Five binuclear Schiff base copper(ii) complexes [Cu(2)(L)(OAc)].3DMF (), [Cu(2)(L)(OAc)](2).3DMF (), [Cu(2)(L)(BNPP)].3CH(3)CN (), [Cu(2)(L)(Fa)].2DMF () and [Cu(2)(L)(Pa)].DMF () (H(3)L = N,N'-bis(3,5-tert-butylsalicylidene-2-hydroxy)-1,3-propanediamine, OAc = acetic acid, BNPP = bis(4-nitrophenyl)phosphate, Fa = 2-tetrahydrofuroic acid, Pa = benzoic acid) have been synthesized and characterized by X-ray single-crystal structure analysis. Variable-temperature magnetic susceptibility studies (2-300 K) indicate the existence of ferromagnetic coupling between the copper(ii) ions in complexes and , and antiferromagnetic coupling in complexes and . The interaction of these complexes with CT-DNA has been studied by using absorption and emission spectral methods. The apparent binding constant (K(app)) values for complexes , , and are 4.67 x 10(5), 9.48 x 10(5), 4.30 x 10(5) and 3.90 x 10(5) M(-1), respectively, which show that the complexes bind to DNA by moderate intercalative binding modes. Furthermore, all these complexes can cleave plasmid DNA to nicked DNA in a sequential manner as the concentrations or reaction times are increased in the absence of reducing agent. Their cleavage activities are promoted in the presence of hydrogen peroxide. The cleavage mechanisms between the complexes and plasmid DNA are likely to involve singlet oxygen (1)O(2) and OH as reactive oxygen species.