Electronic Structures of Porphyrins and their Interfaces with Metals Studied by UV Photoemission

Abstract

The electronic structures of porphyrins and their interfaces with metals were investigated by UV photoemission spectroscopy (UPS). The UPS spectral features of porphine, 5,10,15,20-tetraphenylporphynatozinc(II) (ZnTPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (H2T(4-Py)P) and 5,10,15,20-tetraphenylporphyrin (H2TPP) could be assigned by comparison with MOPAC PM3 calculations. The electronic structures of the porphyrins can be regarded as the superposition of those of porphine and the substituents. The UPS results for ZnTPP/metal (Mg, Al, Ag, Au) interfaces indicated that the energies of the levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface. The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. The level positions of ZnTPP at the interface exhibited drastic change by exposure to oxygen. This change could be explained by the change of the work function of metal surface upon oxidization. For H2T(4-Py)P and H2TPP/metal (Mg, Ag, Au) interfaces, similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state. These findings require the modification of the traditional concept of a common vacuum level at organi/metal interface. The relation between the observed electronic structures and the conduction type of the porphyrins are also discussed.