Regulating Ligand-to-Metal Charge Transfer in UiO-66(Zr)-NH₂ With Site-Isolated Cu⁺ Centers for Efficient...

Site-isolated metal centers within metal-organic frameworks (MOFs) offer a promising strategy to regulate charge-transfer dynamics and enhance photocatalysis. Here, we investigate the influence of atomically dispersed Cu ions on the photophysics and photocatalytic activity of the zirconium-based MOF UiO-66(Zr)-NH₂. Cu was incorporated via wet impregnation, producing Cu@UiO-66(Zr)-NH₂ composites with preserved crystallinity. Structural and spectroscopic analyses confirm that Cu is stabilized predominantly as site-isolated Cu⁺ species interacting with the amino-functionalized linker. Photoluminescence studies reveal that Cu incorporation introduces an additional ligand-to-Cu charge-transfer pathway that competes with the intrinsic ligand-to-cluster transfer to the Zr₆O₄(OH)₄ nodes, resulting in strong photoluminescence quenching. Time-resolved photoluminescence and transient infrared spectroscopy demonstrate that Cu sites efficiently capture photogenerated electrons, suppressing energy dissipation and accelerating charge-separation processes. These electronic effects significantly enhance photocatalytic oxygen reduction for H₂O₂ production under simulated solar irradiation. The H₂O₂ yield upon using 5 mg of MOF increased from ∼1.5 µmol  h^-1 for pristine UiO-66(Zr)-NH₂ to ∼8.2 µmol  h^-1 for the 7 wt% Cu-loaded material. Mechanistic investigations indicate Cu⁺ centers promote oxygen adsorption and activation, favoring the two-electron oxygen reduction pathway. This work demonstrates that site-isolated Cu ions can modulate ligand-to-metal charge-transfer processes in MOFs, offering a rational strategy to design efficient photocatalysts.