Photooxidation of DNA by a cobalt(II) tridentate complex

[Co(bzimpy) 2], where bzimpy is 2,6-bis(benzimidazol-2-yl)pyridine was synthesized and characterized by ESI–MS (electrospray ionization mass spectrometry), UV-visible and fluorescence spectra. Absorption titration and thermal denaturation experiments indicate that the complex binds to DNA with moderate strength. Viscosity measurement shows that the mode of binding could be surface binding. Fluorescence study shows that the fluorescence intensity of the complex decreases with increasing concentrations of DNA, which is due to the photoelectron transfer from guanine base to excited MLCT (metal to ligand charge transfer) state of the complex. Photoexcitation of the complex in the MLCT region in the presence of plasmid DNA has been found to give rise to nicking of DNA.