Self-assembly lanthanides formation

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

This last section deals with a few examples of the formation of self-assembly structures from lanthanide complexes, which are formed by using transition metal ions. This area of research is very novel and relatively few examples have been developed to date. While the lanthanide ions have been used to mediate the formation of supramolecular structures, such as helicates, many of which can have both/-/ mdf-d metal ions, etc. the focus here will be on the use of lanthanide complexes and ligand structures similar to those described above [170-173]. 

Compound 61 can be depicted in a cartoon manner as A in Scheme 6. Our idea was to form the linear f-d-f assembly (B) by simply using a Cu(II), Fe(II) or Zn(II) ion as the bridging unit where these would coordinate to the phen ligand and as such bring two of the lanthanide complexes together. While the formation of a self-assembly was successful, the desired structure (B in Scheme 6) was not exclusively formed. Using Cu(II) we showed that upon addition of 61, at pH 7.4, the Eu(III) emission was switched off. Moreover, the absorption spectra were shifted to the red and the singlet... 

In the first part of this chapter, we have tried to demonstrate that the lanthanide ions can give rise to the formation of a variety of mononuclear supramolecular systems that can be employed for sensing and imaging, and in the formation of novel self-assembly structures that can be further exploited for use in materials-based systems. The next part of this chapter demonstrates that such supramolecular structures can be expanded to polynuclear lanthanide complexes and self-assemblies. 

Several other polymetallic structures formed by self-assembly have been reported in the literature. For instance the class of metallacrowns [48] has received increasing attention over the last few years. These inorganic analogues of crown ethers arc self-assembled from simple building blocks and transition metal ions, mainly Cu " and Fe " (Fig. 9.8). The lanthanide ion, situated in the central cavity, acts as a template in the formation of the heterometallic structure. The first lanthanide-containing stmctures reported were made of a hexanuclear cluster in which the lanthanide ion is surrounded by five transition metals... 

Figure 9.8 Selective formation of 15-MC-5 by self-assembly of lanthanide and transition metal ions with aminohydroxamate ligands...

Tilney J. A., T. J. Sorensen, B. P. Burton-Pye, S. Faulkner, Self-assembly between dicarboxylate ions and a binuclear europium complex formation of stable adducts and heterometaUic lanthanide complexes. Dalton Trans. 40, 12063—12066 (2011). 

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