Photocatalytic electron-transfer oxidation of triphenylphosphine and benzylamine with molecular oxygen via formation of radical cations and superoxide ion

Photooxygenation of triphenylphosphine (Ph₃P) to triphenylphosphine oxide (Ph₃P=O) with molecular oxygen (O ₂) occurs under photoirradiation of 9-mesityl-10-methylacridinium perchlorate ([Acr⁺-Mes]ClO₄^-) which acts as an efficient electron-transfer photocatalyst. Photooxidation of benzylamine (PhCH₂NH₂) with O₂ also occurs efficiently under photoirradiation of Acr⁺-Mes to yield PhCH₂N=CHPh and hydrogen peroxide (H₂O₂). Each photocatalytic reaction is initiated by intramolecular photoinduced electron transfer from the Mes moiety to the singlet excited state of the Acr⁺ moiety to produce the electron-transfer state (Acr^•-Mes^•+). The Mes^•+ moiety oxidizes Ph₃P and PhCH ₂NH₂ to produce the radical cations (Ph₃P ^•+ and PhCH₂NH₂^•+, respectively), whereas the Acr^• moiety reduces O₂ to O₂^•+. The produced Ph₃P^•+ binds with O₂^•+ as well as O₂, leading to the oxygenated product (Ph₃P=O). On the other hand, proton transfer from PhCH₂NH₂^•+ to O₂ ^•+ occurs, followed by hydrogen transfer, leading to the dehydrogenated dimer product, PhCH₂N=CHPh. In each case, the radical intermediates were detected by laser flash photolysis and ESR measurements to clarify the photocatalytic mechanism.