Application of UV-Vis and NMR Spectra of Porphyrin Dications to the Investigation of the Structural Flexibility of Porphyrins Under Solution Conditions

In this study, the influence of core protonation on UV-vis, 13C NMR, and 1H NMR spectra of meso-tetra(aryl)porphyrins has been investigated. The porphyrins have meso substituents with different electron-donating abilities and a steric hindrance at the porphyrin periphery. The changes in the position of the Soret band in UV-vis spectra and chemical shift of the β carbon and protons in 13C NMR and 1H NMR spectra provide evidence on the degree of out-of-plane deformation of porphyrin core. Also, the position of the higher wavelength absorption bands (Q bands) in UV-vis spectra of the porphyrin dications depends directly on the degree of coplanarity between the meso substituents and porphyrin mean plane. Introduction of bulky methyl groups and chlorine atoms onto the ortho-position of phenyl substituents leads to a significant decrease in the saddling of porphyrin core and degree of coplanarity of meso aryl groups with the porphyrin mean plane. Concomitant use of the UV-vis and NMR spectra provide valuable information on the relative molecular flexibility of different porphyrins towards planar to saddle conformational changes upon deprotonation of the used porphyrins. Furthermore, the spectral changes can be used to compare the dihedral angles between the meso aryl groups and the macrocycle mean plane of different porphyrins.