Anion-Exchange-Controlled Chiroptical Properties in Viologen-Linked Glutamide Nanofibrillar Assembly for...

Supramolecular assemblies of dicationic 4,4'-bipyridinium (viologen: V2+) moieties modifying glutamide (G) derivatives, i.e., GV2+, are promising candidates for nanomaterials enabling redox-induced electroresponsive chiroptical switching in the visible region (ECSv), with potential applications in next-generation chiroptical management technologies. In this study, we investigated the dependence of the chiroptical properties of GV-X assemblies on the counteranion X- using ultraviolet-visible (UV-vis) absorption and circular dichroism (CD) spectroscopy to enhance their versatility. Density functional theory (DFT) and time-dependent (TD) DFT calculations reveal that the chiroptical properties of GV2+ interacting with I-, Br-, or Cl- ions (GV-I, GV-Br, and GV-Cl) depend on the charge-transfer interactions of V2+ and X-, determined by the electron-donating strength of X-. The counterion-exchanged GV-X assemblies, synthesized from as-prepared GV-PF6 salts in acetonitrile solution or prepared by excess addition of salts including targeted X- ions, showed composition-dependent chiroptical properties. Notably, the GV-Br assembly with a nanofibrillar structure exhibited ECSv, characterized by color and chiroptical signal changes without sign inversion, differing from those with sign inversion of the previous GV-BrI assembly. The ability to modulate chiroptical properties through anion-exchange and charge-transfer interactions provides a pathway for designing custom materials with specific optical properties for targeted applications.