Visible-light-driven quantitative photooxidation of anthracene to anthraquinone under mild conditions

Abstract

This paper discloses a mild and atom-economic photo-oxidation protocol for visible-light-driven quantitative oxidation of anthracene (AN) to anthraquinone (AQ), which can obtain excellent AN conversion (∼100%) and AQ yield (99.2%) without any catalysts and additives under 10 h of visible light illumination. In addition, the protocol also works well with other chemical molecules that have a similar structure. In this protocol, AN is directly oxidized from AN to AQ without the observation of the intermediate anthrone (AO). Based on a series of control tests, as well as free radical quenching and electron paramagnetic resonance (EPR) spin trapping studies, it can be inferred that the process of photo-oxygenation in AN is most likely initiated by its photo-excited state, denoted as AN*. This photo-excited state has the ability to activate molecular oxygen (O2) by an electron transfer mechanism, resulting in the formation of a superoxide free radical (O2•-). Following this, the process of photo-oxidation is progressively controlled by the oxygenated product AQ, which acts as an active photo-catalyst. AQ is derived from the oxidation of AN by O2•- and its presence accelerates the process of photo-oxidation due to its rapid accumulation. Furthermore, the yield of AQ can be improved by enhancing the light intensity of the light source, timely separating the yellow solid oxidation product AQ formed during the reaction, and pre-adding the oxidation product AQ to the protocol as an active photocatalyst.