Organogel Actuators Based on Redox-Responsive Foldamers: Amplification of Molecular Machine’s Motion to Macroscopic Scale

The amplification of nanoscale motions of artificial molecular machines to the macroscopic level is a major challenge. In this study, a redox-responsive donor−acceptor (DA)-type foldamer, in which tetrathiafulvalene and viologen units are alternatively connected, was installed in an organogel network as a “working unit” to induce actuation. The ion gels were successfully synthesized via thermal azide−alkyne cycloaddition reaction between the azide-functionalized foldamer and the tetra-alkyne-ester cross-linker in an ionic liquid. The reaction efficiency of the cross-link was estimated from the stress−elongation curve. It was confirmed that the foldamers retained their electrochemical activity in the gel. Redox-responsive actuation of the gel was confirmed in a 0.1 M tetrabutylammonium·PF6 acetonitrile solution. Through chemical oxidation, the characteristic length of the gel could be increased up to 125% of its initial length, indicating that the volume of the gel could be increased to 195%. Reversible gel size changes in response to the redox reactions were confirmed. The change in the gel size was induced by equilibrium shift of the foldamer conformation between the folded state and extended state. The actuation force during the reduction of the partially oxidized gel was ca. 3.4 kPa, indicating that the gel can generate 35 gf cm−2. This actuation force proves the great potential of the redox-responsive foldamers to act as a molecular machine for realizing macroscopic actuation.