Lanthanide luminescence

it is of great significance to select or design suitable ligand with the appropriate energy level. To illustrate the sensitizations effect of ligands, we show here two representative examples of such luminescent lanthanide MOFs. The first 

The luminescence of Ln ion from the f-f transitions can be classified as two types of transitions the parity-allowed magnetic dipole transitions and the parity-forbidden electric dipole transitions. When the Ln ion is inserted into a chemical enviromnent, noncentrosymmetric interactions allow the mixing of electronic states of opposite parity into the 4f wave functions, and electric dipole transitions become partly allowed. The intensity of some of these transitions is particularly sensitive to the nature of the metal ion environment, and these transitions are called hypersensitive transition a typical example is the Dq p2 transition of Eu [106]. Thus, the luminescence of lanthanide ions can provide valuable information about the local enviromnent and make them very suitable for acting as a structural probe deciphering the symmetry of the chemical environment and the coordination sphere. 

As remarked above, the luminescence of Eu ions is most appropriate as structural probe for the determination of the number of metal ion sites in a compound, their symmetry, their hydration numbers, and their coordination sphere. Especially, the ratio of the intensities of the Dq Fi and Dq 

The second example MOF Eu3(2,6-pydc)3(2,6-Hpydc)(S04)(H20)3-3(H20)3 (2,6-pydc = pyridine-2,6-dicarboxylate) [108] crystallizes in the triclinic space group and possesses a 2D metal-organic framework based on hexanuclear Eus SBUs. The asymmetric unit comprises three independent Eu ions, three pydc dianions, one Hpydc anion, one sulfate, three coordinated, and three guest water 

All of these criteria more-or-less follow simple common sense ideas about the characteristics of the two types of transition. Together, they enable distinctions to be drawn in most cases. Although the detailed study of the bands is in its infancy, it is interesting to note that f - d transition energies seem to follow a spectrochemical series, just as do d - d in the transition metal ions. So, the following 5f- 6d transition energies, all xl0 cm have been reported for complexes of U and other similar ions  

It has long been known that many lanthanide ions fluoresce under ultraviolet light, the fluorescence coming from f - f transitions some ions which do not normally fluoresce at room temperature do so when they are cooled. This fluorescence property has led to lanthanide ions being incorporated in the phosphor of domestic fluorescent tubes and in the screens of colour televisions. When an ion is in an electronically excited state there is a competition between deactivation by radiative and non-radiative processes. For an ion to be a good emitter, any non-radiative process must be a poor second in the competition. If studies are carried out using aqueous solutions, it is found that the lanthanide ions at the centre of the lanthanide series are 

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Lanthanide luminescence apphcations have already reached industrial levels of consumption. Additionally, the strongly specific nature of the rare-earths energy emissions has also led to extensive work in several areas such as photostimulable phosphors, lasers (qv), dosimetry, and fluorescent immunoassay (qv) (33). 

Sensitizers of near-IR lanthanide luminescence that are effective in the visible region of... 

Anion concentrations can also be monitored through lanthanide luminescence. Once again, a wide range of pathways can be responsible for luminescence quenching. For instance, complex (60) with a pendant phenanthridinium group exhibits halide ion-dependent luminescence properties as a result of collisional quenching of the phenanthridinium-centered singlet state by halide ions.165... 

Figure 9.49 Examples of antenna-chelator compounds that have been used for lanthanide luminescence.

Lanthanide chelates also can be used in FRET applications with other fluorescent probes and labels (Figure 9.51). In this application, the time-resolved (TR) nature of lanthanide luminescent measurements can be combined with the ability to tune the emission characteristics through energy transfer to an organic fluor (Comley, 2006). TR-FRET, as it is called, is a powerful method to develop rapid assays with low background fluorescence and high sensitivity, which can equal the detection capability of enzyme assays (Selvin, 2000). 

As in the structure of the TMT chelator group, pyridine derivatives long have been known to be enhancers of lanthanide luminescence (Thomas et al., 1978). One such compound,... 

Figure 9.53 DPA derivatives have been used as potent enhancers of lanthanide luminescence. Three DPA groups can coordinate with a terbium ion. The iodoacetate derivative of DPA has been used to label covalently molecules for lanthanide luminescence.

Another antenna group that is particularly effective for lanthanide luminescence is carbosty-ril derivatives (7-amino-4-methyl-2(lH)-quinolinone), which can be attached to many chelating... 

Figure 9.55 A carbostyril-DTPA europium chelator is a strong enhancer of lanthanide luminescence. The chemical structure and three-dimensional structure of the chelator are shown.

In 2000, most European countries switched from their traditional currencies to the euro. Lanthanide luminescence is used as a means of preventing counterfeit euro banknotes from passing into the money chain. Excitation of euro banknotes with ultraviolet light results in fluorescence in the red, green and blue regions due to complexes of europium (Eu3+), terbium (Tb3+) and thulium (Tm3+), respectively, that are present in the banknotes. 

Biinzli JG (2009) Lanthanide luminescent bioprobes (LLBs). Chem Lett 38 104—109... 

R. A. Evangelista, A. Poliak, and E. F. G. Templeton, Enzyme-amplified lanthanide luminescence for enzyme detection in bioanalytical assays, Anal. Biochem. 197, 213-224 (1991). 

Influence of Charge-transfer and 4/ " 5ti States on the Temperature Quenching of Lanthanide Luminescence... 

During recent years it has become clear that the temperature quenching of lanthanide luminescence is determined by the properties of the c.t. or 4f 5d state. Let us first mention shortly the mechanisms that have been proposed for temperature quenching of lanthanide ion emission. 

It has been proposed (5) that in the case of the Ln + ions an excited state involving a metal d state is responsible for the absence of luminescence. It is then necessary that this state has a large Franck-Condon shift and is situated at energies not very much higher than those of the 4/-c.t. state. Whatever the solution of this problem may be, it is clear that either c.t. or 4f 5d states play an important role in the quenching process of the luminescence. The conclusion of all the material presented in this section is that this is true for all types of lanthanide luminescence. 

Jenkins AL, Uy OM, Murray GM. Polymer-based lanthanide luminescent sensor for detection of the hydrolysis product of the nerve agent Soman in water. Anal Chem 1999 71 373-378. 

A number of non-podand (i.e., those without an apical atom or group) acyclic ligand systems have been developed for lanthanide luminescence applications. Many of these are designed as helicating ligands such that the lanthanide ion is well encapsulated despite the linearity of the ligand. 

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. 

Gruber, H. J. Kada, G. Pragl, B. Riener, C. Hahn, C. D. Harms, G. S. Ahrer, W. Dax, T. G. Hohenthanner, K. Knaus, H. G. Preparation of thiol-reactive Cy5 derivatives from commercial Cy5 succinimidyl ester. Bioconjug. Chem. 11(2), 161-166. Gudgin Dickinson, E. F. Poliak, A. Diamandis, E. P. Time-resolved detection of lanthanide luminescence for ultrasensitive bioanalytical assays. J. Photochem. Photobiol. B Biol. 1995,27, 3-19. 

The use of europium chelates, with their unusually long fluorescence decay times, as labels for proteins and antibodies has provided techniques that are referred to as time-resolved fluoroimmunoassays (TRFIA). Fluorophores as labels for biomolecules will be the topic of Sect. 3. Nevertheless, TRFIAs always have to compete with ELISA (enzyme-linked immunosorbent assays) techniques, which are characterized by their great versatility and sensitivity through an enzyme-driven signal amplification. Numerous studies have been published over the past two decades which compare both analytical methods, e.g., with respect to the detection of influenza viruses or HIV-1 specific IgA antibodies [117,118]. Lanthanide luminescence detection is another new development, and Tb(III) complexes have been applied, for instance, as indicators for peroxidase-catalyzed dimerization products in ELISAs [119]. 

The sensitisation process associated with producing lanthanide luminescence consists of a number of steps, including excitation of the antenna and energy transfer to the lanthanide. The details of this process and the considerations required in designing complexes working on this principle are discussed in the following sections. 

Scheme 3 Final steps of sensitisation process - lanthanide luminescence...

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