ESIPT-Modulated Supramolecular Host–Guest System with Single-Crystal Structural Insights for Selective Multichannel Heavy-Metal Ion Detection and as a Potential AnCE Inhibitor

A multifunctional supramolecular host–guest system based on an ESIPT-active Schiff base ligand (<i>E</i>)-<i>N</i>′-(5-bromo-2-hydroxybenzylidene)-2-fluorobenzohydrazide monohydrate (<b>FHMB</b>) has been designed and confirmed through single-crystal X-ray diffraction analysis for multichannel heavy-metal ion sensing and explored as a potential angiotensin-converting enzyme (AnCE) inhibitor. <b>FHMB</b> exhibits excitation-dependent dual emission arising from keto–enol tautomerism, enabling sensitive modulation of fluorescence through excited-state intramolecular proton transfer regulation.

Complexation with Zn²⁺, Cd²⁺, and Pb²⁺ ions induces distinct turn-on fluorescence responses, characterized by distinct color outputs, allowing for simultaneous and differential detection in aqueous media. In contrast, FHMB exhibits exceptional selectivity toward Zr⁴⁺, producing a pronounced ratiometric fluorescence response under different sensing conditions. Binding stoichiometry and thermodynamics were elucidated using Job’s plot, Benesi–Hildebrand, and BindFit analyses, while ¹H NMR, SEM–EDX, and elemental mapping confirmed metal-induced coordination and supramolecular reorganization.