Dynamics of Interconversion of Contact and Solvent-Separated Radical-Ion Pairs

Abstract

The important intermediates in photoinduced bimolecular electron transfer reactions in solution are the contact radicalion pair (CRIP, A'-D'+) and the solvent-separated radical-ion pair (SSRIP, A'-(S)D+).lg2 Their role in the photochemistry of ground state charge-transfer (CT) acceptor (A)/donor (D) complexes, as an example, is illustrated in Scheme 1. Excitation of the AD complex gives a CRIP that may undergo solvation to form a SSRIP (ksOlv) or return electron transfer to the ground state (k-et)q. Within the SSRIP, return electron transfer (k-& further separation (ksep) to form free (fully separated) radical ions (FRI, A'+ D'+), and feedback (desolvation) to the CRIP (k-solv) may occur. Previous studies of radical-ion-pair dynamics have relied mainly on time-resolved absorption ~pectroscopy.~ However, the CRIP, SSRIP and FRI can all potentially contribute to the transient absorption signals? and the complete dynamics of the interconverting radical-ion pairs cannot be determi~~ed.~g.'~ Here we describe an approach that allows all of the rate constants shown in Scheme 1 to be determined, including the rates of interconversion of the CRIP and SSRIP (ksolv, k-solv), thus establishing their relative energies. Excitation of a CT complex usually results in a transient absorption decay due to reactions of the CRIP and the SSRIP and a relatively long-lived residual absorption due to the FRI? The absorption decay may be resolved into two exponential components, depending upon the relative magnitudes of the appropriate rate constants (Scheme 1). In principle, four independent experimental parameters are available from such experiments, Le., two time constants (11 and 12), the ratio of their preexponential factors (Rabs). and the quantum yield for