Application of the electronic spectra of porphyrins for analytical purposes: The effects of metal ions and structural distortions

We investigated the UV-Vis absorption, singlet-1 and singlet-2 fluorescence, as well as the formation of several metalloporphyrins from equilibrial and kinetic aspects in aqueous solution. Among these complexes were numerous typical out-of-plane and several in-plane metalloporphyrins, and between the two categories, a few border-line cases. On the basis of our results, we have complemented the categorization introduced by Barnes and Dorough for the metalloporphyrins. According to our observations, also in metalloporphyrins, the distortion, i.e., the planarity or nonplanarity of the macrocycle, is basically responsible for the spectral characteristics, while the electronic structure of the metal center is a secondary factor, with a considerable importance mainly in the in-plane complexes. The type of complexes can be spectrophotometrically determined on the basis of their UV-Vis absorption and fluorescence spectra. Beside the spectral and photophysical effects of metalation, also those of the structural distortions were studied, which can originate also from metalation, protonation or overcrowded peripheral substitution of the free-base porphyrins, as well as from the axial ligation of metalloporphyrins.Our observation may be useful for different spectrophotometric analytical detection and determination methods, e.g. size-selective metal detection using free-base porphyrins (or other ringed chelate ligands), as well as the determination of Lewis bases as potential axial ligands, using metalloporphyrins.

► Categorization of metalloporphyrins.
► Spectral effects of the metalation and the peripheral substitution of porphyrins.
► Spectrophotometric and size-selective detection of metal ions.
► Detection of Lewis bases as potential axial ligands using metalloporphyrins.