Tunability of anion binding strength based on acyl-thiourea receptors containing isatin group

Some acyl-thiourea derivatives containing isatin group were synthesized and their interactions with anions were investigated using UV-vis spectroscopy and 1H NMR titrations in DMSO and DMSO-d6, respectively. These compounds have a same molecular framework, functionalising with different groups lead to different anion binding strength of these receptors. Receptor 1 showed a higher binding affinity for AcO− than for F−, due to the cooperative multiple hydrogen bond interactions of AcO− with the acyl-thiourea group and N-H group in the indole unit of receptor 1. Displacing the N-H proton in the indole unit with -CH3 group, receptor 2 showed no obviously discriminative responses for F−, AcO− and H2PO4− due to lack of such additional binding. In the case of receptor 3, which was functionalised with strong electron-withdrawing group, it showed selectively chromogenic response for F− based on double deprotonation of the receptor in DMSO, whereas AcO− and H2PO4− induced single deprotonation only.