Frechet type dendrons

FuUerodendrimers also allow an evaluation of the accessibility of the Cgo core unit by studying bimolecular deactivation of its excited states by external quenchers. Recently Ito, Komatsu, and co-workers have used this approach to investigate a series of fuUerodendrimers (9-11) in which Frechet-type dendrons have been connected to a fullerene moiety via an acetylene linker (Fig. 5) [35]. 

Fig.1. Rotaxane dendrimers from crown ether type macrocycle ring, bipyridinimn derivatives rod component and Frechet-type dendron stoppers...

Delivery of physically encapsulated small molecules at targeted sites by dendrimers has been superbly envisioned in a recent report by McGrath and Junge [95]. Second generation Frechet type dendrons linked to a central azo-benzene-derivative (32, Fig. 15) underwent reversible cis/trans isomerization of... 

Scheme 3. Frechet-type dendrons of generations one to three (2a-c) with a functional group (X) at the focal point... 

Recently, Bo and coworkers [317] reported on dendronized PF with carbazole end groups in the peripheral Frechet-type dendrons 214a-c. Polymers emitted blue light with high < pL of 86-96% in solution for all three polymers 214a-c (G0-G2). In the solid state, < PL depended on the size of the attached dendron (29, 55, and 64% for GO, Gl, and G2, respectively) (Chart 2.50). 

Fujiki and coworkers [290] synthesized asymmetrically substituted PFs, bearing a bulky Frechet-type dendron and a less bulky 3,6-dioxaoctyl group in position 9. The polymers 215-217 showed a pure blue PL emission with rather low green emission band (at 520 nm) for the films annealed at 200°C for 3 h (in vacuum) (Chart 2.51). 

The properties of the hybrid diblock structures can be altered drastically by simply taking advantage of the high terminal functionality of the dendritic block. For example unusual diblock structures useful for the modification of surfaces have been prepared by ATRP of polystyrene (PS) initiated from the benzylic halide focal point of Frechet-type dendrons with terminal isophthalate ester groups [9b], Well-defined copolymers with narrow molecular weight distributions were obtained and excellent agreement was observed between calculated... 

ABA triblock hybrids with Frechet-type dendrons as terminal A blocks and polystyrene as the central B block have been prepared by anionic bidirectional growth of polystyrene followed by attachment of reactive dendrons at the chain... 

The concept of making brush-type polymers in which a linear polymer is funtionalized with dendritic side-chains was first suggested by Tomalia in a 1987 patent, though actual experimental work on his approach was only reported recently recently [15]. Hawker and Frechet were first to document the preparation of a vinyl copolymer containing a few pendant Frechet-type dendrons (Figure 7.9). 

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

Stoddart and co-workers have made use of Frechet-type dendrons as dendritic stoppers for self-assembled [n]rotaxanes [65]. The solubility enhancement that results from incorporating dendritic wedges at the termini facilitated the purification of these materials by column chromatography despite the polycationic nature of their bipyridinium backbone. Again, the dendritic wedges did not alter the electrochemical characteristics of the viologen subunits. Flowever, the enhanced solubility resulting from the presence of the dendritic components en-... 

Similar results have been presented by Miller and co-workers, who capped the ends of oligo(dihexylfluorene)s and poly(dihexylfluorene)s with Frechet-type dendrons (Figure 7.17C) [73], Annealing experiments coupled with emission studies revealed that G-3 and G-4 dendrons were effective at preventing excimer formation, even when the poly(fluorene) spacer was 50-80 repeat units long. 

Another approach was followed by Bolm et al. [24], who prepared dendron ligands consisting of a chiral pyridyl alcohol connected to the core of Frechet type dendrons [25]. The chiral dendron ligands were used for the in situ generation of ethylzinc dendron ligand complexes which catalyze the addition of diethylzinc to benzaldehyde (see e.g., 15, Scheme 16). The size of the dendron appeared to have practically no influence on the enantioselectivity of this reaction. 

PREPARATION OF POLYETHER DENDRIMERS BY ASSEMBLY OF FRECHET-TYPE DENDRONS AROUND A CORE... 

Many examples of this class of compounds have been reported [13a-c], The first one was constituted by Frechet-type dendrons bearing a 3,5-dioxybenzoate focal point. It can self-assemble around Er3+, Tb3+, or Eu3+ ions leading to the formation of dendrimer 9 [38], Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy-transfer Forster mechanism. The much lower sensitization effect found for Eu3+ compared with Tb3 + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu3+ can quench the donor excited state by electron transfer [39]. 

The reversibility of oxidative addition of these Ir complexes to Cjq was proven with temperature-dependent NMR measurements [19], These measurements were carried out with Cgo[lr(CO)Cl(PEt3)2]2 [19], or in another study with a dendritic complex derived from Ir(CO)Cl(PPh2R)2 vvith R being a Frechet-type dendron of the first or second generation [21],... 

The third class of AuNPs-dendrimer nanomaterials is Nanoparticle Cored Dendrimers (NCDs). The first report on this topic describes a new strategy in which Frechet-type dendrons with a single thiol group at the focal point were used as a surface stabilizer of Au NPs. These dendronized Au NPs were synthesized using a modification of the two-phase Brust-Schiffrin method, giving rise to highly stable and very monodisperse Au NPs of small size (about 2.4—3.1 nm) [129] (Figure 3.13). 

Figure 3.13 Synthesis of gold nanoparticle-cored dendrimers using Frechet-type dendrons.

Lochmann et al. were the first to attempt postmodification of the internal molecular framework of Frechet-type dendrons [46]. Multifunctionalisation of the interior of the dendron via metallation (superbase followed by reaction with various electrophiles) proved to be of a somewhat random nature owing to a lack of regioselectivity. It is admittedly suitable for engineering the solubility properties of the dendrons, but not for producing dendritic structures having functional groups in defined positions of the internal scaffold of the molecule. 

Supramolecular or coordinative [120] complexation A method of preparing dendrimers with central metal complex entity accomplished by Kawa and Frechet consists in the complexation of a metal cation with suitably functionalised den-dron ligands. Self-assembly of three Frechet-type dendrons, each with a carboxy-... 

The local chirality of a core unit is not necessarily manifested in a macroscopic chirality of the dendrimer. This is clearly apparent from the example of the dendrimers constructed by Meijer et al. from a glycerol unit as chiral core unit and four Frechet-type dendrons of different generations (Fig. 4.66). No optical activity could be measured with available instrumentation for the two enantiomeric forms (S)-l and (i )-l [2 a]. 

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