Molecular orbitals of phthalocyanine

Abstract

The pi molecular orbitals of metal-free phthalocyanine (H 2Pc) are obtained by Hueckel-type MO calculations. The bonding scheme of the two inner hydrogen atoms has been considered explicitly. Thus, the symmetry of the system is D 2 h . In one model, the localized hydrogen model, the hydrogens are assumed to bond directly to two opposite nitrogens. In the other, the shared hydrogen model, each hydrogen is assumed to be shared by two neighboring nitrogens in a form of hydrogen bond. The localized hydrogen model cannot reproduce the observed absorption spectrum within a reasonable range of the parameters. The shared hydrogen model has successfully predicted the observed absorption frequencies. By comparison with the MO levels of tetrabenzporphin, the intense absorption of phthalocyanine in the visible range is attributed to the large Coulomb energies of the four bridge nitrogens.