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.