OPTICAL ABSORPTIONS IN POLY(PARA-PHENYLENE VINYLENE) AND POLY(2,5-DIMETHOXY-1,4-PARAPHENYLENE VINYLENE) OLIGOMERS

On the basis of geometries optimized at the Austin model 1 level, we use the intermediate neglect of differential overlap method coupled with a single configuration interaction technique to calculate the optical absorption spectra of oligomers of poly(paraphenylene vinylene), PPV, and their dimethoxy-substituted derivatives. Our results are in good agreement with experimentally measured visible and UV absorption spectra for oligomers and polymers, and provide a detailed interpretation of the electronic transitions involved, distinguishing the roles of localized and delocalized pi molecular orbitals. In addition to the lowest energy transition, from highest occupied to lowest unoccupied delocalized pi orbitals (2.5 eV in PPV), we calculate further structure in the absorption at higher energies associated with the delocalized pi orbitals. These match well the measured absorption features in PPV compounds, at 4.5 eV (tetramer), 4.1 eV (pentamer) and 3.8 eV (hexamer, polymer).