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

Abstract

The amplification of nanoscale motions of artificial molecular machines to the macroscopic level is a major challenge. In this study, a redoxresponsive 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 crosslinker 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•PF 6 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.

Anion‐Coordination Foldamer‐Based Polymer Network: from Molecular Spring to Elastomer

“…A comparison of the tough elastomer P‐L 6U Cl to some related materials with mechanically interlocked molecules (MIMs) as the monomer is depicted in Figure 3e [5c–f,16,20] . P‐L 6U Cl shows significantly enhanced mechanical properties than the crosslinked o ‐phenylene foldamer‐based polymer networks, [9b] and has higher strength and Young's modulus than those of crosslinked gels [8,10] . On the other hand, it displays relatively larger tensile strength and Young's modulus but weaker maximum tensile strain than the systems with monomers linked by polymers [5f,20b] .…”

“…[5c-f,16,20] P-L 6U Cl shows significantly enhanced mechanical properties than the crosslinked o-phenylene foldamer-based polymer networks, [9b] and has higher strength and Young's modulus than those of crosslinked gels. [8,10] On the other hand, it displays relatively larger tensile strength and Young's modulus but weaker maximum tensile strain than the systems with monomers linked by polymers. [5f,20b] Besides, the extension-contraction motion of the polymer networks in this work follows a different mechanism from MIMs.…”

“…The extension‐contraction motions of such molecular springs by changing the helical conformation have been realized in solution and/or in the solid state triggered by different stimuli [7] . However, the powerful fundamental motion of foldamers is less incorporated into polymer networks to exert impact on their macroscopic properties [8–10] . Zhang, Guan and co‐workers utilized helical peptide chains to endow the crosslinked gels with both tough and resilient properties [8] .…”

Confined Space Nanoarchitectonics for Dynamic Functions and Molecular Machines

“…In particular, those with dynamic functions, such as responding to external stimuli or changing morphology, are of particular interest as having more advanced functions [207][208][209][210]. The ultimate example would be a molecular machine in which molecules and supramolecules work like machines [211][212][213][214]. Such a dynamic function would be greatly influenced by the environment around its functional center.…”

Anion‐Coordination Foldamer‐Based Polymer Network: from Molecular Spring to Elastomer

“…[7] However, the powerful fundamental motion of foldamers is less incorporated into polymer networks to exert impact on their macroscopic properties. [8][9][10] Zhang, Guan and co-workers utilized helical peptide chains to endow the crosslinked gels with both tough and resilient properties. [8] Li, Guan and co-workers designed a series of aromatic oligoamide foldamers to cross-link polymer backbone, and the intramolecular hydrogen bonding of the foldamer helped to delay or retard the creep and relaxation processes, thus increasing the toughness of the films.…”