Introduction: It is unanimously known that cyanide (CN⁻) is one of the most toxic ions because it can interfere with the body’s physiological phenomena, causing endocrine disorders, respiratory failure, hypoxia, vascular necrosis, and even death. Therefore, it is essential to develop cost-effective, sensitive, rapid, and efficient methods for sensing CN⁻ ions.

Methods: The chemosensor was synthesised via a multi-step reaction, and its photophysical properties were analysed using UV–visible and fluorescence analyses. Binding was confirmed through Job’s plot, ¹H-NMR, HR-MS, and FT-IR, with TD-DFT validation using Gaussian 09W software.

Results: The chemosensor exhibited high selectivity and specificity for CN⁻, with a distinct naked-eye colour change from yellow to blue. Additionally, a “turn-off” fluorescence response was observed, attributed to the inhibition of the Intramolecular Charge Transfer (ICT) process. The fluorescence quenching efficiency for CN⁻ was 73.21%, with a Stern–Volmer quenching constant of 1.22 × 10⁵ M⁻¹. The detection limit was determined to be 5.47 µM. Furthermore, FT-IR spectroscopy, ¹H-NMR titration, and HR-MS analysis were conducted to study the plausible binding site. TD-DFT calculations and topological analysis were performed to investigate molecular orbitals and their localisation in the free probe and its CN⁻ complex.

Conclusion: The optical behaviour of the chemosensor towards CN⁻ was noted in this study. The “naked-eye” changes led to the development of test kit and smartphone analysis. Moreover, the fluorescent “turn-off” behaviour could be exploited for confocal fluorescence imaging of CN^- in living cells.