An Efficient Design for the Rigid Assembly of Four Bidentate Chromophores in Water‐Stable Highly Luminescent Lanthanide Complexes

Abstract

The unique electronic properties of lanthanide ions (longlived luminescence and sharp emission spectra) make them particularly suitable for the development of diagnostic tools in medical analysis. [1,2] Lanthanide complexes are increasingly used for sensor development [3] and as luminescent probes in time-resolved high-throughput assays and fluorescence imaging because of their ability to discriminate between background fluorescence and the target signal.[ ).[ [15][16][17][18] As the Laporte-forbidden 4f-4f transition prevents direct excitation of lanthanide luminescence, lanthanide ions require sensitization by suitable organic chromophores. Furthermore, for practical applications under physiological conditions, the lanthanide ion should be incorporated into highly stable complexes. The efficiency of ligand-to-lanthanide energy transfer, which requires compatibility between the energy levels of the ligand excited states and the accepting levels of lanthanide ions, is crucial in the design of highperformance probes.[19] Moreover, high quantum yield cannot be obtained without the prevention of nonradiative deactivation of the lanthanide excited states by OÀH oscillators of coordinated or closely diffusing water molecules. The incorporation of suitable chromophores in carefully designed polydentate ligands leads to increased stability of the lanthanide chelate in solution and allows for the metal center to be well protected from water molecules. However, the tendency of lanthanide ions to adopt high coordination numbers and their lack of stereochemical preferences make the design of such ligands very challenging.[20] A successful strategy adopted by several investigators relies on tripodal architectures for the organization of three tridentate binding units in nine-coordinate Ln iii complexes. [21][22][23][24][25] The design of polydentate ligands that allow the arrangement of four bidentate moieties around a lanthanide ion has received less attention in spite of the excellent luminescent properties observed for tetrakis complexes of bidentate chomophores, such as quinolinates [26] or tropolonates. [27] Recently, octadentate ligands incorporating four bidentate chromophores have been shown to yield lanthanide complexes with very efficient emissions in the visible or NIR regions. [15,28] The structures of these complexes have not been elucidated, but the highly flexible structure of the backbone that connects the bidentate units suggests nonoptimal protection of the metal center in such systems.Herein, we describe a new and particularly efficient way of assembling four picolinate chromophores around a lanthanide center with a multidentate ligand that yields highly luminescent and water-stable lanthanide complexes. The decadentate ligand N,N,N'N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (H 4 tpaen) is readily obtained in five steps from commercially available pyridine-2,6-dicarboxylic acid and ethylenediamine in a yield of 26 % (Scheme 1).The water-soluble complexes of tpaen were isolated in 50-60 % yie...

Lanthanide‐Based Coordination Polymers Assembled by a Flexible Multidentate Linker: Design, Structure, Photophysical Properties, and Dynamic Solid‐State Behavior

“…The resulting solution was washed with a saturated NaHCO 3 solution (2 60 mL), dried over anhydrous Na 2 SO 4 , filtered, and evaporated to dryness to give a yellow oil that was purified by flash chromatography on neutral aluminum oxide (230 mL, CH 2 Cl 2 /EtOH gradient from 99.5:0.5 to 98:2). The product (1.26 g, 1.70 mmol) was heated to reflux overnight in a 6 m HCl solution (50 mL [29] or b, c) M/L The synthesis of the Eu-containing polymer was also performed in the presence of triethylamine as a base. The powder obtained displayed a very different PXRD pattern.…”

“…A possible explanation of the effect of the base in favoring one isomer could be the presence of potassium-coordinated intermediates. [29] …”

“…Previously, we found that 10 À2 m solutions of the decadentate tetrapodal ligand tpaen 4À (H 4 tpaen = N,N,N'N'-tetrakis[(6-carboxypyridin-2-yl)meth-A C H T U N G T R E N N U N G yl]ethylenediamine), in which four picolinate building blocks are connected by an ethylendiamine spacer, in water react with lanthanide ions to afford only mononuclear tencoordinate complexes, even in the presence of excess metal (Scheme 2). [29] Mononuclear complexes can still be obtained when low-denticity binding units (e.g., bidentate [29,41] or tridentate [30,41] ) are connected to a spacer by highly flexible arms to yield a strain-free multidentate linker (Scheme 2), but the preorganization remains small in some of these systems and polymetallic complexes can form depending on the conditions. [30] In this study, a long flexible four-carbon-atom aliphatic spacer was chosen to connect four picolinate building blocks into the potentially decadentate linker tpabn 4À (H 4 tpabn = N,N,N',N'-tetrakis[(6-carboxypyridin-2-yl)methyl]butylenediamine).…”

“…Predisposed flexible multidentate ligands in which an anchor ensures that the geometry and orientation of dangling chelating units have been successfully used to carefully control the coordination properties of lanthanide ions in the rational design of monometallic complexes. [25][26][27][28][29] Particularly relevant is the recent thermodynamic study of Piguet and co-workers that introduces a new tool (the effective concentration) for the quantitative estimation of the preorganization of multidentate linkers, which could be useful in any multicomponent assembly process. [30] However, flexible multidentate linkers have been rarely used in the construction of lanthanide-based coordination polymers [31][32][33][34][35][36] in spite of their expected increased thermal stability, ability to provide insight into supramolecular isomerism or polymorphism, and potential to support structural dynamism in a functional porous framework.…”

“…Picolinates were chosen as suitable building blocks for the construction of a potential decadentate linker because of their efficient sensitization of lanthanide ions [29,40] and their versatile chemistry. Previously, we found that 10 À2 m solutions of the decadentate tetrapodal ligand tpaen 4À (H 4 tpaen = N,N,N'N'-tetrakis[(6-carboxypyridin-2-yl)meth-A C H T U N G T R E N N U N G yl]ethylenediamine), in which four picolinate building blocks are connected by an ethylendiamine spacer, in water react with lanthanide ions to afford only mononuclear tencoordinate complexes, even in the presence of excess metal (Scheme 2).…”