Substituent and solvent effects on the hyperporphyrin spectra of diprotonated tetraphenylporphyrins

UV-visible spectra have been studied for a series of p-substituted tetraphenylporphyrins (TPPs) titrated with strong acid in various solvents. Substiment effects on the Soret and Q(I) absorption peaks and the fluorescence emission peaks have been treated by Hammett correlations. In general, there are only small effects with electron-withdrawing substituents, but electron-donating substituents lead to lower energy transitions, with especially strong effects observed in the case of the diprotonated porphyrins with good electron-donating substituents (hyperporphyrin effects). The hyperporphyrin effects are attributed to the crossing of a pi molecular orbital on the substituted phenyl group above the usual porphyrin pi highest occupied molecular orbital. For the neutral TPPs, this crossing is estimated to occur with a substituent as electron-donating as p-methoxy, and for the diprotonated TPPs, the crossing occurs at approximately unsubstituted TPP. Distinctive solvent effects on the spectra and the Hammett correlations are observed.