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

Supramolecular assemblies of dicationic 4,4′-bipyridinium (viologen: V 2+ ) moieties modifying glutamide ( G ) derivatives, i.e., G V 2+, 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 G V-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 G V 2+ interacting with I –, Br –, or Cl – ions ( G V-I, G V-Br, and G V-Cl) depend on the charge-transfer interactions of V 2+ and X –, determined by the electron-donating strength of X – . The counterion-exchanged G V-X assemblies, synthesized from as-prepared G V-PF 6 salts in acetonitrile solution or prepared by excess addition of salts including targeted X – ions, showed composition-dependent chiroptical properties. Notably, the G V-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 G V-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.