Experimental and DFT studies on the DNA-binding trend and spectral properties of complexes [Ru(bpy)2L] (L = dmdpq, dpq, and dcdpq)

The trend in DNA-binding affinities and the spectral properties of a series of Ru(II) polypyridyl complexes, Ru(bpy)₂(dmdpq)]²⁺ (1), Ru(bpy)₂(dpq)]²⁺ (2), Ru(bpy)₂(cndpq)]²⁺ (3) (bpy = 2,2′-bipyridine; dpq = dipyrido3,2-d:2′,3′-f]quinoxaline; dmdpq = di-methyl-dpq; dcdpq = di-cyano-dpq), have been experimentally and theoretically investigated. The DNA-binding constants K_b of the complexes were determined systematically with spectrophotometric titration. The density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out for these complexes. The experimental results show that these complexes bind to DNA in intercalation mode, and the order of their intrinsic DNA-binding constants K_b is K_b(1) < K_b(2) ? K_b(3). The substituents on the intercalative ligands of the complexes play a very important role in the control of DNA-binding affinities of the complexes, in particular, the stronger electron-withdrawing substituent (-CN) on the intercalative ligand can greatly improve the DNA-binding property of the derivative complex. The trend in DNA-binding affinities as well as the spectral properties of metal-ligand charge-transition (¹MLCT) of this series of complexes can be reasonably explained by applying the DFT and TDDFT calculations and the frontier molecular orbital theory.