Rational Regulation of the Linkage Polarity in Imine-Linked Covalent Organic Frameworks for Efficient Photocatalytic Nitrogen Reduction to Ammonia

Imine-linked covalent organic frameworks (COFs) have gained considerable attention for photocatalytic reduction of nitrogen (N 2 ) to ammonia (NH 3 ). However, the intrinsic polarization of the linking C═N bonds leads to a high energy barrier for π-electron delocalization, giving rise to high exciton binding energy and slow charge separation/transfer efficiency. In this study, the C═N linkages of the imine-linked COFs TA-BT were intentionally protonated, oxidized, and methylated by a facile postmodification strategy, thereby yielding three novel polarized COFs with different linkage polarity, namely, TA-BT-H, TA-BT-O, and TA-BT-I. The resulting COFs exhibit a higher dielectric constant and lower exciton binding energy as compared to pristine TA-BT, accelerating charge separation/transfer. Without adding sacrificial agents or cocatalysts, the NH 3 yield rates of TA-BT-H, TA-BT-O, and TA-BT-I reach 376.5, 498.2, and 320.8 μmol g –1 h –1, significantly higher than pristine TA-BT (185.6 μmol g –1 h –1 ) and surpassing most previously reported photocatalysts. The mechanism study indicates that the polarized COFs have undergone an alteration in the N 2 adsorption sites, thereby enhancing adsorption and activation. Furthermore, the N 2 reduction pathway favors an alternating hydrogenation mechanism. This finding highlights the significant potential of the linkage polarity in imine-linked COFs for photocatalytic N 2 reduction to NH 3 .