Lanthanides quantum yields

T (H2O) = 1.54 ms, T (D2O) = 2.61 ms, < ,ot 0.32, < ,ot (H2O) = 0.32. (Eu)36 r (H2O) = 0.62 ms, x (D2O) = 2.25 ms, brightness (relative to (Eu)DTPA-csl24) = 0.57, quantum yield was not substantially increased in this DOTA derivative compared to previously reported acyclic complexes (DTPA-csl24). One important feature of this article is the discussion about the lanthanide quantum yield ( Ln). This parameter has been often poorly determined however, it is a key parameter in LRET experiment since the distance R, inferred from energy transfer experiments via donor luminescence decay or intensity measurements, depends on knowing the donor ( Ln) quantum yield R oc Very often this has been explicitly or implicitly... 

Flowever, there is a trade-off in using near-IR emissive lanthanides, in that luminescence lifetimes are shorter, and quantum yields lower, compared to complexes of Tb and Eu. This arises because the near-IR emissive lanthanides are quenched by lower harmonics of the O-H oscillator, increasing the Franck-Condon overlap with the metal excited state. For neodymium, matters are further complicated by the manifold of available metal-centered excited states, which leads to particularly effective quenching by C-H oscillators. Thus, complexes in which there are few C-H oscillators close to the metal are desirable if the luminescence lifetime is to be optimized (e.g. 44).76 97-101... 

An important measure of the luminescence is the quantum yield. In effect, this is the probability that a photon will be emitted by the lanthanide given that one photon has been absorbed by the antenna ligand. Since measurement of absolute quantum yields is particularly difficult, the overall quantum yield ( ) is normally measured with reference to certain standards (26) these are routinely [Ru(bpy)3]2+ in water or SulfoRhodamine 101 in methanol for Eu3 +, and quinoline sulfate in 0.1 M HC1 or fluorescein in 1 N NaOH for Tb3+ (27,28). A method has been developed that measures energy transfer from the lanthanide complex to an acceptor of known quantum yield (28). 

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. 

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. 

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). 

Currently the best lanthanide complexes have millisecond excited state lifetimes in aqueous solution (52,56,93) and quantum yields of approximately 0.3 (Eu3 + ) and 0.6 (Tb3+) (27). While almost all of the... 

Latva, M. Takalo, H. Mukkala, V.- . Matachescu, C. Rodrfguez-Ubis, J.-C. Kankare, J. Correlation between the lowest triplet state energy level of the ligand and lanthanide(III) luminescence quantum yield. J. Luminesc. 1997, 75, 149-169. 

Xiao, M. Selvin, P. R. Quantum yields of luminescent lanthanide chelates and far-red dyes measured by resonance energy transfer. J. Am. Chem. Soc. 2001,123, 7067-7073. 

Brunet, E. Juanes, 0. Sedano, R. Rodriguez-Ubis, J.-C. Lanthanide complexes of polycarboxylate-bearing dipyrazolylpyridine ligands with near-unity luminescence quantum yields the effect of pyridine substitution. Photochem. Photobiol. Sci. 2002, 1, 613-618. 

Many aza-crown macrocydic ligands [118] have been used to produce stable, well-shielded lanthanide complexes that have good photophysical properties. The aza-crown macrocydic ligand 19 has a terpyridine moiety incorporated into it to act as a sensitiser. Quantum yields of 0 = 0.18 and 0 = 0.21 were determined in water for the Eu(III) and Tb(III) complexes respectively. 19 is an... 

Due to the modifications of the electronic cloud induced by complexation, the quantum yield and the excitation spectrum are also modified. As the direct determination of the absolute quantum yield is very difficult to achieve, one usually finds in the literature quantum yield values determined by comparison to well-known standards, such as quinine sulfate. For example, some values can be found in Georges (1993) or in Klink et al. (2000) for some europium complexes but may be found also in many other papers on lanthanide luminescence. Studies on the correlations between the photophysical properties of a given type of europium complexes and the energy levels can be found in Latva et al. (1997), Klink et al. (2000). A correlation has been found between the excitation properties and the stoichiometry of various Eu(III) complexes (Choppin and Wang, 1997). Note that the changes in the excitation maximum induced by complexation usually amount to a few tenths of nanometers, which requires high resolution for detection. In the case of Eu(III), a correlation has been found between the frequency... 

Haase and co-workers have extensively studied the synthesis of high-quantum-yield nanophosphors such as Eu-, Tb- or Er-doped lanthanide phosphates (Riwotzki et al., 2000, 2001 Lehmann et al., 2004 Kompe et al., 2003 Lehmann et al., 2003). For example, the photoluminescence (PL) quantum yield of the CePO Tb core nanocrystals (4-6 nm) diluted... 

---