Divergent Nanotube Synthesis through Reversible Macrocycle Assembly

ConspectusNanotubes offer a unique combination of structural precision, tunable interior environments, and high aspect ratios that will be useful for many applications. Despite these desirable attributes, widespread explorations into the properties and applications of chemically designed nanotubes have been limited by challenges related to their synthesis. This realization has motivated developing a unified synthetic nanotube design, which would enable wide-reaching explorations into one-dimensional molecular architectures. In principle, supramolecular polymerization is a viable method to prepare such systems, but historically, this approach has yielded materials with poor mechanical properties and/or low aspect ratios whose chemical diversity is limited. This Account describes the development of an acid-mediated approach to macrocycle assembly that overcomes these limitations to yield robust, yet reversible, high-aspect-ratio nanotubes. Imine-linked macrocycles are prepared in high yield from readily accessible precursors by coupling dynamic imine exchange to an out-of-equilibrium macrocycle stacking event. Upon protonation, these macrocycles assemble into high-aspect-ratio nanotubes through electrostatic, solvophobic, and π-πinteractions. The interplay between covalent and noncovalent processes are critical to guide macrocycle synthesis and assembly. Including basic pyridine groups into the macrocycle backbone leads to cooperative assembly, even in the presence of <1 equiv of acid per macrocycle. This design was elaborated to enable a general one-pot nanotube synthesis from many functional aromatic dialdehydes. The development of structure-property relationships for nanotube assembly strength and ion conductivity are made possible because protonation-induced macrocycle assembly is modular and robust. For instance, supramolecular interactions endow synthetic nanotubes with robust cohesion and mechanical properties that surpass many covalent linear polymers. Tailoring the nanotube interior using site-selective chemical functionalization results in ion-conducting materials. Pyridinium-based nanotubes universally exhibit the ability to conduct protons and nanotubes functionalized with interior glycol groups promote efficient Li-ion transport. Overall, this versatile class of one-dimensional nanostructures shows substantial promise to merge the desirable properties of carbon nanotubes and biological filaments, all while being synthetically tailorable for many designed applications.