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.