Credit: ©2008 RSC

Some interlocked molecules in which linear components pass through cyclic ones — so-called rotaxanes — can mimic the expansions and contractions of muscle fibres. Whereas most previous examples have relied on transition-metal complexation and redox chemistry, researchers in Australia have now made a molecular 'muscle' that can be controlled photochemically.

Christopher Easton and co-workers1 at the Australian National University and Adelaide University made a rotaxane comprising two identical components interlocked in a head-to-tail fashion. Each component contains a photoisomerizable stilbene group that is attached, on one side, to a cyclodextrin ring and, on the other, to a bulky blocking group that prevents the molecule unravelling. When either of the stilbenes are in the trans form, they fit inside the cyclodextrin of the other component, but when in the cis form, size constraints cause the cyclodextrin to move off the stilbene and onto a different binding site. This movement results in a contraction of the whole rotaxane molecule.

Conversion between the two stilbene isomers, and therefore the expanded and contracted positions, occurs by exposing the system to different wavelengths of light. Both of these states, as well as an intermediate where one stilbene is cis and one is trans, have been isolated and characterized by NMR spectroscopy.