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Electrochemical polymerization of thianaphthene
Journal of Electroanalytical Chemistry 510 (2001) 29 – 34 www.elsevier.com/locate/jelechem
Electrochemical polymerization of thianaphthene Fan Wang a, Gaoquan Shi b,*, Fengen Chen b, Jingkun Xu b, Jiaxin Zhang b a
Department of Chemistry and Key Laboratory of Life Organic Phosphorus of Education Commission, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China b Department of Chemistry, Qujing Normal Institute, Qujing City 655000, People’s Republic of China Received 8 January 2001; received in revised form 23 April 2001; accepted 6 May 2001
Abstract Thianaphthene has been electrochemically polymerized in pure boron trifluoride diethyl etherate (BFEE) solution or in a mixed electrolyte of BFEE and concentrated sulfuric acid (SA). The addition of a certain amount of sulfuric acid into BFEE accelerated the polymerization and also increased the current efficiency of the electrosynthesis. Poly(thianaphthene) (PTN) in the dedoped state is soluble in usual strong polar organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and N-methyl pyrrolidinone (NMP). Its structure has been examined by infrared, H1-NMR and UV spectra. Fluorescent spectral studies indicate that the polymer is a strong blue light emitter. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thianaphthene; Electrochemical polymerization; Structure; Fluorescence
1. Introduction Extensive work has been devoted to the synthesis of polythiophenes [1]. The high conductivity, chromic and fluorescent properties of these materials provide potential applications in fabricating many kinds of microelectrical devices [2]. Polythiophenes constitute a large family, and one of its members, poly(isothianaphthene) has been known for a long time [3,4]. However, only a few publications concerning the electrosynthesis and characterization of poly(thianaphthene) (PTN) have been published [5]. Boron tetrachloride diethyl etherate (BFEE) can exist in diethyl ether as a polar molecule, [(C2H5)3O+]BF− 4 , which furnishes a conducting medium [6]. This solution was found to be a good electrolyte for electrochemical polymerization of aromatic compounds such as thiophene and benzene [7 – 11]. In BFEE, the oxidation potentials of the aromatic monomers are much lowered and the mechanical properties of the conducting polymer films were improved. In this paper we report the experimental results of electrochemical polymerization of thianaphthene in pure BFEE, or in a mixed medium * Corresponding author. Tel.: + 86-25-359-2369; fax: +86-106277-1149. E-mail address: [email protected] (G. Shi).
of BFEE and sulfuric acid (SA). A new soluble conducting polymer with strong fluorescence in the blue light region was obtained.
2. Experimental
2.1. Materials BFEE (Changyang Chem. Plant., Beijing) was purified by distillation. Thianaphthene (Acros, 98%) and sulfuric acid (Yuehua Chem. Plant, Tianjing 98%) were used as received. Commercial HPLC grade acetonitrile (MeCN) with purity higher than 99.9% was purchased from the Tianjing Shiyou Biological and Medical Technology Company. Tetrabutyl ammonium tetrafluoroborate (TBABF, 98.0%) was a product of Fluka and had been dried at 60 °C for 24 h before use.
2.2. Electrosyntheses and polymer characterizations Electrochemical syntheses and examinations were performed in a one-compartment cell with the use of a Model 283 potentiostat –galvanostat (EG&G Princeton Applied Research) under computer control. The working electrode was a Pt electrode with a diameter of 1
0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 1 ) 0 0 5 3 4 - 4
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F. Wang et al. / Journal of Electroanalytical Chemistry 510 (2001) 29–34
mm. The counter electrode was a Pt wire. All the potentials were measured versus an Ag AgCl electrode (immersed directly in the solution). A correction of 0.069 V is needed to bring the measured potentials in BFEE originally versus Ag AgCl to potentials versus the standard hydrogen electrode [7]. To obtain a sufficient amount of polymer for structural studies, stainless steel (AISI 304) sheets with a surface area of 4 cm2 each were employed as working and counter electrodes, which were polished with abrasive paper (1800 mesh), and cleaned in an ultrasonic water bath and rinsed with methanol and acetone. The typical electrolytic solution was pure BFEE or a mixture of BFEE+5 – 10% SA (by volume) and contained 0.1 mol l − 1 thianaphthene. All solutions were deaerated by a dry nitrogen stream and maintained at a
slight nitrogen overpressure during the experiments. The amount of polymer deposited on the electrode was controlled by the integrated charge passed through the cell. The as-formed PTN is in a doped state and dark blue in color. It is powdery and not soluble in any solvent. For spectral examinations, the polymer was dedoped with 25% ammonia for 3 days, and then washed repeatedly with water and acetone. Finally it was well dried under vacuum at 60 °C. The dedoped PTN is yellow–brown in color and soluble in strong polar organic solvents such as DMSO, DMF and NMP. The current efficiency (CE, that is, the charge consumed by the growth of the polymer film relative to the total charge passed in the cell) was measured by weighing the polymer (in the dedoped state) deposited on the electrode (wp), according to Eq. (1) CE = [(nFwp/M)/Q]× 100%
Fig. 1. Anodic polarization curves of thianaphthene in pure BFEE (a), BFEE+5% SA (by volume, b), BFEE+ 10% SA (by volume, c), MeCN+ 0.1 mol l − 1 TBABF (d) or BFEE+0.1 mol l − 1 TBABF (e). Potential scan rate: 20 mV s − 1.
(1)
where F is the Faraday constant (96 500 C mol − 1), Q the integrated charge passed in the cell during the film growth, n the number of the electrons transferred per monomer attached to the polymer (here estimated at 2), and M the molar mass of the monomer. The dopant contents of PTNs were evaluated by comparing the mass of PTN before and after dedoping. FT-IR spectra were taken by using a GX FT-IR spectrometer (Perkin–Elmer) with KBr pellets. UV spectra were recorded by using a UV-2100S UV–vis spectrometer (Shimadzu). The H1-NMR spectrum was recorded on an AC-P200Q NMR spectrometer (Bruker) and CD3SOCD3 was used as the solvent. The fluorescence spectrum was recorded using a Perkin– Elmer LS 50B luminescence spectrometer. The conductivity of the as-formed PTN was measured by the conventional four-probe technique.
3. Results and discussion
3.1. Electrochemical polymerization
Fig. 2. Cyclic voltammograms of 0.1 M thianaphthene in pure BFEE (a), or in the mixed electrolytes of BFEE + 5% SA (b, by volume) and BFEE+10% SA (c). Potential scan rate: 50 mV s − 1.
Fig. 1 shows the anodic polarization curves of thianaphthene in different electrolytes. The oxidation of thianaphthene in pure BFEE is initiated at 0.44 V (Fig. 1a). The addition of 5% (by volume) concentrated SA increased the oxidation potential of the monomer to 0.55 V (Fig. 1b). However the polymerization rate was increased as the potential was higher than about 0.75 V. The polymer synthesized in pure BFEE adhered weakly to the electrode surface and most of the polymer precipitated on the bottom of the cell or dispersed in the solution. The color of the solution changed quickly from transparent into blue after the potential was applied. The current efficiency was measured as only 13%. On the other hand, the polymer deposited from the solution of BFEE containing 5% SA could adhere well
F. Wang et al. / Journal of Electroanalytical Chemistry 510 (2001) 29–34
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Fig. 3. IR spectra of PTN prepared by using pure BFEE (a), BFEE +5% SA (b) or BFEE +10% SA (c) as electrolyte.
Fig. 4. UV – vis spectra of thianaphthene (a) and dedoped PTN (b) prepared by using pure BFEE as electrolyte. Solvent: DMSO.
to the electrode surface and no polymeric powder precipitated on the cell bottom. The current efficiency was also increased to about 58%. This is mainly due to the addition of SA which increased the dopant content of PTN from ca. 12 to ca. 28% (by weight) and decreased the solubility of the polymer. With a further increase of the content of concentrated SA to 10%, the polymerization rate decreased dramatically and only a trace amount polymer was found on the electrode surface (Fig. 1c). The reason for this phenomenon is not very clear. One possible explanation is that the addition of SA increased the conductivity of the electrolyte and accelerated the polymerization, while it also increased
the chance of stopping chain growth. The addition of 0.1 mol l − 1 (C4H9)4NBF4 into BFEE also increased the oxidation potential of thianaphthene from ca. 0.4 to ca. 1.0 V (Fig. 1d) and the current efficiency of polymerization was measured at lower than 20%. The oxidation potential of thianaphthene in MeCN+ 0.1 mol l − 1 (C4H9)4NBF4 was measured at ca. 1.46 V (Fig. 1e). This value is much higher than those observed in BFEE, and BFEE + SA systems. On the basis of these observations, we chose BFEE or BFEE + SA as the electrolyte for synthesizing PTN. The successive cyclic voltammograms of 0.1mol l − 1 thianaphthene in pure BFEE and in the media of BFEE
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BFEE accelerated the polymerization and increased the wave current density as shown in Fig. 2b. Furthermore, the redox reaction of the polymer also became more reversible and an oxidation wave appeared at about 1.0 V. However, as the content of SA was increased to 10%, the polymerization rate decreased sharply on the second CV cycle (Fig. 2c) and only a trace amount of PTN was formed.
3.2. Structure of polythianaphthene
Fig. 5. 1H-NMR spectra of thianaphthene (A) and dedoped PTN prepared by using BFEE as electrolyte (B).
mixed with SA are illustrated in Fig. 2. The potential scans shown led to the formation of a film on the electrode (blue to black as the deposit thickened). In pure BFEE, the polymer showed a broad reduction wave at about 0.40 V on the first CV scan (Fig. 2a). However, the oxidation wave did not show clearly (may be too broad and weak to be seen), indicating that the redox reaction of the polymer is an irreversible process. The increase of the reduction wave currents implied that the amount of the polymer on the electrode increased. The potential shift of this maximum provides information about the increase of the resistance in the polymer film and the over-potential needed to overcome the resistance [12]. The addition of 5% SA into
The typical transmission IR spectra of dedoped PTNs prepared from pure BFEE (a), or from the mixed electrolyte of BFEE+ 5–10% SA (b and c) are shown in Fig. 3. In these spectra, the characteristic bands of the thianaphthene ring are found at 3057, 1633, 1436, 755, 728 cm − 1 and confirm the formation of poly(thianaphthene) [13]. The bands are relatively broad mainly because the polymer has a broad chain length distribution. Furthermore, there are shoulder bands close to 1700 cm − 1 in Fig. 3b and c. This is possibly due to overoxidation of the polymers in the mixed electrolytes resulting in the formation of carboxide groups. The UV spectra of thianaphthene (a) and PTN (b, in the dedoped state) prepared from pure BFEE medium and dissolved in DMSO are shown in Fig. 4. The UV spectrum of the monomer shows peaks at 297, 289, and 266 nm. The spectrum of the polymer has a much broader band with several fine structures at 261, 293, 350 and 380 nm. The overall absorption tails off to about 600 nm. The longer wavelength indicated longer polymer sequences [14]. The UV spectral results also confirmed that a conjugated polymer with broad molar mass distribution was obtained. The UV spectra of the PTNs deposited from the mixed electrolytes are similar to that shown in Fig. 4b, indicating similar chain structures.
Fig. 6. Fluorescence spectrum of dedoped PTN prepared by using BFEE as electrolyte. Solvent: DMSO.
F. Wang et al. / Journal of Electroanalytical Chemistry 510 (2001) 29–34
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protons are moved to much lower fields, like strong electron withdrawing group-substituted thianaphthene derivatives [15], which confirms the formation of a conjugated delocalizing structure. The proton number ratios of a:b:c:d are calculated to be 1.00:2.04:0.32:0.55. Accordingly, the ring coupling reaction during the polymerization process eliminated the protons at the ‘c’ and ‘d’ positions of the monomer, while the ‘a’ and ‘b’ protons remained unchanged. Thus, the structure of the polymer can be reasonably postulated as shown in the insert of Fig. 5B. The excitation spectrum of the polymer has several peaks at about 310, 350 and 390 nm as shown in Fig. 6. The emission spectrum is characterized by a dominant maximum at 420 nm and a shoulder band at about 510 nm. These results indicate that the polymer is a good blue light emitter. The PTN prepared from the mixed electrolytes presents almost the same fluorescence spectra.
3.3. Morphology and conducti6ity The polymer film prepared electrochemically by using pure BFEE as the electrolyte is rough and irregular as is shown in its scanning electron micrograph (Fig. 7a). The film is cauliflower-like and has many cracks. On the other hand, the film obtained from the system by using BFEE+ 5% SA as the electrolyte was compact (Fig. 7b). The size of the cauliflowers is much smaller than that shown in Fig. 7a and no cracks appeared. However, the film cannot be peeled off to a free-standing state and can be removed only by scraping with a knife. Fig. 7c shows the transmission electron micrograph of the scraped polymer which shows that it is sheet-like. Dedoped PTN is soluble in DMSO and can be cast into a compact, smooth and transparent film. The conductivity of the as-formed PTN film prepared from pure BFEE was found to be 10 S cm − 1. However, the PTN film synthesized from the mixed electrolyte of BFEE+5% SA was found to be 35 S cm − 1. This value is comparable to that of poly(isothianaphthene) ( 50 S cm − 1) [4]. Fig. 7. Scanning electron micrographs of the PTN films deposited on the electrode surface from BFEE (a), BFEE + 5% SA (b), and a transmission electron micrograph of the PTN scraped from the electrode surface (c).
Fig. 5A is the H1-NMR spectrum of the monomer. This figure shows four groups of protons: 7.38 (2H), 7.47 (1H), 7.77 (1H), 7.90 (1H) and 8.01 (1H). The assignments of the NMR lines are shown in the insert of this spectrum. Fig. 5B presents the H1-NMR spectrum of PTN. This spectrum also shows four groups of protons, while the proton lines are much broader and have fewer fine structures than the corresponding proton lines shown in Fig. 5A. The chemical shifts of the
4. Conclusions Thianaphthene can be electrochemically polymerized in BFEE. However, the current efficiency is low and the film quality is poor. Addition of 5% (by volume) of concentrated sulfuric acid into BFEE increased the current efficiency of polymerization by about 4 times. A compact PTN film was formed on the electrode and the conductivity of the film was measured as 35 S cm − 1. The polymer in the doped state is dark blue in color and insoluble. However, the polymer in the dedoped state is soluble in strong polar solvents and a compact
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and smooth film can be obtained by casting its solution. The PTN prepared in this system has a strong fluorescence in the blue light region. The material may have the potential to be used in many commercial applications, including lasers [16,17].
Acknowledgements We thank the Natural Science Foundation of China for support of this work (nos. 29773019 and 50073012)
References [1] J. Roncali, Chem. Rev. 92 (1992) 711. [2] H.S. Chan, S.C. Ng, Prog. Polym. Sci. 123 (1998) 1167. [3] F. Wudl, M. Kobayashi, A.J. Heeger, J. Org. Chem. 49 (1984) 3382.
.
[4] M. Kobayashi, N. Colaneri, M. Moysel, F. Wudl, A.J. Heeger, J. Chem. Phys. 82 (1985) 5717. [5] H. An, M. Seki, R. Yosomiya, Makromol. Chem. Rapid Commun. 8 (1987) 325. [6] D.D. Eley, in: P.H. Plech (Ed.), Chemistry of Cationic Polymerization, Macmillan, New York, 1983, p. 393. [7] G. Shi, S. Jin, G. Xue, C. Li, Science 267 (1995) 994. [8] G. Shi, C. Li, Y. Liang, Adv. Mater. 11 (1999) 1145. [9] G. Shi, B. Yu, G. Xue, C. Li, J. Chem. Soc. Chem. Commun. (1994) 2549. [10] X. Wang, G. Shi, Y. Liang, Electrochem. Commun. 1 (1999) 536. [11] C. Li, G. Shi, Y. Liang, J. Electroanal. Chem. 455 (1998) 1. [12] T.F. Otero, E.D. Larreta-Azelain, Polymer 29 (1988) 1522. [13] Sadtler Standard Spectra, Sadtler Research Laboratories INC, No. 303K, 1966. [14] L. Guo, G. Shi, Y. Liang, Synth. Met. 104 (1999) 129. [15] Sadtler Standard Spectra, Sadtler Research Laboratories INC, Nos. 25113, 5512, 7959, 10313, 1966. [16] T. Granlund, M. Berggren, M. Andresson, A. Ruzeckas, V. Sundstro¨ m, G. Bjo¨ rk, M. Granstro¨ m, O. Ingana¨ s, Chem. Phys. Lett. 288 (1998) 879. [17] R.L. Pilston, R.D. MaCullough, Synth. Met. 111/112 (2000) 433.
Electrochemical polymerization of thianaphthene Fan Wang a, Gaoquan Shi b,*, Fengen Chen b, Jingkun Xu b, Jiaxin Zhang b a
Department of Chemistry and Key Laboratory of Life Organic Phosphorus of Education Commission, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China b Department of Chemistry, Qujing Normal Institute, Qujing City 655000, People’s Republic of China Received 8 January 2001; received in revised form 23 April 2001; accepted 6 May 2001
Abstract Thianaphthene has been electrochemically polymerized in pure boron trifluoride diethyl etherate (BFEE) solution or in a mixed electrolyte of BFEE and concentrated sulfuric acid (SA). The addition of a certain amount of sulfuric acid into BFEE accelerated the polymerization and also increased the current efficiency of the electrosynthesis. Poly(thianaphthene) (PTN) in the dedoped state is soluble in usual strong polar organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and N-methyl pyrrolidinone (NMP). Its structure has been examined by infrared, H1-NMR and UV spectra. Fluorescent spectral studies indicate that the polymer is a strong blue light emitter. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thianaphthene; Electrochemical polymerization; Structure; Fluorescence
1. Introduction Extensive work has been devoted to the synthesis of polythiophenes [1]. The high conductivity, chromic and fluorescent properties of these materials provide potential applications in fabricating many kinds of microelectrical devices [2]. Polythiophenes constitute a large family, and one of its members, poly(isothianaphthene) has been known for a long time [3,4]. However, only a few publications concerning the electrosynthesis and characterization of poly(thianaphthene) (PTN) have been published [5]. Boron tetrachloride diethyl etherate (BFEE) can exist in diethyl ether as a polar molecule, [(C2H5)3O+]BF− 4 , which furnishes a conducting medium [6]. This solution was found to be a good electrolyte for electrochemical polymerization of aromatic compounds such as thiophene and benzene [7 – 11]. In BFEE, the oxidation potentials of the aromatic monomers are much lowered and the mechanical properties of the conducting polymer films were improved. In this paper we report the experimental results of electrochemical polymerization of thianaphthene in pure BFEE, or in a mixed medium * Corresponding author. Tel.: + 86-25-359-2369; fax: +86-106277-1149. E-mail address: [email protected] (G. Shi).
of BFEE and sulfuric acid (SA). A new soluble conducting polymer with strong fluorescence in the blue light region was obtained.
2. Experimental
2.1. Materials BFEE (Changyang Chem. Plant., Beijing) was purified by distillation. Thianaphthene (Acros, 98%) and sulfuric acid (Yuehua Chem. Plant, Tianjing 98%) were used as received. Commercial HPLC grade acetonitrile (MeCN) with purity higher than 99.9% was purchased from the Tianjing Shiyou Biological and Medical Technology Company. Tetrabutyl ammonium tetrafluoroborate (TBABF, 98.0%) was a product of Fluka and had been dried at 60 °C for 24 h before use.
2.2. Electrosyntheses and polymer characterizations Electrochemical syntheses and examinations were performed in a one-compartment cell with the use of a Model 283 potentiostat –galvanostat (EG&G Princeton Applied Research) under computer control. The working electrode was a Pt electrode with a diameter of 1
0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 1 ) 0 0 5 3 4 - 4
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F. Wang et al. / Journal of Electroanalytical Chemistry 510 (2001) 29–34
mm. The counter electrode was a Pt wire. All the potentials were measured versus an Ag AgCl electrode (immersed directly in the solution). A correction of 0.069 V is needed to bring the measured potentials in BFEE originally versus Ag AgCl to potentials versus the standard hydrogen electrode [7]. To obtain a sufficient amount of polymer for structural studies, stainless steel (AISI 304) sheets with a surface area of 4 cm2 each were employed as working and counter electrodes, which were polished with abrasive paper (1800 mesh), and cleaned in an ultrasonic water bath and rinsed with methanol and acetone. The typical electrolytic solution was pure BFEE or a mixture of BFEE+5 – 10% SA (by volume) and contained 0.1 mol l − 1 thianaphthene. All solutions were deaerated by a dry nitrogen stream and maintained at a
slight nitrogen overpressure during the experiments. The amount of polymer deposited on the electrode was controlled by the integrated charge passed through the cell. The as-formed PTN is in a doped state and dark blue in color. It is powdery and not soluble in any solvent. For spectral examinations, the polymer was dedoped with 25% ammonia for 3 days, and then washed repeatedly with water and acetone. Finally it was well dried under vacuum at 60 °C. The dedoped PTN is yellow–brown in color and soluble in strong polar organic solvents such as DMSO, DMF and NMP. The current efficiency (CE, that is, the charge consumed by the growth of the polymer film relative to the total charge passed in the cell) was measured by weighing the polymer (in the dedoped state) deposited on the electrode (wp), according to Eq. (1) CE = [(nFwp/M)/Q]× 100%
Fig. 1. Anodic polarization curves of thianaphthene in pure BFEE (a), BFEE+5% SA (by volume, b), BFEE+ 10% SA (by volume, c), MeCN+ 0.1 mol l − 1 TBABF (d) or BFEE+0.1 mol l − 1 TBABF (e). Potential scan rate: 20 mV s − 1.
(1)
where F is the Faraday constant (96 500 C mol − 1), Q the integrated charge passed in the cell during the film growth, n the number of the electrons transferred per monomer attached to the polymer (here estimated at 2), and M the molar mass of the monomer. The dopant contents of PTNs were evaluated by comparing the mass of PTN before and after dedoping. FT-IR spectra were taken by using a GX FT-IR spectrometer (Perkin–Elmer) with KBr pellets. UV spectra were recorded by using a UV-2100S UV–vis spectrometer (Shimadzu). The H1-NMR spectrum was recorded on an AC-P200Q NMR spectrometer (Bruker) and CD3SOCD3 was used as the solvent. The fluorescence spectrum was recorded using a Perkin– Elmer LS 50B luminescence spectrometer. The conductivity of the as-formed PTN was measured by the conventional four-probe technique.
3. Results and discussion
3.1. Electrochemical polymerization
Fig. 2. Cyclic voltammograms of 0.1 M thianaphthene in pure BFEE (a), or in the mixed electrolytes of BFEE + 5% SA (b, by volume) and BFEE+10% SA (c). Potential scan rate: 50 mV s − 1.
Fig. 1 shows the anodic polarization curves of thianaphthene in different electrolytes. The oxidation of thianaphthene in pure BFEE is initiated at 0.44 V (Fig. 1a). The addition of 5% (by volume) concentrated SA increased the oxidation potential of the monomer to 0.55 V (Fig. 1b). However the polymerization rate was increased as the potential was higher than about 0.75 V. The polymer synthesized in pure BFEE adhered weakly to the electrode surface and most of the polymer precipitated on the bottom of the cell or dispersed in the solution. The color of the solution changed quickly from transparent into blue after the potential was applied. The current efficiency was measured as only 13%. On the other hand, the polymer deposited from the solution of BFEE containing 5% SA could adhere well
F. Wang et al. / Journal of Electroanalytical Chemistry 510 (2001) 29–34
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Fig. 3. IR spectra of PTN prepared by using pure BFEE (a), BFEE +5% SA (b) or BFEE +10% SA (c) as electrolyte.
Fig. 4. UV – vis spectra of thianaphthene (a) and dedoped PTN (b) prepared by using pure BFEE as electrolyte. Solvent: DMSO.
to the electrode surface and no polymeric powder precipitated on the cell bottom. The current efficiency was also increased to about 58%. This is mainly due to the addition of SA which increased the dopant content of PTN from ca. 12 to ca. 28% (by weight) and decreased the solubility of the polymer. With a further increase of the content of concentrated SA to 10%, the polymerization rate decreased dramatically and only a trace amount polymer was found on the electrode surface (Fig. 1c). The reason for this phenomenon is not very clear. One possible explanation is that the addition of SA increased the conductivity of the electrolyte and accelerated the polymerization, while it also increased
the chance of stopping chain growth. The addition of 0.1 mol l − 1 (C4H9)4NBF4 into BFEE also increased the oxidation potential of thianaphthene from ca. 0.4 to ca. 1.0 V (Fig. 1d) and the current efficiency of polymerization was measured at lower than 20%. The oxidation potential of thianaphthene in MeCN+ 0.1 mol l − 1 (C4H9)4NBF4 was measured at ca. 1.46 V (Fig. 1e). This value is much higher than those observed in BFEE, and BFEE + SA systems. On the basis of these observations, we chose BFEE or BFEE + SA as the electrolyte for synthesizing PTN. The successive cyclic voltammograms of 0.1mol l − 1 thianaphthene in pure BFEE and in the media of BFEE
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BFEE accelerated the polymerization and increased the wave current density as shown in Fig. 2b. Furthermore, the redox reaction of the polymer also became more reversible and an oxidation wave appeared at about 1.0 V. However, as the content of SA was increased to 10%, the polymerization rate decreased sharply on the second CV cycle (Fig. 2c) and only a trace amount of PTN was formed.
3.2. Structure of polythianaphthene
Fig. 5. 1H-NMR spectra of thianaphthene (A) and dedoped PTN prepared by using BFEE as electrolyte (B).
mixed with SA are illustrated in Fig. 2. The potential scans shown led to the formation of a film on the electrode (blue to black as the deposit thickened). In pure BFEE, the polymer showed a broad reduction wave at about 0.40 V on the first CV scan (Fig. 2a). However, the oxidation wave did not show clearly (may be too broad and weak to be seen), indicating that the redox reaction of the polymer is an irreversible process. The increase of the reduction wave currents implied that the amount of the polymer on the electrode increased. The potential shift of this maximum provides information about the increase of the resistance in the polymer film and the over-potential needed to overcome the resistance [12]. The addition of 5% SA into
The typical transmission IR spectra of dedoped PTNs prepared from pure BFEE (a), or from the mixed electrolyte of BFEE+ 5–10% SA (b and c) are shown in Fig. 3. In these spectra, the characteristic bands of the thianaphthene ring are found at 3057, 1633, 1436, 755, 728 cm − 1 and confirm the formation of poly(thianaphthene) [13]. The bands are relatively broad mainly because the polymer has a broad chain length distribution. Furthermore, there are shoulder bands close to 1700 cm − 1 in Fig. 3b and c. This is possibly due to overoxidation of the polymers in the mixed electrolytes resulting in the formation of carboxide groups. The UV spectra of thianaphthene (a) and PTN (b, in the dedoped state) prepared from pure BFEE medium and dissolved in DMSO are shown in Fig. 4. The UV spectrum of the monomer shows peaks at 297, 289, and 266 nm. The spectrum of the polymer has a much broader band with several fine structures at 261, 293, 350 and 380 nm. The overall absorption tails off to about 600 nm. The longer wavelength indicated longer polymer sequences [14]. The UV spectral results also confirmed that a conjugated polymer with broad molar mass distribution was obtained. The UV spectra of the PTNs deposited from the mixed electrolytes are similar to that shown in Fig. 4b, indicating similar chain structures.
Fig. 6. Fluorescence spectrum of dedoped PTN prepared by using BFEE as electrolyte. Solvent: DMSO.
F. Wang et al. / Journal of Electroanalytical Chemistry 510 (2001) 29–34
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protons are moved to much lower fields, like strong electron withdrawing group-substituted thianaphthene derivatives [15], which confirms the formation of a conjugated delocalizing structure. The proton number ratios of a:b:c:d are calculated to be 1.00:2.04:0.32:0.55. Accordingly, the ring coupling reaction during the polymerization process eliminated the protons at the ‘c’ and ‘d’ positions of the monomer, while the ‘a’ and ‘b’ protons remained unchanged. Thus, the structure of the polymer can be reasonably postulated as shown in the insert of Fig. 5B. The excitation spectrum of the polymer has several peaks at about 310, 350 and 390 nm as shown in Fig. 6. The emission spectrum is characterized by a dominant maximum at 420 nm and a shoulder band at about 510 nm. These results indicate that the polymer is a good blue light emitter. The PTN prepared from the mixed electrolytes presents almost the same fluorescence spectra.
3.3. Morphology and conducti6ity The polymer film prepared electrochemically by using pure BFEE as the electrolyte is rough and irregular as is shown in its scanning electron micrograph (Fig. 7a). The film is cauliflower-like and has many cracks. On the other hand, the film obtained from the system by using BFEE+ 5% SA as the electrolyte was compact (Fig. 7b). The size of the cauliflowers is much smaller than that shown in Fig. 7a and no cracks appeared. However, the film cannot be peeled off to a free-standing state and can be removed only by scraping with a knife. Fig. 7c shows the transmission electron micrograph of the scraped polymer which shows that it is sheet-like. Dedoped PTN is soluble in DMSO and can be cast into a compact, smooth and transparent film. The conductivity of the as-formed PTN film prepared from pure BFEE was found to be 10 S cm − 1. However, the PTN film synthesized from the mixed electrolyte of BFEE+5% SA was found to be 35 S cm − 1. This value is comparable to that of poly(isothianaphthene) ( 50 S cm − 1) [4]. Fig. 7. Scanning electron micrographs of the PTN films deposited on the electrode surface from BFEE (a), BFEE + 5% SA (b), and a transmission electron micrograph of the PTN scraped from the electrode surface (c).
Fig. 5A is the H1-NMR spectrum of the monomer. This figure shows four groups of protons: 7.38 (2H), 7.47 (1H), 7.77 (1H), 7.90 (1H) and 8.01 (1H). The assignments of the NMR lines are shown in the insert of this spectrum. Fig. 5B presents the H1-NMR spectrum of PTN. This spectrum also shows four groups of protons, while the proton lines are much broader and have fewer fine structures than the corresponding proton lines shown in Fig. 5A. The chemical shifts of the
4. Conclusions Thianaphthene can be electrochemically polymerized in BFEE. However, the current efficiency is low and the film quality is poor. Addition of 5% (by volume) of concentrated sulfuric acid into BFEE increased the current efficiency of polymerization by about 4 times. A compact PTN film was formed on the electrode and the conductivity of the film was measured as 35 S cm − 1. The polymer in the doped state is dark blue in color and insoluble. However, the polymer in the dedoped state is soluble in strong polar solvents and a compact
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and smooth film can be obtained by casting its solution. The PTN prepared in this system has a strong fluorescence in the blue light region. The material may have the potential to be used in many commercial applications, including lasers [16,17].
Acknowledgements We thank the Natural Science Foundation of China for support of this work (nos. 29773019 and 50073012)
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