Electronic Properties and Orbital-Filling Mechanism in Rb-Intercalated Copper Phthalocyanine

The evolution of the electronic properties of a thin film of copper phthalocyanine deposited on Al(100) and progressively intercalated with rubidium atoms was followed by photoemission and X-ray absorption spectroscopies. Electron donation from the Rb atoms to the C32H16N 8Cu molecules results in the lifting of the degeneracy in the e g ligand-derived molecular orbital and the lowering of the molecular symmetry. For Rb∼2C32H16N8Cu, spectral evidence indicates that both donated electrons reside in the first split-off eg-derived level, thus creating an electronic inequivalence between the C atoms in the benzene rings. For higher Rb concentrations, a reduction of the Cu oxidation state is observed, together with a new Cu-derived state in valence-band photoemission spectra, testifying to the filling of the b1g orbital. Thus, even though b1g is the lowest unoccupied orbital of the neutral molecule, in the film, the Cu-derived b 1g-derived states are occupied only after a partial filling of the eg-derived band has taken place. Despite the fact that the e g-derived spectral weight becomes larger as the rubidium content in the RbxC32H16N8Cu compound increases, no spectral density was observed at the Fermi level, showing that the film remains insulating for all of the investigated stoichiometries.