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-2yl)methyleneamino]benzenethiol and B are 1H-imidazo[4,5-f ]-[1,10]phenanthroline (L 3 ) and 1-(pyren-2-yl)-1H-imidazo[4,5-f ]-[1,10]phenanthroline (L 4 ), were studied in detail in a structureactivity 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 μM), 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 μM. Intracellular reactive oxygen species (ROS) generated in the HeLa cells during photoactivation of the complexes was [a]