Tuning the solubility of hepta(p-benzamide)s via the monomer sequence

The automated synthesis of hepta(p-benzamide) heterosequences on solid support using a modified peptide synthesizer is reported. The oligomers are synthesized from 4-aminobenzoic acid and 4-amino-2-(hexyloxy)benzoic acid, the latter carrying a solubilizing hexyl side chain. It is known from previous studies that both the unsubstituted hepta(p-benzamide) and the fully hexyloxy-substituted hepta(p-benzamide) are insoluble in all common organic solvents. Heterosequences in which both types of monomers alternate are, however, soluble in polar organic solvents such as DMSO. The heterosequence heptamers behave as strong organogelators when DMSO solutions are left at room temperature for several hours. Transmission electron microscopic (TEM) investigations revealed that the gelation was due to the oligomers forming long entangled fibers via a non-covalent aggregation mechanism. We explain these phenomena by a heterosequence triggered switch of aggregation mechanism. The unsubstituted oligomers strongly aggregate via a directional hydrogen-bond driven mechanism which changes to a less directional π-interaction driven aggregation mechanism for the substituted oligomers.We hereby demonstrate that designed heterosequences in non-natural oligoamides can lead to materials with distinctly different conformations which directly affect the intermolecular interactions and their supramolecular organization.