Reductive Quenching of Ruthenium Polypyridyl Sensitizers by Cyanometalate Complexes

Laser flash photolysis/transient absorbance and emission spectroscopy were used to probe the nature of the reductive quenching of ruthenium polypyridyl sensitizers by cyanometalate electron donors in aqueous solution. Quenching rate constants and approximate cage escape efficiencies were measured for a number of donor/sensitizer pairs; octacyanometalates (Mo(CN)₈^4-, W(CN)₈^4-) exhibit much higher cage escape efficiencies, typically 40-80%, than do hexacyanometalates (Fe(CN)₆^4-, Os(CN)₆^4-). Cage escape efficiencies vary with the overall charge of the sensitizer; geminate ion pair recombination competes most efficiently with cage escape when the electron donor and acceptor have 4+ and 4- overall charges, respectively. No dependence of cage escape efficiency on thermodynamic driving force for the back-electron-transfer reaction is observed. Little or no dependence on ionic strength or counterion is observed. Steady-state and time-resolved luminescence experiments with Ru(bpy)₃²⁺(bpy = 2,2'-bi-pyridine) show that the quenching process is dynamic at low cyanometalate concentrations and that association of the cationic sensitizer with the anionic cyanometalates occurs at higher concentrations. Even under these conditions, quantum yields for charge separation approach unity with octacyanometalate electron donors.