Redox-Responsive Artificial Molecular Muscles: Reversible Radical-Based Self-Assembly for Actuating Hydrogels

Interest in the design and development of artificial molecular muscles has inspired scientists to pursue new stimuli-responsive systems capable of exhibiting a physical and mechanical change in a material in response to one or more external environmental cues. Over the past few decades, many different types of stimuli have been investigated as a means to actuate materials. In particular, materials that respond to reduction and oxidation of their constituent molecular components have shown great promise on account of their ability to be activated either chemically or electrochemically. Here, we introduce a novel redox-responsive mechanism of actuation in hydrogels by describing a systematic investigation into the radical-based self-assembly of a series of unimolecular viologen-based oligomeric links, present at only 5 mol % of the polymer linkers in a three-dimensional network. The actuation process results in an overall reversible contraction of a family of hydrogels, down to 35% of their original volume in the first 25 min and ultimately to 9% after a few hours, even while remaining submerged in water. The mechanism of contraction starts with a decrease in electrostatic repulsion upon chemical reduction, leading to a loss of counterions and intramolecular self-assembly of the main-chain viologen subunits. The overall mode of actuation takes place relatively quickly in comparison to hydrogels of similar size, and the rate of contraction is accelerated as higher molecular weight oligoviologen links are implemented. The contraction process ultimately leads to a 2-fold increase in elasticity of the material, and upon exposure to oxygen and water, the hydrogels quickly oxidize and regain their original size and mechanical properties, thus resulting in a reversible actuation process that is capable of lifting objects which are 5–6 times heavier than the contracted hydrogel itself

Topologically Controlled Syntheses of Unimolecular Oligo[<i>n</i>]catenanes

Catenanes are a well-known class of mechanically interlocked molecules that possess chain-like architectures and have been investigated for decades as molecular machines and switches. However, the synthesis of higher-order catenanes with multiple, linearly interlocked molecular rings has been greatly impeded by the generation of unwanted oligomeric byproducts and figure-of-eight topologies that compete with productive ring closings. Here, we report two general strategies for the synthesis of oligo[n]catenanes that rely on a molecular “zip-tie” strategy, where the “zip-tie” is a central core macrocycle precursor bearing two phenanthroline (phen) ligands to make odd-numbered oligo[n]catenanes, or a preformed asymmetric iron(II) complex consisting of two macrocycle precursors bearing phen and terpyridine ligands to make even-numbered oligo[n]catenanes. In either case, preformed macrocycles or [2]catenanes are threaded onto the central “zip-tie” core using metal templation prior to ring-closing metathesis (RCM) reactions that generate several mechanical bonds in one pot. Using these synthetic strategies, a family of well-defined linear oligo[n]catenanes were synthesized, where n = 2, 3, 4, 5, or 6 interlocked molecular rings, and n = 6 represents the highest number of linearly interlocked rings reported to date for any isolated unimolecular oligo[n]catenane