Lanthanide complexes emission spectra

The donor contribution in the acceptor channel (crosstalk) should be as low as possible the impact of this contribution on a bioassay is not obvious to anticipate starting from a lanthanide complex emission spectrum, since many instmmental factors, such as the filter settings (bandpass width), have to be considered. The intensity distribution between the emission lines is critical, particularly for europium complexes, with a strong impact of the ligand structure and symmetry (for terbium complexes, this impact is reduced). Care must be exercised in comparing published emission spectra, since many of the published spectra are not corrected for the photomultiplicator sensitivity (which falls off rapidly between 650 and 800 nm even using a red PMT ). The consequence is that the 690-nm ( Dq p4) band seems much smaller than its true value. Some articles do indeed show spectra corrected for the sensitivity of the detection system (which contains contributions from the PMT, but also from the monochromators and optics). Whenever such corrections have been applied, this is usually indicated in the experimental section of the article. 

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... 

Different lanthanide metals also produce different emission spectrums and different intensities of luminescence at their emission maximums. Therefore, the relative sensitivity of time-resolved fluorescence also is dependent on the particular lanthanide element complexed in the chelate. The most popular metals along with the order of brightness for lanthanide chelate fluorescence are europium(III) > terbium(III) > samarium(III) > dysprosium(III). For instance, Huhtinen et al. (2005) found that lanthanide chelate nanoparticles used in the detection of human prostate antigen produced relative signals for detection using europium, terbium, samarium, and dysprosium of approximately 1.0 0.67 0.16 0.01, respectively. The emission... 

In parallel to these lifetime changes, complexation induces tremendous modifications of the emission spectra, that have been examined for lanthanides in dedicated papers (for example, Hnatejko et al., 2000) or reviewed (Biinzli and Choppin, 1989). Briefly, for Eu(III), taken as a typical example of R(III) ions, the emission arises from the 5Do -> 7Fj transitions, from which the 5Do -> 7F2 (around 616 nm) exhibits hypersensitivity. Therefore, the wealth of information experimentally obtained is highly dependent on the scanning step used and a high resolution is needed to make the best of an emission spectrum (Biinzli and Choppin, 1989). However, even in the case of a low resolution, the changes are spectacular, as illustrated in fig. 7. 

Figure 7.15 Linear emission spectrum of selected lanthanide complexes (Eu, Gd, Tb, Er) and cinnamic acid = 337nm).

Terpyridyl is a terdentate ligand and behaves like bipyridyl, and o-phenanthroline. The three terpyridyl ligands coordinate to the lanthanide when the anion is perchlorate. The emission spectrum of the Eu(III) complex points to D3 symmetry which has been confirmed by the determination of the structure [242],... 

Multilayer devices with lanthanide chelate complexes. In these complexes, efficient energy transfer from the singlet or triplet exciton on the ligand of the complex to the lanthanide atom at its center results in efficient, atomic-like line emission spectra from the latter. By adjusting the identity and concentration of the different lanthanide complex dopants, a line spectrum with white CIE coordinates was achieved.77... 

Without at least relative values of brightnesses and quantum yields, claims of bright , brilliant , intensely luminescent NIR luminescent lanthanide complexes are of little value. Absence of noise in an emission spectrum may indeed mean that the measured complex has a high brightness but may also be the result of a very efficient light detection system and/or a powerful excitation source. 

It is not because the emission spectrum of a luminescent compound can be measured that it necessarily means the compound is a good emitter. There are different criteria to be taken into account before concluding on the efficiency of the luminescence. In the case of lanthanide complexes, it is not because the luminescence of the ion is measured that either the sensitization or the luminescence of the ion is efficient. The luminescence needs to be further investigated to evaluate the efficiency of each step yielding to the emission of the lanthanide ion. 

The luminescence of the lanthanide ions spreads from the UV spectral range up to the NIR, and many lanthanide ions have unique spectral characteristics in the visible region of the spectrum, which also give them distinctive luminescent colors. A lot of applications take advantage of those characteristic emissions for color reproduction and lighting. Phosphors, nanomaterials made of lanthanide complexes or enclosing lanthanide compounds, as well as LEDs based on lanthanide complexes are extensively investigated. 

The lanthanide ions, particularly those near the middle of the series, samarium, europium, terbium, and dysprosium, form complexes that often emit visible radiation when excited in the near-ultraviolet. This emission spectrum can be analyzed by essentially the same procedure as for the absorption spectrum except that the nature of the emission process will generally yield additional information concerning the ground multiplet of the ion. The technique can be applied to solutions and solids but in solution various processes operate to reduce the intensity of the emitted light and to broaden the bands which can result in a reduction of the amount of information that can be obtained. 

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