Columnar Stacking of Partially Fluorinated [4]Helicenes: C-H⋅⋅⋅F Interactions Change the Stacking Orientation.

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F -[4]Helicene forms head-to-tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene-fluoroarene (Ar -Ar ) hetero-stacking to Ar -Ar /Ar -Ar homo-stacking with the help of intermolecular C-H⋅⋅⋅F contacts in the fjord region. As a result, head-to-head columnar stacks appear. Therefore, the conventional Ar -Ar stacking motif is not always applicable to F -[4]helicenes with twisted π-surfaces.