Supramolecular Nanotubes Functioning as Morphology Regulators for Fluid-State Molecular Assemblies

Abstract

Seven supramolecular nanotubes (inner diameters, 35 nm) with different ester groups on their inner surfaces were prepared from synthetic amphiphiles and used as morphology regulators for three fluid-state molecular assemblies: a phospholipid liposome and two surfactant micelles. At temperatures above the gel-to-liquid crystalline phase transition temperature (T g–l) of the liposome (diameter, 360 nm), encapsulation in the nanotubes led to morphological transformation of the liposome into either a much smaller vesicle (diameter, 8 nm) or a long, tubular nanofiber (inner diameter, 5 nm), depending on the carbon number of the ester group on the nanotube inner surface. Before encapsulation, the liposome had a typical lamellar molecular packing with a stacking periodicity (d) = 5.45 nm and T g–l = 42.0 °C, whereas the resulting vesicle had an interdigitated molecular packing with d = 3.13 nm and T g–l = 31.8 °C, although this unusual molecular packing was rearranged back to the original lamellar molecular packing after release from the nanotubes. The morphology of the tubular nanofiber (d = 5.06 nm and T g–l = 48.7 °C) was not changed before and after release from the nanotubes. The nanotubes also induced rearrangement of an encapsulated surfactant micelle harboring an environmentally responsive probe, pyrene, either to a spherical morphology with a typical molecular packing of micelles, in which pyrene existed in a monomeric state, or to a twisted, fibrous morphology with a lamellar molecular packing, in which pyrene formed a J-type aggregate emitting excimer fluorescence. The micelle with the twisted, fibrous morphology exhibited a helicity induced by the confinement in the nanotubes with supramolecular chirality, even though the surfactant itself was achiral.