RON DAGANI

Tinkertoy parts can be assembled into a host of wondrous structures, but they can't do it by themselves. Molecular building blocks, on the other hand, have been shown to use specific noncovalent interactions to assemble themselves into sometimes elaborate supramolecular structures in solution or in the solid state.

	PORPHYRIN PATTERNS Porphyrins with one or two cyanophenyl substituents (nitrogens are dark blue) arrange themselves on a gold surface to form (from left) a triangular, square, or linear structure, depending on the number of "linkers" (cyano groups) and their orientation relative to one another. ADAPTED FROM NATURE © 2001

Now, Japanese researchers have demonstrated a new way to coax specially designed planar molecules to assemble into simple, controllable patterns on a surface [Nature, 413, 619 (2001)]. Their approach ultimately may lead to molecule-based devices that assemble themselves.

The Japanese team, led by surface physicist Takashi Yokoyama of the National Institute for Materials Science, in Nagoya, used porphyrin molecules designed and synthesized by Shinro Mashiko's group at the Communications Research Laboratory, in Kobe. These porphyrin building blocks are functionalized with 3,5-di-tert-butylphenyl groups and either one or two cyanophenyl groups. When two cyanophenyl groups are used, they can be attached to either adjacent or opposite sides of the porphyrin "square."

Because of their dipolar nature, cyano groups on neighboring porphyrins can undergo long-range dipole-dipole interactions, as well as hydrogen-bonding interactions, that cause the porphyrins to aggregate in predictable patterns.

To demonstrate this, Yokoyama deposits one kind of porphyrin on a clean gold surface at room temperature and then observes, at 63 K, the patterns the molecules have formed using a scanning tunneling microscope (STM). When the porphyrins carry a single cyanophenyl group, they aggregate into a trimeric cluster, with the three cyano groups pointing inward. When the cyanophenyl substituents are located 90š apart on the porphyrin, they "link hands" to form a tetrameric cluster. And when the cyanophenyl groups are on opposite sides of the porphyrin square, they link to form a linear structure--a straight wire. The STM images reveal that these wires can be more than 100 nm long.

IN PRINCIPLE, such supramolecular structures should have useful electronic and optoelectronic properties, the researchers note. Yokoyama says they are preparing to investigate this question.

In a commentary on the work in the same issue of Nature, chemistry professor Paul S. Weiss of Pennsylvania State University suggests that one of the next steps might be to add function to the assembled molecules. Because porphyrins can bind metal atoms at their centers, Weiss says, one could choose metal atoms that would allow additional ligands to be linked across the top of the surface assembly, like a cable connecting a series of telephone poles. Such a superstructure might be more suitable for, say, electrical conduction than the surface assembly supporting it.

"Much work remains to make such functional assemblies a reality," Weiss notes. Nevertheless, he's hopeful that the "very neat idea" outlined by Yokoyama and coworkers may allow the supramolecular assemblies to be connected to the outside world and to be used as a "platform on which to construct and align more complex structures."

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