Chemistry of Cyclo[18]Carbon (C18): A Review

Carbon-based allotropes are propelling a technological revolution in communication, sensing, and computing, concurrently challenging fundamental theories of the previous century. Nevertheless, the demand for advanced carbon-based materials remains substantial. The crux lies in the efficient and reliable engineering of novel carbon allotrope. Although C18 has undergone theoretical and experimental investigation for an extended period, its preparation and direct observation in the condensed phase occurred only recently through STM/AFM techniques. The distinctive cyclic ring structure and the dual 18-center π delocalization character introduce various uncommon properties to C18, rendering it a subject worthy of in-depth exploration. In this context, this review delves into past developments contributing to the state-of-the-art understanding of C18 and provides insights into how future endeavours can expedite practical applications. Encompassing a broad spectrum, this review comprehensively investigates almost all facets of C18, including geometric characteristics, electron delocalization, bonding nature, aromaticity, reactivity, electronic excitation, UV/Vis spectrum, intermolecular interaction, response to external fields, electron affinity, ionization, and other molecular properties. Moreover, the review also outlines representative strategies for the direct synthesis and characterization of C18 using atom manipulation techniques. Following this, C18-based complexes are summarized, and potential applications in catalysis, electrochemical devices, optoelectronics, and sensing are discussed.

The Stiffest and Strongest Predicted Material: C2 N Atomic Chains Approach the Theoretical Limits

Though linear atomic chains exhibit extreme properties, it is presently unclear how these properties can be maximized by the choice of elemental composition. Considering that boron, carbon, and nitrogen can form high modulus and high strength atomic chains, here an algorithm is developed to construct 143 possible atomic chains of these elements with 6 or fewer atoms in the primitive cell and explore their stabilities and mechanical properties by first-principles calculations. It is found that the gravimetric modulus (1032 GPa g^-1 cm³ ) and strength (108 GPa g^-1 cm³ ) of the C₂ N chain significantly exceed those of any known material, including the previously stiffest predicted material (C chain, 945 GPa g^-1 cm³ ) and the previously strongest predicted material (BC chain, 92 GPa g^-1 cm³ ), and also approach the theoretical limits of gravimetric modulus (1036 GPa g^-1 cm³ ) and strength (130 GPa g^-1 cm³ ). Mechanistic analyses demonstrate that the higher gravimetric modulus and strength of the C₂ N chain, compared with the C and BC chains, originate from its short, stiff chemical bonding and the abnormal decrease in bond length alternation under tension. The likely ease of fabrication and potential synthesis routes for C₂ N chains are discussed.

Exploring relationships between chemical structure and molecular conductance: from α,ω-functionalised oligoynes to molecular circuits

The quantum circuit rule (QCR) allows estimation of the conductance of molecular junctions, electrode|X-bridge-Y|electrode, by considering the molecule as a series of independent scattering regions associated with the anchor groups (X, Y) and bridge, provided the numerical parameters that characterise the anchor groups (a_X, a_Y) and molecular backbones (b_B) are known. Single-molecule conductance measurements made with a series of α,ω-substituted oligoynes (X-{(CC)_N}-X, N = 1, 2, 3, 4), functionalised by terminal groups, X (4-thioanisole (C₆H₄SMe), 5-(3,3-dimethyl-2,3-dihydrobenzo[b]thiophene) (DMBT), 4-aniline (C₆H₄NH₂), 4-pyridine (Py), capable of serving as 'anchor groups' to contact the oligoyne fragment within a molecular junction, have shown the expected exponential dependence of molecular conductance, G, with the number of alkyne repeating units. In turn, this allows estimation of the anchor (a_i) and backbone (b_i) parameters. Using these values, together with previously determined parameters for other molecular fragments, the QCR is found to accurately estimate the junction conductance of more complex molecular circuits formed from smaller components assembled in series.

Predicted Confinement-Enhanced Stability and Extraordinary Mechanical Properties for Carbon Nanotube Wrapped Chains of Linear Carbon

The demand for high-modulus, high-strength, lightweight materials has continuously driven the bottom-up assembly of carbon nanostructures into high-performance bulk carbon materials, such as graphene sheets and carbon nanotube yarns. Carbyne, often called linear carbon, has a higher predicted gravimetric modulus and gravimetric strength than any other form of carbon, but possibly reacts under near-ambient conditions because of the extended sp¹ hybridization. The successful fabrication of carbon nanotube wrapped single carbyne chain (Shi et al. Nat. Mater. 2016, 15, 634) suggests the possibility of carbyne's bulk production. Herein, we designed a type of carbon assembly that includes a possibly large array of carbyne chains confined within a single-walled nanotube sheath (nanotube wrapped carbynes, NTWCs), in which carbyne chains act as reinforcing building blocks, and the carbon nanotube sheath protects the multiple carbyne chains against chemical or topochemical reaction. We showed that NTWCs exhibit confinement-enhanced stabilities, even when they contain multiple neighboring carbyne chains. We developed a mechanics model for exploring the mechanical properties of NTWCs. On the basis of this model, the gravimetric modulus (and strength) of NTWCs was predicted to increase from 356.4 (50.25) to 977.2 GPa·g^-1·cm³ (71.20 GPa·g^-1·cm³) as the mass ratio of carbyne carbons to sheath carbons increases, which is supported by atomistic simulations. The highest calculated gravimetric modulus and strength of NTWCs are 174.2% and 41.7%, respectively, higher than those of either graphene or carbon nanotubes. The corresponding highest values of engineering modulus and strength of NTWCs with a density of 1.54 g·cm^-3 are 1505 and 109.6 GPa, respectively. Hence, NTWCs are promising for uses in high-modulus, high-strength, lightweight composites.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Laser desorption vs. electrospray of polyyne-threaded rotaxanes: Preventing covalent cross-linking and promoting noncovalent aggregation

Laser-induced cross-linking of polyynes is successfully hindered when the polyyne is encapsulated as part of a rotaxane and therefore protected by a surrounding macrocycle. When the rotaxane is electrosprayed, however, noncovalent aggregate ions are efficiently formed. Aggregates of considerable size (including more than 50 rotaxane molecules with masses beyond 100k Da) and charge states (up to 13 charges and beyond) have been observed. Either protons or sodium cations act as the charge carriers. These aggregates are not formed when the individual components of the rotaxane, i.e., the macrocycle or the polyyne, are separately electrosprayed. This underlines the structural importance of the rotaxane for the aggregate formation. Straightforward force field calculations indicate that the polyyne thread hinders the folding of the macrocycles, which facilitates the bonding interaction between the two components.