Visible colorimetric fluoride and hydroxide sensing by asymmetric tris-urea receptors: combined experimental and theoretical studies.

A series of asymmetric tris-urea receptors with electron withdrawing group (EWG) or electron donating group (EDG), L1-L4, were synthesized and characterized by high resolution mass spectroscopy (HRMS), nuclear magnetic resonance spectroscopy (NMR) and single-crystal X-ray diffraction (XRD) techniques. Receptors with EWG substituent showed different visible colorimetric response to fluoride and hydroxide anions. Binding studies toward various monovalent anions were performed by UV-Vis and NMR titration experiments.