Pyrene-biimidazole based Ru(ii) and Os(ii) complexes as highly efficient probes for the visible and near-infrared detection of cyanide in aqueous media

Abstract

Two pyrenyl-biimidazole based mononuclear Ru(II) and Os(II) complexes of the type [(bpy)₂M(Py-BiimzH₂)]²⁺ (M = Ru^II and Os^II), where Py-BiimzH₂ = 10-(1-H-imidazole-2-yl)-9H-pyreno[4,5-d]imidazole and bpy = 2,2′-bipyridine, have been synthesized and thoroughly characterized in this work using various analytical tools and spectroscopic techniques. These complexes were designed to recognize and sense cyanide ions in pure aqueous media. The single crystal X-ray structure of the Ru(II) complex shows that the compound is crystallized in a monoclinic system with the P2(1)/c space group. Both complexes show intense absorptions throughout the entire UV-vis region and also exhibit luminescence at room temperature. In case of Os(II), both the absorption and emission bands stretched up to the NIR region and thus a more bio-friendly condition for the detection of the anions is provided. Both steady state and time-resolved studies suggest that the emission originates predominantly from the ³MLCT excited state mainly centered in the [M(bpy)₂]²⁺ moiety of the complexes which is only slightly affected by the pyrene moiety. The electrochemical properties of the complexes are characterized using one reversible metal-centered oxidation and several ligand-centered reduction processes. The anion sensing properties of the complexes in both acetonitrile and pure aqueous media were thoroughly examined through different channels such as absorption, steady state and time-resolved emission spectroscopic methods and cyclic and square wave voltammetric measurements. Both complexes possess a very high selectivity towards cyanide ions in aqueous media in the presence of an excess of other anions. Moreover, the complexes display a visual detection of cyanide ions with a very low detection limit of the order of 10⁻⁸ M. Finally, theoretical calculations employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were carried out to elucidate the details of the electronic structure and transitions involved in the complexes and their cyanide adducts.