Structure and Photoinduced Excited State Keto−Enol Tautomerization of 7-Hydroxyquinoline-(CH₃OH)<i>_n</i> Clusters

Abstract

An investigation on the structures and the excited-state proton-transfer reaction has been carried out for size-selected 7-hydroxyquinoline(7HQ)-methanol hydrogen-bonded clusters in supersonic jets. The structures of the clusters were determined based on the analysis of the fluorescence-detected IR spectra in the OH stretching region and by density functional theory calculations. Two isomers were found to exist for 7HQ-(CH3OH)1. One has the form in which the OH group of 7HQ is hydrogen-bonded to the oxygen of CH3OH, and the other has the form in which the OH group of CH3OH is hydrogen-bonded to the nitrogen of 7HQ. For 7HQ-(CH3OH)2 and -(CH3OH)3, their structures were determined to be “enol bridge” form, in which one end of a chain-formed methanol cluster is hydrogen-bonded to the OH group and the other to the nitrogen of 7HQ. Photoinduced excited-state proton-transfer reaction was investigated by measuring the dispersed fluorescence spectra for the clusters. Among them, a visible emission with a maximum at ∼530 nm, which coincides with the tautomer emission of 7HQ generated in methanol solution, was observed for the enol bridge 7HQ-(CH3OH)3 and larger clusters. This result presents a direct evidence for the enol bridge form 7HQ-(CH3OH)3 to be a precursor for the proton-transfer reaction in solution.