Toward Programmed Molecular Lanthanide Probes and Sensors
The peculiar spectroscopic properties of the lanthanide metal ions (4f-block, Lnlll), and particularly Eulll and Tblll, make them suitable for the design of chemical luminescent probes and analytical sensors when Lnlll 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 Lnlll 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 Fell and AgI to give the heterodinuclear double helicate [FeAg(L10)2]3+ and the heterotrinuclear organometallic -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 Znll occupies the pseudo-octahedral capping site and corresponds to a noncovalent tripod which properly organizes the strands for their coordination to Lnlll. 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 Lnlll and ii) the design of the tridentate unit bound to Lnlll. [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.
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