Quantum chemical elucidation of the luminescence mechanism in a europium(iii) co-doped UiO-66 chemosensor selective to mercury(ii)

Abstract

Lanthanide(III) ions can be incorporated into metal–organic frameworks (MOFs) to form Ln@MOFs through post-synthetic procedures. This makes the MOFs efficient luminescent chemical sensors for detecting trace amounts of heavy metals. In this report, a quantum chemical theoretical protocol has been carried out to elucidate the detection principle of the turn-off luminescence mechanism in a Eu@UiO-66(DPA)-type MOF selective to Hg²⁺ ions. UiO-66(DPA) is an iso-reticular MOF of UiO-66 constructed from the Zr₆-cluster [Zr₆(μ₃-O)₄(μ₃-OH)₄]¹²⁺ and the ligands 1,4-benzenedicarboxylate (BDC) and 2,6-pyridinedicarboxylate (DPA) as linkers. The sensitization and energy transfer (ET) in UiO-66(DPA) doped with Eu³⁺ were analyzed using multireference ab initio CASSCF/NEVPT2 methods and time-dependent density functional theory (TD-DFT). The cluster model used in the calculations comprises the Z₆-cluster/BDC/DPA fragments with the DPA ligand coordinating to Eu³⁺ or Hg²⁺ ions. The proposed sensitization pathway involves intersystem crossing from S₁(DPA) to T₁(DPA), a plausible subsequent energy transfer from T₁(DPA) to the ⁵D₁ state of Eu³⁺, and then vibrational relaxation to the emissive ⁵D₀ state. These results also suggest that the electronic states of the BDC ligand can be strengthened by the population of the T₁ electronic states of the DPA antenna via ET. Periodic DFT calculations confirm the electronic state mixture of BDC and DPA linkers in the conduction bands, just above the electronic state of Eu³⁺ ions, which is in concordance with the proposed Eu³⁺ sensitization pathways. The assessed optical properties (absorption and emission) of Hg²⁺@UIO-66(DPA) explain the experimental behavior of this chemosensor when the Hg²⁺ ion replaces the Eu³⁺ ion and the luminescence diminishes.