A Colorimetric Approach to Anion Sensing: A Selective Chemosensor of Fluoride Ions, in which Color is Generated by Anion-Enhanced π Delocalization

A colorless neutral receptor with no particular chromogenic substituents selectively recognizes fluoride ions with a concomitant development of a yellow color (see scheme). The receptor contains two equivalent arms, each equipped with a thiourea fragment. Color originates from the extended π delocalization induced by the strong hydrogen-bonding interaction of the F− ion with the internal NH groups of thiourea moieties.