Jean-Claude Bunzli - Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

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  Aebischer,A., Gumy,F., Bunzli,J.C.G. Physical Chemistry   Chemical Physics 2009 11 (9)   1346-1353

  Abstract: The efficiency with which the   surroundings of trivalent lanthanide ions sensitize their   luminescence (eta(sens)) is a key parameter in the design of   highly emitting molecular edifices and materials. Evaluation of   eta(sens) requires the measurement of the overall and intrinsic   quantum yields obtained upon ligand and metal excitation,   respectively. We describe a modified integration sphere enabling   absolute determination of these quantities on small amounts of   solid samples or solutions (60 mu L). The sphere is tested for   linear response of emitted versus absorbed light intensities with   increasing concentration of Cs-3[Ln(dpa)(3)] solutions (Ln = Eu,   Tb). The overall (Q(L)(Eu) = 29 +/- 2%) and intrinsic (Q(Eu)(Eu)   = 41 +/- 2%) quantum yields obtained for Eu allow the direct   calculation of eta(sens) (71 +/- 6%) while the radiative lifetime   (tau(rad) = 4.1 +/- 0.3 ms) is calculated from Q(Eu)(Eu) and the   observed lifetime. The intrinsic quantum yield matches the value   extracted from emission parameters using the simplified equation   proposed by Werts et al. but, on the other hand, the theoretical   estimate using spontaneous transition probabilities calculated   from Judd-Ofelt (JO) parameters is off by -25% (3.15 ms). In the   case of Cs-3[Tb(dpa)(3)], the molar absorption coefficient of the   D-5(4) <- F-7(6) transition is too small to measure Q Tb Tb   for the solution but this quantity could be determined for the   microcrystalline sample (72 +/- 5%, tau(rad) = 1.9 +/- 0.1 ms).   In this case, the JO theoretical estimate leads to a much too   short tau(rad) value. The large difference in eta(sens) for   microcrystalline samples of Eu (85%) and Tb (42%)   tris(dipicolinates) is attributed to back energy transfer in the   latter compound consecutive to a sizeable overlap between the 5D4   -> 7F6 emission and the absorption spectrum of the   dipicolinate triplet, this overlap being smaller in the case of   the solution. The overall quantum yield of Na-3[Yb(dpa)(3)] in   aqueous solution is very low (0.015 +/- 0.002%) due to both poor   sensitization efficiency (8%) and small intrinsic quantum yield   (Q(Yb)(Yb) = 0.178 +/- 0.003%; tau(rad) = 1.31 +/- 0.02 ms). For   evaluating intrinsic quantum yields of Yb in aqueous solutions of   coordination compounds from lifetimes, a value of 1.2-1.3 ms is   recommended

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