Exploring the effects of solvent polarity on the rate of Forster-type electronic energy transfer in a closely-spaced molecular dyad

A rigid molecular dyad, comprising two disparate boron dipyrromethene dyes operating as terminals and being separated by a spirobifluorene connector, has been subjected to a detailed examination of how changes in solvent influence the rate of intramolecular electronic energy transfer (EET). In all solvents, the probability of EET exceeds 90% but the rate shows a clear increase in non-polar solvents. This effect is traced to small perturbations of the optical absorption spectrum of the acceptor, which affects the overlap integral for Forster-type EET and also disturbs the accompanying transition dipole moment. There is an additional subtle effect whereby the nature of the surrounding solvent affects the transition dipole density, which itself modifies the separation distance.