We associate this phenomenon to the potential aggregation phenomenon that can occur as the photochemical water oxidation reaction proceeds. This decreases substantially the number of Ru-tda active sites in contact with the solution and thus a decrease of overall oxygen generation and rate. Indeed, a dynamic light scattering (DLS) experiment (See Fig. and Table -4) clearly reveals a progressive increase of particle size as the reaction proceeds. Finally, we also tested the effect of increasing the anionic strength up to 1 M pH 7 phbf resulting in a significant decrease of O2 generation (See Fig. ).