POPAM generation

Within their scaffolds, POPAM dendrimers also possess amine groups which can undergo protonation or coordination with metal ions. The spectroscopic and photochemical properties of the first to the fourth POPAM generation with peripheral fluorescing naphthylsulfonamide groups were compared with those of reference compounds A (N-methylnaphthalenesulfonamide) and B (N-(3-di-methylaminopropyl)-2-naphthalene-l-sulfonamide) [22] (Fig. 5.19). 

Fig. 4.1 Third-generation (G3) POPAM dendrimer — with tetrafunctional core unit and AB2 branching (the generations are shown in shades of grey)...

Table 4.1 I ncrease of POPAM parameters with the number of generations [3]...

Divergent synthesis of POPAM dendrimers is based on the concept developed by Vogtle et al. in 1978 (see Section 1.1) [4]. Acrylonitrile is added to a primary mono- or oligo-diamine in a Michael reaction (Fig. 4.2). Suitable reducing agents are Co(II)-borohydride complexes or diisobutylaluminium hydride [5]. Subsequent iterative reaction cycles permit repeated addition of acrylonitrile followed by reduction until the limiting generation is reached. 

In 1993 Meijer et al. and Miilhaupt et al. almost simultaneously accomplished the preparative synthesis of higher generation monodisperse POPAM dendrimers on the basis of this principle [6]. Thanks to their terminal amino groups, POPAM dendrimers can readily be per-functionalised. Thus Balzani, De Cola,... 

For example, Kaifer et al. decorated the periphery of a POPAM dendrimer with ferrocene and cobaltocenium units. Such a fourth-generation dendrimer (Fig. 4.4) proved to be a suitable guest system - with numerous docking sites -for /9-cyclodextrin as host compound (see Section 8.3.6) [8]. 

It also proved possible to attach aza-crown ethers to the periphery of a POPAM dendrimer via amidoferrocenyl units (Fig. 4.5) [9a] and also to decorate the outer shell of a second-generation POPAM dendrimer alternately with eight F-stilbene and eight 4-tert-bulylbenzenesulphonic acid units [9b]. Various other... 

Fig. 4.3 Fi rst- to fourth-generation per-mono-substituted dan-syl-POPAM dendrimers (the fifth generation omitted here was also isolated)...

Fig. 4.4 Fourth-generation POPAM dendrimer with terminal cobaltocenium units...

Dendritic hybrid architectures of the two dendrimer types - POPAM and PAMAM - designated by Majoros et al. as POMAM dendrimerf [17] are structural rarities. One such dendrimer of this type was assembled with PAMAM branching units starting from a POPAM core unit [18]. Vogtle et al. developed POPAM/PAMAM hybrid dendrimers up to the third generation (Fig. 4.8) [19]. 

Fig. 4.21 Second-generation stilbene-decorated POPAM Dendrimers (according to Balzani, Vogtle, et ai)...

In contrast, the rotation per chiral end group of first- to fifth-generation POPAM dendrimers with sterically demanding Boc-protected amino acids as end groups (Fig. 4.75) decreased significantly with increasing generation number and approached the value zero [30]. 

Methyl orange can also be subjected to reversible potoisomerisation [28]. Corresponding decoration of the outer shell of a POPAM dendrimer accordingly affords two control mechanisms - conformational and pH control. As with azo-benzene-functionahsed POPAM dendrimers, the photophysical properties are independent of the generation number, with the exception of the molar extinc-... 

Fig. 5.24 Methyl orange-functionalised third-generation POPAM dendrimer (according to De Cola, Vogtle, Pina, et al.)...

This proven host system was transferred to dendrimers by functionalising the periphery of POPAM dendrimers with the acyclic Hamilton variant (Fig. 6.13). After monofunctionalisation with an amino group on the isophthaloyl system, it could be multiply covalently bonded to the dendrimer. In this way, up to fourth-generation POPAM dendrimers could be peripherally embellished with barbiturate receptors. The mode of action of the individual dendrimers as multivalent... 

A light-controlled dendritic box was investigated in analogous fourth-generation POPAM dendrimers whose periphery alternately bore azobenzene and naphthyl moieties. Measurements of photoisomerisation at a wavelength of 365 nm in dichloromethane showed that not all azobenzene units in the E form are transformed into the Z form, but that, for example, four units remain in the E form and some 28 units isomerise to the Z form. At pH 7, six eosin molecules per host molecule were taken up by this mixture of isomers. On the other hand, the dendrimer having all azobenzene groups in the E form takes... 

First to fifth-generation POPAM dendrimers with terminal adamantyl groups (Fig. 6.21) were dissolved in water in the presence of /J-cyclodextrin. The hydro-phobic effect forces the terminal, lipophilic adamantyl groups as guests into the endo-lipophilic host cavities of /f-cyclodexlrin which offer a complementary fit. 

Incomplete coverage of the surface of such a fifth-generation POPAM dendrimer exposes hydrophobic areas of adamantyl units remaining uncomplexed by cyclodextrins on the dendritic outer shell [38]. Pyrenes were used as neutral fluorescence probes to examine whether this might lead to aggregation in water driven by the hydrophobic effect [39a]. Their inclusion in the dendrimer/cyclo-dextrin aggregate leads to changes in fluorescence intensity and in the vibrational fine structure. Formation of excimers was also observed. 

For molecular encapsulation of guest molecules Reinhoudt et al. synthesised a water-insoluble fifth generation POPAM dendrimer containing a total of 62 tertiary amino groups and whose periphery is functionalised with 64 apolar ada-mantyl groups (see Section 4.1). To ensure that this dendrimer (Fig. 8.3a) is sol-... 

Fig. 8.3 a) Transformation of a fifth-generation adamantyl-terminated POPAM dendrimer into water-soluble dendrimer by covering the adamantyl end groups with /fcyclo-dextrin (in excess) (schematic for the sake of simplicity neither all 64 adamantyl groups... 

Luminescence has already been considered in general terms in Chapter 5. Luminescent POPAM dendrimers of various generations with peripheral dansyl units were studied by Balzani and Vogtle et al. as sensor model systems with regard to the fundamental suitability of dendritic structures for multiplication of signalling groups (multi-labelling) [48]. 

Three representative dendrimers, with different numbers of generations, are shown in Fig. 3.7-7. They are a poly(amidoamine) dendrimer (PAMAM) synthesized by D.A. Tomalia, which incorporates an ethylenediamine core and terminal amino groups (Fig. 3.7-7a), a poly(propyleneamine) product (POPAM) reported by... 

Fig. 3.7-9 MM calculated structures of POPAM dendrimers for generations 1 to 5 (C white N black H omitted for clarity).

Fig. 3.7-10 Lipophilic urea-terminated POPAM dendrimers la, b (generation 2) and lla, b (generation 3) with R = hexyl (a) and R = dodecyl (b) [16]...

Poly(propylene imine) (PPI or POPAM) dendrimers are the first dendritic stmctures found in literature. They were first reported by Buhleier etal, who presented a divergent synthesis of branched and dendritic-like molecules. These simple structures were accomplished using controlled organic chemistry. In this case, a Michael reaction between a primary amine and two equivalents of acrylonitrile resulted in a dinitrile. Further reduction through Co(II)-catalyzed hydrogenolysis led to a terminal diamine (Figure 16). An iterative protocol allowed the con-stmction of what today is defined as a second-generation... 

Figure 24 Third-generation POPAM dendiimer D and the different guests used in this study. Inset the supposed binding motif. (Reproduced from Ref. 6. John Wiley Sons, Inc., 2009.)...

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