Dynamic Pyridinethiol Ligand Shuttling within Iron-Anchored Covalent Organic Frameworks Boosts CO2 Photoreduction

Abstract

Covalent organic framework (COF), as a porous crystalline material, provides a versatile platform for photocatalytic CO2 reduction reaction (PCO2RR). However, the lack of surface redox active sites and rapid photogenerated charge recombination are the major barriers limiting further enhancement of PCO2RR activity. Herein, we designed a novel photocatalytic system constructed from a dynamic D··M-A structure COF. The earth abundant metal iron sites were embedded into the triazinyl COF structure containing bipyridine units (Fe-bpy-COF), and the subsequent supplementation of pyridinethiol to the system allowed efficient CO2 photoreduction without additional photosensitizers. Remarkably, the Fe-bpy-COF system achieved an impressive formate yield of 4052 μmol g-1 h-1 and CO yield of 2123 μmol g-1 h-1, being over approximately 8.2-fold higher than that of the previously reported Re-COF under visible light irradiation. On the basis of 1H NMR titration experiments and steady-state tests of the absorption spectra, the superior photocatalytic performance is accordingly attributed to the dynamic coordination interaction between the pyridinethiol ligands and Fe-bpy-COF host, thus facilitating continuous double-electron transfer from the pyridinethiol ligands to the iron center under visible light and inhibiting photogenerated charge recombination in Fe-bpy-COF. Finally, the reaction pathways of CO2 conversion catalyzed by the reductive iron active species are elucidated by combining experimental results and density functional theory study. This provides unprecedented insights into the design of earth abundant metal-derived COF photocatalysts for efficient and selective CO2 reduction under visible light.