Researchers in the US have shown that mechanical force can bring about unique chemical reactions. Their experiment involved pulling on molecules in solution using ultrasound and suggests that mechanics could open up other new reaction pathways in chemistry. The discovery could also lead to the development of new technologies such as force-activated sensors or reversible adhesives.
Chemists have several ways to help chemical reactions along, such as adding heat or light. But broad additions of energy such as these can bring about unwanted by-products and waste valuable reactants. In other cases, heat and light simply do not work.
It has been known for decades that mechanical force is another way of promoting reactions – a field known as “mechanochemistry”. If you chew a piece of rubber, for example, some of the material’s covalent bonds will break, forming shorter polymers. Chemists have also used mechanical force to select and promote certain reactions, such as opening molecular rings or changing molecular structures. What they have not been able to do is use mechanical force to effect a chemical reaction that could not be driven in any other way.
Stubborn reaction
It is this feat that has now been demonstrated by Christopher Bielawski and colleagues at the University of Texas at Austin. Bielawski’s group focused on a ring-shaped functional group known as triazole (C2H3N3), which is often used in the biological research and materials science. Triazole – specifically the isomer 1,2,3-triazole – is formed during the cycloaddition of an azide (the N3– functional group) and an alkyne (hydrocarbons with a carbon–carbon triple bond) in the presence of copper. Once formed, however, the triazole is unaffected by almost all thermal, chemical and light treatments.
The researchers begin with triazole and then attach polymer chains to either side of the individual molecules. The sample is then put in solution and ultrasound is applied. This causes tiny bubbles to grow and collapse, pulling on nearby polymer chains. According to the team, this generates a tensile force along the polymer backbones that reaches a maximum in the centres – exactly where the triazole molecules are located. The force distorts the bonds, say the researchers, allowing triazole to break into its constituent azide and alkyne.
“The reported reaction [triazole into an azide and alkyne] is one of the very few transformations that is promoted only by mechanical force – the reactivity we describe cannot be achieved using other stimuli, such as heat or light,” says Bielawski.
Bielawski believes his group’s demonstration highlights how materials composed of triazoles could fail under certain mechanical stresses. But he also thinks the work could have practical applications. Biologists who already use triazoles to label biomolecules could now remove those labels, for instance. Meanwhile, physicists could help chemists explore the role mechanics plays in chemical bonding, deepening our understanding of chemical dynamics and, potentially, leading to discoveries of new chemical transformations.
Like “un-pouring concrete”
“It’s the chemical equivalent of un-pouring concrete once it is set,” says Stephen Craig, a chemist at Duke University in North Carolina, US. Craig believes that, in the construction of complex molecules, it might be possible to use mechanical force to protect certain functional groups by temporarily turning them into non-reactive groups, such as triazole. “It is important to protect valuable reactive groups by rendering them unreactive in the early stages of construction, so that they survive until they are needed in the late stages,” he adds.
Nancy Sottos, a materials scientist at the University of Illinois at Urbana-Champaign, US, calls the work “very exciting”, and thinks it could herald applications such as force-activated sensors. “Extrapolating into the future, it could provide a new platform for force-responsive materials,” she says. “Far-reaching possibilities might include polymers with reversible adhesion to a particular surface.”
The research is published in Science 333 1606.
