Unlocking Zenith Electronic State of Ni Sites in Ligand-Engineered COFs for Efficient •O2 --Driven Uranium Photoreduction.

Photocatalytic uranium reduction via the superoxide radical (•O2 -) pathway is a sustainable route, but its efficiency is constrained by rapid charge recombination and poor O2 activation. Herein, we develop a ligand engineering strategy to construct a series of Ni-diketimine-linked COFs by varying the diketone units directly coordinated to the Ni centers, enabling precise tuning of the Ni electronic state. The acenaphthylene-1,2-dione-derived Ni-COF-E reaches a peak optimum within the present series, termed the "Zenith electronic state", as evidenced by the highest ligand-to-metal charge transfer (0.584 e) and the largest charge transfer distance (1.700 Å) among the three Ni-COFs in the S1 excited state. This zenith electron density balances efficient photogenerated electron capture with enhanced O2 adsorption and a lowered barrier for •O2 - formation, which is corroborated by combined experimental and theoretical analyses. Consequently, Ni-COF-E delivers a record •O2 - generation rate of 8.48 µM h-1 and achieves >99% uranium removal from real wastewater. This work establishes direct ligand engineering as a powerful strategy for tuning single-metal sites in COFs and positions the Zenith electronic state as a new design principle for •O2 --mediated photocatalysis.