Dendrons, self-assembling

M. Wyszogrodzka, R. Haag, Synthesis and characterization of glycerol dendrons, self-assembled monolayers on gold a detailed study of their protein resistance, Biomacromolecules 10 (2009) 1043-1054. 

We now examine recent progress reported by Percec, Rosen and colleagues [151] that has clearly demmistrated the first working examples of predictive, Mendeleev-like nano-periodic tables. These Percec nano-periodic tables clearly demonstrate a priori predictions for the mode of [S-l]-type amphiphilic dendron self-assembly into supramolecular dendrimers with 85-90% accuracy. Quite remarkably, as proposed in the original concept [137], these self-assembly modes may be accurately predicted based on simply knowing the CNDPs (size, shape, surface chemistry, and flexibility) for the amphiphilic dendron primary structure, as will be described in the next sectimi. 

A Library of Amphiphilic Dendron Self-Assembly Directed... 

First Nano-periodic Tables for Predicting Amphiphilic Dendron Self-Assembly to Supramolecular Dendrimers Based on the CNDPs... 

Coordination metallacycles of an achiral dendron self-assemble via metal-metal interaction to form luminescent superhelical fibers, M. Enomoto, A. Kishimura, T. Aida, J. Am. Chem. Soc. 2001,123, 5608. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. 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 Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

Self-assembly of aromatic dendron subunits has been tried by the design of coordination to multivalent metal cations (i.e., metal-cored dendrimer complexes). Several metal-cored dendrimer complexes have successfully exhibited luminescence by antenna effects. 

A supramolecular assembly of macromolecules bearing antenna dendron has been reported. Pyrazole-anchored PBE dendrons were synthesized to examine the coordination behavior to transition-metal cations (Cu, Au, Ag) [31]. Self-assembled metallacycles were found. The Cu-metallacycle further formed luminescent fibers about 1 pm in diameter. The luminescence (605 nm) occurred by the excitation of the dendron (280 nm) and the excitation spectrum was coincident with the absorption spectrum of the dendron, suggesting the antenna effect. Interestingly, the luminescence of the Cu-metallacycle fiber disappeared when the fiber was dissociated into the individual metallacycles in C2H2. 

In order to evaluate the release and self-assembly of the diphenylalanine end groups, dendron 22 was incubated in PBS, pH 7.4, without PGA at a concentration of 1.5 mM. Transmission electron microscopy (TEM) analysis revealed that attachment of the diphenylalanine to the dendritic platform prevented self-assembly, and therefore, no organized structures were observed (Fig. 5.17a). Then, dendron... 

Fig. 28. Self-assembling dendrons exhibiting a thermotropic cubic liquid crystalline phase reported by Percec et al. 

SCHEME 40. Mannosylated dendrons having an 8-hydroxyquinoline ligand at the focal point for self-assembly.278... 

Size and shape Certain steric effects can be achieved using characteristically wedge-shaped dendrons. Thus, self-assembling dendrons have been connected to supramolecular aggregates with defined dimeric or hexameric structures. Such aggregates can form columnar superstructures which reveal liquid-crystalline properties. Spherical superstructures arise from the self-assembly process when conical dendrons are used14 14,161 Similar... 

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-... 

Figure 5.10 Diverse applications using hydrogen bonding interactions employed by Rotello and coworkers (Arumugam et al. 2007) (a) self-assembled dendronized polymer and (b) inorganic-organic hybrid material using barbiturate functionalized nanoparticles and Hamilton wedge functionalized block copolymers.

The addition of large linear blocks to dendrons with opposite polarity creates a desymmetrized structure predisposed to sequester insoluble components by aggregation rather than intramolecular hydrogen-bonding. Amphiphilic, linear-dendritic diblock (AB) and triblock (ABA) copolymers self-assemble into multimolecular micelles with CMC values that are well below those of low molecular weight surfactants. Typically, a hydrophilic linear block such as PEG is attached to the focal point... 

Figure 11.18 Dendron chain copolymers, (a) A poly ether dendron displaying a focal PEG chain and (b) notional representation of the self-assembly of the dendron chain copolymers.

Amphiphilic systems containing an extended rigid-rod block, rather than a more flexible linear chain, have also been shown to self-assemble into well-defined structures. Stupp s group explored a series of rod-dendron and dendron-rod-dendron hybrids containing two to three biphenyl ester moieties as a rigid rod that was capped by one or two 3,4,5-tra-alkoxy benzoate dendrons (Lecommandoux et al. 2003). The... 

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