Synthesis, Structures, and Properties of Heptabenzo[7]circulene and Octabenzo[8]circulene

This
study puts forth two new members of fully ortho-benzannulated
[n]­circulenes, heptabenzo[7]­circulene
and octabenzo[8]­circulene, which are new negatively curved nanographenes
and also represent unprecedented structures of septuple [4]­helicene
and octuple [4]­helicene, respectively. The successful synthesis of
them through Scholl reaction in good to excellent yields takes advantage
of the reactivity of naphthalene. Quantum chemistry calculations reveal
that heptabenzo[7]­circulene and octabenzo[8]­circulene are both flexible
π-molecules and adopt saddle-shaped geometry of C2 and D2d symmetry, respectively, at the global energy minimum in agreement
with the single-crystal structures. A serendipitous discovery from
this study is that tetra­(tert-butyl) octabenzo[8]­circulene
in the single crystals self-assemble into a supramolecular nanosheet
with an unprecedented motif of π–π stacking. Such
a new molecular packing mode, in combination with the demonstrated
semiconducting property of octabenzo[8]­circulene, suggests a new supramolecular
two-dimensional material.