Potential modulation reflectance of manganese halogenated tetraphenylporphyrin derivatives assembled on gold electrodes

Electron transfer from/to redox manganese porphyrin self-assembled monolayers (SAMs) on gold electrodes in DMSO solution was investigated by potential-modulation reflectance, i.e. electroreflectance (ER) with cyclic voltammetry and ac impedance. The ER voltammogram reveals more clearly the redox waves of Mn(III)/Mn(II) in the porphyrin monolayers than the cyclic voltammogram (CV). The standard rate constants of electron transfer, i.e. turn-over number of the redox reaction at the formal potential was evaluated from complex plane plots of ER of the manganese redox couples. On the complex plane plot of the ER, the rate constant can be determined from a characteristic frequency of the ac response. The rate constants thus obtained were compared with the values evaluated from the impedance data and further compared with the values calculated by Sagaraʹs treatment from the relationship between the phase retardation and the frequency of the complex ER data. Fairly good agreement was obtained between the rate constants from the three evaluations. The ER response is found to be comparable to the complex capacitance defined as (dQ/dE) for the circuit involving the redox couple. It was found that the rate constants increase with a decrease in a distance between the electrode substrate and the redox manganese porphyrin and depend on the species bound to phenyl groups around the porphyrin unit in the order of Cl>H>F.