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 10(4)-10(6) 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 H-1 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, Delta G(w) was the predominant contributor to the driving force for the reaction. Natural bond order calculations provided natural atomic charges that enabled quantification of Delta G(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.