Primary photoexcitations in oligophenylenevinylene thin films probed by femtosecond spectroscopy

The photoexcited state dynamics of vapor-deposited oligophenylenevinylene f ilms, with chain lengths of n=2, 3, and 4 vinylene units, is investigated w ith a 100-fs time resolution. The spectral signatures of three different sp ecies A(1), A(2) and A(3) are identified in the photoinduced absorption spe ctrum. Their assignment is based on kinetics, chain length dependence, and theoretical modeling. A(1) is attributed to the singlet-singlet S-1-->S-2 t ransition. The S-1 population displays a strongly intensity dependent decay in the first ps due to bimolecular annihilation. A(2) is assigned to tripl et-triplet absorption; the triplet state is populated within the pump pulse duration by a nonconventional mechanism which circumvents the spin-flip co nstraints. A(3) grows superlinearly with the pump intensity, and shows a di stinct formation dynamics. It is assigned to radical ion pairs resulting fr om singlet coalescence. These results provide insight into the fluorescence quenching mechanisms of organic molecules in the solid state.