Ligand Rotation Induced Oxidation State Change and Spin Appearance of the Bis(phthalocyaninato)cerium (CePc₂) Molecule on the Au(111) Surface

We demonstrate that a nonmagnetic molecule is changed into magnetic when its internal structure is rotated by forming an ordered film on a metal surface. We used a double-decker complex, bis(phthalocyaninato)cerium (CePc2), where the valence of the Ce atom changes between +3 and +4 with a small energy barrier. CePc2 is nonmagnetic in bulk and becomes magnetic in the monolayer film on Au(111). The intra-molecular structure and the magnetic property are revealed using scanning tunneling microscopy/spectroscopy. We attribute the mechanism to a change of the rotational angle (0) between the upper and lower Pc ligands, driven by the steric repulsive force originating from forming an almost square lattice on Au(111). The 0 changes from 0 = 45 degrees of the bulk value into 0 = 0 degrees for the film. GGA + U calculation indicates that the change from 0 = 45 degrees to 0 = 0 degrees causes the Ce valence change from the nonmagnetic +4 state into the magnetic +3 state.