2rotaxane

A [2]rotaxane is a mechanically interlocked molecular assembly in which two macrocyclic rings are threaded onto a single linear axle and retained by bulky end groups, or stoppers, that prevent the rings from dethreading. The design confines both rings to the same axle but allows them to move along it and interact with defined sites or stations along the axle.

In a typical [2]rotaxane, the axle is dumbbell-shaped and bears recognition sites that can selectively bind portions of the macrocycles. The macrocycles can shuttle between different stations on the axle in response to external stimuli, enabling controllable molecular motion. The overall topology is such that the rings are mechanically interlocked rather than covalently attached to the axle.

Synthesis of [2]rotaxanes often involves template-directed assembly followed by stoppering. Strategies include metal–ligand coordination, hydrogen bonding, and other noncovalent interactions to assemble the axle and rings, with subsequent introduction of bulky stoppers to lock the structure. Dynamic covalent chemistry can also be employed to seal the completed interlocked architecture. The result is a discrete, stable entity in which the motion of the rings is constrained by the mechanical bond.

Significance and potential applications of [2]rotaxanes lie in their function as molecular machines. They serve as prototypes for controllable nanoscale motion, with potential uses in molecular switches, actuators, sensors, and smart materials. The study of [2]rotaxanes contributes to the broader field of mechanically interlocked molecules, highlighting how topology and noncovalent interactions can govern function at the molecular level.