Modulation of aggregation-induced emission through substituent-driven amide-imidic acid tautomerism in a phenoxybenzohydrazide-based fluorescent probe for selective Fe3+ detection.

A novel aggregation-induced emission (AIE) phenoxybenzohydrazide-based probe, namely, N'-[(4-(dimethylamino)phenyl)methylidene]-2-phenoxybenzohydrazide (L), was designed and synthesized. The probe exhibited strong AIE behavior in aqueous environments, attributed to the hydrogen bonding between the amide nitrogen atom and water molecules. This substituent-driven hydrogen-bonding interaction favours the amide-imidic acid tautomerism and the consequent restriction of intramolecular motion (RIM), leading to an efficient fluorescence enhancement upon aggregation. In contrast, a structurally related analogue, N'-(4-chloro-2-hydroxybenzylidene)-2-phenoxybenzohydrazide (L'), containing a different substituent at the imine carbon center, failed to exhibit AIE characteristics, highlighting the crucial role of electronic substituent effects in regulating hydrogen-bonding-assisted amide-imidic acid equilibrium and aggregation behavior. The probe L exhibits a highly sensitive and selective fluorescence quenching response towards Fe3+ metal ions, with a detection limit of 2.629 µM. The strong response of the probe, even in the presence of competing ions, proves its capability to be used as a reliable tool for environmental monitoring. The coordination modes between Fe3+ and L were confirmed by Job's plot, mass spectrometry, FT-IR spectroscopy, and DFT studies. The practical applicability of L was assessed via real-time estimation of Fe3+ ions in tap, river, and pond water samples. These research findings highlight the effectiveness of AIEgen L as a suitable fluorescent platform for the highly selective and sensitive detection of Fe3+ metal ions.