Quantum yields lanthanide complexes

The strong distance dependence of the lanthanide luminescence sensitisatimi efficiency is clearly illustrated in Fig. 6, in which two fluorescein-based ytterbium(III) complexes are compared. Both complexes were measured in water. Even though the ligand stmctures are slightly different, the intrinsic lanthanide luminescence quantum yields (luminescence decay times [7, 56] of both complexes (around 1 ps in aqueous buffer, 8...10 ps in D2O). Since in both cases the same chromophore is used, the enormous difference is luminescence intensity can therefore be attributed to a change in sens-... 

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). 

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... 

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... 

Because of the higher sensitivity of Ndm ions towards deactivation through O-H oscillators, the complexes with this lanthanide have much lower quantum yields and lifetimes when compared to Ybm. The best photophysical properties are obtained with phthalexon S and since complexes with PS contain 4-5 water molecules, depending on the lanthanide ion, it is quite clear that exclusion of these water molecules from the first coordination sphere will lead to much enhanced luminescent properties. This is indeed demonstrated by bis(cyclen)-substituted PS, H736 (see fig. 36), which increases quantum yields to 0.23 and 1.45% in D2O for Ndm and Ybm, respectively (Korovin and Rusakova, 2002). 

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