Coverage- and temperature-controlled isomerization of an imine derivative on Au(111)

The isomerization behavior of photochromic molecular switches is strongly influenced by adsorption on metal surfaces. For (E)-3,5-di-tert-butyl-N-(3,5-di-tert-butylbenzylidene)aniline (abbreviated as TBI for tetra-tert-butyl imine), it is found that a layer adsorbed on Au(111) can undergo an isomerization from the trans to the cis and back to the trans configuration when continuously increasing the sample temperature and accordingly decreasing the sample coverage. The conformation and adsorption geometry of TBI are determined from near-edge X-ray absorption fine structure measurements in agreement with density functional theory calculations taking into account the van der Waals interaction between adsorbate and metal surface. The coverage- and temperature-controlled conformational transitions are reversible and are driven by the higher packing density of the less stable cis-isomer in combination with the low thermal activation barrier of the trans- to cis-isomerization typical for imine derivatives. This unexpected scenario is corroborated by thermal desorption and vibrational spectroscopy as well as scanning tunneling microscopy.