By tugging on two sides of a specially designed molecule, chemists have been able to change its shape so that it becomes much more reactive. The researchers were able to control the reactivity of the molecule by applying a mechanical force on its chemical bonds. The energy for such a chemical transformation typically comes from light, heat, or electricity. “The key thing is that force will trigger the molecule to become reactive … and that [reactive state] would go on to do useful and productive chemistry,” says Jeffrey Moore, a chemistry and materials-science professor at the University of Illinois at Urbana-Champaign who published the work in Nature.
The finding could lead to self-healing materials, in which molecules under stress would change shape and react to make the material stronger. Another use would be polymers that react and light up right when they are damaged. “Say you’re a parachutist and you want to know whether the cords are still adequate; you could just get a visual read on them,” Moore says. The development could also lead to a new way of doing chemistry by using force instead of heat, light, or catalysts.
The idea of using force to make a molecule reactive has existed in organic chemistry since the 1940s, but until now it essentially boiled down to breaking long plastic polymers into smaller pieces. The process has not been very useful because no one has been able to control where the polymer breaks.
Moore and his colleagues were able to control the structure of a ring-shaped organic molecule, not just break it apart. The researchers attach polymer chains to the two sides of the molecule. Then they apply ultrasound frequency to a solution containing the molecule; the ultrasound creates a mechanical force along the polymer chains and pulls them in opposite directions. The chains tug at the molecule and break a chemical bond in the ring, which triggers a more extensive rearrangement of the molecule and makes it reactive.
Moreover, the researchers found that force rearranges the molecule in a way that is different than if the molecule was exposed to other triggers, like heat and light. “One of the really beautiful, unexpected things about this work is that the reaction that occurs … is not the reaction that you would expect to happen,” says Stephen Craig, a chemistry professor at Duke University. “They pull on one molecule and make it rearrange into something that normally there is no way to rearrange into with heat or light.”
Once the molecule is in this new arrangement, it reacts with another molecule that can be detected with ultraviolet light. Moore says that this concept could be used to make polymers that would give some kind of visual cue when they are just about to break. Or, he says, researchers could further develop the principle and make materials in which, if researchers apply pressure at a certain point, the molecules in that area become reactive and stick together–a novel strategy for creating self-healing materials.
Craig contrasts the new work with the old-fashioned use of force in chemistry, which was, essentially, to break polymers into smaller fragments. What makes the new work exciting, he says, is that it shows that researchers can use force to make molecules come together and create bigger molecules. “As a long-term vision, one can imagine materials that get stronger, rather than weaker, as they experience greater and greater forces,” Craig says.
Others think that this might be the start of a new way of doing chemistry. “Conceptually, what it demonstrates is that stress can become a new tool to do organic chemistry,” says Virgil Percec, a chemistry professor at the University of Pennsylvania. “It’s a very elegant and beautiful demonstration.” By controlling chemical reactions using small mechanical forces on molecules, “we might be able to eliminate the need of environmentally unfriendly catalysts to do various reactions,” he says.
Next, says Moore, his team plans to find out whether the concept holds in solids and not just in polymer solutions.
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