Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Transition dendrimers

It is possible to take advantage of the differing characteristics of the periphery and the interior to promote chemical reactions. For example, a dendrimer having a non-polar aliphatic periphery with highly polar inner branches can be used to catalyse unimolecular elimination reactions in tertiary alkyl halides in a non-polar aliphatic solvent. This works because the alkyl halide has some polarity, so become relatively concentrated within the polar branches of the dendrimer. This polar medium favours the formation of polar transition states and intermediates, and allows some free alkene to be formed. This, being nonpolar, is expelled from the polar region, and moves out of the dendrimer and into the non-polar solvent. This is a highly efficient process, and the elimination reaction can be driven to completion with only 0.01 % by mass of a dendrimer in the reaction mixture in the presence of an auxiliary base such as potassium carbonate. [Pg.144]

Methanofullerene 20 with phenylacetylene dendrimer addends has also been reported [45] (Fig. 10). The UV absorption of fullerodendrimer 20 is particularly strong and is mainly attributed to transitions located on the two dendritic branches of the molecule. The photophysical investigations revealed that the large poly(aryl)acetylene branches act as photon antennae [46]. [Pg.96]

In these dye-functionalized dendrimers, light absorbed by the numerous peripheral coumarin-2 units is funneled to the coumarin-343 core with remarkably high efficiency (toluene solution 98% for the first three generations 93% for compound 8). Given the large transition moments and the good overlap between donor emission and acceptor absorption, energy transfer takes place by Forster mechanism [34]. [Pg.168]

PBE dendrimers with a cyclic polyamine core at the focal point have been synthesized to form transition-metal complexes [27]. Tb + complexes exhibited luminescence by the excitation of the dendrons. [Pg.200]

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. [Pg.201]

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. [Pg.218]

The use of dendrimers as supports to anchor transition metal catalysts has attracted considerable attention over the past decades [48] (see also Chapter 4 of this book). Several groups studied the use of dendrimers immobilised on insoluble supports [49], and this type of material meet the requirements for catalysis in interphases. Alper reported the use of diphosphine functionalised polyamidoamine (PAMAM) dendrimers... [Pg.56]

V. Balzani, S. Campagna, G. Denti, A Juris, S. Serroni, M. Venturi, Designing Dendrimers Based on Transition-Metal Complexes. Light-harvesting Properties and Predetermined Redox Patterns , Acc Chem Res. 1998, 31, 26-34. [Pg.265]

Important physical property subtleties were noted within the dendrimer subset. For example, dendrimers possessing asymmetrical branch cells (i.e. Den-kewalter type) exhibit a constant density versus generation relationship (Figure 1.20). This is in sharp contrast to symmetrical branch cell dendrimers (Tomalia-type PAMAM) that exhibit a minimum in density between G = 4 and G = 7 (NH3 core) [48, 96]. This is a transition pattern that is consistent with the observed development of container properties described in Figure 1.21. [Pg.34]

Therefore, within the PAMAM family, the shapes of dendrimers span the range from stars to spheres. Small G3 dendrimers have a diffuse, open structure while large G10 dendrimers are spheres with a uniform interior, sharp outside transition, and low polydispersity. [Pg.266]

Most, but not all of the terminal units are near the outside of the dendrimer at any given time. The SAXS studies [17] of the segment density distribution have shown that there is an abrupt transition region at the outside of large PAM AM dendrimers. The combination of these two factors suggests that the terminal functionalities of dendrimers are accessible from the outside and available for chemical reactions such as attachment to surfaces, mounting of a catalyst, or for use as a crosslink junction. [Pg.274]

As was described earlier, the internal SDD of dendrimers is remarkably high compared to traditional polymers and is well defined by a narrow transition zone at the outside. When linear polymers are forced together by increasing their concentration in a solvent, they freely interpenetrate each other due to their open structure. Dendrimers, on the other hand, appear to represent a different case, due to their compact size and architectural features. [Pg.274]

Figure 16.17 Amine-terminated polypropylene imine) dendrimers act as tridentate ligands for the complexation of transition metals [217] (a), and can function as templates for the assembly of Troger s base dizinc(ll) bis-porphyrin molecules, (b) [218]... Figure 16.17 Amine-terminated polypropylene imine) dendrimers act as tridentate ligands for the complexation of transition metals [217] (a), and can function as templates for the assembly of Troger s base dizinc(ll) bis-porphyrin molecules, (b) [218]...
Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... [Pg.501]


See other pages where Transition dendrimers is mentioned: [Pg.15]    [Pg.66]    [Pg.194]    [Pg.213]    [Pg.54]    [Pg.61]    [Pg.146]    [Pg.153]    [Pg.198]    [Pg.216]    [Pg.223]    [Pg.452]    [Pg.654]    [Pg.244]    [Pg.69]    [Pg.71]    [Pg.163]    [Pg.277]    [Pg.376]    [Pg.810]    [Pg.257]    [Pg.132]    [Pg.27]    [Pg.29]    [Pg.134]    [Pg.177]    [Pg.264]    [Pg.271]    [Pg.312]    [Pg.324]    [Pg.333]    [Pg.335]    [Pg.346]    [Pg.350]    [Pg.415]    [Pg.485]   
See also in sourсe #XX -- [ Pg.267 , Pg.272 , Pg.283 ]




SEARCH



© 2024 chempedia.info