rotaxanes

Rotaxanes are a class of mechanically interlocked molecules composed of a dumbbell-shaped axle threaded through a macrocyclic ring. The assembly is stabilized when bulky end groups, or stoppers, on the axle prevent the ring from slipping off, creating a mechanical bond rather than a covalent linkage between components.

In a rotaxane, the ring is trapped on the axle by the stoppers, restricting dissociation. If the ring is only threaded but not trapped, the system is referred to as a pseudorotaxane. The term [n]rotaxane is used if the molecule contains n mechanically interlocked rings along a single axle.

Synthesis of rotaxanes commonly uses template-directed strategies that preorganize the ring and axle, followed by an end-capping or stoppering step. Two widely used approaches are threading-then-stoppering, where a preformed macrocycle is threaded onto an axle and bulky groups are appended to lock it in place, and clipping, where the ring is formed around the axle. Assembly often relies on specific interactions such as metal coordination, hydrogen bonding, or π-stacking to guide the components into the correct orientation before fixation.

Properties of rotaxanes arise from the mechanical bond, which allows relative motion of the ring and axle without breaking covalent bonds. The ring can shuttle between defined stations along the axle in response to external stimuli, including changes in pH, redox state, or light, enabling reversible, controllable motion.

Applications span molecular machines, switches, and shuttles, with potential uses in data storage, sensing, and catalysis. The field gained prominence through work by Sauvage, Stoddart, and Feringa on functional, controllable mechanically interlocked molecules, culminating in the Nobel Prize in Chemistry in 2016.