An NBD-armed thiacalix[4]arene-derived colorimetric and fluorometric chemosensor for Ag+: a metal–ligand receptor of anions

A new type of thiacalix[4]arene-based chemosensor 2a and 2b containing amino groups as a binding site and 7-nitrobenzo-2-oxa-1,3-diazole (NBD) as a chromophore have been synthesized and characterized by single crystal X-ray diffraction and by NMR spectroscopic methods. Each receptor acted as a strong colorimetric and fluorometric sensor for silver ions and exhibited significant stepwise deprotonation of NH as evidenced by a bathochromic shift in both the UV-vis absorption and fluorescence emission spectra after addition of an Ag(+) cation. Furthermore, the metal-ligand complex 2a·Ag(+) can be exploited as an ion-pair receptor for the recognition of F(-), AcO(-) or I(-) anions through switching the detection signal output (can been distinguished by 'naked eye'). However, receptor 2b·Ag(+) displays a selective response only to iodide ion. These results could be attributed to the slightly different structural conformation between receptors 2a and 2b, which are derived from a 1,3-alternate-thiacalix[4]arene scaffold, suggesting they play a key role in the selective and binding ability of the host chemosensors.