Role of Cd ²⁺ and Ag ⁺ as Codoping Species in the Mechanism of the Luminescence Activation/Deactivation of a Ln ³⁺ @UiO-66-(COOH) ₂ Chemical Sensor: A Quantum Chemistry Approach

Postsynthetic modification (PSM) as lanthanide incorporation combined with transition metal codoping has proven to be a highly effective strategy for significantly enhancing the luminescence efficiency of metal-organic framework (MOF)-based materials. In this study, we investigated a series of luminescent materials: Ln³⁺/M@UiO-66-(COOH)₂ (M = Cd²⁺, Ag⁺; Ln³⁺ = Eu³⁺, Tb³⁺). These MOFs hold 1,2,4,5-benzenetetracarboxylic acid (H₄btec) as organic linkers, which provide coordination sites that facilitate the incorporation of codopant metal ions into the MOF structure. The primary goal of this work is to elucidate the photophysical mechanisms underlying energy sensitization and emission pathways in these systems, with special emphasis on the influence of transition metal ions on the excited-state dynamics and energy transfer to the Ln³⁺ emitting centers. The results obtained using multiconfiguration calculations (CASSCF/NEVPT2) and time-dependent DFT (TDDFT) reveal that the presence of Cd²⁺ and Ag⁺ ions stabilizes the triplet state T₁ of the linker, favoring a more efficient energy transfer to the excited states of Eu³⁺. This stabilization allows direct sensitization to the ⁵D₂ level, instead of ⁵D₄ as occurs in the absence of codopants, which reduces energy losses associated with nonradiative processes and results in a significant improvement in Eu³⁺ luminescence. Building upon these findings and to broaden the scope of our investigation, we extended the study to a new luminescent sensor where Eu³⁺ was replaced by Tb³⁺. The new Tb³⁺-based system exhibited an even higher luminescence efficiency, which can be attributed to the direct sensitization of the ⁵D₄ excited state, from which radiative deactivation occurs efficiently. Overall, this study demonstrates that combining lanthanide doping with transition metal codoping allows for rational modulation of excited-state processes, providing a strategic framework for the design of high-performance luminescent MOF-based sensors.