Breakthrough harnesses light for controlled chemical reaction

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When chemist Tehshik Yoon looks out his office window, he sees a source of energy to drive chemical reactions. Plants “learned” to synthesize chemicals with sunlight eons ago; Yoon came to the field a bit more recently.But this week, in the journal Science, he and three collaborators detail a way to use sunlight and two catalysts to create molecules that are difficult to make with conventional techniques.In chemistry, heat and ultraviolet (UV) light are commonly used to drive reactions. Although light can power reactions that heat cannot, UV has disadvantages, says Yoon, a chemistry professor at University of Wisconsin-Madison. The UV often used in industry carries so much energy that “it’s dangerous to use, unselective, and prone to making unwanted by-products.”Many chemicals exist in two forms that are mirror images of each other, and Yoon is interested in reactions that make only one of those images.”It’s like your hands,” Yoon says. “They are similar, but not identical; a left-hand glove does not fit the right hand. It’s the same way with molecules in biology; many fail unless they have the correct ‘handedness,’ or ‘chirality.’”The pharmaceutical industry, in particular, is concerned about controlling chirality in drugs, but making those shapes is a hit-or-miss proposition with UV light, Yoon says.He says the new technique answers a question posed by a French chemist in 1874, who suggested using light to make products with controlled chirality. “Chemists could never do that efficiently, and so the prejudice was that it was too difficult to do.”When a graduate student asked for a challenging project seven years ago, Yoon asked him to explore powering reactions compounds with metals that are used to capture the sun’s energy in solar cells. In a solar cell, these metals release electrons to make electricity.”We are taking the electrons that these metals spin out and using their energy to promote a chemical reaction,” Yoon says.Plants do the same thing during photosynthesis, he says: absorb light, release high-energy electrons, and use those electrons to bond water and carbon dioxide into sugars. That reaction is the basis of essentially all of agriculture and all food chains.Once the solar-cell metal supplied electrons, Yoon thought about using a second catalyst to control chirality. He passed the project to Juana Du, another graduate student.”She must have synthesized 70 different catalysts,” says Yoon. …
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