Electron-transfer quenching of excited Ru(bpy)₃²⁺ ions adsorbed on ion-exchange surfaces

Both the decay of the ³CT Ru(bpy)₃²⁺ emission at 600 nm and the bleaching of Ru(bpy)₃²⁺ ground-state optical absorption have been investigated. The ionic species were adsorbed on a negative ion exchange resin (Sephadex SP-C50) which is a tridimensional dextran gel with sulfopropyl groups attached. The ³CT Ru(bpy)₃²⁺ ions are quenched by Cu²⁺, Fe³⁺, ferric nitrilotriacetate (FeNTA), O₂, benzoquinone, and Cr(acac)₃ in a dynamic process. Except for O₂ and benzoquinone, electron-transfer products are produced with quantum yields ranging from 0.1 for FeNTA and Cr(acac)₃ to 0.68 for Cu²⁺ and 1.0 for Fe³⁺. The rates of quenching for the positive ions, based on the total volume of the swollen resin, are higher by about one order of magnitude as compared to the appropriate rates in water. Rates of quenching by the uncharged species seem to be comparable to the rates in polyelectrolyte solutions. The reversed electron-transfer reactions were also measured for Fe³⁺, Cu²⁺, and FeNTA. In all three cases, the back-reaction is very fast, obeying fairly closely a second-order rate law (deviations indicating a possible first-order contribution are observed after >70% have reacted back). Reaction rate constants are 3.4 × 10⁸, 4.1 × 10⁹, and 4 × 10⁹ M^-1 s^-1 for Fe³⁺, Cu²⁺, and iron(III) nitrilotriacetate, respectively. At sufficiently high coverage by Ru(bpy)₃²⁺ and sufficiently high laser pulse intensities, triplet-triplet annihilation was observed for ³CT Ru(bpy)₃²⁺, similarly to the effect measured previously in micelle solutions and in a polyelectrolyte solution. Unlike the complicated nature of the enhanced decay of ³CT Ru(bpy)₃²⁺ in the polyelectrolyte, the enhanced decay in the ion-exchange resin obeys a simple second-order rate law. The results are discussed in the light of previous work in water, polyelectrolytes, and an ion exchanger.