7 research outputs found
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 74â100%. The lifetime and quantum yield of the
sensitized europium luminescence increase from 0.5 ms and 12â13%
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 [LnÂ(Îș<sup>3</sup>-ligand)<sub>2</sub>(Îș<sup>1</sup>-ligand)Â(H<sub>2</sub>O)<sub><i>x</i></sub>] 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 Îș<sup>1</sup>-ligand
to give anhydrous complexes [LnÂ(Îș<sup>3</sup>-ligand)<sub>3</sub>] that exhibit efficient europium luminescence. X-ray structures
of the anhydrous complexes confirm that the lanthanide ion (La<sup>III</sup>, Eu<sup>III</sup>) 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 74â100%. The lifetime and quantum yield of the
sensitized europium luminescence increase from 0.5 ms and 12â13%
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 [LnÂ(Îș<sup>3</sup>-ligand)<sub>2</sub>(Îș<sup>1</sup>-ligand)Â(H<sub>2</sub>O)<sub><i>x</i></sub>] 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 Îș<sup>1</sup>-ligand
to give anhydrous complexes [LnÂ(Îș<sup>3</sup>-ligand)<sub>3</sub>] that exhibit efficient europium luminescence. X-ray structures
of the anhydrous complexes confirm that the lanthanide ion (La<sup>III</sup>, Eu<sup>III</sup>) 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 Ï-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<sup>III</sup> Tetrakis(ÎČ-diketonate) Dimeric Complex: Photophysical Properties, Structural Elucidation by Sparkle/AM1 Calculations, and Doping into PMMA Films and Nanowires
Reaction of Ln<sup>III</sup> with
a tetrakisÂ(diketone) ligand H<sub>4</sub>L [1,1âČ-(4,4âČ-(2,2-bisÂ((4-(4,4,4-trifluoro-3-oxobutanoyl)
phenoxy)Âmethyl)Âpropane-1,3-diyl)ÂbisÂ(oxy)ÂbisÂ(4,1-phenylene))ÂbisÂ(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, (NBu<sub>4</sub>[LnL])<sub>2</sub> (Ln = Eu, Tb, Gd:Eu), in both solid-state and dimethylformamide
(DMF) solution. The photophysical properties of the Eu<sup>III</sup> podate are compared with those of the mononuclear diketonate (NBu<sub>4</sub>[EuÂ(BTFA)<sub>4</sub>], BTFA = benzoyltrifluoroacetonate),
the crystal structure of which is also reported. The new Eu<sup>III</sup> dimeric complex displays bright red luminescence upon irradiation
at the ligand-centered band in the range of 250â400 nm, irrespective
of the medium. The emission quantum yields and the luminescence lifetimes
of (NBu<sub>4</sub>[EuL])<sub>2</sub> (solid state: 51% ± 8%
and 710 ± 2 Όs; DMF: 31% ± 5% and 717 ± 1 Όs)
at room temperature are comparable to those obtained for NBu<sub>4</sub>[EuÂ(BTFA)<sub>4</sub>] (solid state: 60 ± 9% and 730 ±
5 Όs; DMF: 30 ± 5% and 636 ± 1 Όs). Sparkle/AM1
calculations were utilized for predicting the ground-state geometries
of the Eu<sup>III</sup> dimer. Theoretical JuddâOfelt 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 <sup>5</sup>D<sub>1</sub> (44.2%) and <sup>5</sup>D<sub>0</sub> (55.8%) levels. Furthermore,
the newly obtained Eu<sup>III</sup> complex was doped into a PMMA
matrix to form highly luminescent films and one-dimensional nanowires
having emission quantum yield as high as 67%â69% (doping concentration
= 4% by weight); these materials display bright red luminescence even
under sunlight, so that interesting photonic applications can be foreseen