Diffusion modulated donor–acceptor energy transfer in a disordered system

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Abstract
This mainly theoretical paper treats, in a disordered system, the time‐dependence of irreversible donor–acceptor energy transfer in the limit of low acceptor concentrations. The transfer is modulated by diffusion of energetic donor molecules relative to acceptors. Though donor dynamics will be touched upon, the main emphasis of the paper is on the risetime dynamics of the concentration of energetic acceptors in the nanosecond and subnanosecond regimes following excitation of donors with a brief pulse of light. It has been found that the fastest acceptor risetimes occur in the limit of slowest donor diffusion. The reason for this apparent anomaly, which reverses intuition, is that diffusion aids attainment of a steady state, and the steady state donor distribution gives rise to slower acceptor risetimes than transient contributions. The result has been verified in some preliminary experiments where quasidonor diffusion, caused by donor–donor energy transfer, has been retarded by dilution with inert molecules. Faster risetimes also result from shortening the range of the donor–acceptor transfer interaction. A very striking effect on acceptor risetimes can come about from the presence of local order of donors surrounding an acceptor. Providing the peak of the donor radial distribution function is sufficiently close to the acceptor compared with a characteristic transfer distance, acceptor risetimes can be substantially speeded up. Because of the sensitivity of acceptor dynamics to local order and transfer behavior, the possibility is raised of using the acceptor as a ’’probe’’ for these effects in future picosecond and subpicosecond experiments.