Electronic structure of lithium phthalocyanine studied by ultraviolet photoemission spectroscopy

The electronic structure of the stable organic radical compound, lithium ph thalocyanine (LiPc), was investigated using ultraviolet photoemission spect roscopy (UPS). The observed UPS spectra were compared with those of metal f ree phthalocyanine (H2Pc) and zinc phthalocyanine (ZnPc). The spectral feat ures of LiPc and ZnPc were analyzed with the aid of ab initio MO (molecular orbital) and density functional theory (DFT) methods. These three compound s showed similar spectral features, and the binding energy of each spectral structure had almost the same value. In addition to this, the ionization t hreshold of LiPc in the solid state was found to be nearly equal to those o f the other compounds: the values are 4.9(5) eV for LiPc, 5.1(2) eV for H2P c, and 5.0(3) eV for ZnPc. These observations show that these phthalocyanin es have similar electronic structures in the top region of the valence band and that the radical electron exists in the molecular orbital of the phtha locyanine ring, which is the highest occupied molecular orbital (HOMO) of H 2Pc 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 H2Pc and ZnPc. (C) 2000 Elsevier Science B. V. All rights reserved.