Ruthenium(II) and Osmium(II) Mixed Chelates Based on Pyrenyl–Pyridylimidazole and 2,2′-Bipyridine Ligands as Efficient DNA Intercalators and Anion Sensors

We report herein the synthesis and characterization of two monometallic ruthenium­(II) and osmium­(II) complexes of composition [(bpy)₂M­(HImzPPy)] (ClO₄)₂ derived from pyrenylimidazole–10-pyridin-2-yl-9<i>H</i>-9,11-diazacyclopenta­[<i>e</i>]­pyrene (HImzPPy) and 2,2′-bipyridine (bpy) ligands. X-ray crystallographic study shows that both crystals belong to the triclinic system having space group <i>P</i>1̅. The photophysical properties of <b>1</b> and <b>2</b> in acetonitrile indicate that the metal-to-ligand charge-transfer excited state is mainly centered in the [M­(bpy)₂]²⁺ moiety of the complexes and slightly affected by the extended conjugation of the pyrenylimidazole moiety. Both complexes display one-electron reversible metal-centered oxidative processes and a number of quasi-reversible reductive processes. The binding affinities of the complexes toward calf-thymus DNA (CT-DNA) were thoroughly studied through different methods such as absorption, emission, excited-state lifetime, circular dichroism, and thermal denaturation of DNA and a relative DNA binding study using ethidium bromide. All of these experiments account for the intercalative nature of both <b>1</b> and <b>2</b> toward CT-DNA as well as their light-switch behavior. The anion recognition study through different spectroscopic techniques reveals that both complexes act as “turn-on” luminescence sensors for H₂PO₄⁻ and “turn-off” sensors toward F^– and AcO^–. The imidazole N–H proton of the receptors gets deprotonated with the excessive addition of F^– and AcO^–, while it interacts with H₂PO₄^– through hydrogen-bonding interaction. Theoretical calculations (DFT and TD-DFT) were also performed to understand the photophysical properties of the metalloreceptors