Unidirectional rotation in a molecular motor driven by angular-momentum transfer via helical orbitals.

The generation of unidirectional motion has been a long-standing challenge in the engineering of molecular motors and, more generally, machines. A molecular motor is characterized by a set of low-energy states that differ in their configuration, that is, position or rotation. In biology and Feringa-type motors, unidirectional motion is driven by excitation of the molecule into a high-energy transitional state followed by a directional relaxation back to a low-energy state. Directionality is created by a steric hindrance to movement along one of the directions on the path from the excited state back to a low-energy state. Here we showcase a principal mechanism for the generation of unidirectional rotation of a molecule without the need for steric hindrance and transitional excited states. The chemical design of the molecule-consisting of a platform, an upright axle and a chiral rotor moiety-enables a rotation mechanism that relies on the transfer of orbital angular momentum from the driving current to the rotor. The transfer is mediated via orbital currents that are carried by helical orbitals in the axle.