Negatively curved carbons are theoretical carbon allotropes as proposed by embedding heptagons or octagons in a graphitic lattice. Unlike five-membered rings in fullerenes, which induce positive curvature, the seven- or eight-membered rings induce negative curvature, giving rise to a variety of esthetic carbon nanostructures known as Mackay crystals or carbon schwarzites. In addition, hypothetical toroidal carbon nanotubes consisting of five-, six-, and seven-membered rings present positive curvature on the outside and negative curvature on the inside of the torus. These carbon allotropes with negative curvature are predicted to have interesting properties and potential applications on the basis of computational studies but are yet to be synthesized. A promising bottom-up approach to these intriguing but still imaginary carbon structures is organic synthesis of negatively curved polycyclic arenes, which are also known as negatively curved nanographenes. They not only are segments of negatively curved carbon allotropes containing important structural information but also can in principle be used as templates or monomer units for the synthesis of carbon schwarzites and toroidal carbon nanotubes. This Account describes research on the design, synthesis, structure, stereochemical dynamics, and properties of negatively curved nanographenes, with emphasis on our efforts in this field. In our designs of negatively curved nanographenes, a few heptagon- or octagon-embedded π systems were employed as basic structural units, including [7]circulene, heptagon-embedded hexa- peri-benzocoronene, tetrabenzodipleiadiene, and [8]circulene. They present a saddle-shaped geometry and consist of a relatively small number of sp carbon atoms. By expanding or connecting these structural units, we designed and synthesized larger negatively curved nanographenes consisting of up to 96 sp carbon atoms. A method of key importance in the synthesis of negatively curved nanographenes is the Scholl reaction, which enables the formation of multiple carbon-carbon bonds in a single step by intramolecular oxidative cyclodehydrogenation. The unique structures of negatively curved nanographenes were studied by experimental and computational methods. In particular, X-ray crystallography of single crystals revealed remarkably curved π faces accompanied by severe out-of-plane deformation of benzenoid rings, which sheds light on the limit of π bonds and the aromaticity of polycycles. As found mainly from calculations, the flexible polycyclic frameworks of negatively curved nanographenes are associated with stereochemical dynamics that is not available for planar polycyclic aromatics. In addition, some negatively curved nanographenes have been found to function as organic semiconductors in the solid state. We envision that the study of negatively curved nanographenes will serve as an important initial step toward the eventual synthesis of new carbon allotropes with negative curvature and new frontiers of nanocarbon materials.

The past decade has witnessed remarkable success in the synthesis of curved polycyclic aromatics through Scholl reactions which enable oxidative aryl–aryl coupling even in company with the introduction of significant steric strain. These curved polycyclic aromatics are not only unique objects of structural organic chemistry in relation to the nature of aromaticity but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topology. Moreover, they have received considerable attention in the fields of supramolecular chemistry and organic functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aromatics, the outcome of a newly designed substrate in the Scholl reaction still cannot be predicted in a generic and precise manner largely due to limited understanding on the reaction mechanism and possible rearrangement processes. This review provides an overview of Scholl reactions with a focus on their applications in synthesis of curved polycyclic aromatics with interesting structures and properties and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aromatics.

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 C 2 and D 2d 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.

This study presents a new type of negatively curved nanographene (C H ) that contains an unprecedented pattern of heptagons. A tert-butylated derivative of C H was successfully synthesized using tetrabenzodipleiadiene as a key building block. This synthesis involved a ring expansion reaction as a key step to form the seven-membered rings in the framework of tetrabenzodipleiadiene. The single-crystal structure reveals a saddle-shaped molecule with a highly bent naphthalene moiety at the center of the polycyclic backbone. As found from the DFT calculations, this aromatic saddle is flexible at room temperature and has a saddle-shaped geometry as the dominant conformation. The DFT calculations along with experimental results show that the attachment of t-butyl groups to the central tetrabenzodipleiadiene moiety of nanographene C H can stabilize the saddle conformation and make this nanographene less flexible.

Zigzag carbon nanobelts are al ong-sought-after target for organic synthesis.Herein we report new strategies for designing and synthesizing unprecedented zigzag carbon nanobelts,which present awave-like arrangement of hexagons in the unrolled lattice of (n,0) single wall carbon nanotubes (n = 16 or 24). The precursors of these zigzag carbon nanobelts are hybrid cyclic arylene oligomers consisting of metaphenylene and 2,6-naphthalenylene as well as para-phenylene units.T he Scholl reactions of these cyclic arylene oligomers form multiple carbon-carbon bonds selectively at the apositions of naphthalene units resulting in the corresponding zigzag carbon nanobelts.A sm onitored with fluorescence spectroscopy, one of these nanobelts binds C 60 with an association constant as high as (6.6 AE 1.1) 10 6 M À1 in the solution in toluene.C omputational studies combining linear regression analysis and hypothetical homodesmotic reactions reveal that these zigzag nanobelts have strain in the range of 67.5 to 69.6 kcal mol À1 ,a nd the ladderization step through Scholl reactions is accompanied by increase of strain as large as 69.6 kcal mol À1 .T he successful synthesis of these nanobelts demonstrates the powerfulness and efficiency of Scholl reactions in synthesizing strained polycyclic aromatics. Figure 1. (a) [n]Cyclacene and zigzag carbon nanobelts reported very recently;( b) structures of zigzag carbon nanobelts 1-3.