A mechanism of @E02256@ 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 @O04322@ \(k_{D \rightarrow A}\) 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 @R05240@ of the medium, \(\tau _{0}\) the @R05064@ of the donor, \(r\) the distance (cm) between donor (D) and acceptor (A), and \(J\) the @S05818@ (in coherent units \(\text{cm}^{6}\ \text{mol}^{-1}\)) between the @A00043@ of the acceptor and the @F02453@ 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 @R05064@.