CHARGE SEPARATION AND RECOMBINATION IN ISOLATED SUPERMOLECULES

In this paper we consider long-range electron transfer (ET) in a structurally rigid, solvent-free, DBA supermolecule, which consists of a bridged (B) electron donor (D) and electron acceptor (A), exploring the excess vibrational energy (E) dependence of the microscopic ET rates from photoselected states. We have studied charge separation (DBA)* --> D+BA- and charge recombination D+BA- --> DBA in an isolated supermolecule where the charge-transfer D+BA-state constitutes the lowest spin-allowed electronic excitation. The energy-dependent microscopic ET rates in the statistical limit of the radiationless transitions theory were expressed in terms of the averaged Franck-Condon density, for which quantum and classical expressions were presented in the harmonic approximation. Model calculations elucidated some general features of the dependence of the microscopic ET rates on the molecular parameters, i.e., the mode-specific intramolecular reorganization energies and the electronic energy gap DELTAE. An energy gap law for the dependence of the microscopic ET rate at low E on the electronic energy gap was derived, which, for the electronic origins, exhibits a Poissonian \DELTAE\ dependence with an exponential decrease at large \DELTAE\, manifesting universal features of intramolecular and medium-induced radiationless transitions. Classical Franck-Condon factors were found to provide a useful description of the gross features of the microscopic ET rates at high E, where nuclear tunneling effects are minor, and to give a heuristic description of the effects of intramolecular vibrational energy redistribution within the initial vibronic manifold on ET dynamics.