Research Progress of Porphyrin-Based Covalent Organic Frameworks in Photocatalysis

Covalent organic frameworks (COFs) are a class of crystalline organic porous materials formed by molecular building blocks via covalent bonds. According to the dimensions of the framework extended in space, COFs can be divided into two-dimensional and three-dimensional COFs. Owing to their high specific surface area, good stability and strong designability, COFs have broad prospects in the fields of gas adsorption and separation, catalysis, sensing, optoelectronics, and energy storage. In the field of photocatalysis, COFs have the following advantages. First, COFs are highly designable, which can achieve efficient photocatalysis by introducing different functional units to adjust the band gap energy and light absorption range. Secondly, the ordered structure of COFs improves the electron-hole separation efficiency. In addition, the high specific surface area and abundant active sites of COFs can promote the photocatalytic reaction and improve the reaction rate. Finally, COFs are connected by covalent bonds, which make them highly stable and facilitate the maintenance of catalytic activity in the photocatalytic process. Porphyrins are a class of 18 pi-electron conjugated macrocyclic compounds formed by the interconnection of four pyrroles, which can be involved in photosynthesis as the core part of chlorophyll and other biomolecules. As a kind of functional units with large pi-conjugated structure and good photophysical properties, porphyrins usually have strong absorption in the 400 similar to 450 nm (Soret band) and 500 similar to 700 nm (Q band) regions, and their photocatalytic properties can be regulated by coordination with metal ions and modification of functional groups, which has unique advantages in the field of photocatalysis. By introducing porphyrins into COFs, combined with their unique advantages, porphyrin-based COFs have been found great potential in the field of photocatalysis due to the exceptional optical absorption in a broad spectral range, multiple active sites and effective electron-hole separation ability. As a new type of photocatalyst, porphyrin-based COFs have attracted much interest and have been rapidly developed in the field of photocatalysis. In this review, we focus on the discussion of porphyrin-based COFs in the photocatalytic CO2 reduction, photocatalytic water splitting, photocatalytic organic transformation, and photocatalytic reduction of hexavalent uranium. Finally, the prospects and challenges of porphyrin-based COFs in photocatalysis are discussed.