Engineering π···π-Stacking Molecular Nanosheets of Iron Complexes for Improving Photocatalytic CO₂ Reduction.

As a type of emerging two-dimensional (2D) material, molecularly stacking MOLs with nanosized thickness have emerged as a potential platform for photocatalysis. However, the molecular design of precise photogenerated carrier transport pathways is challenging for this type of 2D photocatalyst. Here, we report the strategy of π···π stacking assembly of 2D ultrathin MOLs for boosting CO₂ photoreduction. Single-crystal X-ray diffraction analysis shows that Fe-mbb features vertically stacked Fe complexes held together by isotropic π···π stacking in two dimensions; however, Fe-mbm features anisotropic stacking via π···π and C-H···π interactions in different dimensions, giving rise to the difference in stacking patterns within 2D ultrathin MOLs. As a consequence, the MOL with 2D π···π stacking exhibits superior photocatalytic performance in terms of CO₂ photoreduction to CO due to the enhanced separation efficiency of photogenerated carriers supported by ultrafast spectroscopic techniques. DFT calculations further reveal that the enhanced catalytic efficiency is associated with stronger CO₂ adsorption and lower Gibbs free energy for the formation of key intermediates. These findings indicate that intermolecular stacking is an efficient strategy for the synthesis of emerging 2D materials, thereby meeting the key requirements for accelerating carrier transport in artificial photosynthesis.