Dendrimer functional unit

It has been demonstrated that dendrimers can be used also as fluorescent sensors for metal ions. Poly(propylene amine) dendrimers functionalized with dansyl units at the periphery like 34 can coordinate metal ions by the aliphatic amine units contained in the interior of the dendrimer [80]. The advantage of a dendrimer for this kind of application is related to the fact that a single analyte can interact with a great number of fluorescent units, which results in signal amplification. For example, when a Co ion enters dendrimer 34, the fluorescence of all the 32 dansyl units is quenched with a 32-fold increase in sensitivity with respect to a normal dansyl sensor. This concept is illustrated in Fig. 3. 

Dendrimers containing metal complexes as peripheral units. These dendrimers (Fig. lb) belong to the class of dendrimers functionalized on the surface. Dendrimers coated with up to 48 Ru(Cp)(CO)2R [3], 64 Fe(Cp)2 [4], and 3072 AuCl [5] units have been reported. 

Fig. 16.10 Reaction scheme for immobilization of DNA onto functionalized Si02 substrates. Plasma treated Si02 substrates are denoted as Si OH, APTMS functionalized substrates are denoted as Si NH2, dendrimer functionalized substrates are denoted as Si G(4.5)COOH, and substrates to which DNA capture probes have been immobilized are denoted as Si DNA. Inset Repeat unit of PAMAM dendrimer possessing terminal carboxylic acid functionality...

Baars et al. recently investigated the host-guest properties of polypropylene imine) dendrimers functionalized with tris-3,4,5-tri(tetraethyleneoxy)benzoyl units (Figure 16.15) [191]. These hosts are highly soluble in a broad range of solvents, from apolar solvents such as toluene to polar aqueous media. 

The convergent synthesis of a range of aryl ester dendrimers with peripheral tetrathiafulvalene units was also reported (Devonport et al. 1998). The dendrimers acquire some amount of the cation-radical tetrathiafulvalene tips on reacting with iodine in solutions. Another promising material is polyphenylene dendrimers functionalized with benzophenone units. On being transformed into the potassium ketyl derivatives, the dendrimers bring forth intermolecular biradicals. These technically attractive species contain four-membered cycles that coordinatively combine two potassium cations... 

A special form of homofimctional hnking utihzes so called dendrimers. Dendrimers are nanospherical structures for which the exact size depends on the number of branching points and which carry reactive functional units in their periphery (for example aldehyde-, thiol-, epoxy groups etc). The structure of dendrimers is similar to a tree, and their ramifications consist of repetitive units. It should be noted that their size is limited due to the fact that the packing density of their terminal groups increases. With increasing size, their macroscopic structure approximates the form of a sphere. 

The many possibilities of directing the physical/chemical properties of a den-drimer by introduction of selected functional groups into the molecule lead to an great variety of potential applications of dendrimers. Depending upon the position and the nature of the functional units within the dendrimer structure, it is possible to distinguish between different types of functional dendrimers ... 

The properties of a dendrimer are determined not only by the specific properties of its functional units, but also by their number and structural variety, and by cooperative effects between different functional units. A number of special applications and the development of substances suitable for mimicking biological systems [53] require such multifunctional dendritic systems with more than one kind of functionality. 

The following sections are devoted primarily to the introduction of two different types of functional units. A distinction is made between dendrimers with bi-functionalised molecular periphery (Fig. 3.7 Types A, B, C, D) and those in which one function is located in the core and the other in the branching units or in the periphery (Types E, F). Multifunctional dendrimers of type G with different functional units in the core, scaffold, and periphery have so far played only a minor role and will therefore only be treated briefly here, particularly since compounds of this type will be considered in greater depth in Chapter 6. 

Fig. 3.11 Amphiphilic switchable POPAM dendrimer (according to Majoral et al.), whose periphery contains the two functional units in equal proportions but randomly distributed...

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). 

The above sections demonstrate that the dendrimer chemist meanwhile has recourse to an extensive range of possibilities for introducing specific functional units in one or more selected regions of the molecule (centre, periphery, interior branching scaffold) in order to customise dendrimers or dendrons for projected applications. It should, however, be borne in mind that the synthetic strategies for introduction of two or more different functional units (bifunctionalisation, multifunctionalisation) demand considerable synthetic effort as well as time and expense and their use is limited to certain dendrimers and dendrons, because special conditions have to be fulfilled (e.g. special end groups such as primary amino functions). 

Introduction of the desired functional unit by subsequent functionalisation of the internal functionalities of the dendrimer precursor. 

Dendrimers with a single, gen-eration-specifically incorporated functional unit. 

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