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

Abstract

Abstract 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 e g (LUMO) and a 2u (HOMO) orbitals. Significantly larger positive shifts of the reduction potentials (0.622–0.892 V and 0.527–0.913 V for ΔE 1 and ΔE 2 , respectively) compared to the oxidation ones (0.06–0.397 V and 0.085–0.166 V for ΔE 1 and ΔE 2 , 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 (Δν Soret /Δν Q(0,0) = 1.61–6.32) for the used porphyrins are more in accordance with destabilization of the a 1u orbital upon the reaction.