Semiempirical CNDO-CI calculations were used to study dianions and model complexes of phthalocyanine, tetrabenzoporphyrin, tetraazaporphyrin, and porphyrin. Calculated energies for singlet x -* x* transitions associated with the Q, B, N, and L bands of the dianions were compared with previously reported spectroscopic data for complexes containing Zn2+ and Mg2+. The calculated transition energies agreed well with experimental data. In addition to the most intense x -* x* transitions, weak n -* x* transitions were also examined. The results indicate that the lone-pair orbital structure of tetraazaporphyrin and phthalocyanine, more complex than that of porphyrin and tetrabenzoporphyrin, give rise to pyrrole n -» x* transitions which lie higher than the B bands. Effects of simple metal ions on the spectra of the four macrocycles were examined by carrying out calculations on model complexes. The results indicate that presence of a metal ion influences the oscillator strengths of the major x -* x* transitions more strongly than it influences the transition energies.
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