A fluorescein-based MOF for screening proton reduction centers by fluorescence quenching for photocatalytic H₂...

Photocatalytic hydrogen evolution typically requires a photosensitizer, a proton reduction catalyst, and a sacrificial electron donor. Although redox-active transition metal ions are widely used as proton reduction catalysts to replace expensive Pt, a simple and rapid method to screen highly active centers among them is still lacking. Herein, we prepared a zinc-based MOF using fluorescein and a pyridyl-based mixed ligand. Fluorescein ensures excellent visible-light absorption, while the pyridyl ligand provides accessible coordination sites for metal ions. Fluorescence titration experiments revealed distinct quenching responses toward different redox-active transition metal ions, following the order Fe³⁺ > Ni²⁺ > Cu²⁺ > Co²⁺, indicating efficient photoinduced electron transfer from the MOF to the metal centers. IR and XPS analyses confirmed that Fe³⁺ coordinates with the uncoordinated pyridinic nitrogen atoms of the framework. Under optimized conditions, the Fe³⁺-MOF system achieved a hydrogen production of 582.3 μmol g^-1. When Fe³⁺ was replaced with Ni²⁺, Cu²⁺, and Co²⁺ under identical conditions, the hydrogen evolution amounts were 460.9, 302.3, and 222.5 μmol g^-1, respectively, consistent with the order of fluorescence quenching efficiency. Control experiments with TEA, CV, colloidal Pt, and [Co(bpy)₃]Cl₂ confirmed that reductive quenching by TEA dominates the initial quenching of the excited state, intrinsic reduction potentials do not directly govern activity, and Zn-FLDP acts as a photosensitizer whose coordination enhances efficiency. These results establish fluorescence quenching as a practical screening tool for highly active proton reduction centers.