Lanthanide cores

Ionic interactions have been used to prepare lanthanide-core dendrimers. This has been achieved using a convergent synthesis, in which polyether den-drons with a carboxylic acid group at the focal point were assembled around a lanathanide cation. This involved a metathetical reaction with compounds such as Er(OAc)3, Tb(OAc)3 or Eu(OAc)3 to introduce the appropriate lanthanide ion. 

Two practical advantages of luminescence species engulfed in antenna dendrimer scaffolds are apparent, namely their miscibility with organic media (solvents or/and resins) and their ability to form thin films. For example the lanthanide-cored dendrimer complexes described in this chapter can be regarded as organic-soluble inorganic luminescers. 

The PBE dendron has a glass transition at about 40 °C and is soluble in various organic solvents (e.g., THF, acetone, toluene). It is therefore a moldable, thermoplastic, film-forming material. This practical feature is maintained for the lanthanide-cored dendrimer complexes. The complexes are partially miscible with poly(methyl methacrylate), affording transparent luminescence compositions by mixing in solvent. 

Vinylphenyl-terminated PBE dendrons were prepared as polymerizable den-drons from 4-vinylbenzyl chloride [37]. The vinylphenyl-terminated PBE dendrons are useful to make the lanthanide-cored dendrimer complexes polymerizable. The Ist-generation Tb +-cored dendrimer complex bearing the vinyl-phenyl terminal on the dendron subunits (Fig. 5) was copolymerized with N-iso-propylacrylamide in the presence of methylene bis-acrylamide (as crosslinker) in DMSO to give a green-luminescence transparent gel. The DMSO gel was con-... 

Kawa, M., and Frechet, J.M.J. (1998) Self-assembled lanthanide-cored dendrimer complexes enhancement of the luminescence properties of lanthanide ions through site-isolation and antenna effects. Chem. Mater. 10, 286-296. 

Pitois, C., Hult, A., and Lindgren, M. (2005) Lanthanide-cored fluorinated dendrimer complexes synthesis and luminescence characterization. Journal of Luminescence, 111, 265. 

Carbon-arc lighting The US Army, Navy, and Coast Guard searchlights all use lanthanide-cored carbons. 

Schemes Structure of lanthanide-cored dendrimers of Kawa and Fr chet [17,18]...

For phosphorescence, lanthanide metal ions can be used in a similar manner. Lanthanide ions have very interesting photophysical properties, but often exhibit weak absorption bands, and aggregate to form clusters, which limit their applications. Thus, a dendrimer that can provide a protective shell to isolate a cation and at the same time enhance the emission by transfer from the periphery to the lanthanide ion at the core could be of great interest. Self-assembled lanthanide-cored dendrimers have been prepared to prove such an assumption synthesis was carried out by mixing three equivalents of polyaryl ether dendrons bearing carboxylic acid entity at the focal point with Ln(III) cations [Er(III), Tb(III), and Eu(III)] (Fig. 5.4) [34]. The authors demonstrated that the enhancement of the lanthanide cation emission associated with the dendritic core shell was observed, and an antenna effect from the periphery to the core was shown to promote this process. 

From the standpoint of chemical reactivity, the 5s and 5p shells of lanthanides can be considered to be core electrons. Indeed arguments to this effect could be made for Ln 4f orbitals, given their extremely contracted nature. Shown in Figure 3 is a plot from a DHF calculation of a 4f spinor for Gd(III). Note the maximum in the wavefunction at 0.57 A that is, a value comparable to a hydrogen Is orbital Dolg et al. - examined various lanthanide core sizes and found essentially no difference in state splittings of the Ce atom between all-electron calculations and those in which a 28-electron core ([Ar]3d °) is used, Satisfactory results are also obtained for a 46-electron core ([Kr]4di°). Inclusion of 5s and 5p into the core (i.e, a 54-electron [Xe] core)... 

Because of the greater extension of the 5f radial wavefunctions with respect to those of the shielding 7s and 7p shells, they are more sensitive to changes in the valence-electron situation than for the corresponding lanthanide cores. Nevertheless, their rigidity is remarkable as compared to that for the valence electrons themselves. This can be seen quantitatively in the plots for the Slater electrostatic interaction integrals (Sf, Sf) and spin-orbit radial integral C, which dominate the atomic Hamiltonian for all cases of interest to us here. 

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