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International audience; In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O-2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O-2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

International audience; We describe the first method to control, in a single microfluidic device, the generation, fusion, and mixing of pico- to nanoliter water-in-oil drops using a straightforward LED light stimulus. This is achieved by implementing a photosensitive surfactant in the water phases of a novel dual flow-focusing microfluidic configuration. UV illumination at 365 nm enables dynamic switch of the flow behavior between a stable dual jet regime (−UV) and a stable dual drop regime (+UV), thus allowing us to generate on-demand two distinct populations of water drops. These drops are fused and mixed downstream inside expansion chambers, offering both a dynamic control by light stimulation and a controllable degree of fusion and mixing by the positions of chambers. The mixing inside the fused drops is found to be much faster than diffusion due to the chaotic advection inside the moving drops.; Grâce à l’introduction d’un tensioactif photosensible dans les phases aqueuses d’un nouveau type de dispositif microfluidique à double focalisation de flux, nous décrivons pour la première fois le contrôle de la génération, de la fusion et du mélange de microgouttes aqueuses dispersées dans une phase d’huile, à l’aide d’un stimulus lumineux produit par une simple diode électroluminescente. Une illumination UV à 365 nm fait basculer réversiblement l’écoulement entre un régime à double jet (−UV) et un régime à double train de gouttes (+UV), nous permettant ainsi de générer à la demande, et de manière dynamique, deux populations de gouttes de compositions distinctes. Des chambres d’expansion permettent d’induire la fusion des gouttes générées par la lumière et de contrôler ainsi le mélange de leurs composants. Nous montrons que la position des chambres définit le degré de fusion et de mélange. Ce mélange s’opère de manière bien plus rapide que le temps caractéristique de diffusion des composants, grâce à l’advection chaotique créée au sein des gouttes en mouvement.