Chirality-Specific Unidirectional Rotation of Molecular Motors on Cu(111)

olecular motors have chemical properties thatenable unidirectional motion, thus breaking microscopicreversibility. They are well studied in solution, but much lessis known regarding their behavior on solid surfaces. Here,single motor molecules adsorbed on a Cu(111) surface areexcited by voltages pulses from an STM tip, which leads to theirrotation around a fixed pivot point. Comparison withcalculations shows that this axis results from a chemical bondof a sulfur atom in the chemical structure and a metal atom ofthe surface. While statistics show approximately equal rotationsin both directions, clockwise and anticlockwise, a detailed studyreveals that these motions are enantiomer-specific. Hence, therotation direction of each individual molecule depends on its chirality, which can be determined from STM images. At firstglance, these dynamics could be assigned to the activation of the motor molecule, but our results show that this is unlikely asthe molecule remains in the same conformation after rotation. Additionally, a control molecule, although it lacksunidirectional rotation in solution, also shows unidirectional rotation for each enantiomer. Hence, it seems that theunidirectional rotation is not specifically related to the motor property of the molecule. The calculated energy barriers formotion show that the propeller-like motor activity requires higher energy than the simple rotation of the molecule as a rigidobject, which is therefore preferred