Benzotrithiophene and triphenylamine based covalent organic frameworks as heterogeneous photocatalysts for benzimidazole synthesis

Metal-free covalent organic frameworks (COFs) as visible-light active and recyclable photocatalysts afford an eco-friendly and sustainable option to classical photosensitizers, which usually require noble metals (iridium, ruthenium, rhodium, etc.) to produce photocatalytic activity. Most classical small molecule photosensitizers have poor recyclability with certain limitations. As a result, it is of great significance to develop a metal-free and easily recyclable COF photocatalyst. In this study, we designed and synthesized a new type of COF photocatalyst (BTT-TPA-COF) in which benzotrithiophene and triphenylamine units are alternately connected. It has high specific surface area, permanent porosity and good stability. In addition, this design strategy can effectively adjust the band gap, energy level and photoelectric performance of BTT-TPA-COF. As a metal-free photocatalyst, BTT-TPA-COF exhibits high-efficiency photocatalytic activity, excellent substrate tolerance and excellent recyclability for the synthesis of 2-arylbenzimidazole compounds. This research not only puts forward a design strategy for high-efficiency photocatalysts, but also broadens the application range of COF materials in photocatalytic organic reactions.