Photophysical Study of DNA-Bound Complexes Containing Two Covalently linked [Ru(2,2‘-bipyridine)3]2+-Like Centers

The changes in the absorption, steady-state emission, and luminescent lifetime properties, upon binding of binuclear complexes of type [(bpy) 2 Ru(Mebpy)−(CH 2 ) n −(bpyMe)Ru(bpy) 2 ] 4+ (bpy = 2,2‘-bipyridine; Mebpy− = 4-methyl-2,2‘-bipyridine-4‘-; 1b, n = 5; 1c, n = 7) to double-stranded DNA, have been compared relative to those for the monometallic analogue [Ru(bpy) 2 (Me 2 bpy)] 2+ ( 1a ) (Me 2 bpy = 4,4‘-dimethyl-2,2‘-bipyridine). Mc Ghee von Hippel analysis indicates that the binuclear complexes 1b and 1c bind more than 100 times as strongly to DNA as does 1a . Luminescence lifetime analysis in the presence and absence of DNA resolves at least two distinct binding modes which exhibit markedly different accessibility to oxygen and dissimilar behavior under physiological salt conditions. The binding to DNA by the binuclear complexes shows a much greater resistance to increased NaCl concentration relative to that of the monometallic complex, while plots of log K obs versus log [Na + ] indicate that for both the mononuclear and binuclear complexes electrostatic binding dominates. Absorption spectra measured reveal a complex mode of binding for the bimetallic complexes 1b and 1c under high-loading conditions (e.g., [nucleotide]:[ruthenium center] ∼ 1.)