pH effects on optical and DNA binding properties of a thiophene-containing ruthenium(II) complex

A new ruthenium(II) complex, [Ru(bpy)2(Htip)]Cl2 {where bpy = 2,2′-bipyridine and Htip = 2-(thiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline}, has been synthesized and characterized by 1H NMR spectroscopy, elemental analysis, and mass spectrometry. The pH effects on UV–Vis absorption and emission spectra of the complex have been studied, and the ground- and excited-state acidity ionization constant values have been derived. The calf thymus (ct) DNA binding properties of the complex have been investigated with UV–Vis absorption 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 molecular structures and electronic properties of [Ru(bpy)2(Htip)]2+ and deprotonated form [Ru(bpy)2(tip)]+ have also been investigated by means of density functional theory calculations in an effort to understand the DNA binding properties. The results suggest that the complex undergo three-step successive protonation/deprotonation reactions with one of which occurring over physiological pH region, and act as a ct-DNA intercalator with an intrinsic DNA binding constant value on 105 M−1 order of magnitude that is insensitive to pH.