Thermal with Electronic Excitation for the Unidirectional Rotation of a Molecule on a Surface

Exploring the limits of the microscopic reversibilityprinciple,we investigated the interplay between thermal and electron tunnelingexcitations for the unidirectional rotation of a molecule-rotor onthe Au(111) surface. We identified a range of moderate voltages andtemperatures where heating the surface enhances the unidirectionalrotational rate of a chemisorbed DMNI-P rotor. At higher voltage,inelastic tunneling effects dominate, while at higher temperature,the process becomes stochastic. At each electron transfer event duringtunneling, the quantum mixing of ground and excited electronic statesbrings part of the surface thermal energy in the excited electronicstates of the molecule-rotor. Thermal energy contributes thereforeto the semiclassical unidirectional rotation without contradictingthe microscopic reversibility principle.