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 polari ty (i.e., with increasing peak energy (v) for charge-transfer fluoresc ence), can be quantitatively accounted for in terms of a dominating co ntribution of (DBA)-D(+)BA(-) mixing, involving intensity borrowing f rom local (DBA) electronic excitations. In these DBA molecules, the t raditional two-level D(+)BA(-)-DBA coupling scheme is inapplicable. Th e analysis of the (v) dependence of k(rad) for a certain DBA in a seri es of solvents results in the (DBA)-D(+)BA(-) couplings V*, which are in good agreement with the V parameters extracted from oscillator st rengths for charge-transfer absorption. The V parameters, which obey the relation V proportional to exp(-alpha N) (where N is the number o f bonds in the bridge), determine the rates for nonradiative (DBA) -- > D(+)BA(-) charge separation and recombination from electronically ex cited states. The (DBA)-D(+)BA(-) mixing is maximized for the isolate d, solvent-free DBA molecule. For the isolated molecules analyzed here in, the fraction of (DBA) admixture within the charge-transfer state is similar to-0.02, being even smaller for the solvated molecules.