A mechanism of excitation transfer which can occur between molecular entities separated by distances considerably exceeding the sum of their van der Waals radii. It is described in terms of an interaction between the transition dipole moments (a dipolar mechanism). The transfer rate constant kD → A is given by: \[k_{D \rightarrow A} = \frac{K^{2}\ J\ 8.8\times 10^{-28}\ \text{mol}}{n^{4}\ \tau _{0}\ r^{6}}\] where K is an orientation factor, n the refractive index of the medium, τ 0 the radiative lifetime of the donor, r the distance (cm) between donor (D) and acceptor (A), and J the spectral overlap (in coherent units cm6 mol-1) between the absorption spectrum of the acceptor and the fluorescence spectrum of the donor. The critical quenching radius, r 0, is that distance at which kD → A is equal to the inverse of the radiative lifetime.