Lifetimes for Radiative Charge Recombination in Donor-Acceptor Molecules

In this paper we demonstrate that the marked solvent dependence of the rates k_rad 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 k_rad 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∗ α exp(-αN) (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.