Hindered rotation of a copper phthalocyanine molecule on C60:: Experiments and molecular mechanics calculations

If copper phthalocyanine (CuPc) molecules are deposited on a Au(111) surface covered with a monolayer of C-60, the molecules are found to adsorb individually onto the close-packed layer of C-60. As the adsorption site of the CuPc is not symmetric with respect to the underlying C-60 layer, the CuPc molecule has six equivalent orientations according to the hexagonal packing of the C-60. Scanning tunneling microscopy (STM) measurements reveal that at room temperature, the molecules may hop between these adsorption sites due to thermal activation and therefore appear as a ring with six maxima. This paper reviews the STM measurements carried out by Stohr in the light of molecular force field calculations. The potential energy for an individual CuPc molecule has been calculated as a function of the lateral position on the C-60 layer based on the van der Waals interactions between molecules. For each position, the minimal energy has been evaluated, taking the vertical and the rotational degrees of freedom into account. A map of the potential energy not only reveals the preferred adsorption sites, it also indicates several favorable paths between these minima. The calculations consistently explain the experimental observations at low and ambient temperature.