Controlled self-assembly of asymmetric dumbbell-shaped rod amphiphiles: Transition from toroids to planar nets

We report here the synthesis and self-assembly of a series of asymmetric, dumbbell-shaped rod amphiphiles containing hydrophilic oligoether dendrons at one end and hydrophobic alkyl dendrons at the other end of the stiff rod segment with systematic variation in the length of the hydrophobic alkyl chain ranging from ethyl to tetradecyl groups. These rod amphiphiles were observed to self-assemble, in aqueous solution, into nanoscopic supramolecular aggregates that change significantly on variation in the length of hydrophobic alkyl chain, as characterized by cryo-transmission electron microscopy (cryo-TEM) experiments. The rod amphiphile based on the ethyl group (f(alkyl/EO) = 0.45) exhibits a spherical micellar structure, while the amphiphile with the hexyl group (f(alkyl/EO) = 0.71) shows a toroidal structure. Further increasing the alkyl length to decyl (f(alkyl/EO) = 0.97) and tetradecyl (f(alkyl/EO) = 1.23) gives rise to the formation of two-dimensional networks and vesicles, respectively. Remarkably, the toroidal structure showed to change into two-dimensional networks on heating in a reversible way due to a lower critical solution temperature (LCST) behavior of the oligoether dendritic exterior. This structural change takes place directly without passing through any intermediate structures. The results described here demonstrate that systematic variation in the hydrophobic chain length of the rod amphiphiles can provide access to a wide variety of self-assembled nanostructures in aqueous solution. The primary driving force for this unique self-assembling behavior is proposed to be the energy balance between repulsive interactions of the hydrophilic oligoether dendrons and attractive hydrophobic interactions of the rod segments.