Correlation of frontier orbital positions and conduction type of molecular semiconductors as derived from UPS in combination with electrical and photoelectrochemical experiments

He I photoelectron spectra collected from selected phthalocyanine thin films on Au substrates have been used to determine the position of the highest occupied orbitals and threshold ionization potentials in the solid state. Relative as well as absolute positions are given. Chemical modifications of these dyes (substitution of nitrogen heteroatoms to create the pyrido- or pyrazinotetraazaporphyrin complexes, addition of electron-withdrawing substituents to the ring periphery) and changes in the crystal structure of the solid affect the size of the frontier orbital (band) gap and especially the absolute HOMO and LUMO energies. The influence of different substituents with varying electron-donating or electron-accepting properties is investigated in detail. Photoemission data are compared with redox potentials and molecular orbital (MO) calculations from the literature. The frontier orbital positions strongly correlate with the conduction type of the thin film, the nature of electrical property changes seen during interactions with gases such as O₂ and NH₃, and the photoelectrochemical properties in different aqueous electrolytes.