Scientists from Pennsylvania State University and Rice University have found the smallest switch yet: a single molecule. And because switches lie at the heart of computer memory and logic systems, the results, published in todays issue of Science, may hold promise for the future of molecular computing.

Using a scanning tunneling microscope, the researchers took pictures of the individual moleculesknown as phenylene ethynylene oligomerson a surface coated with a different chemical over time periods of up to 26 hours. The molecule takes on different shapes when it is "on" (bottom image) and "off" (top image). By varying the arrangement of molecules in the supporting layers, the scientists were able to control how often the introduced molecules switched. A more orderly environment seems to limit the ability of molecules to change their shape, or conformation, by rotating around single bonds between atoms. When this rotation is hindered, so is switching, suggesting the two are linked.

Although the scientists had some success in changing the molecules from the "on" state to the "off" one, turning them on proved more difficult. "It had been predicted that single molecules did not switch, but we proved they did and we identified at least part of the mechanism," says Paul Weiss, a professor of chemistry at Penn State University. He cautions that they are a long way from designing applications, such as computer architecture, for the molecular switches. The next step, he says, "is figuring out how to control the molecules' movement between 'on' and 'off.'"