Dendrimer scaffold

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. 

RG. 66. Commercially available dendrimer scaffolds commonly used for glycodendrimer syntheses, and general synthetic strategies for synthesis of glycodendrimers. 

The use of soluble dendrimers with phospine ligands experienced a rapid growth and was also transferred to several other dendrimer scaffolds [64—66]. Many reactions, such as hydrogenation, hydrovinylation, Stille coupling, Knoeve-nagel condensation and Michael addition were reported (Tab. 7.2) [73, 74]. 

Fig. 1.14 Dendrimer scaffold - with three dendrons (schematic, idealised)...

In the synthesis of functional dendrimers, interest has hitherto been focussed on variation of the functional core unit or peripheral groups and the resulting effects on the properties of the dendrimer. For a long time, the only function ascribed to the dendritic branches and their repeating units was that of a scaffold linking periphery and core. It was overlooked that, in the interior of the dendrimer scaffold, an individual characteristic (nano)environment can arise which is largely dependent upon the chemical characteristics and the polarity of the repeating units used to construct the dendrimer. Moreover, they can facilitate cascade processes and serve as a platform for cooperative effects between dendrimer branches [37]. 

Fig. 3.9 On b ifunctionalisation according to Majoral et ai. daughter dendrons are produced divergently starting from specific functions in the interior of the dendrimer scaffold...

However, these bifunctionalisation methods are comparatively laborious and applicable only in special cases, since the monofunctionalisation step is limited to substrates possessing an additional coupling site in protected form for the second functional unit. A more versatile method of local bifunctionalisation, which has no need of a deprotection step and also utilises commercially available dendrimer scaffolds, consists in the functionalisation of POPAM dendrimers bearing amine terminal groups with sulphonyl chlorides and subsequent substitution of the sulphonamide proton with other sulphonyl chlorides [63] or with alkyl- or (dendritic) benzyl bromides [64] (see Fig. 3.13). 

Whereas the bifunctionalisation strategies presented so far are relatively straightforward, the synthesis of multifunctional dendrimers with more than two kinds of functional units requires considerable synthetic effort. Preparation of dendrimers with a functional core and additional functional units in the dendrimer scaffold and in the periphery requires de novo synthesis of the entire dendrimer scaffold, with the synthesis conditions having to be tolerable for all groups (Fig. 3.15). 

Mullen et al. presented a multi-chromophoric dendrimer which absorbs over the entire visible spectrum. Three different dyes are positioned in a rigid polyphenylene dendrimer scaffold in such a manner that an energy gradient is generated between the periphery and the centre of the core and efficient energy transfer to the central chromophore (A) takes place on excitation of the peripheral chromophore (Cl, C2) (Fig. 3.16) [77]. 

Reaction of the end groups of an existing dendrimer scaffold with a mixture of two differently functionalised substrates. 

De novo synthesis of dendrimer scaffold necessary a) Convergent synthesis Dendron assembly using appropriately prefunctionalised monomeric building blocks (e.g. AB2FG) starting from the peripheral functional units of the dendrimer to be... 

Dendrimers with metal complex moieties in their branches require the prior incorporation of specific coordination sites into the dendrimer scaffold. Newkome et al. used such a dendrimer with twelve alkyne units for spot-on introduction of l,2-dicarba-c oso-dodecaborane groups (Fig. 4.59, above right) [127]. Moreover, on-target coordination with dicobalt-octacarbonyl to form a metallodendrimer with twelve dicobalt-hexacarbonyl units was also accomplished. These units can serve as protective groups on the one hand [128], and as catalysts on the other [129]. 

Chiral dendrimer scaffold, made up of chiral branching building blocks or chiral spacers... 

Overall, studies on poly(benzyl ether) dendrimers with various chiral core units show that the influence of the achiral dendrimer scaffold on the chiropti-cal properties of the core unit depend primarily upon the origin of their chirality. 

Studies performed on chiral core dendrimers have provided valuable information about the influence of the achiral dendrimer scaffold on the chiroptical properties of the core unit. Yet they also show that prediction of the chiroptical properties of the dendrimer is difficult, since the chiral relationship between the local chirality of the core unit and the nanoscopic conformation of the overall dendrimer structure is influenced by numerous structural factors. Further studies will be required to attain a fuller understanding of how an individual chiral building block can induce chirality in the entire dendrimer architecture (see Section 4.2.7). 

Fig. 4.70 Dendrocleft (according to Diederich et al.) for stereoselective host/guest interaction depending upon the size of the dendrimer scaffold surrounding the chiral core (marked with a red asterisk)...

In contrast, the diastereoselectivity of the dendritic host increases. This indicates that on shielding of the chiral core unit with sterically more demanding dendrons in higher-generation dendroclefts the monosaccharide guests are no longer bound in the immediate vicinity of the chiral core unit instead, a less specific host/guest interaction takes place with the dendrimer scaffold. 

Chiroptical studies on dendrimers with chiral dendrimer scaffold... 

Fig. 4.72 Dendrimer with completely chiral dendrimer scaffold based on chiral branching units (according to Sharpless et al.)...

Chow and Mak came to a similar conclusion on investigating the chiroptical properties of dendrimers containing enantiomerically pure threitol building blocks obtained from tartaric acid as spacers between the achiral phloroglucin branching units (see Fig. 4.73) [27]. They found that the chiral spacers in the dendrimer scaffold do not influence one another and contribute additively to the overall rotation. Moreover, they also observed that on introduction of both enantiomers one (R,R)-threitol unit precisely compensated the rotational contribution of one (S,S)-threitol unit, provided that the enantiomeric building blocks were located at equivalent positions within the dendrimer scaffold. However, CD-spectroscopic data revealed that the contribution of the exterior threitol units to the total rotation must be slightly different from that of the interior units. 

If the periphery of a POPAM dendrimer bears azobenzene units then dye molecules can be included as guests in the dendrimer scaffold (see also host/ guest chemistry in Section 6.2.3) [23]. Here the E- and Z-isomers (or their enriched versions) differ in their capacity for accommodating guest molecules. In principle, guest molecule inclusion can be controlled (switched) in this way (Fig. 5.21). 

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