The interesting DNA-binding properties of three novel dinuclear Ru(II) complexes with varied lengths of flexible bridges

Three binuclear Ru(II) complexes with two Ru(bpy)₂(pip)]²⁺-based subunits {where bpy = 2,2′-bipyridine and pip = 2-phenylimidazo4,5-f]1,10]phenanthroline} being linked by varied lengths of flexible bridges, were synthesized and characterized by ¹H NMR, elemental analysis, UV-visible (UV-vis) and photoluminescence spectroscopy. The structures of the three complexes were optimized by density functional theory calculations. The interaction of the complexes with calf thymus DNA was investigated by UV-vis and luminescence titrations, steady-state emission quenching by Fe(CN)₆]⁴⁻, DNA competitive binding with ethidium bromide, DNA melting experiments, and viscosity measurements. The experimental results indicated that the three complexes bound to the DNA most probably in a threading intercalation binding mode with high DNA binding constant values three orders of magnitude greater than the DNA binding constant value reported for proven DNA intercalator, mononuclear counterpart Ru(bpy)₂(p-mopip)]²⁺ {p-mopip = 2-(4-methoxylphenyl)imidazo4,5-f]1,10]phenanthroline}.