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

Abstract

The luminescence properties of based-MOFs materials have been improved by incorporating lanthanide ions (Ln3+) via post-synthetic modifications (PSM). In this report, a quantum chemical theoretical protocol has been carried out to elucidate the detection principle of the turn-on luminescence mechanism in the modified MOF labeled as Ln3+@UiO-66-(COOH)2 (Ln3+= Eu3+, Tb3+). The MOF is constituted by the nodes Zr6-octahedron {[Zr6(μ3-O)4(μ3-OH)4]12+}, the ligands (also called linkers) 1,2,4,5-benzenetetracarboxylic acid (H4btec), and the Eu3+ and Tb3+ ions which coordinate through the free carboxylate groups. Multiconfigurational post-Hartree–Fock method via CASSCF/NEVPT2 calculations and the analysis of the rate constants associated with radiative and nonradiative deactivations (kF, kP and kISC), 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 H4btec linker through the 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 a radiative deactivation towards the 7FJ states and the enhanced luminescence. Moreover, the Tb3+ ion was considered as an alternative to Eu3+. In this case, the results showed a similar sensitization channel where the energy transfer could occur likely towards the 5D2 state of Tb3+. This theoretical protocol offers a powerful tool to investigate the photophysical properties of MOFs-based systems doped with lanthanide ions.