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* proportional to exp(-alpha N) (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 similar to-0.02, being even smaller for the solvated molecules.