Naphthalene–Hydrazine Conjugates as Efficient Chemosensors for Zn 2+ and Cu 2+ Ions: Design, Photophysics, and Sensing Mechanism

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Two naphthalene–hydrazine‐based Schiff base ligands, PB‐1 (bearing an anthracene moiety) and PB‐2 (bearing a triphenylamine unit), were synthesized and fully characterized by NMR and mass spectrometry. Their metal ion sensing properties were evaluated using UV–vis absorption and fluorescence spectroscopy in a 1:1 DMSO/HEPES buffer system (pH 7.4). Both probes displayed pronounced chromogenic and fluorogenic responses toward Zn 2 ⁺, demonstrating high sensitivity and selectivity, with detection limits in the nanomolar range. In contrast, only a chromogenic response was observed for Cu 2 ⁺, while no significant responses were detected with other tested metal ions, highlighting their differential sensing ability. Job's plot analysis confirmed a 2:1 ligand‐to‐metal stoichiometry for the Zn 2 ⁺ complexes, with strong binding affinities reflected in high association constants. DFT/TDDFT calculations supported these findings by showing a substantial reduction in the HOMO–LUMO energy gap upon Zn 2 ⁺ coordination, consistent with the observed photophysical changes. Importantly, Zn 2 ⁺ binding was reversible, as evidenced by fluorescence recovery after EDTA treatment, underscoring the ligands’ reusability and practical potential for real‐time Zn 2 ⁺ detection.