Preparation of an EDOT-based polymer: Optoelectronic properties and electrochromic device application

Article abstract of DOI:10.1039/c4ra13060j

Here we present the synthesis, characterization and electropolymerization of a new EDOT-based monomer; 5,10-dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine (DTD). Electrochemical polymerization of DTD was performed potentiostatically by using dichloromethane (DCM) as solvent and tetrabutylammonium hexafluorophosphate (TBPF₆) as supporting electrolyte. Homopolymer [P(DTD)] films and copolymer [P(DTD-co-TPA)] films of DTD prepared by using 4-(2,5-di(thiophen-2-yl)-1H-pyrrole-1-yl)butane-1-amine (TPA) were characterized via CV and UV-vis spectroscopy. Spectroelectrochemical analysis of P(DTD) revealed electronic transitions at 585 nm (π-π transition) with an electronic band gap of 1.69 eV. Electrochromic studies revealed that P(DTD) has competitive properties to EDOT. Furthermore, a dual-type complementary colored polymer electrochromic device based on P(DTD) and P(TPA) was constructed in sandwich configuration. Spectroelectrochemical studies revealed that the oxidized state of the device shows a blue color whereas it is yellow in the reduced state. The maximum contrast (Δ%T) and switching time of the device were measured as 25.5% and 0.5 s for 385 nm and 21% and 1.0 s for 550 nm.

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Spectroelectrochemistry graph of novel TPA/DTD electrochromic device
237x150mm (96 x 96 DPI)

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Preparation of an EDOT-Based Polymer: Optoelectronic Properties
and Electrochromic Device Application
crucial. For instance, polythiophene (PTh) shows good
conductivity and high stability in its oxidized form but suffers
1. Introduction
Conducting polymers (CPs) constitute an important class of
materials that alternates single and double carbon–carbon bonds
along their polymeric chains and they possess the properties of
both organic polymers and inorganic semiconductors. They are
also called as synthetic metals or intrinsically electroactive
conjugated polymers.1,2,3,4,5 The discovery of high electrical
conductivity in doped polyacetylene has stimulated an
enormous amount of work towards the description of the
electronic properties of conjugated organic polymers.6,7,8,9,10
Further studies figure out that they have appreciably fast
charge-discharge kinetics, suitable morphology and fast
doping-undoping processes. Especially thiophene-based
organic conducting polymers continue to fascinate many
scientists due to their potential applications in the development
and construction of new advanced materials such as sensors,
photovoltaic devices, rechargeable batteries, supercapacitors,
artificial muscles and electrochromic devices.11,12,13,14,15,16,17 It
is important to point out that in designing and producing these
devices, maintaining the structural regularity and stability are
from structural defects arising from β-couplings, resulting in
conjugation breaks. Blocking of the 3- and 4- positions, forces
polymerization through the 2- and 5- positions, leading a linear
polymer chain with no α-β coupling with fewer structural
defects than most heterocyclics. The result yields in changes in
the polymer’s absorption characteristics and ultimately its color
in its neutral form. During the oxidation process to form charge
carriers, new electronic states are introduced into the polymers
which exhibit absorptions at lower energies with a concomitant
loss of absorption due to the neutral polymer. As such, these
materials exhibit electrochromism during redox switching.
Among
these
thiophene-based
compounds;
3,4-
ethylenedioxythiophene (EDOT) has accepted as one of the
most successful materials from both fundamental and practical
perspective. It possesses several advantageous properties when
compared to other polythiophene derivates: it combines a low
oxidation potential and moderate band gap with good stability
in its oxidized state. Since the discovery of high conductivity in
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iodine-doped polyacetylene, many interesting conducting sodium sulfide nonahydrate (ACS reagent, ≥ 99.5%, Sigma-
polymers have been developed. Of these, polythiophenes have Aldrich), succinyl chloride (Sigma-Aldrich),
been most studied as electronic materials, with poly(3,4- tetrabutylammonium hexafluorophosphate (TBPF6) (Sigma
ethylenedioxythiophene) (PEDOT) being the most successful Aldrich), thiophene (Sigma-Aldrich), toluene (Sigma-Aldrich),
commercially used conducting polymer.18
o-xylene (Sigma-Aldrich) were used as received from their
In the term of conducting polymers estimated to be used as indicated commercial suppliers without any further
electrochromic materials; importance of long term stability, purification.
switching times, contrast ratios, coloration efficiency,
electrochromic memory are obvious. Hence, it is pivotal to
2.2 Equipment
improve new electrochromic materials possessing the capability The FT-IR spectra were acquired on a Perkin-Elmer Spectrum
of derivatization, in addition to the ascendant properties of 100 Series FT-IR spectrometer. Measurements were collected
EDOT. For this purpose, synthesis and characterization of 5,10- in KBr pellets or in ATR mode using the average of 15 scans.
dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine
(DTD)
were The mass spectral analysis were performed with an Agilent
rendered adequately. Electrochemical polymerization of the Technologies 6890N Network GC System and Agilent
synthesized DTD was also carried out. In order to substantiate Technologies 5975B VL MSD Mass Spectrometer operating at
the copolymer studies; electrochemical studies with 4-(2,5- an ionization potential (EI) of 70 eV. 1H-NMR and 13C-NMR
di(thiophen-2-yl)-1H-pyrrole-1-yl)butane-1-amine (TPA) were spectra were recorded at room temperature on a Varian-
performed in detail. Since there are substantial numbers of Mercury 400MHz high performance digital Fourier Transform
studies on copolymerization of EDOT and thienyl pyrrole (FT)-NMR Spectrometer (Mercury-400BB) and CDCl3 was
derivatives, we choose TPA as comonomer for a proper used as solvent. The 1H-NMR chemical shifts were reported as
comparison. Besides these studies, by using TPA, we δ values in parts per million (ppm) relative to tetramethylsilane
configured out a high quality device in ITO|P(DTD)||Gel (TMS, δ: 0) as internal standard. The 13C-NMR chemical shifts
Electrolyte||P(TPA)|ITO configuration and we investigated the were also reported as δ values in ppm downfield from TMS.
properties of this device.
Melting points (up to 350°C, uncorrected) were obtained on an
Electrothermal Programmable Melting Point Apparatus
(IA9300). Thin layer chromatography (TLC) was used to track
the progress of the experiments steadily. For this purpose,
aluminum sheets (Merck, 20x20, Silica Gel 60 F254) were
utilized. Purification of the desired products from common
impurities -including byproducts, unreacted starting materials-
was performed by column chromatography technique. Hence
silica gel (SiO2) (Merck, Silica Gel 60, 0.063-0.200 mm, 70-
230 mesh ASTM) was selected as the column stationary phase.
In order to pursue electrochemical synthesis and performing
cyclic voltammetry studies, an Ivium potentiostat/galvanostat
was used in all electrochemical measurements. The spectra
were collected with a Diode Array UV-vis spectrophotometer
(Agilent 8453) with a PC interface. Spectrophotometer was
used to conduct the spectroelectrochemical experiments of both
polymer and electrochromic device. The indium tin oxide (ITO)
coated glass plates of thickness of 0.7 mm with resistance of 8–
12 .sq-1 were purchased from Delta Technologies Limited,
USA, and used as such for spectroelectrochemical studies. All
electrochemical experiments were carried out in a one-
component cell. The polymer films were deposited on ITO
plates electrochemically using ITO-coated plate as working
2. Experimental
2.1 Materials
Quinoline (Merck) was dried over anhydrous Na2SO4, filtered
and subsequently distilled under reduced pressure from zinc
dust. Flask containing distilled quinoline was covered by
aluminium foil to protect the solvent from light and stored in
refrigerator at about 5°C. N,N-Dimethylformamide (DMF)
(Sigma-Aldrich) was dried over 4A molecular sieves and
freshly distilled under vacuum prior to use. Anhydrous
potassium carbonate was pre-dried in an oven at 150°C for 3 to
4 hours. Other dry solvents were stored over a suitable drying
agent (usually 4A molecular sieves) under oxygen-free
nitrogen. All of the other solvents, reagents and catalyzers used
in this study; acetone (ACS reagent, ≥ 99.5%,Sigma-Aldrich),
acetonitrile (Sigma-Aldrich), aluminium chloride (Merck),
benzoyl peroxide (Sigma-Aldrich), N-bromosuccinimide
(Sigma-Aldrich), 1,4-diaminobutane (Sigma-Aldrich), carbon
tetrachloride (Fluka), copper chromite (Sigma-Aldrich),
dichloromethane (DCM) (Sigma-Aldrich), diethyl ether (Sigma-
Aldrich), diethyl oxalate (Sigma-Aldrich), ethyl chloroacetate
electrode,
a Pt counter electrode and pseudo reference
(Sigma-Aldrich)
,
ethanol
(absolute,
Sigma-Aldrich),
electrodes (calibrated against Fc/Fc+ (0.3 V)).
hydrochloric acid (ACS reagent, 37%, Sigma-Aldrich), lithium
perchlorate (Sigma-Aldrich), methanol (ACS reagent, ≥ 99.8%,
Sigma-Aldrich), potassium carbonate (anhydrous, Fluka),
potassium hydroxide (ACS reagent, 85%, Sigma-Aldrich),
propionic acid (Sigma-Aldrich), sodium (Sigma-Aldrich),
2.3 Synthesis of 5,10-Dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine
(DTD)
The target monomer DTD was prepared through the synthetic
route shown in Scheme 1. The detailed synthetic procedures
and related spectral data are reported below:
Diethyl 2,2’-thiodiacetate (1): This compound was
synthesized according to a similar procedure developed by
propylene
carbonate
(PC)
(Sigma-Aldrich),
poly(methylmetacrylate) (PMMA) (Sigma-Aldrich), sodium
bicarbonate (Sigma-Aldrich), sodium sulfate (Sigma-Aldrich),
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Kumar [13]. For this purpose, into a 250 mL round-bottomed IR (ATR), νmax/cm-1: 3324 (s, O-H stretching), 2982-2874 (w,
flask containing a stirring bar and fitted with a reflux aliphatic C-H stretching), 1670 (s, ester C=O stretching), 1405,
condenser, ethyl chloroacetate (36.60 g, 31.96 mL, d:1.15 1373 (m, aliphatic C-H bending), 1309 (m, ester C-O-C
g/mL, 300 mmol) was dissolved in anhydrous acetone (100 asymmetric stretching), 1170, 1150 (s, ester C-O-C symmetric
mL) and the solution was stirred at room temperature for a stretching), 1082, 1015, 885, 767, 683.
period of time (~15 minutes). Crystalline sodium sulphide Elemental analysis: anal. calcd. for C10H12O6S (260.0): C 46.15
(Na2S.9H2O) was added (36.00 g, 150 mmol) slowly and the H 4.65 S 12.32; found: C 45.80 H 4.26 S 12.03.
mixture was stirred at 55-60°C using an oil bath. The heating 1,2-Bis(bromomethyl)benzene (3): This procedure was
process was terminated after the observation of the NaCl developed according to a previously described pathway.19 For
precipitation on the wall of the reaction flask. Then, the this purpose, into a round-bottomed flask, o-xylene (5.80 g,
heterogeneous mixture was stirred at 25°C for 24 hours. The 54.0 mmol) and N-bromosuccinimide (NBS) (20.00 g, 112.3
progress of the reaction was monitored by TLC (hexane/ethyl mmol) were dissolved in 25 mL of carbon tetrachloride. A half
acetate, 2:1, v/v). After completion of the reaction, NaCl was amount of benzoyl peroxide (0.01 g, 0.04 mmol) used as free
filtered off under suction using a fritted glass funnel and radical initiator was added to the reaction medium and started
washed thoroughly with cold acetone. After the removal of the to reflux under moderate stirring. After 20 minutes, the other
solvent, the crude product was purified by vacuum distillation half of benzoyl peroxide (0.01 g, 0.04 mmol) was added and the
to yield 24.75g (80%) of diethyl 2,2’-thiodiacetate as slightly mixture was refluxed for an additional 6 hours. The reaction
yellow liquid (observed b.p.: 140-145°C / 10-15 mmHg, lit b.p.: was monitored via TLC analysis (DCM). The flask content
150°C/18 mmHg).
allowed cooling down to room temperature. Thereafter,
1H-NMR (400MHz, CDCl3) δH/ppm:1.20 (t, J=8.8 Hz, 6H, succinimide solids precipitated during the reaction was
ester -CH3), 3.30 (s, 4H, S-CH2), 4.15 (q, J=8.8 Hz, 4H, ester - separated from the solution under suction by using a Buchner
CH2).
funnel. The excess of carbon tetrachloride was removed by
MS (EI) m/z (%) calcd. for C8H14O4S: 206.1; found: 206.1 distillation. The crude mixture was chilled in the refrigerator
(M+, 33), 160 (100), 133 (53), 115 (7), 105 (54), 88 (15), 77 overnight, and the precipitate product was filtered out. A white
(49), 60 (21), 45 (14), 29 (25).
solid was obtained after recrystallization of the crude product
(65%) of 1,2-
IR (KBr), νmax/cm-1: 2984, 2939, 2909 (w, aliphatic C-H), 1732 from diethyl ether to yield 9.26
g
(s, ester C=O stretching), 1447, 1367, 1276 (ester, C-O-C bis(bromomethyl)benzene (observed m.p: 97°C, lit m.p: 99°C).
asymmetric stretching), 1154-1029 (ester, C-O-C symmetric MS (EI) m/z (%) calcd. for C8H8Br2: 264.0; found: 264.0 (M+,
stretching).
11), 183.1 (97), 104.2 (100), 78.2 (19).
Diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate (2): To a IR (ATR), νmax/cm-1: 3052-3021 (w, aromatic, C-H stretching),
round-bottomed flask containing a stirring bar and connected to 2965 (w, aliphatic C-H stretching), 1489 (m, aliphatic C-H
a reflux condenser and a pressure-equalizing dropping funnel, bending), 768 (s, aromatic C-H bending).
clean sodium metal (0.57 g, 24.7 mmol) and 20 mL of absolute Diethyl
ethanol were added attentively. The whole reaction steps were 1,3-dicarboxylate (4): In a 100-mL round-bottomed flask,
performed under nitrogen atmosphere. After the evolution of diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate ( ) (0.75 g,
5,10-dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine-
2
hydrogen gas was observed to decrease, the reaction mixture 2.9 mmol) was dissolved in 15 mL anhydrous DMF under inert
was hold in an ice-water bath and a mixture of 2,2’- (nitrogen) atmosphere. Dry K2CO3 (1.38 g, 10.0 mmol) and
thiodiacetate (1) (2.00 g, 9.7 mmol) and diethyl oxalate (1.42 g, 1,2-bis(bromomethyl)benzene (0.76 g, 2.9 mmol) were added
1.32 mL, d: 1.08 g/mL, 9.9 mmol) were added dropwise over 1 to the solution and the mixture was heated at 100°C for 7 hours.
hour. Presently, color of the mixture became yellow-green and After completion of the reaction, the mixture was poured into
then stirred for an additional 24 hours at room temperature. 100 mL ice-water. The precipitated crude product was filtered
Formation of a solid yellow chunk was observed. The mixture off, washed with 25 mL water once, and with 25 mL 0.1 N
was poured in an ice-water mixture bath and the solution was potassium hydroxide solution twice. Recrystallization was
acidified to pH:2 by adding hydrochloric acid solution. performed from dichloromethane to yield 0.94 g (87%) of
Primarily, the solution became like a milky liquid and then the diethyl 5,10-dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine-1,3-
formation of the crude product has occurred. The solids were dicarboxylate as yellow solid (observed m.p: 146-148°C, lit
separated from the solution by filtration and purified by m.p: 147-148°C).
recrystallization from methanol to yield 1.94 g (77%) of diethyl 1H-NMR (400MHz, CDCl3) δH/ppm: 1.35 (t, 6H, J=6.8 Hz),
3,4-dihydroxythiophene-2,5-dicarboxylate as yellow solid 4.30 (q, 4H, J=7.2 Hz), 5.60 (s, 4H), 7.30 (m, 4H).
(observed m.p: 132-134°C, lit m.p: 134.5°C).
MS (EI) m/z (%) calcd. for C18H18O6S: 362.0; found: 362.0
1H-NMR (400MHz, CDCl3) δH/ppm: 1.20 (t, J=7.3 Hz, 6H, (M+, 75), 317 (23), 248 (28), 217 (20), 189 (29), 175 (21), 161
ester -CH3), 4.30 (q, J=7.4 Hz, 4H, ester -CH2), 9.40 (s, 2H, - (13), 135 (18), 118 (7), 104 (100), 78 (36), 29 (13).
OH).
IR (ATR), νmax/cm-1: 2981-2909 (w, aliphatic C-H stretching),
MS (EI) m/z (%) calcd. for C10H12O6S: 260.0; found: 260.0 1725, 1709 (s, ester C=O stretching), 1557, 1487 (s, aliphatic
(M+, 38), 214 (100), 187 (14), 168 (61), 146 (9), 118 (7), 100 C-H bending), 1372-1242 (s, ester C-O-C asymmetric and
(35), 85 (7), 69 (8), 57 (2), 45 (9), 29 (8.5).
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symmetric stretching), 1059 (s, ether C-O symmetric
stretching), 846, 765.
Elemental analysis: anal. calcd. ForC18H18O6S (362.0): C 59.66
H 5.01 S 8.85; found: C 58.81 H 4.76 S 8.12.
5,10-Dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine-1,3-
dicarboxylic
acid
(5):
Diethyl
5,10-
dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine-1,3-dicarboxylate
(4) (1.00 g, 2.8 mmol) and 15 mL ethanol were put into a round
bottomed flask fitted with a reflux condenser. After the addition
of KOH (85%, 0.54 g, 9.9 mmol) and 1 mL distilled water, the
mixture was refluxed in hot water bath for 6 hours. After
termination of the reaction, the excess of ethanol was removed
by distillation. Unreacted ester was removed by extraction with
ether and then the residue was mixed with crushed ice. The
cooled water phase was acidified with concentrated HCl
(pH:2). The resulting crude product was separated by filtration
to yield 0.79 g (92%) of 5,10-Dihydrobenzo[f]thieno[3,4-
b][1,4]dioxocine-1,3-dicarboxylic acid as slightly yellow
powder (observed m.p: 250°C).
Figure 1.1H-NMR spectrum of DTD.
IR (ATR), νmax/cm-1: 3400-2500 (O-H stretching), 2976-2918
(w, aliphatic C-H stretching), 1672 (s, carboxylic acid C=O
stretching), 1489 (m, aliphatic C-H asymmetric bending), 1453
(m, aliphatic C-H symmetric bending), 1279 (s, C-O
asymmetric stretching), 1039 (m, C-O symmetric stretching).
5,10-Dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine (6): The
dicarboxylic acid derivative; 5,10-Dihydrobenzo[f]thieno[3,4-
b][1,4]dioxocine-1,3-dicarboxylic acid (5) (0.35 g, 1.6 mmol)
was dissolved in 5 mL freshly distilled quinoline and catalytic
amount of copper chromite (0.10 g, 0.32 mmol) was added to
the reaction medium. The homogeneous dark brown solution
was stirred under a nitrogen flow in a preheated oil bath at
180°C for 8 hours. The flask content allowed to cool down to
room temperature, and mixed with ice-water and concentrated
hydrochloric acid solution. This process was repeated several
times in order to remove excess quinoline completely. Organic
phase was extracted then with ethyl acetate, washed with brine
and water. Solvent was removed under reduced pressure after
drying the organic layer over anhydrous sodium sulfate. The
crude product was purified by column chromatography (SiO2,
Scheme 1. Synthetic route to the monomer DTD, conditions: (i) Na2S.9H2O,
acetone, rt, 24 h, 80%; (ii) , NaOEt (freshly prepared from Na(s) and EtOH
(abs.) under N2 atmosphere), EtOH, 0°C, diethyl oxalate, 24 h, then treated with
HCl (pH:2), 77%; (iii) NBS, CCl4, reflux, 6 h, 65% (iv) K2CO3, DMF, reflux, 7 h, 87%;
(vi) KOH, EtOH, 6 h, 92%; (vii) quinoline, Cu2Cr2O5 (cat.), 180°C, 8 h, 50%.
1
2.4 Synthesis of 4-(2,5-Di(thiophen-2-yl)-1H-pyrrol-1-yl)butane-
1-amine (TPA):
hexane/ethyl
acetate,
3:1,
v/v)
to
give
5,10-
The TPA monomer; 4-(2,5-di(thiophen-2-yl)-1H-pyrrole-1-
yl)butane-1-amine was synthesized from the reaction of 1,4-
di(2-thienyl)-1,4-butanedione and butane-1,4-diamine in the
presence of catalytic amounts of propionic acid according to the
reported procedure.20
dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine as light yellow
solid (0.175 g) in 50% yield (observed m.p: 81°C, lit m.p: 80-
83°C). At the end of the reaction and structure of DTD was
characterized.
1H-NMR (400MHz, CDCl3) δH/ppm: 5.35 (s, 4H), 6.50 (s, 2H),
7.19-7.30 (m, 4H, aromatic).
1H-NMR (400MHz, CDCl3) δH/ppm: 7.2 (d; 2H), 6.95 (dd;
2H), 6.9(dd; 2H), 6.25 (s, 2H), 3.95 (t; 2H), 1.5 (m; 4H), 1.30
(m; 4H).
MS (EI) m/z (%) calcd. for C12H10O2S: 218.0; found: 218.0
(M+, 80), 173 (24), 162 (17), 145 (17), 129 (21), 117 (89), 104
(100), 91 (11), 78 (42) (Figure 1).
IR (ATR), νmax/cm-1: 3057-3032 (w, aromatic C-H stretching),
2934-2878 (w, aliphatic C-H stretching), 1373 (s, C-H
symmetric bending), 1180, 1170, 1136 (w, C-O symmetric
stretching).
2.5 Cyclic voltammetry (CV)
In order to assay the electroactivity of the polymers and to
accomplish the oxidation/reduction behaviors of DTD, TPA
and DTD-co-TPA; voltammetric measurements were carried
out using the potentiostat with a conventional three electrode
Elemental analysis: anal. calcd. for C12H10O2S (218.0): C 66.03
H 4.62 S 14.69; found: C 65.40 H 4.03 S 13.96.
cell made from
a
quartz cuvette cell. An Ivium
potentiostat/galvanostat interfaced with a personal computer
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was used in all electrochemical measurements. Additionally, a 2.9 Preparation of the gel electrolyte
blanket of nitrogen was maintained above the solution during
The
gel
electrolyte
was
prepared
by
using
the experiment and a background voltammogram was run to
ensure that no impurity was present before the addition of
substrate. Spectroelectrochemical measurements were carried
out in a three-electrode quartz cell. The spectra were collected
with a Diode Array UV-vis spectrophotometer (Agilent 8453)
with a PC interface. Solvent (DCM) and supporting electrolyte
(TBPF6) couples were used during the electrochemical studies.
TBPF6:ACN:PMMA:PC in the ratio of 3:70:7:20 by weight.
After dissolving TBPF6 in ACN, PMMA was added into the
solution. In order to dissolve PMMA, vigorous stirring and
heating are required. Propylene carbonate (PC), as a plasticizer,
was added to the reaction medium when all of the PMMA was
completely dissolved. The mixture was stirred and heated until
the highly conducting transparent gel was obtained.
2.10 Construction of electrochromic device
2.6 Electrochemical polymerization of DTD and DTD-co-TPA
In this research, copolymer of DTD was utilized as the
cathodically and TPA as the anodically coloring electrochromic
materials. DTD copolymer was deposited on ITO-coated glass
Electrochemical polymerization of DTD, TPA and DTD-co-
TPA were performed by sweeping the potential between -0.5 to
1.5 V, -0.5 to 1.5 V and -0.3 to 1.5 V, respectively, with scan
rate of 250 mV/s in a 0.05 M TBPF6/DCM electrolyte-solvent
couple system. The working and counter electrodes were ITO
and Pt wire, respectively, and the reference electrode was
Ag/Ag+. P(DTD), P(TPA) and P(DTD-co-TPA) films were
washed with DCM in order to remove excess of TBPF6 and
unreacted monomer after the potentiodynamic electrochemical
polymerization. Similar method was used to synthesize the
polymer on an ITO coated glass plate.
substrate via constant potential electrolysis in 0.05
M
TBPF6/DCM supporting electrolyte-solvent couple at +1.5 V.
The TPA coated electrode was prepared at +1.5 V in 0.05 M
TBPF6/DCM the same electrolyte-solvent couple.
Chronocoulometry was employed to match the redox charges of
the two complimentary polymer films to maintain a balanced
number of redox sites for switching. Electrochromic devices
(ECDs) were prepared by arranging two electrochromic
polymer films -one film is in oxidized, the other is in neutral
state-by facing each other in sandwich form and gel electrolyte
was spread on the polymer-coated sides of the electrodes under
atmospheric conditions.
3. Results and discussion
3.1 Electrochemical Properties of Polymers
3.1.1 Cyclic voltammetry studies of DTD and DTD-co-TPA
Scheme 2. Electrochemical synthesis route for P(DTD) and P(TPA).
Cyclic voltammograms of the monomers were gathered in a
solvent-supporting electrolyte system (DCM/TBPF6) at room
temperature and 0.01 M solution of DTD and TPA were
polymerized by repeated potential scanning (250 mV/s)
between -0.5 and +1.5 V. In order to get information about the
electroactivity of the copolymer, we arranged CV studies in the
medium of TPA. For this purpose, 1.25 mL DTD (0.001M) and
1.25 mL TPA (0.001 M) were introduced into the single
compartment standard electrolysis cell. TBPF6 was utilized as
the supporting electrolyte. The cyclic voltammetry study of
copolymer was carried out between -0.3 V and 1.5 V. In all CV
records, an increase in the peak intensities was observed upon
sequential cycles which imply the formation of an electroactive
polymer film on the working electrode surface. As we
compared the voltammograms of homopolymer and copolymer;
the reduction and oxidation potentials of DTD, TPA and DTD-
co-TPA were determined as 0.01 - 0.45 V; 0.17 - 1.2 V; and
0.43 - 1.25 V, respectively. The observation of reduction and
oxidation potentials with different values proved us the
formation of copolymer. At the same time, observation of
DTD-co-TPA's onset potential value at a different region
showed occurrence of the copolymer as well. However, images
of cyclic voltammograms and redox potential values of
2.7 Spectroelectrochemistry
Spectroelectrochemical analyses of the polymers were carried
out to understand the band structure of the product. For
spectroelectrochemical studies, the DTD, TPA and DTD-co-
TPA film were deposited potentiostatically at 1.5 V in (0.05 M)
TBPF6/DCM solvent–electrolyte couple on indium tin oxide-
coated glass slide (ITO). UV-vis spectra of the film were
recorded at various potentials in (0.05 M) TBPF6/DCM.
2.8 Switching
A square wave potential step method coupled with optical
spectroscopy was used to investigate the switching times and
contrast of the polymers. In this double potential step
experiment, the potential was set to an initial potential (where
the conducting polymer was in one of its extreme states) for 5 s
and stepped to a second potential for another 5 s, before being
switched back to the initial potential. During the experiment the
percent transmittance (%T) and switching times at λmax of the
polymer were measured using a UV–Vis spectrophotometer
.
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P(DTD), P(TPA) and P(DTD-co-TPA) which were taken in the 3.1.2 Scan rate dependence of the peak currents
same solvent environments were different from each other as
P(DTD) and P(DTD-co-TPA) films were prepared using
depicted in Figure 2. In order to determine the
spectroelectrochemical and optical properties of the P(DTD),
P(TPA) and P(DTD-co-TPA) with the evaluation of cyclic
voltammograms; we decided to continue our studies by using
DCM as solvent and TBPF6 as supporting electrolyte.
constant potential electrolysis. Cyclic voltammograms of
prepared homopolymer and copolymer films with different
scanning rates were taken and anodic/cathodic changes of the
peak heights were also examined. Figure 3 shows cyclic
voltammograms of P(DTD) and P(DTD-co-TPA) at different
scan rates in DCM. With the increase of scan speed it was
determined that anodic and cathodic current peak values were
changed linearly. The scan rates for the anodic and cathodic
peak currents show a linear dependence (anodic and cathodic
least squares fit of R=0.999, R=0.998; R=0.999, R=0.999,
respectively) as a function of the scan rate as illustrated in
Figure 3 for P(DTD) and P(DTD-co-TPA). This demonstrates
that the electrochemical processes are not diffusion controlled.
Figure 2. Cyclic Voltammograms of a) P(DTD), b) P(TPA) and c) P(DTD-co-TPA)in
DCM containing 0.05 M TBPF6 at 250 mV/s scan rate.
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a)
a)
4
0.6
2
Ipa
Ipc
1
3
2
0
-1
-2
-3
0.4
0.2
0.0
0.2
0.4
0.6
100 200 300 400 500 600 700 800
Scanrate (mV/s)
1
0
0.05 V/s
0.1 V/s
0.2 V/s
0.3 V/s
0.4 V/s
0.5 V/s
0.6 V/s
0.7 V/s
0.8 V/s
-1
-2
-3
1.st cycle
1000.th cycle
0.5
0.0
0.5
1.0
1.5
-0.5
0.0
0.5
1.0
1.5
Potential (V)
Potential (V)
b)
b)
Ipa
0.6
0.6
0.4
0.2
0.0
0.2
0.4
0.6
Ipc
0.6
0.3
0.3
0.0
-0.3
-0.6
20
50
100
150
200
Scanrate (mV/s)
0.0
-0.3
20 mV/s
50 mV/s
100 mV/s
200 mV/s
1.st cycle
1000.th cycle
-0.6
-0.5
0.0
0.5
1.0
1.5
0.5
0.0
0.5
1.0
1.5
Potential (V)
Potential (V)
Figure 3. Redox behaviors of a) P(DTD) and b) P(DTD-co-TPA) at different scan
rates in 0.05 M TBPF6/DCM.
Figure 4. Stability tests of a) P(DTD) and b) P(DTD-co-TPA) via cyclic voltammetry
with a scan rate of 250 mV/s 0.05 M TBPF6/DCM.
3.1.3 Stability
3.2 Electrochromic properties of the P(DTD) and
P(DTD-co-TPA)
Electrochromic stability is frequently related with
electrochemical stability since the degradation of the active
redox couple results in the loss of electrochromic contrast and
therefore the achievement of the electrochromic material.
Cyclic voltammetry technique was functioning as an important
appliance to inspect the long term stability of the conducting
polymers. To interpret the stability of the polymers, the
potential was swept repeatedly between -0.5 and +1.5 V for
P(DTD) and between -0.5 and 1.5 V for P(DTD-co-TPA) with
a scan rate of 250 mVs−1. As shown in Figure 4, after 1000
cycles a decrement about 24.6% and 9.2% for P(DTD) and
P(DTD-co-TPA) was detected, respectively. In the proportion
of this result, the stability of the formed copolymer was
detected to be much higher than the homopolymer.
3.2.1. In-situ polymerization of P(DTD)
Electropolymerization is a standard oxidative method for
preparing electrically conducting conjugated polymers.
Smooth, polymeric films can be efficiently electrosynthesized
onto conducting substrates where their resultant electrical and
optical properties can be probed easily by several
electrochemical and coupled in situ techniques.
We investigated the in situ electrochemical polymerization by
UV-vis spectrophotometer under 1.5 V at TBPF6/DCM at every
10 s time interval (Figure 5).
It can be concluded from Figure 6 that there is a linear increase
in absorbance with time until a certain time (0–40 s). After this
time, linearity continues with the square root of the time due to
polymerization rate becomes controlled by diffusion because of
the consumption of monomer at the electrode double layer
(Figure 6).
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charge carrier bands were determined. As seen in Fig. 7.a π–π*
transition wavelengths (λmax) of P(DTD) were determined as
585 nm. The onset energy for the π– π * transition (electronic
band gap) was found to be 1.69 eV.
0 s
1.2
10 s
20 s
30 s
40 s
50 s
60 s
70 s
80 s
90 s
100 s
1.0
0.8
0.6
0.4
0.2
0.0
In order to interpret the spectroelectrochemical properties of the
copolymer P(DTD-co-TPA), films were deposited onto ITO-
coated glass plates in same solvent medium between -0.5 and
0.9 V via potentiodynamic method. The λmax value for the π-π*
transitions in the neutral state of copolymer was found to be
485 nm, revealing light orange color which was all significantly
different than pure P(TPA) and P(DTD). The electronic band
gap defined as the onset energy for the π-π* transition was
found to be 1.78 eV. Upon increase in the applied voltage, the
formation of a new absorption band at 900 nm was observed
due to the evolution of charge carriers (Figure 7.b).
400
600
800
1000
Wavelenght (nm)
At the eventual state of this study, the color change of the
homopolymer was observed in two different colors between
dark blue and transparent. On the other hand, we detected 4
different colors of copolymer and with this outcome, a
multichromic conducting polymer has been prepared.
Figure 5. In situ electrochemical polymerization of DTD.
1.2
1.1
1.0
998
1.0
0.9
2 =0.
r
0.8
2 =0.996
r
0.7
0.8
0.6
7
8
9
10
(time)0.5
0.6
0.4
0.2
0.0
5
7
9
540 nm
1000 nm
.9
2
0
2
7
=
9
r
.9
0
2
=
r
0
20
40
60
80
100
Time (s)
Figure 6. Absorbance recorded at different wavelengths during the
polymerization. Inserted figure represents absorbance changes recorded with
the square root of the time at different wavelengths (between 40 and 100 s.).
3.2.2 Spectroelectrochemistry of the P(DTD) and P(DTD-co-
TPA)
Spectroelectrochemistry is a technique that examines the
changes in optical properties of conducting polymers upon
voltage change. It also provides useful data about the electronic
structure of the polymer such as band gap (Eg) and the intergap
states that appear upon doping. In order to detect the
spectroelectrochemical and optical properties of the P(DTD)
and P(DTD-co-TPA) with the appraisal of the cyclic
voltammograms; we used DCM as solvent and TBPF6 as
supporting electrolyte. P(DTD) was coated on ITO and spectral
changes were explored by UV-vis spectrometer in a monomer-
free 0.05M TBPF6 in DCM via incrementally increasing applied
potential between -0.7 V and +0.7 V. While between -0.7 V and
0.0 V; P(DTD) film was dark blue, with a higher potential (0.0
V and +0.7 V interval) appliance it became transparent (Figure
7.a ). Following to the applied voltage, decrease in the intensity
of the π–π* transition wavelength (λmax) and formation of
Figure 7. Spectroelectrochemical graphics of a) P(DTD) and b) P(DTD-co-TPA) 0.1
M TBPF6 in DCM.
3.2.3 Electrochromic Switching
Electrochromic switching studies were performed to test the
ability of a polymer to switch rapidly and the ability to exhibit
striking color change. The experiments which were carried out
by spectroelectrochemistry showed the ability of P(DTD) and
P(DTD-co-TPA) to switch between its neutral and doped states
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with a change in transmittance at a fixed wavelength.
Throughout the experiments, the percent transmittance (%∆T)
at the wavelength of maximum contrast was measured by
utilizing UV-vis spectrophotometer.
b)
1.2
0.6
0.0
0.6
For P(DTD) maximum contrast (%∆T) and switching time were
measured from Figure 8. as 43.4% and 2.5 s for 585 nm and 1.5
s and 62.5% for 1000 nm in DCM by adjusting the potential
between -0.7 V and 0.7 V with a residence time of 5 s. The
switching studies were executed for the gathered copolymer.
For this execution, -0.5 V to 0.9 V were applied to P(DTD-co-
TPA) film with 5 seconds intervals in order to calculate the
maximum contrast and switching time values for copolymer.
The maximum optical contrasts (ΔT%) and switching time for
P(DTD-co-TPA) film were measured as 12% and 3s for 480nm
and 29% and 3.5 s for 900 nm by step the potential between -
0.5 V and 0.9 V (Figure 8.b).
0.5
0.4
485 nm
900 nm
0.3
0.8
0.6
0.4
2.6
0.0
2.6
5.2
a)
1.2
0.6
0.0
0
10
20
30
40
50
60
-0.6
1.2
Time (s)
0.9
Figure 8. Potential-Time, Absorbance-Time, Current Density-Time graphics for a)
P(DTD) and b) P(DTD-co-TPA) in DCM/TBPF6.
585 nm
0.6
0.3
3.2.4 Spectroelectrochemistry of Electrochromic Device (ECD)
1.5
1.2
While constructing the electrochromic device, the anodically
coloring polymer film P(TPA) was fully reduced and the
cathodically coloring polymer P(DTD) was fully oxidized.
Upon application of a voltage, one of the polymer films is
oxidized, whereas the other is neutralized, resulting in a color
change. Spectroelectrochemistry experiments were performed
to investigate the changes of the electronic transitions of the
ECD, with the increase of the applied potential. Figure 9
represents the absorption spectrum of the ECD, recorded by
application of different voltages between -0.5 V and 1.5 V. At -
0.5 V the polymer layer was in its neutral state, where the
0.9
0.6
1000 nm
0.3
5.4
2.7
0.0
2.7
0
10
20
30
40
50
60
Time (s)
absorption at 385 nm was due to
휋–π* transition of the
polymer. At this potential, DTD was in oxidized state showing
no pronounced absorption at the UV–Vis region of the
spectrum, thus the color of the device was yellow. As the
applied potential was increased the polymer layer started to get
oxidized and a decrease in the intensity of the absorption was
observed. Meanwhile, DTD layer was in its reduced state,
which was followed by the appearance of the new absorption at
585 nm and dominated the color of the device was blue. A
comparison of the optical properties of P(DTD)/P(TPA) device
with other PEDOT/thienylpyrrole devices in literature are given
in Table 1.
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potential between -0.5 V and +1.5 V with a residence time of 5
s.
Figure 9. Spectroelectrochemical spectrum of the device as applied potentials
between -0.5 and +1.5 V.
Table 1. Optical properties of PEDOT/Thienyl pyrrole devices in the
literature
λmax
(nm)
Switcing
Time (s)
Structure
% ∆T
References
Figure 10. Potential-Time and Absorbance-Time at 385 and 550 nm for P(DTD-co-
TPA) between -0.5 V and 1.5 V.
400
600
386
606
431
617
575
615
445
615
385
550
6
30
13
-
-
P(SNSNO2)/PEDOT
device
21
3.2.6 Stability of ECDs
-
Cyclic voltammetry is employed by monitoring current changes
to figure out the long-term stability for devices. Cyclic
voltammetry studies showed that the electrochromic devices
operated stably with applied voltage of -0.3 V and 2.2 V with
250 mV/s scanrate under atmospheric conditions. P(DTD-co-
TPA) device could be repeatedly switched up to 500 cycles. We
determined that the device had good redox stability (Figure 11).
2.1
-
22
23
20
24
P(PTP)/PEDOT device
-
-
P(DTTP)/PEDOT
device
29
25
20
-
1.5
1.3
0.8
-
P(TPBA)/PEDOT
device
0.2
P(FPTP)/PEDOT
device
19.4
25.5
21
1.4
0.5
1.0
0.1
P(DTD)/P(TPA) device
This work
1.st cycle
500.th cycle
0.0
2.5 Switching of ECDs
-0.1
To investigate switching characteristics of the ECD’s the
transmission and the response time at the maximum contrast
wavelength was monitored during repeated redox stepping
experiments. For the device, maximum contrast (%∆T) and
switching time were measured from Figure 10 as 25.5% and 0.5
s for 385 nm and 21% and 1.0 s for 550 nm by stepping
0.0
0.5
1.0
1.5
2.0
Potential (V)
Figure 11. Cyclic Voltammograms of the device as a function of repeated scans at
250 mV/s: after 1st cycle, after 500 cycles.
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Conclusions
ARTICLE
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thiophene-based
monomers;
5,10-
11. A. Kaynak and R. Foitzik, 2010, 14
.
dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine (DTD) and 4-(2,5-
di(thiophen-2-yl)-1H-pyrrole-1-yl)butane-1-amine (TPA) have
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dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine-1,3-dicarboxylate
with concomitant hydrolysis and decarboxylation steps gave the
desired monomer DTD. When optical and electrochemical
properties of electrochemically synthesized DTD were
investigated, it was found that it possesses similar
characteristics of EDOT. The incorporation of TPA into the
conjugated DTD backbone unit via copolymerization results in
different emission colors. Optical and electrochemical
properties of the copolymer were investigated. We also
successfully established the utilization of dual-type
complementary colored polymer electrochromic devices using
P(DTD)/P(TPA) in sandwich configuration. The switching
ability and spectroelectrochemical properties of the
electrochromic device were investigated by UV-vis
spectrophotometry and cyclic voltammetry. This device
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switching times (1.7 s) with a good switching stability.
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One of the researchers (Metin Ak) gratefully thanks to the
Scientific and Technological Research Council of Turkey
(TUBITAK; project number: 111T074) and PAUBAP
(2011FBE74). Mustafa Güllü and Gülbin Kurtay also wish to
acknowledge here with the grants received from TÜBİTAK
(TBAG/110T071) and AU-BAP (14H0430002).
Notes and references
1Pamukkale University, Faculty of Art and Science, Chemistry Department,
Denizli, Turkey
2Ankara University, Faculty of Science, Chemistry Department, Ankara,
Turkey
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