Single molecule reaction switched back and forth


Rebecca Pool

Tuesday, January 26, 2016 - 13:00
Image: IBM scientists trigger and observe reactions in an individual molecule.
Using scanning probe microscopy, researchers from IBM and the University of Santiago de Compostela have demonstrated reversible Bergman cyclisation for the first time.
By pulsing the applied voltage at the microscope tip, researchers could cycle the reaction by first cleaving a carbon-carbon bond in a molecule and then reforming the broken bond to reverse the reaction.
A reversible Bergman cyclisation action in a single molecule sitting on an ultra thin NaCl film can be directly imaged using atomic force microscopy. [IBM]
This fascinating molecular rearrangement reaction was first described in 1972 by American chemist Robert George Bergman, and is now known to form the basis of how some anticancer drugs react.
To induce and image the Bergman cyclisation, researchers used a home-built low temperature ultrahigh vacuum combined STM and AFM.
Reaction intermediates were first stabilised on an ultrathin NaCl film, used as the substrate, with the microscope tip used to trigger and reverse the reaction.
To image the reaction, researchers modified the tip of their AFM with a single carbon monoxide molecule, a technique first developed by Gerhard Meyer and Leo Gross, IBM, in 2009 to enhance imaging resolution and observe pentacene molecules.
“One main differentiator of our technique, with respect to other established techniques, is that we measure single molecules," highlights Gross. "Another advantage is that we can use the tip to initiate chemical reactions of individual molecules and we can follow the reactions and 
study their products at the atomic scale.”
As Gross highlights: “Working at low temperatures and on inert surfaces such as [these] two-atom-thick layers of salt, we are able to stabilize reactive intermediates that under normal conditions are too short-lived to be studied in detail."
"Not only can we form highly reactive intermediates using the tip to create and cleave bonds within the molecule, we can even switch between different reaction intermediates," he adds. "Remarkably, we can change almost all important properties of these molecules by switching them, affecting their reactivity, structure and their optical, electronic and magnetic behavior.”
The researchers switched between reactive intermediates by means of selective C-C bond formation or cleavage, opening up the field of radical chemistry for on-surface reactions by atomic manipulation.
Video: Studying a fascinating molecular rearrangement reaction known as a Bergman cyclisation.
“This work suggests the great potential of this technique to discover unexpected new reactions," says Gross's colleague, Professor Diego Peña from the University of Santiago de Compostela.
"In conventional solution chemistry, after so many decades of rigorous research, the chances of finding new important reactions are quite limited," he adds. "Compared to this, single-molecule chemistry by tip manipulation is in its infancy, and I expect extremely exciting discoveries in the near future.”
The next steps for the team will be to synthesize large custom-designed molecules and molecular networks with the tip that cannot be made by any other means.
The team is also interested in exploring new applications for molecules, such as molecular logic devices based on single-electron transfer.
Research is published in Nature Chemistry.
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