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ABSTRACT We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)₂Ru(n,n′‐dhbp)]Cl₂withn = 6 and 4 in 1_Aand 2_A, respectively). Full characterization data are reported for 1_Aand 2_Aand single crystal X‐ray diffraction for 1_A. Both 1_Aand 2_Aare diprotic acids. We have studied 1_A, 1_B, 2_A, and 2_B(B = deprotonated forms) by UV‐vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy³MLCT states relative to the acidic forms. Complexes 1_Aand 2_Aproduce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC₅₀light values as low as 0.50 μM with PI values as high as >200vs. MCF7. Computational studies were used to predict the energies of the³MLCT and³MC states. An inaccessible³MC state for 2_Bsuggests a rationale for why photodissociation does not occur with the 4,4′‐dhbp ligand. Low dark toxicity combined with an accessible³MLCT state for¹O₂generation explains the excellent photocytotoxicity of 2.