A mechanism of @[email protected] 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 @[email protected] \(k_{D \rightarrow A}\) 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 @[email protected] of the medium, \(\tau _{0}\) the @[email protected] of the donor, \(r\) the distance (cm) between donor (D) and acceptor (A), and \(J\) the @[email protected] (in coherent units \(\text{cm}^{6}\ \text{mol}^{-1}\)) between the @[email protected] of the acceptor and the @[email protected] spectrum of the donor. The critical quenching radius, \(r_{0}\), is that distance at which \(k_{D \rightarrow A}\) is equal to the inverse of the @[email protected]