Improvement of solubility of 2,2′-binaphthalene derivatives bearing urea groups as anion receptors and their application to a chloride selective electrode

To improve the solubility of 2,2′-binaphthalene derivatives bearing urea groups, a series of bisureas based on 2,2′-binaphthalene with various alkyl and aryl substituents at the terminal nitrogen atom of the ureas has been synthesized via a bisisocyanate intermediate. The solubility of the receptors was evaluated by means of UV–vis spectroscopy. Receptors 3 and 8 bearing sterically bulky tert-butyl and 2,6-dimethylphenyl groups as substituents at urea moieties showed good solubility in common organic solvents. UV–vis spectral titrations revealed that the receptor 3 maintained the binding ability for anions, however, 8 showed lowering of the binding ability for anions. The anion-binding ability of receptor 3 is in the order of F− > AcO− > Cl− > (EtO)2PO2− > Br− > NO3− > ClO4− > I−. Receptor 3 showed strong emission at 440 nm in MeCN and the emission was gradually quenched upon the addition of Cl− and AcO−, indicating that 3 can be useful as fluorescence sensor for anions. The receptor 3 can be applied to a chloride selective electrode and the selectivity of 3 is sufficient for analytical use.