Excited state intramolecular proton transfer dynamics of 1-hydroxy-2-acetonaphthone.

Excited state intramolecular proton transfer (ESIPT) of 1-hydroxy-2-acetonaphthone (HAN) has been in controversy, mainly because its Stokes shift is small compared to those of typical ESIPT molecules. We have investigated excited state dynamics of HAN by time-resolved fluorescence with a resolution high enough to record the nuclear wave packet motions in the excited state. Population dynamics of both the normal and tautomer forms were recorded together with the wave packet motions of the tautomer in the excited state, which confirm the ESIPT of HAN. The population dynamics of the normal and tautomer forms imply that the ESIPT dynamics is biphasic with two time constants <25 and 80 fs. Theoretical analysis of the vibrational modes of the tautomer excited impulsively reveals that major part of the change for the ESIPT reaction is on the naphthalene ring.