Single molecule electric motors

• Applicants: Dr. Lukas Gerhard; Professor Dr. Marcel Mayor
• 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

While macroscopic motors react to a directed driving force with a directed motion, thermal energy washes over state population in any machines at the nanoscale resulting in brownian motion. Besides the astonishing variety of molecular machines found in biological systems, artificially designed molecular motors are still rare. We intend to establish new driving forces for actuating nanoscale devices exploiting the spin angular momentum of an electric current. The idea of a spin-transfer torque motor, in which a spin-polarized current is depolarized by spin flip scattering in a molecule creating a torque was proposed some years ago by Ono et al. However, this concept is still to be experimentally verified. Our project aims to synthesize suitable molecular complexes and to experimentally test this concept. In order to extent this concept to injected currents that are unpolarized, in a second attempt, we will exploit the spin-selective transmission of axial chiral molecules such that a current is polarized within the molecule also leading to a torque. Our third approach relies on a more classical principle using the Lorentz force of the current through the molecule in a magnetic field. In the present project, we plan to built spin-driven molecular motors by synthesizing helical chiral poly-aromatic systems that comprise tripodal platforms building on our experimental experience with similar molecular tripodal structures and our expertise in the synthesis of helical chiral molecules.We will synthesize suitable combinations of tripodal platforms with a designated rotation axis comprising heavy-metal complexes or different chiral poly-aromatic head groups. Here, we rely on our proven expertise in the synthesis of the individual molecular building blocks. Low temperature scanning tunneling microscopy (LT STM) allows to accurately approach single molecules and even to address specific parts of a molecule in a controlled way. We have shown in preliminary work that using STM, the chirality of a single molecule and its rotation direction can be identified. As the working temperature of our STM of 5 K is equal to thermal energies in the range of half an meV, energies of statistically populated states of a metastable system can be determined precisely down to the range of few μeV. Minor chemical modifications of the rotor or the tunneling parameters are expected to be reflected in the rotational behavior and the population of the different states. A working unidirectional molecular motor will serve as starting point for further investigations that aim at a detailed understanding of the process of spin relaxation in helical chiral complexes. A close collaboration between the synthetic chemistry and experimental surface science building on fast feedback on a weekly basis will enable us to extensively test the proposed concepts and to identify suitable combinations of molecular feet and functional head groups.

• DFG Programme: Research Grants
• Co-Investigators: Michal Valásek, Ph.D.; Professor Dr. Wulf Wulfhekel