Hyperporphyrin effects in the spectroscopy of protonated porphyrins with 4-aminophenyl and 4-pyridyl meso substituents.

Spectrophotometric titrations for a full series of 4-aminophenyl/4-pyridyl meso-substituted porphyrins were carried out using methanesulfonic acid in DMSO to study the hyperporphyrin effect across different substitution patterns. The series included zero, one, two (cis and trans), three, and four meso(4-aminophenyl) groups, with the remaining meso substituents being 4-pyridyl groups. The peripheral pyridyl groups consistently protonate before the interior porphyrin pyrrole nitrogens, which protonate before the aminophenyl groups. Aminophenyl substituents increase the basicity of the pyrrole nitrogens and lead to distinctive hyperporphyrin spectra with a broad Soret band and a strong red absorption. The structure proposed to give rise to these spectra is the previously proposed charge-transfer interaction between the aminophenyl and the protonated pyrrole. A novel hyperporphyrin structure involving charge-transfer interactions between two peripheral substituents is also proposed in one case, the triply protonated (+3) porphyrin with three aminophenyls and one pyridyl substituent; two of the aminophenyl groups delocalize the charges on the interior nitrogens, while the third aminophenyl group delocalizes with the protonated pyridyl.