Energy transfer in one-dimensional molecular crystals: Direct and indirect energy exchange in the non-Boltzmann regime

Direct energy transfer between localized states via virtual coupling with the host states in one-dimensional molecular solids is demonstrated experimentally using optical and optically detected electron spin coherence techniques. The relative importance of direct transfer versus indirect transfer (which is characterized by the phonon-assisted promotion of the localized state to the band with the subsequent radiationless decay into a mobile exciton followed by retrapping) has been determined in isotopically mixed 1,2,4,5-tetrachlorobenzene crystals over a temperature range where the thermal energy of the lattice is insufficient to establish Boltzmann equilibria between the localized and delocalized states (non-Boltzmann regime). The results demonstrate that the transfer mechanisms between trap and host states are very sensitive to the trap concentration and the temperature of the lattice, and that direct exchange is dominant at high trap concentrations. Finally, the experimental results are compared with the theoretical expectations for excitation yields and trap-to-trap distances in the isotopically mixed crystals