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Spectroelectrochemical studies of poly(5-cyanoindole) in aqueous medium
ELSEVIER
Synthetic Metals 101 (1999) 117
SPECTROELECTROCHEMICAL
STUDIES
OF
POLY(5CYANOINDOLE)
IN
AQUEOUS
MEDIUM
H. Talbil, A. Pron2, G. Louam and D. Billaudl ILCSM, UMR 75.55, Universite Henri Poincare Nancy I, BP. 239, 54506 Vandoeuvre-l&s-Nancy Cedex, France 2LPMS, UMR 585 (CEA-CNRS- UJF), DRFMC, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France 3Laboratoire de Physique Cristalline, Institut des Materiaux de Nantes, 44072 Nantes, France Abstract The electrochemical redox processes of poly(5-cyanoindole) (PSCN) in acidic aqueous solutions were investigated using in situ Raman and optical absorption spectroscopy. Vibrational assignments were made on the basis of the results obtained on polyindole and PSCN in acetonittile solution. The drastic changes in optical absorption and Raman spectra observed at various oxidation stages were explained by the conversions between at least three different structures. Keywords
: Polymer;
Poly(S-cyanoindole);
Spectroelectrochemistry;
Poly(S-cyanoindole) shows excellent redox switching in aqueous acidic medium and a good sensitivity to pH [l]. The mechanisms of the various transitions and especially the influence of protons are still debated. The purpose of this paper is to obtain information on the structure and to characterize the different modifications on the PSCN the oxidation processes. backbone, induced by Spectmelectmchemical studies is a very useful method in characterizing thin polymer f&n electrodes in situ during electrochemical oxidation. Details of the Raman method and the polymer polymerization process have been published elsewhere [ 1.21. The as-grown films were removed from the growth solution and washed in mixturcs of acetonitrile and water containing increasing amounts of water. The polymer electrode was then immersed into an aqueous 2 mol dmw3 HC104 solution. Then, the potential was cycled with a sweep rate of 10 mV.s-J and Raman spectra were recorded for various selected potentials during oxidation. The spectra recorded during the reduction process are similar to those obtained at the same potential during the oxidation wave. This fact indicates that the process and the modification of the spectra are reversible and does not include degradation of the polymer. The m-situ Raman spectra recorded with a 676.4 nm line for polymers oxidized in HC104 (2mol dme3) are shown in Fig.1. The main characteristic bands observed on the spectra recorded at -0.15 V can be attributed as follow : Raman lines at 1650 and 1578 cm-l are associated with the C-C bond of benzene ring, the band at 1333 cm-l is related to uC8N+FC6H+FC2C3 and that at 1230 cm-l is attributed to CN vibrational modes. The oxidation at 0.4 V is accompanied by the disappearance of bands at 1650 and 1578 cm-l. During the first redox process (from -0.15 to 0.72 V), the most important changes occur in the 1250-1350 cm-l region. The intensity of the band at 1333 cm-l continuously decreases and completely disappears on the spectra recorded at 0.81 V. A new peak appears at 1290 cm-l. Noticeable spectral changes occur during the second redox process (0.72-1.25 V). Two bands appears at 1325 and 1377 cm ml attributed to C-N and C=N stretching vibrations respectively. The intensity of these bands shows a continuous increase with increasing electrode potential. Simultaneously, two new bands, due to the C-C stretching vibrations in benzenic and quinoid rings appears respectively at 1555
Resonance Raman spectroscopy. and 1625 cm-l. An additional vibrational band occurs at 1459 cm -l, assigned to a mixed mode related to C=N and C=C stretching vibrations. During the fast redox process, polarons are formed in the polymer chain and there is equilibrium between neutral and radical cation forms. With potential moving in anodic direction between 0.72-0.97 V, polarons are transferred to bipolarons and two types of defects co-exists in the polymer lattice. When the potential is higher than 0.97 V, these non equilibrium polaron defects are completely converted to form the more stable bipolarons. Modifications occurring on the Raman spectra can be explained by the coexistence, with different proportions, of neutral species, radical cations and dications. There is a continuum of oxidation states ranging all the way from the completely reduced form of PSCN to the completely oxidized one. The degree of protonation is not known for all these species. For the completely oxidized form, it is highly probable that there is very little or essentially no protonation of the product.
600
800
Raman
loo0
shifi
1200
wavenumbers
1400
1600
(cm“)
F i g . 1 : Rarnanspectra(676.4nm excitation line) of PSCNat variom stagesof oxidation in HC104 (2 mol.dm‘3). References [l] H. Talbi & D. Billaud, Synth. Met.,93 (2) (1998) 105-l 10. [2] H. Talbi, B. Humbert & D. Billaud, Spectrochim. Acta. In press.
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: so379-6779(98)012971
Synthetic Metals 101 (1999) 117
SPECTROELECTROCHEMICAL
STUDIES
OF
POLY(5CYANOINDOLE)
IN
AQUEOUS
MEDIUM
H. Talbil, A. Pron2, G. Louam and D. Billaudl ILCSM, UMR 75.55, Universite Henri Poincare Nancy I, BP. 239, 54506 Vandoeuvre-l&s-Nancy Cedex, France 2LPMS, UMR 585 (CEA-CNRS- UJF), DRFMC, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France 3Laboratoire de Physique Cristalline, Institut des Materiaux de Nantes, 44072 Nantes, France Abstract The electrochemical redox processes of poly(5-cyanoindole) (PSCN) in acidic aqueous solutions were investigated using in situ Raman and optical absorption spectroscopy. Vibrational assignments were made on the basis of the results obtained on polyindole and PSCN in acetonittile solution. The drastic changes in optical absorption and Raman spectra observed at various oxidation stages were explained by the conversions between at least three different structures. Keywords
: Polymer;
Poly(S-cyanoindole);
Spectroelectrochemistry;
Poly(S-cyanoindole) shows excellent redox switching in aqueous acidic medium and a good sensitivity to pH [l]. The mechanisms of the various transitions and especially the influence of protons are still debated. The purpose of this paper is to obtain information on the structure and to characterize the different modifications on the PSCN the oxidation processes. backbone, induced by Spectmelectmchemical studies is a very useful method in characterizing thin polymer f&n electrodes in situ during electrochemical oxidation. Details of the Raman method and the polymer polymerization process have been published elsewhere [ 1.21. The as-grown films were removed from the growth solution and washed in mixturcs of acetonitrile and water containing increasing amounts of water. The polymer electrode was then immersed into an aqueous 2 mol dmw3 HC104 solution. Then, the potential was cycled with a sweep rate of 10 mV.s-J and Raman spectra were recorded for various selected potentials during oxidation. The spectra recorded during the reduction process are similar to those obtained at the same potential during the oxidation wave. This fact indicates that the process and the modification of the spectra are reversible and does not include degradation of the polymer. The m-situ Raman spectra recorded with a 676.4 nm line for polymers oxidized in HC104 (2mol dme3) are shown in Fig.1. The main characteristic bands observed on the spectra recorded at -0.15 V can be attributed as follow : Raman lines at 1650 and 1578 cm-l are associated with the C-C bond of benzene ring, the band at 1333 cm-l is related to uC8N+FC6H+FC2C3 and that at 1230 cm-l is attributed to CN vibrational modes. The oxidation at 0.4 V is accompanied by the disappearance of bands at 1650 and 1578 cm-l. During the first redox process (from -0.15 to 0.72 V), the most important changes occur in the 1250-1350 cm-l region. The intensity of the band at 1333 cm-l continuously decreases and completely disappears on the spectra recorded at 0.81 V. A new peak appears at 1290 cm-l. Noticeable spectral changes occur during the second redox process (0.72-1.25 V). Two bands appears at 1325 and 1377 cm ml attributed to C-N and C=N stretching vibrations respectively. The intensity of these bands shows a continuous increase with increasing electrode potential. Simultaneously, two new bands, due to the C-C stretching vibrations in benzenic and quinoid rings appears respectively at 1555
Resonance Raman spectroscopy. and 1625 cm-l. An additional vibrational band occurs at 1459 cm -l, assigned to a mixed mode related to C=N and C=C stretching vibrations. During the fast redox process, polarons are formed in the polymer chain and there is equilibrium between neutral and radical cation forms. With potential moving in anodic direction between 0.72-0.97 V, polarons are transferred to bipolarons and two types of defects co-exists in the polymer lattice. When the potential is higher than 0.97 V, these non equilibrium polaron defects are completely converted to form the more stable bipolarons. Modifications occurring on the Raman spectra can be explained by the coexistence, with different proportions, of neutral species, radical cations and dications. There is a continuum of oxidation states ranging all the way from the completely reduced form of PSCN to the completely oxidized one. The degree of protonation is not known for all these species. For the completely oxidized form, it is highly probable that there is very little or essentially no protonation of the product.
600
800
Raman
loo0
shifi
1200
wavenumbers
1400
1600
(cm“)
F i g . 1 : Rarnanspectra(676.4nm excitation line) of PSCNat variom stagesof oxidation in HC104 (2 mol.dm‘3). References [l] H. Talbi & D. Billaud, Synth. Met.,93 (2) (1998) 105-l 10. [2] H. Talbi, B. Humbert & D. Billaud, Spectrochim. Acta. In press.
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: so379-6779(98)012971