Solvent-Directed Assembly of a Pyridinium-Tailored Methyl Oleanolate Amphiphile: Stepwise Growth of Microrods and Nanofibers

Although a few architectures have been fabricated by the self-assembly of natural triterpenoids, the precise control of shape and size is rarely studied. Herein, a methyl oleanolate-bearing amphiphile, 1-[2-(methyl oleanolate)-2-oxoethyl]pyridinium bromide (MOP), has been designed and its assembly behavior was investigated. It was found that the morphologies of MOP assemblies ranged from nanoparticles to rigid microrods and flexible nanofibers in chloroform/p-xylene and methanol/water, respectively. During the assembly process, the systematical variational solvophobic/solvophilic effect resulted in the formation of spherical nanoparticles with opposite dipoles and converse bilayer structures. Moreover, such opposite molecular orientations lead to the inversion of supramolecular chirality and distinct mechanical properties. The driving forces and packing patterns of MOP in each solvent system were clearly demonstrated by the combination of NMR, UV-vis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), theoretical computation, and contact angle experiments, which revealed the roles of triterpenoids and pyridinium cations in the assembly process. This work provides a facile strategy to control the supramolecular structures in triterpenoid-based assemblies by adjusting the solvent polarity and composition.