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, 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 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 C1^2+* excited state traps the halide and prevents its oxidation, but allows for dynamic oxidation of a second iodide ion.