Density functional theory studies of N-protonation of the free base phthalocyanine

The structures and relative energies of twenty-two N-protonated species of the free base phthalocyanine (H2Pc) have been systematically studied with the density functional theory at the B3LYP/6-31G(d) level of theory. The calculations demonstrated that the N-protonation tends to increase the N-C bonds and the C-N-C angles on the protonation sites. The inner protonation at the isoindole-nitrogen atoms causes significant out-of-plane deformation of the macrocycle, ascribed to the steric hindrance of the central cavity. The relative energies of various protonated species were calculated and compared to deduce the preferred sites for protonation. It was revealed that the outer protonation at the meso-nitrogen atoms is energetically more favorable than the inner protonation at the isoindole-nitrogen atoms. Among the studied twenty-two protonated species, the most stable one is H6Pc4+(IS1), for which all the outer meso-nitrogen atoms are protonated. TDDFT calculations have been performed for selected species, and the results were used to analysis the UV-visible spectrum of the concentrated sulfuric acid solution of the free base phthalocyanine.