A theoretical procedure, via quantum chemical computations, to elucidate the detection principle of the turn‐off luminescence mechanism of an Eu‐based Metal‐Organic Framework sensor (Eu‐MOF) selective to aniline, is accomplished. The energy transfer channels that take place in the Eu‐MOF, as well as understanding the luminescence quenching by aniline, were investigated using the well‐known and accurate multiconfigurational ab initio methods along with sTD‐DFT. Based on multireference calculations, the sensitization pathway from the ligand (antenna) to the lanthanide was assessed in detail, that is, intersystem crossing (ISC) from the S1 to the T1 state of the ligand, with subsequent energy transfer to the 5D0 state of Eu3+. Finally, emission from the 5D0 state to the 7FJ state is clearly evidenced. Otherwise, the interaction of Eu‐MOF with aniline produces a mixture of the electronic states of both systems, where molecular orbitals on aniline now appear in the active space. Consequently, a stabilization of the T1 state of the antenna is observed, blocking the energy transfer to the 5D0 state of Eu3+, leading to a non‐emissive deactivation. Finally, in this paper, it was demonstrated that the host‐guest interactions, which are not taken frequently into account by previous reports, and the employment of high‐level theoretical approaches are imperative to raise new concepts that explain the sensing mechanism associated to chemical sensors.

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A methodology that allows us to explain the experimental behavior of a turn-on luminescent chemosensor is proposed and verified in 1-[(1H-1,2,4-triazole-3-ylimino)-methyl]-naphthalene-2-ol] (L1), selective to Al 3+ cations. This sensor increases its emission when interacting with ions upon excitation at 442 nm, which is denoted as the chelation-enhanced fluorescence effect. Photoinduced electron transfer is responsible for the fluorescence quenching in L1 at 335 nm, in Ni 2+ /L1 at 385 nm, and in Zn 2+ / L1 at 378 nm. In Ni 2+ /L, ligand-to-metal charge transfer (LMCT), from the molecular orbital of the ligand to the Ni 3d x 2 − y 2 orbital, can contribute to the quenching of fluorescence. Based on oscillator strength, the highest luminescence intensity of L1 at 401 nm and that of Al 3+ /L1 at 494 nm in relation to the others is evidenced. The consideration of the relative energies of the excited states and the calculation of the rate and lifetime of the electron transfer deactivation are necessary to get a good description of the sensor.

The turn-on luminescent chemosensor [2-Hydroxy-1-naphthaldehyde-(2-pyridyl) hydrazone] (L), selective to Al 3+ ions, was studied by means of density functional theory (DFT) and time-dependent-DFT quantum mechanics calculations. The UV-Vis absorption and the radiative channel from the adiabatic S 1 excited state were assessed in order to elucidate the selective sensing mechanism of L to Al 3+ ions. We found that twisted intramolecular charge transfer (TICT) and photoelectron transfer (PET), which alter the emissive state, are responsible for the luminescence quenching in L. After coordination with Al 3+ , the TICT is blocked, and PET is no longer possible. So, the emission of the coordination complex is activated, and a fluorescence effect enhanced by chelation is observed. For compounds with Zn 2+ and Cd 2+ , the luminescence quenching is caused by PET, while for Ni 2+ , ligand to metal charge transfer is the prominent mechanism. To go into more detail, the metal-ligand interaction was analyzed via the Morokuma-Ziegler energy decomposition scheme and the natural orbital of chemical valence.

We report the synthesis and theoretical study of two new colorimetric chemosensors with special selectivity and sensitivity to Ni2+ and Cu2+ ions over other metal cations in the CH3CN/H2O solution. Compounds (E)-4-((2-nitrophenyl)­diazenyl)-N,N-bis­(pyridin-2-ylmethyl)­aniline (A) and (E)-4-((3-nitrophenyl)­diazenyl)-N,N-bis­(pyridin-2-ylmethyl)­aniline (B) exhibited a drastic color change from yellow to colorless, which allows the detection of the mentioned metal cations through different techniques. The interaction of sensors with these metal ions induced a new absorption band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study via time-dependent density functional theory (TD-DFT) was performed. This method has enabled us to reproduce the hypsochromic shift in the maximum UV–vis absorption band and explain the selective sensing of the ions. For all of the systems studied, the absorption band is characterized by a π → π* transition centered in the ligand. Instead of Ni2+ and Cu2+ ions, the transition is set toward the σ* molecular orbital with a strong contribution of the 3d x 2- y 2 transition (π → 3d x 2- y 2). These absorptions imply a ligand-to-metal charge transfer (LMCT) mechanism that results in the hypsochromic shift in the absorption band of these systems.

Theoretical elucidation of the turn-off mechanism of the luminescence of a chemosensor based on a metal-organic framework (MOF) [Zn 2 (OBA) 4 (BYP) 2 ] (BYP: 4,4 0bipyridine; H 2 OBA: 4,4 0-oxybis[benzoic acid]), selective to nitrobenzene (NB) via quantum chemical computations, is presented. The electronic structure and optical properties of Zn-MOF were investigated through the combination of density functional theory (DFT) and time-dependent DFT methods. Our results indicate that the fluorescence emission is governed by a linker (BPY)-to-linker (OBA) charge transfer (LLCT) involving orbitals π-type. Next, the interaction with the analyte was analyzed, where very interesting results were obtained, that is, the lowest unoccupied molecular orbital is now composed of orbitals from NB, which changes the emissive state of the Zn-MOF. This suggests that the LLCT process is blocked, inducing the fluorescence quenching. Otherwise, the Morokuma-Ziegler energy decomposition and natural orbitals for chemical valence on the Zn-MOF-NB interactions were studied in detail, which illustrate the possible channels of charge transfer between Zn-MOF and NB. Finally, we believe that this proposed methodology can be applied to different chemosensor-analyte systems to evidence the molecular and electronic factors that govern the sensing mechanisms.

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