A Writhed Mӧbius Nanobelt Derived from [7]Helicene

A novel writhed Möbius nanobelt was designed by fusing [7]helicene with a fragment of O,N-heterocyclacene. The 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. Both the nanobelt and its macrocyclic precursor exhibit C2 symmetry, but this symmetry is only reflected in 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 approximately 0.008.