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Single molecule electric motors

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Solid State and Surface Chemistry, Material Synthesis
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 399469443
 
Final Report Year 2023

Final Report Abstract

The project aimed to create molecular electric motors driven by the tunneling current of a Scanning Tunneling Microscope (STM). To achieve this goal, we considered a number of possibilities in which a current could drive a directed motion at the molecular scale. While previous studies on rotational switching primarily focused on two-dimensional molecules, our intention was to explore three-dimensional molecular complexes. In particular, we planned to harness the spin selective transmission of axial chiral molecules to induce a directed rotation between a molecular rotor mounted on a molecular stator which in turn is immobilized on a metal surface. In contrast to the macroscopic world, on the scale of individual molecules, thermal and quantum mechanical effects become significant and the realization of a directed motion becomes non-trivial. In addition, time resolution of STM does not allow to follow molecular dynamics. Consequently, we devised a statistical analysis to investigate molecular rotational switching between multiple individual meta-stable states. In the course of the project, a new enlarged molecular platform was developed and synthesized which would not only facilitate the investigation and implementation of our proposed molecular electric motors but also allowed decoupling of molecular chromophores for single molecule electroluminescence studies. Our synthetic group has developed successful synthesis of previously unknown molecular tripods such as I-VII, and the synthetic methodology discovered during this project provides us versatile synthetic pathways to functional molecular scaffolds that might be used as potential molecular devices such as molecular rotors or molecular light emitters. All new compounds were fully characterized, and provided to the physicists for surface measurements. In order to spatially separate the individual Geländer head groups, they were co-deposited with the tripodal platform which resulted in a well ordered lattice with a well defined local environment and an increased distance between individual rotor head groups. The three-dimensional molecular tripodal structures with Geländer-type head groups did indeed show rotational switching between three discernible current levels in the STM junction, indicating a switching between three meta-stable states of the molecule and allowing for the assignment of a rotation direction. Statistical analysis showed a directional switching with extremely high confidence levels and rotational frequencies of about 0.1 to 1 Hz, depending on the tunneling conditions. This allowed us to study the rotation as a function of tip position, tunneling current and tunneling voltage. We found that the position of the tip crucially influences the energy landscape of rotor head group such that imaging the individual meta-stable states turned out to be impossible. As another consequence, the rotation frequency does not increase but decrease with the tunneling current. Surprisingly, the directionality of the rotation shows a not-linear behavior as a function of the applied bias voltage with a local maximum at around 1.8 V. Most notably, the direction of rotation did not reverse upon reversal of the polarity of the bias voltage, i.e. the current direction, excluding the proposed hypothesis of chirality induced spin selectivity (CISS) as the driving force. Ab-initio DFT calculations carried out by collaborating groups in the Universities of Prague and Regensburg showed that the current through the molecules acquires a momentum in the triple bond of the ethynylene linker connecting the Gel¨nder head group to the tripodal platform, driving the molecule in the opposite direction. The calculations show that the current induced momentum transfer has the same direction for both directions of the current, because orbitals of opposite chirality contribute for the two different current directions. Through our synthesis efforts and experimental studies, we opened up the way for future applications of molecular motors in various fields. The theoretical calculations triggered by our surprising findings shed new light on previous experimental indications for spin-induced momentum transfer and will allow to explore a new class of molecules for current-induced momentum transfer.

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