Picosecond dynamics of electronic energy transfer in condensed phases

Abstract

Energy transfer between donor and acceptor molecules randomly distributed in condensed phases is investigated by time-resolved spectroscopy on the picosecond and nanosecond time scales. The effects of translational diffusion and excitation transfer among the donors is experimentally observed and used to test theoretical models based on a diffusion equation for the donor excitation. The time-resolved data demonstrate that the Förster dipole–dipole model is valid in the cresyl violet (donor):azulene (acceptor) system from 1 ps to at least 10 ns after excitation, and over a 1000-fold range of acceptor concentration. The critical transfer distance obtained from the transient experiments (26.6 Å) is in excellent agreement with the value obtained from the spectral overlap (27.8 Å) at all acceptor concentrations. In fluid solutions the donor decay agrees very well with the approximate solution of the diffusion equation including a sink term for energy transfer. The deviations observed at high donor concentrations suggest that donor–donor excitation transfer is nondiffusive on the picosecond time scale.