Probing the work generated by the autonomous rotation around a single ruthenium atom in an organometallic complex

A variety of rotary molecular motors powered by light, chemical energy or tunnelling electrons have been synthesized and their operation in solution, in gels or on surfaces has been demonstrated. However, the single-molecule mechanics of such machines remain scarcely studied and very little data regarding their quantified performances have been disclosed. Here, we report on the synthesis of a series of molecules incorporating a five-arm rotor subunit and the direct quantitative measurement of the work generated by its rotation around the central ruthenium atom. We have used single-molecule force spectroscopy (SMFS) to detect the autonomous oscillations. We demonstrate that a mechanical load does not prevent rotation and show that the chemical nature of the arms influences the energy barrier, causing differences in the work that the motor can generate. Our results illustrate that SMFS, which is now widely used to probe processes at the scale of a few tens of nanometers in biomacromolecules, can detect motions around a single atom in a tiny synthetic molecule.