Polymers via Copolymerization: Copolymer of
1-(4-fluorophenyl)-2,5-di(thiophen-2-yl)-1
H-Pyrrole
with 3,4-Ethylene dioxythiophene
OZLEM TURKARSLAN, METIN AK, CIHANGIR TANYELI, IDRIS M. AKHMEDOV, LEVENT TOPPARE Department of Chemistry, Middle East Technical University, 06531, Ankara, Turkey
Received 13 February 2007; accepted 12 April 2007 DOI: 10.1002/pola.22166
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A copolymer of 1-(4-fluorophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole (FPTP) with 3,4-ethylene dioxythiophene (EDOT) was electrochemically synthesized and characterized. While poly(FPTP) (P(FPTP)) has only two colors in its oxidized and neutral states (blue and yellow), its copolymer with EDOT has five different colors (purple, red, light gray, green, and blue). Electrochromic devices based on P(FPTP-co-EDOT) and poly(3,4-ethylenedioxythiophene) (PP(FPTP-co-EDOT) were constructed and charac-terized. The oxidized state of the device shows blue color whereas it shows purple for the reduced state. At several potentials the device has good transparency with green and gray colors. Maximum contrast (D%T) and switching time of the device were measured as 23% and 1.1 s at 555 nm.VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4496–4503, 2007
Keywords: conducting polymers; copolymerization; electrochemistry
INTRODUCTION
An electrochromic material is the one that changes color reversibly by an electrochemical reaction and the phenomenon is called electro-chromism.1 Electrochromic materials have been studied for different technological applications, such as construction of mirrors,2 earth-tone cha-meleon materials,3 and electrochromic devices
(ECDs).4–7
Studies on ECDs began with inorganic com-pounds such as tungsten trioxide (WO3) and
irid-ium dioxide (IrO2).8 Because of the different
colors observed during switching among their dif-ferent redox states,9 organic materials (viologens, metallophtalocyanines, and conducting polymers) have recently received much attention for electro-chromic applications.10 Conducting polymers in particular have several advantages over inor-ganic compounds. These include outstanding coloration efficiency, fast switching ability,11 mul-tiple colors with the same material12 and fine-tuning of the bandgap (and the color) through chemical-structure modification.13
For conducting polymers the electrochromism is related to doping–undoping process. The dop-ing process modifies the polymer electronic struc-ture, producing new electronic states in the band gap, causing color changes. Electronic absorption
In memoriam of Gursel Sonmez who died on 16 January 2006, at the age of 37. He was a distinguished scientist spe-cialized on the electrochromic properties of conducting poly-mers.
Correspondence to: L. Toppare (E-mail: toppare@metu. edu.tr)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 4496–4503 (2007) V
shifts bathochromically upon doping, and the color contrast between the undoped and doped states is related to the polymer band gap.14 A major focus in the study of electrochromic poly-meric materials has been that of controlling their colors by main-chain and pendant group struc-tural modification as well as copolymerization. Copolymerization can lead to interesting combi-nation of the properties.
In this study, we electrochemically synthe-sized copolymer of the 1-(4-fluorophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole (FPTP) with 3,4-ethylene dioxythiophene (EDOT). While P(FPTP) has only two colors in oxidized and neutral states,15 its copolymer with EDOT has five dif-ferent colors. Also, we constructed and charac-terized dual-type electrochromic devices based on P(FPTP-co-EDOT) and poly(3,4-ethylenediox-ythiophene) (PEDOT). Devices were assembled in sandwich configuration of electrochromic materials deposited on ITO glass electrodes and a gel electrolyte. For the construction of devices, PEDOT and P(FPTP-co-EDOT) were used as the cathodically and anodically coloring materials, respectively. Oxidized states of device show blue color where as reduced states reveal purple color. At moderate potentials device has good transparency with green and gray colors. Hav-ing good transparency at moderate potentials, this device can be used in two different combina-tions (transparent-purple or transparent-blue).
EXPERIMENTAL
Chemicals
AlCl3 (Aldrich), dichloromethane (Merck),
succi-nyl chloride (Aldrich), hydrochloric acid (Merck), NaHCO3(Aldrich), MgSO4(Aldrich),
4-fluoroani-line (Aldrich), propionic acid (Aldrich), toluene (Sigma), propylene carbonate (PC), acetonitrile (AN) (Merck), LiClO4(Aldrich),
polymethylmeta-crylate (PMMA), and 3,4-ethylenedioxythiophene (EDOT) were used as received.
Equipments
A three-electrode cell containing an ITO-coated glass slide as the working electrode, a platinum foil as the counter electrode, and a silver wire as the pseudoreference electrode were used for electrodeposition of polymer films via potentio-static or potentiodynamic methods. N2 gas was
passed through the solution to provide an inert atmosphere. All electrochemistry experiments were carried out using a Voltalab PST 50 model potentiostat/galvanostat.
The FTIR spectrum was recorded on a Nico-let 510 FTIR spectrometer. The surface mor-phologies of the copolymer films were analyzed by using JEOL JSM-6400 scanning electron microscope. Varian Cary 5000 UV-vis-NIR trophotometer was used to perform the spec-troelectrochemical studies of the copolymer and the characterization of the devices. Colorimetry measurements were done via Minolta CS-100 spectrophotometer.
Synthesis of 1-(4-Fluorophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole
1-(4-Fluoro-phenyl)-2,5-di(2-thienyl)-1H-pyrrole (FPTP) has previously been synthesized from dehydrative cyclization of diketone with an amine.15,16 About 1.25 g (5 mmol) of 1,4-di(2-thienyl)-butane-1,4-dione (DTBD), 0.66 mL (7 mmol) of 4-fluoraniline, 0.45 mL (5.4 mmol) of propionic acid were dissolved in 25 mL toluene. The mixture was stirred and refluxed for 24 h under argon atmosphere. Toluene was evapo-rated and the product was sepaevapo-rated by column chromatography on silica gel (eluent: dichloro-methane). The desired compound (1 g, 65%) was obtained from elution at the solvent front.
Cyclic Voltammetry
The oxidation/reduction behavior of copolymer was investigated by cyclic voltammetry (CV) using 0.1 M LiClO4/AN supporting
electrolyte-solvent couple. Experiments were carried out in an electrolysis cell equipped with ITO coated glass plate as the working, Pt wire counter and Ag wire pseudoreference electrodes.
Electrochemical Copolymerization of FPTP with EDOT (P(FPTP-co-EDOT))
FPTP (0.05 M) was dissolved in 15 mL acetoni-trile (AN), 0.03 M EDOT and 0.1 M LiClO4 were
introduced into a single compartment electroly-sis cell. Electrolyelectroly-sis was run for 30 min at þ1.2 V at room temperature under inert atmos-phere. Resulting copolymer films were washed with AN to remove LiClO4 after the electrolysis.
Similar method was used to synthesize the co-polymer on ITO coated glass plates (Scheme 1).
Preparation of the Gel Electrolyte
The gel electrolyte was prepared by using LiClO4:AN:PMMA:PC in the ratio of 1.5:70:7:20
by weight. After LiClO4 was dissolved in AN,
PMMA was added into the solution. To dissolve PMMA vigorous stirring and heating was required. Propylene carbonate (PC), as the plas-ticizer, was introduced 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 pro-duced.17
Construction of Electrochromic Devices
In this study, copolymer of FPTP was utilized as the anodically, and PEDOT as the cathodically coloring electrochromic materials. FPTP copoly-mer was deposited on ITO via constant potential electrolysis in 0.1 M LiClO4/AN supporting
elec-trolyte-solvent couple at þ1.2 V. The PEDOT coated electrode was prepared at þ1.5 V in the same electrolyte-solvent couple. Chronocoulome-try was employed to match the redox charges of the two complimentary polymer films to main-tain a balanced number of redox sites for switch-ing. The redox sites of these polymer films were matched by stepping the potentials between 0.4 and þ1.1 V for P(FPTP-co-EDOT), 1.0
andþ1.5 V for PEDOT (vs. Ag/Agþ). ECDs were built by arranging two electrochromic polymer films (one oxidized, the other neutral) facing each other separated by a gel electrolyte.
Figure 1. Cyclic voltammogram of (a) FPTP, (b) FPTP in the presence of EDOT, (c) EDOT, on ITO glass electrode in 0.1 M LiClO4/AN at 250 mV/s. scan
RESULTS AND DISCUSSION
FTIR Spectra
In the FTIR spectrum of FPTP, the following peaks were identified in tmax/cm1: 693
(cyclo-pentadienyl CH out of plane deformation vibration), 841 (2-substituted phenyl ring, out of plane deformation vibration due to neigh-boring H atoms and cyclopentadienyl CHb stretching), 1034 (phenyl ring, ¼CH in plane deformation vibration), 1220 (aromatic amine, CN stretching vibration), 1415 (C¼¼N in plane vibration), 1458 (C¼¼C in plane vibra-tion), 773 and 3102 (cyclopentadienyl CHa stretching vibration). After the potentiostatic copolymerization of FPTP with EDOT, the dis-appearance of peaks at 773 and 3102 cm1 are evidences of the polymerization from 2, 5 posi-tions of thiophene ring. The shoulder observed at 1645 cm1is due to the conjugation and it is considered as another proof of polymerization. The peaks at 2925 and 1144 cm1 belonging to aliphatic CH group and COC group, respectively, indicate that EDOT is incorpo-rated to the polymeric matrice. The peaks appeared at 1089, 1113, and 632 cm1show the presence of dopant ion, ClO4.
Cyclic Voltammetry
When redox behavior of FPTP was investigated via cyclic voltammetry, an electrochromism between yellow and blue colors was observed, while a greenish cloud was formed around the electrode because of the partial dissolution of
linear oligomers. First run of the cyclic voltam-mogram of FPTP in 0.1 M LiClO4/acetonitrile
(AN) showed two oxidation peaks at þ0.51 V and þ0.74 V and a reduction peak at þ0.44 V. After subsequent runs electroactivity increases with increasing scan number. On the other hand, for þ0.74 V peak a decrease in current was observed due to monomer concentration decrease in the diffusion layer [Fig. 1(a)].
To investigate the CV behavior of the copoly-mer, we performed CV studies in the presence of EDOT under same experimental conditions. There was a drastic change in the voltammo-gram, both the augment in the increments between consecutive cycles and the oxidation potential of the material were different than those of FPTP and EDOT, which, in fact, could be interpreted as the formation of a random co-polymer [Fig. 1(b,c)].
Conductivity of the Copolymer Film
The conductivity of electrochemically prepared P(FPTP-co-EDOT) was measured as 2.0 3 103 S/cm via four probe technique.
Scanning Electron Microscopy
Surface morphologies of polymers were investi-gated by scanning electron microscope (SEM). SEM micrograph of P(FPTP-co-EDOT) was dif-ferent than those of P(FPTP)15 and PEDOT (Fig. 2). This was another evidence of copoly-merization.
Electrochromic Properties of P(FPTP-co-EDOT) The best way of examining the changes in opti-cal properties of conducting polymers upon volt-age change is via spectroelectrochemistry. It also gives information about the electronic structure of the polymer such as band gap (Eg) and the
intergap states that appear upon doping. P(FPTP-co-EDOT) film was potentiostatically synthesized at þ1.2 V on ITO electrode. Electro-lyte solution was composed of 0.01 M FPTP, 0.01 M EDOT, and 0.1 M LiClO4/AN. The
spectroe-lectrochemical and electrochromic properties of the resultant copolymer (after being rinsed with acetonitrile) were studied by applying potentials ranging between 0.7 V and (1.1 V in monomer free 0.1 M LiClO4/AN medium. At the neutral
statekmaxvalue because of the p-p* transition of
the copolymer was found to be 546 nm and Eg
was calculated as 1.6 eV using deBroglie quation by inserting the wavelength as the onset of the peak. Upon applied voltage reduction in the in-tensity of the p-p* transitions and formation of charge carrier bands were observed. Thus, appearance of peaks around 834 and 1040 nm could be attributed to the evolution of polaron and bipolaron bands, respectively, (Fig. 3). The band gap of PEDOT andp-p* transition were al-ready reported as 1.6 eV and 620 nm,18 respec-tively. On the other hand, Eg and kmax of the
homopolymer were found as 1.94 eV and 398 nm15 respectively. In conclusion, the band gap energy and absorption maxima of the copolymer
Figure 3. Spectroelectrochemical properties of the P(FPTP-co-EDOT) in 0.1 M LiClO4/AN (a) 2D (b) 3D.
Figure 4. (a) Colors and corresponding L, a, b values of the P(FPTP-co-EDOT) film at different applied potentials in 0.1 M LiClO4/AN (0.1 M) (b) Colors and correspond-ing L, a, b values of the device at different applied potentials.
are different than the values for PEDOT and P(FPTP) as expected. In addition, thekmaxvalue
of the copolymer is red shifted when compared with that of the homopolymer, which is due to increase in conjugation length and the influence of high electron density resulting from the incor-poration of EDOT units.19
The colors of the electrochromic materials were defined accurately by performing colorim-etry measurements. CIE system was used as a quantitative scale to define and compare colors. Three attributes of color; luminance (L), hue (a), saturation (b) were measured and recorded.
The P(FPTP-co-EDOT) has distinct electrochro-mic properties. It shows five different colors in neutral and oxidized states. These colors and corresponding L, a, b values were given in Fig-ure 4(a).
Electrochromic Switching
The ability of a polymer to switch without delay and exhibit a sharp color change is very signifi-cant. Double potential step chronoamperometry was carried out in a monomer free solution with 0.1 M LiClO4 in acetonitrile to estimate the
response time of the device. The potential was stepped between 0.7 and þ1.1 V with a resi-dence time of 5 s. During the experiment, the %transmittance at the wavelength of maximum contrast was measured using a UV-vis spectro-photometer. The optical contrast was monitored by switching the copolymer between 0.7 and þ1.1 V at 546 nm. As seen in Figure 5, copoly-mer has good stability, fast switching time (1.1 s) and high optical contrast (32%) when com-pared with the homopolymer which has 1.3 s and 2.3% switching time and optical contrast,15 respectively.
Spectroelectrochemistry of Electrochromic Devices A dual-type ECD consists of two electrochromic materials (one anodically coloring, the other cathodically coloring) deposited on transparent ITO, placed in a position to face each other and
Figure 5. Electrochromic switching, optical absorb-ance change monitored at 546 nm for P(FPTP-co-EDOT) between0.7 V and þ1.1 V.
Figure 6. Spectroelectrochemical properties of the dual type ECD as a function of applied potential from1.4 V to þ1.6 V (a) 2D (b) 3D.
a gel electrolyte was applied in between. The anodically coloring polymer film P(FPTP-co-EDOT) was fully reduced and the cathodically coloring polymer (PEDOT) was fully oxidized prior to construction of electrochromic devices (ECDs).
Optoelectrochemical spectra of the dual type ECD as a function of applied potential (from 1.4 to þ1.6 V) are given in Figure 6. Maximum absorption at 555 nm revealing purple color was observed due to p-p* transition. At that state PEDOT did not reveal an obvious absorption at the UV–vis region of the spectrum and device revealed purple color. At moderate potentials de-vice have good transparency and colors of the device are green and gray at 0.0 V and þ0.3 V, respectively. When the applied potential de-creased, because of reduction of PEDOT layer, blue color became dominant and a new absorp-tion was observed at 620 nm. The observed colors with the colorimetry parameters L, a, b values are shown in Figure 4(b).
Switching of ECDs
One of the most important characteristics of ECDs is the response time. It is the time needed to perform switching between the two colored states. Having good transparency at moderate potentials, device can be used for two different combinations, transparent-purple when switched from 0.2 to þ1.6 V and transparent-blue when switched from 0.2 to 1.4 V. For this purpose chronoabsorptometry was employed by stepping
the potential with a residence time of 5 s. Dur-ing the experiment, the % transmittance (%T) at the wavelength of maximum contrast was meas-ured by a UV-vis spectrophotometer. For the de-vice, maximum contrast (D%T) and switching time was measured as 23% and 1.1 s at 555 nm, 14% and 1.1 s at 620 nm (Fig. 7).
Stability of ECDs
Redox stability is a significant characteristic of electrochromic devices with long lifetimes. The main causes of device failure are different applied voltages and environmental conditions. Cyclic voltammetry method was used to
esti-Figure 7. Electrochromic switching, optical absorbance change monitored for ECD (a) between0.2 to þ1.6 V at 555 nm. (b) between 0.2 and 1.4 V at 620 nm.
Figure 8. Cyclic Voltammogram of the device as a function of repeated scans 500 mV/s: after 1st cycle (plain), after 500 cycles (dash).
mate the long-term stability of the devices. After 500 cycles almost all electroactivity (with 9% loss) of the device retain intact (Fig. 8). These results showed that both ECDs have good envi-ronmental and redox stability.
CONCLUSIONS
Copolymer of the 1-(4-fluorophenyl)-2,5-di(thio-phen-2-yl)-1H-pyrrole (FPTP) with 3,4-ethylene dioxythiophene (EDOT) was electrochemically synthesized and characterized. Resulting copoly-mer film has distinct electrochromic properties. While P(FPTP) has only two colors in oxidized and neutral states (blue and yellow), its copoly-mer with EDOT has five different colors (purple, red, light gray, green, blue). At the neutral state kmax value because of the p-p* transition of the
copolymer was found to be 546 nm and Eg was
calculated as 1.6 eV. Double potential step chro-noamperometry experiment shows that copoly-mer film has good stability, fast switching time (1.1 s), and high optical contrast (32%).
Also dual-type electrochromic device based on P(FPTP-co-EDOT) and poly(3,4-ethylenedioxy-thiophene) (PEDOT) was constructed and char-acterized. ECD has good environmental and re-dox stability. Oxidized state of device shows pur-ple color whereas reduced state reveals blue color. At interval potentials device has good transparency and colors of the device are yellow and gray. Having good transparency at moder-ate potentials, this device can be used for two different combinations (transparent-purple or transparent-blue). For the device, maximum contrast (%T) and switching time was measured as 23% and 1.1 s at 555 nm, 14% and 1.1 s at 620 nm. These properties of our device make it good candidate for electrochromic window appli-cations.
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