On the possibility of interpretation of the spectra of tetrapyrrole compounds and their radical ions on the basis of quantum-chemical calculations by the density functional method

Using the time-dependent formalism of the density functional theory (time-dependent density functional theory (TDDFT)), the energies and intensities of the electronic transitions of radical anions of Mg porphyrin and Mg phthalocyanine are calculated quantum chemically. Based on these calculations, an interpretation of the electronic spectra of these compounds is proposed. It is shown that, for the neutral Mg porphin molecule, the results of the TDDFT calculations are consistent with the generally accepted notions and with the experimental data only if hybrid density functionals are applied. In this case, the best results are obtained for an exchange-correlation functional whose parameters are close to the standard parameters of the B3LYP functional. The spectra of the electronic transitions of radical anions of the considered tetrapyrrole molecules calculated by the TDDFT (B3LYP) method are consistent both with previous semiempirical calculations and with the experimental data.