A Foldamer-Based Chiroptical Molecular Switch That Displays Complete Inversion of the Helical Sense upon Anion Binding

Chiral indolocarbazole dimers fold into a helical conformation by virtue of intramolecular hydrogen bonds, as demonstrated by 1H NMR and CD spectra and optical rotations. In particular, the optical properties of the dimers were found to be extremely sensitive to the nature of the solvent, depending on whether they are folded or not. The helical sense of the dimers can be reversibly switched by binding sulfate ion, which gives rise to complete inversion of the CD spectra. The binding mode and absolute stereochemistry of the sulfate complexes was unequivocally determined by single-crystal X-ray structures, which are all consistent with the CD and 1H NMR spectra in solution.