Primary photoexcitations in oligophenylenevinylene thin films probed by femtosecond spectroscopy

The photoexcited state dynamics of vapor-deposited oligophenylenevinylene films, with chain lengths of $n=2,$ 3, and 4 vinylene units, is investigated with a 100-fs time resolution. The spectral signatures of three different species A₁, A₂ and A₃ are identified in the photoinduced absorption spectrum. Their assignment is based on kinetics, chain length dependence, and theoretical modeling. A₁ is attributed to the singlet-singlet S₁→S₂ transition. The S₁ population displays a strongly intensity dependent decay in the first ps due to bimolecular annihilation. A₂ is assigned to triplet-triplet absorption; the triplet state is populated within the pump pulse duration by a nonconventional mechanism which circumvents the spin-flip constraints. A₃ grows superlinearly with the pump intensity, and shows a distinct formation dynamics. It is assigned to radical ion pairs resulting from singlet coalescence. These results provide insight into the fluorescence quenching mechanisms of organic molecules in the solid state.