Cryptates lanthanide

Even couples of lanthanide ions show this quenching process. The Ce(III) and Eu(III) ions, for example, quench each other s luminescence [127]. Here a MMCT state with Ce(IV)-Eu(II) character is responsible. In solid [Ce <= 2.2.1] cryptate there occurs energy migration over the cryptate species. Also here [Eu c 2.2.l] acts as a quencher [128]. The quenching action is restricted to short distances (about 12 A [129]). 

Homogeneous Time Resolved Fluorescence (HTRF) (Cisbio International) is an assay based on the proximity of a lanthanide cryptate donor and a fluorescent acceptor molecule whose excitation wavelength overlaps that of the cryptate s emission. The utility of this technique is based on the time resolved fluorescence properties of lanthanides. Lanthanides are unique in the increased lifetime of their fluorescence decay relative to other atoms, so a delay in collection of the emission intensity removes the background from other fluorescent molecules. An example of the HTRF assay is a generic protein-protein interaction assay shown in Fig. 2. 

HTRF (homogeneous time-resolved fluorescence) LANCE (lanthanide chelate excitation) Cisbio International PerkinElmer Life Sciences Lanthanide TR-FRET using Eu3+ cryptate/chelate-donor fluorophore and cross-linked aUophycocyanin-acceptor fluorophore... 

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Alpha, B. Ballardini, R. Balzani, V. Lehn, J.-M. Perathoner, S. Sabbatini, N. Antenna effect in luminescent lanthanide cryptates a photophysical study. Photochem. Photobiol. 1990, 52(2), 299—306. 

Platas, C. Avecilla, F. de Bias, A. Rodriguez-Blas, T. Geraldes, C. F. G. C. Toth, E. Merbach, A. E. Biinzli, J.-C. G. Mono and bimetallic lanthanide(III) phenolic cryptates obtained by template reaction solid state structure, photophysical properties and relaxivity. J. Chem. Soc., Dalton Trans. 2000, 611-618. 

Avecilla, F. de Bias, A. Bastida, R. Fenton, D. E. Mahia, J. Macias, A. Platas, C. Rodriguez, A. Rodriguez-Blas, T. The template synthesis and X-ray structure of the first dinuclear lanthanide(III) iminophenolate cryptate. Chem. Commun. 1999, 125-126. 

These complexes, unlike the crown ether complexes but similar to the aza-crown and phthalocyanine complexes, are fairly stable in water. Their dissociation kinetics have been studied and not surprisingly they showed marked acid catalysis.504 Association constant values for lanthanide cryptates have been determined.505,506 A study in dimethyl sulfoxide solution by visible spectroscopy using murexide as a lanthanide indicator showed that there was little lanthanide specificity (but surprisingly the K values for Yb are higher than those of the other lanthanides). The values are set out in Table 9.507... 

Table 9 Log Association Constant Values for Lanthanide Cryptates in DMSO507...

A number of X-ray crystal determinations have made the principles of lanthanide cryptate structural chemistry fairly clear. In [La(N03)2(2,2,2-cryptate)][La(N03)6] (Figure 8), the La3+ ion is 12-coordinated with two bidentate nitrate ions coordinating in two of the three spaces between the cryptate chains the third space is thus too compressed to be occupied also.508 [Sm(N03)(2,2,2-cryptate)][Sm(N03)5(H20)] shows only one such space occupied511 and the structure of [Eu(C104)2,2,2-cryptate](C104)2MeCN is similar to the samarium cryptate.512,513 Intemuclear distances in these complexes are shown in Table 10. 

Table 10 Lanthanide-Ligand Distances in Cryptate Complexes...

Lanthanide cryptates presenting interesting photophysical properties have been obtained [2.32, 2.33] (see also Sect. 8.2). 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

The formation of luminescent lanthanide complexes relies on a number of factors. The choice of coordinating ligand and the method by which the antenna chromophore is attached to it, as well as the physical properties of the antenna, are important. In order to fully coordinate a lanthanide ion, either a high-level polydentate ligand such as a cryptate 1 or a number of smaller ligands (such as 1,3-diketones, 2) working in cooperation are required. Both 1 and 2 are two of the simplest coordination complexes possible for lanthanide ions. In both cases there are no antennae present. However, the number of bound solvent molecules is decreased considerably from nine (for lanthanide ions in solution) to one to two for the cryptate and three for the 1,3-diketone complexes. 

By redesigning the above acyclic podand-type ligand 3 into a cyclic cryptate, the issue of stability can be resolved resulting in kinetically stable complexes (Scheme 4) [102]. The Tb(III) and Eu(III) complexes of cryptate 5 show an increase in lanthanide emission lifetimes of 0.72 ms and 0.41 ms, respectively, upon excitation at 310 nm. Similar results are found with the phenanthroline analogue 6 with Eu(III). A large number of modifications of these cryptates have been reported, all showing enhancements in the lanthanide ion emission [103-106]. 

Mathis, G., 1998. Biological applications of rare earth cryptates. In Saez Puche, R., Caro, P. (Eds.), Rare Earths. Editorial Complutense, Madrid, pp. 285-297. Matsumoto, K., Yuan, J.G., 2003. Lanthanide chelates as fluorescent labels for diagnostics and biotechnology. In Sigel, A., Sigel, H. (Eds.), Metal Ions in Biological Systems, vol. 40. Marcel Dekker, New York, pp. 191-232 (chapter 6). 

Fig. 4.20. Stability constants of lanthanide cryptates in propylene carbonate at 298 K and p. = 0.1 M (NettCKL). From data reported by J.-C.G. Biinzli, in Handbook on the Physics and Chemistry of Rare Earths, eds K.A. Gschneidner, Jr., L. Eyring, Vol. 9, Ch. 60, North Holland, Amsterdam, 1987.

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