Targeted proton transfer within a molecule: Smallest conceivable switch

 For a long time miniaturization has been the magic word in electronics. Dr. Willi Auwaerter and Professor Johannes Barth, together with their team of physicists at the Technische Universitaet Muenchen (TUM), have now presented a novel molecular switch in the journal Nature Nanotechnology. Decisive for the functionality of the switch is the position of a single proton in a porphyrin ring with an inside diameter of less than half a nanometer. The physicists can set four distinct states on demand.

Porphyins are ring-shaped molecules that can flexibly change their structure, making them useful for a wide array of applications. Tetraphenylporphyrin is no exception: It likes to take 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 tunneling 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 tunneled electron initiates the proton transfer.

The molecular switch has a surface area of only one square nanometer, 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. Knud Seufert played a key role with his experiments: "To operate a four-state switch by moving a single proton within a molecule is really fascinating and represents a true step forward in nano-scale technologies."

This research was funded by the European Research Council (ERC Advanced Grant MolArt, No. 247299), the Excellence Cluster Munich-Centre for Advanced Photonics (MAP) and the Institute for Advanced Study of the TU Muenchen.

Story Source:

The above story is reprinted from materials provided by Technische Universitaet Muenchen.

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Journal Reference:

Willi Auwärter, Knud Seufert, Felix Bischoff, David Ecija, Saranyan Vijayaraghavan, Sushobhan Joshi, Florian Klappenberger, Niveditha Samudrala, Johannes V. Barth. A surface-anchored molecular four-level conductance switch based on single proton transfer. Nature Nanotechnology, 2011; DOI: 10.1038/NNANO.2011.211

Positioning and pinching slow proton movement in catalyst

Twisting and pinching slow a catalyst's ability to generate energy from hydrogen, according to scientists at Pacific Northwest National Laboratory's Center for Molecular Electrocatalysis. In converting hydrogen to electricity, the catalyst uses claw-like extensions, called ligands, to get needed items in place. However, the ligands opening and closing during the reaction can't be speeded up, making it the rate-limiting step; the ligands' tendency to grasp the protons a bit too tightly can also slow the reaction.

Why It Matters: Reducing our nation's reliance on fossil fuels relies on creating technologies to quickly and efficiently convert the power generated at wind farms and other sources into a use-any-time fuel, such as hydrogen molecules.  Scientists are striving to design affordable, abundant catalysts that turn energy into hydrogen and then release the energy when desired. This study, which appears in the Journal of the American Chemical Society, answers important questions about how the chemical energy in the hydrogen would be converted into electrical energy.

The team examined a nickel-centered catalyst with two ligands, six-sided molecular bits that extend off the edge of the catalyst and resemble crab claws. The specific catalyst is [Ni(PCy2NBn2H)2]2+ (PCy2NBn2 = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane).

The catalyst's ligands snag a hydrogen molecule and move it to nickel at the heart of the catalyst. The nickel cracks the molecular hydrogen into 2 electrons and 2 protons. The electrons remain at the center and are then free to do work, just like the electrons generated by fossil fuel combustion. The ligand picks up the protons using a pendant amine, a small string of atoms containing nitrogen that dangle off the ligand.

During this process, the ligand twists between two different conformations, known as "boat" and "chair." Changing between boat and chair is the rate-limiting step in the reaction, meaning that the reaction can only go as fast as this change occurs. This proved to be an unexpected result for the team.

But, there were other surprises to be found in the data as well. When the ligand picks up the protons it uses the same pendant amine that was involved in taking the hydrogen to the center.

"Intuitively, I would have expected the molecules to completely scramble the protons," said Dr. Wendy Shaw, recipient of the U.S. Department of Energy's Early Career Award and physical chemist on this study. "That the protons moved on the same ligand every time is really interesting and could have implications for future catalysts."

Further, the team found that the pendant amine isn't always ready to give up the protons. Under certain conditions, the molecule holds on tightly to the protons, pinching them. The amines can pinch the protons so hard that they can't move.

In this study, the researchers used a complementary experimental and theoretical approach. The experimental scientists determined the conformation of the catalyst and the rates of reaction using nuclear magnetic resonance spectroscopy experiments in PNNL's Physical Sciences Laboratory. The theorists described the proton movement using complex calculations, including density functional theory and molecular dynamics simulations. The theoreticians used supercomputers at two user facilities. The team used resources at EMSL, Environmental Molecular Sciences Laboratory, and NERSC, the National Energy Research Scientific Computing Center.

"This paper is an example of what a research team can do when they are working closely together," said Dr. Morris Bullock, Director of the Center for Molecular Electrocatalysis at PNNL. "Together the experimentalists and theorists really answer the question of how the protons move."

The experimentalists and theorists, together in the Center for Molecular Electrocatalysis, are using this research as they tackle two new challenges in proton movement. First, they are conducting a more detailed examination of proton pinching. Second, the team is studying how pendant amines take the next step, moving the proton from the edge of the molecule to the environment beyond. This relay hand-off is critical for fuel cells and other alternative energy technologies.

"What we found has really helped us think about how to design new catalysts," said Shaw.

More information: O'Hagan M, WJ Shaw, S Raugei, S Chen, JY Yang, UJ Kilgore, DL DuBois, and R Bullock. 2011. "Moving Protons with Pendant Amines: Proton Mobility in a Nickel Catalyst for Oxidation of Hydrogen." Journal of the American Chemical Society. Article ASAP.

Provided by Pacific Northwest National Laboratory (news : web)