Electron-hole interaction effects in the absorption spectra of phenylene-based conjugated polymers

A recently introduced microscopic model of the polyphenylenes is discussed and applied to study the absorption spectra of poly(paraphenylene) (PPP) and poly(phenylenevinylene) (PPV) oligomers. Emphasis is placed on the relation between the ultraviolet-visible spectra of oligomers and the effective electron-hole interaction potential. The long-range Coulomb interaction is treated on an equal footing with the intramonomer interactions. A physical understanding is achieved, within a single model, of the origin, polarization, positions, and intensities of the main absorption bands of short (biphenyl, stilbene) and long oligomers of PPP and PPV. For the infinite polymer chain the model leads to the generic appearance of three dispersing absorption bands. A fourth, dipole-forbidden, singlet excitation band is rendered allowed if charge-conjugation symmetry is broken. A family of four distinct, tightly bound, triplet state excitons is also found.