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.
Wen et al. (Mon,) studied this question.