SHORT COMMUNICATION Synthesis and

Chemical Papers 64 (1) 114–117 (2010) DOI: 10.2478/s11696-009-0087-6

SHORT COMMUNICATION

Synthesis and characterization of conducting copolymer , 3 -bis(thiophene-3-ylmethylene)benzene-1,3-diamine-co-3,4ethylenedioxythiophene)

of (

1

Altu˘ g K¨ umb¨ ul, Ersen Tura¸ c, Tu˘ gba Dursun, Ertu˘ grul S ¸ ahmetlio˘ glu* Department of Chemistry, Nigde University, 51100, Nigde, Turkey Received 27 March 2009; Revised 19 June 2009; Accepted 22 June 2009

Electrochemical copolymerization of N 1 ,N 3 -bis(thiophene-3-ylmethylene)benzene-1,3-diamine (TMBA) with 3,4-ethylenedioxythiophene (EDOT) was carried out in a CH3 CN/LiClO4 (0.1 M) solvent–electrolyte via potentiodynamic electrolysis. Chemical structure of the monomer was determined by nuclear magnetic resonance (1 H NMR) and Fourier transform infrared (FTIR) spectroscopy. The resulting copolymer was characterized by cyclic voltammetry (CV), FTIR, scanning electron microscopy (SEM), and thermogravimetry analyses (TGA). Conductivity measurements of the copolymer and PEDOT (poly(3,4-ethylenedioxythiophene)) were carried out by the four-probe technique. c 2009 Institute of Chemistry, Slovak Academy of Sciences Keywords: electrochemical polymerization, conducting copolymers, thiophene, EDOT

Among electroactive organic polymers, polythiophene derivatives occupy a prime position due to their high conductivity, good redox reversibility, swift change of color with the potential, and stability in environment (Sahin et al., 2005). Conducting polymers are materials exhibiting electrical and optical properties of metals or semiconductors while retaining the attractive mechanical properties and processing advantages of polymers (Heeger, 2001). Conductive polymers can be synthesized by various techniques including electrochemical and chemical polymerization (Malinauskas, 2001). Electrochemical polymerization is usually carried out in a single- or dual-compartment cell equipped with three electrodes and containing a supporting electrolyte, monomer and an appropriate solvent. The electrochemical technique has received wider attention both because of its simplicity and the added advantage of obtaining a simultaneously doped conductive polymer (Kumar & Sharma, 1998; Turac et al., 2008). Conjugated organic polymers are either insulators or semiconductors. Controlled additions of known, usually small, non-stoichiometric quantities of chemi-

cal species result in drastic changes in the electronic, electrical, magnetic, and optical properties of the polymer. Doping is a reversible process which generally does not cause degradation. Both doping and undoping processes (involving dopant counterions which stabilize the doped state) can be carried out chemically or electrochemically. The p-doping process is partial oxidation of the π-backbone of an organic polymer whereas n-doping is partial reduction of the πbackbone of an organic polymer (MacDiarmid, 2002). Polymers of thiophene and pyrrole based compounds are environmentally and thermally stable materials; therefore, they can be used as non-linear optical devices (Chan et al., 1993), polymer light emitting diodes (Andersson et al., 1995), gas sensors (Ohmori et al., 1993), organic transistors (Horowitz, 1998), and electrochromic devices (Mortimer et al., 2006). Electrochromism is a phenomenon where a material shows reversible and visible change in color associated with an electrochemically induced oxidation– reduction reaction (Camurlu et al., 2005; Tarkuc et al., 2006, 2007, 2008; Ak et al., 2008). Design and synthesis of new conjugated polymers are a significant

*Corresponding author, e-mail: [email protected]

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A. K¨ umb¨ ul et al./Chemical Papers 64 (1) 114–117 (2010)

+ H 2N

NH2

S

i

EDO T

N

CHO

N

ii

HC

CH

S

S

N O

O S

I

N

CH

O

HC

S

S

O S

II

Fig. 1. Synthesis of monomer TMBA (I ) and its copolymer with EDOT (II ). Reaction conditions: i) CH3 OH, 60 ◦C, 24 h; ii) CH3 CN, LiOCl4 , electrochemical copolymerization.

part of polymer research (Ertas et al., 2004) and they have attracted great attention since 1977 (Skotheim et al., 1998). Among conducting polymers, polythiophene and its derivatives have become a subject of considerable interest as electrochromic materials due to their chemical stability, ease of synthesis and structural versatility (Rosseinsky & Mortimer, 2001). EDOT as a substituted monomer is a popular choice since it produces a low band gap polymer with high stability and good conductivity (Wagner et al., 2005). EDOT can give rise to non-covalent intramolecular interactions with adjacent thiophenic units and thus induce self-rigidification of the p-conjugated system in which it is incorporated (Akoudad et al., 1999; Raimundo et al., 2001; Akoudad & Roncali, 1999). In the present work, synthesis of monomeric N 1 ,N 3 -bis(thiophene-3-ylmethylene)benzene-1,3-diamine and its structure analysis (using 1 H NMR and FTIR) is described. Subsequently, its electrochemical copolymerization with EDOT via the potentiodynamic method using LiClO4 as the supporting electrolyte in CH3 CN was performed to afford poly(N 1 ,N 3 -bis(thiophene-3-ylmethylene)benzene1,3-diamine-co-3,4-ethylenedioxythiophene) (Fig. 1). The resulting copolymer was characterized using data from CV, SEM, TGA, FTIR, and conductivity measurements. 1,3-Phenylenediamine, thiophene-3-carboxaldehyde, methanol, potassium bromide, and acetonitrile were purchased from Merck (Darmstadt, Germany); lithium perchlorate, dichloromethane, 3,4-ethylenedioxythiophene, and silica gel were purchased from Sigma–Aldrich (St. Louis, USA) and they were used without further purification. 1 H NMR spectra were recorded on a Bruker DPX400 spectrometer at 25 ◦C using CDCl3 as solvent and TMS as internal standard. FTIR spectra (in KBr discs, in the range of 4000–400 cm−1 ) were obtained using a JASCO FT/IR-300E spectrometer. Cyclic voltammograms were recorded in CH3 CN/LiClO4 (0.1 M) electrolyte-solvent couple with a system consisting of a potentiostat (CH Instruments 600) and a CV cell containing a Pt-flake working electrode, Pt wire counter electrode, and an Ag wire pseudo reference electrode. Measurements were carried out at room temperature. Thermogravimetric analyses were performed under nitrogen atmosphere in the range of 25–1000 ◦C with the heating rate of 5 ◦C min−1 using

a Perkin–Elmer Pyris Diamond 6.0 model TG/DTA. Particle morphology of polymer films was examined by means of scanning electron microscopy (Carl Zeiss EVO-40), operating at 20 kV. Electrical conductivity measurements of PEDOT and its copolymer were carried out by the four-probe technique at room temperature using a home made instrument. Compact thin pellets for these measurements were prepared under the pressure of 8–10 Pa. Measured conductivities of copolymer II and PEDOT were 3 × 10−5 S cm−1 and 2 × 10−5 S cm−1 , respectively. N 1 ,N 3 -bis(thiophene-3-ylmethylene)benzene1,3-diamine (TMBA) (I) A mixture of 1,3-phenylenediamine (10 mmol, 1.08 g), thiophene-3-carboxaldehyde (20 mmol, 1.76 mL) and methanol (40 mL) was stirred and refluxed in a round-bottomed flask equipped with a magnetic stirrer for 24 h at 60 ◦C. Evaporation of methanol, followed by flash column chromatography (silica gel column, elution with dichloromethane) afforded the desired compound I (1.92 g, 65 %, m.p. 172 ◦C). All attempts to separate individual stereoisomers were unsuccessful regardless of the column type and the solvent used. 1 H NMR (CDCl3 ), δ: 8.57 (2H, CH—N), 8.21 (2H, H-4 in thiophene), 7.67 (4H, H-2, H-4, H-6 in benzene), 7.16 (4H, H-2, H-5 in thiophene). FTIR (KBr), ν˜/cm−1 : 3088 (C—H of thiophene), 3029 (C—H in benzene), 2936 (C—H in CH—N), 1621 (CH—N), 1566–1446 (C—C), 1187 (C—S), 1049 (C—H in-plane bending), 879 (C—H out-of-plane bending). Poly(N 1 ,N 3 -bis(thiophene-3-ylmethylene) benzene-1,3-diamine-co-3,4ethylenedioxythiophene) (II) To TMBA (50 mg) dissolved in CH3 CN (5 mL) in a single compartment electrolysis cell, EDOT (5 µL) was added. LiClO4 was used as the supporting electrolyte. Cyclic voltammetry runs were performed between −1.5 V and +1.5 V with the scan rate of 500 mV s−1 . Polymer was washed with double distilled water to obtain residual supporting electrolyte. FTIR (KBr), ν˜/cm−1 : 3010 (C—H in benzene), 2876 (C—H in CH—N), 1616 (CH—N), 1561–1440 (C—C), 1183 (C—S), 1145 (C—O), 1119, 1042 (C—H in-plane bending), 632.


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Fig. 2. Cyclic voltammograms of copolymer II (a) and PEDOT (b) in CH3 CN/LiClO4 .

Fig. 3. SEM images of copolymer II (a) and PEDOT (b).

As seen in Fig. 2a, oxidation peak of the monomer drastically shifted to +0.29 V when EDOT was added into the medium. The voltammogram of poly(TMBAco-EDOT) is quite different compared to that of pristine EDOT (Fig. 2b) (pure PEDOT–Epa : 0.17 V, Epc : −0.88 V). Both the current increments between the consecutive cycles and the redox potentials of the copolymer compared to those of pure PEDOT and poly(TMBA-co-EDOT) were different. This shift indicates a reaction between EDOT and the thiophene moiety of poly(TMBA-co-EDOT). In the electropolymerization process including cyclic voltammetry with the increasing polymer thickness (or cycle number), the current reaches higher values hence both the anodic and the cathodic peaks are getting broad thus proving the electropolymerization process. In the FTIR spectra of poly(TMBA-co-EDOT) (II ), peaks at 3088 cm−1 and 879 cm−1 originating from the C—Hα stretching of the thiophene moiety disappeared completely. This proves polymerization at 2,5 positions of the thiophene moiety of the monomer and EDOT. The new broad band at about 1616 cm−1 was caused by poly-conjugation. The strong absorption peaks at 1119 cm−1 and 632 cm−1 were attributed

Fig. 4. TGA of copolymer II.

to the incorporation of ClO− 4 ions into the polymer film during the doping process. Thus, the results of the FTIR studies clearly indicate copolymerization of TMBA with EDOT. The surface morphology of II and PEDOT was investigated by scanning electron microscopy (SEM). The solution side of the II film revealed globular morphology (Fig. 3a) which is completely different than the PEDOT surface morphology (Fig. 3b). TGA of II, measured under nitrogen atmosphere in the range of 25–1000 ◦C, revealed a 5 % mass loss



at 120 ◦C, 50 % mass loss at 290 ◦C, and complete decomposition at 550 ◦C (Fig. 4). Acknowledgements. The authors thank E. Gokturk for SEM experiments.

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