Synthesis, characterization, photophysical studies and interaction with DNA of a new family of Ru(II) furyl- and thienyl-imidazo-phenanthroline polypyridyl complexes

A new family of Ru(II) polypyridyl complexes (C1 to C6) containing furyl- or thienyl-imidazo-phenanthroline ligands (4–6) were synthesized using microwave irradiation and characterized by elemental analysis, 1H NMR, UV–Vis absorption and fluorescence spectroscopy, FAB, ESI-MS and MALDI-TOF-MS spectrometry. On the other hand, the novel furyl- or thienyl-imidazo-phenanthroline derivatives (5–6) were synthesized through the Radziszewski reaction and completely characterized by the usual spectroscopic techniques. The interaction of the complexes with calf thymus DNA in the absence and in the presence of different quenchers (ethidium bromide, potassium hexacyanoferrate(II) and methyl viologen) has been studied by absorption spectroscopy, steady-state and single-photon timing luminescence measurements. Their electronic spectra show visible absorption peaks at 457–463 nm, with red luminescence at 603–613 nm. The emission quantum yields of these complexes are between 0.006 and 0.016 in air-equilibrated DMSO solution. Luminescence lifetimes in water lie within the 0.4–1.0 μs range, with a non-exponential behavior due to aggregation of the probe. Ru(II) complexes C3, C4, C5 and C6 show intrinsic dsDNA-binding constants of 2.74 × 105, 3.02 × 105, 1.32 × 105 and 1.63 × 105 M−1, respectively. The planar extended structure of the imidazo-phenanthroline ligands and the collected spectroscopic data suggest a partial intercalative binding mode of the novel metal probes to double-stranded DNA.