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)₂Ru²⁺ 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)][ClO₄]₂·H₂ 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°.

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