Electronic structure of Ni(II) porphyrins and phthalocyanine studied by soft X-ray absorption spectroscopy

Investigations of chemical bonding and electronic structure features for polycrystalline Ni(II) phthalocyanine (NiPc), Ni(II) tetrabenzoporphyrin (NiTBP), Ni(II) 2,3,7,8,12,13,17,18-octaethylporphyrin (NiOEP), Ni(II) 5,10,15,20-tetraisobutylporphyrin (NiTiBuP), Ni(II) 5,15-diphenylporphyrin (NidPP) and Ni(II) 5,15-diisobutylporphyrin (NidiBuP) have been performed by means of high-resolution soft X-ray absorption spectroscopy. It has been found that the low-lying unoccupied electron states in nickel porphyrins are mainly generated by the NiN4 quasi-molecule: the lowest empty state is the weakly antibonding molecular orbital (MO) of σb1g symmetry with nearly pure Ni 3dx2-y2 character, while the next empty state is the eg MO, which has a hybridized Ni 3dπ-N 2pπ character and reflects π chemical bonding (π-back-donation) between nickel and neighbouring nitrogen atoms of the pyrrole rings. The differences observed in the Ni 2p spectra of nickel porphyrins and phthalocyanines indicate a limited usefulness of the NiN4 quasi-molecule and a need for its extension to the whole porphyrin (phthalocyanine) macrocycle. As a result, the differences in the Ni 2p spectra of the complexes and in their electronic structure are associated with a torsion of the opposite pyrrole ring planes (NiOEP, NiTiBuP) and the presence of N atoms in meso positions (NiPc). These conclusions are further confirmed by a comparative analysis of the Ni 2p and N 1s absorption spectra for NiPc and NiOEP.