Redox Active Ion-Paired Excited States Undergo Dynamic Electron Transfer

Ion-pair interactions between a cationic ruthenium complex, [Ru­(dtb)2(dea)]­[PF6]2, C1 2+ where dea is 4,4′-diethanolamide-2,2′-bipyridine and dtb is 4,4′-di-tert-butyl-2,2′-bipyridine, and chloride, bromide, and iodide are reported. A remarkable result is that a 1:1 iodide:excited-state ion-pair, [C1 2+ , I – ]+*, underwent diffusional electron-transfer oxidation of iodide that did not occur when ion-pairing was absent. The ion-pair equilibrium constants ranged 104–106 M–1 in CH3CN and decreased in the order Cl– > Br– > I–. The ion-pairs had longer-lived excited states, were brighter emitters, and stored more free energy than did the non-ion-paired states. The 1H NMR spectra revealed that the halides formed tight ion-pairs with the amide and alcohol groups of the dea ligand. Electron-transfer reactivity of the ion-paired excited state was not simply due to it being a stronger photooxidant than the non-ion-paired excited state. Instead, work term, ΔG w was the predominant contributor to the driving force for the reaction. Natural bond order calculations provided natural atomic charges that enabled quantification of ΔG w for all the atoms in C1 2+ and [C1 2+ , I – ]+* presented herein as contour diagrams that show the most favorable electrostatic positions for halide interactions. The results were most consistent with a model wherein the non-ion-paired C1 2+* excited state traps the halide and prevents its oxidation, but allows for dynamic oxidation of a second iodide ion.