Self-assembled nanotubes and nanoribbons of aromatic diamide-esters

Supramolecular nanostructures provide excellent opportunities to enhance our understanding of the basic mechanism of self-organization and hence to deal with the fundamental issues of nanoscale construction e.g. desired morphology and well defined size. The ability to manipulate the ordered structures at molecular scale, affected by competing intermolecular or molecule-environment interactions, provide further insight for a more fundamental understanding of ordering and growth phenomena. In particular, self-assembly of organic molecules into highly ordered nanostructures is of significant interest due to their potential applications as functional soft materials.

In this context, we investigated the ordering and growth phenomena of achiral aromatic diamide molecules, which involve three distinct strongly interacting groups (SIGs) (one aromatic-ester ring and two amide groups per molecule). We demonstrated that solutions of 3,5-bis-(5-hexylcarbamoylpentyloxy)-benzoic acid decyl ester (BHPB-10), can form metastable nanostructures on solid substrates and in bulk. Specific solvents affect interactions between particular SIGs, thus promoting various nano-structures: lamellae, nanoribbons, helical ribbons, or nanotubes.

In cyclohexane, a solvent allowing for both inter-amide hydrogen bonds and mutual attraction of rings, formation of nanotubes with a diameter of 28 ± 5 nm was observed in the bulk and on the surfaces. By contrast, in cyclohexanone, which suppresses inter-amide hydrogen bonds, flat nanoribbons with specific width of 12 ± 4 nm were formed on solid substrates after drying.

By annealing in cyclohexane vapour, we followed the process of surface structures switching from nanoribbons to nanotubes and observed helical ribbons as the precursor of nanotubes. We also turned nanotubes back into nanoribbons by adding cyclohexanone, thus demonstrating reversible switching along the route: tubes → lamellae → at ribbons → helical ribbons → tubes. Additionally, we employed the pathway explored via studying BHPB-10 to another molecule of the same family (BHPB-14), and demonstrated switching from 2D flat nanoribbons to 3D helical nanoribbons.
Our understandings for the self-assembly in the achiral molecules provide insight in the influence of complementary inter-molecular specific SIG-based interactions and demonstrate an effective pathway for tailoring the shape and size of nanostructures derived from the same building unit.