Dendrimers structural concept

The existence of a hollow core as well as a densely packed exterior layer in PAMAM dendrimers was proven by studying their conformational behavior [3]. Meijer et al. introduced the concept of dendritic boxes by synthesis and characterization of dendrimer with flexible core and solid shell structure. The flexible core was based on poly(propyleneimine) dendrimer. The rigid shell was obtained by modification of terminal groups with bulky amino acid derivatives, that is (r-Boc)-protected L-phenylalanine. Dendrimer structure was fully studied by H and CNMR, IR and UV techniques. Additionally, solid-phase behavior of the shell was confirmed by spin-lattice (Ti) and spin-spin (T2) relaxation measurements and molecular mechanics calculations [3,18,19]. 

The interest in hyperbranched polymers arises from the fact that they combine some features of dendrimers, for example, an increasing number of end groups and a compact structure in solution, with the ease of preparation of hn-ear polymers by means of a one-pot reaction. However, the polydispersities are usually high and their structures are less regular than those of dendrimers. Another important advantage is the extension of the concept of hyperbranched polymers towards vinyl monomers and chain growth processes, which opens unexpected possibilities. 

This manuscript describes the dendritic macromolecules for optical and optoelectronic apph-cations, particularly stimulated emission, laser emission, and nonlinear optics. Dendrimers have been designed and synthesized for these applications based on simple concepts. A coreshell structure, through the encapsulation of active imits by dendritic branches, or a cone-shaped structure, through the step-by-step reactions of active imits, can provide particular benefits for the optical high-gain media and nonlinear optical materials. It also described experimental results that support the methods presented for designing and fabricating functionalized dendrimers for optoelectronic applications, and theoretical results that reveal the intermolecular electronic effect of the dendritic structure. 

Percec, V., Cho, W.D., Mosier, P.E., Ungar, G., and Yeardley, D.J.P. (1998) Structural analysis of cylindrical and spherical supramolecular dendrimers quantifies the concept of monodendron shape control by generation number./. Am. Chem. Soc. 120, 11061-11070. 

Since they were first mentioned in 1978, the concept of cascade or dendritic structures has witnessed a meteoric rise. On the one hand, thanks to their aesthetic beauty, and on the other hand, because of their broad applicability, dendrimers are in considerable demand. As the concept is not limited to one class of substances and, furthermore, allows a simple access on several routes, there is -besides the researchers who started dendrimer chemistry - a growing number of research groups which are dendritically expanding their special areas of interest in different ways and describing new or varying already existing properties. 

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. 

More recently, mathematically defined, structure controlled, covalent megamers have been reported. They are a major subclass of megamers also referred to as core-shell tecto dendrimers) [126-128], Synthetic methodologies to these new architectures have been reported to produce precise megameric structures that adhere to mathematically defined bonding rules [91, 129], It appears that structure controlled complexity beyond dendrimers is now possible. The demonstrated structure control within the dendrimer modules, and now the ability to mathematically predict and synthesize precise assemblies of these modules, provide a broad concept for the systematic construction of nanostructures with dimensions that could span the entire nanoscale region (Figure 1.24). 

The preceding sections have demonstrated that dendrimers of lower generation are akin to branched polymeric structures. It is therefore to be expected that their flow behavior in dilute solution may be described in terms of the well-known concepts of dilute polymer solutions [14, 15]. Hence, dissolved dendrimers should behave like non-draining spheres. From an experimental comparison of and the immobilization of solvent inside the den-drimer can be compared directly since in this case the dendrimer may be approximated by a homogeneous sphere. Therefore R = 3/5 Rl where Ry, denotes the hydrodynamic radius of the dendrimer. This has been found experimentally [19]. 

As a start to a tetralogy on dendrimers, the volumes Dendrimers and Den-drimers II have already appeared in print. This mini-series continues now with the latest volume Dendrimers III and will be completed by the fourth volume Dendrimers IV within the next few months. Volume III offers dendrimers based on novel design concepts leading to highly stiffened and shape persistent dendritic structures as well as to new families of rather soft and floppy dendrimers and focuses on new functional properties and materials aspects. As an example, the question of host-guest interactions with dendrimers, whose existence has been under intense debate for a long time, finds its final - and positive - answer in this volume. As a consequence, dendrimers clearly represent a subset of supramolecular chemistry. 

In their theoretical considerations of molecular structure, de Gennes and Her-vet assumed an ideal dendrimer with extended branches with all terminal groups arranged at its periphery in a kind of outer ring around the dendrimer core [9]. According to this model, dendrimers should exhibit a lower segment density at the core, which increases to a maximum value on moving to the periphery. This concept is known as the dense-shell model (Fig. 1.17). 

Most studies performed partly on molecular models [33] but also on real POPAM and PAMAM dendrimers support the latter model concept [34]. Careful studies on the three-dimensional structure of flexible dendrimers in solution were performed by Ballauff et al. by means of SANS (Small Angle Neutron Scattering) [35] (see Section 7.6). 

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... 

Brunner s concept of attaching dendritic wedges to a catalytically active metal complex represented the first example of asymmetric catalysis with metal complex fragments located at the core of a dendritic structure [5,6]. Important early examples of catalysts in core positions were Seebach s TAD-DOL systems (TADDOL = 2,2-dimethyl-a,a,a/,a/-tetraphenyl-l,3-dioxolane-4,5-dimethanol) [38,39]. In general, the catalytic performance of such systems was either unchanged with respect to the simple mononuclear reference system or significantly lower. In no case has the potential analogy of this core fixation and the existence of efficient reactive pockets in enzymes been vindicated. This may be due to the absence of defined secondary structures in the dendrimers that have been employed to date. 

Avnir et al. llbl have examined the classical definitions and terminology of chirality and subsequently determined that they are too restrictive to describe complex objects such as large random supermolecular structures and spiral diffusion-limited aggregates (DLAs). Architecturally, these structures resemble chiral (and fractal) dendrimers therefore, new insights into chiral concepts and nomenclature are introduced that have a direct bearing on the nature of dendritic macromolecular assemblies, for example, continuous chirality measure44 and virtual enantiomers. ... 

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