Understanding the Deactivating/Activating Mechanisms in Three Optical Chemosensors Based on Crown Ether with Na⁺/K⁺ Selectivity Using Quantum Chemical Tools

The optical properties and transduction mechanisms in three reported optical chemosensors based on crown ether with selectivity turn-on luminescence toward Na⁺ over K⁺, were investigated using Density Functional Theory/Time-Dependent Density Functional Theory (DFT/TD-DFT). The analysis of the structural stability of the conformers enables us to understand the optical properties of the sensors and their selectivity toward Na⁺. The UV-Vis absorption and the radiative channels of the adiabatic S₁ excited state were assessed. In these reported sensors, the Photoinduced Electron Transfer (PET) from the nitrogen and the oxygen (O-atoms of the substituted N-phenylaza group) lone pairs to fluorophore groups lead to a nonradiative deactivation process in the fluorophore to p-conjugated anilino-1,2,3-triazol ionophore. This Intramolecular Charge Transfer (ICT) deactivation produced the luminescence quenching in the free sensors and K⁺/C1 complexes. The Na⁺/sensor interaction produced a Chelation Enhanced Fluorescence (CHEF) due to the inhibition of the PET and ICT, which was confirmed via the calculated oscillator strength of the emission process. The K⁺/sensor interaction displayed the possibility of PET in C3; however, this fact was inconclusive to affirm the quenching of luminescence, the CHEF in C2 and C3 and the selectivity toward Na⁺ over K⁺ in these systems. For this reason, simulation of the absorption and emissions spectra (calculated oscillator strength), calculation of the kinetic parameters (in charge transfers and radiative deactivations process), analysis of the metal-ligand interaction character, and the analysis of the structural stability of the conformers were determinant factors to understand the selectivity and the optical properties of these chemosensors. The results suggest that these theoretical tools can also be used to predict the optical properties and Na⁺/K⁺ selectivity of optical chemosensors.