Linearly Polymerized Benzene Arrays As Intermediates, Tracing Pathways to Carbon Nanothreads

High-pressure structure and reactivity of crystalline bibenzyl: Insights and prospects for the synthesis of functional double-core carbon nanothreads

The high-pressure synthesis of double-core nanothreads derived from pseudo-stilbene crystals represents a captivating approach to isolate within the thread chromophores or functional groups without altering its mechanical properties. These entities can be effectively utilized to finely tune optical properties or as preferential sites for functionalization. Bibenzyl, being isostructural with other members of this class, represents the ideal system for building co-crystals from which we can synthesize double-core nanothreads wherein bridging chromophores, such as the azo or ethylene moieties, are embedded in the desired concentration within a fully saturated environment. To achieve this, a critical step is the preliminary characterization of the high-pressure behavior of crystalline bibenzyl. We report here an accurate investigation performed through state-of-the-art spectroscopic techniques, Raman and Fourier transform infrared spectroscopy, and x-ray diffraction up to 40 GPa. Our findings reveal a strongly anisotropic compression of the crystal, which determines, at pressures between 1 and 2 GPa, consistent modifications of the vibrational spectrum, possibly related to a torsional distortion of the molecules. A phase transition is detected between 9 and 10 GPa, leading to a high pressure phase where, above 24 GPa, the nanothread formation is observed. However, the observed reaction was limited in extent and required significantly higher pressures in comparison to other members of the pseudo-stilbene family. This comprehensive study is imperative in laying the foundation for future endeavors, aiming to synthesize double-core nanothreads from pseudo-stilbene crystals, and provides crucial insights into the high-pressure behavior and phase transitions of crystalline bibenzyl.

One dimensional ferroelectric nanothreads with axial and radial polarization

Long-range ferroelectric crystalline order usually fades away as the spatial dimension decreases, and hence there are few two-dimensional (2D) ferroelectrics and far fewer one-dimensional (1D) ferroelectrics. Due to the depolarization field, low-dimensional ferroelectrics rarely possess the polarization along the direction of reduced dimensionality. Here, using first-principles density functional theory, we explore the structural evolution of nanoribbons of varying widths constructed by cutting a 2D sheet of ferroelectric α-III2VI3 (III = Al, Ga, In; VI = S, Se, Te). We discover a one-dimensional ferroelectric nanothread (1DFENT) of ultrasmall diameter with both axial and radial polarization, potentially enabling ultra-dense data storage with a 1D domain of just three unit cells being the functional unit. The polarization in 1DFENT of Ga2Se3 exhibits an unusual piezoelectric response: a stretching stress along the axial direction will increase both the axial and radial polarization, referred to as the auxetic piezoelectric effect. Utilizing the intrinsically flat electronic bands, we demonstrate the coexistence of ferroelectricity and ferromagnetism in 1DFENT and a counterintuitive charge-doping-induced metal-to-insulator transition. The 1DFENT with both axial and radial polarization offers a counterexample to the Mermin-Wagner theorem in 1D and suggests a new platform for the design of ultrahigh-density memory and the exploration of exotic states of matter.

Ordered Van der Waals Hetero-nanoribbon from Pressure-Induced Topochemical Polymerization of Azobenzene

Pressure-induced topochemical polymerization of molecular crystals with various stackings is a promising way to synthesize materials with different co-existing sub-structures. Here, by compressing the azobenzene crystal containing two kinds of intermolecular stacking, we synthesized an ordered van der Waals carbon nanoribbon (CNR) heterostructure in one step. Azobenzene polymerizes via a [4 + 2] hetero-Diels-Alder (HDA) reaction of phenylazo-phenyl in layer A and a para-polymerization reaction of phenyl in layer B at 18 GPa, as evidenced by in situ Raman and IR spectroscopies, X-ray diffraction, as well as gas chromatography-mass spectrometry and the solid-state nuclear magnetic resonance of the recovered products. The theoretical calculation shows that the obtained CNR heterostructure has a type II (staggered) band gap alignment. Our work highlights a high-pressure strategy to synthesize bulk CNR heterostructures.

Theory of Borazine-Derived Nanothreads: Enumeration, Reaction Pathways, and Piezoelectricity

Nanothreads are one-dimensional macromolecules formed by pressure-induced polymerization along stacks of multiply unsaturated (or highly strained) molecules such as benzene (or cubane). Borazine is isoelectronic to benzene yet with substantial bond polarity, thus motivating a theoretical examination of borazine-derived nanothreads with degrees of saturation of 2, 4, and 6 (defined as the number of four-coordinated boron and nitrogen atoms per borazine formula unit). The energy increases upon going from molecular borazine to degree-2 borazine-derived threads and then decreases for degree-4 and degree-6 nanothreads as more σ bonds are formed. With the constraint of no more than two borazine formula units within the repeat unit of the framework's bonding topology, there are only 13 fully saturated (i.e., degree-6) borazine-derived nanothreads that avoid energetically costly homopolar bonds (as compared to more than 50 such candidates for benzene-derived threads). Only two of these are more stable than borazine. Hypothetical pathways from molecular borazine to these two degree-6 borazine-derived nanothreads are discussed. This relative paucity of outcomes may assist in kinetic control of reaction products. Beyond the high mechanical strength also predicted for carbon-based threads, properties such as piezoelectricity and flexoelectricity may be accessible to the polar lattice of borazine-derived nanothreads, with intriguing prospects for expression in these extremely thin yet rigid objects.

High-Pressure Reaction Profiles and Activation Volumes of 1,3-Cyclohexadiene Dimerizations Computed by the Extreme Pressure-Polarizable Continuum Model (XP-PCM)

Quantum chemical calculations are reported for the thermal dimerizations of 1,3‐cyclohexadiene at 1 atm and high pressures up to the GPa range. Computed activation enthalpies of plausible dimerization pathways at 1 atm agree well with the experiment activation energies and the values from previous calculations. High‐pressure reaction profiles, computed by the recently developed extreme pressure‐polarizable continuum model (XP‐PCM), show that the reduction of reaction barrier is more profound in concerted reactions than in stepwise reactions, which is rationalized on the basis of the volume profiles of different mechanisms. A clear shift of the transition state towards the reactant under pressure is revealed for the [6+4]‐ene reaction by the calculations. The computed activation volumes by XP‐PCM agree excellently with the experimental values, confirming the existence of competing mechanisms in the thermal dimerization of 1,3‐cyclohexadiene.

Theoretical Studies of Furan and Thiophene Nanothreads: Structures, Cycloaddition Barriers, and Activation Volumes

This theoretical study examines the formation, structure, and stability of two of the most ordered nanothreads produced yet, those derived from furan and thiophene. The energetic consequences and activation barriers of the first two steps of oligomerization via a Diels-Alder mechanism were examined. The ca. 20 GPa difference in the synthetic pressures (lower for furan) is explainable in terms of the greater loss of aromaticity by the thiophene. The effects of pressure on the reaction profiles, operating through a volume decrease along the reaction coordinate, are illustrated. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with opposing, cumulative dipole moments. The computed activation volumes are consistently more negative for furan, in accordance with the lower onset pressure of furan polymerization. The energetics of three ordered polymer structures were examined. The syn polymer, with all O/S atoms on the same side, if not allowed to distort, is at a high energy relative to the other two due to the O/S lone pair repulsion, understandably greater for S than for O at the 2.8/2.6 Å separation. Set free, the syn isomers curve or arch in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The syn polymer can also stabilize itself by twisting into zig-zag or helical energy minima. The release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of the crystalline order in the polymer as it is returned to ambient pressure.

Crystalline C3N3H3 tube (3,0) nanothreads

The diamond nanothread is predicted to have comparable or larger strength and stiffness than the carbon nanotube but severely suffers from intrathread bonding disorder in practice. Here, we synthesized a bulk crystalline diamond nanothread material C₃N₃H₃ with a C/N-ordered tube (3,0) structure via high-pressure topochemical polymerization of s-triazine. The π-stacked s-triazine experienced a selective concerted bonding of three neighbored C···N pairs, completely different from the classical Diels–Alder reactions of aromatics under high pressure. This study obtained the structure-specific diamond nanothread with an unambiguous elemental reaction, which is the basis to realize the expected excellent properties of diamond nanothreads. Following the identified reaction selectivity under high pressure, more structure-specific carbon materials can be designed.

Carbon nanothread (CNTh) is a “one-dimensional diamond polymer” that combines high tensile strength and flexibility, but it severely suffers from intrathread disorder. Here, by modifying the reactivity and the stacking ordering of the aromatic precursor, crystalline C₃N₃H₃ CNTh with perfect hexagonal orientation and stacking was synthesized at 10.2 GPa and 573 K from s-triazine. By Rietveld refinement of X-ray diffraction data, gas chromatography mass spectrometry investigation, and theoretical calculation, we found that synthesized CNTh has a tube (3,0) structure, with the repeating s-triazine residue connected solely by C–N bonds along the thread. A “peri-cage” reaction, the concerted bonding between six C and N atoms, instead of [4 + 2] or [1,4] addition reactions, was concluded for the formation of CNThs, and the critical bonding distance between the nearest intermolecular C and N was ∼2.9 Å. The formation of a “structure-specific” crystalline CNTh with C and N orderly distributed highlighted the importance of reaction selectivity and stacking order of reactant molecules, which have great significance for understanding the polymerization of aromatic molecules under high pressure and developing new crystalline CNThs.

Towards custom built double core carbon nanothreads using stilbene and pseudo-stilbene type systems

Until recently, saturated carbon nanothreads were the missing tile in the world of low-dimension carbon nanomaterials. These one-dimensional fully saturated polymers possess superior mechanical properties by combining high tensile strength with flexibility and resilience. They can be obtained by compressing aromatic and heteroaromatic crystals above 15 GPa exploiting the anisotropic stress that can be achieved by the diamond anvil cell technique. Recently, double-core nanothreads were synthesized by compressing azobenzene crystals, achieving the remarkable result of preserving the azo group as a linker of the resulting double thread. Herein, we demonstrate the generality of these findings through the synthesis of double carbon nanothreads from trans stilbene and azobenzene-stilbene mixed crystals. Employment of Fourier transform infrared spectroscopy and synchrotron X-ray diffraction enabled a comprehensive characterization of the reactivity identifying threshold conditions, kinetics and structure-reaction relationship. In particular, the reaction is anticipated by a phase transition characterized by a sudden increase of the monoclinic angle and a collapse along the b axis direction. Large bidimensional crystalline areas extending several tens of nanometers are evidenced by transmission electron microscopy also confirming the monoclinic unit cell derived from X-ray diffraction data in which threads possessing the polymer 1 structure, as suggested by density functional theory calculations, are packed. The most exciting result of this study is the demonstration of viable synthesis of double nanothreads where the number and the nature of chromophoric groups linking the threads can be tuned by preparing starting crystals of desired composition, thanks to the isomorphism typical of the pseudo-stilbene molecules. This is extremely important in tailoring nanothreads with tunable optical properties and an adjustable band gap, also exploiting the possibility of introducing substituents in the phenyl groups.

Mechanistic Insights into the Pressure-Induced Polymerization of Aryl/Perfluoroaryl Co-Crystals

Recently discovered diamond nanothreads offer a stiff, sp3-hybridized backbone unachievable in conventional polymer synthesis that is formed through the solid-state pressure-induced polymerization of simple aromatics. This method enables monomeric A-B...

Pressure-Induced Polymerization: Addition and Condensation Reactions

Under pressure of 1-100 GPa, unsaturated organic molecules tend to form covalent bond to each other for a negative enthalpy change, which often produces polymeric materials with extended carbon skeleton. The polymerization reactions typically happen in crystal, which promotes the topochemical process. This review summarized the topochemical polymerization processes of several alkynes, aromatics, and alkynylphenyl compounds, including the critical crystal structures before the reaction, bonding process, and the structure of the products. Secondly, this review also summarized the condensation reaction identified in the polymerization process, including the elimination of small molecules such as NH3, etc.

Synthesis of double core chromophore-functionalized nanothreads by compressing azobenzene in a diamond anvil cell

Carbon nanothreads are likely the most attracting new materials produced under high pressure conditions. Their synthesis is achieved by compressing crystals of different small aromatic molecules, while also exploiting the applied anisotropic stress to favor nontopochemical paths. The threads are nanometric hollow structures of saturated carbon atoms, reminiscent of the starting aromatic molecule, gathered in micron sized bundles. The examples collected so far suggest that their formation can be a general phenomenon, thus enabling the design of functionalities and properties by suitably choosing the starting monomer on the basis of its chemical properties and crystal arrangement. The presence of heteroatoms or unsaturation within the thread is appealing for improving the processability and tuning the electronic properties. Suitable simple chromophores can fulfill these requirements and their controlled insertion along the thread would represent a considerable step forward in tailoring the optical and electronic properties of these mechanically extraordinary materials. Here, we report the synthesis and extensive characterization of double core nanothreads linked by azo groups. This is achieved by compressing azobenzene in a diamond anvil cell, the archetype of a wide class of dyes, and represents a fundamental step in the realization of nanothreads with tailored photochemical and photophysical properties.

One-step high-pressure synthesis of 2D crystalline double nanothreads linked by azo groups.

Scalable Synthesis of Crystalline One-Dimensional Carbon Nanothreads through Modest-Pressure Polymerization of Furan

Carbon nanothreads, which are one-dimensional sp³-rich polymers, combine high tensile strength with flexibility owing to subnanometer widths and diamond-like cores. These extended carbon solids are constructed through pressure-induced polymerization of sp² molecules such as benzene. Whereas a few examples of carbon nanothreads have been reported, the need for high onset pressures (≥17 GPa) to synthesize them precludes scalability and limits scope. Herein, we report the scalable synthesis of carbon nanothreads based on molecular furan, which can be achieved through ambient temperature pressure-induced polymerization with an onset reaction pressure of only 10 GPa due to its lessened aromaticity relative to other molecular precursors. When slowly compressed to 15 GPa and gradually decompressed to 1.5 GPa, a sharp 6-fold diffraction pattern is observed in situ, indicating a well-ordered crystalline material formed from liquid furan. Single-crystal X-ray diffraction (XRD) of the reaction product exhibits three distinct d-spacings from 4.75 to 4.9 Å, whose size, angular spacing, and degree of anisotropy are consistent with our atomistic simulations for crystals of furan nanothreads. Further evidence for polymerization was obtained by powder XRD, Raman/IR spectroscopy, and mass spectrometry. Comparison of the IR spectra with computed vibrational modes provides provisional identification of spectral features characteristic of specific nanothread structures, namely syn, anti, and syn/anti configurations. Mass spectrometry suggests that molecular weights of at least 6 kDa are possible. Furan therefore presents a strategic entry toward scalable carbon nanothreads.

First-principles study of carbon nanothreads derived from five-membered heterocyclic rings: thiophene, furan and pyrrole

Structural, mechanical and electronic properties of carbon nanothreads derived from five-membered ring heterocyclic compounds are presented and discussed, demonstrating their enhanced stability and promising set of features.

Pressure-Induced Polymerization of Polycyclic Arene-Perfluoroarene Cocrystals: Single Crystal X-ray Diffraction Studies, Reaction Kinetics, and Design of Columnar Hydrofluorocarbons

Pressure-induced polymerization of aromatic compounds leads to novel materials containing sp³ carbon-bonded networks. The choice of the molecular species and the control of their arrangement in the crystal structures via intermolecular interactions, such as the arene-perfluoroarene interaction, can enable the design of target polymers. We have investigated the crystal structure compression and pressure-induced polymerization reaction kinetics of two polycyclic 1:1 arene-perfluoroarene cocrystals, naphthalene/octafluoronaphthalene (NOFN) and anthracene/octafluoronaphthalene (AOFN), up to 25 and 30 GPa, respectively, using single-crystal synchrotron X-ray diffraction, infrared spectroscopy, and theoretical computations based on density-functional theory. Our study shows the remarkable pressure stability of the parallel arene-perfluoroarene π-stacking arrangement and a reduction of the interplanar π-stacking separations by ca. 19-22% before the critical reaction distance is reached. A further strong, discontinuous, and irreversible reduction along the stacking direction at 20 GPa in NOFN (18.8%) and 25 GPa in AOFN (8.7%) indicates the pressure-induced breakdown of π-stacking by formation of σ-bonded polymers. The association of the structural distortion with the occurrence of a chemical reaction is confirmed by a high-pressure kinetic study using infrared spectroscopy, indicating one-dimensional polymer growth. Structural predictions for the fully polymerized high-pressure phases consisting of highly ordered rods of hydrofluorocarbons are presented based on theoretical computations, which are in excellent agreement with the experimentally determined unit-cell parameters. We show that the polymerization takes place along the arene-perfluoroarene π-stacking direction and that the lateral extension of the columns depends on the extension of the arene and perfluoroarene molecules.

Hierarchically Structured Allotropes of Phosphorus from Data-Driven Exploration

The discovery of materials is increasingly guided by quantum-mechanical crystal-structure prediction, but the structural complexity in bulk and nanoscale materials remains a bottleneck. Here we demonstrate how data-driven approaches can vastly accelerate the search for complex structures, combining a machine-learning (ML) model for the potential-energy surface with efficient, fragment-based searching. We use the characteristic building units observed in Hittorf's and fibrous phosphorus to seed stochastic ("random") structure searches over hundreds of thousands of runs. Our study identifies a family of hierarchically structured allotropes based on a P8 cage as principal building unit, including one-dimensional (1D) single and double helix structures, nanowires, and two-dimensional (2D) phosphorene allotropes with square-lattice and kagome topologies. These findings yield new insight into the intriguingly diverse structural chemistry of phosphorus, and they provide an example for how ML methods may, in the long run, be expected to accelerate the discovery of hierarchical nanostructures.

High density mechanical energy storage with carbon nanothread bundle

The excellent mechanical properties of carbon nanofibers bring promise for energy-related applications. Through in silico studies and continuum elasticity theory, here we show that the ultra-thin carbon nanothreads-based bundles exhibit a high mechanical energy storage density. Specifically, the gravimetric energy density is found to decrease with the number of filaments, with torsion and tension as the two dominant contributors. Due to the coupled stresses, the nanothread bundle experiences fracture before reaching the elastic limit of any individual deformation mode. Our results show that nanothread bundles have similar mechanical energy storage capacity compared to (10,10) carbon nanotube bundles, but possess their own advantages. For instance, the structure of the nanothread allows us to realize the full mechanical energy storage potential of its bundle structure through pure tension, with a gravimetric energy density of up to 1.76 MJ kg⁻¹, which makes them appealing alternative building blocks for energy storage devices.

Carbon nanothreads are promising for applications in mechanical energy storage and energy harvesting. Here the authors use large-scale molecular dynamics simulations and continuum elasticity theory to explore mechanical energy storage in carbon nanothreads-based bundles.

Thermal transport of carbon nanomaterials

The diversity of thermal transport properties in carbon nanomaterials enables them to be used in different thermal fields such as heat dissipation, thermal management, and thermoelectric conversion. In the past two decades, much effort has been devoted to study the thermal conductivities of different carbon nanomaterials. In this review, different theoretical methods and experimental techniques for investigating thermal transport in nanosystems are first summarized. Then, the thermal transport properties of various pure carbon nanomaterials including 1D carbon nanotubes, 2D graphene, 3D carbon foam, are reviewed in details and the associated underlying physical mechanisms are presented. Meanwhile, we discuss several important influences on the thermal conductivities of carbon nanomaterials, including size, structural defects, chemisorption and strain. Moreover, we introduce different nanostructuring pathways to manipulate the thermal conductivities of carbon-based nanocomposites and focus on the wave nature of phonons for controlling thermal transport. At last, we briefly review the potential applications of carbon nanomaterials in the fields of thermal devices and thermoelectric conversion.

Modulating the H-bond strength by varying the temperature for the high pressure synthesis of nitrogen rich carbon nanothreads

Carbon nanothreads are among the most attractive new materials produced under high pressure conditions. Their synthesis can be achieved by compressing the crystals of aromatic molecules exploiting both the anisotropic stress produced by the unidirectional applied force and that intrinsic to the crystal arrangement. We explored here the transformation of pyridine into a nitrogen rich carbon nanothread crystal by varying the pressure and temperature conditions with the twofold purpose of disclosing the microscopic mechanism of transformation and optimizing the yield and quality of the produced crystalline nanothreads. The best conditions for the synthesis were identified in the 14-18 GPa range at temperatures between 400 and 500 K with a product yield greater than 30%. The comparison of experiments performed under different P-T conditions allowed us to understand the role of high temperature, which is necessary to weaken or even destroy the complex H-bond network characterizing the pyridine crystal and preventing the correct approach of the aromatic rings for nanothread formation. X-ray diffraction data confirm the excellent 2D hexagonal packing of the nanothreads over several tens of microns, whereas the sharp absorption lines observed in the IR spectrum strongly support a substantial order along the threads. Diffraction results suggest a polytwistane structure of the threads derived from a Diels-Alder [4 + 2] polymerization involving molecules arranged in a slipped parallel configuration along the pyridine crystal a and b axes. Electron microscopy evidences an arrangement of the nanothreads in bundles of tens of nanometers.

Thermal transports of one-dimensional ultrathin carbon structures

Carbon atomic chain, linear benzene polymers, and carbon nanothreads are all one-dimensional (1D) ultrathin carbon structures. They possess excellent electronic and mechanical properties; however, their thermal transport properties have been rarely explored. Here, we systematically study their thermal conductance by combining the nonequilibrium Green's function and force field methods. The thermal conductance varies from 0.24 to 1.00 nW K^-1 at 300 K, and phonon transport in the linear benzene polymers and carbon nanothreads is strongly dependent on the connectivity styles between the benzene rings. We propose a simple 1D model, namely force-constant model, that explains the complicated transport processes in these structures. Our study not only reveals intrinsic mechanisms of phonon transport in these carbon structures, but also provides an effective method to analyze thermal properties of other 1D ultrathin structures made of only several atomic chains.

Evidence for Orientational Order in Nanothreads Derived from Thiophene

Nanothreads are one-dimensional sp³ hydrocarbons that pack within pseudohexagonal crystalline lattices. They are believed to lack long-range order along the thread axis and also lack interthread registry. Here we investigate the phase behavior of thiophene up to 35 GPa and establish a pressure-induced phase transition sequence that mirrors previous observations in low-temperature studies. Slow compression to 35 GPa results in the formation of a recoverable saturated product with a 2D monoclinic diffraction pattern along (0001) that agrees closely with atomistic simulations for single crystals of thiophene-derived nanothreads. Paradoxically, this lower-symmetry packing signals a higher degree of structural order since it must arise from constituents with a consistent azimuthal orientation about their shared axis. The simplicity of thiophene reaction pathways (with only four carbon atoms per ring) apparently yields the first nanothreads with orientational order, a striking outcome considering that a single point defect in a 1D system can disrupt long-range structural order.

From Molecules to Carbon Materials—High Pressure Induced Polymerization and Bonding Mechanisms of Unsaturated Compounds

With the development of high-pressure apparatus, in situ characterization methods and theoretical calculations, high-pressure technology becomes a more and more important method to synthesize new compounds with unusual structures and properties. By compressing compounds containing unsaturated carbon atoms, novel poly-ionic polymers, graphanes and carbon nanothreads were obtained. Their compositions and structures were carefully studied by combining multiple cutting-edge technologies, like the in situ high-pressure X-ray and neutron diffraction, transmission electron microscopy, pair distribution function, solid-state nuclear magnetic resonance and gas chromatography-mass spectroscopy. The reaction mechanisms were investigated based on the crystal structure at the reaction threshold pressure (the pressure just before the reaction taking place), the long-range and short-range structure of the product, molecular structure of the intermediates, as well as the theoretical calculation. In this review, we will summarize the synthesis of carbon materials by compressing the unsaturated compounds and its reaction characteristics under extreme conditions. The topochemical reaction mechanism and related characterization methods of the molecular system will be highlighted. This review will provide a reference for designing chemical reaction and exploring novel carbon materials under high-pressure condition.

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

Methane and other hydrocarbons are major components of the mantle regions of icy planets. Several recent computational studies have investigated the high-pressure behaviour of specific hydrocarbons. To develop a global picture of hydrocarbon stability, to identify relevant decomposition reactions, and probe eventual formation of diamond, a complete study of all hydrocarbons is needed. Using density functional theory calculations we survey here all known C-H crystal structures augmented by targeted crystal structure searches to build hydrocarbon phase diagrams in the ground state and at elevated temperatures. We find that an updated pressure-temperature phase diagram for methane is dominated at intermediate pressures by CH 4 :H 2 van der Waals inclusion compounds. We discuss the P-T phase diagram for CH and CH 2 (i.e., polystyrene and polyethylene) to illustrate that diamond formation conditions are strongly composition dependent. Finally, crystal structure searches uncover a new CH 4 (H 2 ) 2 van der Waals compound, the most hydrogen-rich hydrocarbon, stable between 170 and 220 GPa.

Decomposition and oligomerization of 2,3-naphthyridine under high-pressure and high-temperature conditions

The chemical reaction of 2,3-naphthyridine, a nitrogen-containing aromatic compound, was investigated at pressures ranging from 0.5 to 1.5 GPa and temperatures from 473 to 573 K. A distinct decrease in the amount of residual 2,3-naphthyridine was observed in the samples recovered after reaction at ˃523 K at 0.5 and 1.0 GPa, and ˃548 K at 1.5 GPa. The formation of o-xylene and o-tolunitrile accompanied a decreasing N/C ratio of the reaction products, indicating decomposition of the aromatic ring and release of nitrogen. Precise analysis of the reaction products indicated the oligomerization of decomposed products with the residual 2,3-naphthyridine to form larger molecules up to 7mers. Nitrogen in the aromatic ring accelerated reactions to decompose the molecule and to oligomerize at lower temperatures than those typically reported for aromatic hydrocarbon oligomerization. The major reaction mechanism was similar between 0.5 and 1.5 GPa, although larger products preferentially formed in the samples at higher pressure.

Local Structure and Bonding of Carbon Nanothreads Probed by High-Resolution Transmission Electron Microscopy

Carbon nanothreads are a new one-dimensional sp³-bonded nanomaterial of CH stoichiometry synthesized from benzene at high pressure and room temperature by slow solid-state polymerization. The resulting threads assume crystalline packing hundreds of micrometers across. We show high-resolution electron microscopy (HREM) images of hexagonal arrays of well-aligned thread columns that traverse the 80-100 nm thickness of the prepared sample. Diffuse scattering in electron diffraction reveals that nanothreads are packed with axial and/or azimuthal disregistry between them. Layer lines in diffraction from annealed nanothreads provide the first evidence of translational order along their length, indicating that this solid-state reaction proceeds with some regularity. HREM also reveals bends and defects in nanothread crystals that can contribute to the broadening of their diffraction spots, and electron energy-loss spectroscopy confirms them to be primarily sp³-hybridized, with less than 27% sp² carbon, most likely associated with partially saturated "degree-4" threads.

Topology-Driven Single-Molecule Conductance of Carbon Nanothreads

Highly conductive single-molecule junctions typically involve π-conjugated molecular bridges, whose frontier molecular orbital energy levels can be fine-tuned to best match the Fermi level of the leads. Fully saturated wires, e.g., alkanes, are typically thought of as insulating rather than highly conductive. However, in this work, we demonstrate in silico that significant zero-bias conductance can be achieved in such systems by means of topology. Specifically, caged saturated hydrocarbons offering multiple σ-conductance channels afford transmission far beyond what could be expected based upon conventional superposition laws, particularly if these pathways are composed entirely from quaternary carbon atoms. Computed conductance of molecular bridges based on carbon nanothreads, e.g., polytwistane, is not only of appreciable magnitude; it also shows a very slow decay with increasing nanogap, similarly to the case of π-conjugated wires. These findings offer a way to manipulate the transport properties of molecular systems by means of their topology, alternatively to the traditionally invoked electronic structure.

Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra

A one-dimensional (1D) sp³ carbon nanomaterial with high lateral packing order, known as carbon nanothreads, has recently been synthesized by slowly compressing and decompressing crystalline solid benzene at high pressure. The atomic structure of an individual nanothread has not yet been determined experimentally. We have calculated the ¹³C nuclear magnetic resonance (NMR) chemical shifts, chemical shielding tensors, and anisotropies of several axially ordered and disordered partially saturated and fully saturated nanothreads within density functional theory and systematically compared the results with experimental solid-state NMR data to assist in identifying the structures of the synthesized nanothreads. In the fully saturated threads, every carbon atom in each progenitor benzene molecule has bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called "degree-6" nanothread), while the partially saturated threads examined retain a single double bond per benzene ring ("degree-4"). The most-parsimonious theoretical fit to the experimental 1D solid-state NMR spectrum, constrained by the measured chemical shift anisotropies and key features of two-dimensional NMR spectra, suggests a certain combination of degree-4 and degree-6 nanothreads as plausible components of this 1D sp³ carbon nanomaterial, with intriguing hints of a [4 + 2] cycloaddition pathway toward nanothread formation from benzene columns in the progenitor molecular crystal, based on the presence of nanothreads IV-7, IV-8, and square polymer in the minimal fit.

Protective Carbon Overlayers from 2,3-Naphthalenediol Pyrolysis on Mesoporous SiO₂ and Al₂O₃ Analyzed by Solid-State NMR

Hydrothermally stable carbon overlayers can protect mesoporous oxides (SiO₂ and Al₂O₃) from hydrolysis during aqueous-phase catalysis. Overlayers made at 800 °C by pyrolysis of 2,3-naphthalenediol deposited out of acetone solution were analyzed by solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. Power absorption due to sample conductivity was prevented by diluting the sample in nonconductive and background-free tricalcium phosphate. While pyrolysis on SiO₂ produced a predominantly aromatic carbon film, at least 15% of nonaromatic carbon (sp³-hybridized C as well as C=O) was observed on γ-Al₂O₃. These species were not derived from residual solvent, according to spectra of the same material treated at 400 °C. The sp³-hybridized C exhibited weak couplings to hydrogen, short spin-lattice relaxation times, and unusually large shift anisotropies, which are characteristics of tetrahedral carbon with high concentrations of unpaired electrons. Moderate heat treatment at 400 °C on SiO₂ and Al₂O₃ resulted in yellow-brown and nearly black samples, respectively, but the darker color on Al₂O₃ did not correspond to more extensive carbonization. Aromatic carbon bonded to hydrogen remained predominant and the peaks of naphthalenediol were still recognizable; however, some of the chemical shifts differed by up to 5 ppm, indicating significant differences in local structure. On SiO₂, additional sharp peaks were detected and attributed to 1/3 of the 2,3-naphthalene molecules undergoing fast, nearly isotropic motions.

The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR

Carbon nanothreads are a new type of one-dimensional sp³-carbon nanomaterial formed by slow compression and decompression of benzene. We report characterization of the chemical structure of ¹³C-enriched nanothreads by advanced quantitative, selective, and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy. The width of the NMR spectral peaks suggests that the nanothread reaction products are much more organized than amorphous carbon. In addition, there is no evidence from NMR of a second phase such as amorphous mixed sp²/sp³-carbon. Spectral editing reveals that almost all carbon atoms are bonded to one hydrogen atom, unlike in amorphous carbon but as is expected for enumerated nanothread structures. Characterization of the local bonding structure confirms the presence of pure fully saturated "degree-6" carbon nanothreads previously deduced on the basis of crystal packing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise between 20% and 45% of the sample. Furthermore, ¹³C-¹³C spin exchange experiments indicate that the length of the fully saturated regions of the threads exceeds 2.5 nm. Two-dimensional ¹³C-¹³C NMR spectra showing bonding between chemically nonequivalent sites rule out enumerated single-site thread structures such as polytwistane or tube (3,0) but are consistent with multisite degree-6 nanothreads. Approximately a third of the carbon is in "degree-4" nanothreads with isolated double bonds. The presence of doubly unsaturated degree-2 benzene polymers can be ruled out on the basis of ¹³C-¹³C NMR with spin exchange rate constants tuned by rotational resonance and ¹H decoupling. A small fraction of the sample consists of aromatic rings within the threads that link sections with mostly saturated bonding. NMR provides the detailed bonding information necessary to refine solid-state organic synthesis techniques to produce pure degree-6 or degree-4 carbon nanothreads.

Kinetic theory for the formation of diamond nanothreads with desired configurations: a strain-temperature controlled phase diagram

Diamond nanothreads (DNTs) are a brand-new one-dimensional carbon nanomaterial that was synthesized recently by compressing benzene. Compared with sp2 carbon nanotubes, DNTs may possess a much higher interfacial load-transfer ability. However, previous studies have shown that the mechanical properties of DNTs are highly sensitive to the composition of Stone-Wales (SW) transformed sites. Up to now, it remained unclear what roles SWs play in the structure stability and how to engineer its molecular structure for novel mechanical properties. Using ab initio calculations, here we show that the most stable structure of a DNT is composed of alternative SW and hydrogenated carbon nanotube (3,0) units, suggesting that SW plays an essential role in stabilizing DNT. Interestingly, we found that the SW transition barrier is a nearly linear function of the applied strain, enabling strain engineering of its molecular structure. To do so, we propose a strain-temperature-stretching rate phase diagram to guide the construction of desired molecular structures to achieve superplastic behavior of DNTs. Our findings not only enrich our understanding of this novel carbon material, but also provide a strategy to control its structural and mechanical properties for novel applications, such as energy absorption, energy storage and materials reinforcement.

Mechanochemistry in [6]Cycloparaphenylene: A Combined Raman Spectroscopy and Density Functional Theory Study

Raman spectroscopy under high pressures up to 10 GPa and density functional computations up to 30 GPa are combined to obtain insights into the behavior of a prototypical nanohoop conjugated molecule, [6]cycloparaphenylene ([6]CPP). Upon increasing pressure, the nanohoop undergoes deformations, first reversible ovalization and then at even higher pressures aggregates are formed. This irreversible aggregation is caused by the formation of new intermolecular σ-bonds. Frequencies and derivatives of the Raman frequency shifts as a function of pressure are well reproduced by the computations. The frequency behavior is tied to changes in aromatic/quinonoid character of the nanohoop. The modeling at moderate high pressures reveals the deformation of the [6]CPP molecules into oval-like and peanut-like shapes. Surprisingly the pressure derivatives of the observed Raman mode shifts undergo a sudden change around a pressure value that is common to all Raman modes, indicating an underlying geometrical change extended over the whole molecule that is interpreted by the computational modeling. Simulations predict that under even larger deformations caused by higher pressures, oligomerization reactions would be triggered. Our simulations demonstrate that these transformations would occur regardless of the solvent, however pressures at which they happen are influenced by solvent molecules encapsulated in the interior of the [6]CPP.

Surprising Stability of Cubane under Extreme Pressure

The chemical stability of solid cubane under high-pressure was examined with in situ Raman spectroscopy and synchrotron powder X-ray diffraction (PXRD) in a diamond anvil cell (DAC) up to 60 GPa. The Raman modes associated with solid cubane were assigned by comparing experimental data with calculations based on density functional perturbation theory, and low-frequency lattice modes are reported for the first time. The equation of state of solid cubane derived from the PXRD measurements taken during compression gives a bulk modulus of 14.5(2) GPa. In contrast with previous work and chemical intuition, PXRD and Raman data indicate that solid cubane exhibits anomalously large stability under extreme pressure, despite its immensely strained 90° C-C-C bond angles.

Carbon Nitride Nanothread Crystals Derived from Pyridine

Carbon nanothreads are a new one-dimensional sp³ carbon nanomaterial. They assemble into hexagonal crystals in a room temperature, nontopochemical solid-state reaction induced by slow compression of benzene to 23 GPa. Here we show that pyridine also reacts under compression to form a well-ordered sp³ product: C₅NH₅ carbon nitride nanothreads. Solid pyridine has a different crystal structure from solid benzene, so the nontopochemical formation of low-dimensional crystalline solids by slow compression of small aromatics may be a general phenomenon that enables chemical design of properties. The nitrogen in the carbon nitride nanothreads may improve processability, alters photoluminescence, and is predicted to reduce the bandgap.

All the Ways To Have Substituted Nanothreads

We describe a general, symmetry-conditioned way of enumerating isomers of saturated singly substituted one-dimensional nanothreads of the (CH)₅E and (CH)₅CR type, where E is a heteroatom and R is a substituent. Four nanothreads - so-called tube (3,0), polytwistane, the zipper polymer, and polymer I, are treated in detail. The methodology, combining symmetry arguments and computer-based enumeration, is generally applicable to isomerism problems in polymers.

Chemistry through cocrystals: pressure-induced polymerization of C2H2·C6H6to an extended crystalline hydrocarbon

The 1 : 1 acetylene–benzene cocrystal, C₂H₂·C₆H₆, was synthesized under pressure in a diamond anvil cell (DAC) and its evolution under pressure was studied with single-crystal X-ray diffraction and Raman spectroscopy.

The structure and synthesis of organic crystalline polymers: hints from ab initio computation

The optical properties and structures of extended covalently bonded hydrocarbon polymers were studied by the DFT method and compared with experimental data.

Mechanochemical Synthesis of Carbon Nanothread Single Crystals

Synthesis of well-ordered reduced dimensional carbon solids with extended bonding remains a challenge. For example, few single-crystal organic monomers react under topochemical control to produce single-crystal extended solids. We report a mechanochemical synthesis in which slow compression at room temperature under uniaxial stress can convert polycrystalline or single-crystal benzene monomer into single-crystalline packings of carbon nanothreads, a one-dimensional sp³ carbon nanomaterial. The long-range order over hundreds of microns of these crystals allows them to readily exfoliate into fibers. The mechanochemical reaction produces macroscopic single crystals despite large dimensional changes caused by the formation of multiple strong, covalent C-C bonds to each monomer and a lack of reactant single-crystal order. Therefore, it appears not to follow a topochemical pathway, but rather one guided by uniaxial stress, to which the nanothreads consistently align. Slow-compression room-temperature synthesis may allow diverse molecular monomers to form single-crystalline packings of polymers, threads, and higher dimensional carbon networks.

High-Pressure Chemistry and the Mechanochemical Polymerization of [5]-Cyclo-p-phenylene

Evidence for the surprising formation of polymeric phases under high pressure for conjugated nanohoop molecules was found. This paper represents one of the unique cases, in which the molecular-level effects of pressure in crystalline organic solids is addressed, and provides a general approach based on vibrational Raman spectroscopy combining experiments and computations. In particular, we studied the structural and supramolecular chemistry of the cyclic conjugated nanohoop molecule [5]cyclo-para-phenylene ([5]CPP) under high pressures up to 10 GPa experimentally and up to 20 GPa computationally. The theoretical modeling for periodic crystals predicts good agreements with the experimentally obtained Raman spectra in the molecular phase. In addition, we have discovered two stable polymeric phases that arise in the simulation. The critical pressures in the simulation are too high, but the formation of polymeric phases at high pressures provides a natural explanation for the observed irreversibility of the Raman spectra upon pressure release between 6 and 7 GPa. The geometric parameters show a deformation toward quinonoid structures at high pressures accompanied by other deformations of the [5]CPP nanohoops. The quinonoidization of the benzene rings is linked to the systematic change of the bond length alternation as a function of the pressure, providing a qualitative interpretation of the observed spectral shifts of the molecular phase.

One-dimensional diamondoid polyaniline-like nanothreads from compressed crystal aniline

One-dimensional diamondoid polyaniline-like nanothreads combine the outstanding mechanical properties of carbon nanotubes with the versatility of NH₂ groups.

Low-dimensional nanomaterials such as highly ordered polyaniline (PANI) have attracted considerable interest due to their expected extraordinary electronic and optoelectronic properties. In spite of several attempts, the attainment of atomically well-ordered PANI is a long-standing challenge. Pressure-induced polymerization of aromatic molecules in the crystal phase has been demonstrated as a practicable route for the synthesis of highly ordered polymers but this approach has never been tested to produce PANI. Here we show the synthesis of diamondoid polyaniline-like nanothreads at 33 GPa and 550 K by compressing aniline in crystal phase-II. Infrared spectroscopy, transmission electron microscopy, X-ray diffraction data, and DFT calculations support the formation of this totally new polyaniline-like nanothread. The NH₂-enriched carbon nanothread combines the outstanding mechanical properties of carbon nanotubes with the versatility of NH₂ groups decorating the exterior of the nanothreads representing potential active sites for doping and as linkers for molecules with biological interest and inorganic nanostructures. The synergy of all of these properties emphasizes the strong potential of this material to be applied in a broad range of areas, from chemistry to materials engineering.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons at ambient and high pressures

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons (PAHs) was found at 500–773 K and ambient and high (3.5 GPa) pressures. The most intensive oligomerization at 1 bar and 3.5 GPa occurs at 740–823 K. PAH carbonization at high pressure is the final stage of oligomerization and occurs as a result of sequential oligomerization and polymerization of the starting material, caused by overlapping of π-orbitals, a decrease of intermolecular distances, and finally the dehydrogenation and polycondensation of benzene rings. Being important for building blocks of life, PAHs and their oligomers can be formed in the interior of the terrestrial planets with radii less than 2270 km.

Deuterium Isotope Effects in Polymerization of Benzene under Pressure

The enormous versatility in the properties of carbon materials depends on the content of the sp² and sp³ covalent bonds. Under compression, if intermolecular distances cross a critical threshold, then unsaturated hydrocarbons gradually transform to saturated carbon polymers. However, the mechanism of polymerization, even for benzene, the simplest aromatic hydrocarbon, is still not understood. We used high-pressure synchrotron X-ray, neutron diffraction, and micro-Raman spectroscopy together with density functional calculations to investigate the isotope effects in benzene isotopologues C₆H₆ and C₆D₆ up to 46.0 GPa. Raman spectra of polymeric products recovered from comparable pressures show the progression of polymerization exhibiting a pronounced kinetic isotope effect. Kinetically retarded reactions in C₆D₆ shed light on the mechanism of polymerization of benzene. We find that C₆D₆-derived products recovered from P < 35 GPa actively react with moisture, forming polymers with higher sp³ hydrogen contents. Significant isotopic shift (≥7 GPa) in persistence of Bragg reflections of C₆D₆ is observed.

The best features of diamond nanothread for nanofibre applications

Carbon fibres have attracted interest from both the scientific and engineering communities due to their outstanding physical properties. Here we report that recently synthesized ultrathin diamond nanothread not only possesses excellent torsional deformation capability, but also excellent interfacial load-transfer efficiency. Compared with (10,10) carbon nanotube bundles, the flattening of nanotubes is not observed in diamond nanothread bundles, which leads to a high-torsional elastic limit that is almost three times higher. Pull-out tests reveal that the diamond nanothread bundle has an interface transfer load of more than twice that of the carbon nanotube bundle, corresponding to an order of magnitude higher in terms of the interfacial shear strength. Such high load-transfer efficiency is attributed to the strong mechanical interlocking effect at the interface. These intriguing features suggest that diamond nanothread could be an excellent candidate for constructing next-generation carbon fibres.