Scalable, Divergent Synthesis of a High Aspect Ratio Carbon Nanobelt

A new armchair carbon nanobelt synthesized by tuning the regioselectivity of the Scholl reaction of quinquephenyl

A new armchair carbon nanobelt is successfully synthesized by tuning the regioselectivity of the Scholl reaction of 1,1':2',1'':4'',1''':2''',1''''-quinquephenyl. This nanobelt exhibits a preferential binding affinity towards C70 over C60 as found from photoluminescence titration experiments.

Carbon Nanomaterial Fluorescent Probes and Their Biological Applications

Fluorescent carbon nanomaterials have broadly useful chemical and photophysical attributes that are conducive to applications in biology. In this review, we focus on materials whose photophysics allow for the use of these materials in biomedical and environmental applications, with emphasis on imaging, biosensing, and cargo delivery. The review focuses primarily on graphitic carbon nanomaterials including graphene and its derivatives, carbon nanotubes, as well as carbon dots and carbon nanohoops. Recent advances in and future prospects of these fields are discussed at depth, and where appropriate, references to reviews pertaining to older literature are provided.

Generalized kekulenes and clarenes as novel families of cycloarenes: structures, stability, and spectroscopic properties

Cycloarenes constitute a captivating class of polycyclic aromatic hydrocarbons with unique structures and properties, but their synthesis represents a challenging task in organic chemistry. Kekulenes and edge-extended kekulenes as classic types of cycloarenes play an important role in the comprehension of π electron distribution, but their sparse molecular diversity considerably limits their further development and application. In this work, we propose two novel classes of cycloarenes, the generalized kekulenes and the clarenes. Using density functional theory, we carry out a comprehensive study of all possible isomers of the generalized kekulenes and clarenes with different sizes. By applying a simple Hückel model, we show that π delocalization plays a crucial role in determining the relative stability of isomers. We also discover that π-π stacking is commonly present in certain larger clarenes and provides a considerable additional stabilization effect, making the corresponding isomers the lowest-energy ones. Among all considered typical looped polyarenes, generalized kekulenes and/or clarenes are revealed to be the energetically most stable forms, suggesting that these novel cycloarenes proposed here would be viable targets for future synthetic work. The simulated 1H NMR spectra and UV-vis absorption spectra provide valuable information about the electronic and optoelectronic properties for the most stable generalized kekulene and clarene species and may support their identification in future synthesis and experimental characterization.

Enumeration, Nomenclature, and Stability Rules of Carbon Nanobelts

With recent breakthroughs and advances in synthetic chemistry, carbon nanobelts (CNBs) have become an emerging hot topic in chemistry and materials science. Owing to their unique molecular structures, CNBs have intriguing properties with applications in synthetic materials, host-guest chemistry, optoelectronics, and so on. Although a considerable number of CNBs with diverse forms have been synthesized, no systematic nomenclature is available yet for this important family of macrocycles. Moreover, little is known about the detailed isomerism of CNBs, which, in fact, exhibits greater complexity than that of carbon nanotubes. The copious variety of CNB isomers, along with the underlying structure-property relationships, bears fundamental relevance to the ongoing design and synthesis of novel nanobelts. In this paper, we propose an elegant approach to systematically enumerate, classify, and name all possible isomers of CNBs. Besides the simplest, standard CNBs defined by chiral indices (n, m), the nonstandard CNBs (n, m, l) involve an additional winding index l. Based on extensive quantum chemical calculations, we present a comprehensive study of the relative isomer stability of CNBs containing up to 30 rings. A simple Hückel-based model with a high predictive power reveals that the relative stability of standard CNBs is governed by the π stabilization and the strain destabilization induced by the cylindrical carbon framework, and the former effect prevails over the latter. For nonstandard CNBs, a third stability factor, the H···H repulsion in the benzo[c]phenanthrene-like motifs, is also shown to be important and can be incorporated into the simple quantitative model. In general, lower-energy CNB isomers have a larger HOMO-LUMO gap, suggesting that their thermodynamic stability coincides with kinetic stability. The most stable CNB isomers determined can be considered the optimal targets for future synthesis. These results lay an initial foundation and provide a useful theoretical tool for further research on CNBs and related analogues.

Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne peri-Annulations: Making Phenalenyl Ketones without Oxidants

Although Bu3Sn-mediated radical alkyne peri-annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a "weak link" that undergoes C-O fragmentation in the final cascade step. Breaking a C-O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities (i.e., Bu3Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.

Flexible Phenanthracene Nanotubes for Explosive Detection

Phenanthracene nanotubes with arylene-ethynylene-butadiynylene rims and phenanthracene walls are synthesized in a modular bottom-up approach. One of the rims carries hexadecyloxy side chains, mediating the affinity to highly oriented pyrolytic graphite. Molecular dynamics simulations show that the nanotubes are much more flexible than their structural formulas suggest: In 12, the phenanthracene units act as hinges that flip the two macrocycles relative to each other to one of two possible sites, as quantum mechanical models suggest and scanning tunneling microscopy investigations prove. Unexpectedly, both theory and experiment show for 13 that the three phenanthracene hinges are deflected from the upright position, accompanied by a deformation of both macrocycles from their idealized sturdy macroporous geometry. This flexibility together with their affinity to carbon-rich substrates allows for an efficient host-guest chemistry at the solid/gas interface opening the potential for applications in single-walled carbon nanotube-based sensing, and the applicability to build new sensors for the detection of 2,4,6-trinitrotoluene via nitroaromatic markers is shown.

Thermal treatment of water-soluble particles formed by compounds composed of carbon nanobelts and C60 molecules

It was previously shown that spherical particles are self-assembled by compounds composed of C60-(6,6)CNB-C60, where CNB stands for "carbon nanobelt", by mixing two individual solutions of C60 and (6,6)CNB molecules dissolved in 1,2-dichlorobenzene at room temperature. The particles are monodisperse in water thanks to their high absolute value of the zeta potential in water. In this report, we investigate the effect of thermal treatment of the particles on some changes in the physical properties and structures. We find that the particles become electrically conductive after thermal treatment at 600 °C for 1 h. We suppose that the change in the electrical characteristics might have been caused by the structural change of (6,6)CNBs into opened-up ribbons composed of fused benzene rings, which construct networks supported by C60 molecules in the particles, judging by the change in the absorption and mass spectra of the particles after thermal treatment and analysis of a possible change in the structure of C60-(6,6)CNB-C60 based on quantum chemical calculations employing the PM6 method, with which it is known that nanostructures such as carbon nanotubes (CNTs) and (6,6)CNBs can be correctly estimated.

Infinitenes as the Most Stable Form of Cycloarenes: The Interplay among π Delocalization, Strain, and π-π Stacking

The recent successful preparation of infinitene has sparked widespread attention due to its aesthetic appeal and synthetic challenge. Spectroscopic measurements and follow-up computational investigations suggest that infinitene holds fundamental significance and potential applications in chiroptics, optoelectronics, asymmetric synthesis, and supramolecular chemistry. However, unlike other looped polyarenes enriched with sizes and shapes, the infinitene molecule seems, so far, the only known example of this fascinating new form of nanocarbons, whose further exploitation would be considerably limited because of the lack of molecular diversity. Here, we introduce a whole new family of generalized infinitenes with different sizes and topologies. Three types of infinitene structures are rationally designed by joining two units of coronene, kekulene, or their extended analogs. The constructed molecules of varying sizes, each with a large number of possible topoisomers, are systematically studied by DFT calculations. Comprehensive analysis using a simple energy decomposition model uncovers that the stability of infinitenes is governed by the interplay among π delocalization, steric strain, and π-π stacking. While the first two factors are crucial to the stability of smaller infinitenes, the latter is the primary stabilizing interaction for larger infinitenes. Most importantly, we show that larger-sized infinitenes are actually the energetically most favorable form among all known looped polyarenes; their substantial thermodynamic stability surpassing that of circulenes, various carbon nanobelts, and kekulene-like macrocycles renders them promising targets for synthesis. The simulated ¹H NMR, UV-vis, and circular dichroism spectra along with optical rotations for the most stable infinitene species may help their identification in future synthetic efforts.

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.

Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar's Sextets

This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar's π-sextets. To illustrate the utility of this concept to the design of Kekulé diradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar's sextets in the open-shell form overcomes electron pairing and leads to the emergence of a high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP calculations, the singlet diradical character in this core system is characterized by the y₀ value of 0.98 (y₀ = 0 for a closed-shell molecule, y₀ = 1 for pure diradical). The efficiency of the new design strategy was evaluated by comparing the Kekulé system with an isomeric non-Kekulé diradical of identical size, i.e., a system where the radical centers cannot couple via resonance. The calculated singlet-triplet gap, i.e., the ΔE_ST values, in both of these systems approaches zero: -0.3 kcal/mol for the Kekulé and +0.2 kcal/mol for the non-Kekulé diradicaloids. The target isomeric Kekulé and non-Kekulé systems were assembled using a sequence of radical periannulations, cross-coupling, and C-H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. Both molecules are NMR-inactive but electron paramagnetic resonance (EPR)-active at room temperature. Cyclic voltammetry revealed quasi-reversible oxidation and reduction processes, consistent with the presence of two nearly degenerate partially occupied molecular orbitals. The experimentally measured ΔE_ST value of -0.14 kcal/mol confirms that K is, indeed, a nearly perfect singlet diradical.

Closed Aromatic Tubes-Capsularenes

In this study, we describe a synthetic method for incorporating arenes into closed tubes that we name capsularenes. First, we prepared vase-shaped molecular baskets 4-7. The baskets comprise a benzene base fused to three bicycle[2.2.1]heptane rings that extend into phthalimide (4), naphthalimide (6), and anthraceneimide sides (7), each carrying a dimethoxyethane acetal group. In the presence of catalytic trifluoroacetic acid (TFA), the acetals at top of 4, 6 and 7 change into aliphatic aldehydes followed by their intramolecular cyclization into 1,3,5-trioxane (¹ H NMR spectroscopy). Such ring closure is nearly a quantitative process that furnishes differently sized capsularenes 1 (0.7×0.9 nm), 8 (0.7×1.1 nm;) and 9 (0.7×1.4 nm;) characterized by X-Ray crystallography, microcrystal electron diffraction, UV/Vis, fluorescence, cyclic voltammetry, and thermogravimetry. With exceptional rigidity, unique topology, great thermal stability, and perhaps tuneable optoelectronic characteristics, capsularenes hold promise for the construction of novel organic electronic devices.

Room temperature synthesis of water-soluble spherical particles of a uniform diameter composed of carbon nanobelts and C60 molecules

A carbon nanobelt (CNB) is a loop of fused benzene rings and a C₆₀ molecule is a football shaped fullerene composed of 60 carbon atoms. In this study, we synthesize uniform spherical particles composed of (6,6)CNBs and C₆₀ molecules in 1,2-dichlorobenzene at room temperature via bottom-up self-assembly, setting the molar concentrations of (6,6)CNBs and C₆₀ molecules at appropriate values, and find that the particles are monodisperse even in water. The present room temperature synthetic methodology may well be applied to the creation of nano/micro structures/materials using basic carbon nano units such as cycloparaphenylene (CPP, carbon nanorings) and fullerenes; e.g., C₆₀, C₇₀ and C₅₉N.

Low-Valent Zirconocene-Mediated Synthesis of Porphyrin(2.1.2.1)s and Its Extension to Synthesis of a Porphyrin(2.1.2.1) Nanobarrel

Rosenthal's-reagent-mediated intramolecular cyclometallation of α,α-dialkynyldipyrrin nickel(II) complex and subsequent acid treatment afforded a 1,3-butadiene-embedded porphyrin(2.1.2.1), 6, which served as a reactive diene towards dienophiles such as dimethyl acetylenedicarboxylate (DMAD) and benzyne to give corresponding Diels-Alder adducts. Diels-Alder reaction of 6 and benzdiyne gave adducts 14, 15 a, and 15 b along with a trace amount of porphyrin(2.1.2.1) barrel 13. Stepwise routes using 14 or 15 a/15 b as a substrate allowed for the synthesis of 13 as a single stereoisomer. The nanobarrel structure for 13 was revealed by X-ray diffraction, where its cavity held two chloroform molecules via C-H⋅⋅⋅π interaction. DFT calculations revealed that the electrostatic attraction was dominant with binding energy of 32.8 kcal mol^-1 .

Synthesis, Aromatization and Cavitates of an Oxanorbornene-Fused Dibenzo[de, qr]tetracene Nanobox

Oxanorbornene-fused double-stranded macrocycles, represented by kohnkene, are not only synthetic precursors toward short segments of zigzag carbon nanotubes but also typical cavitands processing an intrinsic cavity. However, their capability to bind guest molecules in solution remained unexplored. Herein we report a new member of oxanorbornene-fused double-stranded macrocycles, which is named a nanobox herein because of its shape. Reductive aromatization of this oxanorbornene-fused nanobox leads to observation of a new zigzag carbon nanobelt by high resolution mass spectroscopy. The fluorescence titration and NMR experiments indicate that this nanobox encapsulates C₇₀ in solution with a binding constant of (3.2 ± 0.1) × 10⁶ M⁻¹ in toluene and a high selectivity against C₆₀ and its derivatives. As found from the X-ray crystallographic analysis, this nanobox changes the shape of its cross-section from a rhombus to nearly a square upon accommodating C₆₀.

A new oxanorbornene-fused nanobox encapsulated C₇₀ selectively in solution with a binding constant of (3.2 ± 0.1) × 10⁶ M⁻¹. Reductive aromatization of this nanobox led to observation of a new zigzag carbon nanobelt by mass spectroscopy.

N-doped nonalternant aromatic belt via a six-fold annulative double N-arylation

The design and synthesis of nitrogen (N)-doped molecular nanocarbons are of importance since N-doped nanocarbons have received significant attention in materials science. Herein, we report the synthesis and X-ray crystal structure of a nitrogen-inserted nonalternant aromatic belt. The palladium-catalyzed six-fold annulative double N-arylation provided an aromatic belt bearing six nitrogen atoms in one step from cyclo[6]paraphenylene-Z-ethenylene, the precursor of the (6,6)carbon nanobelt. The C_3i-symmetric structure of the aromatic belt in the solid state was revealed using X-ray crystallography. The multistep (electro)chemical oxidation behavior of the belt, which was facilitated by the six p-methoxyaniline moieties, was studied, and a stable dication species was successfully identified by X-ray crystallography. The present study not only shows the unique structure and properties of the N-doped nonalternant aromatic belt but also expands the scope of accessibility of synthetically difficult belt molecules by the conventional intramolecular contraction pathway.

Nitrogen-doped nonalternant aromatic belt was synthesized via palladium-catalyzed six-fold annulative double N-arylation reaction. The highly symmetric structure and multistep oxidation behavior of the N-belt were confirmed.

Latest News: Reactions of Group 4 Bis(trimethylsilyl)acetylene Metallocene Complexes and Applications of the Obtained Products

Recently published reactions of group 4 metallocene bis(trimethylsilyl)acetylene (btmsa) complexes from the last two years are reviewed. Complexes like Cp'2 Ti(η2 -Me3 SiC2 SiMe3 ) and Cp2 Zr(py)(η2 -Me3 SiC2 SiMe3 ) with Cp' as Cp (cyclopentadienyl) and Cp* (pentamethylcyclopentadienyl) have been considered (py=pyridine). These complexes can liberate a reactive low-valent titanium or zirconium center by dissociation of the ligands and act as ''masked'' MII complexes (M=Ti, Zr). They represent excellent sources for the clean generation of the reactive coordinatively and electronically unsaturated complex fragments [Cp'2 M]. This is the reason why they were used for many synthetic and catalytic reactions during the last years. As an update to several review articles on this topic, this contribution provides an update with recent examples of preparative organometallic and organic chemistry of these complexes, acting as reagents for a wide range of coordinating and coupling reactions. In addition, applications and investigations concerning reaction products derived from this chemistry are mentioned, too.

Large π-Extended and Curved Carbon Nanorings as Carbon Nanotube Segments

ConspectusDesigning and synthesizing topologically unique molecules is a long-term challenge for synthetic chemists. Classical polycyclic aromatic hydrocarbons (PAHs) are a large group of π-conjugated planar organic compounds with rich photophysical and electronic properties, while nonplanar/curved PAHs have different molecular orbital arrangements and demonstrate unique properties. The chemistry of curved aromatic molecules has been of significant interest to explore the relationship between π conjugation and molecular geometry, which offers an attractive combination of fundamental problems, potential applications, and aesthetic appeal. Remarkable advances have been made in the last few decades during the discovery of novel curved aromatic molecules, including corannulenes, fullerenes, and carbon nanotubes (CNTs). Especially, there has been increasing interest in making single-chirality CNTs and their curved molecular components (known as finite segments of CNTs) with a fixed geometry. The most representative examples of such organic molecules are cycloparaphenylenes (CPPs) and related carbon nanorings, which possess cylindrical topologies and nanoscale conjugated segments similar to CNTs. CPPs, as the shortest cross-section and the simplest structure of armchair CNTs, have been synthetically accessible since 2008. Recent years have witnessed breakthroughs and rapid development in the synthesis of CPP-based nanorings as well as their derived molecules. In these molecules, the distortion from aromatic planarity can induce radially oriented π systems and further affect their electronic, optical, self-assembly, and charge-transport characteristics. These unique and interesting carbon nanorings are potentially useful in a variety of optoelectronic and biomedical materials. It is well-known that extension of the π-conjugated system facilitates the delocalization of π electrons and the redistribution of electronic clouds, leading to rich diversification of physical properties in the fields of electronics, optics, and supramolecular chemistry. Therefore, the precise design and controllable synthesis of carbon nanorings with large π conjugation will promote important advances in synthetic chemistry. To date, a number of π-extended carbon nanorings have been reported, and they exhibit novel physicochemical properties resulting from their fascinating topologies and structures. However, challenges still remain in the synthesis of π-extended carbon nanorings and their structural analogues and exploration of their unique properties.In this Account, we give a brief overview of our efforts to synthesize large π-extended carbon nanorings using different strategies and explore their novel applications. In 2013 we started our research on the synthesis of carbon nanorings with large π-conjugated structures. This research project has led to (i) the successful preparation of a series of carbon nanorings with inserted PAHs, especially with various nanographenes inserted, such as hexa-peri-hexabenzocoronene; (ii) the design and synthesis of a series of carbon nanorings consisting solely of PAHs; and (iii) the initial synthesis of π-extended carbon-nanoring-based polymers as the long polymeric segments of CNTs, in which macrocyclic CPPs as the basic repeating blocks were covalently coupled together. Herein we describe in detail how these challenging π-extended carbon nanorings were synthesized, and their interesting physical properties are discussed.

Synthesis and Chiral Resolution of Twisted Carbon Nanobelts

Twisted carbon nanobelts could display persistent chirality, which is desirable for material applications, but their synthesis is very challenging. Herein, we report the successful synthesis and chiral resolution of such a kind of molecules (1-H and 1) with a figure-eight configuration. 1-H was synthesized first by macrocyclization through Suzuki coupling reaction followed by benzannulation via Bi(OTf)₃-mediated cyclization reaction of vinyl ether. Oxidative dehydrogenation of 1-H gave the fully π-conjugated 1. Their twisted structures were confirmed by X-ray crystallographic analysis and calculations, and they can be resolved by chiral high-performance liquid chromatography. The isolated enantiomers showed persistent chiroptical properties according to the circular dichroism measurements, with moderate |g_abs| values (0.0016 for 1-H and 0.005-0.007 for 1). Their photophysical properties were also briefly studied.

Selective Oxidation of Belt[4]arene[4]tropilidene and Its Application to Construct Hydrocarbon Belts of Truncated Cone Structure with Expand Cavity

Despite their unique structures, tantalizing properties, and potential applications in carbon nanoscience and technology, the synthesis and functionalization of zigzag hydrocarbon nanobelts have remained largely unexplored until recently. Reported herein is the selective transformations of belt[4]arene[4]tropilidenes and their application in the construction of novel belts. The oxidation of belt[4]arene[4]tropilidene with benzeneseleninic anhydride under controlled conditions selectively afforded mono- to tetrakis(α-diketone)-functionalized belt intermediates. A subsequent condensation reaction with 1,2-phenylenediacetonitrile and 1,2-phenylenediamine produced a diversity of unprecedented belts with various macrocyclic cavities.