Laser photolysis studies of the phenolic H-atom transfer mechanism for a triplet π,π∗ ketone in solution revisited

The mechanism of the H-atom transfer from phenols or thiophenols to triplet π,π∗ 5,12-naphthacenequinone (5,12-NQ) has been examined by means of laser flash photolysis at 295 K. Based on the Hammett plots and the Rehm-Weller equation for the quenching rate constants, the phenolic H-atom transfer from phenols or thiophenols to triplet π,π∗ 5,12-NQ is shown to proceed via the electron transfer followed by proton transfer. The previously proposed mechanism for H-atom transfer of π,π∗ triplets, that proton transfer is followed by electron transfer, was not verified in the present systems.