Terbium quantum yields

Due to the competing non-radiative decay routes for the lanthanide excited state, there is an intrinsic limit to the overall quantum yield in luminescent lanthanide complexes. It has been estimated that these values are 0.50 and 0.75 for europium and terbium, respectively (27). Although quantum yields exceeding these have been reported (31,32), care should be taken in analyzing quantum yield results in the literature, as these are often given for the energy transfer process alone, and not the overall quantum yield, and in other cases it is unclear as to which process(es) the quoted quantum yield refers to. 

The related bis(pyrazolyl)borate ligand [L5]- forms 10-coordinate complexes of the type [Ln(L5)2(N03)]. The terbium complex in particular has promising luminescence characteristics, with a lifetime of 2.6 ms and quantum yield of 0.36 in methanol (49). However, the complexes are not stable in water. 

An attempt to synthesize binuclear complexes led to the development of the back-to-back ligand [L7]2-. The lanthanide coordination environment of the resultant [ ( )2 2( 7)] complexes is the same as that in the parent mononuclear [L4]- complexes, as shown in Pig. 7. Despite the possibility of displacement of the nitrate anions by solvent molecules, a reasonable lifetime (0.9 ms) and quantum yield (0.13) is recorded for aqueous solutions of the terbium complex (49). 

Due to the presence of hard anionic oxygen atoms, phenolate and carboxylate groups are often employed as donors in lanthanide coordination chemistry. Ligand [L18]4- is reported as an excellent triplet sensitizer for lanthanide luminescence (61). Indeed aqueous lifetimes of 0.57 and 1.61 ms are reported for europium and terbium, respectively quantum yields of 0.20 and 0.95 respectively refer to the efficiency of the energy transfer process alone. 

Ligands L33 and L34 are both good sensitizers of europium, whilst the former also efficiently sensitizes terbium emission (29). The quantum yield of [EuL34]3+ in aqueous solution, 0.247, is among the highest reported for any europium complex. The lower triplet energy of L34 results in back transfer of energy from the terbium excited state to... 

A closely related ligand is [L41]3- where one side of the cyclen ring is replaced by a pyridyl group (72). In addition to the high quantum yield of the terbium complex ( = 0.51), the complex exhibits in vivo and in vitro tissue selectivity, allowing it to be used for imaging of adenocarcinoma cells. 

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. 

The europium complexes are highly luminescent, with quantum yields of up to 0.19 in methanol. The ligand triplet state is too low in energy to efficiently populate the terbium excited state, and no metal-centered emission is observed. 

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 authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

Chauvin, A.-S., Gumy, F., Imbert, D., and BunzU, J.-C.G (2004) Europium and terbium tris(dipicolinates) as secondary standards for quantum yield determination. Spectroscopy Letters, 37 (5), 517-532. 

Low-temperature studies of solid tris-bipyridine europium, terbium, lanthanum(III), and sodium clathrochelates were performed on UV excitation [390]. The lanthanum (III) and sodium clathrochelates were examined to get more information on the luminescent properties of the ligand. These clathrochelates showed luminescence at temperatures below 100 K upon longwave UV excitation corresponding to ligand-centred absorption. The quantum yield of the ligand phosphorescence is co 0.02. An increase of temperature results in a drastic decrease in the luminescence intensity, and at 100 K it becomes equal to zero (Fig. 69). 

Terbium clathrochelate showed green emission of very high intensity. The emission spectrum contains the D4 —> Fj transition bands of the encapsulated terbium ion. The same but less intense emission spectrum was observed at higher temperatures. The luminescence quantum yield is close to 1 at 4.4 K and is approximately 0.05 at room temperature [390]. The decrease in intensity of the terbium(III) ion luminescence starts at 100 K (higher than that of free macrobicyclic tris-bipyridine ligand and lower than that of the corresponding europium(III) compound, Fig. 69). It may be... 

The powder contains 0.5 and 0.3 water molecule per europium(III) clathrochelate molecule at 300 K and 4.2 K, respectively, and the quantum yields were calculated to be 0.56 and 0.65, respectively. These values practically coincide with the experimental data. The observed deviation of the quantum yield from 100% for this solid sample is caused by the presence of solvate water molecules [391], The decrease in the luminescence quantum yield in aqueous solutions is also largely caused by an increase in the number of water molecules around the clathrochelate complex. Close to unity luminescence yield for terbium(III) clathrochelate powder may be attributed to the absence of water in this sample. The scheme of the light conversion process absorption - energy transfer emission (A-ET-E) is shown in Fig. 71. 

While Co " competes with Zn for binding at both sites in bovine lens leucine aminopeptidase, Mg " " competes for only one of the sites. When cobalt(ii) is substituted at site 1, the specific activity with L-leucine-p-nitroanilide is increased by more than ten-fold. Cobalt(ii) has been replaced for zinc in thermolipin and simultaneously terbium(iii) for calcium the observed visible fluorescence (excitation at 280 nm) provided quantum yield measurements which indicated that the terbium-cobalt distance was 13.7 A, in good agreement with the Ca—Zn distance determined from a recent crystal structure. Vallee and co-workers suggest that this technique... 

Europium and terbium tris(dipicolinates) are known to be highly luminescent, with overall quantum yields of 24% and 22%, respectively, and lifetimes of 1.67 and 1.43 ms, respectively. This property is taken advantage of in simple luminescent methods for the determination of nanomolar concentrations of lanthanides (Barela and Sherry, 1976). In fact, both the quantum yield and lifetime heavily depend on the concentration of the chelates and the pH since the latter governs the speciation in solution (Eigure 38). Three or six water molecules are coordinated onto the metal ion in the lower bis and monospecies, respectively. As a consequence, the corresponding quantum yields and lifetimes are drastically reduced with respect to the tris-species. Eor instance, a quantum yield of only 4.5% has been determined for [Eu(dpa)2] (Aebischer et al., 2009) while t( Do) = 0.35 ms only for this complex (Chauvin et al., 2004). 

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