Rotaxane-Based Molecular Muscles

CONSPECTUS: More than two decades of investigating the chemistry of bistable mechanically interlocked molecules (MIMs), such as rotaxanes and catenanes, has led to the advent of numerous molecular switches that express controlled translational or circumrotational movement on the nanoscale. Directed motion at this scale is an essential feature of many biomolecular assemblies known as molecular machines, which carry out essential life-sustaining functions of the cell. It follows that the use of bistable MIMs as artificial molecular machines (AMMs) has been long anticipated. This objective is rarely achieved, however, because of challenges associated with coupling the directed motions of mechanical switches with other systems on which they can perform work. A natural source of inspiration for designing AMMs is muscle tissue, since it is a material that relies on the hierarchical organization of molecular machines (myosin) and filaments (actin) to produce the force and motion that underpin locomotion, circulation, digestion, and many other essential life processes in humans and other animals. Muscle is characterized at both microscopic and macroscopic length scales by its ability to generate forces that vary the distance between two points at the expense of chemical energy. Artificial muscles that mimic this ability are highly sought for applications involving the transduction of mechanical energy. Rotaxane-based molecular switches are excellent candidates for artificial muscles because their architectures intrinsically possess movable filamentous molecular components. In this Account, we describe (i) the different types of rotaxane "molecular muscle" architectures that express contractile and extensile motion, (ii) the molecular recognition motifs and corresponding stimuli that have been used to actuate them, and (iii) the progress made on integrating and scaling up these motions for potential applications. We identify three types of rotaxane muscles, namely, "daisy chain", "press", and "cage" rotaxanes, and discuss their mechanical actuation driven by ions, pH, light, solvents, and redox stimuli. Different applications of these rotaxane-based molecular muscles are possible at various length scales. On a molecular level, they have been harnessed to create adjustable receptors and to control electronic communication between chemical species. On the mesoscale, they have been incorporated into artificial muscle materials that amplify their concerted motions and forces, making future applications at macroscopic length scales look feasible. We emphasize how rotaxanes constitute a remarkably versatile platform for directing force and motion, owing to the wide range of stimuli that can be used to actuate them and their diverse modes of mechanical switching as dictated by the stereochemistry of their mechanical bonds, that is, their mechanostereochemistry. We hope that this Account will serve as an exposition that sets the stage for new applications and materials that exploit the capabilities of rotaxanes to transduce mechanical energy and help in paving the path going forward to genuine AMMs.