PROTON-TRANSFER AND SOLVENT POLARIZATION DYNAMICS IN 3-HYDROXYFLAVONE

The excited-state intramolecular proton-transfer (ESIPT) dynamics of 3-hydroxyflavone (3HF) and 3-deuteroxyflavone (3DF) in nitrile solvents has been studied by using static and picosecond emission spectroscopies. ESIPT rates were determined for 3HF and 3DF in acetonitrile over a range of temperatures. Room temperatures rates of 1.7 x 10(11) s-1 and apparent activation energies of approximately 2.9 kcal/mol were obtained. Little or no kinetic isotope effect was observed. ESIPT rates for 3HF in benzonitrile were also determined over a range of temperatures. The room temperature rate was 3.1 x 10(11) s-1 and the apparent activation energy was approximately 2.0 kcal/mol. These results are analyzed in terms of a simple model in which solvent reorganization is needed to facilitate ESIPT, and once the appropriate solvent configuration is reached, proton tunneling occurs very rapidly. From this analysis we conclude the following. (1) There is a significant solvent polarization induced barrier in acetonitrile. (2) The ESIPT rate in acetonitrile is partially limited by the rate of barrier crossing, and partially limited by the rate of solvent reorganization. (3) There is little or no solvent-induced barrier in benzonitrile. (4) The ESIPT rate in benzonitrile is entirely limited by the rate of solvent reorganization.