RATE EXPRESSIONS FOR EXCITATION TRANSFER .1. RADIATIONLESS TRANSITIONTHEORY PERSPECTIVE

The ideas concerning preparation of an initial state and its subsequen t temporal evolution are discussed with reference to interchromophore electronic excitation (energy) transfer (EET), particularly the ''reso nance'' case. The cases of strong and weak coupling, and their consequ ences; are discussed. It is shown that when the interactions between t wo identical chromophores lie in the weak coupling regime, then a rate of excitation transfer may be defined on an:experimental time scale a nd is given by a Fermi golden rule expression; the quasicontinuous fin al 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 c oupling limit is shown to result in excimer or exciplex-type emission. A rate equation formalizing donor-acceptor EET in general is determin ed by consideration of the Green's function methods of scattering theo ry, The role of nuclear coordinates, including the possibility of Herz berg-Teller coupling, and the influence of nonradiative processes are examined.