Effect of an External Electric Field on Electronic Excitation Energy Transfer in Molecular Crystals

While several mechanisms can be imagined by which an external electric field can affect electronic excitation energy transfer in crystals, the simplest possibility involves changing the energy level spacings of neighboring molecules. Energy transfer from one molecule to another is affected by an energy mismatch or loss of resonance. In a band model, presumably valid at low temperatures, this loss of resonance decreases the exciton bandwidth and reduces the exciton group velocity. When the bandwidth is comparable to the inhomogeneous broadening, an abrupt decrease in the rate of energy transfer can be expected, reflecting the phenomenon of Anderson localization. At higher temperatures, the energy mismatch can be accommodated by phonon emission and absorption processes. The dependence of the energy transfer rate constant on electric field and temperature for the case of coherent energy transfer and for the limit of incoherent energy transfer is predicted for a simple model system.