Luminescent lanthanide helicates self-assembled from ditopic ligands bearing phosphonic acid or phosphoester units

A series of hexadentate ditopic receptors incorporating benzimidazole moieties have been designed, which are fitted with phosphonic acid or phosphoethylester coordinating units. In addition, poly(oxyethylene) pendants have been introduced on the benzimidazole backbone of two ligands to increase water solubility. The ligands self-assemble with lanthanide ions under stoichiometric conditions, yielding triple-stranded homobimetallic helicates, as ascertained by mass spectrometry and UV-visible titrations. The helicates display large thermodynamic stability, for example, log beta(23) approximately 21-24 for all the Eu(III) complexes. Photophysical measurements reveal sensitization of the metal-centered luminescence in the europium and terbium complexes, which is modulated by the nature of the ligand. Hydration numbers determined by the lifetime method are essentially zero. The Eu((5)D(0)) lifetimes are long and reach values up to 3.2 ms, while quantum yields as high as 25% are obtained in water at pH 7.4. Back transfer limits the sensitization efficiency for Tb(III) luminescence, and both lifetimes and quantum yields are much smaller. The properties of the helicates are discussed with respect to those self-assembled from ligands bearing carboxylate coordinating units

Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence

We report on new anionic tridentate benzimidazole-pyridine-tetrazolate ligands that form neutral 3:1 complexes with trivalent lanthanides. The ligands are UV-absorbing chromophores that sensitize the red luminescence of europium with energy-transfer efficiency of 74–100%. The lifetime and quantum yield of the sensitized europium luminescence increase from 0.5 ms and 12–13% for the as-prepared solids to 2.8 ms and 41% for dichloromethane solution. From analysis of the data, the as-prepared solids can be described as aqua-complexes [Ln­(κ³-ligand)₂(κ¹-ligand)­(H₂O)_<i>x</i>] where the coordinated water molecules are responsible for the strong quenching of the europium luminescence. In solution, the coordinated water molecules are replaced by the nitrogen atoms of the κ¹-ligand to give anhydrous complexes [Ln­(κ³-ligand)₃] that exhibit efficient europium luminescence. X-ray structures of the anhydrous complexes confirm that the lanthanide ion (La^III, Eu^III) is nine-coordinate in a distorted tricapped trigonal prismatic environment and that coordination of the lanthanide ion by tetrazolate is weaker than by carboxylate

Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence

We report on new anionic tridentate benzimidazole-pyridine-tetrazolate ligands that form neutral 3:1 complexes with trivalent lanthanides. The ligands are UV-absorbing chromophores that sensitize the red luminescence of europium with energy-transfer efficiency of 74–100%. The lifetime and quantum yield of the sensitized europium luminescence increase from 0.5 ms and 12–13% for the as-prepared solids to 2.8 ms and 41% for dichloromethane solution. From analysis of the data, the as-prepared solids can be described as aqua-complexes [Ln­(κ³-ligand)₂(κ¹-ligand)­(H₂O)_<i>x</i>] where the coordinated water molecules are responsible for the strong quenching of the europium luminescence. In solution, the coordinated water molecules are replaced by the nitrogen atoms of the κ¹-ligand to give anhydrous complexes [Ln­(κ³-ligand)₃] that exhibit efficient europium luminescence. X-ray structures of the anhydrous complexes confirm that the lanthanide ion (La^III, Eu^III) is nine-coordinate in a distorted tricapped trigonal prismatic environment and that coordination of the lanthanide ion by tetrazolate is weaker than by carboxylate

Modulating the Photophysical Properties of Azamacrocyclic Europium Complexes with Charge-Transfer Antenna Chromophores

International audienceTwo europium complexes with bis(bipyridine) azamacrocyclic ligands featuring pendant arms with or without π-conjugated donor groups are synthesized and fully characterized by theoretical calculations and NMR spectroscopy. Their photophysical properties, including two-photon absorption, are investigated in water and in various organic solvents. The nonfunctionalized ligand gives highly water-stable europium complexes featuring bright luminescence properties but poor two-photon absorption cross sections. On the other hand, the europium complex with an extended conjugated antenna ligand presents a two-photon absorption cross section of 45 GM at 720 nm but is poorly luminescent in water. A detailed solvent-dependent photophysical study indicates that this luminescence quenching is not due to the direct coordination of O-H vibrators to the metal center but to the increase of nonradiative processes in a protic solvent induced by an internal isomerization equilibrium

A Eu^III Tetrakis(β-diketonate) Dimeric Complex: Photophysical Properties, Structural Elucidation by Sparkle/AM1 Calculations, and Doping into PMMA Films and Nanowires

Reaction of Ln^III with a tetrakis­(diketone) ligand H₄L [1,1′-(4,4′-(2,2-bis­((4-(4,4,4-trifluoro-3-oxobutanoyl) phenoxy)­methyl)­propane-1,3-diyl)­bis­(oxy)­bis­(4,1-phenylene))­bis­(4,4,4-trifluorobutane-1,3-dione)] gives new podates which, according to mass spectral data and Sparkle/AM1 calculations, can be described as dimers, (NBu₄[LnL])₂ (Ln = Eu, Tb, Gd:Eu), in both solid-state and dimethylformamide (DMF) solution. The photophysical properties of the Eu^III podate are compared with those of the mononuclear diketonate (NBu₄[Eu­(BTFA)₄], BTFA = benzoyltrifluoroacetonate), the crystal structure of which is also reported. The new Eu^III dimeric complex displays bright red luminescence upon irradiation at the ligand-centered band in the range of 250–400 nm, irrespective of the medium. The emission quantum yields and the luminescence lifetimes of (NBu₄[EuL])₂ (solid state: 51% ± 8% and 710 ± 2 μs; DMF: 31% ± 5% and 717 ± 1 μs) at room temperature are comparable to those obtained for NBu₄[Eu­(BTFA)₄] (solid state: 60 ± 9% and 730 ± 5 μs; DMF: 30 ± 5% and 636 ± 1 μs). Sparkle/AM1 calculations were utilized for predicting the ground-state geometries of the Eu^III dimer. Theoretical Judd–Ofelt and photoluminescence parameters, including quantum yields, predicted from this model are in good agreement with the experimental values, proving the efficiency of this theoretical approach implemented in the LUMPAC software (http://lumpac.pro.br). The kinetic scheme for modeling energy transfer processes show that the main donor state is the ligand triplet state and that energy transfer occurs on both the ⁵D₁ (44.2%) and ⁵D₀ (55.8%) levels. Furthermore, the newly obtained Eu^III complex was doped into a PMMA matrix to form highly luminescent films and one-dimensional nanowires having emission quantum yield as high as 67%–69% (doping concentration = 4% by weight); these materials display bright red luminescence even under sunlight, so that interesting photonic applications can be foreseen