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

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.