The range dependence of fluorescence anisotropy in molecular energy transfer

In the theory of excitation energy transfer it is generally considered that species initially excited by photoabsorption transfer their energy to other molecules by two distinct mechanisms, known as radiative and radiationless energy transfer. Recently it has been shown that the two mechanisms for energy transfer are in fact indistinguishable, each being the asymptotic limit of a unified mechanism involving virtual photon coupling. The familiar R−6 dependence associated with Förster radiationless transfer is the short‐range limit, while over longer distances retardation effects modify the radial dependence to R−2, and the result is the classical radiative transfer law. For radiationless energy transfer, wide use is made of Galanin’s result concerning fluorescence depolarization losses due to single‐step transfer. Here Galanin’s work is extended to obtain a general formula for the residual fluorescence anisotropy following energy transfer over arbitrary intermolecular distances. Hence a connection is established with the depolarization associated with reabsorption. In particular, it is shown that the anisotropy increases significantly from its initially low value over distances considerably less than those normally associated with radiative energy transfer. The necessary criteria for experimental identification of the transition from radiationless to radiative character are described, and model systems for their realization are considered.