The molecular necklace: a rotaxane containing many threaded α-cyclodextrins

THE importance of non-covalent interactions in biological systems motivates much of the current interest in supramolecular assemblies1. A classic example of a supermolecule is provided by the rotaxanes2–5, in which a molecular 'rotor' is threaded by a linear 'axle'. Previous examples have included cyclic crown ethers threaded by polymers6, paraquat–hydroquinone complexes7 and cyclodextrin complexes8,9. We found recently that α-cyclodextrin will form high yields of a crystalline complex with polyethylene glycol (PEG), and suggested that the PEG penetrates the 'beaker-like' tunnel of the cyclodextrin10,11. We report here the preparation of a compound in which several cyclodextrins are threaded on a single PEG chain and are trapped by capping the chain with bulky end groups. This brings a step closer the 'molecular abacus' proposed by Stoddart and coworkers7. We call this supramolecular assembly a 'molecular necklace'.