2016 July

Pushing a single-molecule switch

Monday, 11th July 2016Publication highlights
(Top) Single porphycene molecule imaged at 5 K with a scanning tunneling microscope. The white star in the left indicates the position where the force-induced switching can occur by bringing the tip to the molecule. (Bottom) Chemical structure of porphycene. The molecule switches between two states through intramolecular hydrogen-atom transfer as indicated by the curved arrows.

(Top) Single porphycene molecule imaged at 5 K with a scanning tunneling microscope. The white star in the left indicates the position where the force-induced switching can occur by bringing the tip to the molecule.
(Bottom) Chemical structure of porphycene. The molecule switches between two states through intramolecular hydrogen-atom transfer as indicated by the curved arrows.

One finds switches everywhere in our modern life, and everyone knows the force that is needed to push them, for example, to turn on a room light from a wall switch—the force from one’s finger is enough. But, how does one push and how much force does one need to apply if the switch were dramatically scaled down to the “Nano-world”, for example, to push a “single-molecule switch” at nano-meter (10-9 m) scale. This “extremely small” question is related not only to basic science but also to potential future technological application of molecular devices. Researchers at Fritz-Haber Institute of the Max-Planck Society, Berlin, together with colleagues in Poland (Warsaw), Spain (San Sebastian) and the UK (Liverpool), have succeeded in operating a single-molecule switch and measuring the force needed to activate it by using a state-of-the-art scanning probe microscope. They found that only a very tiny force, fraction of a nano-Newton (10-9 Newton), was required to switch a single molecule.

The researchers discovered that about 1 nm-size organic molecule (porphycene) attached on a metal surface can be switched with an atomically-sharp metallic tip of a scanning probe microscope (whose very apex has a single atom and sometimes called an “atomic-scale finger”). The molecule changes between two states through an intramolecular hydrogen atom transfer, a so-called tautomerization that is important in nature, which can be triggered by “pushing” the molecule with the sharp tip. The experiments could not only quantify the forces but also revealed that switching can be induced at a very specific position within a single molecule, with a spatial resolution of about 0.02 nm which is less than a typical chemical bond length (about 0.1 nm). Furthermore, the researchers found that the switching mechanism cannot be rationalized by a pure “mechanical” force because switching could not be induced even if a sufficient force was applied, when the tip apex was decorated by a single xenon atom—a chemically inert rare gas that lacks a chemical reactivity. This result indicates that the chemical force (interaction) between the tip apex atom and molecule plays a crucial role in the reaction.

(Top) Artwork of the experiment. (Bottom) Measured force curve during tip approach and retraction.

(Top) Artwork of the experiment.
(Bottom) Measured force curve during tip approach and retraction.

The research team in Spain and UK carried out extensive first principle calculations by using a supercomputer to elucidate the force-induced tautomerization mechanism. From the simulation the researchers found that the force-induced reaction resembles an activation step in a catalytic reaction rather than pure mechanical activation. The researchers believe that the results may also provide a microscopic insight into complex catalytic processes with a fresh perspective, leading to a new method to control chemistry at the atomic level.

Nanoscale molecular devices are a fascinating future application, in which individual functionalized molecules behave as an independent element based on their physical/chemical properties. The research team demonstrated a novel way to operate a molecular switch which should play a central role in such devices. The researcher at Fritz-Haber Institute, who conceived the experiment, believes that our approach will make it possible not only to operate various types of molecular switches, but also to construct “Rube Goldberg machine” from single molecules since the “atomic-scale finger” of a scanning tunneling microscope also allows us to manipulate single atoms and molecules one by one.

Contact address

Dr. Takashi Kumagai
Research group leader
Fritz-Haber Institute of the Max-Planck Society
Department of Physical Chemistry
Email: kuma@fhi-berlin.mpg.de
Web: http://www.fhi-berlin.mpg.de/pc/kumagai/
Tel.: +49 (0)30 8413 5110

Publication

Force-induced tautomerization in a single molecule
Ladenthin et al. Nature Chemistry (http://dx.doi.org/10.1038/nchem.2552)

Please read the Japanese version here.