Tetrabenzo[8]circulene: Aromatic Saddles from Negatively Curved Graphene

An
aromatic saddle was designed from the hypothetical three-dimensional
graphene with the negative Gaussian curvature (Schwarzite P192). Two
aromatic saddles, tetrabenzo[8]­circulene (TB8C) and its
octamethyl derivative OM-TB8C, were synthesized by the
Scholl reaction of cyclic octaphenylene precursors. The structure
of TB8C greatly deviates from planarity, and the deep
saddle shape was confirmed by single-crystal X-ray crystallography.
There are two conformers with the S4 symmetry,
which are twisted compared to the DFT structure (D2d). The theoretical studies propose
that the interconversion of TB8C via the planar transition
state (125 kcal mol–1) is not possible. However,
the pseudorotation leads to a low-energy tub-to-tub inversion via
the nonplanar transition state (7.3 kcal mol–1).
The ground-state structure of TB8C in solution is quite
different from the X-ray structure because of the crystal-packing
force and low-energy pseudorotation. OM-TB8C is a good
electron donor and works as the p-type semiconductor.