What is molecular structure
Molecular quantum switches use hydrogen tunnels with the help of the substrate
The quantum dynamics of hydrogen is central to many problems in nature because it is strongly influenced by its environment. In a recently published article in the JournalPRL Members of the Lise Meitner group at the MPSD deal with hydrogen transfer within an adsorbed molecular switch and show that the substrate surface can play a decisive role in the tunneling reaction.
The development of miniaturized technological components makes molecular-based nanotechnology a constantly growing area of interest. This is where individual molecules become the basic components of electronic devices. The sheer variety of possible molecular architectures and the ability to precisely control molecular synthesis open the door to endless functional components. The biggest challenge, however, is gaining control of these functions at the nano-scale, where quantum mechanical effects come to the fore.
The porphycene molecule is an example of a prototype molecular switch. Porphycene is a structural isomer of porphyrin with strong H bonds in its internal cavity. Its switching ability is based on a reaction that is very fundamental in chemical physics: a double hydrogen transfer, which can swap the positions of the hydrogen atoms in the inner cavity and thus define different states (on / off) of the molecule. This process is known as tautomerization. In order to control and measure the atomic structure and the switching speed of these molecular units, they are typically immobilized by bringing them into contact with metal surfaces. Therefore, researchers need to understand hydrogen dynamics in an environment where the interaction between the atoms within the molecule is qualitatively different from that between the molecule and the surface.
In this context, porphycene has been extensively studied using single molecule experimental techniques. Researchers have observed several puzzling aspects of the rate of tautomerization over different temperature ranges. At certain temperatures, the atoms no longer behave like classical particles, but can instead tunnel through barriers. In analogy to a mountain, the atoms would immediately shoot between two valleys in a straight line under the mountain instead of covering the longer distance over the mountain and back down again.
In her new job, now inPRL published, Yair Litman and Mariana Rossi are investigating this assisted molecular switch using state-of-the-art methodology and new computer algorithms: a combination of density functional theory and ring polymer instantons. This made it possible to study such systems with atomistic simulations on a large scale, which treat both electrons and nuclei as quantum mechanical particles. The authors show that for porphycene adsorbed on Cu (110) and Ag (110) surfaces, the hydrogen transfer reaction is indeed a large contribution from nuclear tunneling, even at temperatures not far below room temperature.
Surprisingly, Litman and Rossi discovered that when the temperature drops, heavy surface atoms such as copper participate in the intramolecular hydrogen tunneling reaction and, at a temperature of around 80 K, can increase the tunneling rate by up to two orders of magnitude. The stronger the interaction of the molecule with the surface (hybridization of electronic orbitals), the more pronounced the participation of the surface atoms in the tunnel event.
It is noteworthy that the authors also explain an unconventional temperature dependence of the tunnel rate, which was previously observed in experiments. It is caused by an intermediate metastable structure in the reaction that only exists for an ultra-short period of ~ 100 picoseconds (a picosecond is one trillionth of a second), so it could not be detected with the experimental techniques used to date.
With these new findings, Litman and Rossi were able to explain different temperature dependence regimes of the rate in the tunneling regime. From this they in turn developed a simple model for predicting the temperature dependence for other metal surfaces on which the switch is adsorbed.
The authors provide important new insights into the fact that certain properties of the surface support can influence the atomic quantum mechanical properties of the switching reaction in these and probably also in other molecules. They also show that single crystal substrates offer an ideal platform on which state-of-the-art theory and experiment can work together to produce a deeper understanding of nuclear quantum dynamics in complex environments. Such fundamental insights can guide the design and interpretation of experimental architectures in molecular nanotechnology development.
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