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

Fullerenes and their derivatives are known to photosensitize the production of singlet oxygen (¹O₂), but their role in generating hydroxyl radical (^•OH) under visible light has not been reported. Here, we demonstrate that fullerol can mediate the electron transfer from Rhodamine B dye to O₂ under visible light irradiation, achieving simultaneous dye decolorization and ^•OH-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 quencher), and coumarin (^•OH probe) provided indirect evidence for the generation of ^•OH under visible light. Furthermore, spin trapping technique directly detected the oxidizing species such as ^•OH, HO₂^•, and ¹O₂ in the visible light irradiated solution of RhB/fullerol mixture. It was proposed that the photochemical oxidation mechanism depends on pH: ^•OH production is favored at acidic pH through fullerol-mediated sequential electron transfer while ¹O₂ 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-driven and ¹O₂-driven mechanism by a simple pH adjustment.