Excitation relaxation in films of dipolar N, N-dimethylaminobenzylidene 1,3-indandione molecules

Abstract

Ultrafast relaxation of self-trapped (ST) excitons in polycrystalline film of N, N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecules, which possess pronounced donor–acceptor properties, is studied by using femtosecond luminescence up-conversion spectroscopy. Exciton relaxation dynamics is compared to excitation decay obtained for DMABI molecules embedded in a rigid transparent matrix. Excitons in the film undergo rapid self-trapping on the time scale below 100 fs. A strong nonradiative recombination of ST excitons is observed on the subpicosecond time scale, which is shown to occur concurrently with their final localization. This final localization is attributed to the cooling of locally heated (up to 600 K) ST excitons, that occurs with the time constant τ c=800 fs. The fast nonradiative relaxation is assigned to the thermally activated transition of the hot excitons to the ground state during their thermalization. After the fast relaxation stage is completed, the second, slow exciton relaxation stage emerges, which is attributed to one-dimensional migration of excitons and their nonradiative decay on structural defects.