Electronic energy transfer in impurity bands of mixed organic solids

In this paper we consider some implications of the Anderson transition (AT) for the termination of electronic energy transfer (EET) in an impurity band of a low-temperature, isotopically mixed organic solid. The critical impurity concentration C̄ for the occurrence of the AT was found to be compatible with the experimental data for triplet EET. The mechanism of EET in extended states was described in terms of a strong scattering, random phase model, which leads to a minimum diffusion coefficient D_m≃10 ^-4-10^-5 cm² sec^-1. Energy transfer from the impurity band to an energy sink (ETFIBTES) from extended states was described in terms of diffusion-controlled kinetics, providing a proper interpretation for the onset of EET in singlet impurity bands, which corresponds to a kinetic threshold, and the critical concentration for EET in triplet impurity bands, which marks the AT. Finally, we considered ETFIBTES from localized states, when the localization length is comparable to the spacing between the energy sinks, proposing a novel method for the determination of the localization length in disordered organic solids.