Light powers molecular piston and cylinder.

A device of molecular dimensions moves like a simple machine when stimulated by light, according to a new study.

J. Fraser Stoddart of the University of California, Los Angeles and his colleagues at the University of Birmingham in England and the University of Bologna in Italy have synthesized a pair of organic molecules that join together like a cylinder enclosing a piston. The researchers find that by attaching a photosensitive compound to the cylinder, a cyclophane, they can use light to make it expel the piston, a bis-naphthalene.

"If you look at the process, it leaves the impression of a linear motor," Stoddart says. He and his colleagues report their findings in the Nov. 4 Journal of the American Chemical Society.

Multicomponent molecules with this piston-and-cylinder structure are known as pseudorotaxanes. Unlike true rotaxanes, they don't have bulky caps on the ends of the piston that prevent the cylinder from sliding off (SN: 12/13/97, p. 378).

The researchers examined the response to light of the pseudorotaxane in a solution of sodium oxalate. A complex containing the metal rhenium, affixed to the cylinder, absorbs a photon of light and donates an electron to the cylinder. The electron neutralizes one of four positive charges that hold the piston in place, weakening the bond enough to encourage the piston to slide all the way out.

To keep the electron from returning to the rhenium complex immediately, before the piston has time to move, the chemists allow sodium oxalate to donate an electron to replenish the rhenium complex. Even with sodium oxalate supplying electrons, only about 30 percent of the pseudorotaxanes disassemble upon illumination.

When the researchers turn off the activating light and bubble oxygen through the solution, they restore the cylinder's positive charge, and a piston reenters.

Rotaxanes and pseudorotaxanes might one day be used in miniaturized sensors or in information-processing devices, Stoddart says, but such applications will take "lots of development." The current study is mainly "a proof of principle that this assembly of components can move with respect to each other."

Harry W. Gibson, who studies rotaxanes at the Virginia Polytechnic Institute and State University in Blacksburg, agrees that applications are still far away. Chemists often carry out basic chemical studies in a solution, he notes, but useful devices are usually in the solid state. For molecular machines, "there's still a gap to bridge between the conceptual and the practical. Clearly, this is a step in that direction."