Dynamic Electron Transfer
Excited-state electron-transfer reactions are of essential importance as they provide a means to convert solar energy into stored potential energy and chemical bonds. Two mechanism for electron transfer are commonly utilized for bimolecular reactions. In the first, dynamic electron transfer, a light excited chromophore must first diffuse to a redox active species before electron transfer occurs. In the alternative static mechanism, the interaction between chromophore and redox active species is formed first in a non-covalent ground-state adduct. This adduct gives rise to a non-emissive species that undergoes ultrafast light-driven electron transfer without the diffusional step. These non-covalent ground-state adducts are often enhanced through Coulombic attraction and ion-pair formation.
In an article published in the Journal of the American Chemical Society, researchers in the G. Meyer Group have shown the first example of diffusional excited-state electron transfer enabled by ion-pair formation between redox active donors and acceptors.
In this study, ion-pair interactions between a cationic ruthenium chromophore, bearing a ligand specially designed for hydrogen bond with a halide, and chloride, bromide, and iodide were investigated through a range of spectroscopic techniques. Remarkably, a 1:1 iodide:excited-state ion-pair, [C12+, I-]+*, was formed that did not undergo static electron transfer. Instead, the ion-paired complex underwent diffusional electron-transfer with a second iodide that did not occur when ion-pairing was absent. It is worth noting that the addition of less than one equivalent of halide led to ion-pairs with longer-lived excited states, which were brighter emitters and stored more free energy than did the non-ion-paired states.
The researchers utilized a novel method for calculation of partial atomic charges in concert with a complete Coloumb’s law analysis to look at the work terms associated with electron transfer. These terms indicated a favorable binding location within the designated ligand and that the ion-paired iodide was significantly stabilized to electron transfer. Neither the work terms nor the increased photooxidizing power of the excited state could explain the turn on of electron transfer to the ion-paired complex.
Instead, the results are most consistent with a model in which the formation of the ion-pair competes with electron transfer. It was determined that the rate constant for ion-pairing must be more than 3-orders of magnitude larger than that for electron transfer. In this model the non-ion-paired chromophore traps the first halide and prevents its oxidation. After the initial ion-pairing a second iodide cannot be trapped and dynamic oxidation of a second iodide ion is allowed.