Selective turn-on fluorescence sensing of sulfate in aqueous-organic mixtures by an uncharged bis(diamidocarbazole) receptor

A linear, uncharged, hydrogen bonding receptor A with two carbazole-based binding domains was synthesised and evaluated for its anion binding properties in DMSO/H2O mixtures. H-1 NMR titrations revealed that, in DMSO/H2O 0.5%, A forms both 1:1 and 1:2 complexes with SO42-, H2PO4-, PhCOO- and Cl-. In 1:1 complexes the receptor encloses the tetrahedral anions tightly, forming a helical structure, while Cl-binds with a single carbazole unit only. In the presence of 10% of water the 1:2 complexes with SO42- and PhCOO- disappear, and the respective 1:1 binding constants decrease sufficiently to be quantified by UV-Vis titration. In this highly competitive medium, A binds sulfate with K-1:1 = 10(5.47) M-1, i.e., it binds approx. 30, 360 and > 1000 times more strongly than H2PO4-, PhCOO- and Cl-, respectively. Furthermore, the association with sulfate is over 50 times stronger than that for a model diamidocarbazole 1 under identical conditions, suggesting a very strong chelating effect due to the diglycoyl linker. Increasing the amount of water to 25% (the solubility limit of A) lowers the 1:1 binding constant with SO42- to 10(3.73) M-1. Receptor A was shown to act as a selective turn-on fluorescent sensor for sulfate, able to sense sulfate in sulfate-rich mineral water.