Quantum Chemistry Study of Luminescence Quenching in the Eu³⁺@UiO-67 Sensor Induced by Ag⁺ Ions

The Eu³⁺/Ag⁺@UiO-67 system consists of a Zr₆O₄(OH)₄-based UiO-67 metal-organic framework (MOF) functionalized with 2,2'-bipyridine-5,5'-dicarboxylic acid (H₂bpydc) linkers, where Eu³⁺ and Ag⁺ ions are incorporated via post-synthetic metal-N coordination. The presence of both lanthanide and transition metal ions creates a co-doped architecture in which the photophysical properties are strongly influenced by metal-ligand interactions. This work focuses on elucidating, using theoretical tools, the luminescence-quenching mechanism induced by Ag⁺ co-doping in the Eu³⁺@UiO-67 system. To achieve this objective, a comprehensive theoretical protocol combining periodic DFT-based density of states (DOS) studies, TD-DFT excited-state analysis, and multiconfigurational CASSCF/NEVPT2 calculations was employed. Special attention was devoted to evaluating the efficiency of intersystem crossing (ISC) in the linker and the energy transfer (ET) pathways required to populate the emissive excited states of Eu³⁺. The results reveal that Ag⁺ insertion disrupts the linker to lanthanide ET process by promoting electronic states localized on the Ag⁺ centers, thereby reducing the effective T₁ population and sensitization efficiency. DOS analysis further supports this mechanism, showing orbital overlap between linker and Ag⁺ states near the conduction band edge, consistent with the experimentally observed luminescence "turn-off" effect induced by the co-dopant.