Activity correlation among singlet-oxygen quenching, free-radical scavenging and excited-state proton-transfer in hydroxyflavones: Substituent and solvent effects

Abstract

Substituent and solvent effects on activities of singlet-oxygen (1O2) quenching, free-radical scavenging and excited-state proton-transfer (ESPT) in hydroxyflavones were investigated by means of laser, stopped-flow and steady-state spectroscopies together with density functional calculations. Some correlations were found among the three reaction activities. Substitution of OH group at the 5-position in hydroxyflavones increases the ionization energy, hinders the electron transfer from the molecule to 1O2 and free-radicals, and causes deactivation of the 1O2 quenching and free-radical scavenging in ethanol, although the ESPT is very efficient. OH substitution at the 7-position has a negligible influence on the 1O2 quenching and free-radical scavenging activities, but induces solvent-assisted ESPT, which is deactivated by substitution of a catechol structure at B-ring. A synergetic effect between the electron transfer and ESPT-induced potential-surface distortion is likely present on the 1O2 quenching and free-radical scavenging activities. The 1O2 quenching activity of 3,3’,4’-trihydroxyflavone monotonously increases with increasing water concentration in the ethanol solution, whereas addition of a small amount of water respectively activates and deactivates the ESPT and free-radical scavenging, whose activity changes are saturated by addition of a large amount. The reason for these results can be explained in terms of the nodal-plane model, ionization energy difference and potential-curve distortion/displacement with regard to the ESPT and solvation.