Beyond Closed-Shell Photochemistry: Singlet Oxygen Generation Driven by 10H-Phenothiazine Radical Cation

Abstract Phenothiazines are widely studied as closed-shell photosensitizers; however, their open-shell oxidized forms remain largely unexplored in photochemical contexts. Here, we investigate the photophysics and reactivity of the radical cation of 10H-phenothiazine (PHT•+) using time-resolved spectroscopy, electrochemical control, and lipid membrane models. We show that visible excitation of PHT•+ triggers reactive oxygen species formation and induces oxidative damage in biomimetic assemblies. Spectroscopic evidence of PHT•+ double-to-quartet excited-state intersystem crossing, the effect of D2O, quenching by histidine of phosphorescence at 1270 nm, and theoretical calculations support the generation of singlet oxygen (1O2), thereby establishing the phenothiazine radical cation as a photoactive species capable of transferring energy to molecular oxygen. These results expand the mechanistic framework of phenothiazine photochemistry and identify open-shell excited states as viable intermediates for oxygen activation in aqueous environments.