Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel Photosensitizer and Electron Shuttle.

Quinones are important electron shuttles as well as micropollutants in the nature. Acetylacetone (AA) is a newly recognized electron shuttle in aqueous media exposed to UV irradiation. Herein, we studied the interactions between AA and hydroquinone (QH) under steady-state and transient photochemical conditions to clarify the possible reactions and consequences if QH and AA coexist in a solution. Steady-state experimental results demonstrate that the interactions between AA and QH were strongly affected by dissolved oxygen. In O-rich solutions, the phototransformation of QH was AA-independent. Both QH and AA utilize O as the electron acceptor, but in O-insufficient solutions, AA became an important electron acceptor for the oxidation of QH. In all cases, the coexistence of AA increased the phototransformation of QH, whereas the decomposition of AA in O-saturated and oversaturated solutions was inhibited by the presence of QH. The underlying mechanisms were investigated by a combination of laser flash photolysis (LFP) and reduction potential analysis. The LFP results show that the excited AA serves as a better electron shuttle than QH. As a consequence, AA might regulate the redox cycling of quinones, leading to significant effects on many processes, ranging from photosynthesis and respiration to photodegradation.