Energy Gap Dependence of the Efficiency of Charge Separation upon the Sacrificial Reductive Quenching of the Excited States of Ru(II)-Diimine Photosensitizers in Aqueous Solution

The yield of oxidized and reduced products released into the bulk solution upon electron−transfer quenching is the critical parameter governing the efficacy of a redox photosensitizer. The cage escape efficiency (ηce) is a measure of the competition between back electron transfer (k bt) and diffusional separation (k ce) of the geminate redox pair formed within the solvent cage in the quenching act. Values of ηce have been obtained for the sacrificial reductive quenching of the excited states of homo-and heteroleptic Ru(II)-diimine complexes of bpy (2,2−bipyridine), bpz (2,2′−bipyrazine), and bpm (2,2′−bipyrimidine) by TEOA and EDTA. The values of ηce vary with the ground-state reduction potentials of the complexes, describing a strong U-shaped dependence for EDTA, but a much weaker dependence for TEOA. Plots of log(k bt/k ce) vs ΔGbt° exhibit well-defined bell-shaped curves, indicating that k bt traverses the “normal” and “inverted Marcus” regions if k ce has a constant value for all the one-electron reduced photosensitizers.