Lifetimes for Radiative Charge Recombination in Donor-Acceptor Molecules

In this paper we demonstrate that the marked solvent dependence of the rates kmd for radiative recombination in some donor (D)-bridge (B)-acceptor (A) molecules, which increase with decreasing solvent polarity (i.e., with increasing peak energy (v) for charge-transfer fluorescence), can be quantitatively accounted for in terms of a dominating contribution of (DBA)*-D+BA-mixing, involving intensity borrowing from local (DBA)* electronic excitations.In these DBA molecules, the traditional two-level D+BA--DBA coupling scheme is inapplicable.The analysis of the (v) dependence of krad for a certain DBA in a series of solvents results in the (DBA)*-D+BA-couplings V , which are in good agreement with the V parameters extracted from oscillator strengths for charge-transfer absorption.The V parameters, which obey the relation V' 0: exp(-aN) (where N is the number of bonds in the bridge), determine the rates for nonradiative (DBA)* -D+BA-charge separation and recombination from electronically excited states.The (DBA) *-D+BA-mixing is maximized for the isolated, solvent-free DBA molecule.For the isolated molecules analyzed herein, the fraction of (DBA)* admixture within the charge-transfer state is -0.02, being even smaller for the solvated molecules.