Reaction-Induced Fluorescence Switching of an Indole-Based Schiff Base for Selective Cyanide Sensing

The fluorescence response of a Schiff base ligand (L), synthesized from 4-(aminomethyl)indole and 2,3-dihydroxybenzaldehyde, was investigated in the presence of cyanide ions in a CH₃CN/H₂O (7:3, v/v) medium. Mechanistic investigations indicate that cyanide recognition proceeds through nucleophilic addition at the imine center rather than conventional non-covalent interaction, altering the electronic structure of the molecule and generating a strongly emissive species. This transformation was confirmed by ¹H NMR titration and further supported by ESI-MS analysis. Density functional theory (DFT) calculations further revealed a marked redistribution of frontier molecular orbitals together with an increase in the HOMO-LUMO energy gap from 4.24 to 5.01 eV, consistent with the experimentally observed fluorescence activation. As a consequence of this reaction-driven process, ligand L displayed a pronounced emission enhancement at 474 nm upon exposure to CN⁻, whereas other tested anions produced negligible responses. Quantitative fluorescence measurements yielded an association constant of 5.66 × 10⁴ M⁻¹ and a detection limit of 0.055 µM. A 1:1 probe-cyanide stoichiometry was established from continuous variation studies and further supported by ESI-MS analysis. These findings demonstrate that reaction-induced structural transformation of the Schiff base framework enables selective fluorescence-based cyanide sensing.