Role of electronic structure on DNA light-switch behavior of Ru(II) intercalators

A series of ruthenium(II) complexes possessing ligands with an extended pi system were synthesized and characterized. The complexes are derived from Ru(bpy)3](2+) (1, bpy = 2,2'-bipyridine) and include Ru(bpy)2(tpphz)](2+) (2, tpphz = tetrapyrido3,2-a:2',3'-c:3',2'-h:2',3'-j]phenazine), Ru(bpy)2(dppx)](2+) (3, dppx = 7,8-dimethyldipyrido3,2-a:2',3'-c]phenazine), Ru(bpy)2(dppm2)](2+) (4, dppm2 = 6-methyldipyrido3,2-a:2',3'-c]phenazine), and Ru(bpy)2(dppp2)](2+) (5, dppp2 = pyrido2',3':5,6]pyrazino2,3-f]1,10]phenanthroline). The excited-state properties of these complexes, including their DNA "light-switch" behavior, were compared to those of Ru(bpy)2(dppz)](2+) (6, dppz = dipyrido3,2-a:2',3'-c]phenazine). Whereas 2, 3, and 4 can be classified as DNA light-switch complexes, 5 exhibits negligible luminescence enhancement in the presence of DNA. Because relative viscosity experiments show that 2-6 bind to DNA by intercalation, their electronic absorption and emission spectra, electrochemistry, and temperature dependence of the luminescence were used to explain the observed differences. The small energy gap between the lowest-lying dark excited state and the bright state in 2-4 and 6 is related to the ability of these complexes to exhibit DNA light-switch behavior, whereas the large energy gap in 5 precludes the emission enhancement in the presence of DNA. The effect of the energy gap among low-lying states on the photophysical properties of 1-6 is discussed. In addition, DFT and TD-DFT calculations support the conclusions from the experiments.