Lollipop Molecules Make Better Switches

Honolulu, Hawaii--In the drive for ever-smaller computer chip components, making devices with molecules that switch like transistors do is all the rage. So far, such devices have had problems: Either they've required freezing temperatures to operate, or they haven't passed enough electrical current to be useful. But now a team has come up with new molecule-based switches that work at room temperature while passing large currents. The new molecules are already giving the nascent field of molecular computing a lift.

In recent months, researchers have demonstrated several types of molecular switches. One team at the University of California, Los Angeles, used a set of molecules called catenanes, made from pairs of intertwined molecular rings. By changing electrical voltages applied to electrodes sandwiching the catenanes, the team showed that they could alter the number of electrons on the rings. This caused the rings to rotate around one another, which altered the ability of current to flow through the catenanes from one electrode to the other. But in constructing the devices, the researchers had to layer the rings atop hydrocarbon molecules. This setup placed the rings closer to one electrode than the other and made it difficult for charges to move through the mix.

To improve the design, the same team, led by chemist Fraser Stoddart and postdoc Julie Perkins, constructed a new class of switching molecules. Called pseudorotaxanes, they are shaped like molecular lollipops. Like the catenanes, the pseudorotaxanes are sandwiched between a pair of electrodes. Positively charged ring-shaped compounds nestle close to an electron-donating portion of the lollipop stem. When an electrical potential is applied between the electrodes, it yanks electrons off the stem area and spurs the ring-shaped compounds to jump to another spot. This jump lowers the electrical resistance of the molecular switch and allows current to flow more easily between the surrounding electrodes, the researchers reported here on 15 December at the International Chemical Congress of Pacific Basin Societies.

In addition, the physical structure of the new switch design fosters current flow. Like the catenanes, the pseudorotaxanes still need hydrocarbons to help them assemble properly between the electrodes. But, in this case, the lollipops and hydrocarbons sit side by side, allowing the lollipops to be centered between the two electrodes and prompting nearly 100 times more current to flow between the electrodes.

"I think this is pioneering stuff," says chemist Peter Stang of the University of Utah in Salt Lake City. He notes that the pseudorotaxanes aren't ready to compete with Pentium chips just yet, as they tend to break down after a week or so of use. Stoddart says he believes that may be due in part to the fact that the rings can slide off the ends of the lollipops. His team is already at work on new versions of the molecules with bulky groups on both ends to prevent the rings from sliding off.

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