Oxidative polymerization of 3,4-ethylenedioxythiophene using

Reactive & Functional Polymers 70 (2010) 75–80

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Oxidative polymerization of 3,4-ethylenedioxythiophene using transition-metal tetrasulfonated phthalocyanine Mohammad Reza Nabid *, Shima Asadi, Mitra Shamsianpour, Roya Sedghi, Samira Osati, Nasser Safari Department of Chemistry, Faculty of Science, Shahid Beheshti University G.C., 1983963113 Tehran, Iran

a r t i c l e

i n f o

Article history: Received 22 November 2008 Received in revised form 22 October 2009 Accepted 25 October 2009 Available online 28 October 2009 Keywords: PEDOT Conducting polymers Water-soluble Electroactive Metallophthalocyanine

a b s t r a c t A method for the synthesis of water-soluble polyethylenedioxythiophene (PEDOT) by using transitionmetal tetrasulfonated phthalocyanine (TSPc) and hydrogen peroxide as effective catalysts in the presence of sulfonated polystyrene (SPS) is reported. The reactions were carried out with different catalysts, such as iron, cobalt and manganese phthalocyanine. Metallophthalocyanines have shown good activities for polymerization, although they degraded easily under oxidizing conditions. In order to determine the role of pH during the polymerization, the reaction was carried out under different pH conditions 2, 2.5 and 3 and the best results were obtained at pH 2. The conductivity of our product was obtained and compared with similar commercial material. PEDOT was characterized by UV–vis and FT-IR spectroscopies and also cyclic voltammetry. Cyclic voltammetry (CV) demonstrated that the synthesized polymer has convenient electroactivity. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In the last few decades, conducting polymers (CPs) have been very actively pursued [1]. Several discoveries brought the CPs to full commercialization with applications in displays [2], sensors [3], polymer light-emitting diodes [4], photovoltaics [5] and electrochromic devices [6,7]. Since the discovery of conducting polymers, they have become extremely attractive materials, due to their tunable band gaps and redox properties, processability, lightness, resistance against corrosion and low cost [8]. Poly (3,4-ethylenedioxythiophene) (PEDOT) has received a tremendous growth of interest in the last decade [9] and has became one of the most investigated conducting polymers for academic research and also for various industrial applications. Such interest results from its remarkable electroconducting properties associated with a high chemical stability. The solubility problem of PEDOT has been avoided by using a water-dispersible polyelectrolyte [10]. The SPS in the PEDOT/SPS complex acts as a template and has two functions. The first is to act as the source for the charge balancing counter ion to stabilize the p-doped PEDOT chains. The second function of SPS is keeping PEDOT segments dispersed in the aqueous medium [11]. The use of enzymes as biological catalysts in the synthesis of conducting polymers has also attracted great interest in recent years [12]. We used horseradish peroxidase enzyme (HRP) for the oxidative polymerization of aniline derivatives in the presence * Corresponding author. Fax: +98 21 22431661. E-mail address: [email protected] (M.R. Nabid). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.10.007

of hydrogen peroxide [13,14]. Although HRP is a promising biocatalytic approach for the synthesis of conducting polymers, it shows low activity toward monomers and low stability at pH below 4.5. To improve the enzymatic synthesis, Sakharov et al. reported the use of laccase and palm tree peroxidase for the synthesis of PANI at pH 3.5 [15,16]. A variety of transition-metal catalysts have been successfully employed in the polymerization processes [17,18]. These complexes were successfully used as effective catalysts to control the radical polymerization of vinyl monomers [19,20]. The oxidative polymerization of aromatic amines is an important method to provide a wide range of conducting polymers. The structure of porphyrin and phthalocyanine are similar to the active site of the enzyme, so we decided to use these catalysts for the polymerization of EDOT. Recently, we reported the synthesis of PEDOT with porphyrin as a catalyst [21]. In this work, we investigate the preparation of PEDOT with a phthalocyanine as a catalyst. Phthalocyanines (Pcs) constitute a remarkably versatile and robust class of compounds with diverse technological applications [22–24]. Metallophthalocyanines (MPcs) have been used as efficient biomimetic catalysts for oxidation, reduction and other reactions of organic compounds [25–31]. The driving forces for the use of phthalocyanines are (i) the resemblance of their macrocyclic structure with that of porphyrins widely used by nature in the active sites of oxygenase enzymes; (ii) their rather cheap and facile preparation in a large scale; (iii) their chemical and thermal stability [32]. Recently, we reported a novel route for the polymerization of aniline with hydrogen peroxide by using anionic water-soluble

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M.R. Nabid et al. / Reactive & Functional Polymers 70 (2010) 75–80

iron (III) tetrasulfonated tetraphenyl porphyrin [33] and water-soluble tetrasulfonate metallophthalocyanine as catalysts [34]. The structure of metallophthalocyanines (MPcs) is similar to metalloporphyrins (MPPs) and it seems that they are efficient catalysts for the polymerization of EDOT due to their higher stability and cheaper prices. The former can also have promising potential for industrial applications [35]. In this paper, in order to extend our studies, we presented the polymerization of EDOT using watersoluble tetrasulfonated metallophthalocyanine. The polymerization was carried out at different conditions in the presence of sulfonated polystyrene (SPS) as a template. We used iron (III) tetra (p-sulfonatophenyl) Phthalocyanine (FeIIITSPc), cobalt (III) tetra (p-sulfonatophenyl) phthalocyanine (CoIIITSPc) and manganese (III) tetra(p-sulfonatophenyl) phthalocyanine (MnIIITSPc) as effective catalysts for the polymerization of EDOT. The general structure of water-soluble tetrasulfonated metallophtalocyanine is shown in Fig. 1.

Table 1 The conductivity of synthesized PEDOT/SPS and commercial one. Name

wt.%

PEDOT:PSS

r (S cm 1)

Commercial PEDOT/SPS Synthetic PEDOT/SPS

2.8 0.07

0.054 0.55

2.8

0.054

1 10 5 5 10 4 (Pellet) 3.1 10 7 (Film) 2 10 5 (Pellet) 1.2 10 7 (Film)

merization, 2.5 ml of 0.02 M hydrogen peroxide was added in increments. The reaction mixture was stirred for 4 h in order to complete the polymerization. The final solution was dark blue. This solution was transferred to an individual regenerated tube and dialyzed (cut off molecular 3000) for 24 h in a distilled water solution to remove any unreacted monomers and oligomers. The same procedure was carried out for the reactions catalyzed with CoIIITSPc and MnIIITSPc.

2. Experimental 2.3. Instrumental characterization 2.1. Materials and reagents Poly (sodium styrene sulfonate) (MW of 70,000) and PEDOT/SPS (2.8 wt.%) were purchased from Aldrich Chemical Co. (Milwaukee, WI) and was used without any further purification. Hydrogen peroxide (30 wt.%), EDOT and all other reagents were obtained from Merck Company (Whitehouse Station, NJ). EDOT was purified by vacuum distillation. The metallophthalocyanines were prepared in our laboratory through the synthesis of tetraphenylphthalocyanine [35]. 2.2. Polymer synthesis The polymerization of EDOT in the presence of SPS was catalyzed with FeIIITSPc at ambient temperature in the presence of hydrogen peroxide. Accordingly, a total of 9.3 mg (0.045 mmol) of SPS (based on the molecular repeat unit) were dissolved in 10 ml of water (pH 2, adjusted with concentrated HCl). This was followed by the addition of 15 ll (0.14 mmol) EDOT and 2.25 ml of 2.7 10 5 M FeIIITSPc to the solution. To commence the poly-

The FT-IR measurements were carried out with the help of a BOMEM MB-Series FT-IR spectrometer in the form of KBr pellets. UV–vis spectra were obtained using a Shimadzu UV-2100 spectrophotometer. The cyclic voltammetry (CV) measurements were performed with a l AUTOLAB Polarograph type III and SAMA 500 ElectroAnalyzer System. The cyclic voltammograms were recorded at ambient temperature using a three electrode cell: platinum as an auxiliary electrode, Ag/AgCl as the reference electrode and Pt (0.2 cm2 surface area) as the working electrode. The cyclic voltammograms were scanned from 0.25 to 1.5 V and 0.5 to 1.5 V at various scan rates in the range of 25–200 mV/s. The conductivity was measured by a Keithley 213 with a digital multimeter system. The resultant polymer was mixed with HCl solution for 4 h until the doping process completed. After doping, the pellet and film of the dried polymer were prepared for conductivity measurements. 3. Results and discussion Synthetic methods for the preparation of conducting polymers have been extensively studied in recent years. PEDOT/SPS is one of the most widely used materials in organic electronics. In this study the oxidative polymerizations of EDOT with a catalytic

Fig. 1. General structure of tetrasulfonated metallophthalocyanine.

Fig. 2. UV–vis spectra of the mixture of reaction: (a) before and (b) after the addition of H2O2.


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Fig. 3. UV–vis spectra of PEDOT/SPS catalyzed by (a) FeTSPP and (b) Fe-TSPc. Fig. 5. The absorption spectra of the PEDOT/SPS complexes synthesized with: (a) CoIIITSPc, (b) FeIIITSPc and (c) MnIIITSPc catalysts.

amount of water-soluble tetrasulfonated metallophthalocyanines were carried out at ambient temperature under acidic conditions. According to the obtained results, the catalysts that were used act more efficiently than enzymes. Porphyrin and phthalocyanine are appropriate alternatives for enzymes because of their special characterizations. The cation radical of the catalyst is responsible for initiating the formation of the EDOT cation radical, which attacks the other cation radical of the monomer to form a dimmer by the separation of two hydrogens and hence the reaction further propagates to give PEDOT. The polymerization was performed in the presence of strong anionic polyelectrolyte such as SPS and dark blue solutions were obtained that show a strong polaron absorption band. The advantage of this work is the synthesis of the water-soluble PEDOT/SPS with lower ratio of SPS to PEDOT against commercial polymer. Therefore, with a lower concentration of polymers we achieved a higher conductivity (Table 1). Another advantage of the approached polymer over current commercial sample is the synthesis in a better condition (in pH 2). Fig. 2 shows the UV–vis spectra of the Fe-TSPc catalyst and 3:1 M ratio of EDOT to SPS in an aqueous solution of pH 2, before (Fig. 2a) and after (Fig. 2b) the addition of hydrogen peroxide. The characteristic band at 750 nm is indicative of the conducting form of PEDOT. The peak at 270 nm in Fig. 2a corresponds to SPS. As can be seen in Fig. 2b, initiation of the polymerization by the addition of H2O2 led to the appearance of an absorption band at

Fig. 4. The absorption spectra of the PEDOT/PSS complex synthesized in different pH values: (a) 2, (b) 2.5 and (c) 3.

750 nm. This was also accompanied by the development of a dark blue color, with a simultaneous increase in the absorption intensity of the peaks over time. The UV–vis spectra of PEDOT catalyzed by metallophthalocyanine (TSPc) and metalloporphyrine (TSPP), (from our previous work) are shown in Fig. 3. This comparison indicates that the amount of polymerized EDOT is higher when, under the same experimental conditions, metalloporphyrin is used.

3.1. Optimization of the reaction conditions The polymerization of EDOT is strongly pH-dependent. To determine the role of pH during the polymerization, the reaction

Fig. 6. UV–vis spectra of PANI/SPS during dedoping and redoping with a base and an acid in the pH ranges of (a) 2–12 and (b) 12–2.

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Fig. 7. FT-IR spectra of (a) EDOT, (b) PEDOT/ FeIIITSPc, (c) PEDOT/ MnIIITSPc and (d) PEDOT/CoIIITSPc.

was carried out under different pH conditions ranging from 2, 2.5 and 3. Fig. 4 shows the UV–vis spectra of PEDOT/SPS prepared at different pHs. In this work, the best results were obtained at pH 2, and optimal pH is needed to provide the conducting form of PEDOT. 3.2. Polymerization with different metallophthalocyanines Fe, Mn and Co phthalocyanines have shown good activities for oxidation, although they are easily degraded under oxidizing conditions [36]. As shown in Fig. 5, the amount of polymerization with MnIITSPc as a catalyst is significantly lower than CoIIITSPc and FeIIITSPc. It seems that, when Mn (III) is the central metal of the catalyst, the obtained complex is formed very fast but as the reaction proceeds it gradually degrades, which ultimately result in lower polymerization amount compared to CoIIITSPc and FeIIITSPc.

ing with NaOH solution. At pH 2, the PEDOT in the complex is in the doped state as reflected by the presence of the polaron band transition at about 770 nm. As the pH of the complex increase, the polaron band at 770 nm gradually disappears, and an absorption band at 550 begins to emerge. At pH 12, a grey solution of PEDOT/SPS complex is formed, indicating that the PEDOT has been fully dedoped. The dedoped PEDOT can be redoped by titrating with HCl solution. A reversible color change is observed, and the spectra are given in Fig. 6b. This pH induced redox reversibility confirms the presence of the electroactive form of PEDOT in the PEDOT/SPS complex. Furthermore, isobestic point can be clearly observed at 630 nm.

3.4. FT-IR spectroscopy

To determine the reversible redox behavior of the PEDOT/SPS complex, the UV–vis spectra of a complex prepared at pH 2 was studied with varying pH. Fig. 6a gives the shift in the absorption spectra of the complex when increasing pH from 2 to 12 by titrat-

The structure of the polymer was studied by Fourier transform infrared spectroscopy. Fig. 7 compares FT-IR spectra of pure EDOT and PEDOT/SPS complexes synthesized by FeIIITSPc, MnIIITSPc and CoIIITSPc catalysts. The bands at 1690, 1386, 1080, 940 and 840 cm 1 are derived from PEDOT [37]. The band at 3111 cm 1 is characteristic of C–H vibration of EDOT and disappears completely upon polymerization. Furthermore, the peaks observed at 1008

Fig. 8. Cyclic voltammograms of the PEDOT/SPS complex.

Fig. 9. Comparison of cyclic voltammograms of the commercial PEDOT/SPS (a) and synthesized one (b).

3.3. Doping–dedoping reversibility of PEDOT/SPS


and 1036 cm 1 correspond to symmetric and asymmetric S'O stretching and a band at 670 cm 1 which was attributed to the SO3 group that confirms the presence of SPS in the complex. 3.5. Cyclic voltammetry The electrochemical properties of the PEDOT/SPS complexes were determined by using cyclic voltammetry. Fig. 8 shows the cyclic voltammograms (CV) of the PEDOT/SPS complex obtained by Fe-TSPc catalyst. Recording the cyclic voltammograms was done at different scanning rates between 25 and 200 mV/s. We observed that there was an appreciable change in the cathodic and anodic peak current values. These voltammograms show a highly reversible capacitive response at potentials between 1500 and 1500 mV, which is characteristic of conducting polymers. The anodic peaks were seen at 0.706 and 0.132 V and a well-defined pair of peaks located at 0.895 and 0.025 V are the potential values of the corresponding cathodic peaks. These recorded CV curves suggest that the PEDOT/SPS complexes produced by metallophthalocyanine are electrochemically active. The electrochemical behavior of the synthesized product and the commercial one was compared as shown in Fig. 9. Comparison of the curves reveals that not only the intensity of our product’s cathodic and anodic peak currents increases but also we observe two pairs of peaks against similar commercial material. Similar results have been observed when we measured electrochemical behavior of our product with the same concentration of commercial material (2.8 wt.%) as shown in Fig. 10a and b. Fig. 10a is re-

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lated to our product and Fig. 10b is the CV of commercial one with the scan rates of 25–200 mV/S. These recorded CV curves suggest that our PEDOT/SPS complexes produced by metallophthalocyanine are electrochemically active. Table 1 offers the electrical conductivity of synthetic PEDOT/SPS films, pellet and commercial one. It is worthwhile to stress that not only with the same concentration of polymers (2.8 wt.%) but also with the lower concentration of resultant material (0.07 wt.%), we achieved better conductivity against commercial PEDOT/SPS. 4. Conclusions The synthesis of a conducting macromolecular complex of PEDOT/SPS was presented by transition-metal tetrasulfonated phthalocyanine catalysts. This approach provides a distinct advantage over similar reactions employing native enzymes due to higher stability and lower price of the catalysts. The results have also demonstrated that the metallophthalocyanines (MPcs) are efficient catalysts for the polymerization of EDOT. Although metalloporphyrins (MPPs) are better catalysts for the polymerization of EDOT, because of higher thermal stability and cheaper prices of MPcs compared to MPPs, the former have more potential for industrial applications. The product shows a better conductivity against commercial sample. Acknowledgement We gratefully acknowledge financial support from the Research Council of Shahid Beheshti University. References

Fig. 10. Comparison of cyclic voltammograms of the PEDOT/SPS complex (a) and commercial one (b) with the same concentration (2.8 wt.%).

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