Luminescent lanthanide complex

Keywords Chirality Lanthanide complex Luminescence Molecular recognition Receptor... 

It is well known, that in aqueous solutions the water molecules, which are in the inner coordination sphere of the complex, quench the lanthanide (Ln) luminescence in result of vibrations of the OH-groups (OH-oscillators). The use of D O instead of H O, the freezing of solution as well as the introduction of a second ligand to obtain a mixed-ligand complex leads to either partial or complete elimination of the H O influence. The same effect may be achieved by water molecules replacement from the inner and outer coordination sphere at the addition of organic solvents or when the molecule of Ln complex is introduced into the micelle of the surfactant. 

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... 

Figure 7 Luminescent lanthanide complexes with representative luminescence lifetimes, and hydration states (derived from luminescence measurements) where appropriate.

Assays based on luminescent lanthanide ions were developed initially in the 1970s, when instrumentation became available which could distinguish long-lived luminescence from a shortlived background. Leif and co-workers reported the first attempts to use lanthanide complexes (in this case europium complexes with 1,10-phenanthroline and 7-diketonates, i.e., [Eu(phen)(diketo-nate)3]) as tags for antibodies.107 These proved kinetically unstable in the pH regime required... 

Lanthanide complex with the enhancing ligand L is luminescent... 

Potential Applications for the Use of Lanthanide Complexes as Luminescent Biolabels Graham R. Motson, Jean S. Fleming and Sally Brooker... 

The complexing ligand of the lanthanide cation also provides a way for the luminescent lanthanide complex to be linked to a particular target molecule, thus acting as a luminescent probe for the target... 

Armelao L, Quici S, Barigelletti F, Accorsi G, Bottaro G, Cavazzini M, Tondello E (2010) Design of luminescent lanthanide complexes from molecules to highly efficient photo-emitting materials. Coord Chem Rev 254 487-505... 

Cross J-P, Lauz M, Badger PD et al (2004) Polymetallic lanthanide complexes with PAMAM-naphthalimide dendritic ligands luminescent lanthanide complexes formed in solution. J Am Chem Soc 126 16278-16279... 

Lanthanide Complexes of Encapsulating Ligands at Luminescent Devices... 

POTENTIAL APPLICATIONS FOR THE USE OF LANTHANIDE COMPLEXES AS LUMINESCENT BIOLABELS... 

Whilst there have been a number of excellent reviews on the design, synthesis, structural determination, and photophysics of luminescent lanthanide complexes (1-6), detailed information for coordination chemists on how these compounds may be used as analytes in the biochemical and biological sciences has been somewhat less readily... 

Of particular interest to this review is the use of luminescent lanthanide (especially europium and terbium) materials as non-radioactive markers (3,5,8,10 12,16). In addition to the safety advantages over radioactive labels, lanthanide complexes remain luminescent as long as the complex remains intact, allowing repeated detection of analytes over a much longer time period than radioactive labels. 

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. 

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). 

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