Excited-state intramolecular proton transfer in jet-cooled 1-hydroxy-2-acetonaphthone

Abstract

The fluorescence excitation and dispersed emission spectra of the intramolecularly hydrogen bonded 1-hydroxy-2-acetonaphthone (HAN) isolated molecule have been investigated in a free jet expansion. The excitation spectrum is well resolved until 900 cm−1 above the origin located at 388.6 nm. Above this excess energy no fluorescence is observed indicating the onset of an efficient nonradiative process. The emission from the vibrationless level of the S1 state exhibits a dual character. The short wavelength fluorescence is highly structured and its vibrational structure correlates with that of the S1 state. The long wavelength part shifted by 2200 cm−1 is more intense and diffuse. Deuteration of the hydroxy proton removes totally the short wavelength part of the emission while the long wavelength part remains unchanged. The results are interpreted in terms of excited state intramolecular proton (or hydrogen atom) transfer from the initial enol form to the tautomeric keto conformer through a small energy barrier between two closely lying local potential minima.