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⁺ ( 1 ²⁺ ), [Ru(dpma)(bpy)(dppz)] ²⁺ ( 2 ²⁺ ), and [Ru(dpma)(phen)(dppz)] ²⁺ ( 3 ²⁺ )-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 (PF ₆ ) 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 (PF ₆ ) ₂ and 3 (PF ₆ ) 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 (PF ₆ ) ₂ and calf thymus DNA is observed. The intrinsic binding constants ( K _b ) of 2 (PF ₆ ) ₂ and 3 (PF ₆ ) ₂ with calf thymus DNA are determined to be (1.4 ± 0.4) × 10 ⁵ and (9.5 ± 0.15) × 10 ⁴ M ^-1 , respectively. In addition, these spectroscopic results are compared with those of the prototype complex [Ru(bpy) ₂ (dppz)] ²⁺ ( 4 ²⁺ ), and density functional theory and time-dependent density functional theory calculations are employed to elucidate these experimental findings.