Gradient Polarization Induces Three-Dimensional Asymmetric Electron Distribution in Covalent Organic Frameworks for Dramatically Enhanced Photocatalytic Overall Nitrogen Fixation.

Abstract

Constructing a donor-acceptor (D-A)-based covalent organic frameworks (COFs) is an effective approach to enhance photocatalytic efficiency, yet the spatial conformation imposes inherent trade-offs between in-plane and interplane carrier transport. Here, we present a spatial gradient polarization strategy that combines distinct electronegativities of (NH)₂─C═S and C═O groups in COFs to establish a 3D asymmetric electron distribution and the gradient polarization. Mechanistic studies have shown that the gradient polarization triggers the interlayer dipole rearrangement in a nonpolar orientation, thereby constructing a cooperative in-plane and out-of-plane polarization field. This polarization field decouples the constraints on carrier transport and drives rapid charge transfer in an anisotropic manner, breaking the traditional perception of D-A intramolecular polarization within the plane. Moreover, the resulting polarized microenvironment activates N≡N bonds, optimizes hydrogen-bond networks, and stabilizes reaction intermediates, enabling the sequential conversion pathway of nitrogen. Ultimately, the covalent organic framework containing (NH)₂─C═S and C═O groups (SBO) exhibited excellent photocatalytic performance with yields of NH₄ ⁺ and NO₃ ^- of 11.59 and 7.18 mg g^-1 h^-1, surpassing all reported systems. Additionally, under natural sunlight, SBO also achieves unprecedented NH₄ ⁺ (87.17 mg m^-2 h^-1) and NO₃ ^- (73.63 mg m^-2 h^-1) yields. Our research not only offers a strategic blueprint for modulating the 3D polarization in COFs but also bridges the gap in the relationship between the molecular structure of materials and the spatial carrier migration.