Toward Programmed Molecular Lanthanide Probes and Sensors

Abstract

The peculiar spectroscopic properties of the lanthanide metal ions (4f-block, Ln^lll), and particularly Eu^lll and Tb^lll, make them suitable for the design of chemical luminescent probes and analytical sensors when Ln^lll is introduced into organized molecular architectures possessing controlled and protected coordination sites. The tridentate receptors 2,6-bis(1-alkylbenzimidazol-2-yl)pyridine (Lⁱ, i=2-7) play a crucial role since they react with Ln³⁺ to give mononuclear building blocks [Ln(Lⁱ)₃]³⁺ where the metal ion is protected by the wrapping of the aromatic ligands. In these ultraviolet to visible light-converting devices (Ln=Eu, Tb), the size of the internal cavity is controlled by the three closely packed strands which produce i) a promising discrimination between Ln^lll according to their sizes and ii) efficient light-harvesting resulting form the strong absoprtion of ultraviolet light by the ligands. These building blocks have been introduced into more sophisticated homodinuclear triple-helical complexes [Ln₂(L⁸)₃]6+ and statistical mixtures containing the heterodinuclear directional visible to visible light-converting device [EuTb(L⁸)₃]6+. Improved molecular programming results form the use of acyclic segmental ligand possessing different binding units coded for one particular ion. The segmental ligands Lⁱ (i=9,10) discriminate Fe^ll and Ag^I to give the heterodinuclear double helicate [FeAg(L¹⁰)₂]³⁺ and the heterotrinuclear organometallic [2]-catenate [FeAg₂(L⁹)₂]⁴⁺ where each metal ion occupies a coordination site satisfying its stereochemical requirements. The receptor L¹¹ recognizes 3d- and 4f-block metal ions to give the heterodinuclear d-f-triple-helical complexes [LnZn(L¹¹)₃]⁵⁺ where Zn^ll occupies the pseudo-octahedral capping site and corresponds to a noncovalent tripod which properly organizes the strands for their coordination to Ln^lll. In these new self-assembled lanthanide podates, predetermined structural and physical properties result form i) the judicious choice of the d-block ion associated with Ln^lll and ii) the design of the tridentate unit bound to Ln^lll. [LnFe(L¹¹)₃]⁵⁺ (Ln=La-Eu) display thermochromism and spin-crossover behavior while light emission can be increased by a factor 10⁴ when going from [EuZn(L¹¹)₃]⁵⁺ to [EuZn(L¹²)₃]⁵⁺. The design of tailored receptors for the preparation of stable lanthanide light-converting devices is discussed together with new characterization methods applied to complicated assembly processes in solution.