Programming Charge Dynamics in Photocatalytic Covalent Organic Frameworks through Heterometallic Symmetry Breaking

The symmetric potential landscape inherent to covalent organic frameworks (COFs) often impedes exciton dissociation and accelerates charge recombination, thereby limiting their photocatalytic efficacy. While donor-acceptor architectures and single-metal incorporation can modulate electronic properties, achieving long-range directional charge separation remains a fundamental challenge. Here, we report a general strategy to break the intrinsic electronic symmetry of COFs by engineering a heterogeneous distribution of metal sites within their pores. By leveraging an atomically precise salen-based zinc COF, we enable the partial substitution of zinc with the more electronegative cobalt, which induces an asymmetric metal-site distribution. This symmetry breaking generates a programmable built-in electric field that optimizes charge separation and carrier mobility. Density functional theory (DFT) calculations and photoelectrochemical analyses reveal that a Co/Zn ratio of nearly 1:5 induces the most pronounced asymmetry, creating the strongest in-plane electric field. The optimized heterometallic COF photocatalyst achieves a CO₂-to-CO conversion rate of 6917 μmol·g^-1, representing a 124-fold enhancement over the pristine ZnCOF and ranking among the highest values reported for COF-based photocatalysts. The universality of this approach is confirmed using other metal pairs (Cu/Zn and Ni/Zn), establishing heterometallic symmetry breaking as a powerful design principle for engineering charge dynamics in porous crystalline materials for solar energy conversion.