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.