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A series of ruthenium (II) complexes have been prepared which contain two phenanthroline ligands and a third bidentate ligand which is one of a set of derivatives of the parent dipyrido3, 2-a: 2', 3'cphenazine (DPPZ) ligand. The spectroscopic properties of these complexes in the presence and absence of DNA have also been characterized. The derivatives have been prepared by condensation of different diaminobenzenes or diaminopyridines with the synthetic intermediate bis (1, 10-phenanthroline) (1, 10-phenanthroline-5, 6-dione) ruthenium (II). Ru (phen) ₂DPPz (2+), like Ru (bpy) ₂DPPz^ (2+), acts as a molecular "light switch" for the presence of DNA, displaying no detectable photoluminescence in aqueous solution but luminescing brightly on binding to DNA. None of the DPPZ derivatives prepared show comparable "light switch" enhancements, since some luminescence may be detected in aqueous solution in the absence of DNA. For some complexes, however, luminescence enhancements of a factor of 20-300 are observed on binding to DNA. For these and the parent DPPZ complexes, the large enhancements observed are attributed to a sensitivity of the ruthenium-DPPZ luminescent charge-transfer excited state to quenching by water; although these complexes show little or no luminescence in water, appreciable luminescence is found in acetonitrile. Other derivatives show little solvent sensitivity in luminescence, and these, like Ru (phen) ₃^ (2+), display moderate enhancements (20-70%) on binding to DNA. Ru (phen) ₂DPPz^ (2+) and its derivatives all show at least biexponential decays in emission. Two binding modes have been proposed to account for these emission characteristics: a perpendicular mode where the DPPZ ligand intercalates from the major groove such that the metal-phenazine axis lies along the DNA dyad axis, and another, side-on mode where the metal-phenazine axis lies along the long axis of the base pairs.
Hartshorn et al. (Wed,) studied this question.