We describe the synthesis and electrochemical oxidation properties of ferrocene dimers bridged by a fused oligothiophene spacer (thienothiophene, benzodithiophene, and dithienothiophene), which are designed as new types of ferrocene dimers for the study of the oxidation process via a mixed‐valence state. The electrochemical properties of all ferrocene dimers were examined by a cyclic voltammetry technique. The voltammograms showed a reversible two‐electron oxidation wave derived from the two ferrocene fragments with a potential difference between the first and second oxidation waves (ΔE1/2 = E1/2(+1/+2) – E1/2(0/+1)), which ranges from 0 to +127 mV. This result indicates that the chemical structure of the fused oligothiophene spacer can be attuned to the two‐electron oxidation process of two ferrocene terminals via a mixed‐valence state.

The formation reaction of peroxodiphosphate (denoted by PDP and meaning P2O84−, HP2O83− etc.) by the reaction of molecular oxygen (O2) with the phosphinate ion (PH2O2−) was extremely accelerated by irradiation with visible light of aqueous solutions containing the tris(2,2′-bipyridine)ruthenium(II) ion ([Ru(bpy)3]2+). The [Ru(bpy)3]2+ acted as a photocatalyst during the reaction. The reaction mechanism consists of a chain reaction being accompanied by the O2-quenching of a photoexcited ruthenium(II) complex ([Ru(bpy)3]2+*) to make the superoxide (O2−•) and [Ru(bpy)3]3+, followed by the reduction of [Ru(bpy)3]3+ by PH2O2− to make [Ru(bpy)3]2+ and PH2O2•. However, such PDP once formed in the presence of O2 began to decrease with a rate law of -d[PDP]/dt = kobsd[PDP] after the solution was saturated with N2 gas, and when the solution was again saturated with air, the PDP began to increase again with light irradiation and remained constant in the dark. Such behaviors are discussed while presenting the reaction mechanisms.

The chromate (VI) ion HCrO4− at pH 2—4 was not appreciablly reduced by the phosphinate ion PH2O2− in the absence of molecular oxygen (O2), but was reduced in the presence of O2, forming peroxodiphosphate ions (denoted by PDP and meaning all forms of P2O84−, HP2O83− etc.) in much higher concentrations than those of the added chromate(VI). Both the rates of the chromate decay and the PDP formation increased with decreasing pH. When the chromate(VI) ion disappeared completely, PDP formation stopped and its concentration remained constant. However, such PDP once formed in the presence of O2 began to decompose after the solution was saturated with N2 gas; its decomposition ceased again when the solution was again saturated with air. Such behaviors are discussed while presenting the reaction mechanisms.