Xun Li: "Probing the motion of a molecular rotor by AFM"
Liege University, Belgium
Xun Li,1 Damien Sluysmans,1 Floriane Devaux,1 Yohan Gisbert,2 Claire
Kammerer,2 Gwénaël Rapenne,2 Anne-Sophie Duwez1
1 UR Molecular Systems, University of Liège, Sart-Tilman 4000, Belgium
2 CEMES, Université de Toulouse, CNRS, Toulouse, France
Controlling internal motions of artificial molecular machines has become highly important for the design of functional devices with unprecedented properties. In this regard, different types of rotary molecular motors have been synthesized. For instance, a series of electron-fueled organometallic rotors were proposed, consisting of (i) a fivearmed rotary component and (ii) a tripodal stator made of a scorpionate ligand. Both components are connected by a ruthenium center allowing the rotation of the rotor with respect to the stator. Various molecular rotors were already synthesized with the possibility of selectively modifying the end-groups of the rotor arms.
Here we probed the motion of single rotary motors in solution and at room temperature using atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS). The stator was modified to facilitate its anchoring on a gold substrate using Au-S interactions, and one arm of the rotor was chemically linked to a poly(ethylene oxide) chain used as a tether for the attachment to the AFM tip during pulling experiments. By recording the force as a function of the tip displacement, we generated force-extension curves, in which information on the relative rotation between rotor and stator is obtained. We determined the minimum force against which the motor can rotate.
To confirm our hypothesis, we replaced the spacer on one the arm of the rotor with different substitutes. We found that the signature in the force curves vary proportionally to the arm lengths. From these experiments, we can conclude that we can detect the rotation of the motors at room temperature and in a solution at a single molecule level. We can also measure the force exerted by the motor against a mechanical load and the corresponding work generated.