Bis(2,2′-bipyridine)ruthenium(II) complexes with imidazo[4,5-f ][1,10]-phenanthroline or 2-phenylimidazo[4,5-f ][1,10]phenanthroline

Abstract

Imidazo[4,5-f]1,10-phenanthroline (ip), 2-phenylimidazo[4,5-f][1,10]phenanthroline (pip) and their (bipy) 2 Ru 2+ complexes (bipy = 2,2′-bipyridine) have been synthesized and characterized. The oxidation potentials of [Ru(bipy) 2 (ip)] 2+ and [Ru(bipy) 2 (pip)] 2+ were found to be 1.254 and 1.284 V vs. saturated calomel electrode respectively; the reduction of ip and pip appears to be irreversible at ca. -0.85 V. The photophysical properties of the complexes were perturbed in the presence of calf thymus DNA. The distinct changes including hypo- or hyper-chromicity at different UV/VIS absorption bands, enhancements of integrated emission intensity and excited-state lifetime, and efficiency of emission quenching by [Fe(CN) 6 ] 4- all indicate the stronger binding affinity to DNA of [Ru(bipy) 2 (pip)] 2+ over that of [Ru(bipy) 2 (ip)] 2+ , consistent with the greater planar area and extended π system of the pip ligand. The luminescence of the complexes showed monoexponential decay at any [DNA]∶[Ru] ratio. The circular dichroism signals of the dialysates of the racemic complexes against calf thymus DNA suggest that the complexes bind to the DNA with enantioselectivity favouring the Δ isomers. These phenomena all suggest that the complexes bind through intercalation of ip or pip into base pairs. The crystal structure of [Ru(bipy) 2 (ip)][ClO 4 ] 2 ·H 2 O was determined; it contains the planar bidentate ligand ip and two twisted bipy ligands with torsional angles between each bipyridine ring pair of 5.7 and 8.6°.