Aqueous-phase selective oxidation of styrene to benzaldehyde over an S-scheme photocatalyst: experimental and computational mechanistic insights

Abstract

The development of simple yet efficient photocatalysts capable of tuning the oxidation of molecular oxygen into targeted reactive oxygen species (ROS) remains a grand challenge for enabling targeted organic transformations. Here, we report metal-free nitrogen-doped carbon quantum dots/graphitic carbon nitride (N-CQDs/gCN) S-scheme heterojunction that achieves highly selective photooxidation of olefins in aqueous media under visible light illumination. Strategic incorporation of N-CQDs into the gCN not only enhances its charge separation and redox potential, but also improves its water dispersibility and stability, enabling unprecedented ROS control. Mechanistic experiments reveal that the active pathway involves in situ generation of singlet oxygen via oxidation of superoxide anion radicals, distinct from conventional triplet energy-transfer routes. The reaction proceeds through [2 + 2] cycloaddition between singlet oxygen and a styrene radical cation, supported by theoretical calculations. This mechanism is further corroborated by intermediate studies, in which both styrene oxide and 1-phenyl-1,2-ethanediol undergo conversion to benzaldehyde with 99 % selectivity, thereby broadening the functional scope of the photocatalyst. The catalyst tolerates a broad substrate scope, including electron-rich, electron-deficient, and sterically hindered olefins, achieving up to 99 % conversion with 73–99 % selectivity. Operational simplicity, aqueous compatibility, and exceptional recyclability position this system as a sustainable platform for selective photoredox oxidations.