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

Rational Approaches toward the Design and Synthesis of Carbon Nanothreads

ConspectusCarbon-based materials─often with superlative electronic, mechanical, chemical, and thermal properties─are often categorized by dimensionality and hybridization. Most of these categories are produced in high-temperature conditions that afford equilibrium-dictated structures, but limit their diversity. In contrast, an emerging class of one-dimensional (1D) carbon materials, coined nanothreads, are accessible through kinetically controlled solid-state reactions of small multiply unsaturated molecules. While abundant in molecular organic synthesis, exerting kinetic control over reactivity is a revolutionary approach to access dense carbon networks. Owing to their internal diamond-like core, these materials are calculated to span a wide range of mechanical and optical properties, with the introduction of functional groups and/or heteroatoms leading to tailorable band gaps and the potential to access electronic states that are not featured in traditional polymers or nanomaterials. Accessing these properties requires the ability to precisely control solid-state molecular reaction pathways, chemical connectivity, and heteroatom/functional group density. Carbon nanothreads are often synthesized through the pressure-induced polymerization of aromatic molecules (e.g., benzene, pyridine, and thiophene) upon compression to 23-40 GPa. While the high pressures required to achieve these crystalline materials often preclude making synthetically viable quantities of product, the use of lessened aromatic reactants, along with light and/or heat, enables more mild reaction pressures. Success to date in forming nanothreads from diverse reactants suggests that physical organic principles govern the reaction, along with topochemical relationships, enabling the emergence of a new field of carbon chemistry that combines the control of organic chemistry with the range of physical properties only possible in extended periodic solids.In this Account, we describe our efforts to rationally synthesize carbon nanothreads with desired structures and present our approaches to dictate reactivity in the organic solid state that enable the formation of crystalline 1D carbon materials. In particular, we focus on the principles being pursued by our group to expand the chemical diversity of materials being accessed, while highlighting efforts that both enhance selectivity over reaction pathways and reduce pressure requirements for polymerization. We begin by leveraging starting materials with lessened or no aromaticity (relative to benzene) to design new backbones while enabling lower pressures such as 15-20 GPa to achieve nanothread formation. Next, we discuss efforts to utilize photochemical activation as a means to dictate the reaction pathway and/or affect the mechanism while also achieving ordered crystalline solids at reduced pressures. Lastly, we highlight efforts to demonstrate kinetic control in solid-state reactions by leveraging supramolecular chemistry (e.g., aryl/perfluoroaryl interactions, hydrogen bonds, π-π stacking) to preorganize starting materials into polymerizable molecular stacks. The resultant design principles provide multiple opportunities to attain previously inaccessible sp3-rich 1D polymeric carbon nanomaterials with unique structures and properties from widely available small molecules. Moreover, the kinetic control provided in the organic solid state enables a priori functionalization and the design of a rich diversity of materials with emergent properties in stark contrast to many well-developed carbon materials.

Double-core nanothread formation from α-furil via a pressure-induced planarization pathway

The packing and geometry of compressed small molecule precursors largely dictate the kinetically controlled formation processes of carbon nanothread materials. Structural ordering and chemical homogeneity of nanothread products may deteriorate through competing reaction pathways, and molecular phase transitions can disrupt precursor stacking geometries. Here, we report the formation of well-ordered, double-core nanothreads from compressed α-furil via a unique polymorphic transition pathway that serves to facilitate a pressure-induced reaction. At ∼1.6 GPa, α-furil transforms to the photoactive trans-planar conformation, which was previously theorized but not observed. Crystalline packing of the trans-planar structure provides closely overlapping molecular stacks that result in topochemical-like Diels-Alder cycloaddition reactions between furan rings upon further compression. The controlled reaction pathways on both sides of the molecule produce two linked "cores" of chemically homogenous nanothreads, and successive nucleophilic addition reactions crosslink a large fraction of the diketone bridges between monomers.

Insights on the Bonding Mechanism, Electronic and Optical Properties of Diamond Nanothread-Polymer and Cement-Boron Nitride Nanotube Composites

The success of composite materials is attributed to the nature of bonding at the nanoscale and the resulting structure-related properties. This study reports on the interaction, electronic, and optical properties of diamond nanothread/polymers (cellulose and epoxy) and boron nitride nanotube/calcium silicate hydrate composites using density functional theory modeling. Our findings indicate that the interaction between the nanothread and polymer is due to van der Waals-type bonding. Minor modifications in the electronic structures and absorption spectra are noticed. Conversely, the boron nitride nanotube-calcium silicate hydrate composite displays an electron-shared type of interaction. The electronic structure and optical absorption spectra of the diamond nanothread and boron nitride nanotube in all configurations studied in the aforementioned composite systems are well maintained. Our findings offer an electronic-level perspective into the bonding characteristics and electronic-optical properties of diamond nanothread/polymer and boron nitride nanotube/calcium silicate hydrate composites for developing next-generation materials.

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.