Isostructural cubane-shaped catalysts [Co-4(III)(mu-O)(4)(mu-CH3COO)(4)(p-NC5H4X)(4)], 1-X (X = H, Me, t-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O-2 productivity with quantum efficiency up to 80% at lambda > 400 nm, thus opening a new perspective for in vitro molecular photosynthesis