Excitation mechanism in the photoisomerization of a surface-bound azobenzene derivative: Role of the metallic substrate

Two-photon photoemission (2PPE) spectroscopy is employed to elucidate the electronic structure and the excitation mechanism in the photoinduced isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). Our results demonstrate that the optical excitation and the mechanism of molecular switching at a metal surface is completely di®erent compared to the corresponding process for the free molecule. In contrast to direct (intramolecular) excitation operative in the isomerization in the liquid phase, the conformational change of the surface-bound TBA is driven by a substrate-mediated charge transfer process. We find, that photoexcitation above a threshold hn ≈2.2 eV leads to hole formation in the Au d-band followed by a hole transfer to the highest occupied molecular orbital (HOMO) of TBA. This transiently formed positive ion resonance subsequently results in a conformational change. The photon energy dependent photoisomerization cross section exhibit an unusual shape for a photochemical reaction of an adsorbate on a metal surface. It shows a threshold like behavior below hn ≈2.2 eV and above hn≈4.4 eV. These thresholds correspond to the minimum energy required to create single or multiple hot holes in the Au d-bands, respectively. This study provides important new insights into the use of light to control the structure and function of molecular switches in direct contact with metal electrodes