Adsorbate Motors: Tricking Microscopic Reversibility on Surfaces

Project Information

AMOS

Grant agreement ID: 101097326

DOI

10.3030/101097326

EC signature date 17 May 2023

Start date 1 January 2024

End date 31 December 2029

Funded under

• European Research Council (ERC)

Total cost

€ 2 499 909,00

EU contribution

€ 2 499 909,00

Coordinated by

UNIVERSITAET GRAZ
Austria

Periodic Reporting for period 1 - AMOS (Adsorbate Motors: Tricking Microscopic Reversibility on Surfaces)

Reporting period: 2024-01-01 to 2026-06-30

Summary of the context and overall objectives of the project

Uni-directional motion, which is the preference of one direction of motion against all others (for instance only forward and no backward motion) is a fundamental property in our daily lives and no machine could work without it. At the atomic level however, the basic laws of chemistry speak against uni-directional motion of single atoms and molecules. In fact, the 100-years old ‘principle of microscopic reversibility’ postulates that in thermodynamic equilibrium the average rate of every process is equal to the average rate of its reverse process. In other words: If a molecule can move forward into a new potential energy minimum, it can also move backwards to where it started from. The AMOS project attempts to trick this principle and aims to control unidirectional motion on well-defined surfaces at the atomic scale. It is based on molecular motors that overcome microscopic reversibility by controlled modulation of the potential energy surface in space and time. This means that the energetic landscape, i.e. the total energy of the system as a function of the reaction coordinates as for instance the position or orientation of the molecule, is controlled with high precision. We will achieve this in this project in two ways: first, internally, with sophisticated molecular structures that force them to rotate in one direction only upon stimulation, so-called Feringa motors. Second, the potential energy surface will be modulated via the environment – specifically the surface on which the motors are adsorbed. This concept, which leads to ‘adsorbate motors’, does not require motor units incorporated into rather complicated chemical structures and therefore time-consuming synthetic procedures, but is based on the combination of molecules with a surface. If the parameters are chosen correctly, this can result in uni-directional motion upon modulation of the potential energy surface, for instance by internal proton transfer in the molecules.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To achieve not only detailed understanding about the mechanisms of molecular motors, but also optimize their functionality in view of upscaling, the chemical structure of these molecules will be tuned and their motor activity will be studied in dependence on their geometric and electronic structure. The latter is important as excited states play a key role in the molecular transformations. Understanding and controlling them in view of excitation energies and excited state lifetimes is a central challenge for further insight into such nanomachines. Specifically, we will change the chemical structure of the molecular motors via side groups, the first experiments are currently ongoing. These can not only modify the motor mechanism itself, e.g. by sterically changing energy barriers between different states, but also the molecular configuration when adsorbed on a surface. Furthermore, by adding side groups that do not sterically, but electronically affect the molecular structure, additional control over the motor mechanism can be achieved. Another important approach for the variation of molecular structures is the combination of several motors within the same molecule, either in small structures or in large polymers. Such assemblies are of great interest in view of cooperative effects where the nanomachine is more than just the sum of its (motor) parts. We have achieved the first results on molecule with multiple motor units incorporated in them. Voltage pulses from the STM tip can excite these molecules and lead to a change in their configuration. Beyond the molecular structures, the AMOS project will attempt to tune the environment of each individual molecule via the surface. Experiments with long-distance motion of molecules have revealed that atomic defects modulate the speed of a single molecule by orders of magnitude. Besides the option of intrinsic or extrinsic defects in and on a surface, this can be done via the surface orientation or the choice of material. We found that changes of the material have a profound effect on the motor function, due to the modified surface lattice constant. Moreover, the material choice defines the reactivity of the surface with respect to the motor molecules with great impact on molecular configurations, stabilities, adsorption energies, and electronic states lifetimes. We have obtained first results of motor molecules on ultrathin films on a metal surface, indicating substantial changes in the motor function.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In terms of energy input and molecular ‘fuel’, we use different stimuli to trigger motion of the molecular motors. On the one hand, voltage pulses from the tip of a scanning tunneling microscope (STM), which leads to a current flow through the molecule. The corresponding electrons can than either tunnel into or out of molecular states, causing molecular transformations, or excite the molecules via inelastic scattering processes. Importantly, also light of different wavelengths is employed to induce photo-chemical processes and consequently molecular motion. It offers great versatility in view of intensity, energy, polarization and duration of light pulses. Additionally, and in contrast to single-molecule excitation with the STM tip, light can remotely trigger many molecules simultaneously, while characterization is performed at the single-molecule level. The statistics are important here and we could realize a detailed study of molecular switching on a surface by advanced computer vision methods. This enabled us to efficiently analyze large numbers of molecules and to identify subtle differences in the switching rates. Additionally, the AMOS project could realize in this first reporting period a motor function of adsorbate motors on different surfaces, which shows that these novel motor molecules are versatile and can work on various substrates. This gives evidence that the potential energy surfaces can be tuned via the crystal structure of the surface, which affects molecular motion. Furthermore, a simple electrospray deposition setup could be established for the clean and intact deposition of large molecules. The role of atomic surface defects for the motion of adsorbed molecules across the surface was investigated, showing these have a profound effect on the molecular speed.