Dendrimers with branching

Especially, the divergent and convergent strategies have been applied to many dendrimer syntheses. Nowadays, there are dendrimers with branching units which are based on silicon, phosphorus and other heteroatoms,1101 dendrimers which are built up supramolecularly (rather than covalently)1111 and even pure hydrocarbon dendrimers.112,131 A dendrimer of the tenth generation with 3072 gold ions at its periphery, which is 15 nanometers in diameter, has been prepared successfully by Majoral et al.1101... 

From their first synthesis on, dendrimers with branched construction are a focus for a variety of research due to their rational and versatile structure. Since proximity of donor-acceptor exponentially affects the efficiency of energy transfer in Dexter type and sixth power of distance in Forster type, appropriate placement of chromophores in light harvesting systems is essential. Hence, dendritic architecture provides a suitable scaffold for these types of systems by enabling the arrangement of each component in an array of spectral overlap. Further, it was postulated that energy dissipation due to collisions with solvent is diminished in dendritic scaffolds to a great extent. ... 

Dendrimers have distinctive properties, such as the ability to entrap small molecules in their core region and very low intrinsic viscosities in solution. Such properties require molecules to have achieved a particular size, and not all molecules with branches radiating from a core are large enough to develop the characteristic properties of true dendrimers. Branched molecules below this critical size are called dendrons and are the equivalent in dendrimer chemistry of oligomers in polymer chemistry. 

The presence of a large number of chain-ends in the fully synthesized dendrimer molecules makes them highly soluble and also readily miscible, for example with other dendrimer solutions. The solubility is controlled by the nature of the end-groups, so that dendrimers with hydrophilic groups, such as hydroxyl or carboxylic acid, at the ends of the branches are soluble in polar solvents, whereas dendrimers with hydrophobic end-groups are soluble in non-polar solvents. The density of the end-groups at the surface of the dendrimer molecule means that they have proportionately more influence on the solubility than in linear polymers. Hence a dendritic polyester has been shown to be more soluble in tetrahydrofuran than an equivalent linear polyester. 

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... 

The effect of core shielding of a porphyrin moiety by peripheral dendrons has been carefully investigated on two series of Zn-phthalocyanine-cored dendrimers with aryl-ether branches [60]. Generation 0,1, and 2 (dendrimer 27) species, terminated with ester groups, are soluble in organic solvents, while the species terminated with carboxylate units (e.g., 28) are soluble in water. 

Dendrimers with Metal Centers as Linkers and Branching Points... 

Recent reports of dendrimers with a versatile C60 core [212] allowing an unusual core branching multiplicity of 12, and reaching dense-packing limits already in the first generation, continues to prove that there are many more unique dendrimers with varied architectures and associated interesting properties that need to be discovered. 

Following the convergent procedure, dendrimers of type 58,59 and 60 have been prepared from the chiral core triol 54 and achiral Frechet-type [62] ben-zylic branch bromides. In the series of dendrimers with aromatic spacers (60) and without spacers (58), the optical activity [a]D decreased on going from the 1st (not shown in Fig. 21) to the 2nd generation, whereas with aliphatic spacers... 

Comparison of the optical activity showed that the dendrimer 63 with branches of (S)-configuration has a specific rotation and a molecular ellipticity which clearly deviate from the expected values [88,90]. All other 2nd-generation dendrimers (even those with additional spacers between the branches and the core) have specific rotations that are comparable to those expected by simple addition of appropriate values for their building blocks. The deviation may therefore signal the presence of chiral conformational substructures in the 2nd-generation dendrimer 63. 

The first attempt to construct a dendrimer with an electroactive Ru-polypyridi-ne core was based on the reaction of Ru(bpy)2Cl2 with a branched polyether-substituted phenanthroline ligand (11) [27]. In the potential window +2/-2V, this compound shows a one-electron oxidation process and three distinct one-electron reduction processes that, by comparison with the behavior of the... 

Two dendrimers based on Fe-porphyrin core carrying peptide-like branches of different sizes have been synthesized in order to have more open and a more densely packed (23) structures [43]. The electrochemical behavior has been examined in CH2C12 and in aqueous solution. In the less polar solvent, the two dendrimers show similar potentials for the Fem/Fen couple, suggesting that the iron porphyrins in both the more open and the more densely packed dendrimers experience similar microenvironments. On the contrary, in water the behavior of the two dendrimers is very different since the reduction from Fem to Fe11 is much easier for the densely packed dendrimer. This result can be explained considering that in the dendrimer with the relatively open structure the aqueous solvation of the iron porphyrin is still possible, whereas in the densely packed one the contact between the heme and the external solvent is signifi-... 

First generation dendrimers (tetranuclear complexes) of the same family with branches containing different metals have also been synthesized and energy migration patterns leading to one or two peripheral units have been obtained [58]. 

The same research group extended their studies, proposing a systematic investigation of the core activities for different carbohydrate densities during biosensing processes.279 Three new mannosylated dendrimers with a Ru(bipy)3 core unit were synthesized, and the influence of the number and size of dendritic branches on the rate of electron and energy transfer, as well as the lectin-biosensing abilities (Scheme 41),... 

In continuing efforts, the same authors constructed tris(galactoside)-modified DAB dendrimers by the accelerated convergent strategy. In this particular approach, the saccharides were connected first to a small TRIS branching derivative to form a cluster which served as a building block, once suitably functionalized.417 N-Succini-midyl-activated ester 561 was then coupled onto G(l) and G(2)-PPI dendrimers with... 

Figure 11. The implications for the total yield, assuming an idealized dend-rimer functionalization for the example of a dendrimer with AB2-branching and bifunctional core unit (G = generation).

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