Australian researchers have developed a novel molecular muscle driven by light.

Biological machines, such as muscles, are commonly found in nature and are essential to many processes in the human body. Taking this as their inspiration, a team led by Christopher Easton at the University of Adelaide, Australia, developed a light-driven molecular muscle based on stilbene and -cyclodextrin.

-Cyclodextrin is a macrocyclic compound which forms host-guest complexes with hydrophobic guests of an appropriate size to be incorporated into -cyclodextrin's central cavity. trans-Stilbenes can act as guests to -cyclodextrin, but the cis isomer cannot as it is much less compact.

Easton and his team created rotaxane 'daisy chain' dimers, in which a stilbene bonded to an -cyclodextrin molecule is threaded through another -cyclodextrin bonded to a stilbene, and then the stilbenes are capped with bulky alkyl groups to prevent the components separating. Initially both the stilbenes are in the trans conformation, and both stilbenes act as guests within the -cyclodextrin. On irradiation with 350nm light, one or both of the stilbenes may isomerise, causing them to be released from the -cyclodextrin and resulting in the extension of the rotaxane. This may be reversed by irradiation with 254nm light, causing the 'muscle' to contract or expand with light irradiation.

'Our molecular muscle switches between states but its particular feature of expanding and contracting as it changes states is of more interest in the development of so-called smart materials that respond to their environment,' says Easton. 'It is a relatively small step to envisage our muscle being incorporated into a polymer that shrinks and expands depending on its exposure to light.'

Jean-Pierre Sauvage of the Louis Pasteur University, Strasbourg, France, says the work is fascinating, saying 'Until now, molecular chemists had proposed to trigger related motions by chemical or electrochemical signals. The present system, based on photonic inputs only expands the field in a very significant fashion.'