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Paving the Way to Nanomachines
March 27, 2014   
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Just a single foreign atom located in the vicinity of a molecule can change the spatial arrangement of its atoms. In a spectacular experiment, a group of Polish researchers working as part of an international team was able to permanently change the positions of the nuclei of hydrogen atoms in a porphycene molecule by pushing a single copper atom toward and away from the molecule.

Asubatomic bit formed by two protons tunneling inside a simple organic molecule can be switched by pushing a single copper atom toward and away from the molecule. The experiment to demonstrate the phenomenon was carried out by a team of researchers from the Fritz Haber Institute of the Max Planck Society in Berlin, the University of Liverpool, and the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw. The experiment was reported in a paper published in Nature Chemistry magazine.

The researchers made use of the specific properties of the porphycene molecule. Porphycene (C20H14N4) is a porphyrin derivative. Chemical compounds belonging to this group occur naturally. They are found, for example, in human blood, where they are involved in reactions related to carrying oxygen. Their molecules have the form of planar carbon rings with hydrogen atoms outside and four nitrogen atoms inside, located in the corners of a tetragon.

In the center of a porphycene molecule, in an empty space surrounded by nitrogen atoms, there are two protons (nuclei of hydrogen atoms) that can move between the nitrogens. It is interesting that both protons are always displaced together. Research carried out for over a decade by a team led by Prof. Jacek Waluk from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw suggests that the movement of protons is not simply a displacement in space. The protons change their positions due to the quantum tunneling effect: in keeping with the uncertainty principle, they just disappear from one place and reappear in another.

In the Berlin laboratory, the porphycene molecules supplied by Waluk’s team were deposited individually onto the surface of a perfect copper crystal. The job was not easy and required the development of appropriate techniques—without them porphycene molecules tended to form groups (aggregates).

The next step were experiments in a vacuum and at very low temperature (5 K, which means five degrees above absolute zero). A single porphycene molecule lying on the copper substrate was observed with a scanning tunneling microscope. The instrument made it possible to record changes in the electron density of the molecule, and thus to monitor changes in its shape. The images obtained with this technique made it possible to determine the current positions of both protons. Therefore the researchers were able to observe the movement of atoms inside the molecule in the course of a chemical reaction.

“We were pretty much surprised to find that, after being deposited on the copper substrate, hydrogen ions in the porphycene molecule formed a configuration that had never been observed before, in spite of many years of research into this compound,” said Waluk. “Instead of being located in the opposite corners of the tetragon formed by the nitrogen atoms, both protons took positions next to each other. Quite surprisingly, we found a new porphycene tautomer.”

Using the tip of the scanning tunneling microscope, in subsequent attempts a single copper atom was moved closer to the porphycene molecule, from different sides. It turned out that, depending on the position of the copper atom, both protons in porphycene, moving between the nitrogen atoms, were located alternately on one side and then on the other side of the molecule. Thus, the porphycene molecule acted as a binary switch, controlled with a single copper atom only. A change in the position of the copper atom by less than a ten-billionth of a meter was sufficient to initiate the transition between the states.

The research shows that the vicinity of a molecule can substantially affect its physical and chemical properties. The results of the study show that, under certain conditions, the environment of molecules can be controlled with atomic precision. At the same time, the observed sensitivity to changes in the environment opens the way for developing methods for the regulation of processes occurring in single molecules.

“It seems likely that the molecule’s sensitivity to its vicinity, as discovered by us, is a common phenomenon in nature. The phenomenon can be exploited, for instance, in designing nanomachines processing information on a single-molecule level,” said Waluk.
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