On the inoperativeness of the ESIPT process in the emission of 1-hydroxy-2-acetonaphthone: a reappraisal.

For a molecule which contains an intramolecular hydrogen bond (IMHB) in its chemical structure to undergo an excited singlet intramolecular proton transfer (ESIPT) process, on photoexcitation, there must occur a simultaneous increase, in a substantial manner, in the acidity of the proton donor and the basicity of the proton acceptor forming the IMHB [J. Am. Chem. Soc. 2001, 123, 11940]. For the reason that those changes occur on photoexcitation of the 2-hydroxyacetophenone but not for 1-hydroxy-acetonaphthone, one draws the conclusion that, while ESIPT is operative in the 1(pi,pi*)(1) electronic state of the monocyclic compound 2-hydroxyacetophenone, it is not operative in its bicyclic homolog 1-hydroxy-2-acetonaphthone. We have shown the photophysics of 1-hydroxy-2-acetonaphthone in its first excited electronic state to be governed by two stable, easily reconverted enol structures, the presence of which causes the peaks in the free-jet fluorescence excitation spectrum for the compound to split into two of similar strength. In this paper, we rationalize photophysical evidence for 1-hydroxy-2-acetonaphthone obtained by femtosecond spectroscopy over the past 13 years in the light of existing photophysical patterns based on steady-state spectra for the compound [J. Am. Chem. Soc. 1993, 115, 4321].