The combined use of a tetranuclear Ru(II) dendrimeric photosensitizer (1) and of a tretraruthenium substituted polyoxotungstate (2) as the catalyst enables photo-induced water oxidation at 550 nm producing O₂ with an outstanding quantum yield of 0.30.

Photoinduced water oxidation to molecular oxygen takes place in systems made of [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as the photosensitizer, [Co₄O₄(O₂CMe)₄(py)₄ ] (py = pyridine) as the molecular catalyst and Na₂S₂O₈ as the sacrificial electron acceptor. The photochemical quantum yield of the process reaches the outstanding value of 30% and depends on pH and catalyst concentration. Transient absorption spectroscopy experiments aimed to clarify the first events of the photocatalytic process are also reported.