Excitonic engineering in low-symmetry covalent organic frameworks through B ← N Lewis pair integration for enhanced photocatalytic performance

Uranium (VI) contamination in water poses serious environmental and health risks. However, its photocatalytic reduction is often hindered by inefficient exciton dissociation in conventional donor-acceptor (D-A) covalent organic frameworks (COFs), originating from intrinsic symmetry-related limitations. The highly symmetric architectures of D-A COFs tend to induce dipole cancellation, thereby weakening the built-in electric field (IEF) required for efficient charge separation. Herein, we propose a dual-strategy design that incorporates highly polarized B ← N Lewis pairs into D-A COFs with deliberately reduced structural symmetry. This synergistic approach breaks inversion symmetry, establishes a strong macroscopic polarization, and promotes interlayer charge delocalization and orbital overlap. The resulting framework exhibits significantly reduced exciton binding energy ( E b ), improved visible-light absorption, and prolonged charge carrier lifetime. As the representative example, 2,3-COF-BF, constructed from low-symmetry monomers and post-synthetically planarized through B ← N Lewis pair coordination, demonstrates exceptional photocatalytic performance in U(VI) reduction. This study underscores the effectiveness of integrating symmetry breaking with dipole engineering for exciton manipulation, providing a promising molecular-level strategy for high-performance photocatalysis.