Excitation of Metastable Intermediates in Organic Photoredox Catalysis: Z-Scheme Approach Decreases Catalyst Inactivation

Despite the numerous applications of eosin Y as an organic photoredox catalyst, substantial mechanistic aspects of the photoredox process have remained elusive. Through deductive, steady-state kinetic studies, we first propose a mechanism for alkaline, aqueous photoredox catalysis using eosin Y, triethanolamine, and oxygen, integrating photo- and nonphotochemical steps. The photoredox cycle begins with a single-electron transfer (SET) induced when eosin Y absorbs green light. This photoinduced SET leads to the formation of a metastable radical trianion that can be fully reduced to inactivated leuco eosin Y via H+/e(-)/H+ transfer or regenerated to eosin Y via ground-state SET to oxygen. Since the radical trianion absorbs violet light, we tested the effect of radical trianion photoexcitation on catalyst regeneration. We found that excitation of the metastable radical trianion in the presence of a threshold concentration of oxygen enabled similar to 100% regeneration of eosin Y. The response to violet light supports the important role of the metastable radical trianion and indicates that the photoredox cycle can be closed via a secondary photoinduced SET event. The idea of photoredox cycles with two consecutive photoinduced electron transfer (PET) steps is not intuitive and is introduced as a tool to increase photocatalyst turnover by selectively favoring regeneration over "death". This alludes to the Z-scheme in biological photosynthesis, where multiple PET reactions, often triggered by different frequencies, promote highly selective biochemical transformations by precluding unproductive SET events in plants and bacteria. We expect that the simple Z-scheme model introduced here will enable more efficient use of organic photoredox catalysts in organic and materials chemistry.