Europium 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... 

At times the distortion of the geometry of a complex can be an advantage. For example, the distortion of the coordination antiprism of the europium atoms in [Eu(acac)3phen] results in the reduction of the symmetry of the complex to such an extent that all the characteristic electronic transitions become allowed and the full multiplicities are seen in the emission spectrum of the complex [100]. 

The molecular structures 38-43 shown in Table 5.9 are typical of red, low-molar-mass electroluminescent materials. The number of efficient low-molar-mass electroluminescent materials emitting in the red is relatively limited. The europium complexes (38 and 39) " exhibit narrow emission spectra in the red part of the visible spectrum. The first europium complexes synthesised, such as compound 38, were not stable enough to be deposited by physical vapour... 

Study of these and other transitions can thus yield valuable information. Crystals of the nine-coordinate complex [Eu(tmhd)3(terpy)] contain two slightly different molecules present in the crystal, its luminescence spectrum showing a broad but imresolved Do Fo transition. In solid [Eu(tmhd)3(Me2phen)] (Ln = La, Eu, Tb, Ho), there are two different square-antiprismatic isomers in the unit cell, and in this case emissions from both isomers can be distinguished in the fluorescence spectrum of the europium complex, which shows an unusually high splitting of the Dq Fq transition. ... 

The excitation spectra of europium and terbium (III) clathrochelates contain a strong dominant band at 27 000 cm-h The CTB at 23 809 cm-i (4.2 K) is mirror-symmetric to the luminescence band. The red emission is detected for the exited europium(III) clathrochelate. The emission spectrum of this complex (Fig. 70) also contains a set of lines corresponding to the °Do —> transitions,... 

The terbium complexes allow more flexibility in the choice of suitable acceptors and more particularly of multiple acceptors in the scope of multiplexing, since the terbium emission lines are more evenly disposed in the emission spectrum (compared to europium emission) displaying lines at 490, 545, 585, and 620 nm. Thus, either fluorescein, rhodamin, or indocyanines derived acceptors can be used, their respective emission falling in workable windows between successive emission lines or in the NIR window. Furthermore, green fluorescent protein (GFP) or GFP-like acceptors have been recently used to design assay involving a fusion protein substrate [17]. 

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. 

The energy levels in europium are particularly favorable for analysis of the emission spectrum since the transitions occur between levels which have small (generally <3) values of J and by far the majority of the complexes that have been studied have contained europium(III) as the central ion. The energy levels of europium(III) and terbium(III) are given in table 25.9 from which it can be seen that the lowest transitions for europium, Do- F/. (/ = 0,1,2) would be much easier to analyze than the lowest transitions for terbium, D4- F/ (/= 6,5,4, 3,2). 

The use of the fluorescence spectrum as an aid in structure indication for favorable cases can be illustrated by the work of Rose and Abramson (1965). The complex octakis(4-picoline-N-oxide)europium(IIl) hexafluorophosphate, since it involves unidentate ligands, would be expected to be either dodecahedral or square antiprismatic. The emission spectrum of this compound at 77 K is quite simple and consists of two peaks in the region and a very much... 

The terdentate ligand, terpyridyl, follows the same pattern as bipyridyl and ortho-phenanthroline (Durham et al., 1969). The maximum number of coordinated ligands (three) occurs with the perchlorate. Since only ionic perchlorate was observed in the infrared spectrum, the molecule was presumed to be nine-coordinate. The emission spectrum of the europium complex was consistent with the expected D3 symmetry with a very slight distortion and this has been confirmed by a complete structure determination (Frost et al., 1969). 

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