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Electrochemical synthesis of electroactive polytriphenylamine
Synthetic Metals 108 Ž2000. 245–247 www.elsevier.comrlocatersynmet
Short communication
Electrochemical synthesis of electroactive polytriphenylamine Andreas Petr a , Carita Kvarnstrom ¨ b, Lothar Dunsch a
a,)
, Ari Ivaska
b
Group of Electrochemistry and Conducting Polymers, Institute of Solid State and Materials Research Dresden, D-01107 Dresden, Germany b ˚ Akademi, FIN-20500 Abo-Turku, ˚ Laboratory of Analytical Chemistry, Abo Finland Received 17 May 1999; accepted 30 June 1999
Abstract In this paper the electrochemical synthesis of polytriphenylamine is shown for the first time. The electrochemical oxidation of triphenylamine ŽTPA. in nonpolar solvents leads to the formation of oligomers of TPA in solution and the formation of a polymer film with TPA as repeating unit. Cyclic voltammetry of these electrochemically produced TPA films is compared to that of chemically produced polytriphenylamine what demonstrates a similar electrochemical behaviour. From FTIR spectra of the polymer film it is concluded that the electrochemically formed polymer is more crosslinked than the chemically produced one. Therefore, it is a very stable redox polymer and a promising candidate for application in batteries, electrochromic displays, sensors or organic electronic devices. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Polytriphenylamine; Electrochemistry; Polymerization; FTIR
1. Introduction As polytriphenylamine ŽPTPA. is a good candidate for the use in electrochromic displays, organic LED and for charge storage several patents pushed forward the great expectations for industrial application of that polymer w1– 4x. Until now the preparation of PTPA was only possible by chemical multistep synthesis w5,6x. In former investigations of the electrochemical oxidation of triphenylamine ŽTPA. only tetraphenylbenzidine as a dimerization product was found as oxidation product by cyclic voltammetry w7,8x. As it is demonstrated here there is a way to produce electrically conducting PTPA films on top of an electrode by electrochemical oxidation of TPA. These electrically conducting films of PTPA exhibit promising properties like very high stability during oxidationrreduction cycles as well as electrochromic and hole conducting properties.
2. Experimental Potentiodynamic preparation of films was done with an EG & G Princeton Applied Research potentiostatrgalvanostat Model 273, while for galvanostatic film preparation a potentiostat PG285 ŽHEKA-Elektronik, Lambrecht, Ger)
Corresponding author.
many. was used. The input voltage was provided by a home made program driving a DA plug-in board ŽACAO, Strawberrysoft, Sunnyvale, CA, USA.. A silver chloride coated silver wire was used as pseudo-reference electrode. All potentials mentioned in this paper are referred to the ferrocene couple. For IR measurements a Mattson Cygnus 100 FTIR spectrometer with an MCT detector was used. The external reflection system built from a variable angle specular reflectance attachment from Specac is described in Ref. w9x. TPA, acetonitrile and toluene were used as purchased of analytical grade ŽFluka.. The supporting electrolyte tetrabutylammoniumhexafluorophosphate ŽBu 4 NPF6 . of the same supplier were dried for 2 h at 708C in a vacuum oven.
3. Results and discussion The oxidation of TPA in polar solvents at low current densities leads generally to the formation of tetraphenylbenzidine as no further reaction at the nitrogen like in aniline is possible. However, in less polar electrolytes and at higher current densities, oligomers of TPA are formed resulting in adhering PTPA films at the electrodes. One reason for this new reaction pathway might be the lower
0379-6779r00r$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 3 7 - X
246
A. Petr et al.r Synthetic Metals 108 (2000) 245–247
Fig. 1. Cyclic voltammogram of an anodically produced PTPA film under potentiodynamic conditions on a Pt electrode in 0.1 m Bu 4 NPF6 racetonitrile. Scan rate: 50 mVrs.
solubility of the formed TPA cation and its dimer in nonpolar solvents than in polar solvents. Therefore, a precipitation of oligomers at the electrode surface in nonpolar solvents occur what is accessible to further electrochemical reactions. In polar solvents all formed oligomers diffuse into the bulk solution. The feature of our synthesis is the use of nonpolar solvents resp. mixtures of nonpolar solvents with polar solvents. The best results were obtained in acetonitrile ŽMeCN.rtoluene Ž1:4. solutions containing 0.06 m Bu 4 NPF6 and 0.05 m TPA. The formation of polymer films occurred by potentiostatic cycling between y0.1 and 1.1 V vs. ferrocene couple. The same is obtained galvanostatically with a current density of 0.5–1 mArcm2 and changing the direction of the current every second. A cyclic voltammogram of a PTPA film on platinum electrode prepared in the potentiodynamic way is shown in Fig. 1. In Fig. 2 a cyclic voltammogram of a galvanostatically produced PTPA film is given. The cyclic voltammograms are nearly the same but galvanostatically produced PTPA films are more symmetric in their anodic and cathodic behaviour. Both cyclic voltammograms are very similar to that of the chemically prepared films w6,10x. Therefore, the redox reaction in both kinds of PTPA seems
Fig. 3. External reflection FTIR spectrum of an electrochemically produced PTPA film under potentiodynamic conditions on a Pt electrode.
to be the same and the formation of charged species energetically equal. The structural equivalency was checked spectroscopically. The FTIR spectrum of an electrochemically produced PTPA film is shown in Fig. 3. This spectrum was taken from a film on a Pt foil by external reflection. The peaks at 1596 and 1275 cmy1 are broader than in the monomer and shifted from their position in the monomer spectra to higher wave numbers. In the region 900 to 1200 cmy1 , the peak profile has changed due to higher concentration of para substituted phenylene rings present in the film in comparison to mono-substituted phenyl in TPA. The increase in number of new bonds in the para position of the benzoic structures points to a coupling of the TPA units via phenyl substituents. Therefore, the broadened absorption peaks in comparison to the chemically produced PTPA indicate a more crosslinked structure of the electrochemically produced PTPA. It is to be stated finally that the electrochemical synthesis is much simpler than the chemical one as this is a one-step reaction avoiding Grignard reagents.
4. Conclusions
Fig. 2. Cyclic voltammogram of an anodically produced PTPA film under galvanostatic conditions on a Pt electrode in 0.1 m Bu 4 NPF6 racetonitrile. Scan rate: 50 mVrs.
It is shown for the first time that the electrochemical oxidation of TPA in nonpolar electrolytes yields electrically conducting films of PTPA at the electrode surface. The films are stable in air and show only little decrease in redox activity during cycling. Electrochemically produced PTPA films are more crosslinked than chemically produced ones as demonstrated by IR spectroscopy. The electrochemical polymerisation of PTPA is much more simple than the multistep chemical synthesis. Furthermore, the electrochemically formed films are preferable as no additional crosslinking is necessary to produce films stable in organic solvents. Therefore, this new electrochemical route for the preparation of a promising material is under further study.
A. Petr et al.r Synthetic Metals 108 (2000) 245–247
Acknowledgements The authors thank the Deutscher Akademischer Austauschdienst ŽDAAD. and the Finnish Academy of Science for financial support of this work.
References w1x Y. Murofushi, M. Ishikawa, Patent JP 6119584, 1985. w2x H. Miyagi, Y. Murofushi, M. Ishikawa, Patent JP 61168625, 1985.
247
w3x H. Miyagi, H. Tabata, I. Ito, E. Sekine, T. Aikawa, Patent JP 63063780, 1986. w4x K. Kaeriyama, M. Suda, M. Sato, Y. Osawa, M. Yasohiko, M. Kawai, Patent JP 63168974, 1987. w5x M. Kawai, M. Ishikawa, US Patent 4565860, 1986. w6x M. Ishikawa, M. Kawai, Y. Ohsawa, Synth. Met. 40 Ž1991. 231–238. w7x R.F. Nelson, R.N. Adams, Am. Chem. Soc. Div. Fuel Chem. 11 Ž1. Ž1967. 74–76, Preprint. w8x W.H. Bruning, R.F. Nelson, L.S. Marcoux, R.N. Adams, J. Phys. Chem. 71 Ž1967. 3055. w9x C. Kvarnstrom, ¨ A. Ivaska, Synth. Met. 62 Ž1994. 125–131. w10x Y. Ohsawa, M. Ishikawa, T. Miyamota, Y. Murofushi, M. Kawai, Synth. Met. 18 Ž1987. 371–374.
Short communication
Electrochemical synthesis of electroactive polytriphenylamine Andreas Petr a , Carita Kvarnstrom ¨ b, Lothar Dunsch a
a,)
, Ari Ivaska
b
Group of Electrochemistry and Conducting Polymers, Institute of Solid State and Materials Research Dresden, D-01107 Dresden, Germany b ˚ Akademi, FIN-20500 Abo-Turku, ˚ Laboratory of Analytical Chemistry, Abo Finland Received 17 May 1999; accepted 30 June 1999
Abstract In this paper the electrochemical synthesis of polytriphenylamine is shown for the first time. The electrochemical oxidation of triphenylamine ŽTPA. in nonpolar solvents leads to the formation of oligomers of TPA in solution and the formation of a polymer film with TPA as repeating unit. Cyclic voltammetry of these electrochemically produced TPA films is compared to that of chemically produced polytriphenylamine what demonstrates a similar electrochemical behaviour. From FTIR spectra of the polymer film it is concluded that the electrochemically formed polymer is more crosslinked than the chemically produced one. Therefore, it is a very stable redox polymer and a promising candidate for application in batteries, electrochromic displays, sensors or organic electronic devices. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Polytriphenylamine; Electrochemistry; Polymerization; FTIR
1. Introduction As polytriphenylamine ŽPTPA. is a good candidate for the use in electrochromic displays, organic LED and for charge storage several patents pushed forward the great expectations for industrial application of that polymer w1– 4x. Until now the preparation of PTPA was only possible by chemical multistep synthesis w5,6x. In former investigations of the electrochemical oxidation of triphenylamine ŽTPA. only tetraphenylbenzidine as a dimerization product was found as oxidation product by cyclic voltammetry w7,8x. As it is demonstrated here there is a way to produce electrically conducting PTPA films on top of an electrode by electrochemical oxidation of TPA. These electrically conducting films of PTPA exhibit promising properties like very high stability during oxidationrreduction cycles as well as electrochromic and hole conducting properties.
2. Experimental Potentiodynamic preparation of films was done with an EG & G Princeton Applied Research potentiostatrgalvanostat Model 273, while for galvanostatic film preparation a potentiostat PG285 ŽHEKA-Elektronik, Lambrecht, Ger)
Corresponding author.
many. was used. The input voltage was provided by a home made program driving a DA plug-in board ŽACAO, Strawberrysoft, Sunnyvale, CA, USA.. A silver chloride coated silver wire was used as pseudo-reference electrode. All potentials mentioned in this paper are referred to the ferrocene couple. For IR measurements a Mattson Cygnus 100 FTIR spectrometer with an MCT detector was used. The external reflection system built from a variable angle specular reflectance attachment from Specac is described in Ref. w9x. TPA, acetonitrile and toluene were used as purchased of analytical grade ŽFluka.. The supporting electrolyte tetrabutylammoniumhexafluorophosphate ŽBu 4 NPF6 . of the same supplier were dried for 2 h at 708C in a vacuum oven.
3. Results and discussion The oxidation of TPA in polar solvents at low current densities leads generally to the formation of tetraphenylbenzidine as no further reaction at the nitrogen like in aniline is possible. However, in less polar electrolytes and at higher current densities, oligomers of TPA are formed resulting in adhering PTPA films at the electrodes. One reason for this new reaction pathway might be the lower
0379-6779r00r$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 3 7 - X
246
A. Petr et al.r Synthetic Metals 108 (2000) 245–247
Fig. 1. Cyclic voltammogram of an anodically produced PTPA film under potentiodynamic conditions on a Pt electrode in 0.1 m Bu 4 NPF6 racetonitrile. Scan rate: 50 mVrs.
solubility of the formed TPA cation and its dimer in nonpolar solvents than in polar solvents. Therefore, a precipitation of oligomers at the electrode surface in nonpolar solvents occur what is accessible to further electrochemical reactions. In polar solvents all formed oligomers diffuse into the bulk solution. The feature of our synthesis is the use of nonpolar solvents resp. mixtures of nonpolar solvents with polar solvents. The best results were obtained in acetonitrile ŽMeCN.rtoluene Ž1:4. solutions containing 0.06 m Bu 4 NPF6 and 0.05 m TPA. The formation of polymer films occurred by potentiostatic cycling between y0.1 and 1.1 V vs. ferrocene couple. The same is obtained galvanostatically with a current density of 0.5–1 mArcm2 and changing the direction of the current every second. A cyclic voltammogram of a PTPA film on platinum electrode prepared in the potentiodynamic way is shown in Fig. 1. In Fig. 2 a cyclic voltammogram of a galvanostatically produced PTPA film is given. The cyclic voltammograms are nearly the same but galvanostatically produced PTPA films are more symmetric in their anodic and cathodic behaviour. Both cyclic voltammograms are very similar to that of the chemically prepared films w6,10x. Therefore, the redox reaction in both kinds of PTPA seems
Fig. 3. External reflection FTIR spectrum of an electrochemically produced PTPA film under potentiodynamic conditions on a Pt electrode.
to be the same and the formation of charged species energetically equal. The structural equivalency was checked spectroscopically. The FTIR spectrum of an electrochemically produced PTPA film is shown in Fig. 3. This spectrum was taken from a film on a Pt foil by external reflection. The peaks at 1596 and 1275 cmy1 are broader than in the monomer and shifted from their position in the monomer spectra to higher wave numbers. In the region 900 to 1200 cmy1 , the peak profile has changed due to higher concentration of para substituted phenylene rings present in the film in comparison to mono-substituted phenyl in TPA. The increase in number of new bonds in the para position of the benzoic structures points to a coupling of the TPA units via phenyl substituents. Therefore, the broadened absorption peaks in comparison to the chemically produced PTPA indicate a more crosslinked structure of the electrochemically produced PTPA. It is to be stated finally that the electrochemical synthesis is much simpler than the chemical one as this is a one-step reaction avoiding Grignard reagents.
4. Conclusions
Fig. 2. Cyclic voltammogram of an anodically produced PTPA film under galvanostatic conditions on a Pt electrode in 0.1 m Bu 4 NPF6 racetonitrile. Scan rate: 50 mVrs.
It is shown for the first time that the electrochemical oxidation of TPA in nonpolar electrolytes yields electrically conducting films of PTPA at the electrode surface. The films are stable in air and show only little decrease in redox activity during cycling. Electrochemically produced PTPA films are more crosslinked than chemically produced ones as demonstrated by IR spectroscopy. The electrochemical polymerisation of PTPA is much more simple than the multistep chemical synthesis. Furthermore, the electrochemically formed films are preferable as no additional crosslinking is necessary to produce films stable in organic solvents. Therefore, this new electrochemical route for the preparation of a promising material is under further study.
A. Petr et al.r Synthetic Metals 108 (2000) 245–247
Acknowledgements The authors thank the Deutscher Akademischer Austauschdienst ŽDAAD. and the Finnish Academy of Science for financial support of this work.
References w1x Y. Murofushi, M. Ishikawa, Patent JP 6119584, 1985. w2x H. Miyagi, Y. Murofushi, M. Ishikawa, Patent JP 61168625, 1985.
247
w3x H. Miyagi, H. Tabata, I. Ito, E. Sekine, T. Aikawa, Patent JP 63063780, 1986. w4x K. Kaeriyama, M. Suda, M. Sato, Y. Osawa, M. Yasohiko, M. Kawai, Patent JP 63168974, 1987. w5x M. Kawai, M. Ishikawa, US Patent 4565860, 1986. w6x M. Ishikawa, M. Kawai, Y. Ohsawa, Synth. Met. 40 Ž1991. 231–238. w7x R.F. Nelson, R.N. Adams, Am. Chem. Soc. Div. Fuel Chem. 11 Ž1. Ž1967. 74–76, Preprint. w8x W.H. Bruning, R.F. Nelson, L.S. Marcoux, R.N. Adams, J. Phys. Chem. 71 Ž1967. 3055. w9x C. Kvarnstrom, ¨ A. Ivaska, Synth. Met. 62 Ž1994. 125–131. w10x Y. Ohsawa, M. Ishikawa, T. Miyamota, Y. Murofushi, M. Kawai, Synth. Met. 18 Ž1987. 371–374.