Dendrimers types

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an anfine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these poly(propylene imine) dendrimers was later commercialized. 

Scheme 2. Classification of rotaxane dendrimers Type I, II and III rotaxane dendrimers incorporating rotaxane-like features at the core, termini, and branches, respectively...

The dendrimer-type tetranuclear Ru(II)-Os(II)3 complex (22, protonated form) shows an interesting electrochemical behavior due to the presence of free basic sites in its bridging ligands [41]. The protonated form shows a 3-1 oxidation pattern due to the simultaneous oxidation of the three Os-based units, followed by the one-electron oxidation of the Ru-based unit. On addition of base, the six chelating moieties (three on the Ru center and one on each Os center) undergo deprotonation. This causes changes in the oxidation potential of the metal ions, with a consequent switching from 3-1 to 1-3 in the oxidation pattern. 

Other than the epoxy groups available on one Priostar dendrimer type and a methyl ester available on a PAMAM dendrimer, the commercial suppliers generally don t offer a selection of spontaneously reactive dendrimers for bioconjugation purposes. For this reason, most of the applications published for coupling biomolecules to dendrimers have used various modification or activation steps to create the appropriate reactive groups for conjugation (e.g., Leon etal., 1996). 

Figure 7.25 Dendrimer-type gadolinium imaging agent Gadomer-17 . (Adapted with permission from Figure 43 of Caravan, P. Ellison, J. J. McMurry, T. J. Lauffer, R. B. Chem. Rev., 1999, 99(9), 2293-2352. Copyright 1999, American Chemical Society.)...

A very important question in the context of dendrimers and their utility as host molecules relates to the existence of cavities within these macromolecules. The presence of internal voids in dendrimers is closely related to their conformational behaviour and to the degree of back-folding of the terminal branches into the interior of the dendrimer. The issue of back folding was already briefly touched upon in section 16.2.1. Next to the purely theoretical calculations mentioned there, several calculations have been performed on specific dendrimer types. 

Recently the divergent iterative approach, welt known in oiganic synthesis, has been successfully extended to prepare polynuclear dendrimer-type complexes containing up to 22 metal centers using 2,3-dpp as bridging ligand. The two... 

The following Sections give a detailed account of the strategies most commonly used for synthesis of the various dendrimer types and the synthetic utility for targeted design of functional dendrimers is evaluated. 

Table 4.2 I ncrease of PAMAM parameters with the number of generations (dendrimer type, see Fig. 4.6) [3]. The second column lists the number of terminal groups of 1 ->3-branched aliphatic Newkome type dendrimers (see Figs. 1-5 and 4.60 for typical formulae)...

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]. 

Van Koten et al. successfully prepared silicon-based dendrimers with a polyca-tionic branching shell. The dendrimer type shown in Fig. 4.37 consists of a non-polar core (tetraarylsilane) bearing an ionic layer which, in turn, is surrounded by a less polar polyether shell [67]. Such dendrimers are predestined for potential inclusion of anionic guests - with anion exchange - in the positively charged host scaffold. 

The last two dendrimer types are less well known, because in the case of D the chirality is low or in the case of E the interpretation of chiral effects probably proves difficult (see Section 4.2.7). 

The dendrimer type with a stilbene scaffold and long-chain end groups mentioned in Section 4.1.5.3 was shown to have a disc-like flattened molecular shape in solution by SAXS and SANS studies performed by Ballauff et al. [51]. 

The overall conclusion drawn from the above considerations is that it is not easy, using somewhat haphazard dendritic modification, to significantly improve existing catalysts which have been optimised over the course of decades. Given the wide scope for variation (dendrimer type, generation, cavity size, flexibility, philicity balance, see above), far more specific adaptations to the particular catalytic conditions are possible and necessary. The approaches to the topic adopted so far will hardly suffice for successful longer term planning. 

Such a self-sorting process can also lead to the formation of structurally well-defined assemblies of the dendrimer type [66], We have demonstrated this in a first example, using the dimerization of tri-ureas 26 derived from triphenylmethanes [67]. It has been shown that this dimerization is independent of the dimerization of tetra-urea calix[4]arenes. It is neither disturbed by tetra-ureas nor does it disturb their dimerization. In addition, Figure 5.20 even shows that the dimerization of rigidified tetra-ureas 25 also occurs independently [68]. 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

Polystyrene B-114401 and poly(acrylate) B-115402 are connected to a dendrimer at its focal core. These are prepared with dendrimer-type macroinitiators with a benzyl bromide at the focal point, from which are initiated the copper-catalyzed living radical polymerizations of styrene and acrylates, respectively. For B-114, various functional groups (R) were introduced into the periphery. 

Another tetrafunctional ester (MI-36) is the smallest number of a series of dendrimer-type initiators such as MI-46 and MI-53 for 6- and 12-arm star polymers, respectively.414 419 420 These initiators induce the living radical polymerizations of MMA with Ni-2 to give the corresponding multiarmed polymers with controlled molecular weights although the arm number with MI-53 is slightly lower than 12 due to incomplete initiation from all the carbon—bromine bonds. 

Tris-or(/j< -lithiation of tris[2-(diethylsulfamoyl)phenyl]bismuthine with ferf-butyllithium followed by treatment with 3 equiv. of diaryliodobismuthine yields a branched tetrameric bismuthine, which, on similar treatment, leads to a dendrimer-type bismuthine [97CC2295],... 

As compared with hole-transporting materials, fewer electron-transporting materials have been reported. A well-known green emitter, Alq3, has been used as a good electron transporter. Other reported electron-transporting materials include oxadiazole derivatives, a dendrimer-type oxadiazole, a triazole derivative, tris(phe-nylquinoxaline), silole derivatives, benzimidazole derivative, and boron-containing compounds (Table 7.3). 

A range of other dendrimer types that include many different core functions (Fig. 3.7-8) as well as other branching atoms, such as sihcon or phosphorus, are also known. At present, derivatives of the first two dendrimer types are also commercially available [5]. 

Dendrimer Type Polymer Used Molecule Pu rpose/Application References... 

As we have just seen, electrostatic interactions can reinforce the formation of axial base complexes in metalloporphyrins, but other thermodynamically favored processes, such as chelation around a bidentate base like pyrazine, can be used. This approach has led Kuroda to the preparation of self-assembled dendrimer-type structures 86-89 represented in Figure 13.50 ". In these structures, the zinc porphyrin dimer formed by multihydrogen bonding of tetracar-boxylate derivatives of TPP is considered as a chelate. 

Synthetic Overview of the Preparation of Common Dendrimer Types 121... 

Dendrimer type End group Matrix Ion suppression Solvent system References... 

Jiang, H. Manolache, S. Wong, A. C. Denes, F. S., Synthesis of Dendrimer-Type Poly(ethylene glycol) Structures from Plasma-Functionalized Silicone Rubber Surfaces. J.Appl. Polym. Sci. 2006,102, 2324-2337. 

Tanaka K, Siwu ERO, Minami K et al (2010) Noninvasive imaging of dendrimer-type N-glycan clusters in vivo dynamics dependence on oligosaccharide structure. Angew Chem hit Ed 49 8195-8200... 

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