Atomically precise control over rotation of a rare-earth molecular motor complex on Au(111)*

Rare-earth metals are vital for many high technological applications from light emission devices to quantum information science. Here, we have developed a molecular motor formed by rare-earth metal and counterion complex that can be rotated with precise control over rotation angle and direction of rotation. In this system, a triflate counterion attached underneath a tri-blade Europium based molecular rotor acts as a pivot that enables fixed-axis rotation on Au(111) surface. The motor rotates either by thermal activation or by an electric field applied from a scanning tunneling microscope tip. Although the preferential rotation angle should be 60 degrees, an additional counterion attached to a side of the motor acting as a counterweight results in three-fold rotations with precise 120-degree steps. Moreover, attachment of side counterion permits 100% control of rotation direction and induces rotational chirality. This work demonstrates that charged counterions can be used to control dynamics of rare-earth molecular systems on materials surfaces.