Unusually Short H⋅⋅⋅H Contacts in Intramolecularly Cyclized Helically Fused Anthracenes

The intramolecular coupling of dichloro-substituted helically fused anthracenes using the Yamamoto coupling yielded cyclized products with sterically congested molecular structures. The X-ray analysis and DFT calculations showed that the aromatic framework adopted a nonplanar structure with a twisted conformation about the newly formed single bond, which acts as a chiral axis. Interestingly, the X-ray structure obtained through the Hirshfeld atom refinement revealed short interatomic distances between the inner hydrogen atoms (1.648-1.692 Å), much shorter than the sum of their van der Waals radii. Owing to these unusually short contacts, the 1H NMR spectrum exhibited a significant deshielding (12.5 ppm) and a large nuclear Overhauser effect (44 %). Additionally, the IR spectrum displayed a high-frequency shift of the C-H stretching vibration. These observations, along with the noncovalent interaction plot indicative of a characteristic steric environment, strongly support the presence of steric hindrance. Moreover, dynamic NMR measurement of the mesityl-substituted derivative yielded a barrier to helical inversion of 84 kJ mol-1. The optical properties and crystal packing of the cyclized products are also reported.

Chasing Turns and Twists: Unraveling the One-Step Synthesis, Intricate Pathways, and Structural Revelations of N-Aryl Aza-quasi[8]circulenes

An efficient one-step synthesis of N-Aryl aza-quasi[8]circulenes is reported starting from bis(biaryl)carbazoles. The intermediacy of N-aryl aza[7]helicene is investigated, and the Scholl oxidative cyclization route is invoked here to overcome the large strain during the formation of N-aryl aza-quasi[8]circulenes from N-aryl aza[7]helicene. Notably, this transformation occurs without the need for directing groups and proceeds from a more helical to a less helical pathway. Both the N-aryl aza[7]helicene and N-aryl aza-quasi[8]circulene are confirmed by single crystal X-ray structural analysis. The enantiomers of N-aryl aza[7]helicene are separated by chiral HPLC and analyzed by circular dichroism spectroscopy to investigate their chiroptical properties. However, N-aryl aza-quasi[8]circulene racemizes rapidly. The radical cations generated from aza-quasi[8]circulene through chemical oxidation exhibit broad absorption in the near-IR region and air stability up to 24 h. Optical and electrochemical studies with aza[7]helicene and aza-quasi[8]circulene derivatives highlight their potential in organic electronic devices.

Electrostatic Potential for Exploring Electron Delocalization in Infinitenes, Circulenes, and Nanobelts

The π-conjugation, aromaticity, and stability of the newly synthesized 12-infinitene and of other infinitenes comprising 8-, 10-, 14-, and 16-arene rings are investigated using density functional theory. The π-electron delocalization and aromatic character rooted in infinitenes are quantified in terms of molecular electrostatic potential (MESP) topology. Structurally, the infinitene bears a close resemblance of its helically twisted structure to the infinity symbol. The MESP topology shows that infinitene possesses an infinity-shaped delocalization of the electron density that streams over the fused benzenoid rings. The parameter ∑i=13Δλi, derived from the eigenvalues (λ_i) corresponding to the MESP minima, is used for quantifying the aromatic character of arene rings of infinitene. The structure, stability, and MESP topology features of 8-, 10-, 12-, 14-, and 16-infinitenes are also compared with the corresponding isomeric circulenes and carbon nanobelts. Further, the strain in all such systems is evaluated by considering the respective isomeric planar benzenoid hydrocarbons as reference systems. The 12-infinitene turns out to be the most aromatic and the least strained among all the systems examined.

π-Expansion of 2,3,6,7-Tetraazanaphthalene with Two Embedded Heptagons: Highly Twisted Structure and Lone-Pair/π* Interaction in the Crystal

Synthesis and characterization of doubly diphenylene-fused 2,3,6,7-tetraazanaphthalene 1 are described. Single-crystal X-ray diffraction analysis showed the highly twisted structure of 1 with a degree of twisting of 13.0°/Å, which is one of the largest values for a π-system. In the crystal, molecules of 1 formed an orthogonal one-dimensional column with π-stacking of diphenylene moieties and a short intermolecular C···N distance due to lone-pair/π* interaction, which is a rare example of lone-pair/π* interaction in a supramolecular assembly.

Synthesis of octagon-containing molecular nanocarbons

Nanocarbons, such as fullerenes, carbon nanotubes, and graphenes, have long inspired the scientific community. In order to synthesize nanocarbon molecules in an atomically precise fashion, many synthetic reactions have been developed. The ultimate challenge for synthetic chemists in nanocarbon science is the creation of periodic three-dimensional (3D) carbon crystals. In 1991, Mackay and Terrones proposed periodic 3D carbon crystals with negative Gaussian curvatures that consist of six- and eight-membered rings (the so-called Mackay-Terrones crystals). The existence of the eight-membered rings causes a warped nanocarbon structure. The Mackay-Terrones crystals are considered a "dream material", and have been predicted to exhibit extraordinary mechanical, magnetic, and optoelectronic properties (harder than diamond, for example). To turn the dream of having this wonder material into reality, the development of methods enabling the creation of octagon-embedding polycyclic structures (or nanographenes) is of fundamental and practical importance. This review describes the most vibrant synthetic achievements that the scientific community has performed to obtain curved polycyclic nanocarbons with eight-membered rings, building blocks that could potentially give access as templates to larger nanographenes, and eventually to Mackay-Terrones crystals, by structural expansion strategies.

Configurationally stable dithia[7]helicene and dithia-quasi[8]circulene fused dithiolones

Stereochemically stable dithia[7]helicene and dithia-quasi[8]circulene decorated with 1,3-dithiol-2-one motifs were synthesized by simple and double oxidative dehydrocyclisation.

Facile Synthesis of Azahelicenes and Diaza[8]circulenes through the Intramolecular Scholl Reaction

Carbazole-based aza[7]helicenes and hetero[9]helicenes were successfully obtained via the intramolecular Scholl reaction of 3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-bis(naphthylphenyl)-appended carbazole gave a triple helicene via an unexpected simultaneous double aryl rearrangement. DFT calculations suggested that the rearrangement proceeded via an arenium cation intermediate. In addition, the reaction of methoxy-appended substrate gave an azahepta[8]circulene via the concurrent C-C bond formation. These helical dyes showed circularly polarized luminescence. The azahepta[8]circulene was further transformed into deeply saddle-distorted dibenzodiaza[8]circulenes as the first example of its solution-based synthesis and unambiguous structural determination.

Synthesis of Saddle-Shape Octaaminotetraphenylene Octahydrochloride

Apart from being experimentally and theoretically interesting, tetraphenylene has potential applications in different fields, including supramolecular chemistry, material science, and asymmetric catalysis. Although a wide range of substituted tetraphenylenes have been reported, octaamine-based tetraphenylene derivatives have not been reported because of their instability. Here, stable octaaminotetraphenylene octahydrochloride is synthesized from the bromination of tetraphenylene to octabromotetraphenylene, which is subsequently aminated into octaiminotetraphenylene. Finally, the imino derivative is deprotected to yield octaaminotetraphenylene octahydrochloride.

Quadruply BN-Fused Tetrathia[8]circulenes with Flexible Frameworks: Synthesis, Structures and Properties

Quadruply BN-fused tetrathia[8]circulenes were synthesized through four-fold electrophilic borylation. The single-crystal X-ray diffraction analysis revealed that the BN-fused tetrathia[8]circulene with peripheral phenyl groups exhibits crystal polymorphism, in which the circulene core adopts both planar and saddle conformations in the solid state. The experimental and theoretical studies revealed that the weaker aromaticity of azaborine compared with benzene renders the flexibility of the BN-fused tetrathia[8]circulenes.

Improved Synthesis of ortho-Phenylene-bridged Cyclic Tetrapyrroles and Oxidative Fusion Reactions Toward Substituted Tetraaza[8]circulenes

Hetero[8]circulenes have emerged as novel functionalized heteronanographenes that show various promising functions such as bright fluorescence, charge transporting, and redox reactivities. One of the effective synthetic strategy is the fold-in type oxidative fusion reaction of ortho-phenylene-bridged cyclic tetrapyrroles, whose construction, however, is not well-sophisticated in terms of reproducibility and possibility for versatile derivatization. In this paper, a "reverse" coupling strategy has been developed, which enabled synthesis of opp-type low symmetric analogues of cyclic tetrapyrroles. Oxidative fusion reaction conditions to afford tetraaza[8]circulenes have also been reinvestigated and improved. Substituent effects of cyclic tetrapyrroles and tetrabenzotetraaza[8]circulenes are studied for solid-state structures and packing structures, redox potentials, and optical properties.

Aggregation-Induced Emission in Tetrathia[8]circulene Octaoxides via Restriction of the Dynamic Motion of their Negatively Curved π-Frameworks

Curved polycyclic aromatic hydrocarbons exhibit intriguing properties, but their flexible and dynamic nature is often considered to be unfavorable for achieving excellent emission properties. In this study, we prepared tetrathia[8]circulene octaoxides that adopt a negatively curved conformation. We found that these molecules exhibit aggregation-induced emission (AIE) that originates from the restriction of the dynamic motion of their saddle-shaped π-frameworks in the solid and aggregated states.

On-Surface Synthesis of a π-Extended Diaza[8]circulene

Heterocyclic [8]circulenes are an important class of polycyclic aromatic hydrocarbon molecules because of their unique structural properties and promising applications. However, the synthesis of heterocyclic [8]circulenes is still limited and thus is an important synthetic challenge. Here we describe the first example of a π-extended diaza[8]circulene surrounded by and fused with six hexagons and two pentagons, which was successfully synthesized only by a combined in-solution and on-surface synthetic strategy. State-of-the-art scanning tunneling microscopy with a CO-functionalized tip and density functional theory calculations revealed its planar conformation and unique electronic structure.

Facile synthesis of fluorescent hetero[8]circulene analogues with tunable solubilities and optical properties

Hetero[8]circulenes are an interesting class of polycyclic heteroaromatic molecules having rigid and planar structures, which are promising in light of their potential applications for OLEDs, OFETs and so forth. Although their synthetic methods have been developed in some specific cases, a facile synthetic protocol of novel hetero[8]circulenes with tunable properties is highly desirable. We herein report the unexpected formation of methoxy-substituted quasi-aza[8]circulene and its conversion into unprecedented triazaoxa[8]circulene. The structures and optical properties were comparatively studied. Remarkably, triazaoxa[8]circulene is highly soluble in THF, acetone and DMSO mainly because of effective hydrogen-bonding of the NH moieties to these solvents. Their highly soluble nature in various solvents enabled us to study the solvent effects of these molecules. In particular, triazaoxa[8]circulene displays a high fluorescence quantum yield of 0.72 in DMSO. Furthermore, enantiomeric separation of highly distorted quasi-aza[8]circulene was successfully achieved by chiral HPLC. Thus, these novel hetero[8]circulene derivatives are practically useful fluorescent nanographene-like molecules with intriguing optical properties.

Toward Negatively Curved Carbons

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 sp² carbon atoms. By expanding or connecting these structural units, we designed and synthesized larger negatively curved nanographenes consisting of up to 96 sp² 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.

Singly and Doubly 1,2-Phenylene-Inserted Porphyrin Arch-Tape Dimers: Synthesis and Highly Contorted Structures

Singly and doubly 1,2-phenylene-inserted Ni^II porphyrin arch-tape dimers 3 and 9 were synthesized from the corresponding β-to-β 1,2-phenylene-bridged Ni^II porphyrin dimers 5 and 11 via Ni⁰ -mediated reductive cyclization and DDQ/Sc(OTf)₃ -promoted oxidative cyclization as key steps, respectively. Owing to the fused eight-membered ring(s), 3 showed a more contorted structure than those of previously reported arch-tape dimers 2 a and 2 b possessing a fused seven-membered ring. Furthermore, 9 displayed much larger molecular contortion. As the molecular contortion increases, the Q band of the absorption spectrum becomes more red-shifted and the electrochemcial HOMO-LUMO gap becomes smaller, reaching at 1294 nm and 0.77 eV in 9, respectively. The effect of molecular contortion on the electronic properties was studied by means of DFT calculations.

Synthesis, Structure, and Properties of Tetrabenzo[7]circulene

Tetrabenzo[7]circulene, a new member of aromatic saddles, was conveniently synthesized from 2-(1-naphthoyl)benzoic acid with the seven-membered ring constructed at an early stage of the synthesis. This method, upon minor modification, was also useful for synthesis of thiophene-annulated [7]circulenes. The structures of tetrabenzo[7]circulene and [7]circulene were compared in terms of symmetry, flexibility, and curvature on the basis of DFT calculations and X-ray crystallography. It was also found that tetrabenzo[7]circulene functioned as a p-type semiconductor in thin-film transistors and cocrystallized with C₆₀.

Chirality in curved polyaromatic systems

Chiral non-planar polyaromatic systems that display zero, positive or negative Gaussian curvature are analysed and their potential to ‘encode’ chirality of larger sp²-carbon allotropes is evaluated. Shown is a hypothetical peanut-shaped carbon allotrope, where helical chirality results from the interplay of various curvature types.