We use time-resolved two-photon photoemission to study two molecular
photoswitches at the Au(111) surface, namely azobenzene and its derivative
tetra-tert-butyl-azobenzene (TBA). Electronic states located at the
substrate–adsorbate interface are found to be a sensitive probe for the
photoisomerization of TBA. In contrast to TBA, azobenzene loses its switching
ability at the Au(111) surface. Besides the different adsorption geometries of
both molecules, we partly attribute the quenching in the case of azobenzene to
a shift of the highest occupied molecular orbital (HOMO) with respect to the
gold d-bands, which renders the hole transfer involved in the
photoisomerization mechanism of TBA inefficient
