Knowledge Bank

Electronic excitation energy transfer in molecular crystals has been studied in the past by varying the concentration and the nature of scattering and trapping centers. Externally applied electric fields can be used to study energy transfer under better controlled conditions by creating energy mismatches between neighbouring molecules. A model has been developed for a linear crystal composed of two molecules per unit cell with an antiferroelectric arrangement of dipole moments. Expessions are derived for the trapping rate functions in the various scattering regimes. The model predicts that an externally applied electric field should decrease the exciton motion, and hence increase the exciton luminescence. Such an increase has been observed in 4,4-dichloro-benzophenone and 4,4 -dimethylbenzophenone. The functional form of the change in exciton luminescence with applied field is determined in the model by the processes that limit the coherence of the exciton motion.