Biphotonic mechanisms with visible light excitation can generate highly reactive species for thermodynamically challenging reactions. Particularly prominent is the consecutive photoinduced electron transfer mechanism, in which excited radical ions are the presumed key catalytically active species. They benefit from extremely high redox powers but suffer from excited state lifetimes in the low picosecond range and are susceptible to photodegradation. This makes mechanistic elucidation difficult and sometimes leads to debate around the identity of the true catalytically active species. In this study, we address these challenges by presenting a fully resolved mechanism, in which a spectroscopically observable and quantifiable ground state p-terphenyl radical anion is generated through sensitized blue-to-ultraviolet triplet-triplet annihilation upconversion, followed by reductive quenching of the highly excited singlet species. Due to its ground-state reactivity, the p-terphenyl radical anion profits from a persistence time approaching the millisecond time scale. This allows it to avoid the problematic photodegradation pathways often encountered with excited-state organic radicals while still exhibiting a competitively high reduction potential. Our results establish blue-to-ultraviolet upconversion as a robust strategy for generating long-lived super-reductants via a consistent, controllable mechanism that enables the activation of small inert substrate molecules, including CO2. This work is relevant in the broader context of advancing photochemistry through mechanistic research that complements synthetic approaches.
Wagner et al. (Sun,) studied this question.