Porphyrin derivative molecules (MP-tBPP) on a Au(111) reconstructed surface have been investigated using scanning tunneling microscopy. The molecules were found to adsorb with two kinds of orientation at the elbow positions of the reconstructed surface. One orientation had the two di-tert-butylphenyl groups of a molecule aligning along a row of elbow positions, and the other was the same alignment rotated approximately 60°. Flipping behaviors from one orientation to the other were observed in successively obtained scanning tunneling microscopy images. The activation energy for the flipping behaviors was estimated to be approximately 60 meV from the flipping rates. The difference in the molecular populations with each of the orientations represents the potential energy of the adsorbed molecules having asymmetric double minima corresponding to the two orientations. The difference between the potential energies of the molecules in these orientations was approximately 12 meV, which was calculated from the ratio between the populations in the orientations and from the difference in the flipping rates.

doi:10.1021/jp111492r