Electronic energy levels in individual molecules, thin films, and organic heterojunctions of substituted phthalocyanines

Ultraviolet photoelectron spectroscopy was performed on phthalocyanine samples in the gas phase and as thin vapor-deposited solid films. The results are compared to those of molecular orbital calculations and electrochemical redox experiments in solutions and as solid films. Electron-withdrawing groups or substituents are introduced to the phthalocyanine system to increase the ionization potential and electron affinity. An almost parallel shift of photoelectron spectra is obtained. Unsubstituted phthalocyaninatozinc (PcZn), complexes of tetrapyridotetraazaporphyrins (TPyTAPZn, TPyTAPVO) and hexadecafluorophthalocyanines (F₁₆PcZn, F₁₆PcVO, F₁₆PcAlF) have been studied. The alignment of energy levels in organic heterojunctions prepared from the substituted phthalocyanines and the unsubstituted zinc complex is discussed in detail. The offset between the highest occupied electronic levels, the establishment of a surface dipole, and the redistribution of charge in the heterointerface region beyond molecular contact is shown.