Controlling rotations and translations of single molecules on a surface represents a fascinating scientific goal, which can be now reached for selected compounds [1]. Under the tip of a scanning tunneling microscope (STM), inelastic tunneling electrons and local electric fields can produce controlled rotations or translations. A deeper understanding of the physical mechanisms inducing unidirectional movements on a surface is however still needed, which would allow the development of nanoscale mechanical molecular devices able to perform calculations, store energy, or produce work.In this talk, recent STM experiments will be presented, where the mechanical properties of selected molecules on the Au(111) surface are investigated by the interplay of inelastic tunneling electrons, thermal excitation, and electric field effects.Azulene based precursors, specifically designed to have a high molecular dipole moment, partially cleave and reassemble on Au(111) forming metal-organic structures with decreasing dipole moments [2]. Furthermore, ring closure reactions of vinylheptafulvene molecular switches can be induced, strongly varying the molecular dipole moment [3]. Both examples allow us to study the role of dipole moment and charge distribution in voltage-pulse manipulation on Au(111).Thanks to their internal charge separation and the possibility to both chemisorb and physisorb on the Au(111) surface, DMBI-based zwitterionic molecules represent ideal model systems to study directed movements on surface, allowing the comparison of unidirectional rotations and translations in the same experimental conditions [4]. Combining long-time electronic excitations with the slow controlled increasing of the substrate temperature, we investigated the interplay of thermal and electronic excitations for the unidirectional rotation.[1] Single Molecule Mechanics on a Surface: Gears, Motors and Nanocars, Eds. F. Moresco and C. Joachim, Advances in Atom and Single Molecule Machines Series, Springer 2023[2] T. Kühne, K. H. Au-Yeung, F. Eisenhut, O. Aiboudi, D. A. Ryndyk, G. Cuniberti, F. Lissel, F. Moresco, Nanoscale, 12, 24471 (2020)[3] K. H. Au-Yeung, T. Kühne, O. Aiboudi, S. Sarkar, O. Guskova, D. A. Ryndyk, T. Heine, F. Lissel, F. Moresco, Nanoscale Advances 4, 4351 (2022)[4] K. H. Au-Yeung, S. Sarkar, T. Kühne, O. Aiboudi, D. A. Ryndyk, R. Robles, N. Lorente, F. Lissel, C. Joachim, F. Moresco, ACS Nano 17, 3128 (2023)
