A Writhed Möbius Nanobelt Derived from [7]Helicene

A novel writhed Möbius nanobelt was synthesized using a helical building block derived from [7]helicene and a C-shaped building block derived from pyrene. These two building blocks were connected through nucleophilic aromatic substitution to form an oxanorbornene-containing macrocycle, which was then converted to the nanobelt by reductive aromatization and subsequent oxidation. The structure of the Möbius nanobelt was confirmed with X-ray crystallography. Both the nanobelt and its macrocyclic precursor exhibit C₂ symmetry, but this symmetry is only reflected by the ¹H NMR signals for the tetra(4-t-butylphenyl)dinaphthopyrene moiety in the nanobelt, not in its precursor. This difference is attributed to the distinct arrangements of the pendent 4-t-butylphenyl groups, caused by the crowdedness and restricted rotation of the C-C single bonds in the nanobelt. Theoretical calculations suggest that the nanobelt does not exhibit global ring currents but has localized aromatic ring currents. Additionally, when an enantiopure form of the [7]helicene derivative was used, the nanobelt was obtained in an enantiopure form, showing an absorption dissymmetry factor of 4 × 10^-3.