Ionic liquids as a novel medium for photochemical reactions. Ru(bpy) 3 2+/viologen in imidazolium ionic liquid as a photocatalytic system mimicking the oxido-reductase enzyme

Ionic liquids are suitable media which stabilize charged intermediates favoring those mechanisms that occur through charge separation. We have used ionic liquids to develop a photocatalytic system to perform the reduction of a carbonyl group to alcohol, thus mimicking the behavior of the reductase enzymes. The photochemical cycle is based on the well-known electron transfer from the Ru(bpy)₃ ²⁺ complex in its excited state, acting as electron donor to MV²⁺, which acts as electron acceptor. The initial electron transfer process can be promoted upon selective Ru(bpy) ₃ ²⁺ excitation by visible light. By means of laser flash photolysis we have provided evidence of the nature and lifetimes of the intermediates involved in the photocatalytic system. Thus, the initial electron transfer between Ru(bpy)₃ ²⁺ triplets and viologen MV ²⁺ forms the MV^•+ radical cation, which upon accepting an H^• atom from a suitable hydrogen atom donor, forms the corresponding dihydropyridine MVH⁺ reducing agent.