Symmetry Breaking at Locally Active Fe Site for Switchable CO₂ Photoreduction Over Isostructural Ultrathin MOLs

Ultrathin metal-organic layers (MOLs) have emerged as a type of promising two-dimensional (2D) platforms for artificial photosynthesis, yet their activity is frequently limited by rapid recombination of photogenerated carriers in presence of structural symmetry. Hence, switching on the reactivity through breaking geometric symmetry to create unsymmetric active sites remains a significant challenge. Herein, we demonstrate a switching strategy via one-atom substitution to construct two isostructural ultrathin MOLs with distinct coordination symmetry at the iron active site. Single-crystal x-ray diffraction and spectroscopic analyses reveal that symmetry breaking at the iron site in the MOL effectively enhances CO₂ adsorption and facilitates photogenerated carrier separation. Under visible-light irradiation, the MOL with unsymmetrical sites achieves an exceptional CO production amount (ca. 21.20 mmol·g^-1), which is as high as 15.8 times more than that of its symmetrical counterpart. Time-resolved transient absorption spectroscopy corroborated by DFT calculations indicates that symmetry breaking not only accelerates the separation and transport of photogenerated charge carriers, but also lowers the Gibbs free energy of CO₂ adsorption. This work elucidates how the atomically precise modification of local coordination symmetry switches the photocatalytic performance in an 'off/on' manner and provides a viable design strategy toward emerging 2D materials for artificial photosynthesis.