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Two new ruthenium(Ⅱ) complexes, [Ru(btz) 3 ](ClO 4 ) 2 (1) and [Ru(btz)(dppz) 2 ](ClO4 ) 2 (2) (btz = 4,4′-bithiazole, dppz = dipyrido[3,2-a:2′,3′-c]phenazine), have been synthesized and characterized by elemental analysis, 1 H NMR, ES-MS and X-ray crystallography. The DNA binding behaviors of two complexes have been studied by spectroscopic and viscosity measurements. The results suggest that complex 1 binds to CT-DNA via an electrostatic mode, while complex 2 via an intercalative mode. Under irradiation at 365 nm, both complexes were found to promote the cleavage of plasmid pBR 322 DNA from supercoiled form Ⅰ to nicked form Ⅱ. The mechanism studies reveal that singlet oxygen 1 O 2 and hydroxyl radical (OH • ) play a significant role in the photocleavage process. ruthenium(Ⅱ) complex, DNA binding, DNA photocleavage, PDT