Two new mixed ligand complexes of cobalt(iii) containing ancillary biimidazole ligands, [Co(phen)(2)H(2)biim](ClO(4))(3).0.5(H(7)O(3))(ClO(4)) (1) and [Co(bpy)(2)H(2)biim](ClO(4))(3).3H(2)O (2) have been synthesized and characterized by various spectroscopic and electrochemical techniques. Both complexes and , have been characterized by the single crystal X-ray diffraction method. While cobalt complex was found to crystallize in the triclinic system with the P1[combining macron] space group, complex was found to crystallize in the orthorhombic system with the Pbca space group. The unit cell packing of complex shows the presence of a water cluster, which is buttressed by oxygen atoms present in the perchlorate anions and water with an average OO bond distance of 2.41-2.86 A. Interaction of these complexes with calf thymus DNA (CT-DNA) was investigated by electronic spectral titrations. The results suggest that complexes and bind to DNA through the groove via hydrogen bonding. This is due to the presence of -NH in the ancillary ligand biimidazole, which favors hydrogen bonding with DNA base pairs. The intrinsic DNA binding constant values for complexes and were found to be (5.18 +/-0.23) x 10(4) M(-1) and (1.26 +/- 0.13) x 10(4) M(-1) respectively. Groove binding of these complexes with DNA is further supported by viscosity measurements, thermal denaturation and circular dichroism studies. Among the two cobalt complexes, complex possesses greater DNA binding strength due to the presence of the conjugated aromatic structure of the phen ligand which increases the hydrophobic interactions with DNA. DNA cleavage experiments using plasmid DNA pUC 18 show that these complexes exhibit efficient photonuclease activity in the presence of molecular oxygen.