Evidence of Ground-State Proton-Transfer Reaction of 3-Hydroxyflavone in Neutral Alcoholic Solvents

Abstract

Photophysical behavior of 3-hydroxyflavone (HF), a molecule that exhibits excited-state intramolecular proton-transfer reaction and has been studied extensively in the past, has been reinvestigated in hydrogen bond donating solvents. In neat alcohols or in an acetonitrile−methanol mixture, a long wavelength absorption band (λmax ≈ 410 nm) has been observed for the first time. It is found that selective excitation of this band does not produce the characteristic fluorescence of the “normal” form or the tautomer; instead, an emission characterized by a featureless band (λmax ≈ 480 nm) is observed. The absorption and emission bands have been found to disappear completely in the presence of water. The influence of temperature suggests that the species responsible for the 410 nm absorption band is produced in higher concentrations at higher temperatures. On the basis of the present results and the available literature, the long-wavelength absorption band of HF has been attributed to its anionic form, generated in the alcoholic media by solvent mediated deprotonation of the 3-hydroxy group of the molecule in the ground state.