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 using product studies, laser flash photolysis (LFP), and fluorescence measurements, and shown to react via excited state intramolecular proton transfer (ESIPT), to give the corresponding o-quinone methide (13) (lambda(max) = 310, 420 nm in 1 : 1 H2O-CH3CN). The same intermediate was also observed on LFP of o-hydroxybenzyl alcohol (10), a system known to give o-quinone methides 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 CH3CN; 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 low fluorescence emission quantum yield and subnanosecond fluorescence lifetime. These results provide new insights into ESIPT from phenol to alkene carbon, a process that has not received much attention although 7 is a close relative of o-hydroxybenzophenone (8), an archetypical system for the illustration of ESIPT. (C) 1999 Elsevier Science S.A. All rights reserved.