Inducing the rotation of a single phenyl ring with tunneling electrons

We report on the electron induced intramolecular rotation of a single phenyl ring of an azobenzene derivative adsorbed on a Au(111) surface using a low-temperature scanning tunneling microscope (STM). By proper functionalization of each of the two azobenzene's phenyl rings with CN end groups, we are able, to identify two distinct isomers at the metal surface corresponding to two possible alignments of the functional groups in the trans conformer. Tunneling electrons induce molecular motion and intramolecular conformational changes both on isolated molecules and H-bonded molecular islands. Particular enhancement is observed for the electrons resonantly tunneling through affinity levels, which is consistent with electronic molecular excitations as the basic mechanism for this manipulation process. On the basis of quantum chemical calculations of a free azobenzene molecule, we propose a dynamical model for the ring-rotation pathways, which includes the electric field in the STM junction to effectively couple electronic excitation with intramolecular rotations.