Dynamic pyridinethiol ligand shuttling within iron-anchored covalent organic frameworks boosts CO2 photoreduction†

Abstract

A covalent organic framework (COF), as a porous crystalline material, provides a versatile platform for the 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. Earth-abundant metal iron sites were embedded into a 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 studies. This provides unprecedented insights into the design of earth abundant metal-derived COF photocatalysts for efficient and selective CO₂ reduction under visible light.