Multivalent-Anion-Induced Inversion of Chiroptical Signals in a Cyanine Dye Bound to Nanotubular Supramolecular...

In this study, we investigated the effect of salts on the induced chirality of the cyanine dye NK2707 when bound to supramolecular assemblies formed by cationic glutamide lipids (G-Py⁺). NK2707 bound to G-Py⁺ self-assembled structures formed diverse aggregates and exhibited exceptionally large circular dichroism (CD) signals. The J-aggregates of NK2707 showed an unusually intense R-chiral-induced CD signal attributable to Davydov splitting; this signal reversed to an S-chiral CD response upon addition of the sodium salt of a multivalent anion. This inversion of a CD signal at the 10⁶ level is unprecedented. In contrast, NaCl addition produced no appreciable spectral change, suggesting that this effect strongly depends on anion valence. We attribute the CD inversion to multivalent-anion bridging of pyridinium groups, which alters and stabilizes the chiral orientation of G-Py⁺ and, in turn, reverses the induced CD. Furthermore, although molecular orientation typically decreases above the gel-liquid-crystal phase transition temperature, Na₂SO₄ addition enabled G-Py⁺ to retain its chiral orientation even in the liquid-crystal state. Na₂SO₄ promoted J-aggregate formation in the gel state and H-aggregate formation in the liquid-crystal state, maintaining a CD peak on the order of 10⁶ while shifting the wavelength by approximately 200 nm. Overall, the G-Py⁺/NK2707 system can be dynamically modulated by multivalent anions and temperature, providing a versatile platform for controlling chiroptical properties through salt-induced changes in molecular orientation.