Dendrimers architecture

Dendrimer architectures are largely determined by the types of branching units they display. To be realised in a dendrimer synthesis, these branching units have to fulfil certain conditions. 

Flory was the first to hypothesize concepts [28,52], which are now recognized to apply to statistical, or random hyperbranched polymers. However, the first purposeful experimental confirmation of dendritic topologies did not produce random hyperbranched polymers but rather the more precise, structure controlled, dendrimer architecture. This work was initiated nearly a decade before the first examples of random hyperbranched4 polymers were confirmed independently in publications by Odian/Tomalia [53] and Webster/Kim [54, 55] in 1988. At that time, Webster/Kim coined the popular term hyperbranched polymers that has been widely used to describe this type of dendritic macromolecules. 

PHOTOINDUCED ENERGY TRANSFER THROUGH DENDRIMER ARCHITECTURE... 

Gadolinium-dendrimer conjugates have been used as blood pool contrast agents in vivo for nuclear magnetic resonance imaging (MRI) of tumors [65]. The efficacy of the conjugates in such applications is dependent on their biodistribution properties, and these properties vary as a function of dendrimer molecular weight and chemical composition [50]. Dendrimer architecture and synthesis... 

The general chemistry used in this approach involves the combination of a limited amount of an amine-terminated dendrimer core reagent with an excess of carboxylic acid terminated dendrimer shell reagent [31]. These two charge differentiated species are allowed to self-assemble into the electrostatically driven supramolecular core-shell tecto(dendrimer) architecture. After equilibration, covalent bond formation at these charge neutralized dendrimer contact sites is induced with carbodiimide reagents (Scheme 1). 

Polymeric micelles, stars and dendrimers in solution consist of a number of polymer chains that form relatively compact aggregates that exhibit internal dynamics and overall diffusion. Whereas the association of polymer in a micelle is usually driven by physical interactions, the star and dendrimer architecture is generally achieved by chemical bonds. 

While the dendrimer architecture is the most popular form of polyamidoamine polymers for gene delivery, linear polyamidoamines have also been evaluated for their effectiveness as gene transfer agents to... 

Studies performed on chiral core dendrimers have provided valuable information about the influence of the achiral dendrimer scaffold on the chiroptical properties of the core unit. Yet they also show that prediction of the chiroptical properties of the dendrimer is difficult, since the chiral relationship between the local chirality of the core unit and the nanoscopic conformation of the overall dendrimer structure is influenced by numerous structural factors. Further studies will be required to attain a fuller understanding of how an individual chiral building block can induce chirality in the entire dendrimer architecture (see Section 4.2.7). 

Compared to polymers, dendrimer architectures offer favourable conditions for fixation of catalytically active moieties thanks to their monodispersity, variability, structural regularity of the molecular scaffold, and numerous functionalisation possibilities. Catalytic units can be fixed - multiply if required - on the periphery, in the core of a dendrimer, or at the focal point of a dendron. If the dendrimers are suitably functionalised at the periphery, appropriate metal complexes can be directly attached to the surface of the molecule. In contrast, dendrimers functionalised in the core or at the focal point shield the catalytically active site through their shell structure in a targeted manner, for example to attain substrate selectivity in the case of reactants of different sizes [1]. The corresponding concepts of exodendral and endodendral fixation of catalysts were inttoduced in the context of functionalistion of carbosilane, polyether, and polyester dendrimers [2]. Exodendral fixation refers to attachment of the catalytic units to the... 

Fig. 3. Dendrimer architectural components as a function of molecular genealogy associated with CMDPs that contribute to endo- and exo-receptor properties...

R. Sadamoto, N. Tomioka, T. Aida, Photoinduced Electron Transfer Reactions through Dendrimer Architecture , J. Am. Chem. Soc., 118, 3978 (1996)... 

A similar quenching by methylviologen through the dendrimer architecture was also observed in the Zn-phthalocyanine-cored dendrimer 54 [125]. 

Frechet and coworkers [85,86] synthesized a series of dendrimers whose energy transfer mechanism is exclusively through-space. By designing dendrimers in which the donor periphery chromophores are effectively separated from the interior acceptors, the dendrimer architecture becomes simply a structural scaffold upon which chromophores can be placed. Chromophores were carefully chosen to satisfy the requirements of FOrster energy transfer (i.e. emission of donor overlaps absorbance of acceptor), so that any photons absorbed by a molecule on the periphery undergo intramolecular singlet energy transfer to the core moiety and emission from that core ensues. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Sadamoto R, Tomioka N, Aida T. Photoinduced electron transfer reaction through dendrimer architecture. J Am Chem Soc 1996 118 3978-3979. 

The implieations of the nature of dendrimer intraconnectivity for electronic coupling in charge transport have been eonsidered. The dendrimer architecture has also been exploited for the synthesis of small, robust eatalytie metal nanoparticle arrays. 

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