An anionic chromogenic sensor based on the competition between the anion and a merocyanine solvatochromic dye for calix[4]pyrrole as a receptor site

The interaction of Brooker’s merocyanine (BM), a merocyanine dye, with calix[4]pyrrole (CP) was studied in acetonitrile. BM is violet in solution, but its interaction with CP changes the color of the solution due to the formation of CP–BM species associated through hydrogen bonding. A displacement assay was then carried out in the presence of different anions (F−, Cl−, Br−, I−, H 2 PO 4 - , HSO 4 - , and NO 3 - ). It was verified that F−, and to a lesser extent Cl− and H 2 PO 4 - , displace BM through the formation of a complex with CP, coloring the solution. Addition of HSO 4 - makes the solution almost colorless because it is sufficiently acidic to transfer a proton to BM, removing it from the receptor site in CP and protonating the dye, thereby allowing the visual detection of the anion in relation to the other anions.