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

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Vol. 409; p. 113122
• Main Authors: Nagaoka, Shin-ichi, Bandoh, Yuki, Matsuhiroya, Satoki, Inoue, Kazumasa, Nagashima, Umpei, Ohara, Keishi
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.03.2021
• Subjects: Excited-state proton-transfer; Free-radical scavenging; Hydroxyflavones; Singlet-oxygen quenching; Solvent effect; Substituent effect; Free-radical scavenging; Substituent effect; Solvent effect; Excited-state proton-transfer; Singlet-oxygen quenching; Hydroxyflavones
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/j.jphotochem.2020.113122

[Display omitted] •Hydroxyflavones function as UV protectors and antioxidants in biological systems.•We have studied the excited-state proton-transfer and antioxidant reactions.•The observed results demonstrate correlation among their reaction activities.•The observed results also demonstrate noteworthy substituent and solvent effects.•The two effects could enable controls and regulations of these functions. 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.