Fig. 2. Visible light photocatalysis.

(A) Ruthenium and iridium complexes, such as 1–3, readily absorb visible light and can mediate numerous photochemical transformations (13, 14). (B) Photoexcited Ru*(bpy)₃²⁺ can act as an electron shuttle, interacting with sacrificial electron donors D (path i) or acceptors A (path ii) to yield either a strongly reducing or oxidizing catalyst toward organic substrates S. Ru*(bpy)₃²⁺ can also directly transfer energy to an organic substrate to yield electronically excited species (path iii). Abbreviations: bpy, 2,2'-bipyridine; bpz, 2,2’-bipyrazine; ppy, 2-phenylpyridine.