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

Using 5-methyl salicylaldehyde (<b>2</b>) as a reactant to react with different amines, 2-aminobenzimidazole (<b>1a</b>), 2-aminobenzothiazole (<b>1b</b>), and 2-aminopyridine (<b>1c</b>), respectively, three types of Schiff base fluorescent probes <b>3a</b>-<b>3c</b> 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 <b>3a</b>-<b>3c</b> can exhibit high selectivity, sensitivity, and anti-interference properties towards Al³⁺ in aqueous media. Among them, the limit of detection (LOD) of probe <b>3b</b> for Al³⁺ is 2.81 × 10^-7 M. Notably, the response times of probes <b>3a</b>-<b>3c</b> 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 <b>3a</b>-<b>3c</b> and Al³⁺ has been verified to be 1:1 through the Job's plot. After coordination with Al³⁺, the C=N isomerization of compounds <b>3a</b>-<b>3c</b> 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 <b>3a</b>-<b>3c</b> can be potentially applied in the selective and high-precision detection of Al³⁺ in environmental systems.