Porphyrin-based covalent organic frameworks with undulated layers for efficient photocatalytic CO₂ reduction

Two-dimensional (2D) porphyrin (Por)-based covalent organic frameworks (COFs) provide an attractive and effective strategy for photocatalytic CO₂ reduction, but the layered structure due to π-π stacking is challenging for the exposure of active sites and transfer of mass and photogenerated carriers. In this study, a series of 2D conjugated porphyrin-based COFs were prepared using porphyrin blocks with linking units having different degrees of twisting. According to the experimental and theoretical calculation results, owing to the large spatial steric hindrance between the two carbazole units connected by the N-N single bond, a greatly undulating layered structure was formed in NN-Por-COF, which enhanced mass transfer and exposed more catalytic sites. The introduction of carbazole also modulated the electronic structure of the porphyrin Co center, which lowered the reaction energy barrier. The optimization of the structural and electronic effects led to the excellent photocatalytic CO₂ reduction performance of NN-Por-COF, with CO conversion rates as high as 22.38 and 3.02 mmol g^-1 h^-1 under pure and diluted (10%) CO₂ atmosphere, respectively, which are superior to those of most of the reported porphyrin-based photocatalysts.