Enhanced pre-sensitization in metal-free covalent organic frameworks promoting hydrogen peroxide photosynthesis

Pre-sensitization caused by the intersystem crossing (ISC) of the photosensitizers for selective generation of singlet state oxygen (1O2) plays a crucial role in 2-electron O2 photoreduction toward high-yield hydrogen peroxide (H2O2), which has long been ignored. Here, the metal-free, crystalline imine-linked triazine covalent organic frameworks with cyano group-substituted (CN-COF) or non-substituted (H-COF) edge units have been developed to regulate the pre-sensitization. The incorporation of cyano group endowed CN-COF with a strong D-π-A structure, which caused a broad visible-light responsive absorption and dramatically increased charge separation, while providing highly stable photogenerated electrons. Such collaborative effect facilitated the ISC process from singlet to triplet excited states, which enabled the activation of O2 from ·O2− (H-COF) to 1O2 (CN-COF) under visible light irradiation, resulting in significantly improved activity and selectivity for H2O2 production. The CN-COF exhibited a high H2O2 yield rate of 2623 μmol h−1 g−1, and a remarkable apparent quantum efficiency of 9.8 % at 420 nm with an excellent selectivity of 94 %. This work suggests a promising cyanation approach to enhance the photosensitization of O2 to 1O2 based on metal-free organic semiconductors toward efficient O2-to-H2O2 photosynthesis.