Theoretical study of the mechanisms of activation/deactivation of luminescence in a UiO-66 type sensor modified with Ln³⁺ (Eu and Tb) as dopant ions

The luminescence properties of MOF-based materials have been improved by incorporating lanthanide ions (Ln³⁺) <i>via</i> post-synthetic modifications (PSM). In this report, a quantum chemical theoretical protocol was carried out to elucidate the detection principle of the <i>turn-on</i> luminescence mechanism in a modified MOF labeled as Ln³⁺@UiO-66-(COOH)₂ (Ln³⁺ = Eu³⁺ or Tb³⁺). The MOF is composed of Zr₆-octahedron {[Zr₆(μ₃-O)₄(μ₃-OH)₄]¹²⁺} nodes; ligands (also called linkers) of 1,2,4,5-benzenetetracarboxylic acid (H₄btec); and Eu³⁺ and Tb³⁺ ions, which coordinate through the free carboxylate groups. The multiconfigurational post-Hartree-Fock method <i>via</i> CASSCF/NEVPT2 calculations and the analysis of rate constants associated with radiative and nonradiative deactivations (<i>k</i>_F, <i>k</i>_P and <i>k</i>_ISC) were used to understand the photophysical processes governing the sensing mechanisms in the Ln³⁺@UiO-66-(COOH)₂ sensor. The most likely sensitization channel is the population of the first excited triplet (T₁) state of the H₄btec linker through intersystem crossing (S₁ → T₁), followed by energy transfer (ET) from H₄btec (T₁) to Eu³⁺ (⁵D₄), followed by vibrational relaxation (VR) processes from the ⁵D₄ state to the ⁵D₀ (Eu³⁺) state, producing radiative deactivation towards ⁷F_<i>J</i> states and enhanced luminescence. Moreover, the Tb³⁺ ion was considered an alternative to Eu³⁺. In this case, the results showed a similar sensitization channel in which energy transfer could occur, likely towards the ⁵D₂ state of Tb³⁺. This theoretical protocol offers a powerful tool to investigate the photophysical properties of MOF-based systems doped with lanthanide ions.