CHARGE SEPARATION AND RECOMBINATION IN ISOLATED SUPERMOLECULES

In this paper we consider long-range electron transfer (ET) in a struc turally rigid, solvent-free, DBA supermolecule, which consists of a br idged (B) electron donor (D) and electron acceptor (A), exploring the excess vibrational energy (E) dependence of the microscopic ET rates f rom photoselected states. We have studied charge separation (DBA) --> D+BA- and charge recombination D+BA- --> DBA in an isolated supermole cule where the charge-transfer D+BA-state constitutes the lowest spin- allowed electronic excitation. The energy-dependent microscopic ET rat es in the statistical limit of the radiationless transitions theory we re expressed in terms of the averaged Franck-Condon density, for which quantum and classical expressions were presented in the harmonic appr oximation. 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 e lectronic energy gap DELTAE. An energy gap law for the dependence of t he microscopic ET rate at low E on the electronic energy gap was deriv ed, 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 a t high E, where nuclear tunneling effects are minor, and to give a heu ristic description of the effects of intramolecular vibrational energy redistribution within the initial vibronic manifold on ET dynamics.