Aryl radical anions: key intermediates on the route to sustainable green-light ionizations

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Aqueous photoionizations produce hydrated electrons, which have already been successfully used for the reductive detoxification of halogenated organic waste and for the direct reduction of nitrogen or carbon dioxide; however, all these procedures had to rely on UV-C light (< 254 nm) for electron generation. Our proposal is concerned with photoionizations that require only green light (532 nm) and are based on catalytic cycles such that nothing but a bioavailable sacrificial donor is consumed.Starting from our very recently published exploratory investigations on the ruthenium-tris(bipyridyl) dication (the first example of a completely green-light driven cyclic photoionization, but with a very low quantum yield) and on the naphthalene radical anion (which exhibits the highest quantum yield known to date for the green-light ionization of an unstable intermediate, but cannot be generated with green light), we intend to fulfil that assignment by a combination of both approaches in a micellar environment. To that end, the ruthenium compound shall serve us as a light-harvesting compound and an arene with a larger number of rings, e.g., a pyrene derivative, as a redox catalyst. By an energy transfer from the green-light excited ruthenium complex followed by an electron transfer from the sacrificial donor ascorbate, we store the energy of the first photon in the aryl radical anion, which we then ionize with a second green photon, recovering the redox catalyst in that process. The function of the micelle is to ensure the desired order of the reaction sequence and to suppress side reactions, both through noncovalent interactions.We anticipate aryl radical anions to exhibit good photoionizability because their rigid molecular skeleton should decelerate the radiationless deactivation of their excited states, which competes with the electron ejection, and their high energy content results in a high excess energy of electron formation. Through intensity dependent measurements of the electron yields by two-pulse laser flash photolysis, together with lifetime measurements of the excited radical anions, we intend to study both effects quantitatively.As we expect, this project will allow us to procure completely green-light driven electron sources with as high an efficiency as possible on one hand, and on the other hand will provide important new insight into the factors that gouvern the quantum yields of photoionizations.