Vibrational studies of poly(paraphenylene): comparison with oligophenyls

Abstract Experimental Raman and infrared results are recalled for poly( pera -phenylene) (PPP) polymerized using different procedures. A comparison is made between oligophenyls and PPP. 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 . These frequencies are practically independent of the chain length, but the relative intensity between the 1220 and 1280 cm −1 modes is found to depend on the number of phenyl rings, on the excitation wavelength and on the concentration level in the case of doped systems. We present a vibrational model able to interpret these experimental data, based on the use of a number of force constants defined in terms of internal coordinates. Starting from force constants used to fit Raman and i.r. modes in benzene and by adding an appropriate number of parameters, a reasonble agreement has been obtained for all the series from biphenyl to PPP. In view of the theoretical results, arguments are given concerning the β-electron localization along the chains.