Modulating In Vitro Photodynamic Activities of Copper(II) Complexes

The photodynamic activities of selected copper(II) complexes [Cu(L 1 )B]ClO 4 ( 1 , 2 ) and [Cu(L 2 )B]ClO 4 ( 3 , 4 ), where L 1 = 2‐[(pyridin‐2‐yl)methyleneamino]phenol, L 2 = 2‐[(pyridin‐2‐yl)methyleneamino]benzenethiol and B are 1 H ‐imidazo[4,5‐ f ][1,10]phenanthroline (L 3 ) and 1‐(pyren‐2‐yl)‐1 H ‐imidazo[4,5‐ f ][1,10]phenanthroline (L 4 ), were studied in detail in a structure‐activity relationship (SAR)‐based approach to explore the role of S‐coordination to Cu II in photodynamic therapy (PDT) to human cervical cancer (HeLa) cells. Although all of the complexes were toxic in the dark (IC 50 ≈ 10 µ m ), we observed enormous enhancement (about 10 fold) in the cytotoxicity of the complexes in visible light (400–700 nm, 10 J cm –2 ), resulting in IC 50 ≈ 1.0 µ m . Intracellular reactive oxygen species (ROS) generated in the HeLa cells during photoactivation of the complexes was examined by 2′,7′‐dichlorofluorescein diacetate (DCFH‐DA) assay. The degree of photocytotoxicity of the complexes is related to the 1 O 2 generation. S‐coordinated complexes exhibited enhanced photocytotoxicity due to the presence of the low‐lying, long‐lived triplet excited state and hence increased ability to generate 1 O 2 from 3 O 2 in a type‐II photoprocess. We observed dual photosensitization of S‐coordination and the pyrenyl moiety for complex 4 , leading to a remarkable PDT effect in HeLa cells with IC 50 ≈ 0.3 µ m, and the cell death was apoptotic in nature. Overall, a SAR‐based approach in modulating the in vitro photodynamic activity of the copper(II) complexes is of paramount importance in designing copper‐based next‐generation PDT agents.