A Hydrazone-Based Fluorescent Chemosensor for Al³⁺: Turn-On Response Induced by ESIPT Suppression

Abstract

The Schiff-base tert-butyl (Z)-2-((2-hydroxynaphthalen-1-yl)methylene)hydrazine-1-carboxylate (KH) was synthesized via a condensation reaction between tert-butyl carbazate and 2-hydroxy-1-naphthaldehyde in methanol. The compound KH functions as an efficient fluorescent chemosensor for Al3+ ions, exhibiting a strong and selective fluorescence enhancement in methanol upon Al3+ addition. The limit of detection (LOD) for Al3+ was calculated to be 3.6 × 10-7 M, which is comparable to the values reported for structurally related probes. Al3+ addition also induced a distinct bathochromic shift and increased absorption intensity in the UV–Vis spectrum. Spectroscopic titration studies revealed that KH binds Al3+ as a polydentate ligand with an association constant of Ka = 1.75 × 106 M-1, and Job’s plots confirmed a 1:1 binding stoichiometry. Free KH shows negligible fluorescence due to an efficient excited-state intramolecular proton transfer (ESIPT) pathway mediated by the naphthol proton, which produces non-emissive tautomeric species. Coordination with Al3+ deprotonates the phenolic group, suppressing ESIPT and eliminating this major non-radiative decay route. Concurrently, metal–ligand coordination rigidifies the molecular structure and induces a chelation-enhanced fluorescence (CHEF) effect, collectively promoting radiative decay. DFT and TD-DFT calculations on KH and the Al3+–K complex were performed to rationalize the photophysical behavior and to probe the ESIPT and sensing mechanisms. The computed data closely match the experimental photophysical results. Analysis of optimized geometries of the enol and keto tautomers in both ground (S0) and excited (S1) states indicates that ESIPT is significantly more favorable in S1, consistent with the calculated potential energy profiles. X-ray crystallographic analysis showed hydrogen bonding between the phenolic OH and the –C=N– nitrogen, supporting that the molecular framework is structurally predisposed for ESIPT. Hirshfeld surface analysis and 2D fingerprint plots further quantified the intermolecular interactions within the crystal lattice. Additionally, NBO analysis of the Al3+–K complex is presented.