Electron transfer of manganese halogenated tetraphenylporphyrin derivatives assembled on gold electrodes

Disulfide-linked manganese halogenated tetraphenylporphyrin derivatives separated by spacer methylene-chain groups (C-n), (MnPFPP-C-n-S)(2), (MnDCPP-C-n-S)(2), and (MnTTP-C-n-S)(2) [n = 2, 6, 12], were synthesized. The formation and redox response of self-assembled monolayers of these derivatives on a gold electrode were examined by electrochemistry combined with optical techniques in aqueous solution as well as in dimethyl sulfoxide (DMSO) solution. The self-assembly depended on the spacer methylene-chain length and the halogen species bound to the porphyrin. The fluorinated porphyrin with long methylene-chain length exhibited the largest surface concentration on the electrode. Electron transfer of the monolayers on the electrode was assumed to occur between Mn(III) and Mn(II) occluded in the porphyrin ring from results of cyclic voltammetry (CV) and potential modulation reflectance, that is, electroreflectance (ER) in DMSO and aqueous solutions. The ER could detect the electron transfer at higher sensitivities than CV especially in aqueous solution. The porphyrin with the longer methylene chain induced the larger positive shift of the formal potential in addition to the larger surface concentration of the porphyrins in the monolayer. The shift in the formal potential also depended on the halogen species in the order MnTTP-C-n-S monolayer > MnDCPP-C-n-S monolayer > MnPFPP-C-n-S monolayer. Rate constants of the electron transfer from the monolayer to the electrode in DMSO and aqueous solutions, which can be calculated from the ER versus modulation frequency relation, increased with decreasing chain length and changed with the halogen species bound to the porphyrin in the order MnDCPP-C-n-S monolayer > MnTTP-C-n-S monolayer > MnPFPP-C-n-S monolayer. The effect of the halogen species may be explained by steric hindrance against aggregation among the porphyrin units in the monolayers especially in aqueous solution.