Hydrocarbon dendrimer

A large family of new dendrimers has been synthesized following this divergent method. Hawker and Frechet developed polyaryl(-benzyl)ether dendrimers [27], Miller andNeenan [28], and also Moore and Xu [29] prepared hydrocarbon dendrimers. The latter have reported the largest monodisperse organic hydrocarbon dendrimer with a molecular mass of 18 kDa and a diameter of 12.5 nm [30]. 

We reported the first synthesis of a symmetrical, four-directional, saturated hydrocarbon dendrimer containing 36 carboxylic acid moieties equidistant... 

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... 

Figure 11.3 Chemical structures of the (a) arborol dendrimer and (b) water-soluble, all-hydrocarbon dendrimer.

Miller and Neenan succeeded in the very same year in producing the first hydrocarbon dendrimers based exclusively on arene units, also using a convergent synthetic strategy [16] (Fig. 1.7). 

Hardly any purely aliphatic hydrocarbon dendrimers have hitherto been prepared (see also Section 6.2.3.3), probably owing to fundamental synthetic problems in the formation of unsymmetrical C-C bonds. The dendritic hydrocarbons described so far are nearly all made up of condensed or coupled arene or/ and multiply bonded components, which is easier to achieve synthetically. As a rule, such molecules are generally much more conformationally rigid than their aliphatic analogues made up of [CH2]n units. 

Meier et al. described hydrocarbon dendrimers with trans-stilbene chromo-phores in the core and in the periphery. Owing to the flexible nature of the arms intra- and intermolecular C-C bonds could be formed by irradiation [28] (see Section 4.1.5.3 for further information). 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

Scheme 5.10. Employment of Suzuki coupling leads to the construction of all aromatic hydrocarbon dendrimers.

A unique class of structures referred to as iptycenes has recently been reported by Hart et al. [129-131]. These architectures represent the first examples of all-hydrocarbon dendrimer prototypes. Starting with either benzene or triptycene as the initiator core, branch junctures were constructed in situ by Diels-Alder adduct formation to give the heptaiptycene (A) and the nonaiptycene (tri-... 

Fig. 28. Hydrocarbon dendrimers (iptycenes) (A) heptaiptycene, (B) nonaiptycene, (C) nonadecaiptycene, (D) supertriptycene...

Polypropylene amines, poly(amidoamines) (PAMAM) and silicon dendrimers set records for the number of generations, whereas the polyacetylene dendrimer prepared by Moore et al. [6] with the formula C1398H1278 and a mass of 18079.53 holds the heavyweight championship title of pure hydrocarbons. Mullen et al. have recently published a series of aromatic hydrocarbon dendrimers consisting of phenylene-bridged benzene rings. [7] Interestingly, these species form two-dimensional planes rather than three-dimensionally extended structures. 

Figure 2. Moore et al. [6] have built up the heaviest pure hydrocarbon dendrimer known to date with a...

Fig. 2 Early t/is-terminated, branched architectures (i.e.. 3 and 4) were described as possessing micellar architecture and character and study of the properties of nuinerous encapsulated molecular guests proved the micellar character of the first, all-alkyl hydrocarbon dendrimer 7.

R., and Bair, H.E. (1992) Synthesis and characterization of a series of monodisperse, 1,3,5-phenylene-based hydrocarbon dendrimers including C276H186 and their fluorinated analogs. 

Phenylene dendrimers 1,3,5-phenylene based hydrocarbon dendrimers M /Mn = 1.08 soluble in common organic solvents spectroscopy, thermal analysis described. [253]... 

A series of monodisperse, 1,3,5-phenylene-based dendrimers (62) and their fluorinated analogues were obtained at the AT T Laboratories [96]. These materials consist of 4, 10, 22 or 46 benzene rings linked symmetrically. In contrast to the former procedure, they are prepared stepwise. A palladium-catalyzed coupling of arylboronic acids with 3,5-dibromo-(trimethylsilyl)-benzene is used. The aryltrimethylsilane is subsequently converted to a new arylboronic acid and the next step is performed. Coupling of arylboronic acid dendrimer arms to 1,3,5-tribromobenzene or 1,3,5-tris(3,5-dibromophenyl)benzene is the final step of the synthesis. The largest hydrocarbon dendrimer is soluble to the extent of 190 g/L in toluene and is stable to 500°C. 

Dendrimers made up of fluorinated phenyl rings were s)mthesized by a similar sequence of reactions. The solubility and thermal properties of these dendritic polyphenylenes have been evaluated. The largest hydrocarbon dendrimer was, for instance, soluble in toluene and stable up to 500°C [26]. 

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