Intramolecular excited-state proton transfer in jet-cooled 2-substituted 3-hydroxychromones and their water clusters

Abstract

Intramolecular excited-state proton transfer (ESPT) of 3-hydroxychromone (3-HC), 3-hydroxyflavone (3-HF) and 2-(2-naphthyl)-3-hydroxychromone (2-NHC), and of their water clusters, was investigated in a supersonic expansion. The visible tautomer fluorescence excitation spectra of these compounds exhibit considerably well-resolved vibrational structures, while no significant uv fluorescence excitation spectrum due to the normal form was observed. The upper limit of the rate constant of the tautomer formation was estimated to be 1.77×1012 s−1 for 3-HC, 6.5×1011 s−1 for 3-HF, and 1.54×1011 s−1 for 2-NHC in supersonic expansion by simulation of the linewidth of the respective origin bands in the visible fluorescence excitation spectra. The order of rate constants of the tautomer formations in the supersonic free jet are consistent with that of the ESPT of these compounds in nonpolar solution. The fluorescence excitation and dispersed fluorescence spectra demonstrate 1:1 and 1:2 water complex formations of 3-HF and 2-NHC and no ESPT in these water clusters. However, no evidence of water complex formation was obtained in jet-cooled 3-HC.