Isomerism and Silver Ion-Regulated Triterpenoid-Based Nanostructures for In Situ Synthesis of Ag Nanoparticles with Plasmonic Chirality

Supramolecular chirality arising from periodic asymmetrical packings of building units plays a significant role in development of smart chiral materials, but the thorough mechanism of chirality control on self-assembly has seldom been understood. Here, three isomeric amphiphiles p/m/o-Py-BL (para/meta/ortho-pyridine-appended betulin) were synthesized from a natural pentacyclic triterpenoid. The presence or absence of supramolecular chirality of assembled p/m/o-Py-BL was regulated by isomeric effects, determined by conformation and strength of intramolecular H-bonds. The helical topologic transformation and chiroptical activity transitions modulated by Ag+ were also reported, ascribed to the transformed conformation of building units, converting a network of chiral nanofibers into helical nanoribbons. In addition, the helical nanoribbons of p-Py-BL/Ag+ were capable of in situ synthesis of Ag nanoparticles with plasmonic chirality. These findings provided a blueprint for regulating supramolecular handedness by isomerism and metal ions, affording a deeper understanding on chiral self-assembly processes and facilitating the applications of triterpenoid-based assemblies in chiral nanomaterials.