Europium complexation measurements

The dipole moment measurement of scandium and europium complexes in solution showed the complexes to be planar. In the solid state, the values of metal atoms are squeezed out of... 

For those complexes where the chromophore is not coordinated to the metal center directly, the orientation of the chromophore is important to ensure efficient energy transfer. The series of ligands L29-L32 were investigated for correlations between structural parameters found in the solid state (see, for example, Fig. 12) and solution (by NMR spectroscopy) and photophysical properties (69,70). It was found that both chromophore-metal separation and the angle of orientation have a direct influence on the quantum yield of the europium complexes. For example, the difference in quantum yield between [Eu(L29)]3+ and [Eu(L30)]3+ (0.06 and 0.02, respectively) cannot be attributed solely to the chromophore-metal separation, so may also depend on the better orientation of the chromophore in the L29 system as measured by the angle a between the metal center, the amide nitrogen atom, and the center of the phenyl ring. 

Structure determinations of salts containing [R c P5W30O110]12-, R = Y, Eu, and a redetermination of the sodium derivative, showed that the central cavity also contained a water molecule coordinated to the encrypted cation (Dickman et al., 1996 Kim et al., 1999). This observation, also noted for the Ca and structures, provides an explanation for the fact that the encrypted cation does not lie in the pseudo-equatorial mirror plane of the polytungstate framework. Luminescence lifetime measurements on the europium complex have led to ambiguous conclusions regarding the number of coordinated water molecules (Soderholm et al., 1995 Lis et al., 1996). As shown in Table 9 the 31P chemical shifts are significantly affected by the acidity of the solution. Remarkably, at intermediate acidities... 

Several of the complexes in Figure 2 were examined further for their resistance to dissociation. The europium complexes Eu(THED)3+ and Eu(TCMC)3+ were more difficult to study quantitatively by H NMR because of their broad H resonances. Decomposition was monitored by use of a UV-vis assay. Excess Cu2+ was added to solutions containing the lanthanide macrocycles. The Cu2+ ion served the dual purpose of trapping the free macrocycle and as an indicator to monitor the amount of macrocycle that had dissociated. All Cu(II) macrocyclic complexes gave an absorbance peak in the UV-vis spectrum that was characteristic of the Cu(II) macrocycle complex. For all macrocycles, Cu2+ was an effective trap formation of the Cu(II) macrocyclic complex went to completion in the presence of 0.10 mM La3+ or 0.10 mM Eu3+, 0.10 mM ligand and excess Cu2+ (1.0 mM). The increase in the concentration of Cu(II) macrocycle complex over time is a measure of the inertness of the lanthanide complex to dissociation. For the La(THED)3+ complex, the reaction rate (51) was independent of the concentration of Cu2+, consistent with the following mechanism ... 

Fluoroimmunoassay makes use of the above behaviour. One of the common commercial methods is dissociation-enhanced fluoroimmunoassay (DELFIA). In this, a nonfluorescent Eu(III) EDTA-like complex is attached by a simple chemical reaction to an antibody or antigen, in a process called labelling. An immunoreaction is next initiated to bind the target, and then a (3-diketone and trioctylphosphine oxide (TOPO) mixture are added to the immunocomplex formed, at pH 3, to promote release of the Eu(III) from the antibody and its complexation as the strongly fluorescent complex [Eu((3-diketonate)3(TOPO)2], which is then measured by time-resolved fluorescence methods. The signal size relates to the amount of europium complexed, which in turns relates directly to the amount of the specifically formed target immunocomplex. This process is represented schematically in Figure 9.5. 

The equilibrium constant for the complexation of cholesterol with Eu(fod)3 has been evaluated from measurements on a series of solutions at varying total concentrations but identical molar ratio. The same analysis provides values for the chemical shifts of protons in the uncomplexed steroid, and in the steroid-europium complex the latter value is not accessible by direct measurement, since the complex is always in equilibrium with the free steroid. The mathematical equations presented in this paper should be generally applicable. In another paper the validity of various procedures for interpreting lanthanide-induced shifts is explored comments on cholesterol are included. No one mathematical model at present available is considered to have general validity. 

To verify the possible effects of the total amount of adsorbed complex on the luminescent lifetimes of the samples, Hfetime measurements were performed for the 1%, 5%, and 10% matrices pressed for 0.5 min. The measured luminescent Hfetimes are 507, 551, and 581 )Xs, respectively. So, it is evident, for the pressed hybrid samples, the luminescence Hfetime is increased with an increased amount of adsorbed luminescent complex. This suggests that not only adsorption but also coordination occurs between the europium complex and the nitrogen atoms of the organic moiety, since a large complex/matrix ratio implies a minor amount of nitrogen atoms per complex molecule, that is, a minor amount of nitrogen atoms enters the europium coordination sphere. 

Knall et al. have tested a series of europium complexes with antenna chromo-phores in different polymer matrices in terms of sensitivity, response time, and dynamic range for the sensing of water vapor. A copolymer containing a Eu " chelate that can be spin-cast as thin film on glass slides was developed [110]. Scheme 6 shows the chemical structure of the polymerizable precursor 29. This complex can be copolymerized with norbomene-2,3-dicarboxylic acid dimethyl ester by means of ring opening metathesis polymerization. The sensor spots respond to water vapor by reversible luminescence quenching, which can be analyzed by means of phase sensitive measurements of luminescence lifetime [110]. 

It is well known that europium complexes emit the most intense fluorescence near 610 nm. Using this wavelength for measurement, the fluorescence excitation spectra of europium complexes in solution were recorded in the range 200.0 - 400.0 nm. The maximum excitation wavelengths for different europium complexes were summarized in table I. Using 645.0 nm and 545.0 nm for fluorescence measurement of samarium and terbium complexes, respectively, the corresponding maximum excitation wavelengths were also summarized in Table 1. 

However, all the authors mentioned a rapid decrease of the luminescence intensity with further heating above the threshold temperature, which was attributed to thermal decomposition of the complex. Among the different tested complexes, it seems that phenanthroline exhibited best thermal stability, because no luminescence intensity decrease was observed up to ISO C, which allowed more efficient water elimination. Accordingly, best lifetime values measured on films doped with europium complex were reported by Li et al. for complexation with phenanthroline (Li et al., 2001). The lifetime characterized as the first e-folding decay time was measured to be 1.40 ms. However, these authors also mentioned a bi-exponential luminescence decay, which indicated that Eu " ions occupy two kinds of spectroscopic sites. This feature probably indicates that all ions were not similarly encapsulated in the phenanthroline cage. 

Lanthanides also have potential as DEFRET energy donors. Selvin et al. have reported the use of carbostyril-124 complexes (53) with europium and terbium as sensitizers for cyanine dyes (e.g., (54)) in a variety of immunoassays and DNA hybridization assays.138-140 The advantage of this is that the long lifetime of the lanthanide excited state means than it can transfer its excitation energy to the acceptor over a long distance (up to 100 A) sensitized emission from the acceptor, which occurs at a wavelength where there is minimal interference from residual lanthanide emission, is then measured. 

The system SOI-— C0 contaning several trivalent rare earths has been studied [274] and the exchange constants Kr, were measured. A complex species of the type [M(COs)4]5- was shown to be present in solution. Poluektov and Kononenko [275] showed a displacement of the 393.5 mfi peak of the europium aquo ion to 394.5 mju and a decrease of intensity of about 1.45 times due to complexation with carbonate ion. 

Thiocyanate. — On the basis of /-orbital hybridization Diamond [351] predicted the formation of stronger actinide complexes with thiocyanate ion than for the rare earths. Subls and Chopfin [352] have studied the ion exchange behaviour of many actinide and rare earth thiocyanate complexes and have shown that europium is eluted much sooner than americium from Dowex-1 with ammonium thiocyanate. The stability constants for the formation of MSCN2+ and M(SCN)2 complexes for Nd3+, Eus+, Pu3+, Am3+, Cm3+, and Cf34 have been measured [353] and are tabulated in Table 25. It is apparent from the table that the formation... 

Reaction of metallic europium with cyclopentadiene in liquid ammonia yields [478] a yellow europous-dicyclopentadienyl (eq. 41) complex. Magnetic measurements on this compound (jn = 7.62 Bohr magneton) and its infrared spectrum definitely confirm the divalent state of europium. 

In the case of the Eu(II) luminescence in methanol, the available data, that refer to the lifetime increase through complexation (see sect. 5) have been tentatively explained within the frame of OH bonds exclusion from the first Eu(II) solvation shell. Interestingly, the values of tobs and rrad for various Eu(II) complexes in methanol were obtained. The discussion above has shown the great interest of such measurements which, in the case of Eu(II), are difficult to perform due to the instability of divalent europium, and which should be more systematically performed in the case of trivalent europium. As compared to the solvated Eu2+ ion... 

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