Photo-oxidation of water sensitised by ruthenium(II) tris(bipyrazine)

The excited state of ruthenium(II) tris(bipyrazine) (Ru(bipyz)₃^2+*) is quenched by the sacrificial electron acceptors, S₂O₈^2- and Co(NH₃)₅Cl²⁺. Under acidic solutions (pH 0), Ru(bipyz)₃^2+* is quenched by protons and therefore quite short-lived (τ = 50 ns). At pH 0, steady-state irradiation of the Ru(bipyz)₃²⁺ in the presence or absence of either S₂O₈^2- or Co(NH₃)₅Cl²⁺ did not produce any permanent products. In addition, no O₂ evolution was observed when an O₂ catalyst was added. At pH 6, Ru(bipyz)₃^2+* is much longer-lived (τ = 1.04 μs) and steady-state irradiation of a Ru(bipyz)₃²⁺ solution containing S₂O₈^2-, rather than Co(NH₃)₅Cl²⁺, did produce changes in absorbance, emission and pH, due to the oxidative degradation of the sensitiser. Microsecond flash photolysis work indicated that Ru(bipyz)₃^2+* is oxidatively quenched by the S₂O₈^- ions leading to the generation of Ru(bipyz)₃³⁺, a very strong and unstable oxidant. Steady-state irradiations carried out on the Ru(bipyz)₃²⁺/S₂O₈^- photochemical system at pH 6, in the presence of an O₂ catalyst, resulted in O₂ generation (φ(O₂) = 0.0025), however photodegradation of the Ru(bipyz)₃²⁺ sensitiser still took place, albeit at a reduced rate.