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Synthesis, characterization and electrochromic properties of conducting copolymers of 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate with thiophene and pyrrole
EUROPEAN POLYMER JOURNAL
European Polymer Journal 40 (2004) 2421–2426
www.elsevier.com/locate/europolj
Synthesis, characterization and electrochromic properties of conducting copolymers of 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate with thiophene and pyrrole U. Bulut a, L. Toppare b
a,*
, F. Yılmaz b, Y. Yag˘cı
b,*
a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Department of Chemistry, Istanbul Technical University, Maslak, 80626 Istanbul, Turkey
Received 2 June 2004; received in revised form 1 July 2004; accepted 2 July 2004 Available online 26 August 2004
Abstract 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOPT) was synthesized via the reaction of 3-thionylcarboxylic acid with glycerol, and electrochemically polymerized either with thiophene and pyrrole by using tetrabutylammonium tetrafluoroborate (TBAFB) as the supporting electrolyte in acetonitrile (AN). Characterization of the resulting copolymers was performed via cyclic voltammetry, FTIR, thermal gravimetry analysis (TGA), and scanning electron microscopy (SEM). Electrical conductivities were measured by the four-probe technique. Spectroelectrochemical analysis shows that the copolymer of the monomer with thiophene has an electronic band gap (due to the p–p* transition) of 2.00 eV, with a dark red color in the fully reduced form and a green color in the fully oxidized form. The copolymer exhibited a long-term switching stability up to 1800 double switches. 2004 Elsevier Ltd. All rights reserved. Keywords: Electrochemical copolymerization; Conducting copolymers; Electrochromic properties
1. Introduction Conducting polymers, synthesized via electrochemical methods have attracted great interest because of their wide range of potential applications in the areas such as
*
Corresponding authors. Tel.: +90 312 210 3251; fax +90 312 210 1280. E-mail addresses: [email protected] (L. Toppare), yusuf @itu.edu.tr (Y. Yag˘cı).
optical displays [1,2], rechargeable batteries [3,4], electrochromic devices [5], and light-emitting diodes [6]. The preparation of graft and block copolymers with desired end groups, such as thiophene or pyrrole, is a nice method to overcome some difficulties in processing conducting copolymers arising from poor mechanical and physical properties [7,8]. For this, functional initiator approach in conventional free radical polymerization is widely used because of its simplicity and applicability to a vast number of monomers. Living polymerization techniques [9] are employed to prepare polymeric
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.07.005
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U. Bulut et al. / European Polymer Journal 40 (2004) 2421–2426
initiators, where living polymer ends are quenched with a reagent having functional groups [10–13]. Conducting polymers comprised of a number of functionalized polythiophenes, polypyrroles, polyanilines, etc., have become recognized as an interesting class of electrochromic materials [14], where electrochromism is defined as a reversible and visible change in the transmittance and/ or reflectance of a material upon oxidation and reduction [15]. The definition has been broadened from that of a color change in the visible spectrum to a multi-spectral energy modulation that might cover ultraviolet (UV), near infrared (NIR), mid infrared (mid-IR), and microwave regions, with ‘‘color’’ corresponding to the response of detectors [16]. Electrochromic techniques are readily employed in a multitude of applications such as display panels, camouflage materials, variable reflectance mirrors, and variable transmittance windows [17]. A number of conjugated polymers have colors both in the oxidized and reduced states, since the band gap lies within the visible region [18]. The color exhibited by the polymer is determined by the band gap energy, defined as the onset of the p–p* transition. An important point in the study of electrochromic polymeric materials has been that of controlling their colors by main-chain and pendant group structural modification. Polyheterocycles have proven to be of special interest for this due to their stability under ambient and use conditions. One of the strategies to control the electrochromic properties is copolymerization, which can result in an interesting combination of the properties observed in the corresponding homopolymers [19]. As synthetic organic chemistry allows the derivatization of the monomer structure, controlling the electronic properties of the conjugated backbone allows to change electrochromic properties of the polymers. In this paper, we present the synthesis of a new monomer, 2,3-bis-[(3thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOPT), and its copolymerization either with thiophene (Th) and pyrrole (Py), where the resultant copolymers were characterized via cyclic voltammetry, TGA, DSC, SEM and conductivity measurements. Moreover, the spectroelectrochemical and electrochromic properties, such as the relative luminance, change of color upon
O
O
OH S
SOCl2
redox switching, and long-term switching stability of the copolymer of TOPT with Th were investigated [P(TOPT-co-Th].
2. Experimental 2.1. Materials Acetonitrile (ACN) (Merck), and tetrabutylammonium tetrafluoroborate (TBAFB) (Sigma) were used without further purification. Thiophene (Th) (Aldrich) and pyrrole (Py) (Aldrich) were distilled before use. Boron fluoride-ethyl ether (BFEE) (Aldrich), 3-thiophenecarboxylic acid (Aldrich), thionyl chloride (Aldrich), glycerol (Aldrich), NaHCO3 (Aldrich), and MgSO4 (Aldrich) were used as received. 2.2. Synthesis of 2-[(3-thienylcarbonyl)oxy]propyl 3thiophene carboxylate A mixture of 3-thiophenecarboxylic acid (0.7000 g, 5.4 mmol) and thionyl chloride (0.765 g, 6.5 mmol) was placed in 2.5 · 10 5 m3 round bottom flask carrying a drying tube and refluxed gently at 100 C for 30 min. A water aspirator vacuum to the top of the condenser was applied to remove any thionyl chloride, and glycerol (1.7 mmol) was added to the mixture. The reaction was allowed to stand on the bath for 15 min. more and then cooled, diluted with 5 · 10 5 m3 ether and transferred to a separatory funnel. The mixture was washed with water and twice with NaHCO3 solution and dried over MgSO4. The solvent was removed over rotatory evaporator. A yellowish-oily product was obtained. The resultant product was further purified via column chromatography, and white crystals were obtained with 47% yield. 1H NMR spectrum of the monomer was taken using a Bruker Instrument-NMR Spectrometer (DPX-400) with CDCl3 as the solvent and tetramethylsilane as the internal standard. FTIR Spectra were taken by a Nicolet 510 FT-IR Spectrometer by dispersing samples in KBr disk. The representation of synthesis is demonstrated in Scheme 1.
HO Cl
O
OH
O
OH O
O O
S S
O S
Scheme 1. Synthesis of TOPT.
S
U. Bulut et al. / European Polymer Journal 40 (2004) 2421–2426
2.3. Cyclic voltammetry The voltammograms were recorded in acetonitrile (ACN)/tetrabutylammonium tetrafluoroborate (TBAFB) solvent-electrolyte couple using a system consisting of a potentiostat (Wenking POS 2), an X–Y recorder and a CV cell containing Pt foil working and counter electrodes, and a Ag/Ag+ reference electrode. Measurements were taken at room temperature under nitrogen atmosphere. 2.4. Synthesis of graft copolymers by electrochemical polymerization
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working electrode, a Pt wire as the counter and Ag/ Ag+ as the reference electrodes. The electrochromic measurements (Solartron 1285 potentiostat/galvanostat), spectroelectrochemistry (HP8453A UV–VIS spectrophotometer), and switching studies of the polymer film deposited on ITO coated glass were carried out in TBAFB (0.1 M)/ ACN. The color coordinates of the film both in the reduced and oxidized state were measured via GretagMacbeth Coloreye XTH Spectrophotometer.
3. Results and discussion 3.1. Synthesis of polymers
2.4.1. Synthesis of the conducting copolymer of TOPT with thiophene For the synthesis of conducting copolymer of TOPT, thiophene was used as the comonomer. 40 mg of TOPT, 8.8 · 10 5 mol, was dissolved in 5 · 10 5 m3 acetonitrile (ACN), and 5 · 10 4 moles of thiophene was introduced into a 3-compartment electrolysis cell, where the supporting electrolyte was TBAFB (0.05 M). Constant potential electrolysis was run at 1.9 V for 60 min. at room temperature under inert atmosphere to obtain films 40 lm. Films were washed with ACN to remove TBAFB subsequent to the electrolysis. Surface morphologies of the resultant films were examined by scanning electron microscopy (SEM), JEOL JSM-6400. Electrical conductivity measurements were carried out by four-probe technique. Thermal behavior of the samples was investigated via a Du Pont 2000 Thermal Gravimetry Analyser. 2.4.2. Synthesis of conducting copolymers of TOPT with pyrrole For the synthesis of conducting copolymer of TOPT, pyrrole was used as the comonomer. The electrolysis procedure and the medium was the same as it is described above for thiophene comonomer. The applied potential was 1.1 V vs. Ag/Ag+ reference electrode at room temperature under inert atmosphere. Films were washed with ACN to remove TBAFB after the electrolysis. 2.4.3. Synthesis of conducting copolymer of TOPT with thiophene on ITO The potentiodynamic electrochemical polymerization of TOPT (0.01 M) in boron fluoride-ethyl ether (BFEE) was carried out in a 3-electrode cell with an ITO coated glass electrode and a platinum wire counter electrode in the presence of 6.25 · 10 5 mol Th. The system is cycled between 0 and +1.75 V vs. Ag/Ag+ reference electrode for 5 min with 0.75 V/s scan rate. The film of P(TOPT-co-Th) was produced potentiodynamically between 0 and +1.75 V using 10 mM TOPT, 6 mM Th and 0.1 M BFEE solution in a 3-compartment CV cell, where an ITO coated glass electrode was used as the
The copolymers of TOPT in the presence of either Th or Py were deposited both potentiodynamically and potentiostatically. However, the homopolymer of TOPT could merely been achieved potentiodynamically, and was not in sufficient amount for further characterization. The electrochromic studies were carried out only for the copolymer of TOPT with Th [P(TOPT-co-Th)]. 3.2. 1H NMR and FT-IR characterization 1
H NMR (d, ppm) data for TOPT: 8.2 (m, 3H, thiophene), 7.5 (m, 3H, thiophene), 7.2 ppm (m, 3H, thiophene), 4.3 ppm (d, 4H, methylene), 3.9 ppm (quintet, 1H, methylene). 13 C NMR (d, ppm) data for TOPT: 161.91, 133.77, 132.94, 128.45, 126.74, 126.37, 63.11, 50.20. The FTIR Spectra of TOPT and the copolymer of TOPT with Th showed the following absorption peaks: For TOPT: 3100 cm 1 (aromatic C–H), 2950–2850 cm 1 (aliphatic C–H), 1725 cm 1 (C@O stretching), 1260–1000 cm 1 (C–O–C symmetric and asymmetric stretching), 845 and 750 cm 1out of plane bendings (b and a hydrogens of thiophene respectively). For [P(TOPT-co-Th)]: A broad peak with a maximum at 3650 cm 1, shading the aromatic C–H stretching at 3080 cm 1, 2963–2920 cm 1 (aliphatic C–H), 1755–1740 cm 1 (C@O stretching), 1647 cm 1 (C@C stretching), a broad intense peak at 1083 cm 1 (BF4 , dopant anion), shading the peaks due to the C–O–C symmetric and asymmetric stretching at around 1240– 1000 cm 1, 831 cm 1 (b-hydrogen of thiophene). The existence of the characteristic peaks of the monomer shows that the film is not polythiophene. Furthermore, the vanishing of the peak at 750 cm 1 indicates the copolymerization, since the polymerization advances through 2,5-positions of the thiophene ring. 3.3. Cyclic voltammetry As seen from Fig. 1(a), the monomer reveals an oxidation peak at +2.40 V. Upon thiophene addition, the
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Fig. 1. Cyclic voltammogram of (a) monomer (TOPT), (b) TOPT in the presence of thiophene, (c) TOPT in the presence of pyrrole on a bare Pt electrode in 0.1 M TBAFB/CH3CN.
reversible electroactivity of thiophene decreases through the first 10 runs. Thereon, the oxidation peak height increases and the peak shifts to +2.25 V. When Fig. 1(b) is compared with the voltammogram of pristine thiophene, the redox peaks are not at the same positions (pure PTh–– Ep,a: +0.98 V, Ep,c: +0.70). This shift is known to be an indirect indication for the reaction between thiophene and the thiophene moiety of the monomer. In the case of pyrrole present in the system, the usual reversible redox peaks were observed at different potentials than that of PPy (Fig 1(c)) (pure PPy––Ep,a: +0.63 V, Ep,c: +0.07). Besides, the peak height it reaches is lower than that of pure polypyrrole with the same scan number. 3.4. Thermal properties
Fig. 2. TGA thermogram of P(TOPT-co-Th).
TGA thermogram of the copolymer of TOPT with Th, P(TOPT-co-Th), showed one weight loss at 273 C (Fig. 2). At this temperature, 28% of the copolymer was decomposed, whereas 18% of PTh decomposes at 246 C under the same conditions. After 680 C, there was still 53% of the sample not decomposed, while it is 50% for PTh. In the case of copolymerization with Py, a weight loss was observed at 250 C, where 8% of the sample was decomposed, and 40% of the sample remained after 750 C. 3.5. Scanning electron microscopy (SEM) In the SEM micrographs of the BF4 doped copolymers of TOPT, a significant difference of morphology, as a chain-like structure, was observed compared to the cauliflower structure of the pure PTh and PPy (Fig. 3).
Fig. 3. SEM micrograph of (TOPT-co-Th).
U. Bulut et al. / European Polymer Journal 40 (2004) 2421–2426
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This may be yet an indication of copolymerization of TOPT either with thiophene or pyrrole. 3.6. Conductivities of the films In Table 1, room temperature (20 C) conductivities of the copolymers with thiophene and pyrrole are given. Compared to pristine polypyrrole and polythiophene, they are one order of magnitude low. This is expected since the presence of monomer moieties destroys the conjugation in both pure PPy and PTh, which may be another indication of copolymerization. 3.7. Electrochromic properties The electrochemical switching of the copolymer of TOPT with Th in BFEE was studied. Fig. 4 shows the potentiodynamic synthesis of [P(TOPT-co-Th)] at a scan rate of 1 V s 1 on a Pt electrode. Both the anodic (Ep,a) and the cathodic (Ep,c) peak potentials are not well-defined in the voltammogram. However, the film produced in BFEE has lower peak potentials compared to the film deposited in ACN solution, since the electrochemical polymerization in BFEE is facilitated as the aromatic resonance energy is lowered, and the abstraction of an electron from the a-position of the heterocyclic ring is promoted [20]. It is known that BFEE exists in diethyl
Table 1 Room temperature conductivities of the films (S/cm) Samples
Conductivity
PTh P(TOPT-co-Th) PPy P(TOPT-co-Py)
5 1 20 0.6
Fig. 4. Cyclic voltammogram of 0.01 M P(TOPT-co-Th) in BFEE at a scan rate of 1 V s 1.
ether as a polar adduct, and the presence of a small amount of water results in the formation of H+[(BF3OH)] , which provides a conducting medium, where the [(BF3OH)] anion serves as the dopant during the polymerization [21]. Spectroelectrochemistry studies were performed on an HP 8453 Diode-array UV–VIS Spectrophotometer. The spectroelectrochemical analysis, the electrochromism and switching studies of the film produced in
Fig. 5. Spectroelectrochemistry of P(TOPT-co-Th) (synthesized in BFEE potentiostatically on ITO coated glass) as a function of wavelength at applied potentials between 0.0 and +1.0 V in 0.1 M TBAFB (0.1 M)/ACN. Spectra were recorded at the following potentials: (a) 0.0 V, (b) +0.2 V, (c) +0.4 V, (d) +0.6 V, (e) +0.8 V, (f) +1.0 V vs. Ag/Ag+.
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Table 2 Electrochemical, electronic, and electrochromic properties Polymer
Ep,aa
Ep,ca
kmax (nm)
Eg (eV)
Color (red)b
Color (ox)b
L
PTh
0.63
0.58
495
1.92
Bright red
Pale blue
P(TOPT-co-Th)
–
–
470
2.00
Dark red
Green
(ox) 57 (red) 51 (ox) 52 (red) 58
a b
ab 7 52 60 44
b 2 46 33 32
Volts vs. Ag/Ag+. Reduced state (red), oxidized state (ox).
BFEE were carried out in TBAFB (0.1 M)/ACN, since the redox responses of polymer films cycled in BFEE are not well defined. The spectroelectrochemical analyses were carried out in order to investigate the electronic structure of the copolymer, and its optical behavior upon redox switching. The film deposited onto ITO coated glass electrode was placed in a UV cuvette for UV–Vis experiment after it is washed with monomerfree electrolyte solution. The absorbance was monitored in situ as a function of potential ranging from 0.0 to +1.0 V (Fig. 5), and from +1.0 to 0.0 V to compare the two spectra to confirm the reversibility of the film. The kmax and the energy band gap values of the PTh and the copolymer for the p–p* transitions were determined, and recorded in Table 2. P(TOPT-co-Th) has a peak absorbance at 470 nm (kmax) and has a band gap (Eg) of 2.00 eV. The color coordinates of the film both in the reduced and oxidized state were measured and the corresponding color coordinates (CIE L * a * b values) were recorded in Table 2. The film switches between a dark red color in the reduced state and a green oxidized state. Besides, the copolymer could be cycled up to 1800 times, completing each cycle in 7 s, between 0 and +1.35 V with a scan rate of 375 mV/s before it loses its ability to switch, where as PTh can be cycled for 240 times under same conditions. Consequently, the copolymer synthesized has quite different electronic and electrochromic properties as compared to pure PTh.
4. Conclusion The syntheses of conducting copolymers of TOPT monomer either with thiophene or pyrrole were achieved in the presence of the TBAFB supporting electrolyte. Free standing, stable and electrically conducting copolymers were obtained. It has also been shown that the copolymerization of TOPT monomer with thiophene in BFEE system yields electroactive and electrochromic films, which could be cycled for 1800 times. The spectroelectrochemical analysis illustrates that the copolymer film synthesized in BFEE reveals a reversible cycling.
Acknowledgment This study is partially supported by TUBA and DPT2003K1209120-02 grants.
References [1] Gustafsson J, Inganas O, Anderson A. Synth Met 1994;62:17. [2] De Paoli M, Panero S, Passerini S, Scrosatti B. Adv Mater 1990;2:480. [3] Bittihm R, Ely G, Woeffier F, Munstedt H, Narmann H, Naegele D. Makromol Chem Makromol Symp 1987;8:51. [4] Mermillod M, Tonguy J, Petiot F. J Electrochem Soc 1986;133:1073. [5] Sotzing G, Reynolds JR, Steel P. Chem Mater 1990;8:882. [6] Kraft A, Grimsdale AC, Holmes AB. Angew Chem Int Ed 1998;37:402. [7] DePaoli MA, Waltman RJ, Diaz AF, Bargon J. J Polym Sci Polym Chem Ed 1985;23:1687. [8] Wang HL, Toppare L, Fernandez JE. Macromolecules 1990;23:1053. [9] Mishra MK, Yag˘cı Y. In: Handbook of radical vinyl polymerization. New York: Marcel Dekker; 1998. [10] Selampnar F, Akbulut U, Yımaz T, Gu¨ngo¨r A, Toppare L. J Polym Sci Polym Chem 1997;35:3009. [11] Alkan S, Toppare L, Hepuzer Y, Yag˘cı Y. J Polym Sci Part A Polym Chem 1999;37:4218. ¨ nen A, Yag˘cı Y. Polym Bull [12] Kızılyar N, Toppare L, O 1997;40:639. ¨ ztemiz S, Toppare L. J Macromol Sci Pure Appl Chem [13] O 2000;3:277. [14] Sapp SA, Sotzing GA, Reynolds JR. Chem Mater 1998;10:2101. [15] Verghese MM, Ram MK, Vardhan H, Malhorta BD, Ashraf SM. Polymer 1997;38:1625. [16] Schwendan I, Wang JH, Welsh DM, Tanner DB, Reynolds JR. Adv Mater 2001;13:634. [17] Schwendan I, Hickman R, So¨nmez G, Schottland P, Zong K, Welsh DM, et al. Chem Mater 2002;14:3118. [18] Sotzing GA, Reddinger JL, Katritzky AR, Soloducho J, Musgrave R, Reynolds JR, et al. Chem Mater 1997;9:1578. [19] Thompson BC, Schottland P, Zong K, Reynolds JR. Chem Mater 2000;12:1563. [20] Jin S, Xue G. Macromolecules 1997;30:5753. [21] Alkan S, Cutler CA, Reynolds JR. Adv Funct Mater 2003;13:331.
European Polymer Journal 40 (2004) 2421–2426
www.elsevier.com/locate/europolj
Synthesis, characterization and electrochromic properties of conducting copolymers of 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate with thiophene and pyrrole U. Bulut a, L. Toppare b
a,*
, F. Yılmaz b, Y. Yag˘cı
b,*
a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Department of Chemistry, Istanbul Technical University, Maslak, 80626 Istanbul, Turkey
Received 2 June 2004; received in revised form 1 July 2004; accepted 2 July 2004 Available online 26 August 2004
Abstract 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOPT) was synthesized via the reaction of 3-thionylcarboxylic acid with glycerol, and electrochemically polymerized either with thiophene and pyrrole by using tetrabutylammonium tetrafluoroborate (TBAFB) as the supporting electrolyte in acetonitrile (AN). Characterization of the resulting copolymers was performed via cyclic voltammetry, FTIR, thermal gravimetry analysis (TGA), and scanning electron microscopy (SEM). Electrical conductivities were measured by the four-probe technique. Spectroelectrochemical analysis shows that the copolymer of the monomer with thiophene has an electronic band gap (due to the p–p* transition) of 2.00 eV, with a dark red color in the fully reduced form and a green color in the fully oxidized form. The copolymer exhibited a long-term switching stability up to 1800 double switches. 2004 Elsevier Ltd. All rights reserved. Keywords: Electrochemical copolymerization; Conducting copolymers; Electrochromic properties
1. Introduction Conducting polymers, synthesized via electrochemical methods have attracted great interest because of their wide range of potential applications in the areas such as
*
Corresponding authors. Tel.: +90 312 210 3251; fax +90 312 210 1280. E-mail addresses: [email protected] (L. Toppare), yusuf @itu.edu.tr (Y. Yag˘cı).
optical displays [1,2], rechargeable batteries [3,4], electrochromic devices [5], and light-emitting diodes [6]. The preparation of graft and block copolymers with desired end groups, such as thiophene or pyrrole, is a nice method to overcome some difficulties in processing conducting copolymers arising from poor mechanical and physical properties [7,8]. For this, functional initiator approach in conventional free radical polymerization is widely used because of its simplicity and applicability to a vast number of monomers. Living polymerization techniques [9] are employed to prepare polymeric
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.07.005
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U. Bulut et al. / European Polymer Journal 40 (2004) 2421–2426
initiators, where living polymer ends are quenched with a reagent having functional groups [10–13]. Conducting polymers comprised of a number of functionalized polythiophenes, polypyrroles, polyanilines, etc., have become recognized as an interesting class of electrochromic materials [14], where electrochromism is defined as a reversible and visible change in the transmittance and/ or reflectance of a material upon oxidation and reduction [15]. The definition has been broadened from that of a color change in the visible spectrum to a multi-spectral energy modulation that might cover ultraviolet (UV), near infrared (NIR), mid infrared (mid-IR), and microwave regions, with ‘‘color’’ corresponding to the response of detectors [16]. Electrochromic techniques are readily employed in a multitude of applications such as display panels, camouflage materials, variable reflectance mirrors, and variable transmittance windows [17]. A number of conjugated polymers have colors both in the oxidized and reduced states, since the band gap lies within the visible region [18]. The color exhibited by the polymer is determined by the band gap energy, defined as the onset of the p–p* transition. An important point in the study of electrochromic polymeric materials has been that of controlling their colors by main-chain and pendant group structural modification. Polyheterocycles have proven to be of special interest for this due to their stability under ambient and use conditions. One of the strategies to control the electrochromic properties is copolymerization, which can result in an interesting combination of the properties observed in the corresponding homopolymers [19]. As synthetic organic chemistry allows the derivatization of the monomer structure, controlling the electronic properties of the conjugated backbone allows to change electrochromic properties of the polymers. In this paper, we present the synthesis of a new monomer, 2,3-bis-[(3thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOPT), and its copolymerization either with thiophene (Th) and pyrrole (Py), where the resultant copolymers were characterized via cyclic voltammetry, TGA, DSC, SEM and conductivity measurements. Moreover, the spectroelectrochemical and electrochromic properties, such as the relative luminance, change of color upon
O
O
OH S
SOCl2
redox switching, and long-term switching stability of the copolymer of TOPT with Th were investigated [P(TOPT-co-Th].
2. Experimental 2.1. Materials Acetonitrile (ACN) (Merck), and tetrabutylammonium tetrafluoroborate (TBAFB) (Sigma) were used without further purification. Thiophene (Th) (Aldrich) and pyrrole (Py) (Aldrich) were distilled before use. Boron fluoride-ethyl ether (BFEE) (Aldrich), 3-thiophenecarboxylic acid (Aldrich), thionyl chloride (Aldrich), glycerol (Aldrich), NaHCO3 (Aldrich), and MgSO4 (Aldrich) were used as received. 2.2. Synthesis of 2-[(3-thienylcarbonyl)oxy]propyl 3thiophene carboxylate A mixture of 3-thiophenecarboxylic acid (0.7000 g, 5.4 mmol) and thionyl chloride (0.765 g, 6.5 mmol) was placed in 2.5 · 10 5 m3 round bottom flask carrying a drying tube and refluxed gently at 100 C for 30 min. A water aspirator vacuum to the top of the condenser was applied to remove any thionyl chloride, and glycerol (1.7 mmol) was added to the mixture. The reaction was allowed to stand on the bath for 15 min. more and then cooled, diluted with 5 · 10 5 m3 ether and transferred to a separatory funnel. The mixture was washed with water and twice with NaHCO3 solution and dried over MgSO4. The solvent was removed over rotatory evaporator. A yellowish-oily product was obtained. The resultant product was further purified via column chromatography, and white crystals were obtained with 47% yield. 1H NMR spectrum of the monomer was taken using a Bruker Instrument-NMR Spectrometer (DPX-400) with CDCl3 as the solvent and tetramethylsilane as the internal standard. FTIR Spectra were taken by a Nicolet 510 FT-IR Spectrometer by dispersing samples in KBr disk. The representation of synthesis is demonstrated in Scheme 1.
HO Cl
O
OH
O
OH O
O O
S S
O S
Scheme 1. Synthesis of TOPT.
S
U. Bulut et al. / European Polymer Journal 40 (2004) 2421–2426
2.3. Cyclic voltammetry The voltammograms were recorded in acetonitrile (ACN)/tetrabutylammonium tetrafluoroborate (TBAFB) solvent-electrolyte couple using a system consisting of a potentiostat (Wenking POS 2), an X–Y recorder and a CV cell containing Pt foil working and counter electrodes, and a Ag/Ag+ reference electrode. Measurements were taken at room temperature under nitrogen atmosphere. 2.4. Synthesis of graft copolymers by electrochemical polymerization
2423
working electrode, a Pt wire as the counter and Ag/ Ag+ as the reference electrodes. The electrochromic measurements (Solartron 1285 potentiostat/galvanostat), spectroelectrochemistry (HP8453A UV–VIS spectrophotometer), and switching studies of the polymer film deposited on ITO coated glass were carried out in TBAFB (0.1 M)/ ACN. The color coordinates of the film both in the reduced and oxidized state were measured via GretagMacbeth Coloreye XTH Spectrophotometer.
3. Results and discussion 3.1. Synthesis of polymers
2.4.1. Synthesis of the conducting copolymer of TOPT with thiophene For the synthesis of conducting copolymer of TOPT, thiophene was used as the comonomer. 40 mg of TOPT, 8.8 · 10 5 mol, was dissolved in 5 · 10 5 m3 acetonitrile (ACN), and 5 · 10 4 moles of thiophene was introduced into a 3-compartment electrolysis cell, where the supporting electrolyte was TBAFB (0.05 M). Constant potential electrolysis was run at 1.9 V for 60 min. at room temperature under inert atmosphere to obtain films 40 lm. Films were washed with ACN to remove TBAFB subsequent to the electrolysis. Surface morphologies of the resultant films were examined by scanning electron microscopy (SEM), JEOL JSM-6400. Electrical conductivity measurements were carried out by four-probe technique. Thermal behavior of the samples was investigated via a Du Pont 2000 Thermal Gravimetry Analyser. 2.4.2. Synthesis of conducting copolymers of TOPT with pyrrole For the synthesis of conducting copolymer of TOPT, pyrrole was used as the comonomer. The electrolysis procedure and the medium was the same as it is described above for thiophene comonomer. The applied potential was 1.1 V vs. Ag/Ag+ reference electrode at room temperature under inert atmosphere. Films were washed with ACN to remove TBAFB after the electrolysis. 2.4.3. Synthesis of conducting copolymer of TOPT with thiophene on ITO The potentiodynamic electrochemical polymerization of TOPT (0.01 M) in boron fluoride-ethyl ether (BFEE) was carried out in a 3-electrode cell with an ITO coated glass electrode and a platinum wire counter electrode in the presence of 6.25 · 10 5 mol Th. The system is cycled between 0 and +1.75 V vs. Ag/Ag+ reference electrode for 5 min with 0.75 V/s scan rate. The film of P(TOPT-co-Th) was produced potentiodynamically between 0 and +1.75 V using 10 mM TOPT, 6 mM Th and 0.1 M BFEE solution in a 3-compartment CV cell, where an ITO coated glass electrode was used as the
The copolymers of TOPT in the presence of either Th or Py were deposited both potentiodynamically and potentiostatically. However, the homopolymer of TOPT could merely been achieved potentiodynamically, and was not in sufficient amount for further characterization. The electrochromic studies were carried out only for the copolymer of TOPT with Th [P(TOPT-co-Th)]. 3.2. 1H NMR and FT-IR characterization 1
H NMR (d, ppm) data for TOPT: 8.2 (m, 3H, thiophene), 7.5 (m, 3H, thiophene), 7.2 ppm (m, 3H, thiophene), 4.3 ppm (d, 4H, methylene), 3.9 ppm (quintet, 1H, methylene). 13 C NMR (d, ppm) data for TOPT: 161.91, 133.77, 132.94, 128.45, 126.74, 126.37, 63.11, 50.20. The FTIR Spectra of TOPT and the copolymer of TOPT with Th showed the following absorption peaks: For TOPT: 3100 cm 1 (aromatic C–H), 2950–2850 cm 1 (aliphatic C–H), 1725 cm 1 (C@O stretching), 1260–1000 cm 1 (C–O–C symmetric and asymmetric stretching), 845 and 750 cm 1out of plane bendings (b and a hydrogens of thiophene respectively). For [P(TOPT-co-Th)]: A broad peak with a maximum at 3650 cm 1, shading the aromatic C–H stretching at 3080 cm 1, 2963–2920 cm 1 (aliphatic C–H), 1755–1740 cm 1 (C@O stretching), 1647 cm 1 (C@C stretching), a broad intense peak at 1083 cm 1 (BF4 , dopant anion), shading the peaks due to the C–O–C symmetric and asymmetric stretching at around 1240– 1000 cm 1, 831 cm 1 (b-hydrogen of thiophene). The existence of the characteristic peaks of the monomer shows that the film is not polythiophene. Furthermore, the vanishing of the peak at 750 cm 1 indicates the copolymerization, since the polymerization advances through 2,5-positions of the thiophene ring. 3.3. Cyclic voltammetry As seen from Fig. 1(a), the monomer reveals an oxidation peak at +2.40 V. Upon thiophene addition, the
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Fig. 1. Cyclic voltammogram of (a) monomer (TOPT), (b) TOPT in the presence of thiophene, (c) TOPT in the presence of pyrrole on a bare Pt electrode in 0.1 M TBAFB/CH3CN.
reversible electroactivity of thiophene decreases through the first 10 runs. Thereon, the oxidation peak height increases and the peak shifts to +2.25 V. When Fig. 1(b) is compared with the voltammogram of pristine thiophene, the redox peaks are not at the same positions (pure PTh–– Ep,a: +0.98 V, Ep,c: +0.70). This shift is known to be an indirect indication for the reaction between thiophene and the thiophene moiety of the monomer. In the case of pyrrole present in the system, the usual reversible redox peaks were observed at different potentials than that of PPy (Fig 1(c)) (pure PPy––Ep,a: +0.63 V, Ep,c: +0.07). Besides, the peak height it reaches is lower than that of pure polypyrrole with the same scan number. 3.4. Thermal properties
Fig. 2. TGA thermogram of P(TOPT-co-Th).
TGA thermogram of the copolymer of TOPT with Th, P(TOPT-co-Th), showed one weight loss at 273 C (Fig. 2). At this temperature, 28% of the copolymer was decomposed, whereas 18% of PTh decomposes at 246 C under the same conditions. After 680 C, there was still 53% of the sample not decomposed, while it is 50% for PTh. In the case of copolymerization with Py, a weight loss was observed at 250 C, where 8% of the sample was decomposed, and 40% of the sample remained after 750 C. 3.5. Scanning electron microscopy (SEM) In the SEM micrographs of the BF4 doped copolymers of TOPT, a significant difference of morphology, as a chain-like structure, was observed compared to the cauliflower structure of the pure PTh and PPy (Fig. 3).
Fig. 3. SEM micrograph of (TOPT-co-Th).
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This may be yet an indication of copolymerization of TOPT either with thiophene or pyrrole. 3.6. Conductivities of the films In Table 1, room temperature (20 C) conductivities of the copolymers with thiophene and pyrrole are given. Compared to pristine polypyrrole and polythiophene, they are one order of magnitude low. This is expected since the presence of monomer moieties destroys the conjugation in both pure PPy and PTh, which may be another indication of copolymerization. 3.7. Electrochromic properties The electrochemical switching of the copolymer of TOPT with Th in BFEE was studied. Fig. 4 shows the potentiodynamic synthesis of [P(TOPT-co-Th)] at a scan rate of 1 V s 1 on a Pt electrode. Both the anodic (Ep,a) and the cathodic (Ep,c) peak potentials are not well-defined in the voltammogram. However, the film produced in BFEE has lower peak potentials compared to the film deposited in ACN solution, since the electrochemical polymerization in BFEE is facilitated as the aromatic resonance energy is lowered, and the abstraction of an electron from the a-position of the heterocyclic ring is promoted [20]. It is known that BFEE exists in diethyl
Table 1 Room temperature conductivities of the films (S/cm) Samples
Conductivity
PTh P(TOPT-co-Th) PPy P(TOPT-co-Py)
5 1 20 0.6
Fig. 4. Cyclic voltammogram of 0.01 M P(TOPT-co-Th) in BFEE at a scan rate of 1 V s 1.
ether as a polar adduct, and the presence of a small amount of water results in the formation of H+[(BF3OH)] , which provides a conducting medium, where the [(BF3OH)] anion serves as the dopant during the polymerization [21]. Spectroelectrochemistry studies were performed on an HP 8453 Diode-array UV–VIS Spectrophotometer. The spectroelectrochemical analysis, the electrochromism and switching studies of the film produced in
Fig. 5. Spectroelectrochemistry of P(TOPT-co-Th) (synthesized in BFEE potentiostatically on ITO coated glass) as a function of wavelength at applied potentials between 0.0 and +1.0 V in 0.1 M TBAFB (0.1 M)/ACN. Spectra were recorded at the following potentials: (a) 0.0 V, (b) +0.2 V, (c) +0.4 V, (d) +0.6 V, (e) +0.8 V, (f) +1.0 V vs. Ag/Ag+.
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Table 2 Electrochemical, electronic, and electrochromic properties Polymer
Ep,aa
Ep,ca
kmax (nm)
Eg (eV)
Color (red)b
Color (ox)b
L
PTh
0.63
0.58
495
1.92
Bright red
Pale blue
P(TOPT-co-Th)
–
–
470
2.00
Dark red
Green
(ox) 57 (red) 51 (ox) 52 (red) 58
a b
ab 7 52 60 44
b 2 46 33 32
Volts vs. Ag/Ag+. Reduced state (red), oxidized state (ox).
BFEE were carried out in TBAFB (0.1 M)/ACN, since the redox responses of polymer films cycled in BFEE are not well defined. The spectroelectrochemical analyses were carried out in order to investigate the electronic structure of the copolymer, and its optical behavior upon redox switching. The film deposited onto ITO coated glass electrode was placed in a UV cuvette for UV–Vis experiment after it is washed with monomerfree electrolyte solution. The absorbance was monitored in situ as a function of potential ranging from 0.0 to +1.0 V (Fig. 5), and from +1.0 to 0.0 V to compare the two spectra to confirm the reversibility of the film. The kmax and the energy band gap values of the PTh and the copolymer for the p–p* transitions were determined, and recorded in Table 2. P(TOPT-co-Th) has a peak absorbance at 470 nm (kmax) and has a band gap (Eg) of 2.00 eV. The color coordinates of the film both in the reduced and oxidized state were measured and the corresponding color coordinates (CIE L * a * b values) were recorded in Table 2. The film switches between a dark red color in the reduced state and a green oxidized state. Besides, the copolymer could be cycled up to 1800 times, completing each cycle in 7 s, between 0 and +1.35 V with a scan rate of 375 mV/s before it loses its ability to switch, where as PTh can be cycled for 240 times under same conditions. Consequently, the copolymer synthesized has quite different electronic and electrochromic properties as compared to pure PTh.
4. Conclusion The syntheses of conducting copolymers of TOPT monomer either with thiophene or pyrrole were achieved in the presence of the TBAFB supporting electrolyte. Free standing, stable and electrically conducting copolymers were obtained. It has also been shown that the copolymerization of TOPT monomer with thiophene in BFEE system yields electroactive and electrochromic films, which could be cycled for 1800 times. The spectroelectrochemical analysis illustrates that the copolymer film synthesized in BFEE reveals a reversible cycling.
Acknowledgment This study is partially supported by TUBA and DPT2003K1209120-02 grants.
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