Synthesis and characterization of a new soluble conducting polymer and its electrochromic devices

Organic Electronics 7 (2006) 351–362 www.elsevier.com/locate/orgel

Synthesis and characterization of a new soluble conducting polymer and its electrochromic devices Elif Sahin a

a,b

, Ertugrul Sahmetlioglu a,c, Idris M. Akhmedov a, Cihangir Tanyeli a, Levent Toppare a,*

Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey b Department of Chemistry, Dicle University, 21280 Diyarbakir, Turkey c Department of Chemistry, Nigde University, 51100 Nigde, Turkey

Received 21 January 2006; received in revised form 16 March 2006; accepted 3 April 2006 Available online 2 May 2006

Abstract A new polythiophene derivative was synthesized by both chemical and electrochemical oxidative polymerization of 1(perfluorophenyl)-2,5-di(2-thienyl)-1H-pyrrole (FPTPy). The structures of both the monomer and the soluble polymer were elucidated by nuclear magnetic resonance (1H-NMR) and fourier transform infrared (FTIR). Polymer of FPTPy was also synthesized via potentiostatic electrochemical polymerization in acetonitrile (AN)/NaClO4/LiClO4 (0.1 M:0.1 M) solvent–electrolyte couple. Characterizations of the resulting insoluble polymer were performed by cyclic voltammetry (CV), FTIR, scanning electron microscopy (SEM) and UV–Vis Spectroscopy. Four-probe technique was used to measure the conductivities of the samples. Moreover, the spectroelectrochemical and electrochromic properties of the polymer film were investigated. In addition, dual type polymer electrochromic devices (ECDs) based on P(FPTPy) with poly(3,4-ethylenedioxythiophene) (PEDOT) were constructed. Spectroelectrochemistry, electrochromic switching and open circuit stability of the devices were studied. They were found to have good switching times, reasonable contrasts and optical memories.  2006 Elsevier B.V. All rights reserved. PACS: 85.60.Pg Keywords: Conducting polymers; UV–vis spectroscopy; Electrochemistry; Morphology

1. Introduction Since the discovery of electrical conductivity in polyacetylene [1], the field of conducting polymers has aroused a great deal of interest among scientists both in industry and academia [2,3]. * Corresponding author. Tel.: +90 312 2103251; fax: +90 312 2101280. E-mail address: [email protected] (L. Toppare).

A great deal of attention was focused on conjugated polymers due to the broad range of applications for which they are potentially useful [2]. Photovoltaic devices [4], light emitting diodes [5], field effect transistors [6], electrochromic devices [7], and various types of sensors [8] based on conjugated polymers are under investigation by numerous researchers around the world. As such, the search for new functional and responsive conjugated polymers exhibiting electrochromism [9], photochromism [10], or non-linear

1566-1199/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2006.04.001

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optics properties are especially sought after for such applications in display technology or data storage. A polymer that possesses a combination of several of these properties is especially an attractive target [11]. An electrochromic material is the one that changes color in a persistent but reversible manner by an electrochemical reaction and the phenomenon is called electrochromism. Electrochromism is reversible and visible change in transmittance and/ or reflectance that is associated with an electrochemically induced oxidation–reduction reactions. It results from the generation of different visible region electronic absorption bands upon switching between redox states. The color change is commonly between a transparent (bleached) and a colored state or between two colored states. In case where more than two redox states are electrochemically available, the electrochromic material may exhibit several colors and may be termed as polyelectrochromic. This optical change is affected by a small electric current at low direct current potentials in the order of a fraction of a volt to a few volts [12]. An electrochromic device is a two-electrode electrochemical cell in a sandwich configuration of thin layers. The arrangement of these layers depends on the operation mode, which can be reflective or transmissive [13]. The reflective mode is used to display or to decrease the reflected light, for example, in a car rear-view mirror. In these devices, one of the electrical contacts should be covered with a reflective layer as a mirror. Transmissive mode operation is very similar, but all layers must become fully transparent when desired. For this reason, optically transparent electrodes must be used [14]. This study aims the synthesis of 1-(perfluorophenyl)-2,5-di(2-thienyl)-1H-pyrrole (FPTPy) via the reaction between 1,4-bis(2-thienyl)butane-1,4-dione and 2,3,4,5,6-pentafluoroaniline. The monomer was characterized via 1H-NMR, FTIR. A new soluble conducting polymer was chemically synthesized and characterized. The spectroelectrochormic properties of electrochemically synthesized polymer were determined. Possibility of using the polymer as a ptype conducting material in an electrochromic device was investigated. 2. Experimental 2.1. Materials AlCl3 (Aldrich), succinyl chloride (Aldrich), dichloromethane (DCM) (Merck), p-toluene sulfonic

acid (PTSA) (Sigma), 2,3,4,5,6-pentafluoroaniline (Aldrich), toluene (Sigma), nitromethane (Aldrich), methanol (Merck), ferric(III) chloride (Aldrich), acetonitrile (AN) (Merck), NaOH (Merck), LiClO4 (Aldrich), NaClO4 (Aldrich), propylene carbonate (PC) (Aldrich) and poly(methyl methacrylate) (PMMA) (Aldrich) were used without further purification. 3,4-ethylenedioxythiophene (EDOT) (Aldrich) were used as received. 2.2. Equipments NMR spectrum of the monomer was recorded on a Bruker-Instrument-NMR Spectrometer (DPX400) using CDCl3 as the solvent. The FTIR spectrum was recorded on a Varian 1000 FTIR spectrometer. Mn for the soluble polymer was measured by gel permeation chromatography (GPC PL220). The surface morphologies of the polymer films were analyzed using JEOL JSM-6400 scanning electron microscope. Cyclic voltammograms were recorded in NaClO4 (0.1 M) and LiClO4 (0.1 M)/AN electrolyte-solvent couple with a system consisting of a potentiostat (Wenking POS 73), an X–Y recorder and a CV cell containing indium/tin oxide (ITO)coated glass plate working and Pt counter electrodes, and a Ag/Ag+ reference electrode. Measurements were carried out at room temperature under nitrogen atmosphere. VoltaLab-50 Potentiostat was used to supply constant potential during electrochemical synthesis. Varian Cary 5000 UV–Vis spectrophotometer was used in order to perform the spectroelectrochemical studies of the polymer and the characterization of the devices. Colorimetry measurements were done with Minolta CS-100 spectrophotometer. 2.3. Synthesis of 1-(perfluorophenyl)-2, 5-di (2-thienyl)-1H-pyrrole (FPTPy) The starting material, 1,4-di (2-thienyl)-1,4butanedione 1, was synthesized according to literature procedure [15]. To a suspension of AlCl3 (16 g, 0.12 mol) in CH2Cl2(15 mL), a solution of thiophene (9.6 mL, 0.12 mol) and succinyl chloride (5.5 mL, 0.05 mol) in CH2Cl2 were added dropwise. The mixture was stirred at 18–20 C for 4 h. This was then poured into ice and concentrated HCl (5 mL) mixture. The dark green organic phase was washed with concentrated NaHCO3 (3 · 25 mL), and dried over MgSO4. After evaporation of the solvent, a blue green solid was remained which

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2.5. Cyclic voltammetry (CV)

O + S

Cl

Cl O

AlCl3 CH2Cl2 18-20 ºC 4h

353

S

S

O O 1

NH2 F

F

F

F F PTSA Toluene 110 oC

N

S

S F F

The oxidation/reduction behavior of polymer was investigated by CV in NaClO4 (0.1 M) and LiClO4 (0.1 M)/AN solvent–electrolyte couple. Experiments were carried out in an electrolysis cell equipped with indium/tin oxide (ITO)-coated glass plates as the working, a Pt wire counter and Ag/Ag+ reference electrodes. The measurements were carried out at room temperature under nitrogen atmosphere.

F

2.6. Electrochemical polymerization of FPTPy

F

Preparative electrochemical polymerization was performed by sweeping the potential between 0.0 V and +1.4 V with 500 mV/s scan rate. 50 mg FPTPy were dissolved in AN and NaClO4 (0.1 M) and LiClO4 (0.1 M) were used as the supporting electrolyte. Electrolyses were carried out using Pt working and counter electrodes and a Ag/Ag+ reference electrode at room temperature for 1 h. The free standing films were washed with AN several times to remove unreacted monomer and the electrolyte. A similar method was used to synthesize the polymer on an ITO coated glass plate.

F 1-(perfluorophenyl)-2,5-di(2-thienyl)-1H-pyrrole (FPTPy)

Scheme 1. The synthetic route of the monomer.

was suspended in ethanol. Filtration and washing with ethanol yielded 1, 4-bis(2-thienyl)butane-1,4dione. The monomer (FPTPy) was synthesized from 1,4-bis(2-thienyl)butane-1,4-dione and 2,3,4,5,6pentafluoroaniline in the presence of catalytical amount of p-toluene sulphonic acid (PTSA) [16,17]. A round-bottomed flask equipped with an argon inlet and magnetic stirrer was charged with 1,4-bis(2-thienyl)butane-1,4-dione (5 mmol, 1.25 g), 1.28 g (7 mmol) 2,3,4,5,6-pentafluoroaniline, 0.1 g (0.58 mmol) PTSA and 20 mL of toluene were added. The resultant mixture was stirred and refluxed for 24 h under argon. Evaporation of the toluene, followed by flash column chromatography (SiO2 column, elution with dichloromethane) afforded the desired compound as pale brown powder. The synthetic route of the monomer is shown in Scheme 1.

2.7. Preparation of the gel electrolyte Gel electrolyte was prepared utilizing NaClO4:LiClO4:AN:PMMA:PC in the ratio of 1.5:1.5:70:7:20 by weight. After NaClO4/LiClO4 was dissolved in AN, PMMA was added into the solution. In order to dissolve PMMA, vigorous stirring and heating were required. Propylene carbonate (PC), as plasticizer, was introduced to the reaction medium when all the PMMA was completely dissolved. The mixture was stirred and heated until a highly conducting transparent gel was produced.

2.4. Chemical polymerization of (FPTPy) with iron (III) chloride

2.8. Construction of electrochromic devices

FPTPy (1 · 10 3 M) was dissolved in nitromethane (15 mL) and placed in a three-necked flask. Iron (III) chloride (2 · 10 3 M) is placed in 15 mL nitromethane. Monomer solution was added drop wise to the solution of iron (III) chloride at 0 C. The reaction was carried out for 5 min. with constant stirring. The dark blue oxidized polymer was first washed with methanol, filtered, compensated with 30% NaOH, and dried under vacuum for 1H-NMR analyses.

Poly (3,4-ethylenedioxythiophene) (PEDOT) was potentiostatically deposited on ITO working electrode by applying +1.5 V in AN/NaClO4/LiClO4 (0.1 M) solvent–electrolyte. P(FPTPy) was obtained by sweeping the potential between 0.0 V and +1.4 V versus Ag/Ag+ in AN/NaClO4/LiClO4 (0.1 M:0.1 M). It is important to balance the charge capacities of the devices prior to assembling the devices. Otherwise, there would be incomplete electrochemical reaction and residual charges would remain during

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the redox process [18]. Therefore, redox charges of the anodically and cathodically coloring polymers were matched by chronocoulometry. In order to obtain the complementary operating conditions, anodically coloring polymers were fully reduced and the cathodically coloring polymer was fully oxidized. By sandwiching the gel electrolyte between the anodically and the cathodically coloring polymers, the device was constructed. 3. Results and discussion 3.1. Synthesis For the synthesis of 1,4-di(2-thienyl)-1,4-butanedione, the double Friedel–Crafts reactions propose by Merz and Ellinger [15] was chosen. However we found that the reaction time can be considerable reduced, the reaction mixture being refluxed at 18– 20 C for 4 h (instead of 24 h stirring at ambient temperature) without loss of yield (75%). 3.2. Cyclic voltammetry Cyclic voltammogram of FPTPy in AN/LiClO4– NaClO4 solvent/electrolyte couple indicated an oxidation peak at 1.1 V and a reduction peak at 0.4 V. When the range between 0.0 V and +1.4 V (Fig. 1) was scanned, it was observed that the electroactivity

increased with increasing scan number. This process promotes an electrochromic change on the polymer film to a yellow color, while a greenish cloud is formed around the electrode due to the partial dissolution of neutral oligomers of low molecular weight present in the reduced P(FPTPy). Under these conditions, the monomer yields a polymer which is subsequently oxidized at the same potential to produce polarons balanced with ClO4 counterions. Further reduction of this polymer at 0.4 V peak involves the neutralization of polarons with the loss of ClO4 and the resulting short linear species are dissolved [19]. 3.3. NMR spectra of FPTPy and P(FPTPy) 1

H-NMR spectrum of monomer (Fig. 2(A)) reveals: (Pale yellow viscous product) C18H8NF5, dH (CDCl3): 6.63 (s, 2 H, pyrrolyl), 6.84 (d, 2H, J = 3.34 Hz, 3-thienyl), 6.96 (dd, 2H, J = 4.61 Hz, 3.34 Hz, 4-thienyl), 7.22 (d, 2H, J = 4.61 Hz, 5thienyl). 1 H-NMR spectrum of polymer (Fig. 2(B)) (CDCl3): 6.53–6.47 (2H, broad s, pyrrolyl), 6.86– 6.75 (2H, d, 3-thienyl), 7.19–7.11 (2H, d, 4-thienyl). GPC data revealed Mn = 8.0 · 103 for P(FPTPy) prepared by chemical polymerization. 3.4. FTIR spectra

Fig. 1. Cyclic voltammogram of FPTPy.

FTIR spectrum of the FPTPy shows the following absorption peaks: 3096 cm 1 (aromatic C–H stretching), 3020 cm 1 (C–Ha stretching of thiophene), 1515 cm 1 (C–F stretching of benzene), 1416–1261 cm 1 (aromatic C@C, C–N stretching due to pyrrole and benzene), 1073 cm 1 (aromatic C@C–F), 801 cm 1 (C–Ha out of plane bending of thiophene). Most of the characteristic peaks of FPTPy remained unperturbed upon chemical polymerization. The intensity absorption bands of the monomer at 3020 cm 1 arising from C–Ha stretching of thiophene moiety, disappeared completely. This is an evidence of the polymerization from 2, 5 positions of thiophene moiety of the monomer. Whereas, two new bands related to C–Hb out-ofplane bending of 2,5 disubstituted thiophene and C–S stretching appeared at 836 and 773 cm 1, respectively. The broad band observed at around 1649 cm 1 proves the presence of polyconjugation

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355

Fig. 2. 1H-NMR spectra of monomer (A), polymer in CDCl3(B).

and the new peak at 642 cm 1 indicates the presence of the dopant ion (Cl ). FTIR spectra of electrochemically synthesized P(FPTPy) showed the characteristic peaks of the monomer. The peaks related to C–Ha stretching of thiophene disappeared completely. The new broad band at around 1622 cm 1 was due to polyconjugation. The strong absorption peak at 1129, 1052 cm 1 were attributed to the incorporation ClO4 ions into the polymer film during doping process. Results of the FTIR studies clearly indicated the polymerization of the monomer.

3.5. Conductivities of the films The conductivities of electrochemically and chemically prepared P(FPTPy) were measured as 1.3 · 10 4 S/cm and 4.2 · 10 5 S/cm respectively via four probe technique. 3.6. Scanning electron microscopy (SEM) Analysis of the surface morphologies of films was done using JEOL JSM-6400 scanning electron microscope. SEM micrograph of solution side of

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Fig. 3. SEM micrographs of P(FPTPy).

P(FPTPy) film showed droplet like structures (Fig. 3). 3.7. Electrochromic properties of the electrochemically synthesized conducting polymer We investigated the in situ electrochemical polymerization of FPTPy by UV–Vis spectrophotometer via applying 1.4 V in AN/LiClO4–NaClO4 (0.1 M:0.1 M) at every 10 s time intervals (Fig. 4).

There was a gradual increase in the peak intensity at around 760 and 850 nm for P(FPTPy) revealing the formation of the charge carriers. The best way of examining the changes in optical properties of conducting polymers upon voltage change is spectroelectrochemistry. It also gives information about the electronic structure of the polymer such as its band gap (Eg) and the intergap states that appear upon doping. P(FPTPy) film was also potentiodynamically synthesized on ITO electrode in the presence of 50 mg FPTPy, while the potential was swept between 0.0 V and 1.4 V in AN/NaClO4/LiClO4. The spectroelectrochemical and electrochromic properties of the resultant polymer were studied by applying potentials ranging between 0.0 V and +1.1 V in monomer free AN/NaClO4/LiClO4 medium. At the neutral state kmax value due to the p–p* transition of the polymer was found to be 420 nm and Eg was calculated as 2.11 eV. Upon applied voltage, reduction in the intensity of the p–p* transitions and formation of charge carrier bands were observed. Thus, appearance of peaks around 700 nm and 1000 nm are attributed to the evolution of polaron and bipolaron bands, respectively (Fig. 5). The colors of the electrochromic materials were defined accurately by performing colorimetry measurements. CIE system was used as a quantitative

Fig. 4. In situ electrochemical polymerization of FPTPy.

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Fig. 5. Optoelectrochemical spectrum of (A) two dimensional representation of P(FPTPy) as applied potentials between 0.0 V and +1.1 V in AN/NaClO4/LiClO4 (0.1 M); (a) 0.0 V, (b) +0.5 V, (c) +0.6 V, (d) +0.7 V, (e) +0.8 V, (f) +0.9 V, (g) +1.0 V, (h) +1.1 V. (B) Three dimensional representation.

scale to define and compare colors. Three attributes of color; hue (a), saturation (b) and luminance (L) were measured and recorded. The P(FPTPy) film shows different colors in the fully reduced state (0.0 V), half oxidized state (0.9 V) and fully oxidized state (1.1 V) (Table 1).

3.8. Electrochromic switching The polymers that can switch rapidly and exhibit striking color changes reveal superior results in electrochromic applications. The experiments carried out via spectroelectrochemistry proved the ability

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Table 1 Colorimetric properties of polymer and device Material

Color

L

P(FPTPy)

Yellow (0.0 V) Green (0.9 V) Light purple (1.1 V)

75 75 67

a 4 1 5

b 27 12 5

P(FPTPy)/PEDOT

Brown (0.0 V) Blue (2.6 V)

70 49

1 3

28 13

of P(FPTPy) to switch between its neutral and doped states with a change in transmittance at a fixed wavelength. Square wave potentials, as determined from the spectroelectrochemical experiments, were applied to the polymer. The polymer film was synthesized on ITO-coated glass slides. During the experiment, the % transmittance (%T) at 420 nm was measured using a UV–Vis spectrophotometer. Upon switching P(FPTPy) between 0.0 V and +1.1 V with a residence time of 5 s, optical contrast and the time needed to reach 95% of the total transmittance change for the homopolymer was measured. P(FPTPy) was found to have 18% optical contrast with a switching time of 1.3 s (Fig. 6). 3.9. Spectroelectrochemistry of Electrochromic Devices (ECDs) A dual-type ECD consists of two electrochromic materials (one revealing anodic coloration, the

other revealing cathodic coloration) deposited on transparent ITO, placed in a position to face each other and a gel electrolyte in between. In order to maintain a balanced number of redox sites for switching, the redox charges of the two complementary polymer films were matched by chronocoulometry. Before constructing the ECD, the anodically coloring polymer film, P(FPTPy), was fully reduced and the cathodically coloring polymer (PEDOT) was fully oxidized. Upon application of voltage, the doped polymer will be neutralized, whereas the other component will be oxidized, resulting in the color change. Colorimetry analyses of the ECDs were performed by using the same procedure as described. The luminance (L), hue (a) and saturation (b) values of the devices were measured and recorded in Table 1. Spectroelectrochemistry experiments were performed to investigate the changes of the electronic transitions of the ECD by increasing the applied potential. Fig. 7 represents the absorption spectra of the ECD, recorded during application of different voltages between 0.0 V and +2.6 V. The polymer was in its neutral state at 0.0 V, where the absorption at 420 nm was due to p–p* transition of the polymer. At this potential, PEDOT 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

Fig. 6. Electrochromic switching, optical absorbance change monitored at 420 nm for P(FPTPy) between 0.0 V and 1.1 V in AN/NaClO4/ LiClO4 (0.1 M:0.1 M).

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Fig. 7. Optoelectrochemical spectrum of (A) two dimensional representation of P(FPTPy)/PEDOT ECD at applied potentials between 0.0 V and +2.6 V; (a) 0.0 V, (b) +0.2 V, (c) +0.4 V, (d) +0.6 V, (e) +0.8 V, (f) +1.0 V, (g) +1.2 V, (h) +1.4 V, (i) +1.8 V, (j) +2.0 V, (k) +2.2 V, (l) +2.4 V, (m) +2.6 V. (B) Three dimensional representation.

increased the homopolymer layer started to get oxidized, and a decrease in the intensity of the absorption was observed. Meanwhile, PEDOT layer was in its reduced state, which was followed by the appearance of the new absorption at 607 nm, dominating the color of the device as blue.

3.10. Switching of ECDs One of the most important characteristics of ECDs is the response time needed to perform switching between the two colored states. Another important parameter is the optical contrast, which

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can be defined as the transmittance difference between the redox states [20]. Chronoabsorptometry, a square wave potential step method coupled with optical spectroscopy, was used to evaluate the response time of the device. The potential was set at an initial potential for a residence time of 5 s and than a second potential was applied for the same set period of time until being switched back to the initial potential again. Applied potentials were determined from the spectroelectrochemical studies, where the ultimate states of the devices were achieved. The experiment was carried out at 607 nm for P(FPTPy) /PEDOT device. The device was switched between 0.0 V and +2.6 V. The maximum transmittance difference between the oxidized and reduced states was measured as 29%. The time required to attain 95% of the total transmittance difference was found as 1.2 s for the device (Fig. 8). 3.11. Stability of ECDs Redox stability is an important requirement for production of reliable electrochromic devices with long lifetimes. Main reasons for device failure are different applied voltages and environmental conditions. Cyclic voltammetry was employed by monitoring current alterations to visualize the long

term stability of the ECD. The voltage was continuously swept between 0.0 V and +2.6 V with 500 mV/s scan rate. After 500 cycles, almost all initial electroactivity was maintained proving that ECD has reasonable environmental and redox stability (Fig. 9). 3.12. Open circuit memory of ECDs The color persistence is an important feature since it is directly related to aspects involved in its utilization and energy consumption during use [21]. After setting the device in one of the colored state and removing the applied voltage, it should retain that color with no further current requirement. This is known as open circuit memory. To test this property a potential was applied for one second and the device was left under open circuit conditions for 100 s while monitoring the percent transmittance change at a fixed wavelength. The open circuit memory of P(FPTPy)/PEDOT device was tested at 0.0 V (yellow colored state) and +2.6 V (blue colored state) at 420 nm. As given in Fig. 10, P(FPTPy)/ PEDOT device shows quite good optical memories both in oxidized (with only 5% transmittance change) and reduced states (with almost no transmittance change).

Fig. 8. Electrochromic switching, optical absorbance change monitored at 607 nm for device between 0.0 V and +2.6 V.

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Fig. 9. Cyclic voltammogram of the device as a function of repeated scans 500 mV/s: after 1st cycle (plain), after 500th cycles (dash).

Fig. 10. Open circuit memory of P(FPTPy)/PEDOT ECD monitored at 420 nm, 0.0 V and +2.6 V potentials were applied for one second for each 100 s time interval.

4. Conclusions The synthesis of a new monomer; 1-(perfluorophenyl)-2,5-di(2-thienyl)-1H-pyrrole FPTPy was successfully achieved. P(FPTPy) was synthesized by both chemical and electrochemical oxidative polymerizations. Chemically synthesized homopoly-

mer of FPTPy is soluble in common organic solvents. This property provides several applications. The homopolymer of P(FPTPy) was also synthesized via potentiodynamically in AN/NaClO4/ LiClO4 (0.1 M) solvent–electrolyte couple. Spectroelectrochemical analyses revealed that this polymer has an electronic band gap of 2.11 eV. The contrast

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is measured as the difference between %T in the reduced and oxidized forms and is noted as 18%T at 420 nm. In the second part of the study, dual-type complementary colored polymer ECD made up of P(FPTPy)/PEDOT was constructed and its characteristics were examined. A potential range from 0.0 V to +2.6 V was found suitable for operating the device. The color changes were distinctive and aesthetically pleasing. Good switching time and optical contrast values were obtained. In addition, the device showed good environmental and redox stability. Considering these results, polymer of FPTPy is a feasible candidate for electrochromic layers in ECDs.

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