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Simulation paves the way for the discovery of conformation-driven quantum interference

Imagine an electronic component that can translate the tiniest mechanical input into an exponential change in conductance. This amazing capability could be provided by molecular electronic components exhibiting conformation-dependent quantum interference, i.e. interference triggered by changes in conformation of the molecular building blocks of the electronic component.

Using a combination of experiment and simulation, researchers from the University of Groningen and the University of Heidelberg discovered the first instance of such conformation-dependent quantum interference in tunneling junctions comprising π-conjugated molecules. Note that interference in this case occurs through space between two π-systems which are close enough to interact and not 'simply' through bond.

For many reasons, this effect has escaped experimental detection in the past. In fact, quantum interference in tunneling junctions is by itself already difficult to detect. This is due to the fact that the magnitude of the current density is not only sensitive to quantum interference but to a myriad of other factors as well. A mere drop in magnitude of the current density is an indication but no proof for destructive quantum interference. Transport measurements alone can therefore not proof the existence of quantum interference. The experimental challenge is increased through the conformation-dependence of the effect under investigation. The very fact that through-space quantum interference is so highly sensitive to molecular conformation has confounded experimental detection in the past, leading to a range of inconsistent observations.

These obstacles have been overcome through careful experimental setup using tunneling junctions comprising self-assembled monolayers of the π-conjugated molecules and a combination of tunneling charge-transport measurements with molecular modeling. In particular, density functional theory simulations have allowed to assign the experimental signatures to destructive quantum interference.