Rate and Stability of Photocatalytic Water Oxidation using Ru(bpy)32+ as Photosensitizer

The kinetics of homogeneous photocatalytic water oxidation is reported using Ru(bpy)3Cl2 as photosensitizer, Na2S2O8 as sacrificial electron acceptor, and three different water-oxidation catalysts: the ruthenium catalyst Ru(bda)(isoq)2 (1, H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, isoq = isoquinoline), Co(NO3)2 (2), and Ir(Cp*)(dmiz)(OH)2 (3, Cp* = pentamethylcyclopentadienyl, dmiz =1,3-dimethylimidazol-2-ylidene). At pH 7.0, in a phosphate buffer and under blue light irradiation, the production of O2 at the catalyst is rate determining when 2 or 3 is used as a water-oxidation catalyst. However, when 1 is used as the catalyst under identical conditions the turnover at the water-oxidation catalyst is not the rate-limiting step of the photocatalysis. Instead, the step limiting dioxygen production is the transfer of electrons from the catalyst to the photooxidized photosensitizer Ru(bpy)33+. Due to the instability of Ru(bpy)33+ in neutral aqueous solutions, slow electron transfer results in significant photosensitizer decomposition, which limits the overall stability of the photocatalytic system. When the catalyst 1 is used, decomposition of both the photosensitizer and the catalyst 1 occurs in parallel. However, the photosensitizer and the catalyst also stabilize each other, i.e., the TON increases when more photosensitizer is added, while the photocatalytic turnover number PTON increases when more catalyst 1 is added. These data demonstrate not only that new and more stable water oxidation catalysts should be developed in the future but also that new and more stable photosensitizers are needed.