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

Abstract

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 ∼100% regeneration of eosin Y. The response to violet light supports th...