Redox Active Ion-Paired Excited States Undergo Dynamic Electron Transfer.

Abstract

Ion-pair interactions between a cationic ruthenium complex, [Ru(dtb)2(dea)][PF6]2, C12+ 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, [C12+, 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, ΔGw was the predominant contributor to the driving force for the reaction. Natural bond order calculations provided natural atomic charges that enabled quantification of ΔGw for all the atoms in C12+ and [C12+, 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 C12+* excited state traps the halide and prevents its oxidation, but allows for dynamic oxidation of a second iodide ion.