Curved Polycyclic Aromatic Molecules That Are π‑Isoelectronic to Hexabenzocoronene

Reported here are two types of curved π-molecules
that are
π-isoelectronic to planar hexabenzocoronene (HBC) but are forced
out of planarity either by an embedded seven-membered ring or by atom
crowding at the fjord region. Embedding a heptagon in HBC leads to
a novel saddle-shaped molecule 1, whose π-backbone
is slightly less curved than the previously reported [7]­circulene
in terms of the average Gauss curvature, but surprisingly much more
rigid than [7]­circulene. Overcrowded fjord regions in novel derivatives
of hexabenzoperylene (HBP) 2a,b lead to
both chiral twisted and antifolded conformers. The successful synthesis
of 1 and 2a,b is related to
introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes.
It is found that the red twisted isomer of 2b isomerizes
at elevated temperature to the yellow anti-folded
conformer. This finding along with the study on the thermodynamics
and kinetics of the thermal isomerization has improved the early understandings
on the conformation of HBP. In the crystals, 1 lacks
π–π interactions between neighboring molecules,
while twisted-2a exhibits both face-to-face and edge-to-face
π–π interactions. Twisted-2b is found
to function as a p-type semiconductor in thin film transistors, but
the thin films of 1 appear insulating presumably due
to lacking π–π interactions. By exploring three
different types of curvatures in 1 and the two isomers
of 2b, this study has revealed that the curvature of
π-face plays a role in determining the frontier molecular orbital
energy levels and π–π interactions and thus needs
to be considered when one designs new organic semiconductors.