Mechanistic Diversity in Photoexcited Radical Redox Chemistry: A Critical Evaluation.

"Photoexcited Radical Redox", a subset of photoredox reactions in which electron transfer occurs via excitation of an already oxidized or reduced radical form of the photocatalyst, has been a cause of both great excitement and confusion in recent years. While the ability of photoexcited radical redox to access extreme redox potentials has led to a string of exciting, high-profile, challenging new transformations, serious doubts have been raised as to the feasibility of the mechanisms proposed, leading to an extensive debate over a diversity of different mechanistic rationales. While these discussions are supported by a steadily growing body of experimental evidence, overall understanding of photoexcited radical redox remains murky at best. Each mechanistic model struggles to account for at least some important observations, and many important general questions remain unanswered. In this review, we comprehensively summarize the various mechanistic models that have been proposed to account for observations of (apparent) photoexcited radical redox, provide a detailed, critical assessment of the evidence for and against each of them, and highlight continuing gaps in collective understanding and directions for future investigations.