Schiff Base Compounds Derived from 5-Methyl Salicylaldehyde as Turn-On Fluorescent Probes for Al3+ Detection: Experimental and DFT Calculations

Using 5-methyl salicylaldehyde (2) as a reactant to react with different amines, 2-aminobenzimidazole (1a), 2-aminobenzothiazole (1b), and 2-aminopyridine (1c), respectively, three types of Schiff base fluorescent probes 3a-3c were designed and synthesized for selective detection of Al3+ in aqueous media. The structure of the compounds was acquired by 1H NMR, 13C NMR, and X-ray single-crystal diffraction. Furthermore, their photochromic and fluorescent behaviors have been investigated systematically by fluorescence spectra. Compounds 3a-3c can exhibit high selectivity, sensitivity, and anti-interference properties towards Al3+ in aqueous media. Among them, the limit of detection (LOD) of probe 3b for Al3+ is 2.81 × 10-7 M. Notably, the response times of probes 3a-3c for Al3+ are 90 s, 80 s, and 80 s, respectively, which are much faster than most previously reported probes. The coordination stoichiometry between compounds 3a-3c and Al3+ has been verified to be 1:1 through the Job's plot. After coordination with Al3+, the C=N isomerization of compounds 3a-3c is inhibited, leading to the closure of the excited state intramolecular proton transfer (ESIPT) effect. At the same time, the fluorescence intensity is significantly increased through chelation-enhanced fluorescence mechanism (CHEF), which is confirmed by density functional theory (DFT) calculations. In addition, probes 3a-3c can be potentially applied in the selective and high-precision detection of Al3+ in environmental systems.