Effect of solution medium on the rate constants of excited-state electron-transfer quenching reactions of ruthenium(II)-diimine photosensitizers

In this paper, we review the effects of solution medium (pH, solvent, temperature, ionic strength, specific electrolytes) on the oxidative and reductive quenching rate constants kq of the excited states of Ru(II)-diimine photosensitizers. Diffusion of the donor and acceptor species together to form the precursor complex (ket) and electron transfer within the complex (ket) contribute to the value of kq. Values of kd vary with bulk solution properties; variations of ket can be described within the context of Marcus theory, wherein dynamic solvent effects influence the nuclear frequency factor and electronic coupling, and static properties cause changes in the driving force of electron transfer ΔGeto and the reorganization energy λ. The pH can affect the state of protonation of the excited photosensitizer and/or the quencher, thereby altering ket through changes in ΔGcto and kd through changes in the charges of the reactants. Ionic species are ion-paired by the dominant counterion; the Olson-Simonson treatment allows the electron transfer components of quenching for ion-paired (kip) and nonion-paired (kmp) species to be extracted. The quenching of ∗Ru(bpy)32+ by methylviologen is used to demonstrate specific salt effects, which result in variations in λ; λ is lowest, and kq highest, for the anions with the most weakly-held hydration spheres and the strongest structure-breaking abilities (e.g. ClO4− , I−). Quenching rate constants can be fine-tuned through the variation of solvent, pH, electrolyte, ionic strength, and temperature.