Pseudorotaxanes

Pseudorotaxanes are mechanically interlocked molecules consisting of a linear axle threaded through a cyclic macrocycle. Unlike rotaxanes, they lack bulky end groups on the axle to prevent dethreading, so the macrocycle can slide along the axle.

The complex arises from non-covalent host–guest interactions between the axle’s recognition sites (such as ammonium, imidazolium, or metal-ligand sites) and complementary binding sites on the macrocycle (for example crown ethers, cyclodextrins, or cucurbiturils). Formation is often driven by templating effects, along with hydrophobic, electrostatic, or hydrogen-bonding interactions. Common examples include ammonium-containing axles threaded through crown ethers or cyclodextrins.

Pseudorotaxanes are dynamic and reversible; the macrocycle can dethread under appropriate conditions. They can be converted into rotaxanes by introducing bulky stoppers at the ends of the axle or by chemical locking. They are also used in stimuli-responsive systems, with assembly controlled by factors such as pH, redox state, or solvent environment.

In research, pseudorotaxanes serve as key intermediates in the construction of molecular machines, switches, and sensors, illustrating principles of mechanical bonding and supramolecular assembly. They provide a flexible platform for studying how threading and dethreading processes can be controlled and exploited for functional nanoscale devices.