Bis-Bipyridinium Gemini Surfactant-Based Supramolecular Helical Fibers and Solid State Thermochromism

The processability and functional performance of stimuli-responsive supramolecular materials are key factors in determining their utility and potential for mass adoption, usage, and profitability. However, it is difficult to predict how structural changes to the molecular components of these systems will impact their operation. Here, a series of π-electron-deficient bis-bipyridinium gemini surfactants were synthesized and evaluated to elucidate the structure-property relationships that govern their ability to form helical-fiber-based donor-acceptor hydrogels, impact hydrogel strength, and influence their solid-state thermochromism. When combined with the π-electron-rich donor melatonin, the helical-fiber- and hydrogel-forming ability of the gemini surfactants was largely influenced by the dimensions of the rigid bridging group that connects the two bis-bipyridinium units. Dynamic viscoelastic rheology and linear sweep voltammetric analysis revealed a positive correlation between the length of the gemini-surfactant bridging group and both the hydrogel strength and the magnitude of the charge-transfer interaction between the donor-acceptor pair. Solid-state thermochromic transition temperatures of processed aerogels, xerogel films, and inkjet-printed patterns were positively correlated with the strength of the charge transfer interaction between the donor-acceptor pair and, thus, also with the length of the gemini surfactant bridging group. The results provide impactful insights that will enable the development of new donor-acceptor-based thermochromes with versatile processability and tunable functionality.