Dendritic shells

The ability of a dendritic shell to encapsulate a functional core moiety and to create a specific site-isolated microenvironment capable of affecting the molecular properties has been intensively explored in recent years [19]. A variety of experimental techniques have been employed to evidence the shielding of the core moiety and to ascertain the effect of the dendritic shell [19, 20]. Dendrimers with a fullerene core appear to be appealing candidates to evidence such effects resulting from the presence of the surrounding dendritic branches. Effectively, the lifetime of the first triplet excited state of fullerene derivatives... 

Size, shape, and density The shielding effects of dendritic shells can likewise be caused by steric factors. Thus, the access of foreign molecules to the central functional unit can be hindered or prevented according to size and density of the dendritic shell. Sometimes, even a certain size selectivity is observed. These effects are especially interesting for electrochemically, catalytically active, redox-and photo-active functional units, since interactions with foreign molecules, such as oxygen quenching of the luminescence (photo-active units) or the access of substrates (catalytically active units) can be influenced.14 11 17,221... 

Controlling the size, shape and ordering of synthetic organic materials at the macromolecular and supramolecular levels is an important objective in chemistry. Such control may be used to improve specific advanced material properties. Initial efforts to control dendrimer shapes involved the use of appropriately shaped core templates upon which to amplify dendritic shells to produce either dendrimer spheroids or cylinders (rods). The first examples of covalent dendrimer rods were reported by Tomalia et al. [43] and Schluter et al. [44], These examples involved the reiterative growth of dendritic shells around a preformed linear polymeric backbone or the polymerization of a dendronized monomer to produce cylinders possessing substantial aspect ratios (i.e. 15-100) as observed by TEM and AFM. These architectural copolymers consisting of linear random... 

Frechet and co-workers [32] studied the ability of the dendrimer shell to provide site isolation of the core porphyrin moiety, using benzene-terminated dendrimers Zn[G-n]4P (i.e. 6). From the cyclic voltammograms in CH2C12, the interfacial electron transfer rate between the porphyrin core and the electrode surface decreased with increasing dendrimer generation. However, small molecules like benzyl viologen could still penetrate the shell of 6 to access the porphyrin core as observed from the quenching of porphyrin fluorescence. Their results also revealed that the dendritic shell did not interfere electrochemically or photochemically with the porphyrin core moiety. 

The fact that the dendritic shell can produce localized microenvironments has been used by Diederich et al. who developed water-soluble dendritic cyclophanes (dendrophanes) as models for globular proteins (Figure 16.13) [5, 170, 171], These dendrimers contain well-defined cyclophane recognition sites as initiator cores for the complexation of small aromatic guests [172-174] and steroids [174-176], Enlargement of the cyclophane core could be used as a tool to complex larger steroid molecules. 

All dendrimers consist of inner tertiary amines, located at the branching points of the various dendritic shells (layers). The amine-terminated dendrimers, furthermore, have basic primary amine end-groups. Basicity is therefore one of the most dramatic properties of the polypropylene imine) dendrimers, and has been studied via titration experiments and calculations. Titration experiments of the dendrimers have been performed in water using 1 M hydrochloric acid. Only two equivalence points are observed for DAB-J nJr-(NH2)4 in a ratio of 2 1. From these titrations, pKa values of 10.0 (primary amine groups) and 6.7 (tertiary... 

Figure 5.11 Energy migration pattern across dendritic shells in OsRu3, OsRu6, OsRu21, and C12RuRu6, see text.

Covalent coupling A pre-existing metal complex is coupled covalently with den-drons and is thus surrounded by a dendritic shell. Appropriate examples are the redox-active metallodendrimers of Kaifer et al. [117] or the dendritic porphyrin/ metal complexes prepared and characterised by Diederich et al. [118] and Aida et al. [119]. 

Fig. 5.25 E nergy transfer through the dendritic shell to the azobenzene core unit (according to Aida et al.)...

However, the divergent synthetic strategy also permits covalent construction of a dendritic shell around a function-bearing core unit (Fig. 6.8 further examples are given in Section 4.1). 

Diederich et al. produced a mimic of the electron-transfer protein cytochrome C. Divergent synthesis afforded a water-soluble iron-porphyrin which possesses a covalently attached dendritic shell surrounding the electrically active heme core which is reminiscent of a protein shell (Fig. 8.7). In contrast to the dendri-... 

In addition, luminescent linear backbones have been encapsulated in dendritic shells through this technique,152,328 while the increased solubility im-... 

An efficient protective dendritic shell to enhance photophysical properties... 

An Efficient Protective Dendritic Shell to Enhance Photophysical Properties... 

The modification of the dendritic shell with perfluorinated aromatic end groups along with an aliphatic bis-MPA scaffold was then described to make this phenomenon possible in water for Eu(III) and Tb(III) [35]. In addition, aForstermechanism was shown to occur between the perfluorinated donor groups and lanthanide cations as acceptors. A study at the air-water surface interface was also reported with these dendrimers [36],... 

FIGURE 5.5 Enhanced electron transfer between porphyrins with protective dendritic shells. 

Over the past 5 years, the combination of metallopor-phyrin with dendrimer chemistry led to a faseinating new class of Hb and Mb models," . with the dendritie superstrueture mimicking the encapsulation of the heme in the natural protein environment. Modification of the dendritic shell around the ironfll) porphyrin core modulates the shape, density, and polarity of these model systems, thereby profoundly affecting the thermodynamics and kinetics of the complexes formed with O2 and CO. 

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