Asymmetric urea, thiourea and squaramide receptors as minimal-anion binding motifs

Asymmetric minimal receptors based on 4-nitrophenylurea, 1, and 4-nitrophenylthiourea, 2, were shown to bind a range of anions and are deprotonated by both fluoride and hydroxide to give a colorimetric response. A 4-trifluoromethylphenylurea receptor, 3, was deprotonated by fluoride but not hydroxide but was not discernible by the naked eye. Deprotonation of 1 by fluoride ultimately resulted in the formation of its bicarbonate complex as determined by X-ray crystallography in agreement with DFT calculations. Fluorescence spectroscopy revealed that emission of 2 reaches a maximum at two equivalents of anions except for fluoride where it continues to increase with increasing addition of TBAF allowing 2 to give a fluoride-specific response.