Probing the free rotary oscillations around a single ruthenium atom in an organometallic complex

A variety of rotary molecular machine prototypes powered by light, chemical energy, or electrons have been synthesized and their operation in solution, gels, or on surfaces has been demonstrated. However, little data regarding their performances have been disclosed. Here, we report on the synthesis of molecules incorporating a five-arm rotor and the direct measurement of the work required to block the rotation around the central atom. We used single-molecule force spectroscopy (SMFS) to detect the free rotary oscillations and measure the work performed by the molecules against the mechanical load. We show that the chemical nature of the arms influences the energy barrier, causing differences in the work that the molecules can generate. Our results illustrate that SMFS, which is now widely used to probe linear displacements at a few tens of nanometer scale in macromolecules, can detect rotary motions around a single atom in a tiny synthetic molecule.