SUBSTITUENT EFFECTS IN PORPHYRAZINES AND PHTHALOCYANINES

Ab initio Hartree-Fock calculations with double-sigma basis sets have been carried out on a variety of free-base porphyrinic molecules. A comparative study is presented on the electronic structures of four unsubstituted parent tetrapyrroles, porphyrin, porphyrazine, tetrabenzoporphyrin, and phthalocyanine. The electronic influence of multiple peripheral fluoro, chloro, cyano, nitro, and trifluoromethyl groups on the orbital energies of the porphyrazine macrocycle has been calculated. Among the structurally perturbed phthalocyanines studied are hexadecafluorophthalocyanine, 2,3 -naphthalocyanine, and tetrapyrazinoporphyrazine. Tetraazaporphyrins such as porphyra zines and phthalocyanines exhibit several important differences compared to the porphyrins, in both their electronic and geometric structures. An important structural consequence of meso-tetraaza substitution is the significantly smaller central hole in porphyrazine than in porphyrin. meso-Tetraaza substitution is predicted to result in a significant increase in the core ionization potentials of the central nitrogens of the porphyrin macrocycle. The first ionization potential of porphyrazine is also predicted to be significantly higher than that of porphyrin. Tetrabenzo annulation of porphyrin and porphyrazine results in a significant drop in their first ionization potentials (IPs). The additional linear annulation in 2,3-naphthalocyanine leads to a further lowering of the first IF. Suitable arrays of highly electronegative peripheral substituents are predicted to exert electronic effects of about 2 eV or more on the nitrogen Is and valence ionization potentials of tetrapyrrolic ligands.