Rate expressions for excitation transfer I. Radiationless transition theory perspective

The ideas concerning preparation of an initial state and its subsequent temporal evolution are discussed with reference to interchromophore electronic excitation (energy) transfer (EET), particularly the "resonance" case. The cases of strong and weak coupling, and their consequences, are discussed. It is shown that when the interactions between two identical chromophores lie in the weak coupling regime, then a rate of excitation transfer may be defined on an experimental time scale and is given by a Fermi golden rule expression; the quasicontinuous final state being provided primarily by nonradiative line broadening. In general, such a rate expression applies so long as the donor-acceptor electronic coupling is less than the vibronic band width. The strong coupling limit is shown to result in excimer or exciplex-type emission. A rate equation formalizing donor-acceptor EET in general is determined by consideration of the Green's function methods of scattering theory. The role of nuclear coordinates, including the possibility of Herzberg-Teller coupling, and the influence of nonradiative processes are examined.