Return electron transfer in radical ion pairs of triplet multiplicity

Return electron transfer (RET) in radical ion pairs may populate the reagent ground states or, in the case of triplet pairs, one reagent triplet state. The efficiency of triplet RET is governed by the free energies of singlet and triplet RET and by the topologies of the potential surfaces of parent molecules, radical ions, and accessible triplet states or biradicals. RET in triplet radical ion pairs is exemplified for two distinct relationships between the three potential surfaces.