Despite the simplicity of the operating principle of a motor, it is still a tremendous challenge to transform energy into motion and to conceive a nano-motor composed of a single molecule. Researchers at the CEMES have succeeded in meeting it, using the original electrostatic motor as an inspiring model.

In the field of nanoscience in general and molecular nano-mechanics in particular, one current challenge is the conception and construction of a nanometer-sized molecular motor, i.e. a machine that transforms energy into work in a continuous fashion, through the means of a controlled unidirectional rotation. This motion should be reversible and of sufficient amplitude to allow both its observation and exploitation.

Motors that were synthesized at the CEMES were designed to be addressed individually. Their operating principle requires to connect the molecule by two nano-electrodes acting as electron tanks, as shown on the figure. The molecule is composed of a fixed part (stator) anchored onto the surface, and a mobile part (rotor) carrying oxidizable groups. In the presence of external polarization, the positive electrode injects in the mobile part of the motor charges of identical sign, inducing repulsion from the electrode and therefore rotation. This constitutes an electrostatic motor, whose operating principle was described as early as 1748 by Benjamin Franklin! The motor rotates by consuming the energy that arises from the transport of electrons from a low electric potential zone to a high potential one. The dissymmetry of the system will allow the control of the direction of rotation.

Such motors have a piano stool structure. The fixed part (in black on the figure) is covalently bound to the surface. The mobile part is a platform (in blue) terminated by five electro-active groups. The successive electron transfer processes will take place on the electro-active groups, and will induce the rotation of the mobile part of the molecule in a privileged direction. Between these two parts a ruthenium atom acts as a joint, giving the motor an organometallic character.

We have synthesized several molecules of different size and chemical constitution, illustrating the various criteria that should be met. First of all, the system should be as rigid as possible so as to avoid useless rotations that would waste energy in unwanted movements. Then, the rotation should be easy around the vertical axis but not at the expense of the integrity of the molecule, i.e. dissociation of the fixed and mobile parts should be prevented. Finally, the redox potentials of the various parts of the system should be compatible with the electron transfer processes required.

Several international collaborations have been established with complementary research groups in physics (Ma Planck Institute, Stuttgart and University of Ohio) in order to anchor the motor on specific surfaces and to study this electron-induced rotation. On the longer term these motors could incorporate nanometric robots able to accomplish various tasks, from medicine to everyday life, or to power the nanovehicles we are developing.