Vibrational spectroscopy and comparative study of trans-polyacetylene, poly(p-phenylene) and poly(p-phenylene vinylene)

Abstract

Theoretical vibrational spectra of trans-polyacetylene (PA), poly(p-phenylene) (PPP) and poly(p-phenylene vinylene) (PPV) are presented, based on a single-periodic-chain model and a harmonic potential with parameters transferred from trans-hexatriene, toluene, and p-divinyl benzene, respectively. The out-of-plane degrees of freedom and the infrared absorption intensities were included in the calculations. The calculations evince the absence of the in-plane C-H bending infrared absorption band of the trans-vinylene portion of trans-PA and PPV, which is a consequence of a delocalized π-electron system. The electron-phonon coupling in PPP and PPV is relatively weak - in contrast to trans-PA, where the frequencies of the principal Raman lines are significantly lowered. As a consequence, the PPV is an example of pristine polymer with well delocalized π-electron system, but without significant electron-phonon coupling.