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

Imidazo[4,5-f]1,10-phenanthroline (ip), 2-phenylimidazo[4,5-f][1,10]phenanthroline (pip) and their (bipy)(2)Ru2+ 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 pi 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 calfthymus DNA suggest that the complexes bind to the DNA with enantioselectivity favouring the Delta 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)][ClO4](2) . H2O 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 degrees.