Spectrophotometric titrations for a full series of para-amino/carbomethoxy-substituted tetraphenylporphyrins 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 amino groups, with the remaining para substituents carbomethoxy groups.  With increasing numbers of aminophenyl groups, the relative basicity increased and the hyperporphyrin effect increased, marked by a strong red band and a split Soret band.  The cis diamino derivative was more readily protonated and showed a stronger hyperporphyrin effect compared to the trans isomer, effects which can be explained based on simple resonance forms.  For the monoamino derivative, an initial increase in the Soret band upon titration along with well-defined isosbestic points provided evidence for a monoprotonated porphyrin.  The relative stability of this unusual intermediate is proposed to be due to charge delocalization of the first cation to the single amino group and destabilization of the second protonation by the para-substituted carbomethoxy substituents.

Spectroscopy of protonated tetraphenylporphyrins with amino/carbomethoxy substituents: Hyperporphyrin effects and evidence for a monoprotonated porphyrin

244th National Meeting (2012)

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