Coordination sites luminescence, lanthanide ions

The chromophore is directly coordinated to the lanthanide ion (see Figure 10a). Chromophores are thus designed to ensure an efficient positioning of the triplet energy, which would allow the photosensitization of the lanthanide ion, and enclose adequate coordination sites to guarantee a stable complexation of the cation. For further details, Lanthanides Luminescence Applications and Luminescent Bioprobes. 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

As discussed early in this chapter, quantum confinement has little effect on the localized electronic levels of lanthanide ions doped in insulating nanocrystals. But when the particle size becomes very small and approaches to a few nanometers, some exceptions may be observed. The change of lanthanide energy level structure in very small nanocrystals (1-10 nm) is due to a different local environment around the lanthanide ion that leads to a drastic change in bond length and coordination number. Lanthanide luminescence from the new sites generated in nanoparticles can be found experimentally. The most typical case is that observed in nanofilms ofEu Y203 with a thickness of 1 nm, which exhibits a completely different emission behavior from that of thick films (100-500 nm) (Bar et al., 2003). 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

Binding sites for Ca " and ATP have been explored by the use of metal probes and nucleotide analogues. The Mn ion substitutes for Mg but also binds at the Ca " sites. Such complications have led to the use of lanthanides as probes for the Ca sites. Thus Gd " and Tb " compete with Ca for the high affinity site. Luminescence studies with laser-excited Tb " at the Ca sites show that two water molecules are present in the first coordination shell. Earlier work with Gd shows that the Ca sites are a maximum of 16.1 A apart, and that both sites involve a low level of hydration, consistent with a hydrophobic site. Gd has also been used as an ESR probe, and, under certain conditions, evidence has been produced for two forms of an E-Gd complex, in accord with current mechanistic views. 

Tetraazamacrocyclic ligands incorporating an intracyclic terpyridine chromophore 39 was described [106, 107], as expected the absorption maxima (335 nm) was more advantageous compared to 38. The physical characteristics were described X (H2O) = 1.06 ms, luminescence decays indicated that no water molecule is coordinated to the metal ion suggesting that the nine binding sites provided by the ligand are coordinating the lanthanide. No decomposition of these complexes was observed after 1 day in... 

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