Theory of Energy Migration in Doped Molecular Crystals

The kinetics of diffusion‐controlled sensitized luminescence is analyzed in molecular crystals in which a small concentration of acceptor molecules is embedded. It is shown that if the probability of the excitation energy transfer is determined by the dipole‐dipole interaction between donor and acceptor molecules the host luminescence decay law n(t) changes from Förster's law (n(t) ∼ exp (−α {article}{empty}{document}√ t{document} ))to an exponential one (n(t) ∼ exp (−βt)) with the electronic excitation diffusion length increasing from 0 to l ≧ 5R0, where R0, is the effective radius of the energy transfer by resonance. In addition the host luminescence decay law n(t) is found with account for the electronic excitation migration over donor molecules in molecular crystal doped by donor and acceptor impurities of small concentrations. It is shown that for such a system the function n(t) cannot be derived from any appropriate diffusion equation.