Oxygen-atom substitution in a β-ketoenamine-linked covalent organic framework for boosting photocatalytic oxygenation of sulfides

Covalent organic frameworks (COFs), a class of crystalline porous materials, have been widely adopted as visible light photocatalysts. Benzothiadiazole is a prevalent building block for COFs, and oxygen-atom substitution of the sulfur-atom of benzothiadiazole leads to benzooxadiazole. Here, with pyrrolidine as the catalyst, two highly crystalline β-ketoenamine-linked COFs, TpBTD-COF and TpBO-COF, are synthesized by the condensation of 1,3,5-triformylphloroglucinol (Tp) with 4,4′-(benzothiadiazole-4,7-diyl)dianiline (BTD) and 4,4′-(benzooxadiazole-4,7-diyl)dianiline (BO), respectively. As such, TpBO-COF exhibits augmented visible light absorption and accelerated charge separation and transport compared with TpBTD-COF based on detailed optical and photoelectrochemical characterizations. Moreover, both TpBTD-COF and TpBO-COF can speedily promote selective oxygenation of sulfides with TEMPO as the hole transfer mediator irradiated by blue light. TpBO-COF exhibits higher activity than TpBTD-COF for the oxygenation of sulfides to sulfoxides with dioxygen. This work demonstrates that heteroatom substitution into COFs can boost the photocatalytic activity for selective organic transformations.