ELECTRONIC DISTINCTION BETWEEN PORPHYRINS AND TETRAAZAPORPHYRINS - INSIGHTS FROM X-RAY PHOTOELECTRON-SPECTRA OF FREE-BASE PORPHYRIN, PORPHYRAZINE, AND PHTHALOCYANINE LIGANDS

High-resolution nitrogen core X-ray photoelectron spectra are reported for free base octaethylporphyrin, octaethylporphyrazine, tetrabenzoporphyrin, and phthalocyanine. The spectra have been analyzed and assigned using all-electron ab initio Hartree-Fock calculations with basis sets of double-zeta or better quality. The results show that meso-tetraaza-substitution causes an increase of 0.2-0.45 eV in the core ionization potentials of the central nitrogens of porphyrin ligands, which implies a significantly more positive electrostatic potential in the central metal-binding region of a tetraazaporphyrin relative to a similarly substituted porphyrin.