Homo- and Heterobimetallic Ruthenium(II) and Osmium(II) Complexes Based on a Pyrene-Biimidazolate Spacer as Efficient DNA-Binding Probes in the Near-Infrared Domain

We
report in this work a new family of homo- and heterobimetallic complexes
of the type [(bpy)2M­(Py-Biimz)­M′II(bpy)2]2+ (M = M′ = RuII or
OsII; M = RuII and M′ = OsII) derived from a pyrenyl-biimidazole-based bridge, 2-imidazolyl­pyreno­[4,5-d]­imidazole (Py-BiimzH2). The homobimetallic
Ru­(II) and Os­(II) complexes were found to crystallize in monoclinic
form with space group P21/n. All the complexes exhibit strong absorptions throughout the entire
UV–vis
region and also exhibit luminescence at room temperature. For osmium-containing
complexes (2 and 3) both the absorption
and emission band stretched up to the NIR region and thus afford more
biofriendly
conditions for probable applications in infrared imaging and phototherapeutic
studies. Detailed luminescence studies indicate that the emission
originates from the respective 3MLCT excited state mainly
centered in the [M­(bpy)2]2+ moiety of the complexes
and is only slightly affected by the pyrene moiety. The bimetallic
complexes show two successive one-electron reversible metal-centered
oxidations in the positive potential window and several reduction
processes in the negative potential window. An efficient intramolecular
electronic energy transfer is found to occur from the Ru center to
the Os-based component in the heterometallic dyad. The binding studies
of the complexes with DNA were thoroughly studied through different
spectroscopic techniques such as UV–vis absorption, steady-state
and time-resolved emission, circular dichroism, and relative DNA binding
study using ethidium bromide. The intercalative mode of binding was
suggested to be operative in all cases. Finally, computational studies
employing DFT and TD-DFT were also carried out to interpret the experimentally
observed absorption and emission bands of the complexes.