The two-electron oxidation of methyl viologen. Detection and analysis of two fluorescing products

Abstract

The oxidation of methyl viologen, 1,1′-dimethyl-4,4′-bipyridylium chloride (MV²⁺), in aqueous solution has been carried out γ-radiolytically and photocatalytically (in the presence of colloidal titanium dioxide). Two strongly fluorescing products have been observed, separated and identified: the 1′,2′-dihydro-1,1′-dimethyl-2′-oxo-4,4′-bipyridylium cation [‘2-one’, λ^fl_max(H₂O)= 516 nm] and the 3,4-dihydro-1,1′-dimethyl-3-oxo-4,4′-bipyridylium cation [‘3-one’, λ^fl_max(H₂O)= 528 nm]. Both compounds exhibit characteristic u.v.–visible absorption spectra in aqueous solution with maxima at 222, 260 and 347 nm (ε= 14800, 20200 and 2400 dm³ mol^–1 cm^–1) or 236, 260 (shoulder) and 390 nm (ε= 12000, 5800 and 3700 dm³ mol^–1 cm^–1) for the 2-one or 3-one, respectively. Their relative abundance in the radiolysis experiments is strongly dependent on the absence or presence of secondary oxidants, e.g. dioxygen and ferric ions. The overall low G-values in the radiation chemical studies [G_max(2-one)= 0.04, G_max(3-one)= 0.11] indicates the formation of other as yet unidentified oxidation products. Comparatively high quantum yields are observed for both products upon ultra-bandgap irradiation of aqueous colloidal suspensions of TiO₂[ϕ(2-one)= 0.032, ϕ(3-one)= 0.01], suggesting a simultaneous or consecutive two-hole transfer mechanism rather than a homogeneous free radical pathway is involved. Probable mechanisms of the ·OH-induced oxidation of methyl viologen are discussed as well as the possible implication of these results on the use of MV²⁺ as an electron relay in solar energy devices.