pH-Controlled motions in mechanically interlocked molecules

Mechanically interlocked molecules (MIMs), especially rotaxanes and catenanes, allow large-amplitude movement at the molecular level, making them the perfect prototypes for artificial molecular switches and machines. By applying external stimuli, the noncovalent interactions between their subcomponents can be interrupted and formed reversibly, leading to mechanical motions. Among various stimuli, pH stimulation is one of the most powerful and commonly used means of controlling motions by acids and bases. In this review, we summarize the pH-controlled mechanical motions including translocation in rotaxanes, circumrotation in catenanes and other kinds of motion in more sophisticated mechanomolecules, and discuss their operating mechanisms. In addition, we present several more recent developments of alternative stimuli for pH responsive motions.