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New molecules harness the energy of light.

For Earth, the ultimate source of energy is the sun, which steadily showers this blue-green planet with energy-rich rays. Nature has evolved numerous systems for harnessing that energy, green plants and bacteria representing notable examples.

Chemists who wish to synthesize molecules capable of capturing and using light's energy face a formidable task. Speaking at last week's meeting of the American Chemical Society in New Orleans, two researchers described newly designed molecules that can put light's energy to work.

Karen J. Brewer, a chemist at Virginia Polytechnic Institute and State University in Blacksburg and her colleagues have fabricated an inorganic light-harvesting molecule. The new material, which they call a supramolecular trimetallic complex, uses sunlight to collect electrical charges.

"These molecules are catalysts," Brewer says. Absorbing two photons apiece, they use the energy to store two electrons, which can then spark further chemical reactions-for example, splitting water molecules to release hydrogen or making fuels.

The method of transforming light energy into electrical potential borrows partly from green plants and partly from solar cells, Brewer says. In plants, chlorophyll molecules capture sunlight's energy and use it to break down carbon dioxide. Brewer says the new molecules also offer the potential to drive chemical changes, but they use inorganic materials more akin to those in solar cells than to those in plants.

While other groups have developed systems to store light energy in chemical form, most involve multiple molecules. In contrast, the new system represents the first inorganic synthetic molecule to use light to "store two electrons in a single place," Brewer says. This capacity improves photochemical efficiency, perhaps enough to make the synthesis of new types of fuel economically feasible, she says.

"Conceptually, the process is straightforward, but it's tricky to pull off with inorganic chemistry," says Thomas J. Meyer, a chemist at the University of North Carolina at Chapel Hill.

While mimicking photosynthesis, the new reaction's chemical energy is redirected toward novel ends, Meyer says. "Instead of making a woody structure and green leaf, you end up with energy-rich chemicals."

Directing light's energy toward a different goal, chemist Gary B. Schuster of the Georgia Institute of Technology in Atlanta and his colleagues have made a new class of liquid crystal molecules whose state and appearance change when they are exposed to light of different polarizations. By setting off a chain reaction in a liquid crystal material, the new molecules "act as triggers," he says, and may serve as a new type of optical switch.

Once tripped optically, the molecules initiate a domino effect in the liquid crystals, leading potentially to a millionfold amplification, Schuster says. The new light-activated molecular switches might serve in three-dimensional arrays to store information. Potentially, this system could lead to rewritable, optically driven holographic memory devices.

By varying the polarization of light, one can "read, write, and erase information in this system," Schuster says. Such optical memories might eventually find their way into personal computers, videos, and music systems, increasing the data storage available in magnetic tape and compact discs. The new phase-changing material might also find a home in spatial light modulators and coatings for optical fibers now used in computing, telephone, and television systems, Schuster says.

"The beauty of light beams," he notes, "is that you can cross them without making a short circuit."
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Title Annotation:new molecules use light to produce work
Author:Lipkin, Richard
Publication:Science News
Date:Apr 6, 1996
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