Nov. 17, 1997 ANN ARBOR---Imagine your roof covered with a thin film of organic molecules busily converting sunlight into electricity. Visualize tiny molecular flashlights illuminating the DNA of living cells. Picture microscopic optical sensors that change color when exposed to trace amounts of chemicals.
Science fiction? Scientists at the University of Michigan and the University of Illinois at Urbana-Champaign don't think so. They have developed a new class of large dendrimer supermolecules which, they say, could one day be used for all these applications and more. "Normally, light energy disperses randomly throughout a molecule," said Raoul Kopelman, the U-M's Kasimir Fajans Professor of Chemistry, Physics and Applied Physics. "But these molecules have a specific tree-like structure which allows them to funnel light energy through the branches and direct it to a central point."
When photons of ultraviolet light hit a group of light-harvesting atoms on a branch of one of these supermolecules, the absorbed energy travels down the branch in the form of energy packets called excitons. Losing a small amount of energy at each branching point, excitons keep falling toward the center of the molecular tree until they finally drop, one at a time, into a molecular "trap," which is attached to the dendrimer's center. In the "nanostar"---the most optimally designed version of these dendrimers to be developed so far---photosensitive molecules in the trap convert exciton energy back into visible light with up to 99 percent efficiency.
"It works like a miniature quantum well in a semiconducting circuit," said Stephen F. Swallen, U-M postdoctoral fellow in chemistry. "The excitons don't have the extra energy to climb back up the molecule, so they just keep falling into the trap."
Synthesized from repeating molecular units called phenylacetylene monomers, which branch out from a central core, dendrimers are among the largest structurally controlled organic molecule ever created, according to Jeffrey S. Moore, professor of chemistry at the University of Illinois at Urbana-Champaign. The biggest molecule they have synthesized so far contains 127 chromophores or light-harvesting units.
Each dendrimer is custom-made by Moore and his colleagues to Kopelman's specifications to produce different chemical and physical properties for different applications. One of the most significant properties of the new molecules is their ability to resist photobleaching. "Anyone who has ever had a sweater fade or disintegrate after exposure to sunlight has experienced photobleaching," Kopelman said. "Molecules can only absorb and emit photons a limited number of times before they fall apart. Photobleaching is a particularly important factor for these dendrimers, because they interact with light very strongly."
Their specific chemical composition and physical structure make it possible for the dendrimers to resist photobleaching, according to Swallen. "While most organic molecules will decompose if multiple excitons are concentrated at the same spot, the nanostar can protect itself by diverting some excess energy away from the center back to the outer parts of the dendrimer," he explained. "Because the molecule is never hit with more energy than it can handle, it lasts much longer than ordinary molecules when exposed to light."
Research funding for the project is provided by the National Science Foundation and the Office of Naval Research. Collaborators included Michael R. Shortreed of Iowa State University, Zhong-You Shi of the University of Michigan; Weihong Tan of the University of Florida, Gainesville; Zhifu Xu of PPG Industries; and Chelladurai Devadoss and Pamidighantam Bharathi from the University of Illinois, Urbana-Champaign.
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