High ratio nanotubes assembled from macrocyclic iminium salts

Tinuku - The distinct properties of one-dimensional nanomaterials, such as carbon nanotubes and actin filaments, emerge from their high aspect ratios. Stacking macrocycles into nanotubes represents a potentially powerful design strategy to access synthetic nanotubes, but such noncovalent assemblies are typically too weak to support individual, high aspect ratio structures.

Researchers report hexagonal macrocycles that exhibit strong electrostatic stacking upon protonation of their imine linkages. The resulting nanotubes form lyotropic phases and align in flow, a hallmark of high aspect ratio structures. The iminium-linked macrocycle assemblies form and disassemble reversibly in the presence of bases and acids, and are stabilized by covalent cross-linking.

Tinuku High ratio nanotubes assembled from macrocyclic iminium salts

“We report high aspect ratio (>103), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles,” said Chao Sun of the Cornell University at Ithaca and colleagues reported to Proceedings of the National Academy of Sciences.

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak.

Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations.

Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes.

As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.