Selective sensing of cyanide in aqueous solution by quinone-indole ensembles – quantitative effect of substituents on the HBD property of the receptor moiety

Abstract

Naphthoquinone-indole based receptors possessing different substituents (OMe, Me, H, Br and NO2) are rationally designed, synthesized and characterized using 1H and 13C NMR and LC–MS techniques. The structure of the bromo-substituted compound is confirmed by single crystal XRD study. The electronic, fluorescence and 1H NMR spectral studies indicate that these receptors selectively sense cyanide ion in aq. HEPES buffer-DMF (1:1v/v) medium. The results of the spectral investigations are well supplemented by electrochemical and DFT computational studies. The mechanism of sensing involves the formation of H-bond between the indole NH group and cyanide ion. The stoichiometry of the H-bonded complexes is found to be 1:1 (receptor:cyanide) and the binding constants are determined to be in the order of 105M−1. For the first time, a good quantitative correlation is observed between the HBD property of the receptors and the Hammett’s substituent constants (σp), which will allow us to tailor-made the HBD property by opting correct substituents. The detection limit observed (2.1–0.01nM) suggests that these receptors can detect cyanide ion concentration much lower than that recommend by WHO.