Molecular Muscles: From Species in Solution to Materials and Devices

The field of "molecular machines" based on interlocking ring compounds started about twenty years ago with the deliberate synthesis of molecules or molecular assemblies which could be set in motion using various types of signals (mostly photonic, electrochemical, or chemical). The complexity of the systems gradually increased and, twelve years ago, rotaxane dimmers whose length could be controlled were proposed. At that time, they were among the most sophisticated machines. In the present review article, we present a survey of the most significant systems published since the beginning of nanosize molecular muscles. The doubly threaded nature of rotaxane dimers consisting of two "ring-and-string" fragments with the ring of one fragment being threaded by the thread of the other fragment, and vice versa, is certainly very well adapted to contraction and elongation motions. The various muscle-like species discussed are based on significantly different principles: coordination chemistry, the transition metals used playing a central role, hydrophobic forces combined with photoisomerization processes or solvent effects when the threaded rings were cyclodextrins, hydrogen bonding and acceptordonor interactions, the movement being driven by a pH change in this latter case. Particularly promising extensions of the field have been reported in recent years. In a very significant example, contraction and extension of surface-bound "molecular muscles" was shown to induce the bending of a beam that is five orders of magnitude larger in size than the molecular species itself. In the most recent contribution, preparation of a polymer from an elemental nanosize molecular muscle was achieved, leading to a long molecular fragment able to undergo contraction and elongation under well-controlled conditions. The corresponding material, formed by the association of thousands of molecular muscles, was shown to behave as a contractile and extensible system at the macroscopic level. © 2014 The Chemical Society of Japan.