Covalent organic frameworks (COFs) are a promising platform for heterogeneous photocatalysis due to their stability and design diversity, but their potential is often restricted by unmanageable targeted excitation and charge transfer. 

Most COF-based photocatalytic systems for CO₂ reduction focus on integrating metal catalytic sites onto COFs, typically alongside additional homogeneous photosensitizers, but the latter suffer from rapid deactivation and low recoverability, limiting the long-term operational efficiency of these catalytic systems.

In a study published in Small, Prof. CAO Rong, Prof. GAO Shuiying and colleagues from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported a bimetallic COF photocatalyst (PBCOF_RuRe) that employs a novel in situ synthesis strategy to integrate sensitization center (Ru) and catalytic site (Re) onto conjugated COF. The design of PBCOF_RuRe aims to enhance charge transfer efficiency and construct a photocatalytic system with long-term stability.

Researchers first constructed the COF photocatalyst, namely PBCOF_RuRe, through in situ Schiff-base condensation of Pyrene with MBpy (M=Ru, Re) units. In this structure, Ru and Re are anchored within bipyridine as the photosensitive center and catalytic site, respectively. PBCOF_RuRe was confirmed to have excellent crystallinity, porous nature and high CO₂ adsorption capacity, and the Ru and Re coordination environment was elucidated, providing comprehensive structural characterization of PBCOF_RuRe.

PBCOF_RuRe exhibited an extraordinary carbon monoxide yield of 8306.6 μmol g^-1 h^-1 and selectivity exceeding 99.8% in photocatalytic CO₂ reduction, outperforming most reported COFs. This superior performance is attributed to the efficient, rapid charge transfer along its stable conjugated framework, which effectively converting CO₂ into CO. By anchoring metal sites within COFs, the reactivity loss of catalyst is minimized, resulting in superior recoverability and long-term stability compared to physically mixed photosensitizer systems.

Moreover, researchers used femtosecond transient absorption spectroscopy (fs-TAS) to investigate the ultrafast dynamic charge transfer kinetics in PBCOF_RuRe, and the results were correlated with its photocatalytic activity. It was confirmed that the light-induced charge transfer from Ru to Re along the stable conjugated structure of PBCOF_RuRe. And in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identified *COOH as a key reaction intermediate, revealing the mechanism for CO₂ reduction by PBCOF_RuRe.

This study demonstrates the synthetic feasibility of in situ integration of sensitizers and catalytic sites within COFs, highlights the effectiveness of bimetallic heterogeneous photocatalysis, and presents a new strategy for the design and synthesis of advanced photocatalysts.

In situ integration of metallic catalytic sites (Re) and photosensitive centers (Ru) within covalent organic framework (PBCOF_RuRe) for the enhanced photocatalytic reduction of CO₂. (Image by Prof. CAO’s group)