Folding and anion-binding properties of an indolocarbazole dimer with urea appendages

An indolocarbazole dimer with the urea functional group was prepared as an anion receptor which contained eight NH hydrogen bonds donors. Two indolocarbazole units were coupled through a butadiynyl linker to allow for helical folding of the resulting dimer by intramolecular hydrogen bonding and dipole-dipole attractions as proven in the ¹H NMR spectroscopy. The dimer was found to bind anions (Cl^-, Br^-, N₃ ^-, AcO^-, and) by multiple hydrogen bonds in 10% (v/v) MeOH-acetone, showing high selectivity for sulphate ion. The ¹H NMR spectra clearly demonstrated that only indolocarbazole NH protons were involved in the hydrogen bonding with small anions such as chloride, but all of the existing indolocarbazole and urea NHs participated in the hydrogen bonding with sulphate ion. This difference in the binding mode might be responsible for the high affinity and selectivity towards sulphate ion.