Metal center regulation of the porphyrin unit in covalent organic polymers for boosting the photocatalytic CO2 reduction activity

Abstract

Mimicking plant photosynthesis for converting CO₂ into high-energy chemical fuels on photocatalysts powered by sunlight is expected to simultaneously alleviate the current energy shortage and global warming. Among the various photocatalysts developed for solar energy conversion, covalent organic polymers (COPs) have attracted increasing attention because of their predictable synthetic routes, variable electronic structures, and precisely regulated active sites. Here, a series of amido-coupled metal porphyrin (MPor)–Ru(II) pincer complexes (RuN₃) COPs (MPor–RuN₃, M = Co, Ni, Cu or Zn) were synthesized and adopted as photocatalysts for the CO₂ reduction reaction (CO₂RR). It was found that the metal center regulation of the porphyrin unit in COPs can effectively boost the photocatalytic CO₂RR activity with CO yield following an order of CuPor–RuN₃ > NiPor–RuN₃ > ZnPor–RuN₃ > CoPor–RuN₃ with the aid of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. Especially, CuPor–RuN₃ COP delivered the highest CO yield under both visible light (74.3 μmol h⁻¹ g⁻¹) and full spectrum (143.7 μmol h⁻¹ g⁻¹) illumination of Xe-lamp, 4.7 and 4.4 times that of the metal-free control (H₂Por–RuN₃), respectively. The control experiments and theoretic calculations revealed that the metal center of the porphyrin unit not only affects the photoexcited charge separation but also influences the BIH oxidation dynamics on the porphyrin units, which has rarely been discussed before. Both of them then jointly contributed to the significantly improved CO₂RR activity of the CuPor–RuN₃ COP. This work shows an effective method to boost the CO₂RR performance of porphyrin-based COPs by regulating the structure and composition, and provides valuable insights into the photocatalytic mechanism at the molecular level.