One-molecule chemistry gets big reaction

Scientists have long wanted to carry out chemical reactions one molecule at a time. In doing so, they imagined, they would probe the most intimate details of those transformations. Moreover, they’ve hoped they might find ways to create novel compounds that are impossible to make by conventional means. They also have anticipated assembling molecules that might be useful as nanometer-scale devices, they say.

Scanning tunneling microscope (STM) images of a reaction: Two iodobenzene molecules appear as bumps on a copper surface (1). They dissociate after a burst of electrons from the STM (2). The microscope tip then drags iodine away (3), pulls one carbon ring to other (4), and forges a biphenyl molecule (5). Hla, et al.

Now, for the first time, researchers have duplicated with individual molecules the full sequence of steps in a widely used chemical reaction. What’s more, they’ve made a photo album of the molecular makeover as it unfolds.

Karl-Heinz Rieder, Saw-Wai Hla, and their colleagues at the Free University of Berlin have miniaturized a century-old chemical transformation known as the Ullman reaction. Laboratories and industry use that process to link two iodobenzene molecules, Hla explains. Each contains a six-carbon ring known as a phenyl, so a barbell-shaped biphenyl results.

Normally, the Ullman reaction proceeds in beakers or larger vessels inside which copper, a catalyst, blends countless reacting molecules. Heating the mixture to 400ºC first breaks off the iodobenzenes’ iodine atoms and then forges links between the phenyl rings left behind. Iterations of this reaction can produce multi-ring polymers.

In its experiment, the Berlin team began with just two molecules of iodobenzene. These were placed on a copper surface shaped like a step and chilled to 20 kelvins. The researchers then used flows of electrons from the sharp tip of a scanning tunneling microscope, or STM, to break up the molecules. The tip dragged the fragments around, and its electron bursts provided the energy to rejoin the phenyl rings.

Because the STM is able to sense the electron clouds that protrude from individual atoms, the researchers also used the instrument to image each step of the reaction. In the end, they checked their work by tugging one ring of the biphenyl product and finding that the other followed along.

To his knowledge, says Phaedon Avouris of IBM’s T.J. Watson Research Center in Yorktown Heights, N.Y., the new experiment is the first example of “really doing chemistry with the STM” rather than just performing part of a reaction.

In 1995, Avouris and his coworkers used electrons from an STM to selectively snip molecular bonds, those between silicon and hydrogen atoms (SN: 6/24/95, p. 391). Last November at Cornell University, physicists Wilson Ho and Hyojune Lee reported the first example of inducing a single atom, iron, and a single molecule, carbon monoxide, to bond—again using an STM.

“What’s important is that [the German procedure] actually lets us visualize the reaction,” adds Ho, now at the University of California, Irvine. Moreover, this experiment and similar ones that Ho expects to follow will “give us new insights into the nature of chemical bonds and how bonds are formed,” he says.

In general, these STM manipulations must take place at extremely low temperatures to keep the reactant molecules from randomly hopping around. In the new experiment, the deep chill also squelched the thermal energy that drives the breaking and reforming of bonds in a conventional, larger-scale Ullman reaction. The STM electron bursts provided that energy.

Now that they have demonstrated molecular manipulation with a familiar reaction, the Berlin researchers have set their sights on molecular constructions never made before, especially ones that might behave like nanometer-scale transistors or other minuscule devices.

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