Electron Accumulation on Naphthalene Diimide Photosensitized by [Ru(2,2'-Bipyridine)3]2.

In a molecular triad comprised of a central naphthalene diimide (NDI) unit flanked by two [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) sensitizers, NDI2- is formed after irradiation with visible light in deaerated CH3CN in the presence of excess triethylamine. The mechanism for this electron accumulation involves a combination of photoinduced and thermal elementary steps. In a structurally related molecular pentad with two peripheral triarylamine (TAA) electron donors attached covalently to a central [Ru(bpy)3]2+-NDI-[Ru(bpy)3]2+ core but no sacrificial reagents present, photoexcitation only leads to NDI- (and TAA+), whereas NDI2- is unattainable due to rapid electron transfer events counteracting charge accumulation. For solar energy conversion, this finding means that fully integrated systems with covalently linked photosensitizers and catalysts are not necessarily superior to multicomponent systems, because the fully integrated systems can suffer from rapid undesired electron transfer events that impede multielectron reactions on the catalyst.