Visible Light Sensitized Production of Hydroxyl Radicals Using Fullerol as an Electron-Transfer Mediator

Abstract

Fullerenes and their derivatives are known to photosensitize the production of singlet oxygen (O-1(2)), but their role in generating hydroxyl radical ((OH)-O-center dot) under visible, light has not been reported. Here, we demonstrate that fullerol can mediate the electron transfer from Rhodamine B dye to O-2 under visible light irradiation, achieving simultaneous dye decolorization and (OH)-O-center dot-induced degradation of 4-chlorophenol. The hydroxyl radical is proposed to be produced via a consecutive reduction of molecular oxygen by fullerol anion radical, which is formed through the electron transfer from the dye to the triplet state of fullerol. Mechanistic investigations using various probe reagents such as superoxide dismutase (superoxide quencher), t-butanol ((OH)-O-center dot quencher), and coumarin ((OH)-O-center dot probe) provided indirect evidence for the generation of (OH)-O-center dot under visible light. Furthermore, spin trapping technique directly detected the oxidizing species such as (OH)-O-center dot, HO2 center dot, and O-1(2) in the visible light irradiated solution of RhB/fullerol mixture. It was proposed that the photochemical oxidation mechanism depends on pH: (OH)-O-center dot production is favored at acidic pH through fullerol-mediated sequential electron transfer vale O-1(2) is generated as a main oxidant at neutral and alkaline condition through the energy-transfer process. Therefore, the photochemical oxidation can be switchable between (OH)-O-center dot-driven and O-1(2)-driven mechanism by a simple pH adjustment.