A colorimetric anion sensing system utilising competitive C–H…X− hydrogen bonding

Combination of tripodal imidazolium-based anion receptor 1 and azonia betaine dye 2 has enabled a colorimetric detection of halide ions by competitive binding. The imidazolium groups of receptor 1 interact with the phenoxy oxygen of betaine dye 2 in acetonitrile solution by multiple [C–H…O− ] hydrogen bonds. Consequently, the blue-coloured solution of the dye immediately turns to greenish yellow upon addition of receptor 1 due to the receptor–dye complex formation. The reversed colour change, which is caused by displacement of guests, is observed by the addition of Cl− , Br−  or I−  anions to the receptor–dye complex solution.