Unravelling the Role of the Central Metal Ion in the Electronic Structure of Tris(8-hydroxyquinoline) Metal Chelates: Photoemission Spectroscopy and Hybrid Functional Calculations

The electronic structures of tris(8-hydroxyquinolinato)− erbium(III) (ErQ3) and tris(8-hydroxyquinolinato)− aluminum(III) (AlQ3) have been studied by means of core level and valence band photoemission spectroscopy with the theoretical support of hybrid Heyd−Scuseria−Ernzerhof density functional theory, to investigate the role played by the central metal atom. A lower binding energy (0.2 eV and 0.3 eV, respectively) of the O 1s and N 1s core levels has been observed for ErQ3 with respect to AlQ3. Differences in the valence band spectra, mainly related to the first two peaks next to the highest occupied molecular orbital (HOMO), have been ascribed to an energetic shift (to 0.4 eV lower energies for ErQ3) of the σ molecular orbital between the oxygen atoms and the central metal atom. A lower (by 0.5 eV) ionization energy has been measured for the ErQ3. The interpretation of these results is based on a reduced interaction between the central metal atom and the ligands in ErQ3, with increased electronic charge around the ligands, due to the higher ionic radius and the lower electronegativity of Er with respect to Al.