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

The efficiency with which the surroundings of trivalent lanthanide ions sensitize their luminescence (Zsens) is a key parameter in the design of highly emitting molecular edifices and materials. Evaluation of Zsens 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 mL). 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 $(Q^{Eu}_L = 29 \pm 2\%)$ and intrinsic $(Q^{Eu}_{Eu} = 41 \pm 2\%)$ quantum yields obtained for Eu allow the direct calculation of $\eta_{sens}$ $(71 \pm 6\%)$ while the radiative lifetime $(\tau_{rad} = 4.1 \pm 0.3 ms)$ 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 $^5D_4\leftarrow ^7F_6$ transition is too small to measure $Q^{Tb}_{Tb}$ for the solution but this quantity could be determined for the microcrystalline sample $(72 \pm 5\%, \tau_{rad} = 1.9 \pm 0.1 ms)$. 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 $^5D_4 \leftarrow ^7F_6$ 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 $(0.015 \pm 0.002\%)$ due to both poor sensitization efficiency (8%) and small intrinsic quantum yield $(Q_{Yb}^{Yb} = 0.178 \pm 0.003\%; \tau_{rad} = 1.31 \pm 0.02 ms)$. 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.