Self-Assembled Nanotubes in Organic Solvents

The concept of strongly interacting groups in self-assembly of soft matter

Amphiphilic molecules in solution typically produce structures coming from cooperative interactions of many synergetically acting functional units. If all essential interactions are weak, the structure can be treated theoretically based on a free energy expansion for small interaction parameters. However, most self-assembling soft matter systems involve strong specific interactions of functional units leading to qualitatively new structures of highly soluble micellar or fibrillar aggregates. Here we focus on the systems with the so-called strongly interacting groups (SIGs) incorporated into unimer molecules and discuss the effects of packing frustrations and unimer chirality as well as the origins of spontaneous morphological chirality in the case of achiral unimers. We describe several theoretical approaches (overcoming the limitations of weak interaction models) including the concepts of super-strong segregation, geometrical mismatch and orientational frustration. We also review some recently developed phenomenological theories of surfactant membranes and multiscale hierarchical approaches based on all-atomic modeling of packing structures of amphiphilic molecules with SIGs. In particular, we discuss self-assembling structures in systems possessing simultaneously several distinct types of SIGs: solutions of beta-sheet oligopeptides (showing different fibrillar morphologies), aromatic diamide-ester molecules (forming membranes, helical ribbons and tubules), and triarylamine amide derivatives (producing light-controlled supramolecular nanowires).

Selectivity of guest–host interactions in self-assembled hydrogen-bonded nanostructures observed by NMR

We studied the incorporation of various small guest molecules into calix[4]hydroquinone nanotubes and nanoclusters using solid-state proton NMR spectroscopy in combination with quantum chemical calculations. While the molecules exhibit different types of hydrogen bonding and van der Waals interactions, they show different affinities to the nanotube host structures. As the guest molecules are located inside the complexes, they experience a shift in the NMR resonance line caused by screening effects from the aromatic electrons of the host superstructure. The abilities to fill the otherwise empty space within the tubes can hence be measured indirectly by the displacement of the NMR lines relative to the free molecules. In this way, we can probe which guest molecules are recognized by the calix[4]hydroquinones as suitable for filling their nanoporous superstructures. Selective guest-host interactions have been successfully achieved for the three component mixture of water and acetone with either 2-methyl-2-propanol or 2-propanol. In both cases, the alcohols were superior to acetone in filling the CHQ tubes.