Microenvironment Modulation of Cu ₃ ‐based Metal‐Covalent Organic Frameworks via Methyl Substitution for Enhanced Photocatalytic CO ₂ Reduction to CH ₄

Photocatalytic CO 2 reduction offers a compelling strategy for carbon neutrality, yet its efficiency is critically limited by both inefficient charge utilization and insufficient active‐site modulation. In this study, a methyl‐functionalized trinuclear copper building unit was introduced to successfully construct Me 2 ‐Cu 3 ‐PY‐COF, while the unmodified Cu 3 ‐PY‐COF was prepared as the control sample. The introduction of methyl groups enhances electron density, narrows the band gap, and promotes charge separation and transfer, thereby improving photocatalytic CO 2 reduction performance. As a result, Me 2 ‐Cu 3 ‐PY‐COF exhibited a CH 4 production rate of 135.15 µmol·g −1 ·h −1 with a selectivity of 91.6%, which was higher than that of the control sample (86.09 µmol·g −1 ·h −1 , 81.9%). Mechanistic studies reveal that CO 2 is converted to CH 4 via a stepwise hydrogenation pathway, with Cu 3 clusters serving as the active sites undergoing partial reduction from Cu 2+ to Cu + , while imine sites facilitate CO 2 adsorption. Density functional theory (DFT) calculations demonstrate that methyl‐induced electronic modulation improves CO 2 and intermediate adsorption, which in turn promotes multi‐electron reduction. This work promotes the research on the application of trinuclear copper cluster‐based metal‐covalent organic frameworks as photocatalysts.