Photophysical induction of dual fluorescence of quercetin and related hydroxyflavones upon intermolecular H-bonding to solvent matrix

Quercetin (3,3′,4′,5,7-pentahydroxyflavone) in contrast to many hydroxyflavonols exhibits very little or no fluorescence in 298 K liquid solution or in the rigid glass protic solvents at 77 K. However, in the UV beam of the spectrofluorimeter a strong dual fluorescence builds up in a protic solvent on a time scale of 5–30 min. This is demonstrated to be a photophysical induction of fluorescence in the thermally annealed solvent matrix at 77 K. The 5-OH group in the vibrationally excited molecule can execute a motion converting its H-bond from an intramolecular to an intermolecular one, activating the 3-OH intramolecular proton transfer fluorescence.