Terbium studies

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

In conclusion I should like to consider a few of the chemical investigations which might be accomplished in the rare earth field by Mossbauer spectroscopy. The study of nonstoichiometric oxides has been discussed earlier, but there is the problem of finding an appropriate doping nuclide for the praseodymium oxide system. The element most capable of following the changes in oxidation state of the praseodymium is terbium-159, which does have a Mossbauer state, however, with a rather broad resonance (58,0 k.e.v., = 0.13 nsec.). Nevertheless, with a sufiiciently... 

For protein and DNA studies several methods of locating the lanthanide probe onto the chain have been employed. In one example the antenna fluorophore is covalently attached to one leg of the chelate of europium or terbium, which is further attached to the specific site through thiol reactive, e.g. maleimide, or amine reactive, e.g. A-hydroxysuccinimide, groups. A typical example is shown in (3.77). ... 

Continuation of the study of the radioactive elements produced by cyclotron bombardment of lower elements led in 1950 to isolation by tracer techniques of numbers 97 and 98. Bombardment of Am241 with helium ions by S. G. Thompson, A. Ghiorso, and G. T. Seaborg produced 97243 which resembled its analogue, terbium, in its elution from ion-cxchange resins. Since terbium was named from the city of Ytterby, 97 was named from the city in which so many new elements had been discovered, Berkeley, and the name berkelium and symbol Bk have been accepted (89, 90). 

The unsymmetrical derivative, L2, in which one benzimidazole group is replaced by a 2-pyridyl moiety, and its complexes with europium and terbium have been further studied for their photophysical properties (37). The coordinated nitrate anions in the [ ( 2)( ) ]... 

Photophysical studies have been conducted on a number of lanthanide complexes of calix[n]arenes, and a significant number of these are discussed in a recent review (79). The first europium and terbium calixarene complexes showed promising photophysical properties, with terbium luminescence lifetime of 1.5 ms and quantum yield of 0.20 in aqueous solution (80). 

High quantum yields and long lifetimes have been reported for a series of calixcrowns with appended bipyridyl chromophores, such as L48 (82). Photophysical studies in acetonitrile gave lifetimes of 0.95-1.38 and 1.83-1.93 ms, and quantum yields of 0.18 0.32 and 0.32-0.39 for europium and terbium complexes, respectively. 

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

It is convenient to divide the ions studied into four groupings, namely, terbium, europium, neodymium, and other trivalent ions. The reason for this grouping is that terbium, europium, and neodymium have been studied more extensively than the rest, and therefore the number of papers is larger. The greater attention paid to these trivalent ions is probably one of practical or potential practical applications to lasers. 

Perhaps the major stimulus for extensive studies of terbium is the possibility of producing coherent green light by laser action. Indeed, a report of such has been made by Huffman (87). 

Studies of the fluorescent lifetime of terbium in the series Tbo.srare eartho.shexaantipyrene triiodide (50) give some indication of the distance... 

Rieke and Allison (97) studied the fluorescent lifetime of terbium in the chelate terbium trianthranilate (TbAn3). They concluded that the spectra and fluorescent lifetime of the chelate differ markedly from unchelated terbium compounds. Fluorescent-lifetime measurements were made at 25°C, 0°C, and 77°K using a stroboscopic light source with a decay time of 20 /xsec and a comparison was made with TbCl3 4H20. At all... 

Glasses. Pearson and Peterson 98) studied extensively the fluorescent decay properties of terbium in Calibo base glass which has the composition CaO, 20 Li20, 10 B203, 70 mole per cent. All their data was taken using a stroboscopic method. 

The rates of internal conversion from the 5Z)3 to the 5D4 states were also measured. The backup oxide in this case was yttrium. This information was obtained by determining the rise time of the 5Z)4-state green fluorescence as a function of time, when the 5Z>3 state was excited. The rise time of the 5Z)4 state is, of course, the decay time of the 5Z>3 state. It was assumed that the decay of the 5Z)3 was predominantly due to an efficient internal conversion process to the 5D4. Measurements of the decay time of the 5Z)3 state directly were not possible, since the emission from this state is very weak if not, indeed, absent. The result of this study is shown in Fig. 23, where it can be seen that the internal-conversion time remains constant at about 17 fxsec up to a terbium oxide concentration of 1 mole per cent. At higher concentrations, the internal conversion time falls rapidly, until at 10 mole per cent terbium oxide the value is about 1.7 /xsec. This is down by a factor of 10 over samples containing 1 mole per cent or less of terbium oxide. 

The temperature dependence of the fluorescent lifetime was also studied. The series of samples which used aluminum oxide as the backup material were selected for these experiments. Figure 24 shows the fluorescence lifetime of terbium as a function of concentration at IT and 4.2°K compared... 

Huffman (87) studied the transient emissions from terbium in a vinylic resin matrix. His compound was Tb tris-[4,4,4-trifluoro-l-(2-thienyl)-1,3-butaneodione] in polymethylmethacrylate. This may be conveniently abbreviated as TbTTA in PMMA. The compound EuTTA in PMM A had previously been reported by Wolff and Pressley (99) to give laser oscillation. Working with small fibers at 77°K, Huffman found distortions from the normal fluorescent decay curves when the optical pumping was large. He interprets this as evidence for stimulated emission. A comparison of these distorted decays with EuTTA in PMMA indicated a similar behavior, thus tending to substantiate his hypothesis. 

Cabezas and DeShazer (43) studied terbium lifetimes in Hughes borosili-cate glass. Their work was concerned with radiative transfer of energy between rare earth ions. That is, they were interested in finding photon-capture systems that would be useful for possible laser applications. They... 

The sensitization of terbium by gadolinium and a study of the kinetics of the exchange process have been reported by Pearson and Peterson (98, 100). Experiments were conducted at room temperature in a doped Calibo glass using a stroboscopic technique. 

Studies of the excitation spectrum of the gadolinium-terbium system... 

In another study, Kondrat eva (103) made a determination of the luminescent quantum yield of the 5D4 state of the terbium ion in aqueous solution. The method used was based upon fluorescent-lifetime measurements and had previously been used by Rinck (96) and Geisler and Hellwege (96) to determine the quantum yield of rare earths in crystals. Kondrat eva made his studies on chloride and sulfate solutions, using the electronic shutter technique of Steinhaus et al. (66). 

The enhancement of the fluorescent yield of terbium, europium, and gadolinium in heavy-water solutions was studied by Kropp and Windsor (105). They observed substantial increases in emission intensities for both terbium and europium compounds when ordinary water was replaced by deuterated water. No appreciable increase was observed for gadolinium, however. For terbium they also obtained the fluorescent lifetimes of the 5D4 state. 

Kondrat eva and Lazeeva (108b) studied the temperature dependence of the fluorescent lifetime of the 5D4 state of terbium sulfate in H20 and... 

Lead by some earlier work on terbium (54), Peterson and Bridenbaugh (109) examined the effects of rare-earth-ion substitution upon the neodymium -state mean life. The series of compounds studied was Na0.5Gdo.46Ndo.02 rare earth0 02WO4. All data were collected at room temperature using a stroboscopic technique. Figure 28 shows the result of this analysis. 

As with terbium, a great deal of study has been made upon this ion in hopes of using it in a visible laser. Experiments along this line appear to have been very successful, and reports have been made on oscillation in crystals (757), glasses, and liquids (138). 

Axe and Weller (52) studied fluorescence and energy transfer of europium in yttrium oxide. In an experiment somewhat similar to that of Peterson and Bridenbaugh (54) on terbium, Axe and Weller were able to obtain experimental evidence for nonradiative-energy transfer between europium and other trivalent rare earth ions. Their study included both intensity and fluorescent-lifetime measurements. 

The cathodoluminescence of thin films containing rare-earth oxides was studied by Hansen and Myers (140). Films of yttrium oxide doped with rare earths were prepared in vacuum by electron-bombardment evaporation of the oxide powder mixtures. Luminescent rise and decay curves were obtained for activation with europium, gadolinium, terbium, and dysprosium. 

A terbium-europium two-ion coupled system was given extensive study by Peterson and Bridenbaugh (61). The compound studied was Nao.5Tbo.45-Eu0.05WO4. The fact that europium could be sensitized by terbium was first pointed out by Van Uitert and Soden (757). 

A very important study of the kinetics of a terbium-europium coupled system was made by Gallagher and co-workers (153). The compound studied was Eu-Tb-4,4 -dimethoxybenzophenone. This work is particularly... 

Kropp and Windsor (105,107) studied extensively the effects of deutera-tion on the luminescence characteristics of some rare-earth complexes. Solutions of europium and terbium salts in heavy water give fluorescence intensities and lifetimes many times greater than the corresponding solutions in ordinary water. Table X gives the results of their studies on europium... 

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