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

摘要

The important intermediates in photoinduced bimolecular electron transfer reactions in solution are the contact radical-ion pair (CRIP, A~(·-)D~(·+)) and the solvent-separated radical-ion pair (SSRIP, A~(·-)(S)D~(·+)). 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 (k_(solv)) or return electron transfer to the ground state (k_(-et))_(cp). Within the SSRIP, return electron transfer (k_(-et))_(ss), further separation (k_(sep) 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 spectroscopy. 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 determined. 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 k_(solv), k_(-solv)), thus establishing their relative energies.