Excited state (formal) intramolecular proton transfer (ESIPT) in p-hydroxyphenyl ketones mediated by water

Evidence is presented that show p-hydroxyphenyl ketones 6-8 undergo excited state intramolecular proton transfer (ESIPT, via the singlet excited state ), mediated by water, which formally transfers the phenol proton to the car bonyl oxygen of the ketone. ESIPT was not observed in neat CH3CN. The ESIPT process in aqueous media generates the corresponding p-quinone methides 9- 11 land the corresponding conjugate bases (phenolate ions) 12-14), as detec ted by laser flash photolysis (LFP). It competes effectively with intersyst em crossing to the excited triplet state. The respective p-methoxyphenyl ke tones 15 and 16 failed to undergo the reaction consistent with the expected lack of proton transfer in these systems. Results for the biphenyl ketone 8 indicate that formal ESIPT can also take place over an extended range, su ggesting that the process is likely general for all p-hydroxyaromatic keton es which opens up the possibility for designing photoswitchable processes b ased on this general phenomenon. (C) 2000 Elsevier Science S.A. All rights reserved.