Substituent-Tuned Benzoyl Thiourea Chemosensors for Selective Fluorescence Quenching of Cu

Para-substituted benzoyl thiourea-based chemosensors L1 (H), L2 (Me), and L3 (Br) incorporating 2-aminobenzothiazole were successfully synthesized and characterized by FT‒IR, UV‒Vis, DSC, NMR, CHNS, and ESI-MS analyses. All ligands exhibited high selectivity and sensitivity toward Cu2+ ions, showing distinct colorimetric and fluorescence “turn-off” responses accompanied by a visible color change from colorless to yellow. The fluorescence quenching was attributed to an intramolecular charge transfer (ICT) mechanism upon Cu2+ coordination. Among the three, L2 displayed the best sensing performance, with limits of detection of 0.95 µM (UV‒Vis) and 6.00 µM (fluorescence), and the highest binding constant (7.81 × 104 M-1) in a DMSO/H2O (10:1, v/v) medium. Job’s plot confirmed a 1:1 ligand-to-metal binding stoichiometry, while DFT calculations revealed a reduction in the HOMO-LUMO energy gaps and enhanced electron delocalization upon Cu2+ complexation. For practical validation, test strips impregnated with the ligands successfully detected Cu2+ in real water samples, and fluorescence recovery upon EDTA addition confirmed the reversibility of the sensing process. These findings underscore the influence of electronic substituents on sensitivity and stability, identifying L2 as a promising chemosensor for environmental Cu2+ monitoring.