Why, on Interaction of Urea-Based Receptors with Fluoride, Beautiful Colors Develop

[reaction: see text] Urea-based receptors, containing electron-withdrawing chromogenic substituents, in a DMSO solution, in the presence of varying excess of fluoride, do not form H-bond complexes, but undergo stepwise deprotonation of the two N-H fragments, an event which is signaled by the development of vivid colors. Double deprotonation is also observed in the presence of hydroxide. Less basic anions (CH3COO-, H2PO4-) induce deprotonation of only one N-H.