Orientational ordering of nonplanar phthalocyanines on Cu(111): Strength and orientation of the electric dipole moment

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc=C₃₂N ₈H16-2) on Cu(111) by using the x-ray standing wave technique, photoelectron spectroscopy, and quantum mechanical calculations. We find that for submonolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a Cl-down configuration by forming a covalent bond to the substrate. For bilayer coverages the x-ray standing wave data indicate a coexistence of the Cl-down and Cl-up configurations on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.