Nanosecond pump and probe observation of bimolecular exciton effects in rubrene single crystals

Abstract

Transient grating pump and probe experiments are used to investigate excitonic processes on the nanosecond time scale in rubrene single crystals. We find that bimolecular interactions cause a photoinduced excited state density on the order of 0.520cm-3 - corresponding to an average distance of ∼3nm between individual states - to decrease by a factor of 2 after 2ns, following a typical power-law decay. We assign the observed power-law decays to high-density interactions between excited states. Because of the high efficiency singlet exciton fission observed in rubrene, these bimolecular interactions are likely those between triplet excitons or between coherent quantum superpositions of a singlet and a pair of triplet-excitons.