Surfaces help quantum switches

The quantum dynamics of hydrogen are central to many problems in nature, being strongly influenced by the environment in which a reaction occurs. In their contribution to PRL, members of the MPSD’s Lise Meitner group tackle the transfer of hydrogen within a supported molecular switch, demonstrating that surface support can play a decisive role in the tunneling reaction.

In molecule-based nanotechnology, individual molecules become the building blocks of electronic devices. The enormous variety of possible molecular architectures and the ability to precisely manipulate molecular synthesis opens the door to infinite functional components. However, the main challenge is to gain control over these functions at the nanoscale, where the effects of quantum mechanics become important.

The porphycene molecule is an example of a molecular switch prototype. Porphyrene is a structural isomer of porphyrin with strong H bonds in its internal cavity. Its switching capacity is based on a fundamental reaction in chemical physics: a double transfer of hydrogen that can swap the positions of the hydrogens in the internal cavity and thus define different states of the molecule (on / off), a process called tautomerization.

To control and measure the atomic structure and switching rate of these molecular units, they are typically immobilized by placing them in contact with metal surfaces. This situation requires understanding the dynamics of hydrogen within an environment that includes qualitatively different types of interactions between atoms within the molecule and between the molecule and the surface.

In this context, porphycene has been extensively studied using single molecule experimental techniques. The researchers observed several puzzling aspects of the tautomerization rate over different temperature ranges, including temperatures at which atoms no longer behave like classical particles, but can instead cross barriers. Using a mountain as an analogy, the atoms would instantly travel between two valleys in a straight line under the mountain, instead of taking the time to go up and down it.

In their new work just released in PRL, Yair Litman and Mariana Rossi tackle this molecular switch supported with cutting-edge methodology and novel computer algorithms: a combination of density functional theory with ring-polymer instantons. These methods have finally made it possible to study such systems with large-scale atomistic simulations that treat both electrons and nuclei as quantum mechanical particles. The authors show that for porphycene adsorbed on the surfaces of Cu (110) and Ag (110), the hydrogen transfer reaction actually presents a large contribution from nuclear tunneling even at temperatures not much below ambient temperature.

Surprisingly enough, the authors found that as the temperature drops, heavy surface atoms such as copper participate in the tunneling reaction of intramolecular hydrogen and can cause an increase in tunneling speed of up to two orders of magnitude at a temperature of about 80 K. The stronger the interaction of the molecule with the surface (hybridization of electron orbitals), the more pronounced the participation of the surface atoms in the tunneling event.

Notably, the authors also explained an unconventional temperature dependence of the tunneling speed, which was previously observed in experiments. It is caused by the existence of an intermediate metastable structure in the reaction, which exists for such a short period of time (~ 100 picoseconds, with a picosecond equal to one trillionth of a second) that it could not be detected by the experimental techniques previously employed in this system. By understanding this mechanism, the authors could also explain different rate temperature dependence regimes in the tunneling regime and propose a simple model to predict this temperature dependence for this switch adsorbed on other metal surfaces.

These are important new insights that certain characteristics of the surface support can influence the quantum mechanical nuclear properties of the switching reaction in these and possibly other molecules. They also demonstrate that monocrystalline substrates are an ideal platform where cutting-edge theory and experiment can come together to provide a deeper understanding of nuclear quantum dynamics in complex environments. These results are of considerable fundamental importance and may also guide the design and interpretation of experimental architectures in the development of molecular nanotechnology.