Spring 2002 -- Vol. 19, No. 1

Paul Weiss

Research Reveals Potential of Single Molecules to Function as Switches When They Change Shape

Your future computer may have components that function based on the action of single molecules. A step in that direction has been made by showing that single molecules can switch between “on” and “off” states, and then hold in that state for hours at a time.

According to a paper by researchers at Penn State and Rice University published in the 22 June 2001 edition of Science, specially designed single molecules can switch in that manner. In addition, conformational changes—which happen when molecules alter their arrangement by rotation of their atoms around a single bond, effectively changing shape by moving or turning—determine how and when that conductance switching occurs in those molecules.

Researchers determined that limiting conformational changes reduces switching between the “on” and “off” states. So, just as squeezing a lot of people into a small room limits their ability to move freely, researchers determined the same thing was happening at a much smaller scale with molecules. Conformational changes do not occur as frequently when the molecules have less room to move in their host environment, or matrix.

Because switching provides the basis of logic and memory in computer systems, the discovery of what causes such switching in single molecules may help researchers move closer to making molecular computers a reality.

“We essentially tightened the noose around the molecule and showed that once its motion was reduced switching went way down,” says Paul Weiss, associate professor of chemistry at Penn State. “We have not worked out how to make computer architecture or anything close to that, but tackling the very small end, which is our specialty, has been an interesting and exciting project. Our next step is figuring out how to control the molecules’ movement between ‘on’ and ‘off.’ In bundles of thousands of molecules, our collaborator, Mark Reed, in electrical engineering at Yale University, and his group, have been able to accomplish movement between the states. Our work was the first to show that single molecules could function as switches.”

Perspective views of a single molecule inserted into a dodecanethiolate monolayer. The top image in each pair shows the molecule in the “on” state and the bottom image shows the molecule in the “off” state.

According to the research, a dense, well-ordered matrix inhibits the rate at which conductance switching occurs among single molecules within that matrix. In a loose, poorly ordered matrix, those same molecules switch between “on” and “off” much more frequently. Researchers tracked the molecules’ movement between “on” and “off” using scanning tunneling microscopy in matrices of alkanethiolate monolayers. The molecules—known as phenylene ethynylene oligomers and comprised of alternating benzene rings and two carbon atoms with triple bonds between them and a functional group on the central of three rings—were the first single molecules to have their switching documented.

“It had been predicted that single molecules did not switch, but we proved they did and we identified at least part of the mechanism,” Weiss says. “Two important advances are determining the limit at one molecule and establishing that its persistence time—the length of time information can be held in a switch at room temperature—can be hours.”

Researchers found the molecules that underwent conformational changes remained anchored in the same spot on the matrices and that the molecules’ apparent size in images changed when they switched. They appeared to stand higher in the matrix when they were “on,” lower when they were “off,” and the respective states lasted as long as 26 hours—indicating changes in conductance. Also, while the research on 10,000 molecules at once showed that groups of molecules could be switched at will, the researchers focusing on single molecules proved they could turn the molecules off, but turning them on was more problematic.

“Clearly, we have an indication it can be done,” Weiss says. “It’s just a matter of setting up the experiment in the most efficient manner.”

Several technical advances were important to the research. Along with a highly stabilized microscope, the researchers determined how to insert the number of molecules they wanted into the matrix by varying the matrix itself. Also, graduate researchers Zachary Donhauser and Brent Mantooth, along with postdoctoral fellow Kevin Kelly, devised a computer program to track each molecule in the matrix automatically and simultaneously. So, every molecule that switched and the specific time it was on or off was recorded during the sometimes day-long trials. The resulting data, along with frame-by-frame results in a movie format, were available a few minutes after each of the sessions. According to Weiss, that “spectacularly clever approach, a huge undertaking that allowed us to analyze our data quantitatively,” was invaluable to the success of the research.

Funding for the research was provided by the Army Research Office, the Defense Advanced Research Projects Agency, the National Science Foundation, the Office of Naval Research, and Zyvex LLC.

Steve Sampsell

Back to Science Journal Spring 2002 Index

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