Influence of Symmetry on the Luminescence and Radiative Lifetime of Nine-Coordinate Europium Complexes

Homoleptic mononuclear nine-coordinate lanthanum(III) and europium(III) tris-complexes [Ln(N^N^O)3]·nH2O with two tridentate N-benzyl-benzimidazole pyridine-2-carboxylates exhibit a rare C3-symmetry of the lanthanide coordination polyhedron in the solid state, as confirmed by luminescence spectroscopy and by X-ray crystallography (the three N^N^O ligands are arranged "up-up-up" around the lanthanide ion). The symmetry, however, is changed to the more-common C1 upon dissolution of the complexes in dichloromethane, as revealed by luminescence spectroscopy (the three ligands are arranged "up-up-down"). The new europium complexes emit efficient ligand-sensitized metal-centered luminescence with excited-state lifetimes of 1.56-2.18 ms and quantum yields of 25-41% in the solid and in the solution. The change of the symmetry from (a higher) C3 to (a lower) C1 alters the luminescence spectrum, shortens the radiative lifetime, and increases the luminescence efficiency of the europium complexes

Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

Jean-Claude Bünzli, Svetlana Eliseeva, Rosario Scopelliti

A series of tridentate benz- ACHTUNGTRENUNGimidazole-substituted pyridine-2-carboxylic acids have been prepared with a halogen, methyl or alkoxy group in the 6-position of the benzimidazole ring, which additionally contains a solubilising N-alkyl chain. The ligands form neutral homoleptic nine-coordinate lanthanum, europium and terbium complexes as established from X-ray crystallographic analysis of eight structures. The coordination polyhedron around the lanthanide ion is close to a tricapped trigonal prism with ligands arranged in an up–up–down fashion. The coordinated ligands serve as lightharvesting chromophores in the complexes with absorption maxima in the range 321–341 nm (

\epsilon=(4.9–6.0) \times 10^4m^{-1}cm^{-1}

) and triplet-state energies between 21 300 and

18 800 cm^{-1}

; the largest redshifts occur for bromine and electron-donor alkoxy substituents. The ligands efficiently sensitise europium luminescence with overall quantum yields (QEu L ) and observed lifetimes (tobs) reaching 71% and 3.00 ms, respectively, in the solid state and 52% and 2.81 ms, respectively, in CH2Cl2 at room temperature. The radiative lifetimes of the EuACHTUNGTRENUNG(5D0) level amount to trad=3.6–4.6 ms and the sensitisation efficiency hsens=QEu L (trad/tobs) is close to unity for most of the complexes in the solid state and equal to approximately 80% in solution. The photophysical parameters of the complexes correlate with the triplet energy of the ligands, which in turn is determined by the nature of the benzimidazole substituent. Facile modification of the ligands makes them promising for the development of brightly emissive europiumcontaining materials.

2009

2,2’-Bipyrimidine as Efficient Sensitizer of the Solid-State Luminescence of Lanthanide and Uranyl Ions from Visible to Near-Infrared

Jean-Claude Bünzli

Treatment of Ln(NO3)3.nH2O with 1 or 2 equiv 2,2-bipyrimidine (BPM) in dry THF readily afforded the monometallic complexes [Ln(NO3)3(bpm)2] (Ln=Eu, Gd, Dy, Tm) or [Ln(NO3)3(bpm)2]THF (Ln=Eu, Tb, Er, Yb) after recrystallization from MeOH or THF, respectively. Reactions with nitrate salts of the larger lanthanide ions (Ln=Ce, Nd, Sm) yielded one of two distinct monometallic complexes, depending on the recrystallization solvent: [Ln(NO3)3(bpm)2].THF (Ln=Nd, Sm) from THF, or [Ln(NO3)3(bpm)(MeOH)2].MeOH (Ln=Ce, Nd, Sm) from MeOH. Treatment of UO2(NO3)2.6H2O with 1 equiv BPM in THF afforded the monoadduct [UO2(NO3)2(bpm)] after recrystallization from MeOH. The complexes were characterized by their crystal structure. Solid-state luminescence measurements on these monometallic complexes showed that BPM is an efficient sensitizer of the luminescence of both the lanthanide and the uranyl ions emitting visible light, as well as of the YbIII ion emitting in the near-IR. For Tb, Dy, Eu, and Yb complexes, energy transfer was quite efficient, resulting in quantum yields of 80.0, 5.1, 70.0, and 0.8 %, respectively. All these complexes in the solid state were stable in air.

2009

Deciphering Three Beneficial Effects of 2,2 '-Bipyridine-N,N '-Dioxide on the Luminescence Sensitization of Lanthanide(III) Hexafluoroacetylacetonate Ternary Complexes

Jean-Claude Bünzli, Svetlana Eliseeva

Lanthanide hexafluoroacetylacetonate ternary complexes with 2,2'-bipyridine-N,N'-dioxide, [Ln(hfa)(3)(bpyO2)], were synthesized for Ln = Eu, Gd, Tb, and Lu and fully characterized by elemental, thermal, and mass-spectrometric analyses. The X-ray crystal structure of [Eu(hfa)(3)(bpyO2)] center dot 0.5C(6)H(6) reveals an octa-coordinate metal ion lying in a severely distorted trigonal dodecahedron geometry; the Eu-O distances lie in the range 2.36-2.44 angstrom with no significant difference between hfa and bpyO2. angstrom detailed comparative photophysical investigation has been carried out to determine the exact influence of the introduction of bpyO2 in the inner coordination sphere of the metal ion in replacement of the two water molecules in [Ln(hfa)(3)(H2O)(2)]center dot While this replacement is detrimental for Tb, it leads to a 15-fold increase in the overall quantum yield for Eu. This large improvement originates from (i) a better sensitization efficiency, the ancillary ligand being responsible for 3/4 of the energy transfer, (ii) elimination of nonradiative deactivation pathways through'harmonics of O-H vibrations, and (iii) reduction in the radiative lifetime. The latter influence is rarely documented, but it accounts here for a approximate to 25% increase in the intrinsic quantum yield, so that more attention should be given to this parameter when designing highly luminescent lanthanide complexes.

2011

Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

Jean-Claude Bünzli, Svetlana Eliseeva, Rosario Scopelliti

\epsilon=(4.9–6.0) \times 10^4m^{-1}cm^{-1}

) and triplet-state energies between 21 300 and

18 800 cm^{-1}

2009