Forces in mechanically interlocked materials

Forces are at the heart of almost every major biological process. The widespread use of molecular machines in nature has inspired scientists to synthesize mechanically interlocked molecules (MIMs) in which controlled, relatively large amplitude motion of one component relative to another can potentially result in net directional forces. This review examines force-related processes in MIMs, namely, catenanes, rotaxanes and knots, and in the polymers that include them. We first discuss the single-molecule force spectroscopy studies, performed on well-defined mechanically linked systems, one molecule at a time, and show how these experiments have provided unprecedented insights into their operation, dynamics, and comprehensive understanding of their performance metrics. Then the use and effects of these mechanical links on mechanically activated polymers are addressed. We examine the unique mechanochemical reactivity of MIMs and how it is exploited to create force-responsive molecular devices and materials or elicit new mechanochemical reactions in response to external force. Finally, we focus on the use of mechanical links as crosslinks, giving rise to the so-called slide-ring materials. The review highlights the unprecedented mechanochemical properties that are emerging from the integration of MIMs into polymers and the quick pace of progress in the field that should give rise to more advanced systems.