Synthesis and Structure

Contents

Synthesis

Organic synthesis allows for the realization of millions of small molecules and polymers. However, thus far we know of only a handful of well-ordered extended carbon solids, molecules, and nanomaterials with strong covalent bonds, for example diamond, graphite, graphene, fullerenes, and nanotubes. These molecules and nanomaterials can have superlative electronic, mechanical, chemical, thermal, and separation properties that have garnered much attention. New examples with different carbon bonding configurations likely would as well; interest in them is great, as suggested by over 600 theoretical predictions for new carbon structures in the literature. Except in a few cases, extended molecular architectures are largely inaccessible through traditional organic synthesis because they are insoluble in common organic solvents. Generally, they are synthesized by high-temperature solid-state or gas-phase methods, or by exfoliation. Room-temperature solid-state reaction of small multiply unsaturated molecules has the potential to overcome this difficulty, as solubility is then not required. "Soft" room temperature reaction can in general allow for more versatile kinetic control of reaction products, rather than tending to produce the more thermodynamically stable reaction products favored at higher temperatures, such as diamond, graphite, and nanotubes.

Overcoming the requirement of topochemical polymerization to produce single crystal reaction products by solid-state reaction could open a new subfield of the synthesis of extended molecular architectures.

However, it is relatively rare to find organic solid-state reactions that produce single crystal or crystalline reaction products instead of amorphous reaction products lacking well defined structure.  If they are found, they are typically topochemical.  A well-known example  of a topochemical polymerization is the reaction of certain diacetylene molecules into polydiacetylene crystals.  These topochemical reactions typically require commensuration between the geometries of the reactant crystal and the product crystal.   Large changes in geometry and interatomic distances typically destroy single crystal order.  Thus it has typically been difficult to assemble small unsaturated organic molecules arranged in crystals into well-ordered reaction products because bond formation induces large changes in geometry as interatomic distances decrease. Overcoming the requirement for topochemical commensuration to produce well-ordered reaction products in polycrystalline or single crystal form could thus open a new subfield of study of these extended molecular architectures, for which a panoply of potentially interesting and exciting properties have been predicted.

Topochemical diacetylene polymerization
Topochemical diacetylene polymerization only proceeds with good commensuration in the geometry of the reactant and product.  If the R group is not such that the R-R distance is correct, then topochemical polymerization does not occur and single crystal or crystalline reaction products may not be produced.  This is a very restrictive requirement.
Benzene in particular exemplifies a molecule that should not polymerize topochemically; the six new, strong, covalent C–C bonds emanating from each monomer upon polymerization by compression or other means shrink intermolecular separations dramatically, and the products have historically been amorphous.
Paris Edinburgh press
Schematic of benzene polymerization in a Paris-Edinburg press to form nanothreads.  Benzene contained in a metal capsule (figure left) is first frozen into a polycrystalline mass at room temperature at 1-2 GPa (1 GPa =~10,000 bar) and subsequently compressed to 18 to 23 GPa, pressures at which it polymerizes into nanothreads.  The nanothreads are found to be consistently aligned with the stress that is a somewhat higher in the vertical direction than in the horizontal direction because of the applied force.  In other words, there is a uniaxial stress component in the vertical direction.  We hypothesize that this uniaxial stress selects benzene columns in the molecular crystal that are oriented vertically (see figure right) for reaction, thus forming a nanothread single crystal from a polycrystalline mass of benzene.

 

Benzene in particular exemplifies a molecule that should not polymerize topochemically; the six new, strong, covalent C–C bonds emanating from each monomer upon polymerization by compression or other means shrink intermolecular separations dramatically, and the products have historically been amorphous. Yet, through a non-topochemical mechanism that we are beginning to understand, slow compression of benzene over the course of hours in an opposed anvil device ( Nat. Mater. 14, 43 (2015)), even if it is initially a polycrystalline molecular solid, produces macroscopic single crystal packings of nanothreads hundreds of microns across ( JACS 139, 16343 (2017)). X-ray diffraction and high resolution transmission electron microscopy confirm their single crystal nature (see below; the robust nanothreads can sustain much higher electron doses before damage than most polymers). Benzene produces carbon nanothreads, while pyridine forms related carbon nitride nanothreads ( JACS 140, 4969 (2018)) with measured composition C5NH(an “atom economic reaction”). Radical and cycloaddition pathways towards their formation have been traced theoretically, which we will compare with experiment (J. Chem. Th. Comp. 14, 1131 (2018) and JACS 137, 14373 (2015)).

The sp3 bonding of carbon nanothreads, combined with their synthesis through organic solid-state chemistry from benzene arguably makes them ‘hybrids’ that collectively function as both hydrocarbon molecules and nanomaterials.

 

Scale-up

The arc synthesis method of Kratschmer and Huffman facilitated work on fullerenes by affording larger quantities with more accessible methods; it also led directly to the discovery of nanotubes through adaptation of the method. Reduction of the synthesis pressures of nanothreads to the ~5-6 GPa range currently used to synthesize ~106kg/year of industrial diamond would be similarly transformative. Thousands of presses in factories in China and across the world synthesize million kg quantities of diamond (see, for example, this supplier). Reaction pressures of 9 GPa are accessible with the synthetic equipment used for industrial diamonds and would still allow for synthesis of >100g at a time, so this will be our goal. Notably, reaction pressures observed to form amorphous networks from precursors such as benzene, thiophene, and furan scale with aromatic stability and are 21-23 GPa,16 GPa, and 10 GPa, respectively. Reaction pressures to form nanothread crystals by kinetically-controlled slow compression should scale similarly, and are confirmed experimentally to be lower for benzene (~20 GPa). In the longer term it may ultimately be possible to synthesize nanothreads near ambient pressure, just as diamond first synthesized at high pressure is now synthesized by chemical vapor deposition at near ambient pressures.

About 1,000,000 kg per year of diamond is produced by high pressure methods, a much larger quantity than is produced by chemical vapor deposition.

 

Structure

X-ray diffraction pattern of carbon Nanothreads
a, Polytwistane crystal representative of nanothread packing, viewed down the hexagonal c-axis (threads are parallel to it) with a 6.5 Å spacing and (100) planes 5.6 Å apart, and a side view down the b-axis. b,X-ray diffraction pattern collected at Penn State using a laboratory X-ray source with a 300μm CuKα1beam and image plate detector usually used for protein crystallography for a nanothread crystal synthesized from polycrystalline, multiphase benzene.  It shows a “signature” hexagonal pattern that matches c,that predicted for the c-axis of the polytwistane crystal. d, Diffraction after 90° rotation of the nanothread crystal, i.e. approximately along the b-axis, matches the pattern predicted in e.

 

The primary tools for characterizing nanothread structure thus far have been x-ray and neutron scattering, transmission electron microscopy (Nat. Mater. 14, 43 (2015)),  and NMR ( JACS 140, 7658 (2018)).  X-ray diffraction reveals a six-fold pattern that matched that predicted for a pseudohexagonal (in other words very close to hexagonal) crystal packing of nanothreads (see figure above a-c).  This diffraction pattern was collected with an x-ray beam 300 microns in diameter, so there is rotational order over this distance.  This order arises even though the original benzene monomer reactant was polycrystalline.  It appears that the uniaxial stress of the opposed anvil devices (diamond cells and Paris-Edinburgh cells) plays a central role in this ordering, as the crystal c-axis is consistently observed to be within 15 degrees or so of the stress axis.  

Striations observed for Nanothread TEM
Striations observed at an angle to the nanothread crystal c-axis.  The spacing between the striations is consistent with the proposed crystal model. TEM data collected by S. Juhl under the supervision of N. Alem.
Image of parallel nanothreads
Imaging of nanothreads from the side produces the striations observed in TEM images, such as the one above.

TEM also reveals striations and hexagonal symmetry in agreement with the proposed nanothread structure consisting of a hexagonal packing of threads about 6.5 Å in diameter, as shown above. Analyses of 1D and 2D quantitative and selective NMR spectra are also consistent with the proposed nanothread structure; in particular it shows that present nanothread samples are a single-phase mixture of pure "degree-6" nanothreads that are fully saturated and partially saturated "degree-4" nanothreads.  Degree-4 nanothreads are also of interest because their pi orbitals may allow for interesting electrical transport properties, potentially even superconductivity.  

NMR reveals that the fully saturated degree-6 regions in the nanothreads are a minimum of 2.5 nm long, if not longer.