DFT study on deprotonation and protonation of porphyrins: How many protons can the porphyrin core take up?

Abstract Deprotonation and protonation are crucial characteristics of porphyrins and porphyrinoids solutions. This report is indicated to investigate all possible deprotonated and protonated species of the porphine ring using DFT. In particular, based on the analysis of the optimized geometries and electronic structures of protonated species, it was confirmed theoretically that aromaticity and stability of the triacid or the tetraacid resulted from further protonation of the porphyrin diacid. Additional hydrogen atoms attached to the inner nitrogen atoms of protonated porphyrins and gave rise to rearrangement of aromatic system involving α-, β-, and meso-carbons. Crowding inner hydrogen atoms attached to the pyrrole nitrogen atoms lead to distortion of porphyrin core. Substituents of porphyrins rendered it to be possible that the final species of the protonation process might be triacid or tetraacid.