Singlet Oxygen Formation vs Photodissociation for Light-Responsive Protic Ruthenium Anticancer Compounds: The...

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)₂Ru(6,6'-dhbp)]Cl₂ with 6,6'-dhbp = 6,6'-dihydroxybipyridine and N,N = 2,2'-bipyridine (bipy) (1), 1,10-phenanthroline (phen) (2), and 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) (3) show EC₅₀ values as low as 4 μM (for 3) vs breast cancer cells upon blue light irradiation ( Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O^- groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic ( Dalton Trans 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3 and blue light. Singlet oxygen sensor green (SOSG) shows the formation of ¹O₂ in cell culture for cells treated with 3 and blue light. At physiological pH, complexes 1-3 are deprotonated to form 1-3 in situ. Quantum yields for ¹O₂ (ϕ_Δ) are 0.87 and 0.48 for 2 and 3, respectively, and these are an order of magnitude higher than the quantum yields for 2 and 3. The values for ϕ_Δ show an increase with 6,6'-dhbp derived substituents as follows: OMe < OH < O^-. TD-DFT studies show that the presence of a low lying triplet metal-centered (³MC) state favors photodissociation and disfavors ¹O₂ formation for 2 and 3 (OH groups). However, upon deprotonation (O^- groups), the ³MLCT state is accessible and can readily lead to ¹O₂ formation, but the dissociative ³MC state is energetically inaccessible. The changes to the energy of the ³MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1-3 and their basic analogs, 1-3. This energy landscape favors ¹O₂ formation for 2 and 3 and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O^- groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.