Substitution effects on the UV–vis and 1H NMR spectra of the dications of meso and/or β substituted porphyrins with trifluoroacetic acid: Electron-deficient porphyrins compared to the electron-rich ones

Spectral shifts upon core protonation of a series of electron-rich and electron-deficient porphyrins with five and six membered rings at the meso-positions have been studied by UV–vis and 1H NMR spectroscopy. The direction of the shift of the Q(0,0) band of different porphyrins depends on the electron-donating or electron-withdrawing character of the meso-substituent and therefore it may be used to evaluate the relative electron donor ability of the substituents. Also, the shift of the Soret bands was found to be influenced to a lesser extent by the electron donor ability of substituted groups at the porphyrin periphery. Although, large red shifts of the Soret band of H4T(o-NO2)P2+, H4T(p-SCH3P)P2+ and H4T(p-NO2P)P2+ indicate a significant out-of-plane deformation of porphyrin core, negligible upfield shifts of the β protons in their 1H NMR spectra show that the practice of using the change in the chemical shift of the β protons as an indicator of structurally induced changes in the porphyrin ring current should be approached with more caution. Interestingly, high level ab initio and DFT calculations demonstrate that in spite of the better electron donating ability of –OCH3 to benzene ring compared to that of –SCH3, meso-(p-SCH3)phenyl group is a clearly better electron donor to the porphyrin core than meso-(p-OCH3)phenyl one.