Electrochemical study of the dication of porphyrins with carboxylic acids: Shift of the absorption bands compared to that of the redox potentials

Core protonation of a series of meso-tetraarylporphyrins (aryl = phenyl, 4-chlorophenyl and 4-methoxyphenyl) with weak and strong carboxylic acids was studied by cyclic voltammetry and UV–vis absorption spectroscopy. Positive shifts of the first and second oxidation and reduction potentials, observed upon protonation of the porphyrins are in close agreement to the observed red shifts of the Q(0,0) bands in the UV–vis spectra and give evidence for the stabilization of the eg (LUMO) and a2u (HOMO) orbitals. Significantly larger positive shifts of the reduction potentials (0.622–0.892 V and 0.527–0.913 V for ΔE1 and ΔE2, respectively) compared to the oxidation ones (0.06–0.397 V and 0.085–0.166 V for ΔE1 and ΔE2, respectively) indicate much greater stabilization of the LUMOs compared to the HOMOs upon diprotonation of the porphyrins. Also, larger red shifts of the Soret bands relative to the Q(0,0) bands (ΔνSoret/ΔνQ(0,0) = 1.61–6.32) for the used porphyrins are more in accordance with destabilization of the a1u orbital upon the reaction.

Diprotonation of meso-tetraarylporphyrin with protic acids is associated with positive shifts of the reduction and oxidation potentials which in combination with the red shifts of the Soret and Q(0,0) bands give convincing evidence for the stabilization of the a2u and eg orbitals and destabilization of the a1u ones.Figure optionsDownload as PowerPoint slideHighlights
► Effects of core protonation on the oxidation and reduction potentials of porphyrins.
► Significantly larger shifts of the reduction potentials compared to the oxidation ones.
► Close correlation between the electrochemical and spectrophotometric shifts.