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Multicolored electrochromic copolymer based on 1,4-di(thiophen-3-yl)benzene and 3,4-ethylenedioxythiophene
Journal of Electroanalytical Chemistry 653 (2011) 21–26
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Multicolored electrochromic copolymer based on 1,4-di(thiophen-3-yl)benzene and 3,4-ethylenedioxythiophene Mi Ouyang, Genghao Wang, Yujian Zhang, Cheng Hua, Cheng Zhang ⇑ State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, PR China
a r t i c l e
i n f o
Article history: Received 6 October 2010 Received in revised form 25 December 2010 Accepted 14 January 2011 Available online 24 January 2011 Keywords: Multicolored Electrochemical polymerization Conjugated polymer Electrochromism
a b s t r a c t Copolymer of 1,4-di(thiophen-3-yl)benzene (DTB) with 3,4-ethylenedioxy-thiophene (EDOT) was electrochemically synthesized and characterized. Resulting copolymer film presents excellent electrochromic properties. At the neutral state, kmax value due to the p–p transition is found to be 496 nm and energy gap (Eg) is calculated as 1.92 eV. Besides, the copolymer film possesses four various colors (red, yellow, green and blue) under different potentials. Otherwise, the electrochromic switching results show that the copolymer film has fast switching time (0.8 s at 746 nm) and high optical contrast (59% at 1100 nm). Cyclic voltammetry study reveals that the copolymer film has excellent stability. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Electrochromic (EC) materials, possessing unique electrochromic properties, have been proven especially useful for not only electrochromic mirrors [1–3], but also thin-film transistors [4], displays [5,6], sensors [7,8], memory devices [9] and so on. As a class of excellent EC materials, polythiophenes have occupied prime position due to its high conductivity, good redox reversibility, swift change of color with potential, and stability in environment [10]. For commercial applications, much research has focused on multicolor EC materials, but most polythiophenes only display two colors [11]. Copolymerization offers an effect way of controlling the electrochromic properties of conducting polymers. Copolymers can lead to an interesting combination of the properties observed in the corresponding homo-polymers [12]. 3,4-Ethylenedioxythiophene (EDOT) is a popular choice as a substituted monomer since it produces a low band gap polymer with high stability and good conductivity [13]. Structure modification is another wise way to achieve various properties. Various strategies taking full advantage of the different substitution induced backbone conformations have been proposed to fine-tune the colored neutral state of polythiophenes. For thiophene, the substituent at the 3-position of the thiophene ring leads to a significant increase of the polymer conjugation length [14]. The introduction of a phenyl group into the backbone of polythiophene stabilizes the conjugated p-bonds system [15]. While, plac⇑ Corresponding author. E-mail address: [email protected] (C. Zhang). 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.01.014
ing phenyl between two thiophenes may allow the fine tuning of the optical band gap and retain the easy connectivity and good optics of thiophenes [16]. In this manuscript, the monomer of 1,4-di(thiophen-3-yl)benzene (DTB) was successfully synthesized (Scheme 1). Then a novel multicolored electrochromic copolymer based on DTB and EDOT was achieved by electrochemical copolymerization. The resultant copolymer film exhibits red, yellow, green and blue four colors with the variation of the applied potentials, containing all the three primary colors. This property may broaden the commercial applications of the copolymer. Furthermore, the copolymer film presents good stability, comparatively fast switching time (0.8 s at 746 nm) and high optical contrast (59% at 1100 nm). 2. Experimental 2.1. Materials CH2Cl2 (Aldrich), 3-bromothiophene (Aldrich), 1,4-dibromobenzene (Aldrich), tetrabutylammonium perchlorate (TBAP) (Aldrich), methanol (Aldrich) were used without further purification. Magnesium powder (Aldrich) and EDOT (Aldrich) were used as received. Tetrahydrofuran (THF) (Aldrich) was distilled before use, and dried with natrium. 2.2. Synthesis of 1,4-di(thiophen-3-yl)benzene Dry THF (15 mL) was added to 0.04 mol 3-bromothiophene. The system was flushed with N2 for 5 min and maintained under
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Scheme 1. Synthesis of DTB.
an inert atmosphere. 0.05 g Pd(dppf)Cl2 was added rapidly and 0.02 mol Grignard reagent was dropped in. The mixture was stirred at 65 °C for 16 h. 30 mL HCl (2%) was added to terminate the reaction. After filtered, the solid was washed several times with water and methanol, respectively. Recrystallization with methanol gave white crystalline 1,4-di(thiophen-3-yl)benzene, 41%. m.p.: 282– 284 °C. 1H NMR (500 MHz, CDCl3): d (ppm) 7.64 (s, 4H); 7.48 (m, 2H,); 7.43 (m, 2H); 7.39 (m, 2H,). 13C NMR (125 MHz, CDCl3): d (ppm) 143.9; 133.5; 128.1; 126.3; 124.9; 123.1. FT-IR (KBr, cm 1): 3099 s, 2922 m, 2852 w, 1909 w, 1541 m, 1429 m, 1375 m, 1080 m, 1001 m, 686 m, 630 m, 517. HRMS: calculated for C14H10S2 m/z 242.2451, found m/z 242.2821. For C14H10S2 (242.3) calcd.: C, 69.38%; H, 4.16%. Found: C, 69.29%; H, 4.14%.
2.3. Instrumentation All electrochemical experiments were performed on CHI660a electrochemical working station (CHI, Chenhua, Shanghai, China). Spectroelectrochemical studies were carried out on a UV 1800 spectrophotometer. Surface morphologies of polymer films were investigated by a Hitachi S-4800 scanning electron microscope. FT-IR spectra were recorded on a Nicolet 6700 FTIR spectrometer. A Perkin-Elmer Diamond TG/DTA 6300 was applied to conduct the thermogravimetric analysis. A Perkin-Elmer Flash EA 1112 elemental analyzer performed the elemental analysis.
2.4. Electrochemistry The redox behavior of monomers and polymers was investigated by cyclic voltammogram (CV) using a three compartment electrolysis cell with indium tin oxide (ITO) plate as working electrode. The counter electrode was made from a platinum sheet, and a double-junction Ag/AgCl electrode (silver wire coated with AgCl in saturated KCl solution, 0.1 M TBAP in CH2Cl2 solution as the second junction) was applied as the reference electrode. 0.1 M TBAP in CH2Cl2 was used as electrolyte. Electrochemistry experiments were carried out at room temperature.
3. Results and discussion 3.1. Electropolymerization and electrochemical characterization As shown in Fig. 1, the onset oxidation potentials of DTB and EDOT were 1.33 V and 1.38 V vs. Ag/AgCl. The difference between the two potentials was just 0.05 V, which indicated that the electrochemical copolymerization of DTB and EDOT could be achieved at appropriate potential. The onset oxidation potential of the mixture DTB-EDOT was found to be 1.34 V. Fig. 2 displayed the successful electropolymerization of DTB, DTB-EDOT and EDOT on ITO working electrodes. Well-defined polymer oxidation and reduction peaks were observed in all the three CVs. Upon sequential cycles, there were gradual increases in the current intensity, which indicated that the films were formed on the surface of the electrodes. Repeated cycling resulted in a tiny decrease in the onset of oxidation potential. The increase in peak separation potential reflects the increase in resistance as the thickness of the polymer film increased. For DTB, The first anodic peak at around 0.97 V should be associated with the conformational change of the polymer during oxidation, and the second anodic peak at around 1.22 V should be associated with the overall doping process [17]. The CV of EDOT exhibited one obvious oxidation peak at around 0.16 V and two reduction peaks at around 0.11 V and 0.55 V respectively. The redox waves of DTB-EDOT were completely different from those of DTB and EDOT, which should be ascribed to the copolymerization. The CV of DTB-EDOT exhibited an oxidation peak at around 1.04 V and a reduction peak at around 0.56 V. The CVs of polymer PDTB, P(DTB-EDOT) and PEDOT were carried out in monomer free 0.1 M TBAP/CH2Cl2. All the three polymer films were electroactive as shown in Fig. 3. The PDTB film exhibited one main oxidation wave at about 1.25 V and the main reduction peak at around 0.94 V. The PEDOT film displayed main oxidation peak and reduction peak at around 0.24 V and 0.64 V, respec-
2.5. Preparation of polymer films The solutions of 3 mM DTB, 3 mM EDOT and 3 mM/3 mM DTB-EDOT in CH2Cl2 were prepared respectively with 0.1 M TBAP as supporting electrolyte. All polymer films were prepared via potentiostatic electrolysis on ITO electrodes, and the films were electrolyzed at negative potential again in order to equilibrate their redox behavior in monomer-free electrolytic solution, after that, the films were washed with CH2Cl2 for several times to remove the residual supporting electrolyte and the monomers. For elemental analysis, thermogravimetric analysis and FT-IR spectra, neutral polymer samples were peeled off from the ITO electrodes, and dried in vacuum at 70 °C for 10 h.
Fig. 1. Polarization curves of DTB, EDOT and DTB-EDOT in 0.1 M TBAP/CH2Cl2. The scan rate was 5 mV s 1.
M. Ouyang et al. / Journal of Electroanalytical Chemistry 653 (2011) 21–26
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that the electrochemical processes were reversible and not diffusion limited, even at very high scan rates.
3.2. Elemental analysis
Fig. 2. Cyclic voltammogram curves of (A) DTB, (B) DTB-EDOT, (C) EDOT in 0.1 M TBAP/CH2Cl2 at a scan rate of 50 mV s 1.
To confirm the formation of the copolymer, a homo-polymer film mixture of PDTB and PEDOT which composed according to the mDTB/mEDOT proportion in polymer P(DTB-EDOT) was expected for comparison with the P(DTB-EDOT) film. The polymer films were synthesized on ITO electrodes, dedoped at negative potential and washed with CH2Cl2. Then, the films were dried in vacuum at 70 °C for 10 h. The mass percentage of C, H, S and N were recorded by elemental analysis. The atom N was assigned to residual TBAP. The mass percentages of other atoms were calculated and were displayed in Table. 1. Then, mDTB/mEDOT in copolymer film was calculated to be approximately 1/7. Film PDTB and PEDOT were mixed according to this proportion. The elemental analysis of the mixture was also performed (Table. 1). As seen from Table. 1, the result of the mixture is close to that of the copolymer. To be more reasonable, the mixture of polymer PDTB and PEDOT used in IR spectra and TG analysis was prepared according to this proportion.
3.3. FT-IR spectra
Fig. 3. Cyclic voltammograms of PDTB, P(DTB-EDOT) and PEDOT in 0.1 M TBAP/CH2Cl2 at scan rate 100 mV s 1.
tively. While the CV of P(DTB-EDOT) exhibited single and well-defined reversible redox couple, and was different from those of PDTB and PEDOT. Although the intermediate CV response from P(DTBEDOT) could be interpreted as the existence of DTB and EDOT units into the polymer chain, the further evidence is needed to be found. Fig. 4 showed the CV curves of P(DTB-EDOT) at different scan rates. The current response of P(DTB-EDOT) was directly proportional to the scan rate, indicating that the polymer film was electroactive and adhered well to the electrode [18]. The scan rate dependence of the anodic and cathodic peak currents showed a linear dependence as a function of the scan rate. This demonstrated
Fig. 4. Cyclic voltammograms of P(DTB-EDOT) film at different scan rates.
Fig. 5 showed the FT-IR spectra of PDTB, mixture of PDTB and PEDOT, P(DTB-EDOT) and PEDOT. Peaks of PDTB at 1470, 1685 and 1605 cm 1 should be ascribed to the C–C stretching vibrations in the phenyl ring. The peak at 735 cm 1 was attributed to C–H out-of-plane vibration of phenyl ring. The peaks at 780 and 832 cm 1 were characteristic of thiophene ring [19]. The peak at 1089 cm 1 was due to the presence of ClO4 [20]. For the spectrum of PEDOT, stretching vibrations of thiophene ring presented at 1320, 1195 and 1066 cm 1. Oxygenic substituents showed bands at 1512 and 980 cm 1 [21]. The spectra of the mixture and P(DTB-EDOT) contained all the characteristic peaks of both PDTB and PEDOT. But some apparent differences were showed in the two spectra. For example, the peak at 1348 cm 1 in the spectrum of P(DTB-EDOT) is sharper than the peak at 1372 cm 1 in the mixture. Taking the peaks at 1654, 1348 and 1076 cm 1 for examples, the spectrum of P(DTB-EDOT) showed a little red shift, indicating the existence of interaction between DTB and EDOT units in the polymers. These provided another proof to the copolymerization of DTB and EDOT.
3.4. Thermogravimetric analysis Fig. 6 displayed the TG curves of PDTB, PEDOT, P(DTB-EDOT) and the mixture. The mixture of PDTB and PEDOT was prepared according to the mass proportion of 1/7 calculated from elemental analysis. TG analysis was carried out in N2 atmosphere with heating rate 10 °C min 1 from 100 to 600 °C. The initial slight weight loss observed at the beginning should be assigned to the evaporation of the residual solvent or the trapped water in polymer films [22]. The curve of PDTB showed the onset temperature of weight loss at around 285 °C and the maximum decomposition temperature was between 290 °C and 330 °C. PEDOT film began to loose weight at around 315 °C. The curve of P(DTB-EDOT) showed three obvious weight losses. The first weight loss could be tentatively attributed to the loss of residual TBAP as it was explained in similar research work based on conducting polymers [23,24]. Another two losses should due to the degradation of the polymer backbone [25]. The apparent difference between the curves of the mixture and P(DTB-EDOT) indirectly confirms the copolymer structure of the P(DTB-EDOT).
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Table 1 Mass percentage of atoms in the copolymer film and the mixture of PDTB and PEDOT. Atom m%
Copolymer Mixture
C
H
S
N
Cl
O in TBAP
O in copolymer
55.10 53.74
5.54 4.84
13.91 12.35
1.56 1.17
3.95 2.97
7.13 5.53
12.81 19.40
and loose spongy network structure PEDOT film, film P(DTB-EDOT) exhibited an accumulation state of clusters of globules. This difference could be attributed to the effect of copolymerization. 3.6. Spectroelectrochemical characterization
Fig. 5. FT-IR spectra of (A) PDTB; (B) mixture of PDTB and PEDOT (C) P(DTB-EDOT); (D) PEDOT.
Fig. 6. TG curves of PDTB, PEDOT, P(DTB-EDOT), mixture of PDTB and PEDOT under nitrogen atmosphere at a heating rate of 10 °C min 1 from 100 to 600 °C.
3.5. Scanning electron microscopy (SEM) The polymer films of DTB, DTB-EDOT and EDOT were prepared by constant potential electrolysis (polymerization charge: 0.80 °C cm 2) from the solution of 0.1 M TBAP/CH2Cl2 containing relevant monomers on the ITO. The surface morphologies of polymer films were investigated by Scanning electron microscope (SEM) as shown in Fig. 7. Comparison with the smooth PDTB film
Spectroelectrochemical analysis is a powerful way to investigate the optical switches and contrasts of EC conducting polymers upon potential change, which provides insights into the electronic structure of the conducting polymer. The spectroelectrochemical and electrochromic properties of the resultant films were studied with 0.1 M TBAP/CH2Cl2 solution as supporting electrolyte. As shown in Fig. 8, the spectroelectrochemical property of PDTB was investigated by applying potentials ranging from 0.4 to 1.0 V. At the neutral state, kmax value was found to be 427 nm due to the p–p transition of the polymer backbone. Above applied voltage, reduction in the intensity of the p–p transitions and formation of charge carrier bands were observed. Thus, the appearance of peaks around 775 nm and >1100 nm could be attributed to the evolution of polaron and bipolaron bands. For comparison, the spectroelectrochemistry of PEDOT film was shown in Fig. 9. The neutral state PEDOT exhibited the p–p transition peak at around 593 nm, and its absorption intensity decreased with the increase of the applied potential. While the doped PEDOT film presented intense charge carrier bands at around 880 nm and >1100 nm. Fig. 10 depicted the different spectroelectrochemical spectra of the copolymer film under various applied potentials ranging from 0.0 to 1.0 V. A well-defined maximum absorption band centered at 496 nm was observed, which should be attributed to the p–p transition of the neutral state copolymer backbone, and it decreased with the increase of potential. The appearance of charge carrier bands (at around 760 nm and >1100 nm) could be attributed to the evolution of polaron and bipolaron bands. Table 2 summarized the maximum absorption bands and energy gaps of PDTB, PEDOT and the copolymer. As seen from Table 2, the kmax value and Eg of copolymer are located between those of the PDTB and PEDOT, indicating that introduction of EDOT to the polymer chain leaded to an effective decrease in the band gap. The film colors of homo-polymer PDTB and copolymer P(DTBEDOT) at different potentials were presented in Fig. 11. Film PDTB presented yellow and blue at neutral and oxidized state, respectively. While film P(DTB-EDOT) displayed four different hues (yellow and three primary colors) at various potential. These different colors corresponding to various doped and neutral states had
Fig. 7. SEM micrograph of (A) PDTB, (B) P(DTB-EDOT), (C) PEDOT.
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Table 2 kmax and Eg values of the homo-polymers (PDTB and PEDOT) and the copolymer P(DTB-EDOT).
kmax (nm) Eg (eV)
PDTB
P(DTB-EDOT)
PEDOT
427 2.14
496 1.92
593 1.68
Fig. 8. Spectroelectrochemical spectra of PDTB film as applied potentials between 0.4 and 1.0 V in 0.1 M TBAP/CH2Cl2.
Fig. 11. Polymer film Colors at different potentials (A) PDTB; (B) P(DTB-EDOT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Spectroelectrochemical spectra of PEDOT film as applied potentials between 0.0 and 1.0 V in 0.1 M TBAP/CH2Cl2.
Fig. 12. Electrochromic switching, optical absorbance monitored for PDTB at 427, 774 and 1100 nm in 0.1 M TBAP/CH2Cl2 between 0.3 and 0.9 V with a residence time of 5 s.
also been confirmed by the CV tests. This multicolor property possesses significant potential applications in smart windows or displays and so on.
with a residence time of 5 s at different wavelength. The switching time was defined as the time required for reaching 95% of the full change in absorbance after the switching of the potential. The copolymer film exhibited higher optical contrast than that of the homo-polymer film, especially, the optical contrast of copolymer film at 1100 nm was found to be 59%. This should be ascribed to the introduction of EDOT units into the polymer backbone. Otherwise, the copolymer film has fast switching time (0.8 s at 746 nm). The electrochromic switching behavior of PEDOT was given for comparison in Fig. 14. PEDOT has good optical contrast and fast switching time. The incorporation of EDOT units into the DTB polymer led to the improvement of electrochromic property of the copolymer. The good stability of the %DT in time, fast switching property and reasonable contrast ratio make this copolymer a promising material for EC devices.
3.7. Electrochromic switching
3.8. Stability of copolymer film P(DTB-EDOT)
Electrochromic switching time and optical contrast (%DT) are the two most important characteristics for EC materials. Figs. 12 and 13 exhibited electrochromic switching response of the homo-polymer and the copolymer films between 0.3 and 0.9 V
The stability of EC materials toward long-term switching between the neutral and oxidized states is one of the most important factors for the application of EC materials in device utilities. Cyclic voltammetry was used to test the stability of the copolymer film.
Fig. 10. Spectroelectrochemical spectra of P(DTB-EDOT) film as applied potentials between 0.0 and 1.0 V in 0.1 M TBAP/CH2Cl2.
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500 cycles, only slight shrinkage of redox loop is observed after 500 cycles, which implies that P(DTB-EDOT) film has reasonable stability, and could be as a promising candidate material for EC devices. 4. Conclusions
Fig. 13. Electrochromic switching, optical absorbance monitored for P(DTB-EDOT) at 496, 746 and 1100 nm in 0.1 M TBAP/CH2Cl2 between 0.3 and 0.9 V with a residence time of 5 s.
A multicolored copolymer from DTB and EDOT was successfully synthesized via electrochemical methods. At different potentials, the copolymer film exhibited four hues containing three primary colors. Spectroelectrochemical studies and electrochromic characterization demonstrated that copolymerization with EDOT not only decreased the energy gap Eg effectively but also enhanced the electrochromic properties such as optical contrast and switching time. With its multicolor electrochromism (yellow and three primary colors) good stability, comparatively fast switching time and high optical contrast, the copolymer film has expected to be amenable for practical use such as EC paper and other displays. Acknowledgement The authors gratefully thank the supporting of National Basic Research Program of China (2010CB635108), Natural Science Foundation of Zhejiang Province, China (Y4090260), Major Science and Technology, Special and Priority Themes of Zhejiang Province, China (2009C14004) and China Postdoctoral Science Foundation (20100471755). References
Fig. 14. Electrochromic switching, optical absorbance monitored for PEDOT at 593 and 1100 nm in 0.1 M TBAP/CH2Cl2 between 1.0 and 0.9 V with a residence time of 5 s.
Fig. 15. Cyclic Voltammogram of P(DTB-EDOT) as a function of repeated scans 500 mV s 1 in 0.1 M TBAP/CH2Cl2.
Fig. 15 presented the long-term stability of the as-prepared P(DTBEDOT) film. Cyclic voltammograms of P(DTB-EDOT) between potential 0.1 and 1.3 V indicate that the film keeps stable up to
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Multicolored electrochromic copolymer based on 1,4-di(thiophen-3-yl)benzene and 3,4-ethylenedioxythiophene Mi Ouyang, Genghao Wang, Yujian Zhang, Cheng Hua, Cheng Zhang ⇑ State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, PR China
a r t i c l e
i n f o
Article history: Received 6 October 2010 Received in revised form 25 December 2010 Accepted 14 January 2011 Available online 24 January 2011 Keywords: Multicolored Electrochemical polymerization Conjugated polymer Electrochromism
a b s t r a c t Copolymer of 1,4-di(thiophen-3-yl)benzene (DTB) with 3,4-ethylenedioxy-thiophene (EDOT) was electrochemically synthesized and characterized. Resulting copolymer film presents excellent electrochromic properties. At the neutral state, kmax value due to the p–p transition is found to be 496 nm and energy gap (Eg) is calculated as 1.92 eV. Besides, the copolymer film possesses four various colors (red, yellow, green and blue) under different potentials. Otherwise, the electrochromic switching results show that the copolymer film has fast switching time (0.8 s at 746 nm) and high optical contrast (59% at 1100 nm). Cyclic voltammetry study reveals that the copolymer film has excellent stability. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Electrochromic (EC) materials, possessing unique electrochromic properties, have been proven especially useful for not only electrochromic mirrors [1–3], but also thin-film transistors [4], displays [5,6], sensors [7,8], memory devices [9] and so on. As a class of excellent EC materials, polythiophenes have occupied prime position due to its high conductivity, good redox reversibility, swift change of color with potential, and stability in environment [10]. For commercial applications, much research has focused on multicolor EC materials, but most polythiophenes only display two colors [11]. Copolymerization offers an effect way of controlling the electrochromic properties of conducting polymers. Copolymers can lead to an interesting combination of the properties observed in the corresponding homo-polymers [12]. 3,4-Ethylenedioxythiophene (EDOT) is a popular choice as a substituted monomer since it produces a low band gap polymer with high stability and good conductivity [13]. Structure modification is another wise way to achieve various properties. Various strategies taking full advantage of the different substitution induced backbone conformations have been proposed to fine-tune the colored neutral state of polythiophenes. For thiophene, the substituent at the 3-position of the thiophene ring leads to a significant increase of the polymer conjugation length [14]. The introduction of a phenyl group into the backbone of polythiophene stabilizes the conjugated p-bonds system [15]. While, plac⇑ Corresponding author. E-mail address: [email protected] (C. Zhang). 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.01.014
ing phenyl between two thiophenes may allow the fine tuning of the optical band gap and retain the easy connectivity and good optics of thiophenes [16]. In this manuscript, the monomer of 1,4-di(thiophen-3-yl)benzene (DTB) was successfully synthesized (Scheme 1). Then a novel multicolored electrochromic copolymer based on DTB and EDOT was achieved by electrochemical copolymerization. The resultant copolymer film exhibits red, yellow, green and blue four colors with the variation of the applied potentials, containing all the three primary colors. This property may broaden the commercial applications of the copolymer. Furthermore, the copolymer film presents good stability, comparatively fast switching time (0.8 s at 746 nm) and high optical contrast (59% at 1100 nm). 2. Experimental 2.1. Materials CH2Cl2 (Aldrich), 3-bromothiophene (Aldrich), 1,4-dibromobenzene (Aldrich), tetrabutylammonium perchlorate (TBAP) (Aldrich), methanol (Aldrich) were used without further purification. Magnesium powder (Aldrich) and EDOT (Aldrich) were used as received. Tetrahydrofuran (THF) (Aldrich) was distilled before use, and dried with natrium. 2.2. Synthesis of 1,4-di(thiophen-3-yl)benzene Dry THF (15 mL) was added to 0.04 mol 3-bromothiophene. The system was flushed with N2 for 5 min and maintained under
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M. Ouyang et al. / Journal of Electroanalytical Chemistry 653 (2011) 21–26
Scheme 1. Synthesis of DTB.
an inert atmosphere. 0.05 g Pd(dppf)Cl2 was added rapidly and 0.02 mol Grignard reagent was dropped in. The mixture was stirred at 65 °C for 16 h. 30 mL HCl (2%) was added to terminate the reaction. After filtered, the solid was washed several times with water and methanol, respectively. Recrystallization with methanol gave white crystalline 1,4-di(thiophen-3-yl)benzene, 41%. m.p.: 282– 284 °C. 1H NMR (500 MHz, CDCl3): d (ppm) 7.64 (s, 4H); 7.48 (m, 2H,); 7.43 (m, 2H); 7.39 (m, 2H,). 13C NMR (125 MHz, CDCl3): d (ppm) 143.9; 133.5; 128.1; 126.3; 124.9; 123.1. FT-IR (KBr, cm 1): 3099 s, 2922 m, 2852 w, 1909 w, 1541 m, 1429 m, 1375 m, 1080 m, 1001 m, 686 m, 630 m, 517. HRMS: calculated for C14H10S2 m/z 242.2451, found m/z 242.2821. For C14H10S2 (242.3) calcd.: C, 69.38%; H, 4.16%. Found: C, 69.29%; H, 4.14%.
2.3. Instrumentation All electrochemical experiments were performed on CHI660a electrochemical working station (CHI, Chenhua, Shanghai, China). Spectroelectrochemical studies were carried out on a UV 1800 spectrophotometer. Surface morphologies of polymer films were investigated by a Hitachi S-4800 scanning electron microscope. FT-IR spectra were recorded on a Nicolet 6700 FTIR spectrometer. A Perkin-Elmer Diamond TG/DTA 6300 was applied to conduct the thermogravimetric analysis. A Perkin-Elmer Flash EA 1112 elemental analyzer performed the elemental analysis.
2.4. Electrochemistry The redox behavior of monomers and polymers was investigated by cyclic voltammogram (CV) using a three compartment electrolysis cell with indium tin oxide (ITO) plate as working electrode. The counter electrode was made from a platinum sheet, and a double-junction Ag/AgCl electrode (silver wire coated with AgCl in saturated KCl solution, 0.1 M TBAP in CH2Cl2 solution as the second junction) was applied as the reference electrode. 0.1 M TBAP in CH2Cl2 was used as electrolyte. Electrochemistry experiments were carried out at room temperature.
3. Results and discussion 3.1. Electropolymerization and electrochemical characterization As shown in Fig. 1, the onset oxidation potentials of DTB and EDOT were 1.33 V and 1.38 V vs. Ag/AgCl. The difference between the two potentials was just 0.05 V, which indicated that the electrochemical copolymerization of DTB and EDOT could be achieved at appropriate potential. The onset oxidation potential of the mixture DTB-EDOT was found to be 1.34 V. Fig. 2 displayed the successful electropolymerization of DTB, DTB-EDOT and EDOT on ITO working electrodes. Well-defined polymer oxidation and reduction peaks were observed in all the three CVs. Upon sequential cycles, there were gradual increases in the current intensity, which indicated that the films were formed on the surface of the electrodes. Repeated cycling resulted in a tiny decrease in the onset of oxidation potential. The increase in peak separation potential reflects the increase in resistance as the thickness of the polymer film increased. For DTB, The first anodic peak at around 0.97 V should be associated with the conformational change of the polymer during oxidation, and the second anodic peak at around 1.22 V should be associated with the overall doping process [17]. The CV of EDOT exhibited one obvious oxidation peak at around 0.16 V and two reduction peaks at around 0.11 V and 0.55 V respectively. The redox waves of DTB-EDOT were completely different from those of DTB and EDOT, which should be ascribed to the copolymerization. The CV of DTB-EDOT exhibited an oxidation peak at around 1.04 V and a reduction peak at around 0.56 V. The CVs of polymer PDTB, P(DTB-EDOT) and PEDOT were carried out in monomer free 0.1 M TBAP/CH2Cl2. All the three polymer films were electroactive as shown in Fig. 3. The PDTB film exhibited one main oxidation wave at about 1.25 V and the main reduction peak at around 0.94 V. The PEDOT film displayed main oxidation peak and reduction peak at around 0.24 V and 0.64 V, respec-
2.5. Preparation of polymer films The solutions of 3 mM DTB, 3 mM EDOT and 3 mM/3 mM DTB-EDOT in CH2Cl2 were prepared respectively with 0.1 M TBAP as supporting electrolyte. All polymer films were prepared via potentiostatic electrolysis on ITO electrodes, and the films were electrolyzed at negative potential again in order to equilibrate their redox behavior in monomer-free electrolytic solution, after that, the films were washed with CH2Cl2 for several times to remove the residual supporting electrolyte and the monomers. For elemental analysis, thermogravimetric analysis and FT-IR spectra, neutral polymer samples were peeled off from the ITO electrodes, and dried in vacuum at 70 °C for 10 h.
Fig. 1. Polarization curves of DTB, EDOT and DTB-EDOT in 0.1 M TBAP/CH2Cl2. The scan rate was 5 mV s 1.
M. Ouyang et al. / Journal of Electroanalytical Chemistry 653 (2011) 21–26
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that the electrochemical processes were reversible and not diffusion limited, even at very high scan rates.
3.2. Elemental analysis
Fig. 2. Cyclic voltammogram curves of (A) DTB, (B) DTB-EDOT, (C) EDOT in 0.1 M TBAP/CH2Cl2 at a scan rate of 50 mV s 1.
To confirm the formation of the copolymer, a homo-polymer film mixture of PDTB and PEDOT which composed according to the mDTB/mEDOT proportion in polymer P(DTB-EDOT) was expected for comparison with the P(DTB-EDOT) film. The polymer films were synthesized on ITO electrodes, dedoped at negative potential and washed with CH2Cl2. Then, the films were dried in vacuum at 70 °C for 10 h. The mass percentage of C, H, S and N were recorded by elemental analysis. The atom N was assigned to residual TBAP. The mass percentages of other atoms were calculated and were displayed in Table. 1. Then, mDTB/mEDOT in copolymer film was calculated to be approximately 1/7. Film PDTB and PEDOT were mixed according to this proportion. The elemental analysis of the mixture was also performed (Table. 1). As seen from Table. 1, the result of the mixture is close to that of the copolymer. To be more reasonable, the mixture of polymer PDTB and PEDOT used in IR spectra and TG analysis was prepared according to this proportion.
3.3. FT-IR spectra
Fig. 3. Cyclic voltammograms of PDTB, P(DTB-EDOT) and PEDOT in 0.1 M TBAP/CH2Cl2 at scan rate 100 mV s 1.
tively. While the CV of P(DTB-EDOT) exhibited single and well-defined reversible redox couple, and was different from those of PDTB and PEDOT. Although the intermediate CV response from P(DTBEDOT) could be interpreted as the existence of DTB and EDOT units into the polymer chain, the further evidence is needed to be found. Fig. 4 showed the CV curves of P(DTB-EDOT) at different scan rates. The current response of P(DTB-EDOT) was directly proportional to the scan rate, indicating that the polymer film was electroactive and adhered well to the electrode [18]. The scan rate dependence of the anodic and cathodic peak currents showed a linear dependence as a function of the scan rate. This demonstrated
Fig. 4. Cyclic voltammograms of P(DTB-EDOT) film at different scan rates.
Fig. 5 showed the FT-IR spectra of PDTB, mixture of PDTB and PEDOT, P(DTB-EDOT) and PEDOT. Peaks of PDTB at 1470, 1685 and 1605 cm 1 should be ascribed to the C–C stretching vibrations in the phenyl ring. The peak at 735 cm 1 was attributed to C–H out-of-plane vibration of phenyl ring. The peaks at 780 and 832 cm 1 were characteristic of thiophene ring [19]. The peak at 1089 cm 1 was due to the presence of ClO4 [20]. For the spectrum of PEDOT, stretching vibrations of thiophene ring presented at 1320, 1195 and 1066 cm 1. Oxygenic substituents showed bands at 1512 and 980 cm 1 [21]. The spectra of the mixture and P(DTB-EDOT) contained all the characteristic peaks of both PDTB and PEDOT. But some apparent differences were showed in the two spectra. For example, the peak at 1348 cm 1 in the spectrum of P(DTB-EDOT) is sharper than the peak at 1372 cm 1 in the mixture. Taking the peaks at 1654, 1348 and 1076 cm 1 for examples, the spectrum of P(DTB-EDOT) showed a little red shift, indicating the existence of interaction between DTB and EDOT units in the polymers. These provided another proof to the copolymerization of DTB and EDOT.
3.4. Thermogravimetric analysis Fig. 6 displayed the TG curves of PDTB, PEDOT, P(DTB-EDOT) and the mixture. The mixture of PDTB and PEDOT was prepared according to the mass proportion of 1/7 calculated from elemental analysis. TG analysis was carried out in N2 atmosphere with heating rate 10 °C min 1 from 100 to 600 °C. The initial slight weight loss observed at the beginning should be assigned to the evaporation of the residual solvent or the trapped water in polymer films [22]. The curve of PDTB showed the onset temperature of weight loss at around 285 °C and the maximum decomposition temperature was between 290 °C and 330 °C. PEDOT film began to loose weight at around 315 °C. The curve of P(DTB-EDOT) showed three obvious weight losses. The first weight loss could be tentatively attributed to the loss of residual TBAP as it was explained in similar research work based on conducting polymers [23,24]. Another two losses should due to the degradation of the polymer backbone [25]. The apparent difference between the curves of the mixture and P(DTB-EDOT) indirectly confirms the copolymer structure of the P(DTB-EDOT).
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Table 1 Mass percentage of atoms in the copolymer film and the mixture of PDTB and PEDOT. Atom m%
Copolymer Mixture
C
H
S
N
Cl
O in TBAP
O in copolymer
55.10 53.74
5.54 4.84
13.91 12.35
1.56 1.17
3.95 2.97
7.13 5.53
12.81 19.40
and loose spongy network structure PEDOT film, film P(DTB-EDOT) exhibited an accumulation state of clusters of globules. This difference could be attributed to the effect of copolymerization. 3.6. Spectroelectrochemical characterization
Fig. 5. FT-IR spectra of (A) PDTB; (B) mixture of PDTB and PEDOT (C) P(DTB-EDOT); (D) PEDOT.
Fig. 6. TG curves of PDTB, PEDOT, P(DTB-EDOT), mixture of PDTB and PEDOT under nitrogen atmosphere at a heating rate of 10 °C min 1 from 100 to 600 °C.
3.5. Scanning electron microscopy (SEM) The polymer films of DTB, DTB-EDOT and EDOT were prepared by constant potential electrolysis (polymerization charge: 0.80 °C cm 2) from the solution of 0.1 M TBAP/CH2Cl2 containing relevant monomers on the ITO. The surface morphologies of polymer films were investigated by Scanning electron microscope (SEM) as shown in Fig. 7. Comparison with the smooth PDTB film
Spectroelectrochemical analysis is a powerful way to investigate the optical switches and contrasts of EC conducting polymers upon potential change, which provides insights into the electronic structure of the conducting polymer. The spectroelectrochemical and electrochromic properties of the resultant films were studied with 0.1 M TBAP/CH2Cl2 solution as supporting electrolyte. As shown in Fig. 8, the spectroelectrochemical property of PDTB was investigated by applying potentials ranging from 0.4 to 1.0 V. At the neutral state, kmax value was found to be 427 nm due to the p–p transition of the polymer backbone. Above applied voltage, reduction in the intensity of the p–p transitions and formation of charge carrier bands were observed. Thus, the appearance of peaks around 775 nm and >1100 nm could be attributed to the evolution of polaron and bipolaron bands. For comparison, the spectroelectrochemistry of PEDOT film was shown in Fig. 9. The neutral state PEDOT exhibited the p–p transition peak at around 593 nm, and its absorption intensity decreased with the increase of the applied potential. While the doped PEDOT film presented intense charge carrier bands at around 880 nm and >1100 nm. Fig. 10 depicted the different spectroelectrochemical spectra of the copolymer film under various applied potentials ranging from 0.0 to 1.0 V. A well-defined maximum absorption band centered at 496 nm was observed, which should be attributed to the p–p transition of the neutral state copolymer backbone, and it decreased with the increase of potential. The appearance of charge carrier bands (at around 760 nm and >1100 nm) could be attributed to the evolution of polaron and bipolaron bands. Table 2 summarized the maximum absorption bands and energy gaps of PDTB, PEDOT and the copolymer. As seen from Table 2, the kmax value and Eg of copolymer are located between those of the PDTB and PEDOT, indicating that introduction of EDOT to the polymer chain leaded to an effective decrease in the band gap. The film colors of homo-polymer PDTB and copolymer P(DTBEDOT) at different potentials were presented in Fig. 11. Film PDTB presented yellow and blue at neutral and oxidized state, respectively. While film P(DTB-EDOT) displayed four different hues (yellow and three primary colors) at various potential. These different colors corresponding to various doped and neutral states had
Fig. 7. SEM micrograph of (A) PDTB, (B) P(DTB-EDOT), (C) PEDOT.
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Table 2 kmax and Eg values of the homo-polymers (PDTB and PEDOT) and the copolymer P(DTB-EDOT).
kmax (nm) Eg (eV)
PDTB
P(DTB-EDOT)
PEDOT
427 2.14
496 1.92
593 1.68
Fig. 8. Spectroelectrochemical spectra of PDTB film as applied potentials between 0.4 and 1.0 V in 0.1 M TBAP/CH2Cl2.
Fig. 11. Polymer film Colors at different potentials (A) PDTB; (B) P(DTB-EDOT). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Spectroelectrochemical spectra of PEDOT film as applied potentials between 0.0 and 1.0 V in 0.1 M TBAP/CH2Cl2.
Fig. 12. Electrochromic switching, optical absorbance monitored for PDTB at 427, 774 and 1100 nm in 0.1 M TBAP/CH2Cl2 between 0.3 and 0.9 V with a residence time of 5 s.
also been confirmed by the CV tests. This multicolor property possesses significant potential applications in smart windows or displays and so on.
with a residence time of 5 s at different wavelength. The switching time was defined as the time required for reaching 95% of the full change in absorbance after the switching of the potential. The copolymer film exhibited higher optical contrast than that of the homo-polymer film, especially, the optical contrast of copolymer film at 1100 nm was found to be 59%. This should be ascribed to the introduction of EDOT units into the polymer backbone. Otherwise, the copolymer film has fast switching time (0.8 s at 746 nm). The electrochromic switching behavior of PEDOT was given for comparison in Fig. 14. PEDOT has good optical contrast and fast switching time. The incorporation of EDOT units into the DTB polymer led to the improvement of electrochromic property of the copolymer. The good stability of the %DT in time, fast switching property and reasonable contrast ratio make this copolymer a promising material for EC devices.
3.7. Electrochromic switching
3.8. Stability of copolymer film P(DTB-EDOT)
Electrochromic switching time and optical contrast (%DT) are the two most important characteristics for EC materials. Figs. 12 and 13 exhibited electrochromic switching response of the homo-polymer and the copolymer films between 0.3 and 0.9 V
The stability of EC materials toward long-term switching between the neutral and oxidized states is one of the most important factors for the application of EC materials in device utilities. Cyclic voltammetry was used to test the stability of the copolymer film.
Fig. 10. Spectroelectrochemical spectra of P(DTB-EDOT) film as applied potentials between 0.0 and 1.0 V in 0.1 M TBAP/CH2Cl2.
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500 cycles, only slight shrinkage of redox loop is observed after 500 cycles, which implies that P(DTB-EDOT) film has reasonable stability, and could be as a promising candidate material for EC devices. 4. Conclusions
Fig. 13. Electrochromic switching, optical absorbance monitored for P(DTB-EDOT) at 496, 746 and 1100 nm in 0.1 M TBAP/CH2Cl2 between 0.3 and 0.9 V with a residence time of 5 s.
A multicolored copolymer from DTB and EDOT was successfully synthesized via electrochemical methods. At different potentials, the copolymer film exhibited four hues containing three primary colors. Spectroelectrochemical studies and electrochromic characterization demonstrated that copolymerization with EDOT not only decreased the energy gap Eg effectively but also enhanced the electrochromic properties such as optical contrast and switching time. With its multicolor electrochromism (yellow and three primary colors) good stability, comparatively fast switching time and high optical contrast, the copolymer film has expected to be amenable for practical use such as EC paper and other displays. Acknowledgement The authors gratefully thank the supporting of National Basic Research Program of China (2010CB635108), Natural Science Foundation of Zhejiang Province, China (Y4090260), Major Science and Technology, Special and Priority Themes of Zhejiang Province, China (2009C14004) and China Postdoctoral Science Foundation (20100471755). References
Fig. 14. Electrochromic switching, optical absorbance monitored for PEDOT at 593 and 1100 nm in 0.1 M TBAP/CH2Cl2 between 1.0 and 0.9 V with a residence time of 5 s.
Fig. 15. Cyclic Voltammogram of P(DTB-EDOT) as a function of repeated scans 500 mV s 1 in 0.1 M TBAP/CH2Cl2.
Fig. 15 presented the long-term stability of the as-prepared P(DTBEDOT) film. Cyclic voltammograms of P(DTB-EDOT) between potential 0.1 and 1.3 V indicate that the film keeps stable up to
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