Electronic transitions in some phthalocyanine and NH-rhodanine-merocyanine films studied by inelastic-electron-tunneling spectroscopy

Inelastic-electron-tunneling spectroscopy on Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Pb tunneling junctions at 4.2 K has been used to study electronic transitions in the energy regime between 0.5 and 2.5 eV. $\pi \to {\pi }^{*}$ singlet-triplet ($S\to T$) transitions are found and spectral structures between 0.5 and 1.0 eV for the Fe-, Co-, and Zn-phthalocyanines are attributed to transitions involving metal-ligand orbitals. Good agreement with other experimental data and theoretical calcualtions is given. Unexpected spectral features between 1.5 and 2.5 eV are observed which shift irreversibly in energy from one run to the next one. This effect is interpreted in terms of internal-field emission processes whose onset is shifted due to charge trapping in the dye layers.