Temperature, Surface, and Coverage-Induced Conformational Changes of Azobenzene Derivatives on Cu(001)

The adsorption of dimetacyano azobenzene (DMC) and carboxymethylester azobenzene on metallic surfaces has been studied by means of X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, and density functional theory (DFT) calculations. We find that the molecule coverage and chemical character of the substrate and its temperature have an influence on the adsorption state. On a Au(111) substrate at room temperature, DMC physisorbs flat in its trans configuration up to a saturation coverage of one monolayer, while the electronic structure of the adsorbate resembles the one calculated for the free molecule. In a submonolayer evaporated on Cu(001) at 150 K, the majority of DMC molecules is found to be in the same physisorbed state as on Au(111). After annealing the substrate above 250 K most of the molecules chemisorb via their azobenzene center, where N-Cu bonds are formed, while the central azo N=N double bond is weakened. The attractive forces between the center of the molecule and the surface, together with a repulsive phenyl-metal interaction, lead to a butterfly-like bent molecular geometry in which the outer aromatic groups are tilted out of the surface plane. At higher coverages, the increased intermolecular interaction provokes a stronger tilt that finally leads to a cleavage of the central N=N azo double bond.