High Aspect Ratio Nanotubes Assembled from Macrocyclic Iminium Salts
One-dimensional nanostructures such as carbon nanotubes rely on strong and directional interactions that stabilize their high aspect ratio shapes from fracture. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>103), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that are two orders-of-magnitude stronger than the neutral macrocycles, as estimated 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 crosslinking their pendant alkenes. As the largest, and the first macrocyclic chromonic liquid crystals, macrocyclic 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.