Luminescent cryptates

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]). 

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. 

Prodi, L. Maestri, M. Balzani, V. Lehn, J.-M. Roth, C. Luminescence properties of cryptate europium(III) complexes incorporating heterocyclic N-oxide groups. Chem. Phys. Lett. 1991,180, 45-50. 

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. 

Following this study, Korovin and coworkers tested cryptands 23b-f which are used in time-resolved luminescent immunoassays (Mathis, 1993) for the sensitization of Ybm luminescence (Korovin et al., 2002b). From the lifetimes determined in both water and deuterated water, one calculates that the hydration number varies from 2 (23b), to 1.5 (23a, 23e), and finally to 1 (23c, 23d, 23f). Quantum yields were not determined, but luminescence intensities relative to the cryptate with 23a (in water, at room temperature) point to cryptands 23c and 23d being the best sensitizers of the Yb111 luminescence with a seven-fold enhancement, while cryptates with 23e and 23b are only 1.5- to 1.8-times more luminescent. 

Ligand 131 contains both cryptate and crown ether binding units. It reacts with neodymium triflate to give a 1 1 cryptate [Nd c 131]3+ exhibiting NIR luminescence upon excitation at 355 nm. The lifetime of the Nd(4F3/2) level is 347 and 711 ns in methanol and methanol-, respectively, which translates to MeOH 0 using eq. (10b). When barium ions are added to a solution of this complex in acetonitrile, the intensity of the 1.06 pm emission band is reduced substantially, while the lifetime of the Nd(4F3/2) level remains unchanged at about 470 ns. The Ndm ion is therefore not displaced from the cryptand cavity, while its luminescence is modulated by the presence of the Ba2+ ions bound by the crown ether (Coldwell et al., 2006). 

A derivative of the (bpy.bpy.bpy) cryptand, obtained by modifying one of the chains, Lbpy, forms a di-protonated cryptate with EuCb in water at acidic pH, [EuCl3(H2Lbpy)]2+ in which the metal ion is coordinated to the four bipyridyl and two bridgehead nitrogen atoms, and to the three chlorine ions (Fig. 4.25). The polyamine chain is not involved in the metal ion coordination, due to the binding of the two acidic protons within this triamine subunit. In solution, when chlorides are replaced by perchlorate ions, two water molecules coordinate onto the Eu(III) ion at low pH and one at neutral pH, a pH at which de-protonation of the amine chain occurs, allowing it to coordinate to the metal ion. As a result, the intensity of the luminescence emitted by Eu(III) is pH dependent since water molecules deactivate the metal ion in a non-radiative way. Henceforth, this system can be used as a pH sensor. Several other europium cryptates have been developed as luminescent labels for microscopy. 

As for the luminescent pH sensor described above, sensors relying on a change in relaxivity may also be designed that are pH sensitive. An example is the iminocryptand LimBT which encapsulates the Gd(III) ion but has sufficient space between its imine chains to let two water molecules interact with the metal ion (Fig. 4.27). Above pH = 8, one of the water molecules is de-protonated and an hydroxo form of the cryptate is present. Therefore, the relaxivity (which is a measure of the efficiency of a contrast agent) is pH dependent and this cryptate works as a pH sensitive stain. 

This model, in which the nonradiative transitions can be suppressed by a stiff surroundings, can be most elegantly tested by studying the luminescence of rare-earth cryptates (103-105). 

A recent example of this consideration is the success of [Eu C bpy.bpy.bpy] cryptate in luminescence immunoassay (104). This cryptate is used as a label to an antibody that is coupled in a specific way to a biomolecule, the presence of which has to be proved. TTie structure of the molecule is shown in Fig. 47. Excitation is into the bpy molecule, which shows a very high absorption strength in the ultraviolet part of the spectrum (cmax 10 M cm ). From the bpy triplet state the energy is transferred to the Eu ion, which finally emits (104,107). Although the quantum efficiency is only 1% (104), the high bpy absorption strength makes application feasible. 

Following the initial reports of the anteima effect in lanthanide cryptates, many other lanthanide complexes containing additional chromophoric ligands have been investigated and found to be useful as luminescent labels for biological substrates, and in general as sensors based on luminescent properties [21]. 

C. O. Roth, Ph.D. Thesis, Cryptates luminescents de ligands N-oxides University of Strasbourg, Strasbourg, France 1992. 

One of the molecules which plays a role in this field is depicted in Fig. 1.9. The luminescent species is the F.u ion which we met already above. It is surrounded by a cage containing molecules of bipyridine. The whole complex is called a cryptate and its formula is written as [Eu C bpy.bpy.bpy) ". The cage protects the Eu " ion against the (aqueous) surroundings which tries to quench the luminescence. If this cryptate is excited with ultraviolet radiation, the bipyridine molecules absorb the exciting radiation and transfer their excitation energy subsequently to the Eu ion which then shows its red luminescence. 

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