Substituent effect and wavelength dependence of the photoinduced Ru–O homolysis in the [Ru(bpy)2(py-SO3)]+-type complexes

Abstract

[Ru(bpy)₂(py-SO₃)]⁺ (3, bpy = 2,2′-bipyridine, py-SO₃ = pyridine-2-sulfonate) was recently found to undergo py-SO₃ ligand dissociation and py-SO₃˙ radical generation under hypoxic conditions upon irradiation (Chem. Commun., 2015, 51, 428). To explore the substituent effect on the Ru–O homolysis by which the py-SO₃˙ radical may be produced, [Ru(4,4′-(R)₂-bpy)(py-SO₃)]⁺, where R = OCH₃ (1), CH₃ (2), COOCH₃ (4), were synthesized and their photochemical properties were investigated. The py-SO₃˙ radical generation efficiencies followed the order of 4 > 3 > 2 > 1, and the radical generation efficiencies are wavelength dependent. As a result, 3 and 4 may lead to DNA covalent binding and DNA cleavage upon 355 nm irradiation, but merely DNA covalent binding upon 470 nm irradiation. In contrast, 1 and 2 can serve as DNA photo-binding agents only due to their less efficient Ru–O homolysis. The Ru–O homolysis via the ³σ(Ru–O)π*(R-bpy) state is proposed to rationalize the substituent effect and the wavelength dependence, which is supported by TD-DFT calculations. This work gave insights into the mechanism of the Ru–O homolysis and provided guidelines for developing new [Ru(bpy)₂(py-SO₃)]⁺-type complexes with higher Ru–O homolysis efficiency. Such complexes have dual activities of photoactivated chemotherapy (PACT) and photodynamic therapy (PDT) under hypoxic conditions and are therefore promising as a new class of antitumor drugs.