Visible-light photosensitized oxidation of α-terpinene using novel silica-supported sensitizers: Photooxygenation vs. photodehydrogenation

Three silica-immobilized organic photocatalysts, based on rose bengal (RB), antraquinone-2-carboxylic acid (ANT-COOH) and a new cyanoanthracene derivative (DBTP-COOH), were prepared and characterized. Their efficiency for the photooxidation of α-terpinene was compared to that of their soluble counterparts. In solution, the three sensitizers showed high quantum yield of singlet oxygen production. Significant autooxidation to p-cymene occurred in the absence of catalyst while the mechanism of the sensitized reaction strongly depended on sensitizer structure. With DBTP and RB materials, ascaridole was rapidly produced by singlet oxygen addition. In contrast, ANT-based sensitizers favored photodehydrogenation to p-cymene through an electron-transfer step inducing a radical chain reaction, followed by further p-cymene oxidation upon prolonged irradiation. The highest efficiency and selectivity were obtained for photooxygenation with DBTP-based materials, and for photodehydrogenation with ANT-based materials, these properties make them attractive for future applications as immobilized photocatalyst in solar synthesis, waste treatment, and microflow reactors.