One-electron Oxidation Potentials of an NADH Model Compound and a Dimeric Rhodium(I) Complex in Irreversible Systems. A Convenient Determination from the Fluorescence Quenching by Electron Acceptors

A new methodology for experimental determinations of redox potentials of oxidants or reductants in both reversible and irreversible systems is presented. The Rehm-Weller free energy relationship between the activation free energy ΔG− and the free energy change ΔG for the electron transfer quenching of the excited states of oxidants or reductants by a series of reductants or oxidants is converted to a linear correlation between ΔG−ΔG and (ΔG)−1, which can be utilized in convenient determinations of redox potentials of oxidants or reductants. This methodology is established in various reversible systems where the redox potentials of oxidants or reductants determined based on the Rehm-Weller free energy relationship agree well with those determined independently by electrochemical methods. By using this method, one-electron oxidation potentials of two quite different compounds, one is an NADH model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the other [Rh2(dicp)4]2+ (dicp=1,3-diisocyanopropane), in MeCN have been determined as 0.60±0.10 and 0.38±0.10 V vs. SCE, respectively. The relation between the oxidation potential and the oxidation peak potential of BNAH measured by the cyclic voltammetry is discussed in the context of free energy relationships.