Intramolecular excited-state proton transfer in 3-hydroxyflavone. Hydrogen-bonding solvent perturbations

Abstract

The phenomenon of excited-state proton transfer in 3-hydroxyflavone is shown to depend sensitively on traces of H-bonding impurities in hydrocarbon solvents. In extremely dry and highly purified hydrocarbon solvents, a unique tautomer yellow-green fluorescence (region I) is observed from 298 to 77 K, independent of solvent temperature and viscosity, in contradiction to the results of previous research. With traces of water present, three regions of fluorescence of 3-hydroxyflavone of 3-hydoxyflavone (2.0 x 10/sup -5/ M in methylcyclohexane (MCH)) can be observed, the tautomer yellow-green fluorescence (maximum at 523 nm) (region I), another green fluorescence (maximum at 497 nm) (region II) attributed to the solute anion, and a blue-violet fluorescence (maximum at 400 nm) (region III) attributed to the normal electromer of 3-hydroflavone. Excitation spectroscopy confirms the presence of a series of ground-state solvates which are correlated with the diverse luminescence behavior observed with water, alcohol, and ether both as trace impurities and as pure solvents. Potential energy curves for the various molecular species studied, and for various solvation modes, are used to reinterpret laser kinetic studies previously published. In particular the reported biexponential normal molecule fluorescence (III) decay, and tautomer fluorescence (I) rise time, are shown to represent a slowmore » solvent-reorganization step from the polysolvated 3-hydroxyflavone and an ultrarapid intrinsic portion-transfer step for the intramolecularly H-bonded 3-hydroxyflavone.« less