Electrochemical and Spectroelectrochemical Study of A4 and A2B2 Pyrene Dendronized Porphyrins

The electrochemical behaviour of two free-base pyrene dendronized porphyrins, Por-(Py2G1)2, Por-(Py2G1)4, and their zinc metallated forms, Zn-Por-(Py2G1)2 and Zn-Por-(Py2G1)4, has been studied by cyclic voltammetry in CH2Cl2. The investigated compounds exhibit several oxidation processes. The study of five model molecules, equivalent to the nuclei and branches of the dendrimer, allowed the assignment of the different oxidation processes. The first oxidation, centered on the porphyrin ring, was also studied by in situ UV-vis spectroelectrochemistry in CH2Cl2 and THF for all the dendrimers. The electrooxidation in the thin layer spectroelectrochemical cell of the two free-base dendritic molecules shows spectral changes that suggest the formation of porphyrin diprotonated species, both in THF and CH2Cl2. Even though the electrochemical and spectroscopic characteristics of the solutions after exhaustive electrolysis in THF were consistent with the formation of these protonated species; in CH2Cl2, a paramagnetic species was obtained, and no further evidence of the formation of diprotonated species could be found. Two-electrons per molecule were consumed during the electrolysis in THF and only one-electron in CH2Cl2, the starting porphyrin was not regenerated by counterelectrolysis in any case. It was demonstrated that irradiation of the porphyrin itself accounts for the observed evolution of the UV–vis spectra in CH2Cl2. For the Zn dendronized porphyrins, the oxidation was a clean one-electron transfer, giving rise to the radical cation, subsequent reductive electrolysis regenerates the initial product.