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Mixed ligand complexes of the type [Ni(phen)2(bnp)](PF6)2 · 2H2O (1) and [Ni(bpy)2(bnp)](PF6)2 · 2H2O (2) (where phen, 1,10,Phenanathroline; bpy, 2,2 bipyridine; bnp, benzo[1.8]naphthyridinone-2-ol) have been synthesized and characterized by employing analytical and spectral methods. The DNA binding properties of the complexes with calf thymus-DNA were studied by using absorption spectra, viscosity measurements as well as thermal denaturation experiments. The absorption spectra indicated that the Ni(II) complexes intercalate tightly between the base pairs of the DNA with intrinsic DNA binding constants of 1.9 × 104 M−1 for (1) and 2.5 × 104 M−1 for (2) in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.2, respectively. The large enhancement in the relative viscosity of DNA on binding to the bnp supports the proposed DNA binding modes. The oxidative cleavage and photo-cleavage activity of complexes (1) and (2) were studied. The photo-induced DNA cleavage activities of the complexes in the presence and absence of d...