institute of Chemical
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prior to use. Tetrabutylammonium hexafluorophosphate (Bu NPF , 99%; Energy
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Chemical) was dried under vacuum at 60 °C for 24 h before use. Other reagents were all
analytical grade and used as received without further treatment.
Electrosynthesis and Electrochemical Tests. Electrochemical polymerization
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was carried out using 0.2 mol L EOTT in CH Cl ꢀBu NPF (0.1 M),
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2 2 6 6
CH Cl ꢀBmimPF (0.1 M) and BmimPF systems. Prior to polymerization, all the
solutions were deꢀaerated by a dry nitrogen stream for 20 min and maintained under a
slight overpressure during the electrochemical experiments to avoid the effects of
oxygen. Film thickness was controlled by the total charge passed through the cell,
which was read directly from the Iꢀt curves by computer. After polymerization, the
polymer films were washed repeatedly with anhydrous monomerꢀfree dichloromethane
to remove the electrolyte and monomer/oligomers.
All of the electrochemical tests were conducted in monomerꢀfree
solventꢀelectrolytes in a oneꢀcompartment cell. Pt wires with diameter of 1 mm were
used as the working (WE) and counter electrode (CE), respectively. An Ag/AgCl
electrode dipped in the solution served as the reference electrode (RE), and it revealed
sufficient stability during the experiments.
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Characterization. H NMR and C NMR spectra were recorded on a Bruker
Spectrospin AV 400 Spectrometer with TMS as the internal standard and CDCl as the
3
solvent. Infrared spectra were determined with a Bruker Vertex 70 Fourierꢀtransform
infrared (FTꢀIR) spectrometer with samples in KBr pellets. Scanning electron
microscopy (SEM) measurements were made with a VEGA\\LSU scanning electron
microscope (Tescan) with the polymer deposited on the ITOꢀcoated glass. The
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electrical conductivity was measured by the standard fourꢀpoint technique on pressed
pellets at room temperature. Electrochemical, spectroelectrochemical, and kinetic
studies were carried out on a Model 263 potentiostatꢀgalvanostat (EG&G Princeton
Applied Research) and a Cary 50 UVꢀvisꢀNIR spectrophotometer. Thermogravimetric
analysis (TGA) was performed with a Pyris Diamond TG/DTA thermal analyzer
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(PerkinElmer) under a nitrogen stream from 25 to 800 °C at a heating rate of 10 °C/min.
The intrinsic ionic conductivities of the solventꢀelectrolyte systems were measured
with the conductivity meter from Kedida Electronics Co., Ltd in Shenzhen, China. The
viscosities were measured by a digital viscometer (NDJꢀ5S, Shanghai Hengping Meter
Co., Ltd., Shanghai, China).
Spectroelectrochemical measurements were carried out under various applied
potentials (ꢀ1.0~1.0 V with potential increment of 0.1 V) to investigate the absorption
spectra of PEOTT films. The spectroelectrochemical cell consists of a quartz cell (1×1
×
4.5 cm), a Ag/AgCl wire (RE), a Pt wire (CE), and an ITO/glass as the transparent
working electrode (WE). ITOꢀcoated glass slides (5×0.9×0.1 cm, 10 ꢀ per square)
were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. It should be
noted here that the quartz cell filled with monomerꢀfree solventꢀelectrolytes and
ITO/glass without a deposited film was used as the background for
spectroelectrochemical measurements. The measurements were carried out in
monomerꢀfree solventꢀelectrolyte systems.
Electrochromic switching studies were carried out to obtain changes in the optical
contrast with time during repeated potential stepping (ꢀ1.0 and 0.8 V) between the
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reduced and oxidized states. The optical density (ꢁOD) at a specific wavelength (λmax
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is determined by transmittance %T values of electrochemically oxidized and reduced
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films, using Eq. (1):
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ꢁ
OD = log(T /T )
(1)
ox red
The coloration efficiency (CE) is defined as the relation between the
injected/ejected charge as a function of electrode area (Q ) and the change in ꢁOD at a
specific dominant wavelength (λmax) as illustrated by the following Eq. (2):
CE = ꢁOD/Q (2)
d
2
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d
Synthesis.
,4-Dimethoxythiophene. KI (0.173 g, 1.042 mmol) and CuO powder (5.015 g,
63.042 mmol) were added to a dry methanol solution of 30 wt.% sodium methoxide
47.432 g) under nitrogen flux. 3,4ꢀDibromothiophene (15.493 g, 64.040 mmol) was
3
(
added dropwise to the mixture in the round bottom flask. The mixture was stirred and
refluxed at 95 °C for 3 days then was cooled to room temperature and distilled on a
rotary evaporator to remove methanol. The crude product was concentrated with rotary
evaporator and purified by a silica gel column chromatography using petroleum ether
as the eluent to afford lightꢀyellow liquid. The product was obtained as colorless oil
1
(
6.446 g). Yield: 70%. H NMR (400 MHz, CDCl
3
, ppm): δ 6.19 (s, 2H), 3.86 (s, 6H);
1
3
1
C NMR (100 MHz, CDCl , ppm): 147.81, 96.25, 57.54 (See Figure S1 and S2 for H
3
13
NMR and C NMR of 3,4ꢀdimethoxythiophene).
Thieno[3,4-b]-1,4-oxathiane (EOTT). EOTT was synthesized by acidꢀcatalyzed
nucleophilic substitution of 3,4ꢀdimethoxythiophene with 2ꢀmercaptoethanol
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,19
according to previously published procedures, as shown in Scheme 1. Yield: 75%.
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3
H NMR (400 MHz, CDCl , ppm): δ 6.74 (t, J = 4.4 Hz, 2H), 6.47 (t, J = 3.6 Hz, 2H),
13
4
.38 (s, 1H), 3.01 (s, 1H); C NMR (100 MHz, CDCl
3
, ppm): 148.46, 115.84, 115.11,
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02.24, 66.64, 24.39 (See Figure S3 and S4 for H NMR and C NMR of EOTT).
Scheme 1. Synthetic route of EOTT.
ꢀ RESULTS AND DISCUSSION
ꢀ1
Figure 1. Cyclic voltammograms (CVs) of 0.2 mol L EOTT in (A) CH
2 2 4 6
Cl ꢀBu NPF
(0.1 M), (B) CH
respectively.
Cl ꢀBmimPF (0.1 M) and (C) BmimPF at the scan rate of 20 mV/s,
2 2 6 6
Cyclic voltammetry. The electropolymerization behavior of EOTT was
investigated in different solventꢀelectrolyte systems, i.e., neat BmimPF
CH Cl ꢀBu NPF and CH Cl ꢀBmimPF . CH Cl ꢀBmimPF was also employed for
comparison due to the same solvent (CH Cl ) but different electrolytes (different
6
,
2
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2
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2
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+
+
cations Bu N vs. [Bmim] ). The onset oxidation potential (Eonset) of EOTT,
4
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corresponding to the oxidation of EOTT to its radical cation, was both initiated at
2 6
1.09 V vs. Ag/AgCl in CH Cl ꢀBu NPF and CH Cl ꢀBmimPF (See Figure S5 for
2
2
4
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2
anodic polarization curves of EOTT in different media), lower than that in
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MeCNꢀBu
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(0.1 M) (nearly 1.2 V vs. Ag/AgCl ). Interestingly, Eonset was further
. Generally, the lower the
onset of the monomer is, the less possibility the side reactions (overoxidation, e.g.)
brought down to about 1.01 V vs. Ag/AgCl in neat BmimPF
6
E
0
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occur, the higher quality the polymer films are formed.
As shown in Figure 1, CVs of EOTT during the electrochemical polymerization
were carried out in different media. On the first scan, the formation of the current loop
2
8
over Eonset was characteristic for the nucleation process in all the systems. During
subsequent potential scanning, progressive increase of peak currents and decrease of
oxidation onset potentials in CVs could be attributed to the gradually growing amount
of the polymer films with increased number of monomeric units deposited onto the
26,27
electrode surface.
In the cathodic scan process, the shift of peak potentials
demonstrated the increase of electrical resistance in the polymer film and the
26,27
overpotential required to overcome this resistance.
The apparent reduction peaks
may be assigned to the transformation from the quinoid structure to the aromatic
2
6,27
state.
Similar phenomenon was also found during the electropolymerization of
19
4 6
EOTT in MeCNꢀBu NPF .
The detailed information of electrochemical parameters for PEOTT
electrosynthesized and measured in different solventꢀelectrolyte systems were
presented in Table S1. Comparison of CVs in different media (Figure 1) shows that
redox peak currents (electroactivity) in neat BmimPF
CH Cl ꢀBu NPF and CH Cl ꢀBmimPF , which may be attributed to the higher
intrinsic ionic conductivity of BmimPF6 ( = 1.46 mS/cm at 25 °C measured in our lab
6
are much higher than those in
2
2
4
6
2
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and 1.45 mS/cm from the reference ) than those of CH
CH Cl ꢀBmimPF (0.78 and 0.81 mS/cm at 25 °C measured in our lab, respectively). In
addition, BmimPF possesses much higher viscosity ( = 275 mP s at 25 °C measured
in our lab and 305 mP s from the reference ) than those of CH Cl ꢀBu NPF and
2 2 4 6
Cl ꢀBu NPF and
2
2
6
6
η
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2
2
4
6
2 2 6
CH Cl ꢀBmimPF (0.50 and 0.55 mP s at 25 °C measured in our lab, respectively),
which contributes to the slower ion/molecular transport kinetics and the radicalꢀradical
coupling of monomers, and further oxidation of oligomers and polymers deposited onto
25
the electrode will be favorably accumulated under slow diffusion condition.
To optimize the polymerization potentials, a set of currentꢀtransients (Iꢀt) during
the electropolymerization of EOTT at different applied potentials (as shown in Figure
S6) were recorded in the aforementioned media. No polymer film was found on the
working electrode until the applied potential reached the threshold value. Considering
the overall factors affecting the quality of the prepared films, such as moderate
polymerization rate, negligible overoxidation, regular morphology, and good
2
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adherence against the working electrode,
PEOTT films used for all the
characterization were electrodeposited by the chronoamperometry method at the
constant potential of 1.15 V vs. Ag/AgCl.
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Figure 2. FTꢀIR spectra of EOTT (A); PEOTT electrosynthesized in different
solventꢀelectrolyte systems: CH
C) and BmimPF (D) containing 0.2 M EOTT.
Structural characterization. In order to elucidate the structure of PEOTT and
2 2 4 6 2 2 6
Cl ꢀBu NPF (0.1 M) (B), CH Cl ꢀBmimPF (0.1 M)
(
6
0
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interpret the polymerization mechanism, FTꢀIR spectra of EOTT and PEOTT
electrosynthesized from different solventꢀelectrolytes were recorded, as shown in
Figure 2. The detailed peak assignment and comparison for EOTT, PEOTT and
PEDOT are summarized in Table S2. Similar to previous results of EDOT and
30
ꢀ1
PEDOT, one can notice that the peaks at 1180 and 890 cm for EOTT could be
attributed to the =C–H inꢀplane and outꢀofꢀplane deformation vibrations, respectively.
ꢀ1
In addition, the band at 3101 cm for EOTT can be ascribed to the =C–H vibration of
the thiophene ring. However, these three peaks disappeared in the spectrum of polymer,
demonstrating that EOTT was dominantly oxidized to form PEOTT through
6
α,α’ꢀcoupling. In the case of PEOTT obtained in BmimPF , the peaks at 1475 and 1446
ꢀ1
cm could be assigned to asymmertic and symmetric C=C stretching vibration,
ꢀ1
respectively. Further, the vibration at 1311 cm is ascribed to the stretching mode of
single C–C bond. The vibration mode from the C–S bond in the thiophene ring can be
ꢀ1
found at 919 cm . For the ethyleneoxythia group, the absorption peaks originated from
the CH symmetric and asymmetric stretching vibrations could be observed at 2964,
2
ꢀ1
ꢀ1
2
874, and 2935 cm , respectively. In addition, the peak at 1081 cm demonstrated the
C–O vibration in the ethyleneoxythia ring. Besides, the strong absorption peak at 838
ꢀ1
ꢀ
6
cm is probably due to the incorporation of PF ions into the polymer film during the
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doping process.
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Figure 3. SEM micrographs of PEOTT films deposited electrochemically on the
ITO/glass electrode from solventꢀelectrolyte systems: CH Cl ꢀBu NPF (0.1 M) (A and
D), CH Cl ꢀBmimPF (0.1 M) (B and E) and BmimPF (C and F) containing 0.2 M
2
2
4
6
2
2
6
6
EOTT. Deposition charge: 20 mC. ITO/glass dimension: 1.5×0.3×0.1 cm.
Morphology and conductivity. The doped films polymerized from
CH Cl ꢀBu NPF and CH Cl ꢀBmimPF (Figure 3 A & B) presented a ridgeꢀlike
2
2
4
6
2
2
6
outlook. Clusters of granules distributed unevenly on the “flatlands” and “ridge” in
Figure 3 A, while nodules spreaded randomly on the “ridge” and “flatlands” in Figure 3
6
B. In sharp contrast, the polymer synthesized in BmimPF (Figure 3 C) presented
2
7
uniform and smooth morphology. The reason for the achievement of homogeneous
polymer films in BmimPF is probably due to the high viscosity nature of BmimPF
6
6
27
itself. The diffusion rate of EOTT monomer and its radical cations in BmimPF
6
is
2
1,25
rather slow in comparison with conventional organic solvents,
thus the
polymerization rate of EOTT is limited by its slow diffusion in BmimPF
6
, leading to
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slower deposition of the polymer film on the electrode surface. The limited deposition
6
rate, together with the milder chemical condition provided by BmimPF , results in the
smooth and tiny morphology of PEOTT films on the electrode surface. At high
magnifications, the surfaces of doped polymer films (Figure 3 D, E and F) have similar
morphology as described above. Therefore, the employment of ionic liquids as the
electropolymerization medium leads to the formation of smooth PEOTT film, which
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may be potentially applicable in many related fields.
The electrical conductivity of asꢀformed PEOTT films from BmimPF
6
was
NPF
as the
ꢀ1
determined to be ca. 6.7×10 S/cm, higher than those synthesized in CH
Cl
2 2
ꢀBu
4
6
−2
−1
and CH
2
Cl
2
ꢀBmimPF
6
(1.7×10 and 1.2×10 S/cm). The use of BmimPF
6
medium for the electrosynthesis of PEOTT results in remarkable increase of electrical
conductivity, which may be due to the fact that the concentration of dopant anions in
6
BmimPF is much higher than that in conventional media. Generally, the larger the
concentration of anions in the media, the higher is the doping level and thus the larger
25,27
the electrical conductivity of the polymer.
Also, the lowered oxidation potential and
mild polymerization condition of BmimPF
6
probably lead to less structural defects and
better chain arrangement, thus the quality improvement of the asꢀformed PEOTT films.
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Figure 4. CVs of PEOTTꢀcoated Pt electrode in monomerꢀfree solventꢀelectrolyte
systems: CH Cl ꢀBu NPF (0.1 M) (A), CH Cl ꢀBmimPF (0.1 M) (B) and BmimPF
6
2
2
4
6
2
2
6
(C) at potential scan rates from 300 to 25 mV/s with potential decrement of 25 mV/s; (D,
E and F) plots of anodic (ip,a) and cathodic (ip,c) peak currents vs. potential scan rates,
respectively. Deposition charge: 4.4 mC.
Electrochemistry of PEOTT film. The electrochemical behavior of the
PEOTTꢀcoated Pt electrode was studied by CV in different monomerꢀfree media
2
7
(Figure 4), respectively. Similar to previous results, steadyꢀstate CVs presented broad
redox peaks and peak currents were proportional to potential scan rates in these media
(Figure 4 D, E and F), indicating that the redox process is reversible, nonꢀdiffusional
32
and the electroactive material is well adhered to the working electrode surface. The
calculated i /i and Q /Q (anodic and cathodic ratio of peak current, oxidation and
p,a p,c
ox red
reduction ratio of charges, respectively) of PEOTT in BmimPF
than those of PEOTT in CH Cl ꢀBu NPF and CH Cl ꢀBmimPF
redox reversibility in BmimPF
6
are closer to 1.0, better
2
2
4
6
2
2
6
, indicating its better
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The doping level (γ) of PEOTT obtained in those media was determined according
2
3
to the previous literature. If an electrodeposition efficiency of 100% is assumed, the γ
values of 0.13, 0.14 and 0.25 are obtained for PEOTT obtained in CH Cl ꢀBu NPF
CH Cl ꢀBmimPF and BmimPF , respectively. The highest doping level was obtained
2
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for PEOTT obtained in BmimPF , probably because RTIL possesses higher
25
concentration of dopant ions and milder chemical conditions. The doping level of
PEOTT is a little lower compared to those generally observed for thiophene derivatives
23
(
~0.3), but higher than that for PEDTT (~0.1).
Figure 5. Longꢀterm CVs of PEOTTꢀcoated Pt electrode in monomerꢀfree
solventꢀelectrolyte systems: CH Cl ꢀBu NPF (0.1 M) (A), CH Cl ꢀBmimPF (0.1 M)
B) and BmimPF (C) upon repeated cycling at the scan rate of 150 mV/s. Deposition
charge: 4.4 mC.
2
2
4
6
2
2
6
(
6
Redox stability of PEOTT film. The longꢀterm redox stability of PEOTT film
was investigated in different media, as shown in Figure 5. The specific electrochemical
data are presented in Table S3. The scanning cycle was 10,000 in the potential scan
range of ꢀ0.3~0.9 V at the scan rate of 150 mV/s. From Figure 5, the amount of
exchange charge was preserved up to 76.5% after 10,000 cycles in CH
2 2 4 6
Cl ꢀBu NPF .
The decrease in electrochemical activity observed in CH Cl ꢀBu NPF can be
2
2
4
6
attributed to several reasons, such as the breakdown of the polymer backbone because
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of nucleophilic attack on PEOTT and the dissolution of the charged material by the
22,33
polar electrolyte medium.
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Whereas in BmimPF , PEOTT films were able to be
switched many times between oxidized and reduced states with relatively small
changes in electroactivity. Even after 10,000 cycles, the electrochemical activity almost
maintained the initial exchange charge, i.e., 100% of the exchange charge retained.
From these results, PEOTT in ionic liquid displayed better redox stability than that in
the conventional solvent systems, making it a good candidate for organic electronics
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applications.
Thermal stability of PEOTT film. From the thermogravimetric analysis of the
asꢀformed PEOTT from different media (Figure S7), PEOTT obtained in BmimPF
started to decompose at about 297 °C, which was a little higher than those obtained in
CH Cl ꢀBu NPF and CH Cl ꢀBmimPF (255 and 282 °C), demonstrating better
thermal stability of PEOTT electrosynthesized from BmimPF . The detailed
6
2
2
4
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2
2
6
6
thermogravimetric parameters of PEOTT were summarized in Table S4.
Figure 6. Spectroelectrochemistry of PEOTT films on ITO/glass electrode in
monomerꢀfree solventꢀelectrolyte systems: CH
CH Cl ꢀBmimPF (0.1 M) (B) and BmimPF (C), respectively. Applied potentials:
1.0~1.0 V.
2
Cl
2
ꢀBu
4
NPF
6
(0.1 M) (A),
2
2
6
6
ꢀ
Spectroelectrochemistry. Spectroelectrochemical studies of the electrodeposited
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PEOTT in monomerꢀfree solventꢀelectrolyte systems were performed to elucidate the
electronic properties and the results are shown in Figure 6. PEOTT films were
deposited onto an ITOꢀcoated glass, and a selfꢀmade electrochemical cell (1×1×4.5
cm) built from a quartz cuvette was placed in the optical path of the light beam in a
UVꢀvisꢀNIR spectrophotometer. Generally, upon successive increment of the applied
potential, the intensity of πꢀπ* transition was decreased and simultaneously a new band
at longer wavelengths generated, which could be attributed to the emergence of lower
energy charge carriers, namely, polarons and bipolarons (radical cations and dications)
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in the conjugated chain.
It should be noted that the monomer EOTT in CH
2
2
Cl showed a maximum
absorbance at 277 nm, lying between the previously reported parent analogs, EDOT
15
and EDTT, as shown in Figure S8. In order to obtain the UV–vis spectrum of the
polymer, PEOTTꢀcoated ITO/glass was switched between ꢀ1.0~1.0 V in different media.
In general, the spectra were initially obtained in the fully reduced state (at the applied
potential of ꢀ1.0 V for PEOTT) to determine the band gap of the polymer. The band gap
(E
g
= 1240/λonset), defined as the onset of πꢀπ* transition in the fully reduced polymer, is
estimated to be 1.60 eV (774 nm) in BmimPF for PEOTT, which is approximately
6
identical to the previously reported value of PEOTT measured in PC and MeCN and
8
also PEDOT. Neutral PEOTT film produced a grey blue color as a result of the strong
absorption of πꢀπ* transition in the visible region at around 593 nm. The electrons are
delocalized along the conjugated backbones through the overlap of aromatic πꢀorbitals,
12,34
which contributes to the πꢀπ* transition.
The πꢀπ* interband transition arising from
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an extended πꢀsystem with a filled narrow valence band and occurs in the visible range,
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leading to a grey blue coloration.
Stepwise oxidation of the polymer leads to the fading of absorbance at 593 nm and
typical evolution of peaks beyond 750 nm. This indicates that electrons are removed
from neutral PEOTT upon oxidation and lead to the formation of charge carriers in
PEOTT film (pꢀdoping). The doping process successively decreases the electronic
conjugation in PEOTT chains by lowering the number of overlapping aromatic
thiophene rings, and thus increasing the energy of πꢀπ* transition. Further oxidation
would result in the formation of polarons and bipolarons (commonly observed in
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PEDOTs) hence introducing sub band gap energy levels. Consequently, a absorbance
decrease in the visible range is observed for PEOTT as well as additional absorption
bands in the nearꢀinfrared region accompanied by its color changing from grey blue in
the neutral state to turquoise green in its oxidized state.
For a comparative study, spectroelectrochemical study of PEOTT was also
performed in CH
applied voltage of ꢀ1.0 V, the reduced PEOTT shows the maximum absorbance peak
max at 595 and 585 nm with E of 1.59 eV (782 and 780 nm), respectively. As each
2 2 4 6 2 2 6
Cl ꢀBu NPF and CH Cl ꢀBmimPF systems, respectively. At the
λ
g
potential was stepped, PEOTT switched from an absorbing persian blue color in the
neutral state to a highly transmissive dark green color in the oxidized state.
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Figure 7. Electrochromic switching and optical absorbance monitored at 600 and 900
nm between ꢀ1.0 and 0.8 V in CH Cl ꢀBu NPF (0.1 M) (A and D), CH Cl ꢀBmimPF
6
2
2
4
6
2
2
6
(0.1 M) (B and E) and BmimPF (C and F) for PEOTT recorded during double step
spectrochronoamperometry. Deposition charge: 100 mC.
Electrochromic performance. Doubleꢀpotential step chronoabsorptometry of
PEOTT film monitored at 600 and 900 nm under the applied potentials of ꢀ1.0 and 0.8
V was performed in different media to probe its electrochromic kinetic studies and the
results are shown in Figure 7. The electrochromic parameters (optical contrast ratio
(ꢁT%), response time, and CE) of PEOTT films in different solventꢀelectrolyte systems
were summarized in Table 1. The switching time was optimized to yield the maximum
transmittance contrast and the potentials were switched stepwise between neutral and
oxidized states with a residence time of 10 s.
The optical contrast ratios of PEOTT are coincident with typical electrochromic
31
CPs, such as PEDOT and its derivatives, but much higher than PEDTT in BmimPF
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(
<10%). In fact, PEDTT shows rather different conformations from PEDOT. This
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conformational change, together with the inductive and resonance effects of chalcogen
substituents in the polymer, results in the shortening of the effective conjugation length
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Table 1. Electrochromic parameters for PEOTT in different media
Response Time (s)
Oxidation Reduction
Coloration
Wavelength
T
neut
(%)
T
oxi
(%)
Media
ꢁT
Efficiency
(
nm)
2
(cm /C)
600
900
10.7 46.9 36.2
36.8 14.4 22.4
12.8 44.6 31.8
1.2
1.3
182
119
183
91
CH Cl ꢀ
2
2
Bu
4
NPF
6
1.6
3.1
6.1
3.1
4.1
4.1
4.3
6.2
6.4
8.0
6
00
900
00
900
CH Cl ꢀ
2
2
BmimPF
6
6
23.8 13.9
30.3 8.0
9.9
6
22.3
212
116
BmimPF
19.0 35.6 16.6
The response time is defined as the time required reaching 95% of the full change
29
in transmittance after the switching of the potential. In comparison with PEOTT
tested in CH Cl ꢀBu NPF and CH Cl ꢀBmimPF , the response time of PEOTT
2
2
4
6
2
2
6
obtained in BmimPF
6
is longer, mainly because the poor fluidity of highly viscous ionic
21
liquid which considerably slows down the diffusing rate of counterions, but
28
apparently much shorter than PEDTT (>10 s) studied in BmimPF6.
The response time was found to be longer for PEOTT from the oxidized state to
the reduced state than that from the reduced state to the oxidized state in all the three
media. The oxidation of PEOTT films corresponds to the formation of cationic charges
ꢀ
along the polymer chains accompanied by the insertion of PF
6
anion (doping process)
in order to preserve the overall electroneutrality, while the reduction process takes the
27
doped polymer to its neutral form with concurrent release of counter anions. For
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ꢀ
PEOTT, the incorporation of PF
6
in the doping process is easily obtained but its
3
4
extraction from the polymer structure rather slow.
The CE is another important characteristic for electrochromic materials and
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defined as the injected/ejected amount of the charge as a function of Q and the change
d
in ꢁOD at a specific dominant wavelength (λmax). In this study, CE was measured as
2
1
82 and 119 cm /C at 600 and 900 nm in CH
2 2
Cl ꢀBu
4
NPF
6
, respectively. In the ionic
Cl ꢀBu NPF
(137
cm /C) and PEDTT in BmimPF (127 cm /C). The lowered polymerization
2
liquid BmimPF
6
, CE (212 cm /C at 600 nm) was higher than that in CH
2
2
4
6
,
which was also better than that of the parent polymer PEDOT in MeCN/LiClO
4
2
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2
28
6
6
potential and milder chemical condition of BmimPF may result in less structural
defects, better chain arrangement and the quality improvement of the asꢀformed
PEOTT films, which may account for the higher CE as compared to the PEOTT film in
2 2 4 6
CH Cl ꢀBu NPF .
Scheme 2. Diagrammatic representation of S—O and S—S intramolecular interactions
in PEOTT backbone.
ꢀ
DISCUSSION
The expected S…O interactions in segments of allꢀanti conformers were shown in
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Scheme 2. PEOTT exhibited better electrochromic properties than PEDTT in
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17
6
BmimPF . In fact, PEDTT has rather different conformations from PEDOT. In the
case of PEOTT, the partial replacement of oxygen by sulfur atom results in an increase
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of Eox and a hypsochromic shift λmax arising from the decreased rigidity of the
19
conjugated structure compared to PEDOT. Nevertheless, PEOTT still exhibited
8
almost the same E
g
as that of PEDOT, because the steric hindrance to coplanarity is
counterbalanced by the interplay of strong S…O and moderate S…S intramolecular
16
interactions, which led to a stabilization of a quasiplanar conformation (there may be
less aromatic nature and more quinoid in PEOTT backbone). Therefore, changing the
single heteroatom in the peripheral ring tunes the flexibility of the backbone without
changing the band gap of 1.6 eV. To sum up, PEOTT is much more rigid than PEDTT
and a little more flexible than PEDOT, as has been confirmed by spectroscopy, the
8
Xꢀray structure of the dimer, and DFT calculations. All these make PEOTT a
promising candidate material for organic electronics.
ꢀ CONCLUSIONS
In summary, the EDOT sulfur analog, EOTT was synthesized and electropolymerized
6 2 2 6 2 2 4 6
from different media (BmimPF , CH Cl ꢀBmimPF and CH Cl ꢀBu NPF ). The effect
of solvents and supporting electrolytes on the morphological structure,
electropolymerization behavior, electrochemical and spectroelectrochemical properties
and electrochromic performance of asꢀformed PEOTT films were evaluated. Due to
6
several advantanges of RTIL BmimPF itself, such as high intrinsic conductivity and
mild chemical conditions, etc., EOTT could be electropolymerized at lower oxidation
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potentials in BmimPF
6
g
and the resulting PEOTT film with a band gap (E ) of about 1.6
eV displayed compact morphology with better electroactivity, outstanding redox
stability, higher electrical conductivity, and improved thermal stability. These results
0
1
2
3
4
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8
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0
1
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5
6
7
8
9
0
revealed the improved properties of PEOTT obtained in neat BmimPF compared with
6
conventional media, proving that ionic liquids are excellent media for the
electrochemical generation of conducting polymer films. Besides, PEOTT films from
all the soventꢀelectrolytes also exhibited the electrochromic nature by color changing
from grey blue to green, and further kinetic studies revealed that PEOTT had decent
2
contrast ratios (36%), higher coloration efficiencies (212 cm /C in BmimPF ), low
6
switching voltages, moderate response time (1.2 s), excellent stability and color
persistence. With these results, PEOTT may find application in displays,
electrochemical sensors, and organic electronic devices, and also holds promise for the
design of a new variety of of optoelectronic materials.
ꢀ
ASSOCIATED CONTENT
ꢁ Supporting Information
1
13
H NMR and C NMR spectra of 3,4ꢀdimethoxythiophene and EOTT; Anodic
oxidation and chronoamperometric curves; Thermogravimetric analysis of PEOTT;
Spectroelectrochemistry and color changes of PEOTT films electropolymerized and
tested in different solvents and supporting electrolytes; UVꢀvis of EOTT;
Electrochemical, FTꢀIR vibration assignment and thermogravimetric parameters for
PEOTT electropolymerized from the EOTTꢀcontaining and tested in EOTTꢀfree media.
This material is available free of charge via the Internet at http://pubs.acs.org.
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ꢀ
AUTHOR INFORMATION
Corresponding authors
Email: xujingkun@tsinghua.org.cn; lby1258@163.com.
*
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Tel: (+)86ꢀ791ꢀ88537967; Fax: (+)86ꢀ791ꢀ83823320.
Notes
The authors declare no competing financial interest.
ꢀ
ACKNOWLEDGEMENTS
We are grateful to the National Natural Science Foundation of China (grant number
1303073, 51463008), Ganpo Outstanding Talents 555 projects (2013), the Training
5
Plan for the Main Subject of Academic Leaders of Jiangxi Province (2011), the Science
and Technology Landing Plan of Universities in Jiangxi province (KJLD12081), and
the Natural Science Foundation of Jiangxi Province (grant number 20122BAB216011,
20142BAB206028, 20142BAB216029), for their financial support of this work.
Thanks are also due to Prof. Dr. Guangming Nie for his kind assistance with the
electrochromic tests.
ꢀ
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