Methylene-Bridged [6]-, [8]-, and [10]Cycloparaphenylenes: Size-Dependent Properties and Paratropic Belt Currents

Cycloparaphenylenes (CPPs) and carbon nanobelts (CNBs) represent some of the most iconic cyclic molecular nanocarbons in recent chemistry owing to their unique properties derived from rigid, strained, and cyclic π-conjugated systems. In the last decade, the synthesis of various sizes of CPPs and CNBs has been achieved that allowed not only for investigating their size-dependent properties and strategically using such properties in various applications but also understanding the fundamental features of cyclic π-conjugated systems and molecular nanocarbons in general. Herein, we report on the synthesis, size-dependent properties, and paratropic belt currents of methylene-bridged [n]cycloparaphenylenes ([n]MCPP, n = 6, 8, 10). [8]MCPP and [10]MCPP were synthesized by the same strategy we developed for [6]MCPP synthesis. With readily available ethoxy-substituted pillar[8]arene and pillar[10]arene as precursors, [8]MCPP and [10]MCPP were successfully synthesized in three steps consisting of de-ethylation, triflation, and nickel-mediated aryl-aryl coupling. The structural and electronic properties of MCPPs were investigated by nuclear magnetic resonance analyses, absorption/fluorescence measurements, X-ray crystallographic analyses, and computational studies, revealing their interesting size-dependent properties. The differences in the size dependency between MCPPs and CPPs reflect the belt-form features of MCPPs, namely, methylene-bridging effects on MCPPs. Moreover, an interesting paratropic belt current along the MCPP backbone has been uncovered both experimentally and theoretically. The ¹H NMR chemical shifts of MCPPs confirmed the presence of a paratropic belt current, whose strength rapidly decreases with increasing nanobelt size.

Lithium-Mediated Mechanochemical Cyclodehydrogenation

Cyclodehydrogenation is an essential synthetic method for the preparation of polycyclic aromatic hydrocarbons, polycyclic heteroaromatic compounds, and nanographenes. Among the many examples, anionic cyclodehydrogenation using potassium(0) has attracted synthetic chemists because of its irreplaceable reactivity and utility in obtaining rylene structures from binaphthyl derivatives. However, existing methods are difficult to use in terms of practicality, pyrophoricity, and lack of scalability and applicability. Herein, we report the development of a lithium(0)-mediated mechanochemical anionic cyclodehydrogenation reaction for the first time. This reaction could be easily performed using a conventional and easy-to-handle lithium(0) wire at room temperature, even under air, and the reaction of 1,1'-binaphthyl is complete within 30 min to afford perylene in 94% yield. Using this novel and user-friendly protocol, we investigated substrate scope, reaction mechanism, and gram-scale synthesis. As a result, remarkable applicability and practicality over previous methods, as well as limitations, were comprehensively studied by computational studies and nuclear magnetic resonance analysis. Furthermore, we demonstrated two-, three-, and five-fold cyclodehydrogenations for the synthesis of novel nanographenes. In particular, quinterrylene ([5]rylene or pentarylene), the longest nonsubstituted molecular rylene, was synthesized for the first time.

Wavy graphene sheets from electrochemical sewing of corannulene

The presence of non-hexagonal rings in the honeycomb carbon arrangement of graphene produces rippled graphene layers with valuable chemical and physical properties. In principle, a bottom-up approach to introducing distortion from planarity of a graphene sheet can be achieved by careful insertion of curved polyaromatic hydrocarbons during the growth of the lattice. Corannulene, the archetype of such non-planar polyaromatic hydrocarbons, can act as an ideal wrinkling motif in 2D carbon nanostructures. Herein we report an electrochemical bottom-up method to obtain egg-box shaped nanographene structures through a polycondensation of corannulene that produces a new conducting layered material. Characterization of this new polymeric material by electrochemistry, spectroscopy, electron microscopy (SEM and TEM), scanning probe microscopy, and laser desorption-ionization time of flight mass spectrometry provides strong evidence that the anodic polymerization of corannulene, combined with electrochemically induced oxidative cyclodehydrogenations (Scholl reactions), leads to polycorannulene with a wavy graphene-like structure.

A bottom-up synthesis of wavy graphene structures obtained through an anodic polymerization process, combined with an electrochemically triggered oxidative cyclodehydrogenation, of the bowl-shaped polyaromatic hydrocarbon corannulene.

Double-Helix Supramolecular Nanofibers Assembled from Negatively Curved Nanographenes

The layered structures of graphite and related nanographene molecules play key roles in their physical and electronic functions. However, the stacking modes of negatively curved nanographenes remain unclear, owing to the lack of suitable nanographene molecules. Herein, we report the synthesis and one-dimensional supramolecular self-assembly of negatively curved nanographenes without any assembly-assisting substituents. This curved nanographene self-assembles in various organic solvents and acts as an efficient gelator. The formation of nanofibers was confirmed by microscopic measurements, and an unprecedented double-helix assembly by continuous π-π stacking was uncovered by three-dimensional electron crystallography. This work not only reports the discovery of an all-sp²-carbon supramolecular π-organogelator with negative curvature but also demonstrates the power of three-dimensional electron crystallography for the structural determination of submicrometer-sized molecular alignment.

Disentangling the Contributions to the Proton Magnetic Shielding in Carbon Nanohoops and Nanobelts: Evidence for a Paratropic Belt-Current

The proton NMR magnetic shieldings of the recently synthesized D_3d isomers of methylene-bridged [6]cycloparaphenylene (MB[6]CPP) and [12]cyclophenacene hide in themselves the effect of a global paratropic current around the nanobelts, which is induced by a magnetic field parallel to the main symmetry axis of the molecules. The effect is particularly pronounced for the methylene protons of MB[6]CPP, especially for those facing inside the nanobelt. The small experimental chemical shift difference of only 0.2 ppm is incompatible with the separation of the signals caused by the belt curvature, which, by itself, is calculated to be larger than 1 ppm, with both signals shifted upfield with respect to the position detected for the nanobelt. A careful dissection of the proton magnetic shielding in terms of molecular orbital contributions, has permitted a quantitative assessment of the genuine effect on each different proton caused by a substantial paratropic belt-current, which brings all the signals in nice agreement with the experimental spectra.

Two-step synthesis of a red-emissive warped nanographene derivative via a ten-fold C-H borylation

The regioselective ten-fold borylation of warped nanographene (WNG: C80H30) was achieved by modifying the reaction conditions of a previously reported Ir-catalyzed C-H borylation, affording decaborylated WNG in high yield (75%) from pristine WNG. The solid-state structure of decaborylated WNG was confirmed by X-ray crystallography. Corresponding decaarylated WNGs containing electron-withdrawing and -donating groups were synthesized from decaborylated WNG using Suzuki-Miyaura cross-coupling reactions to afford the red-emissive warped nanographene derivative.

Rational synthesis of an atomically precise carboncone under mild conditions

Carboncones, a special family of all-carbon allotropes, are predicted to have unique properties that distinguish them from fullerenes, carbon nanotubes, and graphenes. Owing to the absence of methods to synthesize atomically well-defined carboncones, however, experimental insight into the nature of pure carboncones has been inaccessible. Herein, we describe a facile synthesis of an atomically well-defined carboncone[1,2] (C₇₀H₂₀) and its soluble penta-mesityl derivative. Identified by x-ray crystallography, the carbon skeleton is a carboncone with the largest possible apex angle. Much of the structural strain is overcome in the final step of converting the bowl-shaped precursor into the rigid carboncone under mild reaction conditions. This work provides a research opportunity for investigations of atomically precise single-layered carboncones having even higher cone walls and/or smaller apex angles.

Synthesis and structural features of thiophene-fused analogues of warped nanographene and quintuple helicene

Thiophene-fused analogues of warped nanographene (WNG) and quintuple helicene (QH) were synthesized via a three-step π-extension of corannulene.

Thiophene-fused analogues of warped nanographene (WNG) and quintuple helicene (QH) were synthesized via a three-step π-extension of corannulene. Similar to the synthetic route to WNG, five hexagons and five heptagons were generated by a Scholl reaction of pentakis(thienylphenyl)corannulene to form pentathiaWNG. In contrast, decathiaWNG could not be obtained from pentakis(thienylthienyl)corannulene, and instead decathiaQH was generated from the photocyclization of the precursor. X-ray crystallography of the products revealed their conformations and packing modes in the solid state. The configurational features of decathiaQH were further examined by DFT calculations. The absorption and fluorescence spectra of the sulfur-containing WNG and QH were shifted relative to those of the corresponding sulfur-free analogues.

Calixazulenes: azulene-based calixarene analogues - an overview and recent supramolecular complexation studies

Some of the least studied calixarenes are those that consist of azulene rings bridged by -CH₂- groups. Since Lash and Colby’s discovery of a simple and convenient method for producing the parent all-hydrocarbon calix[4]azulene, there have been two other all-hydrocarbon calix[4]azulenes which have been synthesized in good yields by their method. This allowed studying their supramolecular properties. This report is of our latest work on the solution-state supramolecular complexation of one of these calix[4]azulenes, namely tetrakis(5,7-diphenyl)calix[4]azulene or “OPC4A”, with several electron-deficient tetraalkyammonium salts. As a result of more recent methods developed by us and others employing Suzuki–Miyaura cross-coupling reactions to produce additional functionalized azulenes, the promise of further greater functionalized calixazulenes lies in store to be investigated.

A Water-Soluble Warped Nanographene: Synthesis and Applications for Photoinduced Cell Death

Nanographene, a small piece of graphene, has attracted unprecedented interest across diverse scientific disciplines particularly in organic electronics. The biological applications of nanographenes, such as bioimaging, cancer therapies and drug delivery, provide significant opportunities for breakthroughs in the field. However, the intrinsic aggregation behavior and low solubility of nanographenes, which stem from their flat structures, hamper their development for bioapplications. Herein, we report a water-soluble warped nanographene (WNG) that can be easily synthesized by sequential regioselective C-H borylation and cross-coupling reactions of the saddle-shaped WNG core structure. The saddle-shaped structure and hydrophilic tetraethylene glycol chains impart high water solubility to the WNG. The water-soluble WNG possesses a range of promising properties including good photostability and low cytotoxicity. Moreover, the water-soluble WNG was successfully internalized into HeLa cells and promoted photoinduced cell death.

Bicyclo[6.3.0]undeca-1(11),2,4,6,8-pentaen-10-ylidene: An Aromatic Carbene with Ambiphilic Properties

The title carbene (4) was generated as a highly reactive species in solution by photoirradiation of 10-diazobicyclo[6.3.0]undecapentaene (5) using a high-pressure mercury lamp. Carbene 4 reacts with benzene to afford two isomeric adducts, 10-phenylbicyclo[6.3.0]undecapentaene (10) and tricyclo[9.3.0^3, 10 .0]heptadeca-1,3(10),4,6,8,12,14,16-octaene (11). The reactivity toward benzene is a characteristic of an electrophilic aromatic carbene analogous to cyclopentadienylidene 1. In contrast, the reaction of 4 with methanol produces 7-methoxybicyclo[6.3.0]undeca-1,3,5,8,10-pentaene (15). When [D₁ ]methanol was employed as a reactant, the 10-deuterated analogue was formed. The results clearly indicate the formation of bicyclo[6.3.0]undecapentaenyl cation (7) as a novel 10 π-electronic compound by protonation of 4. Theoretical calculations indicate that the 2- and 7-positions of the cation have the largest positive charge in the cation. Moreover, the carbene was generated in the presence of tert-butyl hydroperoxide in aqueous tetrahydrofuran to afford azulene through oxidation of 7, followed by decarbonylation. The nucleophilic property of carbene 3 is similar to that of cycloheptatrienylidene 2. Thus, 4 can be regarded as a novel ambiphilic aromatic carbene.

Methods for the chemical synthesis of carbon nanotubes: an approach based on hemispherical polyarene templates

Hemispherical polyarenes represent attractive templates from which carbon nanotubes of the same diameter and rim structure (chirality) might be grown by repetitive annulation reactions. The resulting single-index (n,m) nanotubes would have one end open and the other end capped by the original template. Efforts in the author’s laboratory to synthesize (5,5) and (6,6) nanotube end-caps are described. Nitroethylene is shown to serve well as a “masked acetylene” for the conversion of polyarene bay regions into new unsubstituted benzene rings by a Diels-Alder cycloaddition/aromatization process. Benzyne reacts similarly, both in solution and in the gas phase. These annulation reactions are proposed as methods for elongating large-diameter templates that have bay regions on their rims into structurally uniform, single-walled carbon nanotubes. Unfortunately, the bay regions on the strongly curved rim of the small-diameter (5,5) nanotube end-cap 3 resist Diels-Alder cycloadditions with both nitroethylene and benzyne. Pentabenzocorannulene (14) is proposed as a promising candidate for surface-catalyzed cyclodehydrogenation to a surface-bound hemispherical polyarene that could serve as a template for synthesis of pure (5,5) carbon nanotubes.

A practical synthesis and X-ray crystal structure of tribenzo[a,d,j]corannulene

Tribenzo[a,d,j]corannulene (5) has been synthesized in four linear steps from commercially available starting materials on a half-gram scale in 18% overall yield. An X-ray crystal structure of this triply benzannulated geodesic polyarene confirms the prediction that successive benzannulation of corannulene has a gradual flattening effect on the bowl-shaped core.

Corannulene-Helicene Hybrids: Chiral π-Systems Comprising Both Bowl and Helical Motifs

Two distinct structural elements that render π-systems nonplanar, i.e., geodesic curvature and helical motifs, have been combined into new polyarenes that contain both features. The resultant corannulene-[n]helicenes (n = 5, 6) show unique molecular dynamics in their enantiomerization processes, including inversion motions of both the bowl and the helix. Optical resolution of a corannulene-based skeletally chiral molecule was also achieved for the first time, and the influence of the bowl-motif annulation on the chiroptical properties was investigated.

Chemistry at the interior atoms of polycyclic aromatic hydrocarbons

For more than 150 years, chemical reactions that make new covalent bonds to polycyclic aromatic hydrocarbons (PAHs) have been confined almost exclusively to substitution and addition reactions on the perimeters of the compounds ("edge chemistry"). The "interior" atoms of PAHs, those belonging to three rings, almost never engage in new σ-bond-forming reactions. A compound with no edges, C60, was the first polycyclic carbon π-system observed to exhibit such reactivity. More recently, smaller subunits of C60, which we call geodesic polyarenes, have also been found to exhibit "fullerene-type chemistry" at their interior carbon atoms. These reactions are all reviewed together here for the first time. The review ends with speculation that σ-bond-forming reactions may also be observed someday even in certain planar, benzenoid PAHs, although no examples have yet been reported.

Challenges in aromaticity: 150 years after Kekulé's benzene

Guest editors Nazario Martín and Lawrence T. Scott introduce the Challenges in Aromaticity: 150 years after Kekulé's benzene issue of Chemical Society Reviews

Nearly exclusive growth of small diameter semiconducting single-wall carbon nanotubes from organic chemistry synthetic end-cap molecules

The inability to synthesize single-wall carbon nanotubes (SWCNTs) possessing uniform electronic properties and chirality represents the major impediment to their widespread applications. Recently, there is growing interest to explore and synthesize well-defined carbon nanostructures, including fullerenes, short nanotubes, and sidewalls of nanotubes, aiming for controlled synthesis of SWCNTs. One noticeable advantage of such processes is that no metal catalysts are used, and the produced nanotubes will be free of metal contamination. Many of these methods, however, suffer shortcomings of either low yield or poor controllability of nanotube uniformity. Here, we report a brand new approach to achieve high-efficiency metal-free growth of nearly pure SWCNT semiconductors, as supported by extensive spectroscopic characterization, electrical transport measurements, and density functional theory calculations. Our strategy combines bottom-up organic chemistry synthesis with vapor phase epitaxy elongation. We identify a strong correlation between the electronic properties of SWCNTs and their diameters in nanotube growth. This study not only provides material platforms for electronic applications of semiconducting SWCNTs but also contributes to fundamental understanding of the growth mechanism and controlled synthesis of SWCNTs.

Site-selective covalent functionalization at interior carbon atoms and on the rim of circumtrindene, a C36H12 open geodesic polyarene

Circumtrindene (6, C₃₆H₁₂), one of the largest open geodesic polyarenes ever reported, exhibits fullerene-like reactivity at its interior carbon atoms, whereas its edge carbons react like those of planar polycyclic aromatic hydrocarbons (PAHs). The Bingel–Hirsch and Prato reactions – two traditional methods for fullerene functionalization – afford derivatives of circumtrindene with one of the interior 6:6 C=C bonds modified. On the other hand, functionalization on the rim of circumtrindene can be achieved by normal electrophilic aromatic substitution, the most common reaction of planar PAHs. This peripheral functionalization has been used to extend the π-system of the polyarene by subsequent coupling reactions and to probe the magnetic environment of the concave/convex space around the hydrocarbon bowl. For both classes of functionalization, computational results are reported to complement the experimental observations.

Probing Mechanisms of Aryl–Aryl Bond Cleavages under Flash Vacuum Pyrolysis Conditions

Several biaryls have been subjected to flash vacuum pyrolysis (FVP) at 1100°C and 0.8–0.9 hPa. Product compositions are reported for the FVP of 9-phenylanthracene (1), 2-bromobiphenyl (5), biphenyl (8), 1,10-diphenylanthracene (12), 9-(2-naphthyl)anthracene (17), and 9,9′-bianthracenyl (20). The experimental results have been used to evaluate four possible mechanistic pathways for the cleavage of aryl–aryl bonds under these conditions: (1) the ‘explosion’ of substituted phenyl radicals; (2) hydrogen atom attachment to an ipso-carbon atom of the biaryl followed by C–C bond cleavage; (3) direct homolysis; and (4) loss of a fragment as an aryne. None of these mechanisms by itself successfully accommodates all of the experimental facts. The data suggest that aryl–aryl bond cleavages under FVP conditions involve at least two different mechanistic pathways and that the relative contributions of the competing pathways probably vary from one biaryl to the next.

A grossly warped nanographene and the consequences of multiple odd-membered-ring defects

Graphite, the most stable form of elemental carbon, consists of pure carbon sheets stacked upon one another like reams of paper. Individual sheets, known as graphene, prefer planar geometries as a consequence of the hexagonal honeycomb-like arrangements of trigonal carbon atoms that comprise their two-dimensional networks. Defects in the form of non-hexagonal rings in such networks cause distortions away from planarity. Herein we report an extreme example of this phenomenon. A 26-ring C80H30 nanographene that incorporates five seven-membered rings and one five-membered ring embedded in a hexagonal lattice was synthesized by stepwise chemical methods, isolated, purified and fully characterized spectroscopically. Its grossly warped structure was revealed by single-crystal X-ray crystallography. An independent synthetic route to a freely soluble derivative of this new type of 'nanocarbon' is also reported. Experimental data reveal how the properties of such a large graphene subunit are affected by multiple odd-membered-ring defects.

Palladium-catalyzed C-H activation taken to the limit. Flattening an aromatic bowl by total arylation

All 10 C-H positions on the rim of corannulene can be arylated by repetitive palladium-catalyzed C-H activation. To relieve congestion among the 10 tightly packed aryl substituents in the product, the central corannulene adopts a nearly planar geometry.

Diels-Alder cycloaddition of acetylene gas to a polycyclic aromatic hydrocarbon bay region

The direct conversion of a polycyclic aromatic hydrocarbon bay region to a new, unsubstituted benzene ring by Diels-Alder cycloaddition of acetylene gas is reported for the first time. At 140 °C in dimethylformamide, under 1.8 atm pressure of acetylene gas, 7,14-dimesitylbisanthene is slowly converted to 7,14-dimesitylbenzo[ghi]bisanthene (21% conversion in 48 h).