Elizabeth Wilson

Carrying individualism to new heights, scientists report that they've performed a complete chemical reaction with single molecules--using the tip of a scanning tunneling microscope. This demonstration of nearly irreducible fine control has some chemists envisioning new, exotic molecules created by selectively cutting and pasting atoms and molecules together.

Physicist Saw-Wai Hla, physics professor Karl-Heinz Rieder, and their colleagues at the Free University in Berlin developed a molecular-scale version of the well-known Ullmann reaction, a process in which iodobenzene breaks apart with the help of a copper powder catalyst, allowing liberated phenyl molecules to fuse into biphenyl [Phys. Rev. Lett., 85, 2777 (2000)].

Long a staple tool of the physicist, the scanning tunneling microscope (STM) creates pictures of atomic hills and valleys on a surface by measuring the variation in tunneling current between the tip and the surface. But researchers increasingly are using STM tips not only to image atoms and molecules, but to manipulate them as well.

For example, physics professor Wilson Ho , now at the University of California, Irvine, and colleagues dropped carbon monoxide molecules from the tip of an STM on top of an iron atom to form Fe(CO)₂ (C&EN, Nov. 29, 1999, page 9).

Now, Hla and Rieder show they can also use the STM tip to break and create chemical bonds. In their STM version of the Ullmann reaction, a few iodobenzene molecules squat on a copper substrate at 20 K, a temperature so frigid that the reaction won't take place without an energetic push. That push comes in the form of electrons injected from the STM tip into a single iodobenzene molecule. The iodine-phenyl bond promptly breaks, evidenced by striking images showing two lumps where there once was one. The researchers repeat the process with another iodobenzene molecule.

Next, the authors drag the now unmat ed iodine atoms out of the way with the STM tip, leaving the phenyls poised for a union on the Cu. With the STM tip, they pull one phenyl next to another. Though the phenyls are now extremely close together, they again need energy to form a true bond.

The authors add this energy with another blast of electrons, which breaks the phenyls' hold on the copper substrate, leaving them with carbon bonds that are momentarily free to spontaneously bind with each other. Hla and Rieder demonstrate this new pairing by dragging the newly formed biphenyl around the substrate: The two aromatic rings come along together.

This system offers promise of designing molecules with an STM scalpel, though it may be limited to atoms and molecules that can be manipulated--and reactions that can be stimulated--by tunneling electrons, Ho says. And the work gives scientists a valuable way to visualize all the elementary steps of such a reaction.

"This is a great paper--I wish I'd done it," Ho says.

STM images show two iodobenzene molecules on a Cu substrate (top left). Next, a voltage pulse from the STM tip breaks the iodine-phenyl bonds (top right and middle left). The iodine atom is moved out of the way (middle right), and finally, the two phenyls are brought together (bottom).

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