Electronic mechanisms of photophysical induction of luminescence in hydroxyflavones

The molecules from the class of 5-hydroxyflavones exhibit very little or no luminescence in fluid solvents at room temperature or in the rigid glass protic solvents at 77 K. Thus the dynamics of excitation of 5-hydroxyflavones is dominated by the radiationless pathway at these conditions. The powerful radiationless deactivation of the excited state in 5-hydroxyflavones is explained in terms of psedo-Jahn-Teller effect, which involves the strong interaction between two excited states of different symmetry. For 5-hydroxyflavones, a strong dual fluorescence of quercetin builds up in hydrogen-bonding capable solvents at 77 K on a time scale of 5--10 minutes under continuous illumination. The photo-physical induction of dual fluorescence was observed for kaempferol and myricetin as well. In case of 5-hydroxyflavone (5HF) the build up of phosphorescence but not a fluorescence was observed.;Those phenomena are demonstrated to be a photo-physical induction of luminescence in the thermally annealed solvent matrix at 77 K. During this process the 5-OH group in the vibrationally excited molecule executes a torsional motion converting its H-bond from an intramolecular to an intermolecular one. For quercetin, myricetin and kaempferol such conversion activates the 3-OH intramolecular proton-transfer fluorescence. In the absence of 3-OH group, the ketone-like phosphorescence is observed. The uniqueness of this photo-physical induction of luminescence lies in its slow-time evolution. The photo-physical induction of fluorescence on minute time-scale is observed for the first time for organic molecules.;The process of photo-induction of fluorescence is demonstrated to require two consecutive photon excitations as shown for quercetin. As a result a quasi-new quercetin conformational species is created. The fluorescence properties of quercetin after photoinduction are compared to fisetin.;The competition of 3-OH and 5-OH groups of hydroxyflavones for the H-bonding with the common 4-carbonyl group is explored. The influence of the number of electron-donating substitutions (-OH groups) on the electronic properties of flavones is studied using the series (in the order of the increasing number of hydroxy substitutions): flavone, 5-hydroxyflavone, kaempferol, quercetin and myricetin.