Towards electrochemical regeneration of redox photocatalysts? The example of the tris(2,2′-bipyridine)ruthenium(II)/methylviologen system

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The regeneration of redox photocatalysts usually requires the use of sacrificial reagents that are added to the medium in stoichiometric amounts, either as an electron donor or as an electron acceptor, to act as an electron relay to the photocatalyst to ensure its effective regeneration. In the present work, we consider the possibility of direct electrochemical regeneration of the catalyst using a model system made of tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) as photosensitizer and methylviologen (MV2+) as quenching substrate. Based on hydrodynamic amperometry experiments, we demonstrate that this can indeed be possible provided that appropriate potential values are applied to ensure the electrochemical reduction of [Ru(bpy)3]3+ back to [Ru(bpy)3]2+ while avoiding interfering processes arising from electron transfer reactions involving the quencher and/or the products/intermediates generated in the course of the photocatalytic event. Furthermore, using EDTA as a sacrificial electron donor, we show that photocurrents can also be exploited for regenerating the quenching substrate by reduction of the reaction product MV+ (back into MV2+).