Interpretation of vibrational studies in phenyl compounds: From oligophenyls to polyparaphenylene

Abstract Among the conducting polymers extensively studied, those presenting a non-degenerated ground state such as polyparaphenylene, polypyrrole, polythiophene… have attracted a real interest. Although they cannot accommodate soliton-like defects, they present transport properties as interesting as in polyacetylene since an n- or p-doping can increase the conductivity by several orders of magnitude. Experimental Raman and infrared results have been recently presented for polyparaphenylene (PPP) polymerized using different procedures such as those developed by Kovacic or Yamamoto. In particular, a comparison between oligophenyls and PPP can be made. The Raman spectra of these samples are essentially composed of a strong band at ≅ 1600 cm −1 and weaker ones at 1220 and 1280 cm −1 whose frequencies are practically independent on the chain length. On the contrary, relative intensities can vary from one sample to the other since, for example, the ratio between the integrated intensity of the 1220 and 1280 cm −1 Raman modes increases with the number of phenyl rings. Also, from biphenyl to PPP, relative intensities between modes around 1600 cm −1 are observed to vary. In this paper, we present a vibrational model able to interpret these experimental data. The model is based on the use of a number of force constants defined in terms of internal co-ordinates. Starting from force constants used to fit Raman and IR modes in benzene and by adding an appropriate number of parameters, a reasonable agreement has been obtained for all the series from biphenyl to PPP. The reasonant behavior of the Raman spectra of AsF 5 -doped PPP allows the assumption that shifted modes could be associated with a quinoid character on the polymer chain. This interpretation is in agreement with the band gap calculations performed by Bredas.