Using the DFT/B3LYP/6-31G(d,p) method, the relationship between molecular structure and acid-base properties of water-soluble conjugate obtained by azo coupling of the diazonium cation of 5-(4’-aminophenyl)-10,15,20-tris(4’sulfonatophenyl)porphine with 1-amino-8-naphtholate-3,6-disulfonate (H-acid) in alkaline medium was analyzed. It was shown that a stable form of the pentanion of this azo compound is the hydrazone trans-tautomer with 1,2-quinoid structure of the H-fragment. The most acidic is hydrazone NNH-group, the dissociation of which begins in the region above pH 12 and can not be quantified. Dissociation of other weaker acid groups of the conjugate is limited by the upper value of the pH water scale. The pyrrolenine nitrogen atoms of the porphyrin platform H2P, which diprotonates with close constants values, are most basic. It was shown that the reasons of synchronous H2P diprotonation are the effects of substituents and the (H2O)2[H4P] aquacomplex formation, which act together. Further protonation of the H-fragment is limited by the lower value of pH water scale. The aquacomplex is an aggregation-stable zwitterion that does not form porphyrin J-aggregates even in the presence of self-assembly inducers. The reason for the high aggregation stability is the steric effect of the bulk H-fragment.
V. B. Sheinin, D. Ivanov, O. Kulikova