Two novel tridentate polypyridyl mixed-ligand complexes of cobalt(II) and ruthenium(II), [Co(TolylTPy)(H₂Bzimpy)]Cl₂ [TolylTPy＝4'-p-tolyl-2,2':6',2''-terpyridine, H₂Bzimpy＝2,6-bis(benzimi- dazol-2-yl)pyridine] (A) and Ru(TolylTPy)(Bzimpy) (B) were synthesized and characterized by elemental analysis, IR and ¹H NMR. The crystal structure of complex B was determined. The interactions between the complexes and calf thymus DNA have been investigated by absorption and fluorescence spectroscopies. The photoactivated cleavage of pBR322 DNA by the complexes was studied. The spectrophotometric studies suggested that both complexes A and B bind to DNA by electrostatic interactions. The agarose gel electrophoresis showed that complex A converted supercoiled pBR322 DNA to nicked and linear DNA when it was irradiated at 310 nm for 15 min.

Key words: tridentate polypyridyl ligand, cobalt(II), ruthenium(II) complex, DNA, absorption spectra, fluorescence spectra