2-(5-Phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (Hptip) and its RuII complex [Ru(bpy)2(Hptip)](PF6)2 (where bpy = 2,2′-bipyridine) have been synthesized and characterized by elemental analysis, 1H 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 DNA-binding properties of [Ru(bpy)2(Hptip)](PF6)2 were also investigated by means of UV-vis and emission spectroscopy, salt effects, steady-state emission quenching by [Fe(CN)6]4–, 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.