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)].0.5C6H6 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(H2O)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 ã25% increase in the intrinsic quantum yield, so that more attention should be given to this parameter when designing highly luminescent lanthanide complexes. © 2011 American Chemical Society

Highly Luminescent and Triboluminescent Coordination Polymers Assembled from Lanthanide beta-Diketonates and Aromatic Bidentate O-Donor Ligands

The reaction of hydrated lanthanide hexafluoroacetylacetonates, [Ln(hfa)(3)(H2O)(2)], with 1,4-disubstituted benzenes afforded a new series of one-dimensional coordination polymers [Ln(hfa)(3)(Q)](infinity), 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)](infinity) (Ln = Eu, Gd, Tb) consisting of zigzag polymeric chains with Ln Ln Ln angles equal to 128 degrees, while the arrays are more linear in [Eu(hfa)(3)(acetbz)infinity and [Eu(hfa)(3)(dmtph)](infinity), with Ln Ln Ln angles of 165 degrees and 180 degrees, 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 degrees C under a nitrogen atmosphere. Their volatility has been tested in vacuum sublimation experiments at 200-250 degrees C and 10(-2) Torr: the metal organic frameworks with acetbz and dmtph can be quantitatively sublimed, while [Ln(hfa)(3)(acbz)](infinity) 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)](infinity) polymers display bright red or green luminescence due to the characteristic D-5(0)-> F-7(J)(J=0-4) or D-5(4) -> F-7(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)](infinity) with 100-170 nm thickness can be obtained by thermal evaporation (P<3 x 10(-5) Torr, 200-250 degrees 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)](infinity) 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