The increase of europium-based OLED luminance through reducing the excited state lifetime by mixed-ligand complex formation.

An approach to the luminance increase of the europium-based OLED is proposed through the formation of the mixed-ligand complex. The introduction of two diverse anionic ligands around one europium ion forming a mixed-ligand complex is confirmed by powder X-ray diffraction, 1H and 19F NMR spectroscopy, MALDI MS spectroscopy, and luminescence spectroscopy. A decrease in the symmetry of the coordination environment leads to a 50% reduction of the lifetime of the excited state. The obtained OLEDs based on mixed ligand europium complexes are significantly superior in luminance to OLEDs based on individual complexes.

Lanthanide Fluorobenzoates as Bio-Probes: a Quest for the Optimal Ligand Fluorination Degree.

The thorough study of fluorinated benzoates of lanthanides (Eu, Tb, Nd, Er, Yb, Gd, La, Lu) is reported. Their composition in single crystal and powder state revealed two predominant structural motifs. An in-depth luminescence study has been performed on the reported fluorobenzoates, showing, that terbium and europium complexes in solid state possess high luminescence intensity with the quantum yield of up to 69 %. High solubility in most organic solvents, as well as in water, combined with the high luminescence intensity in water solution and non-toxicity allowed the testing of europium complexes as bioprobes in cellulo. Among all tested fluorobenzoates, europium 2-fluorobenzoate dihydrate combined the best luminescent properties, thermodynamic stability, aqueous solubility, and non-toxicity, and was shown to be a viable bio-marker.

Highly Luminescent, Water-Soluble Lanthanide Fluorobenzoates: Syntheses, Structures and Photophysics, Part I: Lanthanide Pentafluorobenzoates.

Highly luminescent, photostable, and soluble lanthanide pentafluorobenzoates have been synthesized and thoroughly characterized, with a focus on Eu(III) and Tb(III) complexes as visible emitters and Nd(III) , Er(III) , and Yb(III) complexes as infrared emitters. Investigation of the crystal structures of the complexes in powder form and as single crystals by using X-ray diffraction revealed five different structural types, including monomeric, dimeric, and polymeric. The local structure in different solutions was studied by using X-ray absorption spectroscopy. The photoluminescence quantum yields (PLQYs) of terbium and europium complexes were 39 and 15 %, respectively; the latter value was increased almost twice by using the heterometallic complex [Tb0.5 Eu0.5 (pfb)3 (H2 O)] (Hpfb=pentafluorobenzoic acid). Due to the effectively utilized sensitization strategy (pfb)(-) →Tb→Eu, a pure europium luminescence with a PLQY of 29 % was achieved.

Deciphering three beneficial effects of 2,2'-bipyridine-N,N'-dioxide on the luminescence sensitization of lanthanide(III) hexafluoroacetylacetonate ternary complexes.

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)]·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 Å with no significant difference between hfa(-) and bpyO2. A 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)(H(2)O)(2)]. 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 ≈25% increase in the intrinsic quantum yield, so that more attention should be given to this parameter when designing highly luminescent lanthanide complexes.

Highly luminescent and triboluminescent coordination polymers assembled from lanthanide ß-diketonates and aromatic bidentate O-donor ligands.

The reaction of hydrated lanthanide hexafluoroacetylacetonates, [Ln(hfa)(3)(H(2)O)(2)], with 1,4-disubstituted benzenes afforded a new series of one-dimensional coordination polymers [Ln(hfa)(3)(Q)](∞), where Ln = Eu, Gd, Tb, and Lu and Q = 1,4-diacetylbenzene (acbz), 1,4-diacetoxybenzene (acetbz), or 1,4-dimethyltherephtalate (dmtph). X-ray single crystal analyses reveal [Ln(hfa)(3)(acbz)](∞) (Ln = Eu, Gd, Tb) consisting of zigzag polymeric chains with Ln-Ln-Ln angles equal to 128°, while the arrays are more linear in [Eu(hfa)(3)(acetbz)](∞) and [Eu(hfa)(3)(dmtph)](∞), with Ln-Ln-Ln angles of 165° and 180°, respectively. In all structures, Ln(III) ions are 8-coordinate and lie in distorted square-antiprismatic environments. The coordination polymers are thermally stable up to 180-210 °C under a nitrogen atmosphere. Their volatility has been tested in vacuum sublimation experiments at 200-250 °C and 10(-2) Torr: the metal-organic frameworks with acetbz and dmtph can be quantitatively sublimed, while [Ln(hfa)(3)(acbz)](∞) undergoes thermal decomposition. The triplet state energies of the ancillary ligands, 21,600 (acetbz), 22,840 (acbz), and 24,500 (dmtph) cm(-1), lie in an ideal range for sensitizing the luminescence of Eu(III) and/or Tb(III). As a result, all of the [Ln(hfa)(3)(Q)](∞) polymers display bright red or green luminescence due to the characteristic (5)D(0) → (7)F(J) (J = 0-4) or (5)D(4) → (7)F(J) (J = 6-0) transitions, respectively. Absolute quantum yields reach 51(Eu) and 56(Tb) % for the frameworks built from dmtph. Thin films of [Eu(hfa)(3)(Q)](∞) with 100-170 nm thickness can be obtained by thermal evaporation (P < 3 × 10(-5) Torr, 200-250 °C). They are stable over a long period of time, and their photophysical parameters are similar to those of the bulk samples so that their use as active materials in luminescent devices can be envisaged. Mixtures of [Ln(hfa)(3)(dmpth)](∞) with Ln = Eu and Tb yield color-tunable microcrystalline materials from red to green. Finally, the crystalline samples exhibit strong triboluminescence, which could be useful in the design of pressure and/or damage detection probes.

Role of the ancillary ligand N,N-dimethylaminoethanol in the sensitization of Eu(III) and Tb(III) luminescence in dimeric beta-diketonates.

Two types of dimeric complexes [Ln2(hfa)6(mu2-O(CH2)2NHMe2)2] and [Ln(thd)2(mu2,eta2-O(CH2)2NMe2)]2 (Ln = YIII, EuIII, GdIII, TbIII, TmIII, LuIII; hfa- = hexafluoroacetylacetonato, thd- = dipivaloylmethanato) are obtained by reacting [Ln(hfa)3(H2O)2] and [Ln(thd)3], respectively, with N,N-dimethylaminoethanol in toluene and are fully characterized. X-ray single crystal analysis performed for the TbIII compounds confirms their dimeric structure. The coordination mode of N,N-dimethylaminoethanol depends on the nature of the beta-diketonate. In [Tb2(hfa)6(mu2-O(CH2)2NHMe2)2], eight-coordinate TbIII ions adopt distorted square antiprismatic coordination environments and are O-bridged by two zwitterionic N,N-dimethylaminoethanol ligands with a Tb1...Tb2 separation of 3.684(1) A. In [Tb(thd)2(mu2,eta2-O(CH2)2NMe2)]2, the N,N-dimethylaminoethanol acts as chelating-bridging O,N-donor anion and the TbIII ions are seven-coordinate; the Tb1...Tb1A separation amounts to 3.735(2) A within centrosymmetric dimers. The dimeric complexes are thermally stable up to 180 degrees C, as shown by thermogravimetric analysis, and their volatility is sufficient for quantitative sublimation under reduced pressure. The EuIII and TbIII dimers display metal-centered luminescence, particularly [Eu2(hfa)6(O(CH2)2NHMe2)2] (quantum yield Q(L)Ln = 58%) and [Tb(thd)2(O(CH2)2NMe2)]2 (32%). Consideration of energy migration paths within the dimers, based on the study of both pure and EuIII- or TbIII-doped (0.01-0.1 mol %) LuIII analogues, leads to the conclusion that both the beta-diketone and N,N-dimethylaminoethanol ligands contribute significantly to the sensitization process of the EuIII luminescence. The ancillary ligand increases considerably the luminescence of [Eu2(hfa)6(O(CH2)2NHMe2)2], compared to [Ln(hfa)3(H2O)2], through the formation of intra-ligand states while it is detrimental to TbIII luminescence in both beta-diketonates. Thin films of the most luminescent compound [Eu2(hfa)6(O(CH2)2NHMe2)2] obtained by vacuum sublimation display photophysical properties analogous to those of the solid-state sample, thus opening perspectives for applications in electroluminescent devices.

The development of a new approach toward lanthanide-based OLED fabrication: new host materials for Tb-based emitters.

To develop the recently proposed approach toward host selection for lanthanide-based emitters, four phosphine oxides PO = PO1-PO4 were investigated which are able to both increase the electron mobility and to sensitize terbium luminescence. New highly soluble and brightly luminescent terbium complexes TbCl3(PO)·H2O and Tb(pobz)3(PO)·(CH3)2CO (pobz- = phenoxybenzoate) with a quantum yield of up to 100% were synthesized and thoroughly characterized. To study the electroluminescence properties of these materials, a series of solution-processed OLED devices were fabricated and their heterostructures were selected based on the HOMO and LUMO energies of PO1-PO4, which were carefully assessed by the combination of DFT and TDDFT methods. Thus, the effectiveness of the proposed approach was proved, and the influence of the anionic ligand was shown. The maximum OLED luminance reached 75 Cd m-2, which is a high value for solution-processed OLEDs based on terbium complexes.