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

Pressure-Induced Amidine Formation via Side-Chain Polymerization in a Charge-Transfer Cocrystal

Compression of small molecules can induce solid-state reactions that are difficult or impossible under conventional, solution-phase conditions. Of particular interest is the topochemical-like reaction of arenes to produce polymeric nanomaterials. However, high reaction onset pressures and poor selectivity remain significant challenges. Herein, the incorporation of electron-withdrawing and -donating groups into π-stacked arenes is proposed as a strategy to reduce reaction barriers to cycloaddition and onset pressures. Nevertheless, competing side-chain reactions between functional groups represent alternative viable pathways. For the case of a diaminobenzene:tetracyanobenzene cocrystal, amidine formation between amine and cyano groups occurs prior to cycloaddition with an onset pressure near 9 GPa, as determined using vibrational spectroscopy, X-ray diffraction, and first-principles calculations. This work demonstrates that reduced-barrier cycloaddition reactions are theoretically possible via strategic functionalization; however, the incorporation of pendant groups may enable alternative reaction pathways. Controlled reactions between pendant groups represent an additional strategy for producing unique polymeric nanomaterials.

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.

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.

Pressure induced modification of the electronic properties of stilbene by two-photon spectroscopy

Carbon nanothreads are the most exciting carbon based nanomaterials recently discovered. Obtained by compressing aromatics around 20 GPa, they are characterized by potentially exceptional mechanical properties. The reaction mechanisms have been partly elucidated through computational studies and x-ray diffraction experiments. However, in all these studies, the electronic modifications to which the molecule is subjected with increasing pressure are neglected as also if, and to which extent, the electronic excited states are involved in the high-pressure reactivity. In fact, the pressure increase induces remarkable changes in the electronic properties of molecular crystals, which are often directly related to the reaction's onset and path. We report the pressure evolution of the two-photon induced emission spectrum of crystalline stilbene, the archetype of a class of molecules from which double-core nanothreads are obtained, with the twofold purpose of gaining insight into the reaction mechanism and monitoring if the structural changes observed in x-ray diffraction studies have a detectable counterpart in the electronic properties of the system. The freezing of the spectral diffusion observed on rising pressure is ascribed to a hampered conformational rearrangement because of the larger stiffness of the local environment. The transition to the high pressure phase where the nanothreads form is revealed by the slope change of the pressure shift of all spectral components, while the progressive intensification with pressure of the 0-0 transition suggests a strengthening of the ethylenic bond favoring the charge delocalization on the benzene moieties, which is likely the trigger of the chemical instability.

From Biomass to Functional Crystalline Diamond Nanothread: Pressure-Induced Polymerization of 2,5-Furandicarboxylic Acid

2,5-Furandicarboxylic acid (FDCA) is one of the top-12 value-added chemicals from sugar. Besides the wide application in chemical industry, here we found that solid FDCA polymerized to form an atomic-scale ordered sp³-carbon nanothread (CNTh) upon compression. With the help of perfectly aligned π-π stacked molecules and strong intermolecular hydrogen bonds, crystalline poly-FDCA CNTh with uniform syn-configuration was obtained above 11 GPa, with the crystal structure determined by Rietveld refinement of the X-ray diffraction (XRD). The in situ XRD and theoretical simulation results show that the FDCA experienced continuous [4 + 2] Diels-Alder reactions along the stacking direction at the threshold C···C distance of ∼2.8 Å. Benefiting from the abundant carbonyl groups, the poly-FDCA shows a high specific capacity of 375 mAh g^-1 as an anode material of a lithium battery with excellent Coulombic efficiency and rate performance. This is the first time a three-dimensional crystalline CNTh is obtained, and we demonstrated it is the hydrogen bonds that lead to the formation of the crystalline material with a unique configuration. It also provides a new method to move biomass compounds toward advanced functional carbon materials.

Photochemically Mediated Polymerization of Molecular Furan and Pyridine: Synthesis of Nanothreads at Reduced Pressures

Nanothreads are emerging one-dimensional sp³-hybridized materials with high predicted tensile strength and a tunable band gap. They can be synthesized by compressing aromatic or nonaromatic small molecules to pressures ranging from 15-30 GPa. Recently, new avenues are being sought that reduce the pressure required to afford nanothreads; the focus has been placed on the polymerization of molecules with reduced aromaticity, favorable stacking, and/or the use of higher reaction temperatures. Herein, we report the photochemically mediated polymerization of pyridine and furan aromatic precursors, which achieves nanothread formation at reduced pressures. In the case of pyridine, it was found that a combination of slow compression/decompression with broadband UV light exposure yielded a crystalline product featuring a six-fold diffraction pattern with similar interplanar spacings to previously synthesized pyridine-derived nanothreads at a reduced pressure. When furan is compressed to 8 GPa and exposed to broadband UV light, a crystalline solid is recovered that similarly demonstrates X-ray diffraction with an interplanar spacing akin to that of the high-pressure synthesized furan-derived nanothreads. Our method realizes a 1.9-fold reduction in the maximum pressure required to afford furan-derived nanothreads and a 1.4-fold reduction in pressure required for pyridine-derived nanothreads. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions. The reduction of the onset pressure is caused by an initial [4+4] cycloaddition followed by increasingly facile thermal [4+2]-cycloadditions during polymerization.

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.

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.

Covalently Linked Porphyrins as One-Dimensional Conductors

We apply first-principles calculations to address the problem of the formation and characterization of covalently linked porphyrin-like structures. We show that upon pressure a rehybridization process takes place which leads to one-dimensional compounds resembling nanothreads, in which carbon atoms are all 4-fold coordinated. We also show that the resulting nanostructures have metallic character and possess remarkable mechanical properties. Moreover, in the case of porphyrin-metal complexes, we find that the covalently linked structures may be a platform for the stabilization of straight metallic wires.

Vibration Control of Diamond Nanothreads by Lattice Defect Introduction for Application in Nanomechanical Sensors

Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene sheets (GSs), have been adopted as resonators in vibration-based nanomechanical sensors because of their extremely high stiffness and small size. Diamond nanothreads (DNTs) are a new class of one-dimensional carbon nanomaterials with extraordinary physical and chemical properties. Their structures are similar to that of diamond in that they possess sp3-bonds formed by a covalent interaction between multiple benzene molecules. In this study, we focus on investigating the mechanical properties and vibration behaviors of DNTs with and without lattice defects and examine the influence of density and configuration of lattice defects on the two them in detail, using the molecular dynamics method and a continuum mechanics approach. We find that Young’s modulus and the natural frequency can be controlled by alternating the density of the lattice defects. Furthermore, we investigate and explore the use of DNTs as resonators in nanosensors. It is shown that applying an additional extremely small mass or strain to all types of DNTs significantly changes their resonance frequencies. The results show that, similar to CNTs and GSs, DNTs have potential application as resonators in nano-mass and nano-strain sensors. In particular, the vibration behaviors of DNT resonators can be controlled by alternating the density of the lattice defects to achieve the best sensitivities.

Scalable High-Pressure Synthesis of sp2-sp3 Carbon Nanoribbon via [4 + 2] Polymerization of 1,3,5-Triethynylbenzene

Pressure-induced polymerization of aromatics is an effective method to construct extended carbon materials, including the diamond-like nanothread and graphitic structures, but the reaction pressure of phenyl is typically around 20 GPa and too high to be applied for large-scale preparation. Here by introducing ethynyl to phenyl, we obtained a sp²-sp³ carbon nanoribbon structure by compressing 1,3,5-triethynylbenzene (TEB), and the reaction pressure of phenyl was successfully decreased to 4 GPa, which is the lowest reaction pressure of aromatics at room temperature. Using experimental and theoretical methods, we figured out that the ethynylphenyl of TEB undergoes [4 + 2] dehydro-Diels-Alder (DDA) reaction with phenyl upon compression at an intermolecular C···C distance above 3.3 Å, which is much longer than those of benzene and acetylene. Our research suggested that the DDA reaction between ethynylphenyl and phenyl is a promising route to decrease the reaction pressure of aromatics, which allows the scalable high-pressure synthesis of nanoribbon materials.

Strain-Tunable Carbon Nanothread-Derived Membranes for Water Desalination

Carbon nanothread-derived nanomeshes are highly flexible two-dimensional (2D) structures with tunable pore size and shape, which allows fine control of their transport properties when applied as membranes. In this work, we use molecular dynamics simulations to investigate the performance of several nanomesh structures as membranes for water desalination through reverse osmosis. Results show that these membranes can operate in a wide range of water flow rate, with an optimal point that yields 100% NaCl rejection and water permeability as high as 106 L·cm^-2·day^-1·MPa^-1, higher than other nanoporous 2D materials reported in the literature. This promising performance is partially due to the elliptical pores of strained nanomeshes, which allow the passage of rotated water molecules while rejecting hydrated salt ions. Our results show that carbon nanothread-derived nanomeshes have great potential for application in water desalination processes and emphasize the importance of engineering pore shape in 2D materials when applied as reverse osmosis membranes.

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR

The molecular structure of nanothreads produced by the slow compression of 13C4-furan was studied by advanced solid-state NMR. Spectral editing showed that >95% of carbon atoms were bonded to one hydrogen (C-H) and that there were 2-4% CH2, 0.6% C═O, and <0.3% CH3 groups. Alkenes accounted for 18% of the CH moieties, while trapped, unreacted furan made up 7%. Two-dimensional (2D) 13C-13C and 1H-13C NMR identified 12% of all carbon in asymmetric O-CH═CH-CH-CH- and 24% in symmetric O-CH-CH═CH-CH- rings. While the former represented defects or chain ends, some of the latter appeared to form repeating thread segments. Around 10% of carbon atoms were found in highly ordered, fully saturated nanothread segments. Unusually slow 13C spin-exchange with sites outside the perfect thread segments documented a length of at least 14 bonds; the small width of the perfect-thread signals also implied a fairly long, regular structure. Carbons in the perfect threads underwent relatively slow spin-lattice relaxation, indicating slow spin exchange with other threads and smaller amplitude motions. Through partial inversion recovery, the signals of the perfect threads were observed and analyzed selectively. Previously considered syn-threads with four different C-H bond orientations were ruled out by centerband-only detection of exchange NMR, which was, on the contrary, consistent with anti-threads. The observed 13C chemical shifts were matched well by quantum-chemical calculations for anti-threads but not for more complex structures like syn/anti-threads. These observations represent the first direct determination of the atomic-level structure of fully saturated nanothreads.

Carbon nanothreads enable remarkable enhancement in the thermal conductivity of polyethylene

Polymer nanocomposites with high thermal conductivity have been increasingly sought after in the electronic industry. Based on molecular dynamics simulations, this work assesses the thermal transport in polyethylene (PE) nanocomposites with the presence of a new one-dimensional nanofiller-a carbon nanothread (NTH). It is found that the axial thermal conductivity of PE nanocomposites increases linearly with the content of regularly aligned NTH fillers, while the aggregated pattern suppresses the enhancement effect. This phenomenon is explained by a stronger filler-filler interaction that reduces the intrinsic thermal conductivity of the NTH. Results show that the randomly dispersed NTHs can hardly promote heat transfer because effective heat transfer channels are lacking. Strikingly, surface functionalization has an adverse effect on the thermal conductivity due to the presence of additional voids. The presence of voids answers a long-standing open question that functionalization of the heat conductive filler only slightly improves the thermal conductivity of the polymer composite. Additionally, the transverse thermal conductivity degrades in the presence of the NTH and exhibits no clear correlation with the filler content or the distribution pattern. Overall, this study provides an in-depth understanding of the heat transfer within the polymer nanocomposites, which opens up possibilities for the preparation of highly conductive polymers.

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.

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.

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.

Nanoarchitecture through Strained Molecules: Cubane-Derived Scaffolds and the Smallest Carbon Nanothreads

Relative to the rich library of small-molecule organics, few examples of ordered extended (i.e., nonmolecular) hydrocarbon networks are known. In particular, sp³ bonded, diamond-like materials represent appealing targets because of their desirable mechanical, thermal, and optical properties. While many covalent organic frameworks (COFs)-extended, covalently bonded, and porous structures-have been realized through molecular architecture with exceptional control, the design and synthesis of dense, covalent extended solids has been a longstanding challenge. Here we report the preparation of a sp³-bonded, low-dimensional hydrocarbon synthesized via high-pressure, solid-state diradical polymerization of cubane (C₈H₈), which is a saturated, but immensely strained, cage-like molecule. Experimental measurements show that the obtained product is crystalline with three-dimensional order that appears to largely preserve the basic structural topology of the cubane molecular precursor and exhibits high hardness (comparable to fused quartz) and thermal stability up to 300 °C. Among the plausible theoretical candidate structures, one-dimensional carbon scaffolds comprising six- and four-membered rings that pack within a pseudosquare lattice provide the best agreement with experimental data. These diamond-like molecular rods with extraordinarily small thickness are among the smallest members in the carbon nanothread family, and calculations indicate one of the stiffest one-dimensional systems known. These results present opportunities for the synthesis of purely sp³-bonded extended solids formed through the strain release of saturated molecules, as opposed to only unsaturated precursors.

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.

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.

Structure and Reactivity of the Ionic Liquid 1-Allyl-3-methylimidazolium Iodide under High Pressure

Poly(ionic liquid)s are an interesting class of compounds because of their unique chemical and physical properties gathering the characteristics of ionic liquids and polymers. Pressure and temperature have been demonstrated to be alternative parameters to obtain polymers from monomeric species using only physical tools. In this work, we investigate the reaction under high pressure and room temperature of the ionic liquid 1-allyl-3-methylimidazolium iodide by using the diamond anvil cell technique in combination with synchrotron X-ray diffraction and electronic and vibrational spectroscopies. The results indicate a chemical reaction happening through the terminal double bond of the allyl group both in crystalline and glassy phases with the onset of the reaction around ∼7 GPa. Vibrational spectra present evidence for an oligomerization reaction in both the phases. The reaction occurring both in glassy and crystal phases indicates a mechanism not driven by collective motions and likely related to local topological arrangements. The results presented herein extend our understanding of ionic liquid instability boundaries under high pressure and contribute to the development of alternative synthetic routes to achieve poly(ionic liquids).

Crystal morphology fixed by interplay of π-stacking and hydrogen bonds – the case of 1-hydroxypyrene

Crystal structure of 1-hydroxypyrene has been determined and its luminescence in the solid state described. An interplay of π-stacking and H-bonds results in a conserved morphology and great flexibility of the crystals. This crystal structure can be described as a set of ‘molecular springs’.

The role of H-bond in the high-pressure chemistry of model molecules

Pressure is an extraordinary tool to modify direction and strength of intermolecular interactions with important consequences on the chemical stability of molecular materials. The decrease of the distance among nearest neighbour molecules can give rise to reactive configurations reflecting the crystal arrangement and leading to association processes. In this context, the role of the H-bonds is very peculiar because their usual strengthening with rising pressure does not necessarily configure a decrease of the reaction activation energy but, on the contrary, can give rise to an anomalous stability of the system. In spite of this central role, the mechanisms by which a chemical reaction is favoured or prevented by H-bonding under high pressure conditions is a poorly explored field. Here we review a few studies where the chemical behaviour of simple molecular systems under static compression was related to the H-bonding evolution with pressure. These results are able to clarify a wealth of changes of the chemical and physical properties caused by the strengthening with pressure of the H-bonding network and provide additional tools to understand the mechanisms of high-pressure reactivity, a mandatory step to make these synthetic methods of potential interest for applicative purposes.