Selective sensing of anions with calix[4]pyrroles strapped with chromogenic dipyrrolylquinoxalines

Abstract

New calix[4]pyrroles bearing dipyrrolylquinoxaline as strapping elements have been synthesized and characterized by spectroscopic means. The binding behavior of these receptors at 25 degrees C was investigated first by proton NMR spectroscopy in CD(3)CN/DMSO-d(6) (9:1 v/v), as well as by UV-vis spectroscopy and isothermal titration calorimetry (ITC) in CH(3)CN/DMSO (97:3 v/v). The receptors displayed a selective colorimetric response when exposed to the fluoride, dihydrogen phosphate, and acetate anions (studied in the form of the corresponding tetrabutylammonium salts) and an enhanced affinity as compared to a comparable calix[4]pyrrole system lacking the dipyrrolylquinoxaline-containing strap.

Figure 1

¹H NMR spectral changes of receptor 4 (2.64 mM) seen upon titration with F⁻ (as its tetrabutylammonium salt) in CD₃CN/DMSO-d₆ (9:1 v./v.) at 25 °C.

Figure 2

(a) Changes in the color of 1.12 mM solutions of receptor 4 in CH₃CN/DMSO (97:3 v./v.) seen upon the addition of various anions (100 equiv. each). (b) The spectral changes seen upon the addition of acetate anion (added as TBA-H₂PO₄) to a 50.1 µM solution of receptor 4.

Figure 3

Binding isotherm corresponding to the ITC titration between TBAF and receptor 4 ([4] = 0.41 mM in CD₃CN/DMSO; 97:3 v./v. at 30 °C).

Scheme 1

Synthesis of target compounds

Scheme 2

Proposed binding mode for the interaction of fluoride anion with receptor 4 as inferred from ¹H NMR spectroscopic analyses. Additional interactions, involving the binding of a second equivalent of fluoride anion to the dipyrrolylquinoxaline pyrrolic NH protons, are considered likely at high fluoride-to-receptor ratios.