1,2,4-Triazole-Based Excited-State Intramolecular Proton Transfer-Driven “Turn-On” Chemosensor for Selective Cyanide Detection with Test Strip Utility and Molecular Keypad Lock: An Experimental and Computational Exploration

Cyanide (CN^-) is a highly toxic anion with significant environmental and biological implications, necessitating the development of sensitive and selective detection platforms. In this work, we reported the design and synthesis of a novel excited-state intramolecular proton transfer (ESIPT)-based chemosensor (<b>TSB</b>) derived from 2-hydroxy-1-naphthaldehyde and 4-amino-1,2,4-triazole. Computational studies, including time-dependent density functional theory (TD-DFT), reduced density gradient scatter plots, and simulated infrared spectra, reveal the prevalence of strong intramolecular hydrogen bonds that facilitate the ESIPT phenomenon. The chemosensor elicits a naked-eye response against CN^- ions in CH₃CN:H₂O (4:1, v/v), exhibiting a color change from colorless to yellow. Meanwhile, "turn-on" behavior was observed in fluorescence spectroscopy with a change in color to teal, after the introduction of CN^- ions, allowing real-time monitoring of the ion. The designed chemosensor demonstrates high selectivity for CN^- over other common anions, with a low detection limit of 0.41 μM. Mechanistic investigations using ¹H nuclear magnetic resonance and Fourier transform infrared analysis, along with DFT analysis, confirm a deprotonation-driven interaction and enhanced hydrogen bonding in the excited state. A binding constant of 1.98 × 10⁵ M^-1 and 1:1 stoichiometry were determined. Additionally, the chemosensor-coated paper strips exhibit significant changes on the introduction of CN^- at different concentrations under ultraviolet light, revealing its utility for test strip development. Also, its application was found in the development of a molecular keypad lock. This study presents a powerful ESIPT-based sensor platform with theoretical and practical relevance for CN^- detection in semiaqueous environments.