EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER IN JET-COOLED 1-HYDROXY-2-ACETONAPHTHONE

The fluorescence excitation and dispersed emission spectra of the intr amolecularly hydrogen bonded 1-hydroxy-2-acetonaphthone (HAN) isolated molecule have been investigated in a free jet expansion. The excitati on spectrum is well resolved until 900 cm(-1) above the origin located at 388.6 nm. Above this excess energy no fluorescence is observed ind icating the onset of an efficient nonradiative process. The emission f rom the vibrationless level of the S-1 state exhibits a dual character . The short wavelength fluorescence is highly structured and its vibra tional structure correlates with that of the S-1 state. The long wavel ength part shifted by 2200 cm(-1) is more intense and diffuse. Deutera tion of the hydroxy proton removes totally the short wavelength part o f the emission while the long wavelength part remains unchanged. The r esults are interpreted in terms of excited state intramolecular proton (or hydrogen atom) transfer from the initial enol form to the tautome ric keto conformer through a small energy barrier between two closely lying local potential minima.