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Oxidized model compounds of polyaniline studied by resonance Raman spectroscopy
synthetic
Metals
84 (1997)
787-788
Oxidized model compounds of polyaniline studied by resonance Raman spectroscopy M. Boyera, S. Quillarda, G. Louam=, S. Lefranta, E. Rebourtb, A. P. Monkmanb aL.P.C., Institw des Mat5iau.x de Nantes, 2 rue de la Houssini&e, 44072 Nantes Ce’dex 03, France* bMolecular Electronics Group, SEAS, University of Durham, Science laboratories, South Road, Durham, DHl3LE,
UK
Abstract
The emeraldine salt structure, the only conducting form of polyaniline is not well-known. This is why, we have carried out resonance Raman spectra (RRS) on one of these models compounds oxidized electrochemically in-situ in order to elucidate the electronic structure of this polymer partially oxidized in an acidic medium. A phenyl end-capped dimer of polyaniline, N,N’ diphenyl-p-phenylenediamine (named PCD), was selected. This dimer has been electrochemically oxidized in acetonitrile containing 0,2 M tetrabutylammonium tenafluoroborate as ground salt and 0.1 M diphenyl phosphate as acid. The choice of laser excitation wavelength was fixed by UV-Vis absorption measurements on PCD in the acidic medium. Numerous similarities between emeraldine salt and radical species created were observed and are discussed in this paper. Keywords : polyaniline,
1.
model compounds,
resonance
raman spectroscopy,
Introduction
Detailed measurements of the electrochemistry, optical and vibrational properties of oligomers were confirmed to form a basis for understanding the structure and behavior of a polymer in its doped and undoped form [ 11. In this paper, our purpose is to obtain information on the electronic structure of emeraldine salt by carrying out RRS measurements on PCD oxidized electrochemically in an acidic medium. Vibrational modes of radical cation of PCD were compared with those of PCD and N,N’ diphenyl-pbenzoquinonediimine named OPCD. In this work, calculated frequencies of the radical cation were reported. The theoretical frequencies are obtained in the frame of a dynamical model based on valence force field calculations. More details about these calculations are reported in the literature [2]. 2.
Experimental
results
2.1. Cyclic volrammetry For all the experiments, a classical three-electrode configuration with a platinum working electrode, a platinum counter electrode, and an Ag/Ag+ reference electrode was used. Cyclic voltammograms were obtained with a PAR 273 potentiostat. The cyclic voltammogram of the oligomer displays two reversible and well-resolved waves. The first anodic peak was assigned to the formation of a radical cation and the second to the imine form [3] The dimer undergoes these two one0379-6779/97/$17,00
Q 1997
PII SO379-6779(96)04146,
Elsevier
Science
Sk
Ail
rights
reserved
in situ electrochemical
spectroscopy.
electron transfers at 300 and 600 mV vs. Ag/Ag+. In this paper, only the Raman results obtained on the radical specie created after the first anodic peak are presented. 2.2. In situ UV - vis. absorption
studies
In order to obtain the convenient wavelength to enhance features of the radical cation, UV-Vis spectra of PCD were measured at different applied potentials. The obtained results are very simila? to those~ reported in the literature [4]. Before oxidation, PCD shows a peak located at 300 run. Upon increase of the applied potential, the sample turns from colorless to blue and the spectrum changes dramatically showing two new peaks at 390 and 700 run which increase together whereas the band at 300 MI decreases. The two new bands are associated to the radical cation. 2.3. In situ Raman spectroelectrochemical
studies
A good method to detail the similarities between the oligomers and the polymer is to use the Raman spectroscopy technique. Raman spectra were recorded on a multichannel Jobin-Yvon T64000 spectrometer equipped with a CCD detector. Spectra are similar to those presented in the literature [5]. They were recorded in-situ at different potentials for several excitation wavelengths namely h = 457.9, 514.5, 676.4 and 1064 run. Only the spectrum obtained at h = 676.4 nm is presented, providing the Raman lines originating from oxidized unit(s). This spectrum was obtained at 500 mV, before the second anodid peak, in order to characterize the radical cation of PCD.
788
M. Boyer etal. /SyntheticMetals
84 (1997)
787-788
Table 1 : Observed and calculated Raman frequencies and vibrational assignments of in plane vibrations of PCD, radical cation of PCD and OPCD at h = 676&l nm (* T means terminal rings and I internal ring) notation mode (Wilson) 8a 8a amine 8b amine amine 9a
PCD Exp. 1619
FCD’O Exp. 1628 1586 1499 1441 1400 1226 1176
Calc. 1602 1609 1430 1571
1424
1221 1187
1229 1171
Table 2 : Raman frequencies of radical cation of PCD and emeraldine salt. (B : Benzenoid, Q : Quinoid). radical cation of PCD 1628 1586 1499 1441 1400/1378 1226 1176
emeraldine salt (bipolaron structure) 1621 1581 1515 1484 1332/1311 1253 1188
Assignments
C C N C C C C
- C stretch. (B) = C stretch.(q) - H bend. = C stretch.(Q) - N stretch.(Q) - N stretch. (B) - H bend. (B)
OPCD
Assignments
Exp. 1622 1587
Calc. 1608 1593
1415 1515 1214 1157
1421 1514 1214 1160
C - C stretching (T)* C = C stretching (I)* N - H bending C = C stretching (I) C=NorC-Nstretching(I) C - N stretching (T) C - H bending (I)
In this case, we compared the frequencies of the radical cation to those of PCD and OPCD (N.N’ diphenyl parabenzoquinone diimine). The assignment proposal of the modes of the radical cation as well as those of PCD [6] and OPCD are presented Table 1. Numerous vibrational modes of the radical cation of PCD can be related to corresponding ones of emeraldine salt (Table 2). In particular, the doublet at 1400/1378 cm” in the radical cation is observed at 1332/1311 cm” in the polymeric salt. The double bands is assigned to the C-N stretching vibration and reveals the existence of a semiquinoid structure. This observation is confiied by the values of the corresponding force constants, obtained in our dynamical calculations. The complete set of force constants will be published elsewhere.
4.
Conclusion
From this work, we can notice that the radical cation of PCD is a good model compound of emeraldine salt. Our results are in agreement with those of Shacklette and al [3] for radical structure proposal of PCD. In order to go further, a study of longer oligomers to get closer to the structure of cmeraldine salt is under way, as well as of phenyl-end-capped tetramer of polyaniline.
References 1. 2. 3.
800
1000
1200
1403
Wavenum ber s (cm-l) Figure 1 : Raman spectra at h = 676,44 nm of a) PCD b) radical cation of PCD c) OPCD
1600
4. 5. 6,
A. P. Monkman, D. Bloor, G. C. Stevens, J. C. H. Stevens, and P. Wilson. Synth. Met., 29 (1989) E277 G. Louam, M. Lapkowski, S. Quillard, A. Pron, J. P. Buisson, and S. Lefrant. J. Phys. Chem. 100 (1996) 6998 L. W. Shacklette, J. F. Wolf, S. Gould, and R. H. Baughman. J. Chem. Phys. 88 (1988) 3955. P. M. MacManus, S. C. Yang, and R. J. Cushman. J. Chem. Sot., Chem. Commun., (1985) 1556. I. Harada, Y. Furukawa, and F. Ueda. Synth. Met., 29 (1989) E303. S. Quillard, G. Louam, J. P. Buisson, S. Lefrant, J. Masters, and A. G. MacDiarmid. Solid State Sciences, 107 (1991)
Metals
84 (1997)
787-788
Oxidized model compounds of polyaniline studied by resonance Raman spectroscopy M. Boyera, S. Quillarda, G. Louam=, S. Lefranta, E. Rebourtb, A. P. Monkmanb aL.P.C., Institw des Mat5iau.x de Nantes, 2 rue de la Houssini&e, 44072 Nantes Ce’dex 03, France* bMolecular Electronics Group, SEAS, University of Durham, Science laboratories, South Road, Durham, DHl3LE,
UK
Abstract
The emeraldine salt structure, the only conducting form of polyaniline is not well-known. This is why, we have carried out resonance Raman spectra (RRS) on one of these models compounds oxidized electrochemically in-situ in order to elucidate the electronic structure of this polymer partially oxidized in an acidic medium. A phenyl end-capped dimer of polyaniline, N,N’ diphenyl-p-phenylenediamine (named PCD), was selected. This dimer has been electrochemically oxidized in acetonitrile containing 0,2 M tetrabutylammonium tenafluoroborate as ground salt and 0.1 M diphenyl phosphate as acid. The choice of laser excitation wavelength was fixed by UV-Vis absorption measurements on PCD in the acidic medium. Numerous similarities between emeraldine salt and radical species created were observed and are discussed in this paper. Keywords : polyaniline,
1.
model compounds,
resonance
raman spectroscopy,
Introduction
Detailed measurements of the electrochemistry, optical and vibrational properties of oligomers were confirmed to form a basis for understanding the structure and behavior of a polymer in its doped and undoped form [ 11. In this paper, our purpose is to obtain information on the electronic structure of emeraldine salt by carrying out RRS measurements on PCD oxidized electrochemically in an acidic medium. Vibrational modes of radical cation of PCD were compared with those of PCD and N,N’ diphenyl-pbenzoquinonediimine named OPCD. In this work, calculated frequencies of the radical cation were reported. The theoretical frequencies are obtained in the frame of a dynamical model based on valence force field calculations. More details about these calculations are reported in the literature [2]. 2.
Experimental
results
2.1. Cyclic volrammetry For all the experiments, a classical three-electrode configuration with a platinum working electrode, a platinum counter electrode, and an Ag/Ag+ reference electrode was used. Cyclic voltammograms were obtained with a PAR 273 potentiostat. The cyclic voltammogram of the oligomer displays two reversible and well-resolved waves. The first anodic peak was assigned to the formation of a radical cation and the second to the imine form [3] The dimer undergoes these two one0379-6779/97/$17,00
Q 1997
PII SO379-6779(96)04146,
Elsevier
Science
Sk
Ail
rights
reserved
in situ electrochemical
spectroscopy.
electron transfers at 300 and 600 mV vs. Ag/Ag+. In this paper, only the Raman results obtained on the radical specie created after the first anodic peak are presented. 2.2. In situ UV - vis. absorption
studies
In order to obtain the convenient wavelength to enhance features of the radical cation, UV-Vis spectra of PCD were measured at different applied potentials. The obtained results are very simila? to those~ reported in the literature [4]. Before oxidation, PCD shows a peak located at 300 run. Upon increase of the applied potential, the sample turns from colorless to blue and the spectrum changes dramatically showing two new peaks at 390 and 700 run which increase together whereas the band at 300 MI decreases. The two new bands are associated to the radical cation. 2.3. In situ Raman spectroelectrochemical
studies
A good method to detail the similarities between the oligomers and the polymer is to use the Raman spectroscopy technique. Raman spectra were recorded on a multichannel Jobin-Yvon T64000 spectrometer equipped with a CCD detector. Spectra are similar to those presented in the literature [5]. They were recorded in-situ at different potentials for several excitation wavelengths namely h = 457.9, 514.5, 676.4 and 1064 run. Only the spectrum obtained at h = 676.4 nm is presented, providing the Raman lines originating from oxidized unit(s). This spectrum was obtained at 500 mV, before the second anodid peak, in order to characterize the radical cation of PCD.
788
M. Boyer etal. /SyntheticMetals
84 (1997)
787-788
Table 1 : Observed and calculated Raman frequencies and vibrational assignments of in plane vibrations of PCD, radical cation of PCD and OPCD at h = 676&l nm (* T means terminal rings and I internal ring) notation mode (Wilson) 8a 8a amine 8b amine amine 9a
PCD Exp. 1619
FCD’O Exp. 1628 1586 1499 1441 1400 1226 1176
Calc. 1602 1609 1430 1571
1424
1221 1187
1229 1171
Table 2 : Raman frequencies of radical cation of PCD and emeraldine salt. (B : Benzenoid, Q : Quinoid). radical cation of PCD 1628 1586 1499 1441 1400/1378 1226 1176
emeraldine salt (bipolaron structure) 1621 1581 1515 1484 1332/1311 1253 1188
Assignments
C C N C C C C
- C stretch. (B) = C stretch.(q) - H bend. = C stretch.(Q) - N stretch.(Q) - N stretch. (B) - H bend. (B)
OPCD
Assignments
Exp. 1622 1587
Calc. 1608 1593
1415 1515 1214 1157
1421 1514 1214 1160
C - C stretching (T)* C = C stretching (I)* N - H bending C = C stretching (I) C=NorC-Nstretching(I) C - N stretching (T) C - H bending (I)
In this case, we compared the frequencies of the radical cation to those of PCD and OPCD (N.N’ diphenyl parabenzoquinone diimine). The assignment proposal of the modes of the radical cation as well as those of PCD [6] and OPCD are presented Table 1. Numerous vibrational modes of the radical cation of PCD can be related to corresponding ones of emeraldine salt (Table 2). In particular, the doublet at 1400/1378 cm” in the radical cation is observed at 1332/1311 cm” in the polymeric salt. The double bands is assigned to the C-N stretching vibration and reveals the existence of a semiquinoid structure. This observation is confiied by the values of the corresponding force constants, obtained in our dynamical calculations. The complete set of force constants will be published elsewhere.
4.
Conclusion
From this work, we can notice that the radical cation of PCD is a good model compound of emeraldine salt. Our results are in agreement with those of Shacklette and al [3] for radical structure proposal of PCD. In order to go further, a study of longer oligomers to get closer to the structure of cmeraldine salt is under way, as well as of phenyl-end-capped tetramer of polyaniline.
References 1. 2. 3.
800
1000
1200
1403
Wavenum ber s (cm-l) Figure 1 : Raman spectra at h = 676,44 nm of a) PCD b) radical cation of PCD c) OPCD
1600
4. 5. 6,
A. P. Monkman, D. Bloor, G. C. Stevens, J. C. H. Stevens, and P. Wilson. Synth. Met., 29 (1989) E277 G. Louam, M. Lapkowski, S. Quillard, A. Pron, J. P. Buisson, and S. Lefrant. J. Phys. Chem. 100 (1996) 6998 L. W. Shacklette, J. F. Wolf, S. Gould, and R. H. Baughman. J. Chem. Phys. 88 (1988) 3955. P. M. MacManus, S. C. Yang, and R. J. Cushman. J. Chem. Sot., Chem. Commun., (1985) 1556. I. Harada, Y. Furukawa, and F. Ueda. Synth. Met., 29 (1989) E303. S. Quillard, G. Louam, J. P. Buisson, S. Lefrant, J. Masters, and A. G. MacDiarmid. Solid State Sciences, 107 (1991)