Synthesis, characterization, photophysical, lipophilicity, and in vitro fluorescence studies of mono-, di-, and trinuclear Ru(II) polypyridyl complexes of pyridinyl benzimidazole derivatives.

The synthesis, characterization, and photophysical properties of mononuclear ruthenium(II) complexes [Ru(bpy)2(py-BIm-Bz)](ClO4)2 (1) and [Ru(phen)2(py-BIm-Bz)](ClO4)2 (2), dinuclear complexes [(bpy)2Ru-μ2-(py-BIm-Xy)-Ru(bpy)2](ClO4)4 (3) and [(phen)2Ru-μ2-(py-BIm-Xy)-Ru(phen)2](ClO4)4 (4), and trinuclear complexes [((bpy)2Ru)3-μ3-(py-BIm-Ms)](ClO4)6 (5) and [((phen)2Ru)3-μ3-(py-BIm-Ms)](ClO4)6 (6) of pyridinyl benzimidazole ligands with 2,2'-bipyridine or 1,10-phenanthroline ancillary ligands as fluorescent imaging probes are reported. The ligand py-BIm-Bz crystallizes with inherent disorder due to the competing π-π interactions between two (2-pyridinyl)benzimidazole moieties aligned in parallel and in the opposite direction. The complex 2 forms non-merohedrally twinned crystal with the twin law matrix [0.259 -0.776 0.741, 0.000 -1.000 0.000, 1.259 -0.776 -0.259] and a batch scale factor (BASF) of 0.05. The electronic absorption spectra of the complexes 1-6 differ typically in the π-π* transitions of the ancillary ligands. The complexes exhibit orange-red fluorescence at 624-634 nm at room temperature with quantum yield (0.096 - 0.117) higher than that of [Ru(bpy)3]2+ and a hypsochromic shift of the emission maxima in frozen acetonitrile (λem = 613-628 nm) due to the rigidochromic effect. The excited state lifetime of these complexes are in the range 72-194 ns with the mononuclear complexes exhibiting the highest values. The complexes 1-6 are nontoxic (IC50 > 275 μM) toward both HeLa and Vero cell lines. They are hydrophilic and the logPo/w values are in the -0.53 to -1.46 range. The confocal microscopic study of cellular localization of the complexes on the HeLa cells co-stained with the nuclear staining DAPI dye shows their localization in the cytoplasm and the nuclear membrane penetration increases with nuclearity.