Electric Fields Act as Catalyst

A joint effort by researchers at the University of Barcelona (UB), Spain; the Australian National University, Canberra; and the University of Wollongong, Australia, has found a way to use oriented electric fields to catalyze a fundamental chemical reaction.

The researchers believe their efforts will allow the fabrication of a variety of important chemical compounds faster and cheaper than via current technology.

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At the heart of the work is the Diels-Alder reaction. First described in the early 1920s, it still is widely used in synthetic organic chemistry to create substituted cyclohexene systems. The reaction between a conjugated diene and a substituted alkene is popular because of the control it gives over the formation of cyclohexene systems with specific stereochemical properties.

The researchers applied an oriented electric field between two nanoelectrodes containing the reacting molecules. This novel nanochemical synthesis approach involves joining individual molecules to create new molecular backbones — a process the researchers describe as snapping Lego pieces together — and then synthesizing challenging chemical compounds.

Figure 1. Using a nanoscale junction and an electric field could revolutionize the way important chemical compounds are synthesized. Source: University of Barcelona.

“While it is often thought that the ability to control reaction rates with an applied electrical potential gradient is unique to redox systems, recent theoretical studies suggest that oriented electric fields could affect the outcomes of a range of chemical reactions, regardless of whether a redox system is involved,” write the researchers in a recent issue of Nature.

This possibility arises because minor charge-separated resonance contributors can stabilize many formally covalent species. When an applied electric field is aligned to electrostatically stabilize one of these minor forms, the degree of resonance increases, resulting in the overall stabilization of the molecule or transition state. This means it should be possible to manipulate the kinetics and thermodynamics of non-redox processes using an external electric field — as long as the orientation of the approaching reactants with respect to the field stimulus can be controlled, they note.

“We’ve provided experimental evidence for this for the first time,” says study leader Ismael Díez-Pérez, assistant professor at UB and senior researcher at the Institute for Bioengineering of Catalonia, Barcelona.

The researchers designed a surface model system to probe the Diels–Alder reaction and coupled it with a scanning tunneling microscopy (STM) break-junction approach. This technique, performed at the single-molecule level, is perfectly suited to deliver an electric-field stimulus across approaching reactants, they say.

The team found a five-fold increase in the frequency of formation of single-molecule junctions resulting from the reaction that occurs when the electric field is present and aligned to favor electron flow from the dienophile to the diene. The results are qualitatively consistent with those predicted by quantum-chemical calculations in a theoretical model of this system, and herald a new approach to chemical catalysis, note the authors.

Although electrostatic catalysis is the least-developed form of catalysis in synthetic chemistry, the researchers believe their modified STM approach is a leap forward because it allows them to record the direct signatures of individual molecules reacting.

“By controlling the orientation of the molecules with respect to the electric field, we accelerated a non-redox reaction for the first time,” explains Díez-Pérez.

“Using external electric fields as the ‘catalyst’ in this way means that challenging chemical reactions can be achieved that otherwise might not be possible by classical synthetic methods,” adds Nadim Darwish, a Marie Curie research fellow at UB. “It is this that opens the door to the synthesis of important chemical compounds,” he adds.

The group also is focused on developing nanoscale tools to study biological systems. Using proximity probe tools such as electrochemical tunneling microscopy and spectroscopy, they hope to develop biosensors and other molecular electronics devices to investigate electron transfer in metal oxides and individual redox proteins. In addition, they are working on light-activated molecular actuators that optically control the activity of biomolecules. The group also has engineered several bioactive compounds that can be regulated by light. These are aimed at targets involved in the development of neurodegenerative disorders.