Rational engineering of triazine-benzene linked covalent-organic frameworks for efficient CO₂ photoreduction

Three large π-conjugated and imine-based COFs, named TFP-TAB, TFP-TTA, and TTA-TTB, were synthesized via the ordered incorporation of benzene and triazine rings in the same host framework to study how the structural units affect the efficiency of CO₂ photoreduction. Results from both experiments and density-functional theory (DFT) calculations indicate the separation and transfer of the photoinduced charges is highly related to the triazine-N content and the conjugation degree in the skeletons of COFs. High-efficiency CO₂ photoreduction can be achieved by rationally adjusting the number and position of both benzene and triazine rings in the COFs. Specifically, TTA-TTB, with orderly interlaced triazine-benzene heterojunctions, can suppress the recombination probability of electrons and holes, which effectively immobilizes the key species (COOH) and lowers the free energy change of the potential-determining step, and thus exhibits a superior visible-light-induced photocatalytic activity that yields 121.7 μmol HCOOH g^-1 h^-1. This research, therefore, helps to elucidate the effects of the different structural blocks in COFs on inherent heterogeneous photocatalysis for CO₂ reduction at a molecular level.