Primary photoexcitations and their interconversion in oligophenylenevinylene nanocrystals: Role of excess energy studied with sub-30 femtosecond resolution

Elementary photoexcitations in polycrystalline films of oligophenylenevinylenes (OPV) are studied by pump-probe spectroscopy with sub-30 fs time resolution. When pumping 0.9 eV above the absorption edge, we find that predominantly charge carriers are formed as primary photoexcitations, whereas the majority of the neutral singlet excited states are formed in a secondary process upon geminate charge recombination. The sublinear pump intensity dependence of singlet states concentration is explained by ultrafast annihilation involving hot singlet states. Upon pumping at the absorption edge, instantaneous stimulated emission is observed instead of charged states absorption, demonstrating that singlet states are the predominant primary photoexcitations at low excess energies.