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.
In the 20th century, photochemistry blossomed from a poorly defined to a highly sophisticated science. Early breakthroughs in exploratory photochemistry and the underlying physical principles led to new diverse, yet inter-related areas of research. The alluring goal of asymmetric synthesis with circularly polarized light proved elusive. The discovery of the electron brought a gradual awakening to the idea of electron transfer. Time-resolved spectroscopy developed from ms to fs resolution. The field of photosynthesis progressed from an interest in function and structure of photosynthetic pigments to the isolation and structure elucidation of photosynthetic reaction centers (rhodobacter sphaeroides), to the detailed kinetics of sequential electron-transfer steps in natural and synthetic light-harvesting systems.