NMR Study of Hyperporphyrin Effects in the Protonations of Porphyrins with 4-Aminophenyl and 4-Pyridyl Meso Substituents

Titrations for a series of porphyrins bearing either 4-aminophenyl or 4-pyridyl meso substituents were performed using methanesulfonic acid in DMSO and followed by proton NMR. Special emphasis was placed on identifying the intermediate protonation stages that are described as hyperporphyrins, that is, where there exist strong charge-transfer interactions between the peripheral aminophenyl groups and the protonated porphyrin ring. In particular, evidence was gathered to support the significance of a novel resonance form involving charge transfer between two peripheral substituents, an aminophenyl group and a protonated pyridinium group. 1H NMR and NOESY spectra provide evidence for the importance of such resonance effects in the triply protonated triamino/monopyridyl hyperporphyrin (A3PyPH3+3)