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 D_{2 d} 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.