Integrating single Ni sites into biomimetic networks of covalent organic frameworks for selective photoreduction of CO

Selective photoreduction of CO 2 into a given product is a great challenge but desirable. Inspired by natural photosynthesis occurring in hierarchical networks over non-precious molecular metal catalysts, we demonstrate an integration of single Ni sites into the hexagonal pores of polyimide covalent organic frameworks (PI-COFs) for selective photoreduction of CO 2 to CO. The single Ni sites in the hexagonal pores of the COFs serve as active sites for CO 2 activation and conversion, while the PI-COFs not only act as a photosensitizer to generate charge carriers but also exert a promoting effect on the selectivity. The optimized PI-COF with a triazine ring exhibits excellent activity and selectivity. A possible intra- and inter-molecular charge-transfer mechanism was proposed, in which the photogenerated electrons in PI-COFs are efficiently separated from the central ring to the diimide linkage, and then transferred to the single Ni active sites, as evidenced by theoretical calculations. Fabrication of biomimetic photocatalytic systems consisting of PI-COFs and molecular Ni complexes for selective reduction of CO 2 is demonstrated.