EFFECT OF ION LEVEL EXCITED ON THE FLUORESCENCE YIELD OF EUROPIUM CHELATES

$^{1}$ Notation of DeShazer, L. G., and G. H. Dieke, J. Chem. Phys. 38, 2196 (1963).Author Institution:A wealth of current information is available dealing with luminescence lifetimes and quantum yields of fluorescence of rare earth chelates, particularly those of Europium. In most previous work, fluorescence from the ion has been excited via absorption of light in the chelate ligand and subsequent transfer to the ion. The quantum yield of fluorescence from $^{5}D_{0}$ state has been determined for $Eu(NO_{3})_{3}$ in methanol as a function of excitation directly to the $^{5}D_{0}$, $^{5}D_{1}, ^{5}D_{2}, ^{5}D_{3}$ or $^{5}L_{7}{^{1}}$ levels of $Eu^{3+}$ and depends upon the level excited. The method is applied here to Europium thenoyltri-fluoracetonate, $Eu(TTA)_{3}$, and europium benzoylacetonate, $EuB_{3}$. The quantum yield of the $^{5}D_{o}$ level of $Eu(TTA)_{3}$ in acetone and ethanol is very high (about 80\% in each case). For $EuB_{3}$ in methanol, the quantum yield of the $^{5}D_{0}$ level is only 30%. In all chelate-solvent systems the yield of the $^{5}D_{0}$ level excited via the $^{5}D_{1}$ is less than the $^{5}D_{0}$ excited directly. For $Eu(TTA)_{3}$ in acetone, the yield is only slightly decreased by excitation to the $^{5}D_{1}$ level. However, for $Eu(TTA)_{3}$ in ethanol and $EuB_{2}$ in methanol a decrease of a factor 2-3 is observed. When higher levels are initially excited, the yield is still less. The results are compared with the yields measured by direct ligand absorption. Determination of the quantum yield of the $^{5}D_{0}$ level together with a knowledge of fluorescence lifetime makes it possible to calculate the radiative lifetime of the $^{5}D_{0}\to^{2}F_{1}$ transitions. For $Eu(TTA)_{3}$ in acetone this re is $\sim 0.5$ msec or 8 times less than Eu(NO$3)$3 in ethanol and 28 times less than $Eu(NO_{3})_{3}$ in water. A knowledge of the intrinsic yield of the $^{5}D_{0}$ level allows one to predict whether adducts (such as $D_{2}O$) will result in a significant enhancement of fluorescence yield of the ion