Multistate luminescent probe: ICT-driven dual ESIPT-AIE for selective fluoride and cyanide ion recognition

Abstract

A highly selective and sensitive fluorescent chemosensor, HNBZT, has been designed and developed for the detection of F⁻ and CN⁻ ions. HNBZT exhibits excited-state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), and aggregation-induced emission (AIE) phenomena on a single molecular platform. Its photophysical and sensing behaviour has been evaluated using absorption and fluorescence spectroscopy. Notably, HNBZT displays a ratiometric fluorescence “turn-on” response to both anions in a CH₃CN solvent system. The binding of F⁻ and CN⁻ ions to HNBZT results in distinct absorption and emission colour changes, attributed to the unique electronic and structural perturbations induced by their respective interactions, facilitating visual detection. The lowest detection limits for F⁻ and CN⁻ ions are 7.6 × 10⁻⁸ M and 1.1 × 10⁻⁷ M, respectively. The coherence between the DLS and FESEM results underscores the consistency and reliability of the observed aggregation behaviour of HNBZT. ¹H NMR titrations of HNBZT with F⁻ and CN⁻ ions reveal distinct binding mechanisms, with F⁻ ions forming hydrogen bonds, while CN⁻ ions induce nucleophilic addition, which is also supported by theoretical studies. The binding stoichiometry of HNBZT with F⁻ and CN⁻ ions is determined to be 1 : 1 using Job's plot analysis.