Dual responsive chemosensors for anion and cation: Synthesis and studies of selective chemosensor for F− and Cu(II) ions

New colorimetric chemosensors, 1-[2-{(4-nitrobenzylidene)amino}phenyl]-3-phenylthiourea (1), 1-[2-{(2-hydroxy-5-nitrobenzylidene)amino}phenyl]-3-phenyl thiourea (2) have been synthesized and characterized by spectroscopic techniques and XRD. The molecular structure of receptor 1 was determined by X-ray crystallography and it has the triclinic space group P1 with cell parameters a = 7.2705 Å (6), b = 11.0178 Å (9) c = 12.0137 (9) Å and Z = 2. Anion binding studies carried out using 1H NMR and UV–visible spectrophotometric titrations revealed that these receptors exhibit selective recognition towards F− over other halide anions. The selectivity for F− among the halides is attributed mainly to the hydrogen-bond interaction of the receptor with F−. Receptors 1 and 2 (5 × 10−5 M) show color change from colorless to brown and yellow respectively in the presence of tetrabutylammonium fluoride (TBAF, 5 × 10−3 M). Moreover, F−-induced color changes remain the same even in the presence of large excess of Cl−, Br−, and I−. Chromogenic receptors 1 and 2 undergo distinct color changes from colorless to violet (1) or bluish green (2) on gradual addition of Cu(II) and can be used as colorimetric probes for spectrophotometric and visual analysis of Cu(II) in the presence of other transition metal ions such as Mn2+, Co2+, Ni2+ and Zn2+. The binding constant for 2 was found to be higher than that for 1 towards F− and Cu(II) ion and this may be due to presence of OH group in 2, which offers extra binding site.