text: Anion and π–aromatic systems (typically electron-deficient) create an interaction that is associated with the repulsive forces of the structures. These repulsive forces involve electrostatic and anion-induced polarized interactions. This force allows for the systems to be used as receptors and channels in supramolecular chemistry for applications in the medical (synthetic membranes, ion channels) and environmental fields (e.g. sensing, removal of ions from water).
  The first X-ray crystal structure that depicted anion–π interactions was reported in 2004. In addition to this being depicted in the solid state, there is also evidence that the interaction is present in solution.