Dimeric Cycloparaphenylenes with a Rigid Aromatic Linker

Nanohoops Favour Light-Induced Energy Transfer over Charge Separation in Porphyrin/[10]CPP/Fullerene Rotaxanes

[2]Rotaxanes offer unique opportunities for studying and modulating charge separation and energy transfer, because the mechanical bond allows the robust, yet spatially dynamic tethering of photoactive groups. In this work, we synthesized [2]rotaxane triads comprising a central (aza)[10]CPP⊃C60 bis-adduct complex and two zinc porphyrin stoppers to address how the movable nanohoop affects light-induced charge separation and energy transfer between the rotaxane subcomponents. We found that neither the parent nanohoop [10]CPP nor its electron-deficient analogue aza[10]CPP actively participate in charge separation. In contrast, the nanohoops completely prevented through-space charge separation. This result is likely due to supramolecular "shielding", because charge separation was observed in the thread that acted as reference dyad. On the other hand, the suppression of electron transfer allowed the observation of energy transfer from the porphyrin triplet to the fullerene triplet state with a lifetime of ca. 25 μs. The presence of the interlocked nanohoops therefore leads to a dramatic switch between charge separation and energy transfer. We suggest that our results explain observations made by others in photovoltaic devices comprising nanohoops and may pave the way toward strategic uses of mechanically interlocked architectures in devices that feature (triplet) energy transfer.

Dual molecular tweezers extending from a nanohoop

The field of nanohoops is mature enough that synthetic protocols exists to tune their size, composition (incorporation of heteroaromatic building blocks), connectivity (para versus meta linkages), and solubility in different media (hydrophobic versus hydrophilic). Here, we report an additional dimension incorporating the concept of fullerene tweezers into a nanohoop. The resulting hybrid nanohoop is highly strained at 77 kcal mol-1, possesses a quantum yield of 0.12, emits at 584 nm, and displays a positive cooperative binding for C60 (4K2 ≫ K1).

Cylindrical and Conical Conjugated Nanobelts by Longitudinal Extension of Cycloparaphenylene and Their Double-Walled Assembly

Constructing carbon nanotubes and related segments with atomic precision remains challenging, despite significant advances, including cycloparaphenylene (CPP), carbon nanobelts, and phenine nanotubes. Here, we report three conjugated nanobelts (NB1-NB3) constructed by longitudinal π-extension of CPP with rings of the same, smaller, or larger size. Each nanobelt comprises three fused aromatic units bridging two axial CPP rings, which mediate the intermolecular assembly. NB1-NB3 have a delocalized electronic structure, showing largely red-shifted emission. The smaller NB2 can insert into NB3 to form a stable double-walled assembly (NB2⊃NB3), which exhibits energy transfer with nearly 100% efficiency.

Pinwheel-like Curved Aromatics from the Cyclotrimerization of Strained Alkyne Cycloparaphenylenes

Carbon nanomaterials composed of curved aromatics, such as carbon nanotubes, are difficult to selectively synthesize and modify precisely. Smaller molecular fragments of curved nanomaterials, such as cycloparaphenylenes, benefit from the precision of bottom-up synthesis, however, efforts to expand the curved molecular framework into even larger structures often rely on restrictive early stage synthetic strategies or difficult to control polymerizations. In this work we report a high yielding, strain-promoted, late-stage modification of a series of [n + 1]CPPs. We show that the conversion of these [n + 1]CPPs into soluble, pinwheel-like multipore carbon nanostructures is achievable via a straightforward and efficient metal-mediated alkyne cyclotrimerization reaction. We provide insight into suitable metals for this transformation, the photophysics of these trimeric molecules, as well as their strain profiles and crystal packing. We also demonstrate the strain-enhanced nature of the reaction under the optimized conditions, showing that strained internal-alkyne [n + 1]CPPs efficiently undergo complete conversion whereas unstrained diphenylacetylene remains completely unreacted. We anticipate that this work will have broader impacts on the study of reactivity in strained-alkyne-containing hydrocarbons, and that access to this new molecular architecture will inspire new research and applications in materials science and related fields.

Molecular cylinders with donor-acceptor structure and swinging motion

The construction of three-dimensional nanocarbon structures with well-defined molecular dynamics is a challenging yet rewarding task in material science and supramolecular chemistry. Herein, we report the synthesis of two highly defective, nitrogen-doped molecular cylinders, namely MC1 and MC2, with a length of 1.4 nm and 2.7 nm, respectively. These molecular cylinders are constructed by connecting the cycloparaphenylene endcaps and fused aromatic pillars using a cyclocondensation reaction, affording a distinct donor-acceptor structure. An X-ray crystallographic analysis reveals a tilted cylindrical shape for MC1, and nuclear magnetic resonance spectroscopy and calculations indicate the occurrence of a dynamic swinging motion in solution. The elongation of conjugation in the cylinders attenuates the charge transfer character in the first excited state, resulting in remarkable length-dependent photophysical properties.

Synthesis of Sb688-: Crafting a Homoatomic Antimony Nanotorus

Pure-element cyclic molecules have garnered extensive attention owing to their intriguing structures and promising applications. Among these, carbon-based cyclic molecules such as cyclo[n]carbon (Cn, n = 10-26) and carbon nanotori have ignited significant interest in both experimental and theoretical investigations. However, systematic investigations of analogous cyclic counterparts of heavier main-group elements are limited, with only a few known by theoretical studies. Furthermore, these corresponding cyclic structures lack synthetic examples in the condensed phase, primarily attributed to their high reactivity resulting from lone electron pairs and the absence of electronic delocalization, which typically aids in stabilizing the structure. In this work, we introduce the pioneering synthesis of a pure antimony-based inorganic nanotorus, denoted as Sb688-, facilitated by the incorporation of C60 for oxidation by utilizing wet-chemistry methodologies. The unique nanotorus structure was meticulously examined via single-crystalline X-ray diffraction, unveiling its composition of 68 antimony atoms and forming a tubular structure with approximate dimensions of 18.5 × 18.4 Å2 in a square shape. Theoretical calculations further revealed that the nanotorus structure, characterized by 16 delocalized electrons distributed across eight 3c-2e σ bonds, effectively saturated the eight two-coordinated Sb atoms within the cluster. This study unveils an innovative approach to synthesizing cyclic compounds solely from pure elements, departing from traditional methods dependent on chemical vapor deposition or surface synthesis, and heralds a profound paradigm shift in physical science.

Figure-Eight Bismacrocycles Derived from a Tetraphenylmethane Core and Oligoparaphenylene Loops

Cycloparaphenylenes have garnered significant interest due to their distinctive chemical and physical characteristics. This study presents the synthesis and comprehensive characterization of two bis-macrocycle molecules joined by cycloparaphenylene and tetraphenylmethane moieties. Both molecules were thoroughly characterized using NMR, MALDI-TOF-HRMS, and X-ray diffraction. UV-vis spectroscopy revealed maximum absorption peaks at 325 and 328 nm, while the two bismacrocycles exhibit fluorescence emissions at 470 and 457 nm, consistent with DFT calculations. The computational analysis also disclosed the HOMO-LUMO gaps of 3.373 and 3.342 eV.

Synthesis and Properties of a Strained Triple Nanohoop

A strained triple nanohoop with a shared central benzene unit is synthesized using a threefold intramolecular ring-closing approach. Among the five possible constitutional isomers, the isomer with the highest D3h symmetry is isolated, the structure of which contains three nanohoop blades and a central hexaphenylbenzene unit. The structure is elucidated using NMR spectroscopy and mass spectrometry. The optical and electrochemical properties are investigated, revealing a moderate fluorescence quantum yield of 40 %. A water-soluble nanomaterial is prepared using a nanoparticle encapsulation method, and a fluorescence quantum yield of 10 % is retained, which demonstrates the potential of the nanomaterial in biological systems.

π-Conjugated Nanohoops: A New Generation of Curved Materials for Organic Electronics

Nanohoops, cyclic association of π-conjugated systems to form a hoop-shaped molecule, have been widely developed in the last 15 years. Beyond the synthetic challenge, the strong interest towards these molecules arises from their radially oriented π-orbitals, which provide singular properties to these fascinating structures. Thanks to their particular cylindrical arrangement, this new generation of curved molecules have been already used in many applications such as host-guest complexation, biosensing, bioimaging, solid-state emission and catalysis. However, their potential in organic electronics has only started to be explored. From the first incorporation as an emitter in a fluorescent organic light emitting diode (OLED), to the recent first incorporation as a host in phosphorescent OLEDs or as charge transporter in organic field-effect transistors and in organic photovoltaics, this field has shown important breakthroughs in recent years. These findings have revealed that curved materials can play a key role in the future and can even be more efficient than their linear counterparts. This can have important repercussions for the future of electronics. Time has now come to overview the different nanohoops used to date in electronic devices in order to stimulate the future molecular designs of functional materials based on these macrocycles.

Host-Guest Chemistry in Binary and Ternary Complexes Utilizing π-Conjugated Carbon Nanorings

The carbon nanorings, possessing a radial π system, have garnered significant attention primarily due to their size-dependent photophysical properties and the presence of a unique curved π-conjugated cavity. This is evidenced by the rapid proliferation of publications. Furthermore, the integration of building blocks into CPP skeletons can confer [n]CPPs with novel and exceptional photophysical and electronic characteristics, as well as chiral properties and host-guest interactions, thereby augmenting the diversity of [n]CPPs. Notably, the curved π surface structures and concave cavity of carbon nanorings enable them to host aromatic or non-aromatic guests with a complementarily curved surface, resulting in interesting binary or ternary complexes. This review provides a comprehensive treatment of literature reports on binary and ternary complexes, focusing on both their host-guest interactions and properties. It is important to note that the scope of this review is limited to host-guest chemistry in binary and ternary complexes based on π-conjugated carbon nanorings.

From π-conjugated macrocycles to heterocycloarenes based on benzo[2,1-b:3,4-b']dithiophene (BDTh): size- and geometry-dependent host-guest properties

π-Conjugated macrocycles have been highly attractive due to their challenging synthesis, fascinating aesthetic structure and unique physical and chemical properties. Although some progress has been made in synthesis, the study of π-macrocycles with different structural characteristics and supramolecular interactions still faces major challenges. In this paper, two new single-bond linked macrocycles (MS-4T/MS-6T) were reported, and the corresponding vinyl-bridged heterocycloarenes (MF-4T/MF-6T) were synthesized by the periphery fusion strategy. Further studies have indicated that the structure of these four macrocycles is determined by both size and curvature, showing unique variations from nearly planar to bowl and then to saddle. Interestingly, the nearly planar MS-4T with a small size and the rigid saddle-shaped MF-6T show no obvious response to fullerenes C60 or C70, while the bowl-shaped MS-6T and MF-4T demonstrate a strong binding affinity towards fullerenes C60 and C70. What's more, two kinds of co-crystals with capsule-like configurations, MS-6T@C60 and MS-6T@C70, have been successfully obtained, among which the former shows a loose columnar arrangement while the latter displays a unique three-dimensional honeycomb arrangement that is extremely rare in supramolecular complexes. This work systematically studies the π-conjugated macrocycles and provides a new idea for the development of novel host-guest systems and further multifunctional applications.

Tuning optical properties of π-conjugated double nanohoops under external electric field stimuli-responsiveness

Carbon nanorings have attracted substantial interest from synthetic chemists due to their unique topological structures and distinct physical properties. An intriguing π-conjugated double-nanoring structure, denoted as [8]CPP-[10]cyclacene, was constructed via the integration of [8]cycloparaphenylene ([8]CPP) into [10]cyclacene. Using the external electric field stimuli-responsiveness of [8]CPP-[10]cyclacene, directional charge transfer can be induced, resulting in the emergence of intriguing properties. The effects of the external electric field in three specific directions were explored, vertically in the [8]CPP unit (Fy), vertically in the [10]cyclacene unit (Fz), and horizontally along the double nanorings diameter (Fx). Interestingly, the external electric field vertically to the [10]cyclacene unit significantly enhanced the first hyperpolarizability (βtot) compared to that vertically to the [8]CPP unit. Notably, [8]CPP-[10]cyclacene under Fx exhibited significantly larger the βtot values (1.48 × 105 a.u.) than those of vertical Fy and Fz. This work opens up a wide range of nonlinear optics, making it a compelling area to explore in the field of carbon nanomaterials.

Monitoring Hierarchical Assembly of Ring-in-Ring and Russian Doll Complexes Based on Carbon Nanoring by Förster Resonance Energy Transfer

We presented the construction of the ring-in-ring and Russian doll complexes on the basis of triptycene-derived carbon nanoring (TP-[12]CPP), which not only acts as a host for pillar[5]arene (P5A) but also serves as an energy donor for building Förster resonance energy transfer (FRET) systems. We also demonstrated that their hierarchical assembly processes could be efficiently monitored in real time using FRET. NMR, UV-vis and fluorescence, and mass spectroscopy analyses confirmed the successful encapsulation of the guests P5A/P5A-An by TP-[12]CPP, facilitated by C-H···π and ···π interactions, resulting in the formation of a distinct ring-in-ring complex with a binding constant of Ka = 2.23 × 104 M-1. The encapsulated P5A/P5A-An can further reverse its role to be a host for binding energy acceptors to form Russian doll complexes, as evidenced by the occurrence of FRET and mass spectroscopy analyses. The apparent binding constant of the Russian doll complexes was up to 3.6 × 104 M-1, thereby suggesting an enhanced synergistic effect. Importantly, the Russian doll complexes exhibited both intriguing one-step and sequential FRET dependent on the subcomponent P5A/P5A-An during hierarchical assembly, reminiscent of the structure and energy transfer of the light-harvesting system presented in purple bacteria.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

Design and Synthesis of New Type of Macrocyclic Architectures Used for Optoelectronic Materials and Supramolecular Chemistry

Supramolecular chemistry has received much attention for decades. Macrocyclic architectures as representative receptors play a vital role in supramolecular chemistry and are applied in many fields such as supramolecular assembly and host-guest recognition. However, the classical macrocycles generally lack functional groups in the scaffolds, which limit their further applications, especially in optoelectronic materials. Therefore, developing a new design principle is not only essential to better understand macrocyclic chemistry and the supramolecular behaviors, but also further expand their applications in many research fields. In recent years, the doping compounds with main-group heteroatoms (B, N, S, O, P) into the carbon-based π-conjugated macrocycles offered a new strategy to build macrocyclic architectures with unique optoelectronic properties. In particular, the energy gaps and redox behavior can be effectively tuned by incorporating heteroatoms into the macrocyclic scaffolds. In this Minireview, we briefly summarize the design and synthesis of new macrocycles, and further discuss the related applications in optoelectronic materials and supramolecular chemistry.

Bismacrocycle: Structures and Applications

In the past half-century, macrocycles with different structures and functions, have played a critical role in supramolecular chemistry. Two macrocyclic moieties can be linked to form bismacrocycle molecules. Compared with monomacrocycle, the unique structures of bismacrocycles led to their specific recognition and assembly properties, also a wide range of applications, including molecular recognition, supramolecular self-assembly, advanced optical material construction, etc. In this review, we focus on the structure of bismacrocycle and their applications. Our goal is to summarize and outline the possible future development directions of bismacrocycle research.

Regulating supramolecular interactions in dimeric macrocycles

Supramolecular behavior is highly dependent on many factors, including complicated microenvironments and weak interactions. Herein, we describe tuning supramolecular architectures of rigid macrocycles by synergistic effects of their geometric configurations, sizes, and guests. Two paraphenylene-based macrocycles are anchored onto different positions in a triphenylene derivative, resulting in dimeric macrocycles with different shapes and configurations. Interestingly, these dimeric macrocycles show tunable supramolecular interactions with guests. In solid state, a 2 : 1 host-guest complex was observed between 1a and C₆₀/C₇₀, while an unusual 2 : 3 host-guest complex 3C₆₀@(1b)₂ can be observed between 1b and C₆₀. This work expands the scope of the synthesis of novel rigid bismacrocycles and provides a new strategy to construct different supramolecular systems.

Stable Crystalline Nanohoop Radical and Its Self-Association Promoted by van der Waals Interactions

A stable nanohoop radical (OR3) combining the structures of cycloparaphenylene and an olympicenyl radical is synthesized and isolated in the crystalline state. X-ray crystallographic analysis reveals that OR3 forms a unique head-to-tail dimer that further aggregates into a one-dimensional chain in the solid state. Variable-temperature NMR and concentration-dependent absorption measurements indicate that the π-dimer is not formed in solution. An energy decomposition analysis indicates that van der Waals interactions are the driving force for the self-association process, in contrast with other olympicenyl derivatives that favor π-dimerization. The physical properties in solution phase have been studied, and the stable cationic species obtained by one-electron chemical oxidation. This study offers a new molecular design to modulate the self-association of organic radicals for overcoming the spin-Peierls transition, and to prepare novel nanohoop compounds with spin-related properties.

Optical Physical Mechanisms of Helicene Carbon Nanohoop with Möbius Topology

Optical and spectral properties of carbon nanohoop with Möbius topology is of great interest in nano-science and nano-technology. And it can be imagined that it has a lot of unexpected potential application prospects. However, theoretical calculations based on some figure-of-eight helicene carbon nanohoop with Möbius topology are still insufficient. Therefore, in this paper, we theoretically study the optical and spectral properties of figure-of-eight helicene carbon nanohoop with Möbius topology. Optical and spectral properties are analyzed with visualization method of transition density matrix and charge density difference, which reveal the unique characterization of carbon nanohoop with Möbius topology. Our results can not only deepen the understanding of the optical physical mechanisms of the nanorings with mobius carbons, but also provide deeper insight on optical properties and potential design on optical nanodevices.

Boron-Cluster Embedded Necklace-Shaped Nanohoops

The combination of carbon-based nanohoops with other functional organic molecular structures should lead to the design of new molecular configurations with interesting properties. Here, necklace-like nanohoops embedded with carborane were synthesized for the first time. The unique deboronization of o-carborane has led to the facile preparation of ionic nanohoop compounds. Nanohoops functionalized by nido-o-carborane show excellent fluorescence emission, with a solution quantum yield of up to 90.0 % in THF and a solid-state quantum efficiency of 87.3 %, which opens an avenue for the applications of the nanohoops in OLEDs and bioimaging.

Modular synthesis, host-guest complexation and solvation-controlled relaxation of nanohoops with donor-acceptor structures

Carbon nanohoops with donor-acceptor (D-A) structures are attractive electronic materials and biological fluorophores, but their synthesis is usually challenging. Moreover, the preparation of D-A nanohoop fluorophores exhibiting high fluorescence quantum yields beyond 500 nm remains a key challenge. This study presents a modular synthetic approach based on an efficient metal-free cyclocondensation reaction that readily produced nine congeners with D-A or donor-acceptor-donor' (D-A-D') structures, one of which is water-soluble. The tailored molecular design of nanohoops enabled a systematic and detailed study of their host-guest complexation with fullerene, optical properties, and charge transfer (CT) dynamics using X-ray crystallography, fluorescence titration, steady and ultrafast transient absorption spectroscopy, and theoretical calculations. The findings revealed intriguing physical properties associated with D-A motifs, such as tight binding with fullerene, moderate fluorescence quantum yields (37-67%) beyond 540 nm, and unique solvation-controlled CT relaxation of D-A-D' nanohoops, where two CT states (D-A and A-D') can be effectively tuned by solvation, resulting in dramatically changed relaxation pathways in different solvents.

An unexpected dual-emissive luminogen with tunable aggregation-induced emission and enhanced chiroptical property

In the literature, organic materials with both aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) effects that can emit with multiple bands both in the solution and aggregated state are rarely reported. Herein we report a novel chiral dual-emissive bismacrocycle with tunable aggregation-induced emission colors. A facile four-step synthesis strategy is developed to construct this rigid bismacrocycle, (1,4)[8]cycloparaphenylenophane (SCPP[8]), which possesses a 1,2,4,5-tetraphenylbenzene core locked by two intersecting polyphenylene-based macrocycles. The luminescent behavior of SCPP[8] shows the unique characteristics of both ACQ effect and AIE effect, inducing remarkable redshift emission with near white-light emission. SCPP[8] is configurationally stable and possesses a novel shape-persistent bismacrocycle scaffold with a high strain energy. In addition, SCPP[8] displays enhanced circularly polarized luminescence properties due to AIE effect.

Organic materials with both aggregation induced emission (AIE) and aggregation-caused quenching (ACQ) effects that can emit with multiple wavelengths in the solution and aggregated state are rarely reported. Here, the authors report a chiral dual-emissive bismacrocycle which shows the unique ACQ and AIE effects inducing redshift emission with near white-light emission.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

Enabling dual aromaticity in fused nanobelts: evaluation of the magnetic behavior of fused [10]CPP units

Cyclo-para-phenylene (CPP) nanobelt structures with curved π-surfaces are of relevance in the development of desirable building units for materials science.

Overcrowded Ethylene-Bridged Nanohoop Dimers: Regioselective Synthesis, Multiconfigurational Electronic States, and Global Hückel/Möbius Aromaticity

The design and preparation of molecular systems with multiple geometric and electronic configurations are the cornerstones for multifunctional materials with stimuli-responsive behaviors. We describe here the regioselective and facile synthesis of two types of overcrowded ethylene-bridged nanohoop dimers, with folded and twisted geometric structures as well as closed-shell, diradical and dication electronic structures. The strained nanohoop structures have a profound effect on the overall molecular and electronic configurations, which resulted in the destabilized diradical state. X-ray crystallographic analysis revealed the folded molecular geometry for the neutral species and twisted geometry for the dication species. The unique molecular dynamics, optical properties, and dynamic redox properties were disclosed in the solution phase by spectroscopic and electrochemical methods. Furthermore, the global Hückel and Möbius aromaticity were revealed by a combination of experimental and theoretical approaches. Our studies shed light on the design of nanohoop-incorporated multiconfigurational materials with unique topologies and functions.

A Conjugated Figure-of-Eight Oligoparaphenylene Nanohoop with Adaptive Cavities Derived from Cyclooctatetrathiophene Core

A fully conjugated figure-of-eight nanohoop is presented with facile synthesis. The molecule's lemniscular skeleton features the combination of two strained oligoparaphenylene loops and a flexible cyclooctatetrathiophene core. Its rigid yet guest-adaptive cavities enable the formation of the peanut-like 1:2 host-guest complexes with C₆₀ or C₇₀ , which have been confirmed by X-ray crystallography and characterized in solution. Further computational studies suggest notable geometric variations and non-covalent interactions of the cavities upon binding with different fullerenes, as well as overall conjugation comparable to cycloparaphenylenes.

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.

Confining the Inner Space of Strained Carbon Nanorings

Strained aromatic macrocycles based on cycloparaphenylenes (CPPs) are the shortest repeating units of armchair single-walled carbon nanotubes. Since the development of several new synthetic methodologies for accessing these structures, their properties have been extensively studied. Besides the fundamental interest in these novel molecular scaffolds, their application in the field of materials science is an ongoing topic of research. Most of the reported CPP-type macrocycles display strong binding toward fullerenes, due to the perfect match between the convex and concave π-surfaces of fullerenes and CPPs, respectively. Highly functionalized CPP derivatives capable of supramolecular binding with other molecules are rarely reported. The synthesis of highly functionalized [n]cyclo-2,7-pyrenylenes leads to CPP-type macrocycles with a defined cavity capable of binding non-fullerene guests with high association constants.

Fullerene Wires Assembled Inside Carbon Nanohoops

Carbon-nanohoop structures featuring one or more round-shaped cavities represent ideal supramolecular hosts for spherical fullerenes, with potential to form host-guest complexes that perform as organic semiconductors in the solid state. Due to the tight complexation between the shape-complementary hosts and guests, carbon nanohoops have the potential to shield fullerenes from water and oxygen, known to perturb the electron-transport process. Many nanohoop receptors have been found to form host-guest complexes with fullerenes. However, there is only a little or no control over the long-range order of encapsulated fullerenes in the solid state. Consequently, the potential of these complexes to perform as organic semiconductors is rarely evaluated. Herein, we present a survey of all known nanohoop-fullerene complexes, for which the solid-state structures were obtained. We discuss and propose instances where the inclusion fullerene guests form discrete supramolecular wires, which might open up possibilities for their use in electronic devices.

Cycloparaphenylene Double Nanohoop: Structure, Lamellar Packing, and Encapsulation of C60 in the Solid State

A new member of the cycloparaphenylene double-nanohoop family was synthesized. Its π-framework features two oval cavities that display different shapes depending on the crystallization conditions. Incorporation of the peropyrene bridge within the nanoring cycles via bay-regions alleviates steric effects and thus allows 1:1 complexation with C₆₀ in the solid state. This nanocarbon adopts a lamellar packing motif, and our results suggest that the structural adjustment of this double nanohoop could enable its use in supramolecular and semiconductive materials.

Recent Advances in Dimeric Cycloparaphenylenes as Nanotube Fragments

Since the discovery of cycloparaphenylenes in 2008, the chemical synthesis of more-complicated molecular systems with curved π-surfaces has been vigorously sought, giving rise to a plethora of new exciting molecules with various topologies and functions. This Synpacts article briefly summarizes recent examples of carbon nanohoop dimers, highlighting three examples as nanotube fragments. Their synthesis, isomerization, photophysical properties, and host–guest chemistry are discussed.

1 Introduction

2 Synthetic Strategy toward Nanotube Dimers

3 Isomerization Dynamics of Nanotube Dimers

4 Photophysical Properties of Nanotube Dimers

5 Host–Guest Chemistry of Nanotube Dimers

6 Conclusions

Stable and twisted 5,6:12,13-dinaphthozethrene from angular π-extension

Herein, we describe a concise and efficient synthesis of an angularly extended stable zethrene derivative 1, designed to have more benzenoid rings in the closed-shell resonance form. This compound exhibited enantiomeric structures in the solid state derived from the benzo[4]helicene structure and rapid interconversion in solution. Its far-red absorption, near-infrared emission and amphoteric redox properties were also revealed.

A Highly Strained All-Phenylene Conjoined Bismacrocycle

Herein, we report the precise synthesis of a 3D highly strained all-phenylene bismacrocycle, termed conjoined (1,4)[10]cycloparaphenylenophane (SCPP[10]). This structure consists of a twisted benzene ring which is bridged twice by phenylene units anchored in two para-positions. The conjoined structure of SCPP[10] was confirmed in real space at the atomic scale by scanning tunneling microscopy. Theoretical calculations indicate that this bismacrocycle has a very high strain energy of 110.59 kcal mol^-1 and the largest interphenylene torsion angle of 46.07° caused by multiple repulsive interactions. Furthermore, a 1:2 host-guest complex of SCPP[10] and [6,6]-phenyl-C₆₁ -butyric acid methyl ester was investigated, which represents the first peanut-shaped 1:2 host-guest complex based on bismacrocycles.

Cycloparaphenylene-Phenalenyl Radical and Its Dimeric Double Nanohoop*

The first example of a neutral spin-delocalized carbon-nanoring radical was achieved by integration of the open-shell phenalenyl unit into cycloparaphenylene (CPP). Spin distribution in this hydrocarbon is localized primarily on the phenalenyl segment and partially on the CPP segment as a consequence of steric and electronic effects. The resulting geometry is reminiscent of a diamond ring, with pseudo-perpendicular arrangement of the radial and the planar π-surface. The phenylene rings attached directly to the phenalenyl unit give rise to a steric effect that governs a highly selective dimerization pathway, yielding a giant double nanohoop. Its π-framework made of 158 sp2 -carbon atoms was elucidated by single-crystal X-ray diffraction, which revealed a three-segment CPP-peropyrene-CPP structure. This nanocarbon shows a fluorescence profile characteristic of peropyrene, regardless of which segment gets excited. These results in conjunction with DFT suggest that adjusting the size of the CPP segments in this double nanohoop could deliver donor-acceptor systems.