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Fig. 23. A schematic representation of the intramolecular energy transfer
(IMET) process in Eu*+ complex.
132 Absorption Spectra of the Europium Ion and Its Complexes
Recently sensitization of the donor triplet level in a system containing benzophenone and europium-JroVhexafluoroacetylacetonate has been reported . This sensitization is explained by a triplet-triplet energy transfer from the sensitizor triplet state to the triplet state of the rare earth complex [632a]. Although such transfer of energy may take place at low concentrations (~10-3 M), the author found (unpublished results, see also ref. 632) an efficient diffusion controlled intermolecular sensitization of the Eu3+ ion by benzophenone at ~10_1 M and higher concentrations. Another interesting phenomenon was observed by Gallagher et al.  for a system containing 4,4'-dimethoxybenzophenone (DMB) and Eu3+. No direct energy transfer from DMB to Eu3+ was noted but when Tb3+ was added to the system exchange of energy from DMB to Eu3+ via Tb3+ took place. Tb3+ acts as an intermediate in the two step energy transfer process. Recently intermolecular transfer of energy from the excited levels of some aromatic aldehyde and ketones in dilute solutions to the resonance level of Eu3+ and Tb3+ was reported .
The most extensively studied europium complexes are the (3-dike-tonates [635—644] although the author has tried to concentrate on complexes containing nitrogen heterocycles [373, 401, 645] as ligands. In both case the complexes fluoresce via the IMET mechanism.
The effect of substitution on the p-diketonate [646, 647] and dipyridyl  ligands as well as change of ligand structure whilst keeping the coordinating atoms the same  has been investigated. It has been found that electron donating substituents in the correct positions on the ligands increase the fluorescence yield considerably by the +/ effect while the reverse effect is noted with the electron withdrawing groups. In the europium complexes once the energy is transferred via the IMET process from the ligand to the 5Do resonance level, the transition pro-bablities to the 7F multiplets are not of the same order of magnitude. It is the 5X>o -► 7F2 transition which is found to be strongly affected by ligand substitution. An approximately 15 fold increase in intensity of the 5A> -► 7F2 transition was observed by the author  on changing phthalate to naphtalate in Na[EuLa]. The naphthalate anion obviously by virtue of it’s better resonance acts as a better donor in the IMET process.
The enhancement of europium fluorescence is also observed by reducing the radiationless energy losses using  an “insulating sheath*’. This technique also increases the quantum efficiency of photoluminescence.
It is also possible  to “bracket” the donor triplet state by the proper choice of ligand, and a selective excitation of one of the resonance leypls of Eu3+ ion can be effected. The fluorescence spectra of benzoyl-acetone and dibenzoylmethide complexes of Eu3+ show transitions from
The Luminescence Spectra of the Europium Ion
both the 5X>i and 5Do levels, whereas the o-hydroxybenzophenone complex shows transitions originating only from the 5Z>o level.
Fluorescence decay. — According to Freeman and Crorsy  both radiationless transfer and radiative emission follow a first order exponential decay law. However, Kuhlman and Weissman  found departure from this first order rate decay in the case of the europium dibenzoylmethide complex. Bhaumik  recently pointed out that the transfer of energy from the triplet state actually takes place to a higher level (possibly the 5Di or higher ones in Eu3+) and there is a definite relaxation time (~2 //sec for europium dibenzoylmethide) for the energy to reach the emitting 5Z>o level. This relaxation time is, however, much faster than the radiative decay time of the 5Di level of Eu3+ and consequently weak lines from the 5Z>i level are observed. Thus in the transfer of energy the 5Di level actually acts as an intermediate.
The fluorescence lifetime (r) of a complex either in solution or in the solid state varies considerably with the method of preparation of the sample. It is also difficult to compare results due to the different experimental conditions used. A few data on the fluorescence lifetimes of several Eu3+ complexes are collected in Table 50. The readers are asked, however, to check the references given for detailed experimental conditions etc.
Table 50. Fluorescence lifetime (r) of some Eu*+ complexes
Complexes Transition X in (Jtsec References
77° K 300° K
EuCl3 • 6H20 Total 130 - 
EuCla • 6D20 99 1650 - 
Eu(BrOB)# • 9H20 99 120 - 
Acetophenone (soln.) 6£>o - ?F2 — 260 
EtOH—MeOH soln. 99 — 226 
Eu(Benzoylacetone), • 2HaO
Solid 99 364 296 
EPA soln. 99 440 150 
EtOH—MeOH soln. 99 427 118