Electron-Rich Nanorelay Enhances Ligand-to-Metal Charge Transfer in Lanthanide Metal-Organic Frameworks during Photocatalytic CO₂ Conversion.

Abstract

Lanthanide metal-organic frameworks (Ln-MOFs) exhibit potential yet limitations for photocatalytic CO₂ conversion. Their "potential" stems from their CO₂ adsorption capability, attributed to the ligand-field effects and specialized pore microenvironments; the "limitation" is due to the inertness of ligand-to-metal charge transfer (LMCT) originating from the 4f electron shielding effect, high ionization energies, and low electron affinities. Introducing electron-rich active components, acting as nanorelays, into Ln-MOF pores offers a viable strategy to overcome these drawbacks, though the underlying mechanisms remain to be elucidated. Herein, the mechanistic pathway underlying efficient photocatalytic CO₂ conversion in the B₁₂H₁₂@Tb-based MOF composite is elucidated, wherein the electron-rich closo-[B₁₂H₁₂]^2- serves as the counteranion and functions within the pores as an electron nanorelay. The crystalline structure of this composite is revealed by single-crystal X-ray diffraction data. In situ and transient techniques, together with theoretical calculation, uncovered the serial mechanism, including how closo-[B₁₂H₁₂]^2- nanorelay facilitates a radical-assisted electron transfer and subsequently improves LMCT within Ln-MOF, as well as how Tb³⁺ serves as the catalytic active center for the intermediate process pathways of CO₂ molecules. These mechanistic studies not only unveil the "black box" of MOF-based composite photocatalyst, but also provide solutions for the development of efficient artificial photosynthesis catalysts.