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

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 <i>ab initio</i> 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 <i>via</i> 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.