Dual-Emissive Tris-Heteroleptic Ruthenium Complexes: Tuning the DNA-Triggered Ratiometric Emission Response by Ancillary Ligands.

Three tris-heteroleptic mononuclear Ru(II) complexes with dual fluorescence and phosphorescence-[Ru(dpma)(bpy)(phen)]2+ (12+), [Ru(dpma)(bpy)(dppz)]2+ (22+), and [Ru(dpma)(phen)(dppz)]2+ (32+)-have been designed and used as ratiometric light-response probes for DNA, where dpma is di(pyrid-2-yl)(methyl)-amine, bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline, and dppz is dipyridophenazine, respectively. Single crystals of complex 2(PF6)2 have been obtained and studied by X-ray analysis. The interactions of these complexes with different DNAs are investigated by means of spectroscopic methods, viscosity measurements, and molecular modeling. In the presence of calf thymus DNA, complexes 2(PF6)2 and 3(PF6)2 show the emergence of a new lower-energy phosphorescence emission band; meanwhile, the higher-energy fluorescence emission band is essentially unchanged, functioning as an intrinsic internal reference. These two complexes exhibit stronger preference for calf thymus DNA over single-strand DNA (d(A)16 and d(C)16). In contrast, no binding interaction between 1(PF6)2 and calf thymus DNA is observed. The intrinsic binding constants (Kb) of 2(PF6)2 and 3(PF6)2 with calf thymus DNA are determined to be (1.4 ± 0.4) × 105 and (9.5 ± 0.15) × 104 M-1, respectively. In addition, these spectroscopic results are compared with those of the prototype complex [Ru(bpy)2(dppz)]2+ (42+), and density functional theory and time-dependent density functional theory calculations are employed to elucidate these experimental findings.