A major feature of advanced nanotechnology will be precise mechanical control of how molecules and molecular fragments react, thus forming a desired product and eliminating the possible occurrence of unwanted side reactions. Earlier this year we cited an advance in which mechanical constraint of two molecules positioned on a surface produced a reaction that would not have occurred were the molecules free to move in solution. In further development of the basic science on which advanced positional mechanosynthesis will be based, scientists have now demonstrated that mechanical force, in this case pulling caused by ultrasound, can cause unique reactions that cannot be made to occur using non-specific means such as heat and light. From an article by Jon Cartwright in Physics World “‘Tug-of-war’ prompts chemical reaction“:
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.
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.
The above research was published in the Sept. 16 issue of Science (“Unclicking the Click: Mechanically Facilitated 1,3-Dipolar Cycloreversions”, abstract). An accompanying commentary notes “By pushing, or in this case literally pulling, the reactions down different pathways, they explore novel concepts for synthesizing organic molecules.” For practical molecular manufacturing applications, some much more precise method of focusing mechanical force than is possible with ultrasound will be needed. Perhaps methods can be developed using scanning probe microscopes or molecular machine systems.