Photoexcitation in poly(phenylenevinylene)

Poly(phenylenevinylene) is a conjugated polymer with a non-degenerate ground state, and its excitations are therefore non-topological, or polaron-like. When prepared via the sulphonium polyelectrolyte salt route, it can be obtained in the form of highly oriented films, and is thus suitable for measurements of the anisotropy between intra- and inter-chain motion. We report here measurements of photoinduced absorption and photoluminescence, which detect charged and neutral photoexcitations, respectively. Photoinduced absorption below the band edge (at 2.5 eV) is characteristic of bipolarons, with absorption bands peaked at 0.6 and 1.6 eV. We consider that these are created following inter-chain separation of charge. In contrast, intra-chain excitation usually results in confined charge pairs, and the formation of polaron-excitons which can decay radiatively.