2,2′-Diaminoazo-benzene, a potential scaffold for the synthesis of bis-ureas and thioureas: Solution phase anion sensing and binding studies

The synthesis and anion interaction studies of four novel (thio)ureas, based on 2,2′-diaminoazo-benzene, are reported. The synthesized (thio)ureas exhibited significant colorimetric responses upon the addition of most basic F−, H2PO4− and AcO− in DMSO. These color changes which are brought about through either H-bonding to, or deprotonation of, were clearly visible to the naked eye and were reversed upon addition of protic solvents. Interestingly, each of these anions gave rise to the unique changes in the absorption spectra which can be considered as being a ‘fingerprint’ identity for each of them. A 1:2 host/guest stoichiometry was determined by the continuous variation method. Furthermore, the thioureas with nitro-substituted aryl rings underwent deprotonation in the presence of highly basic F−.