Unusual temperature dependence of the fluorescence decay in heterostructured stilbene.

Fluorescence spectra as well as the fluorescence decay kinetics of hot-pressed and sublimated films of stilbene have been studied in a wide temperature range, from 15 K up to room temperature. The fluorescence decay kinetics demonstrate unusual elongation of the excitation lifetime with a temperature increase. This is in contrast to the corresponding data of stilbene solutions in chloroform and in a polystyrene (PS) matrix. It is well known that the excitation dynamics of stilbene in solution and in a PS matrix is controlled by the molecular isomerization/twisting process of separate molecules. The data analysis and quantum chemistry calculations of stilbene aggregates suggest that the temperature dependence of the fluorescence kinetics of bulk stilbene solids can be explained by fast exciton diffusion, which yields a thermalized exciton distribution in a relatively small number of fluorescence centres. The temperature dependence of the distribution can thus explain the observed fluorescence decay lifetimes.