Transients in the flash photolysis of aqueous solutions of tris(2,2′-bipyridine)ruthenium(II) ion

Aqueous solutions of Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine) were flash photolyzed and pulse irradiated in the presence and in the absence of other additives. Hydrated electrons are produced in a process which is suggested to involve electron ejection from the first triplet of Ru(bpy)₃²⁺, upon the absorption of a second photon. Most of the effeirtive light (∼70%) was in the range 300-340 nm, while ∼30% of the effective light was at wavelength >340 nm. The efficiency of e_aq^- production relative to the efficiency of production of the lowest triplet charge transfer state was measured under our conditions as 0.0015, independent of [Ru(bpy)₃²⁺] and of flash intensity over a wide range. During our work on the biphotonic process for e_aq^- formation, a number of other reactions were investigated. Thus, hydrated electrons react with Ru(bpy)₃²⁺ with a reaction rate constant of (5.8 ± 0.3) × 10¹⁰ M^-1 s^-1. The metal ions Zn⁺, Co⁺, and Cd⁺ reduce Ru(bpy)₃²⁺ with reaction rate constants (1.5 ± 0.2) × 10⁹, (1.7 ± 0.2) × 10⁹, and (6.1 ± 0.6) × 10⁸ M^-1 s^-1 respectively, producing Ru(bpy)₃⁺. Ru(bpy)₃⁺ reacts with O₂, Cu²⁺, and duroquinone; the reaction rate constants are (7.2 ± 0.8) × 10⁹, (3.4 ± 0.3) × 10⁸, and (4.0 ± 0.5) × 10⁹ M^-1 s^-1, respectively. The possibility for a biphotonic effect leading to H₂ formation is discussed.