Yueze Gao

Carbyne─the one-dimensional sp-hybridized allotrope of carbon─has long been predicted to exhibit unique properties, yet its synthesis remains elusive. To probe its behavior, finite sp-carbon chains such as cumulenes and polyynes have been studied. While much has been established about the ground-state properties of cumulenes and polyynes, our understanding of their photophysical dynamics, which gives critical insight into how electronic relaxation, structural reorganization, and spin formation evolve, has focused almost exclusively on short, linear systems, with little consideration of topology. Here, we investigate long 48-carbon linear and cyclic polyynes in the liquid phase using steady-state and ultrafast, temperature- and polarization-resolved optical and vibrational spectroscopy. We find highly delocalized ground states in both topologies, with softened Peierls distortion and reduced bond-length alternation relative to shorter chains. In contrast, the excited state singlet dipole depolarization mechanism and subsequent intersystem crossing are strongly dependent on chain length and topology. As in shorter polyynes, transient infrared spectroscopy shows that photoexcitation of long linear polyynes induces a cumulene-like excited-state geometry; however, this structural rearrangement is weaker and shorter-lived than in shorter analogues. Comparison with shorter-chain trends and theoretical predictions shows that the Huang–Rhys factor remains far below the ideal infinite-chain value at 48 carbons, suggesting that vibronic coupling grows more slowly at long chain length than expected from simple extrapolation. Our results reveal how topology influences the largely backbone-dominated electronic dynamics of long polyynes and refine our understanding of finite sp-carbon systems at longer chain lengths than previously accessible with such time-resolved methods.

Stereoisomers of C ₁₃ Cl ₂ exhibiting helical orbitals around a ring of carbon atoms were synthesized by atom manipulation on NaCl surfaces. We resolved the enantiomeric geometries of the singlet states by atomic force microscopy and mapped their helical orbital densities by scanning tunneling microscopy. A π-orbital basis of the helical, nonplanar singlets that twists by 90° in one circulation is consistent with a half-Möbius topology. In such a topology, the π-orbital basis changes sign with respect to two circumnavigations and is periodic with respect to four circumnavigations. A quasiparticle on a ring with this boundary condition could be interpreted as carrying a Berry phase of π/2. We demonstrate reversible switching of the topology between the two singlets of oppositely threaded half-Möbius topology and the planar, topologically trivial triplet state. Multireference calculations, including large-scale, sample-based ab initio calculations executed on quantum hardware, revealed that the switching is associated with a helical pseudo–Jahn-Teller effect.

Cyclo[ N ]carbons, molecular rings consisting solely of N carbon atoms, have previously been studied in the gas phase and on surfaces at cryogenic temperatures, but they are generally considered too reactive to be studied under ambient laboratory conditions. In this study, we report the synthesis of a cyclo[48]carbon catenane, in which the C ₄₈ ring is protected by being threaded through three other macrocycles. This cyclo[48]carbon [4]catenane is stable enough for spectroscopic characterization in solution at room temperature. Its mass spectrum displays the expected molecular ions; its ¹³ C nuclear magnetic resonance spectrum gives a single resonance for all 48 sp ¹ carbon atoms at 72.9 parts per million; and its Raman spectrum shows an intense peak at 1890 inverse centimeters, similar to linear polyynes.

Cleaving a benzene ring under mild conditions requires disrupting its aromaticity, which is a significant challenge in modern chemistry. Although the generation of cyclic products from cleaving benzene has been reported in a few cases, the formation of acyclic products remains exceedingly rare. Inspired by the transition metal-mediated cleavage of inert bonds through multielectron reduction and functionalization, we have discovered an unprecedented ring-opening protocol for benzene and toluene. This is achieved by the reactions of their four-electron-reduced scandium arene complexes with metal hexacarbonyls M(CO)₆ (M = Cr, Mo, and W), resulting in the formation of linear hexadiene products. Further mechanistic studies indicate that the transformation involves the dearomatization of benzene into a dianionic 1,3-cyclohexadiene moiety, followed by the cleavage of the carbon-carbon bond to form the linear hexadiene unit. This work demonstrates a new strategy for the ring opening of benzene through a multielectron-reduction/functionalization approach.

Rigid, conjugated molecules are excellent candidates as molecular wires since they can achieve full extension between electrodes while maintaining conjugation. Molecular design plays a significant role, through minimizing the accessible pi surface to minimize interactions between the bridging wire and the electrode. Polyynes are archetypal molecular wires that feature both a rigid and a thin molecular framework with a cross-section of a single carbon atom. Understanding the behavior of polyynes in molecular junctions is essential for testing models of length versus electron transport. We report construction of molecular junctions using polyynes with a well-defined length up to ca. 5 nm in devices characterized by scanning tunneling microscopy break junctions (STM-BJ). The polyynes, Py**[n] (n = 4, 6, 8, 10, 12, 16), are endcapped with pyridyl groups, and we demonstrate good agreement between the length of the molecular junction and the calculated molecular length, with an average discrepancy of just 0.1 nm. This highlights the power of STM-BJ experiments to accurately determine molecular length. The range of molecular lengths studied, from 1.8 to 4.8 Å, mark this as the most accurate determination of β in polyynes to date (β = 2.2 nm–1). We have applied a model based solely on the single and triple bond lengths to interpret these β-values, which predicts β = 1.9 nm–1, consistent with the experimental value. This model also confirms that electronic coupling in polyynes is unaffected by the rotation about the single bonds. At all molecular lengths, we observe conductance in tunneling regime continues due to the long effective conjugation length of polyynes.

Molecular rings of N carbon atoms (cyclo[ N ]carbons, or C _N ) are excellent benchmarking systems for testing quantum chemical theoretical methods and valuable precursors to other carbon-rich materials. Odd- N cyclocarbons, which have been elusive to date, are predicted to be even less stable than even- N cyclocarbons. We report the on-surface synthesis of cyclo[13]carbon, C ₁₃ , by manipulation of decachlorofluorene with a scanning probe microscope tip. We elucidated the properties of C ₁₃ by experiment and theoretical modeling. C ₁₃ adopts an open-shell configuration with a triplet ground state and a kinked geometry, which shows different extents of distortion and carbene localization depending on the molecular environment. Moreover, we prepared and characterized the C ₁₃ dimer, cyclo[26]carbon, demonstrating the potential of cyclocarbons and their precursors as building blocks for carbon allotropes.

Polyynes are chains of sp¹ carbon atoms with alternating single and triple bonds. As they become longer, they evolve towards carbyne, the 1D allotrope of carbon, and they become increasingly unstable. It has been anticipated that long polyynes could be stabilized by supramolecular encapsulation, by threading them through macrocycles to form polyrotaxanes—but, until now, polyyne polyrotaxanes with many threaded macrocycles have been synthetically inaccessible. Here we show that masked alkynes, in which the C≡C triple bond is temporarily coordinated to cobalt, can be used to synthesize polyrotaxanes, up to the C₆₈ [5]rotaxane with 34 contiguous triple bonds and four threaded macrocycles. This is the length regime at which the electronic properties of polyynes converge to those of carbyne. Cyclocarbons constitute a related family of molecular carbon allotropes, and cobalt-masked alkynes also provide a route to [3]catenanes and [5]catenanes built around cobalt complexes of cyclo[40]carbon and cyclo[80]carbon, respectively.

Synthetic carbon allotropes such as graphene¹, carbon nanotubes² and fullerenes³ have revolutionized materials science and led to new technologies. Many hypothetical carbon allotropes have been discussed⁴, but few have been studied experimentally. Recently, unconventional synthetic strategies such as dynamic covalent chemistry⁵ and on-surface synthesis⁶ have been used to create new forms of carbon, including γ-graphyne⁷, fullerene polymers⁸, biphenylene networks⁹ and cyclocarbons^10,11. Cyclo[N]carbons are molecular rings consisting of N carbon atoms^12,13; the three that have been reported to date (N = 10, 14 and 18)^10,11 are doubly aromatic, which prompts the question: is it possible to prepare doubly anti-aromatic versions? Here we report the synthesis and characterization of an anti-aromatic carbon allotrope, cyclo[16]carbon, by using tip-induced on-surface chemistry⁶. In addition to structural information from atomic force microscopy, we probed its electronic structure by recording orbital density maps¹⁴ with scanning tunnelling microscopy. The observation of bond-length alternation in cyclo[16]carbon confirms its double anti-aromaticity, in concordance with theory. The simple structure of C₁₆ renders it an interesting model system for studying the limits of aromaticity, and its high reactivity makes it a promising precursor to novel carbon allotropes¹⁵.

The versatility of carbon is revealed in its all-carbon forms (allotropes), which feature unique properties (consider the differences between diamond, graphite, graphene and fullerenes). Beyond natural sources, there are many opportunities to expand the realm of carbon chemistry through the study of new carbon forms. In this work, the synthesis of oligo-/polyynes is used to model the elusive carbyne. The chemical stabilization of oligoynes by sterically encumbered endgroups, particularly the 3,5-bis(3,5-di-tert-butylphenyl)pyridyl group, is key to assemble an extended series of stable oligoynes. The final member of this series is the longest monodisperse polyyne isolated and characterized so far, featuring 24 contiguous alkyne units (48 carbons). Spectroscopic and X-ray crystallographic analysis show that endgroups influence the properties of oligoyne derivatives, but this effect diminishes as length increases toward the polyyne/carbyne limit. For instance, with ultraviolet–visible spectroscopy, molecular symmetry clearly documents the evolution of characteristics from oligoynes to polyynes (in which endgroup effects are absent). The combined experimental data are used to refine predictions for the D_∞h structure of carbyne.

Two platinum-bridged pentacene dimers undergo efficient singlet fission to form a correlated triplet pair (T ₁ T ₁ ). The internal heavy-atom effect of the platinum allows for ¹ (T ₁ T ₁ )– ³ (T ₁ T ₁ ) mixing leading to the formation of mainly (T ₁ S ₀ ).