Time and internal energy dependent fluorescence spectra of naphthol·water clusters

Time-resolved fluorescence spectra of 1-naphthol·(H2O)n clusters formed in a supersonic jet were measured under conditions of strong and weak cooling. Wavelength selectivity was used to excite very similar size distributions in both cases, thereby allowing controlled study of temperaturelike effects of internal energy in clusters. In both warm and cold clusters, long wavelength fluorescence from 1-naphtholate formed via an intermolecular excited state proton transfer (ESPT) reaction is observed, but this emission was significantly more red-shifted in warmer clusters. The fluorescence spectra of warmer clusters also shift strongly to the red after excitation, on a time scale of about 10 ns. Colder clusters showed no spectral shifts on this time scale. A weak solvent isotope effect was also observed. Similar clusters of 2-naphthol with water were not observed to undergo ESPT under any conditions. The time-dependent spectra indicate that ESPT in warm 1-naphthol·water clusters is not prompt, but continues on a ∼10 ns time scale, as does dynamic stabilization of previously reacted clusters. The ESPT rate is believed to be controlled by the rate at which the water relaxes around the naphthol, via interaction with two electronic excited states. The 10 ns and the two faster [R. Knochenmuss, G. R. Holtom, and D. Ray, Chem. Phys. Lett. 215, 188 (1993)] time scales now known in the spectral dynamics are discussed in terms of water solvation relaxation, and connected to the bulk by means of molecular dynamics simulations