o-Quinone methide formation from excited state intramolecular proton transfer (ESIPT) in an o-hydroxystyrene

The photohydration of o-hydroxy-alpha-phenylstyrene (7) has been studied us ing product studies, laser flash photolysis (LFP), and fluorescence measure ments, and shown to react via excited state intramolecular proton transfer (ESIPT), to give the corresponding o-quinone methide (13) (lambda(max) = 31 0, 420 nm in 1 : 1 H2O-CH3CN). The same intermediate was also observed on L FP of o-hydroxybenzyl alcohol (10), a system known to give o-quinone methid es efficiently, but via a different mechanism. ESIPT of 7 to give 13 is not significantly enhanced by water as the species was also observed in neat C H3CN; addition of water (up to 1 : 1 H2O-CH3CN) did not increase its yield substantially. Efficient ESIPT of 7 in neat CH3CN is corroborated by its lo w fluorescence emission quantum yield and subnanosecond fluorescence lifeti me. These results provide new insights into ESIPT from phenol to alkene car bon, a process that has not received much attention although 7 is a close r elative of o-hydroxybenzophenone (8), an archetypical system for the illust ration of ESIPT. (C) 1999 Elsevier Science S.A. All rights reserved.