Electronic structure of lithium phthalocyanine studied by ultraviolet photoemission spectroscopy

Abstract The electronic structure of the stable organic radical compound, lithium phthalocyanine (LiPc), was investigated using ultraviolet photoemission spectroscopy (UPS). The observed UPS spectra were compared with those of metal free phthalocyanine (H 2 Pc) and zinc phthalocyanine (ZnPc). The spectral features of LiPc and ZnPc were analyzed with the aid of ab initio MO (molecular orbital) and density functional theory (DFT) methods. These three compounds showed similar spectral features, and the binding energy of each spectral structure had almost the same value. In addition to this, the ionization threshold of LiPc in the solid state was found to be nearly equal to those of the other compounds: the values are 4.9 5 eV for LiPc, 5.1 2 eV for H 2 Pc, and 5.0 3 eV for ZnPc. These observations show that these phthalocyanines have similar electronic structures in the top region of the valence band and that the radical electron exists in the molecular orbital of the phthalocyanine ring, which is the highest occupied molecular orbital (HOMO) of H 2 Pc and ZnPc. In contrast to this, the work function of LiPc was about 0.2 eV higher than those of other compounds, and its value was found to be 4.4 3 eV. This means that the unoccupied states of LiPc should differ from the closed shell compounds such as H 2 Pc and ZnPc.