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(mu 3-O)4(mu 3-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.