Host-Sensitized Energy Transfer in Molecular Crystals

The time evolution of the fluorescence intensities of tetracene-doped anthracene and anthracene-doped napthalene crystals was investigated for various concentrations of activators and different methods of excitation. Singlet exciton diffusion, generally invoked to explain energy transfer in these systems, cannot explain the observed time dependence, which indicates that energy transfer is more efficient at short times than at long times. Such a variation of energy-transfer efficiency is characteristic of long-range resonant interaction, but the predictions of the normal mathematical formulation of this theory also disagree with the results. A combined theory of long-range interaction and exciton diffusion can be made to fit the data only with a critical energy-transfer distance $R_0$, which is much greater than that determined from spectral considerations and a diffusion coefficient $D$, which is much smaller than that measured by other techniques.