Fluoride-selective colorimetric sensor based on thiourea binding site and anthraquinone reporter

A structurally simple colorimetric sensor, N-4-nitrobenzene-N′-1′-anthraquinone-thiourea (1), for anions was synthesized and characterized by 1H NMR, ESI mass and IR methods. In acetonitrile, the addition of F− changed 1 solution from colorless to yellow. In the presence of other anions such as CH3CO2−, H2PO4−, HSO4− and Cl−, however, the absorption spectrum of 1 was slightly red shifted with no obvious color changes observed. The association constants of anionic complexes followed the order of F− ≫ CH3CO2− > H2PO4− > HSO4− > Cl− > Br−, which was different from the order of anion basicity. AM1 calculation results indicated that the most stable configuration of 1 existed in the Z–E-conformation with a six-membered ring via intramolecular hydrogen bond. This made thiourea moiety of 1 in an unfavorable conformation to bond with oxygen-anionic substrates such as CH3CO2− and H2PO4−, thus leading to a high selectivity and sensitivity for the detection of F−.