Quantum yield and electron-transfer reaction of the lowest excited state of uranyl ion

The pulsed photoreduction (using N₂ laser photolysis technique) of uranyl ion, UO₂²⁺, both in its ground state and in the lowest excited state (at pH 2 and in 2 M H₃PO₄ aqueous solutions) has been studied. We found that *UO₂²⁺ is quenched by ruthenium tris(bipyridine) ions, Ru(bpy)₃²⁺, and Ru(bpy)₃³⁺ is formed; namely, *UO₂²⁺ is an electron-acceptor species. It is shown here that the rate of decay of *UO₂²⁺ in the presence of Ru(bpy)₃²⁺ exactly corresponds to the formation rate of Ru(bpy)₃³⁺, as measured by the bleaching of the Ru(bpy)₃²⁺ ground-state absorption. The excited state of Ru(bpy)₃²⁺ is found to be quenched by UO₂²⁺, and again Ru(bpy)₃³⁺ is formed, UO₂²⁺ being an electron acceptor in its ground state. The quantum yield of the formation of the lowest excited state of UO₂²⁺, φ_*UO22+, multiplied by the yield of electron transfer, φ_et^Ru→*U, between *UO₂²⁺ and Ru(bpy)₃²⁺ is found to be 0.5 at pH 2 and unity in 2 M H₃PO₄ solution. In the presence of a negatively charged polyelectrolyte, such as poly(vinyl sulfate), PVS, the value of this product is reduced to an upper limit of 0.1. It is shown that φ_*UO22+ is unaffected by the presence of PVS. The back reaction, UO₂⁺ + Ru(bpy)₃³⁺ → UO₂²⁺ + Ru(bpy)₃²⁺, is a second-order reaction, k/ε (λ = 450 nm) = 1.05 × 10⁵ s^-1 cm at pH 2 and 1.78 × 10² s^-1 cm in 2 M H₃PO₄ solution. This reaction is accelerated in the presence of O₂ or PVS. The yields and rates of formations and reactions of the photochemical products are discussed.