Electronic structure of thiaporphyrins: an X-ray photoelectron spectroscopic study
The electronic structure of 5,10,15,20-tetraphenylporphyrin (H2tpp) modified with one and two sulfur core atoms has been investigated by X-ray photoelectron spectroscopy (XPS). The nitrogen Is binding energies (b.e.s) were excellent sensors of the electronic effects of the core-modified porphyrins. They clearly show that there is an increase in electron density on the core nitrogen atoms when the two NH groups in H2tpp are replaced by sulfur atoms, the nitrogen 1s b.e. decreasing by about 1.5 eV. The carbon 1s core level also decreases in b.e. on core modification with sulfur atoms. The results obtained are in good agreement with those from UV/VIS, fluorescence spectroscopy, electrochemical studies, X-ray diffraction and extended-Hückel calculations. They indicate that there is an interaction between the nitrogen and sulfur atoms and a mechanism is suggested for it. No evidence was found for interaction among sulfur atoms and the participation of their d orbitals in the electronic structure. Copper and nickel complexes of the monothiaporphyrin were also subjected to XPS analysis.