Dendrimer systems

The starburst point occurs at a well-defined limit for each dendrimer system, and its occurrence is dependent mainly on (a) the functionality of the core, (b) the multiplicity of the branches and (c) the branch length. However, the volume of the core itself, and the length of the monomer branches also have an influence. The end groups may occupy the outer surface of the dendrimer, or the branches may fold inwards, thus distributing the end-groups within the dendrimer. The factors that control this behaviour of the branches are not fully understood, but include the nature of the solvent and the detailed chemistry of the dendrimer branches. 

SANS is a valuable research technique, since it provides information about the size and shape of the molecular scale structures in the size range 1-10 nm found in polymer or dendrimer systems. Neutrons interact with atomic nuclei, hence are sensitive not only to the details of the way such nuclei are arranged but also to the presence of different isotopes. 

Figure 14.7 Dependence of the zero-shear viscosity, uo, on molecular weight, M, for different dendrimer systems. (1) Dendrimers of different chemical composition but in the same state (i.e. PAMAM, PPI and PBzE dendrimers in bulk D, C and E, respectively). (2) Compositionally identical dendrimers (i.e. PAMAMs) in solutions and in the bulk state (A, B and D, respectively). (3) Compositionally identical dendrimers and linear polymers of comparable molecular weights (i.e. PAMAMs in the bulk state D and F, respectively)...

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

One particular dendrimer system was reported to have increased the loading capacity 10-fold over direct coupling of the oligonucleotide to the surface. Unfortunately, the presumed improvement in hybridization efficiency was not discussed in quantitative terms. Reusability of the array was a primary objective of the work. Again, quantitative information was not provided. As the authors note, the array withstood >7 actual hybridization stripping cycles and 30 simulated (lacking target) cycles. 

The concept of a photo-release and report system has been realized with the 9-(2-methyl-l,3-dithien-2-yl)-9//-thioxanthen-9-ol tethered to a PAMAM dendrimer. Irradiation of the dendrimer system with a U-360 broadband filter results in cleavage of the dithienyl unit to generate, after hydrogen radical transfer, 2-methyl-l,3-dithiane and the tethered thioxanthone 630. The increased fluorescence of the latter confirms the successful photo cleavage. The system offers potential for the generation of high local concentrations of substituted dithianes in the vicinity of the dendrimer surface (Equation 209) <2005JA12458>. 

However, the results obtained in recent years have also established that the structural characteristics of the established dendrimer systems, such as the absence of a well-defined secondary structure, have limited the development of efficient abiotic enzyme mimics based on dendrimers. To achieve this ambitious goal, more efforts in dendrimer synthesis will be necessary. The use of dendritic catalysts in biphasic solvent systems has only just begun and appears to be a particularly fruitful field for further developments. These utilitarian aspects aside, it is the aesthetic attraction of these topologically highly regular macromolecules that continues to fascinate those working in the field of dendrimer catalysis. 

The studies of Rg as well as Rrl and Rh of a number of dendrimer systems establish a transition from v=0.45+0.05 to v=0.25+0.05 dependence on MW as shown in Figs. 4 and 5. This result is qualitatively consistent with the simulation results and has several consequences. Because [q] -R- /M, the value of [q] will increase with the MW of the dendrimer when v>l/3, be independent of MW when v=l/3,and decrease with MW when v< 1/3. This last, unusual, behavior of high MW dendrimers has been pointed out [48]. The maximum in the value of [q] at intermediate MW resembles the behavior of star polymers when values of [q] are plotted against f, the arm functionality at constant arm MW [62,81]. It is also observed in comb and graft copolymers with increasing grafting density, other variables being kept constant [82]. 

An increasing emphasis has been placed on the functionalization of known carbosilane dendrimer systems. Functionalization can impart specific properties to the dendrimers or, in some cases, aid in the elucidation of dendrimer structure. Functionalization of carbosilane dendrimers can occur at the core or at the periphery. 

Goodson and co-workers investigated excitation energy transfer processes in nitrogen cored distyrylbenzene and triarylamine dendrimer systems (Fig. 1.5) by photon echo and polarized fluorescence upconversion spectroscopy. Observed components of less than 1 ps were attributed to a coherent energy transport mechanism. The contributions from his group were recently summarized [15],... 

Scheme 16 Dendrimer system reported by Jiang and Aida to exhibit low-energy photon harvesting [80-82]...

This PET pathway was studied directly in the analogous dendrimer systems with PDI cores (e.g. 77 and 78) [103]. Direct excitation of the PDI cores (540 nm) in dendrimers bearing different numbers of OXZ units (e.g. 0,2,4)... 

These studies provide structural models that should be useful for analyzing the dopamine/dendrimer systems. The next step is to test the effectiveness of these modified materials for encapsulation of dopamine (and related materials) and to determine how effectively they are delivered to the kidney centers. As such experiments proceed, continuing simulation will be useful in providing a quantitative framework for understanding various results. 

Sub-picosecond photo-induced charge injection from molecular tripods into mesoporous Ti02 has been achieved over the distance of 24 A (Scheme 70) 186 three-point attachment to the Ti02 surface and the rigidity of the spacer in molecules of such remarkable architecture as (123) facilitate control of the distance and orientation of the sensitizer with respect to the surface. Details of PET in two dendrimer system have been reported. The first contains a perylenediimide as an acceptor core, a rigid second generation polyphenylene... 

Combinations of the very simple spin-coated reactive polymer films discussed in Sect. 2.1.4 with the micro- and nanopatterning approaches studied and refined in model studies on weU-defined macromolecular (dendrimer) systems are ciu rently being investigated with substantial success. Thus, the lessons learned in these model studies can be applied to practical formats in order to provide reactive micro- and nanopatterned platforms for the development of biosensors, biochips (DNA, proteins, saccharides, and so on) and studies of cell-cell and ceU-substrate interactions. 

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