Benzoquinone based chemodosimeters for selective and sensitive colorimetric and turn-on fluorescent sensing of cyanide in water

Abstract

Dibromobenzoquinone-indole ensembles containing different substituents (OMe, Me, H, Br, and NO2) have been designed, synthesized and characterized using spectral techniques. These chemodosimeters selectively sense cyanide ion colorimetrically with turn-on fluorescence in aqueous HEPES buffer. Absorption and emission spectral studies suggest that the interaction between the receptors and cyanide is strong (binding constant in the order of 104M−1) with 1:1 stoichiometry. The detection limit for these receptors to sense cyanide is found to be in the nano molar range which is far below the permissible limit recommended by WHO for cyanide ion in drinking water. 1H and 13C NMR titration results indicate that the mechanism of sensing of cyanide by these receptors is through nucleophilic addition of cyanide to the CC bond of the indole ring. The electro-reduction potentials for the quinone moiety before and after the addition of cyanide substantiate the proposed mechanism. Mulliken charges computed, using DFT method, suggest that these receptors possess an electropositive C-atom for the addition reaction to occur, while such charge polarization is not there in similar compounds containing benzoquinone or naphthoquinone. The effect of substituents on the λICT, binding constant and δNH of indole has been explained using Hammett's substituents constants (σp).