Fluorescence studies of the excited-state proton transfer in substituted 3-hydroxychromones in supersonic jet

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 were 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. Upper limit of 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 line width of the respective origin bands in the visible fluorescence excitation spectra. 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. © 1993, Walter de Gruyter. All rights reserved.