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Porphyins turned into molecular switches

Posted: 14 Dec 2011     Print Version  Bookmark and Share

Keywords:switch  electronics  research  proton 

Physicists demonstrate a one-square-nanometre four-state porphyrin ring switch that can be changed from one state to another up to 500 times per second. The demonstration, led by Willi Auwaerter and Johannes Barth of the Technische Universitaet Muenchen (TUM), is published in the journal Nature Nanotechnology.

The switch is a Tetraphenylporphyrin ring with an inside diameter of less than half a nanometre. Its functionality comes from the position of a single proton in the ring which can be set in four distinct states as needed. The tetraphenylporphyrin rings takes on a saddle shape and is not limited in its functionality when it is anchored to a metal surface. The molecule holds has a pair of hydrogen atoms that can change their positions between two configurations each. At room temperature this process takes place continuously at an extremely rapid rate.

In their experiment, the scientists suppressed this spontaneous movement by cooling the sample. This allowed them to induce and observe the entire process in a single molecule using a scanning tunnelling microscope. This kind of microscope is particularly well suited for the task since—in contrast to other methods—it can be used not only to determine the initial and final states, but also allows the physicists to control the hydrogen atoms directly. In a further step they removed one of the two protons from the inside of the porphyrin ring. The remaining proton could now take on any one of four positions. A tiny current that flows through the fine tip of the microscope stimulates the proton transfer, setting a specific configuration in the process.

Although the respective positions of the hydrogen atoms influence neither the basic structure of the molecule nor its bond to the metallic surface, the states are not identical. This small but significant difference, taken together with the fact that the process can be arbitrarily repeated, forms the basis of a switch whose state can be changed up to 500 times per second. A single tunnelled electron initiates the proton transfer.

The molecular switch has a surface area of only one square nanometre, making it the smallest switch implemented to date. The physicists are thrilled by their demonstration and are also very happy about new insights into the mechanism behind the proton transfer resulting from their study.

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