Highly selective and sensitive colourimetric detection of Hg2+ ions by unsymmetrical squaraine dyes

A novel class of sulphonate group containing unsymmetrical squaraine dyes has been synthesized. These dyes show a selective and sensitive affinity towards Hg2+ in methanol and are practically insensitive to most other environmentally and biologically relevant metal ions. Isothermal titration calorimetric (ITC) studies and High Resolution Mass Spectral (HRMS) analysis indicate a 1:1 binding mode. 1H and 13C NMR of the isolated Dye…Hg2+ adduct showed an unusual addition reaction of Hg2+ along the phenyl-squarate carbon–carbon bond which was also supported by TD-DFT calculations using PM6 level geometries.

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