Radiative decay channel assessment to understand the sensing mechanism of a fluorescent turn‐on Al³⁺ chemosensor

Abstract

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.

Insights into the selective sensing mechanism of a luminescent Cd(II)-based MOF chemosensor toward NACs: roles of the host–guest interactions and PET processes

“…As we reported in our recent works, the PET mechanisms in the molecular sensors based on Schiffbase can be correctly explained considering the relative energies of S 0 and S 1 electronic states of the interacting system [28,29,62,63]. Based on these results, for the best of our knowledge, the optimized geometry of the S 1 electronic state of Cd-MOF/pNA was employed for the first time to compute the excitations that constitute the emission spectrum and therefore go further the sensing mechanism.…”

“…To gain a deeper understanding of the turn-off fluorescence mechanism, we explored the optical properties of Cd-MOF and 5,5'-((thiophene-2,5dicarbonyl)bis(azanediyl))diisophthalic acid, ligand, we named as Linker, and Cd-MOF/analyte systems that were simulated via TD-DFT approach. In our previous studies, we verified the importance of considering the geometry of both the S 0 and the excited states (such as S 1 or T 1 ) to arrive at a robust interpretation of the sensing mechanisms [29,28,62,63]. Therefore, the optical spectra, i.e., absorption and emission of Cd-MOF were simulated, taking the optimized structures of the S 0 and S 1 states, respectively, as inputs in the TD-DFT calculations.…”

“…In this sense, the photoluminescence emission (fluorescence or phosphorescence) occurs from the lowest excited state of a given multiplicity, according to Kasha's rule [27]. Regarding this, it should be considered the relative energies and structures of the first excited states (S 1 or T 1 ), to explain in detail the sensing mechanism of the chemosensor luminescence [28,29]. In consequence, we consider that there is a lack of rigorous theoretical studies of these processes, not only in the sense of elucidating the feasible Scheme 1 Schematic representation of possible photophysical processes from L-MOFs, where blue balls represent the nodes (metal ions or metal clusters) and the red sticks represent linkers (organic ligands).…”

Sensing mechanism elucidation of a chemosensor based on a metal‐organic framework selective to explosive aromatic compounds

“…A precise description, both of the S 0 state and of the S 1 state, in terms of energy and structure, allowed us to explain in detail the turn-on fluorescent mechanism of the two chemosensor luminescents based on Schiff basis selective to metal ions. [60,61] Considering the importance of knowing that the emissive state of chemosensor is optimized, the S 1 state of the Zn-MOF is used to understand the origin of fluorescence in this system. Thus, the optimized geometry of the S 1 state was taken as input data to calculate the electronic transitions that constitute the emission spectrum of Zn-MOF by means of TD-DFT methods.…”

Sensing Mechanism Elucidation of a Chemosensor Based on a Metal-Organic Framework (MOF) Selective to Explosive Aromatic Compounds

“…A precise description, both of the S 0 state and of the S 1 state, in terms of energy and structure, allowed us to explain in detail the turn-on fluorescent mechanism of the two chemosensor luminescent based in Schiff basis selective to metal ions. [53][54] Considering the importance of knowing the emissive state of chemosensor is optimized S 1 state of the Zn-MOF to understand the origin of fluorescence in this system. Thus, the optimized geometry of the S 1 state was taken as input data to calculate the electronic transitions that constitute emission spectrum of Zn-MOF by means of TD-DFT methods.…”