Lignin C–C bond cleavage induced by consecutive two-photon excitation of a metal-free photocatalyst

Abstract

Photocatalytic lignin valorization has caught widespread attention; yet the reaction systems often employ noble metal complexes, hydrogen atom transfer (HAT) agents, and/or sacrificial electron donors/acceptors that do not comply with atom economy or environmental friendliness. Herein, we discovered that N-phenylphenothiazine (PTH) as a metal-free photocatalyst induced the cleavage of the lignin C_α–C_β bond under ambient conditions free of those additional agents with a high yield and selectivity toward benzoic acid. Transient spectroscopic investigations revealed that the energy-demanding C_α–C_β bond cleavage was induced by the potent oxidant, ²PTH˙⁺*, that was derived from consecutive two-photon excitation of PTH.