An Artificial Light Harvesting Complex
Plants collect sunlight with large molecular complexes that transmit it to reaction centers. As a result, each center has a much larger effective area than corresponds to its geometry. Chemists have developed an artificial collection antenna that works in a similar way and can transport the energy from ultraviolet light in a directed manner. Scientists from the University of Michigan and the University of Illinois have developed a new class of large branched supermolecules that they believe could one day be used for a variety of applications. They cite the conversion of sunlight into electrical energy, the selective illumination of cell components such as DNA, and microscopic optical sensors as examples.
"Normally, light energy is scattered at random by a molecule," said Raoul Kopelman, professor of chemistry, physics and applied physics at the University of Michigan. "But these molecules have a defined tree-like structure that allows them to funnel light energy through the 'branches' and direct it to a central point."
When ultraviolet light falls on a matching group of the supermolecule, the absorbed energy travels down the branch in the form of packets of energy (called excitons). The excitons lose a small amount of energy at each branch point, falling further and further towards the center of the molecular tree until they finally, one by one, end up in a molecular "trap" located at the center of the tree structure. In the most effective variant of the molecule to date, the nanostar, light-sensitive molecules in this center convert the energy back into visible light with an efficiency of up to 99 percent.
"It works like a potential well…" explained Stephen F. Swallen, a chemist at the University of Michigan. “The excitons don't have the extra energy to travel back up the molecule; so they just keep falling into the trap.”
The supermolecules are made up of phenylacetylene monomers that branch out from a central region. They are among the largest organic molecules whose structure can be controlled during synthesis. The largest molecule synthesized to date contains 127 chromophores, or light-capturing moieties.
Each tree structure is manufactured by Moore and his colleagues exactly 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 that has faded or dissolved from exposure to the sun has experienced the effects of photobleaching," Kopelman said."Molecules cannot absorb and emit photons any number of times." After a few cycles, they decay.
According to Swallen, the molecules are able to resist photobleaching due to their chemical composition and physical structure. "Whereas most organic molecules decompose when multiple excitons are concentrated in the same place, the nanostar can protect itself by redirecting some excess energy away from the center to the outer parts of the tree," he explained. "Because the molecule is never hit by more energy than it can handle, it can last much longer than ordinary molecules when exposed to light."
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