Elucidation of Isomerization Pathways of a Single Azobenzene Derivative Using an STM

The predominant pathway for the isomerization between cis- and trans-azobenzeneseither (i) inversion by the bending of an NNC bond or (ii) rotation by the torsion of two phenyl ringscontinues to be a controversial topic. To elucidate each isomerization pathway, a strategically designed and synthesized azobenzene derivative was investigated on a Ag(111) surface. This was achieved by exciting the molecule with tunneling electrons from the tip of a scanning tunneling microscope (STM). Structural analyses of the molecularly resolved STM images reveal that both inversion and rotation pathways are available for isomerization on a metal surface and strongly depend on the initial adsorption structures of the molecule. On the basis of the potential energy diagrams for the isomerization, it is concluded that isomerization pathways on a metal surface are not simply related to the excited states.