Electronic structures and spectroscopic properties of benzoporphyrin protolytic species: A TD-DFT study

Abstract The B-ring benzoporphyrin derivative B3B has been studied for its applications in Photodynamic Therapy. Its protolytic species were fully optimized in water and ethanol by Density Functional Theory based on B3LYP hybrid functional and 6-311G(d,p) basis set. All of its species were found to be thermodynamically favorable, with the enthalpy parameter prevailing over the entropy. The Gibbs free energy values showed that the solvent does not influence on the formation of each species. The anionic and neutral species have a planar macrocycle, while the protonation of iminic nitrogens brings out distortions on the macrocycle of the dicationic species, which assumes a “saddle conformation” with pyrrole rings inclined upward and downward simultaneously. All species have presented a low dipole moment which may explain their poor solubility in aqueous media and their high tendency to self-aggregate. Although the Gouterman model has been widely used to explain the absorption spectrum of B3B, this model is not able to account for all the transitions contributing to the main absorption bands of porphyrins as demonstrated by TD-DFT calculations. The excitation energies and oscillator strengths obtained at TD-DFT theory level are in good agreement with experimental results, especially for the dicationic species. Molecular orbital analysis shows no significant differences in the HOMO orbital of B3B protolytic species. The LUMO orbital, on the other hand, may be responsible for the small spectral differences observed.