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. F₆‐4]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^H?Ar^F) hetero‐stacking to Ar^H?Ar^H/Ar^F?Ar^F 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^H?Ar^F stacking motif is not always applicable to F_n‐4]helicenes with twisted π‐surfaces.