High aspect ratio nanotubes assembled from macrocyclic iminium salts

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Nanotechnology

Background:

• One-dimensional nanostructures like carbon nanotubes require strong interactions for stability.
• Weak noncovalent interactions of molecular macrocycles limit the formation of organic nanotubes.
• Developing synthetic routes for high-aspect-ratio organic nanostructures remains a challenge.

Purpose of the Study:

• To engineer stable, high-aspect-ratio organic nanotubes from stacked macrocyclic units.
• To investigate the role of iminium ion formation in enhancing inter-macrocycle interactions.
• To explore stimuli-responsive assembly and fixation of organic nanotubes.

Main Methods:

• Protonation of imine-linked macrocycles to form iminium salts.
• Utilizing organic solvents like tetrahydrofuran for nanotube formation.
• Employing molecular dynamics simulations to understand interaction strengths.
• Investigating acid- and base-induced assembly and disassembly.
• Demonstrating stimuli-responsive assembly using photoacid generators and sonication.
• Permanent fixation via cross-linking of pendant alkenes.

Main Results:

• High aspect ratio (>10^3) lyotropic nanotubes of stacked macrocyclic iminium salts were successfully synthesized.
• Iminium ion formation significantly strengthened cohesive interactions (two orders of magnitude) compared to neutral macrocycles.
• Nanotube assembly stabilized inherently unstable iminium ions against hydrolysis.
• Assembly and disassembly were reversibly controlled by pH changes (acids and bases).
• Stimuli-responsive assembly was achieved using photoacid generators and sonication of chlorinated solvents.
• Permanent nanotube structures were formed by cross-linking pendant alkenes.

Conclusions:

• Macrocyclic iminium salts serve as effective building blocks for high-aspect-ratio organic nanotubes.
• Protonation-induced cohesive interactions provide a robust mechanism for nanotube stabilization.
• The developed system offers tunable, stimuli-responsive assembly and permanent fixation capabilities.
• These organic nanotubes present a rational design approach to mimic the morphology and rheology of carbon nanotubes and biological tubules.