First Micelle‐Free Photoredox Catalytic Access to Hydrated Electrons for Syntheses and Remediations with a Visible LED or even Sunlight

Abstract

Hydrated electrons are super-reductants, yet can be generated with visible light when two photons are pooled, most efficiently through storing the energy of the first photon in a radical pair formed by the reduction of an excited catalyst by a sacrificial donor. All previous such systems for producing synthetically useable amounts of hydrated electrons with an LED in the visible range had to resort to compartmentalization by SDS micelles to curb the performance-limiting recombination of the pair. To overcome micelle-imposed constraints on sustainability and applications, we have instead attached carboxylate groups to a ruthenium (tris)bipyridyl catalyst such that its pentaanionic radical strongly repels the dianionic radical of the bioavailable donor urate. We have explored the influence of the Coulombic interactions on the electron generation by a time-resolved study from microseconds to hours, including for comparison the unsubstituted complex, which forms a monocationic radical, with and without SDS micelles. The new homogeneous electron source is best with regard to stability and total electron output; it has a broader synthetic scope because it does not entail micellar shielding of less hydrophilic compounds, which particularly facilitates cross-couplings; and it tolerates supramolecular containers as carriers of water-insoluble substrates or products. As an application in the field, we demonstrate the solar remediation of a recalcitrant chloro-organic bulk chemical.

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

“…When HUr 2− is oxidized by the excited Ru II complex, it deprotonates and remains dianionic. Since the metal complex is also anionic in this case, Coulombic repulsion limits undesirable reverse electron transfer between the oxidized donor and the reduced photocatalyst, and consequently, the [Ru(dcob) 3 ] 4− /HUr 2− system enabled the micelle‐free production of synthetically useable amounts of hydrated electrons with an LED for the first time . Blue rather than green light was used to optimize the excitation process.…”

Multiphotonen‐Anregung in der Photoredoxkatalyse: Konzepte, Anwendungen und Methoden

“…Wenn HUr 2− durch den angeregten Ru II ‐Komplex oxidiert wird, gibt es ein Proton ab und bleibt dadurch dianionisch. Die starke Coulomb‐Abstoßung schränkt den Rückelektronentransfer zwischen oxidiertem Donor und reduziertem Photokatalysator (beide anionisch) stark ein, wodurch das [Ru(dcob) 3 ] 4− /HUr 2− ‐System erstmals die mizellfreie Erzeugung syntheserelevanter e aq .− ‐Mengen mit einer LED ermöglicht . Um die Anregung zu optimieren, wurde blaues (statt grünes) Licht verwendet.…”

Controlling Spin‐Correlated Radical Pairs with Donor–Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

“…However,i np hotochemistry one excited state quenched via photoinduced electront ransfer (PET) does not necessarily mean that one reduced or oxidized species is formed. Despite practically quantitative excited-state quenching, the actual yield of free radicals or radicali ons might be close to zero, [20,21] but the determinationo fc age-escape efficiencies h for ag iven PET event requires more sophisticated experimental techniques such as quantitative transienta bsorption spectroscopy.O wing to the lack of systematic quantitative studies in photochemistry,t he exact factorst hat govern the overall efficiencies of PET processes are stillp oorly understood and areliable predictiono fthe h values cannot be made in advance.F or instance, the inherent photoreduction efficiencies for an anionic ruthenium(II) complex by as eries of dianionic electron donors differ by as much as af actor of 30. [20] Most h values for PET reactions with triplet-excited Ru complexes are in the range from 0.05 (5 %) to 0.6 (60 %), [21][22][23][24][25][26][27] clearlyi ndicating that unproductive in-cage recombination is ag eneral energywasting problem.…”