Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

Abstract

The efficiency with which the surroundings of trivalent lanthanide ions sensitize their luminescence (η_sens) is a key parameter in the design of highly emitting molecular edifices and materials. Evaluation of η_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 μL). The sphere is tested for linear response of emitted versus absorbed light intensities with increasing concentration of Cs₃[Ln(dpa)₃] solutions (Ln = Eu, Tb). The overall (Q^Eu_L = 29 ± 2%) and intrinsic (Q^Eu_Eu = 41 ± 2%) quantum yields obtained for Eu allow the direct calculation of η_sens (71 ± 6%) while the radiative lifetime (τ_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 Cs₃[Tb(dpa)₃], the molar absorption coefficient of the ⁵D₄←⁷F₆ transition is too small to measure Q^Tb_Tb for the solution but this quantity could be determined for the microcrystalline sample (72 ± 5%, τ_rad = 1.9 ± 0.1 ms). In this case, the JO theoretical estimate leads to a much too short τ_rad value. The large difference in η_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 Na₃[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%; τ_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.