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

Abstract

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. ¹H 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.