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

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.