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  • Molecular Self-Assembly Yields Giant Lattice

Photonics Spectra
Apr 2003
Gary Boas

A group of researchers at the University of Pennsylvania in Philadelphia and the University of Sheffield in the UK has fabricated a liquid crystal lattice that is one of the most complex structures ever produced through molecular self-assembly. The work contributes to the design of increasingly complex nanostructures for a wide range of applications.

Molecular self-assembly techniques take their cue from nature; for example, from the process that enables proteins and cells to arrange themselves organically in functionally beneficial ways. "The general concept is to synthesize highly branched -- thus, dendritic -- molecules that self-assemble and subsequently self-organize into various lattices," explained Martin Glodde, a postdoctoral researcher with Virgil Percec's group at the University of Pennsylvania.

Essentially, these molecules consist of three parts: the apex group, a functional group at the core of the molecule; a branching unit, based on benzoates or benzyl ethers; and aliphatic chains at the periphery of the structure. "These different parts of the molecules attract each other, and when two or more molecules come together, they are ordered in the way that these parts are next to each other," he said. For example, molecules that are flat and tapered organize themselves into cylindrical columns and form hexagonal, rectangular or oblique two-dimensional lattices.

In this case, however, the molecules were conical. As a result, 12 of the molecules self-assembled into 8500-atom spheres, which subsequently took on a liquid crystal phase and ordered themselves into a regular, repeating pattern. The building block of the three-dimensional lattice -- only the third discovered -- consisted of 30 spheres occupying a volume of almost 20 x 20 x 10 nm.

Researchers have produced a complex structure by molecular self-assembly. The building block of the structure is a 20 x 20 x 10-nm volume consisting of 30 spheres, each formed by 12 dendron molecules. A model of one of the spherical supermolecules is shown in the top right corner.

Besides being one of the most complex structures created by molecular self-assembly, the lattice assumed a structure heretofore unseen in an organic compound. "In this particular case," Glodde said, "[we] found by x-ray analysis at high temperature that these spheres self-organize into a lattice that is normally only found in cases of some metal oxides."

The phenomenon might find application in photonics. To control light in much the same way that computer chips control electrons, photonic crystals must be roughly the same size as the wavelength, but they also must display a precise structure that allows for predictability and reproducibility.

The researchers are the first to design molecules that self-assemble on the order of hundreds or thousands of nanometers, and with the necessary degree of precision.

"This is a considerable step toward self-assembled 3-D photonic crystals," said Goran Ungar of the University of Sheffield.

Other potential applications include drug delivery, adhesives, pesticides, composites, coatings and paints, photographic and imaging media, catalysis, microfabrication and microelectronics. The scientists envision many applications that involve encapsulation of materials, and they are investigating how molecular self-assembly might yield hollow spheres.

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