2-(5-Phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (Hptip) and its Ru II complex [Ru(bpy) 2 (Hptip)](PF 6 ) 2 (where bpy ¼ 2,2 0 -bipyridine) have been synthesized and characterized by elemental analysis, 1 H NMR spectroscopy, and mass spectrometry. The acid-base properties of the complex were studied by UV-visible and luminescence spectrophotometric pH titrations, and ground-and excited-state acidity ionization constants were derived. The DNAbinding properties of [Ru(bpy) 2 (Hptip)](PF 6 ) 2 were also investigated by means of UV-vis and emission spectroscopy, salt effects, steady-state emission quenching by [Fe(CN) 6 ] 4À , DNA competitive binding with ethidium bromide, DNA melting experiments, and viscosity measurements. Density functional theoretical calculations were also carried out in order to understand the DNA binding properties.