Free base and protonated form fluorescence of the mixed 5,10,15,20-arylporphyrins

Abstract

Fluorescence spectra for a series of mixed phenyl-mesityl 5,10,15,20-tetraarylporphyrins – with Mes4, Mes3Ph, Mes2Ph2-trans, Mes2Ph2-cis, MesPh3 and Ph4 type architecture (Mes – mesityl, Ph – phenyl) – in their free base, mono- and diprotonated form are reported. Substitution of a single mesophenyl group with a sterically more demanding mesityl group is found to be sufficient to provide stabilization of the monoprotonated porphyrin species. A decrease in the number of mesityl substituents in the macrocycle leads to a decrease in the fluorescence quantum yield for protonated forms. The progressive replacement of ortho,ortho’-substituted mesityl groups with unhindered phenyls results in the additive pattern of spectral changes for the free base porphyrins, whereas for both protonated species a specific jump at the trans to cis transition is observed, along with a gradual additive pattern before and afterwards. This feature is interpreted as a signature of the transition between the hindered-like and unhindered-like conformations of the tetrapyrrolic macrocycles in the protonated forms. These two molecular conformations provide a different interaction of the two one-electron (a1u,eg) and (a2u,eg) configurations, which leads to changes of their contributions into electronic transitions.