Dendrimers structural characteristics

Scheme 1 summarizes four different approaches used to characterize dendrimer structures by photophysical and photochemical probes 1. Non-covalent, inter-molecularly bound interior probes - to study the internal cavities and the encapsulation abilities of dendrimers. 2. Non-covalent, intermolecularly bound surface probes - to study surface characteristics of dendrimers. 3. Covalently linked probes on dendrimer surfaces - to study the molecular dynamics of dendrimers. 4. Covalently linked probes at the dendrimer central core - to study the site isolation of the core moiety and define the hydrodynamic volume of dendrimers by the concentric dendrimer shells. Critical literature in these four categories will be described using representative examples. 

Nevertheless, the existence of the famed maximum in the dendrimer [>7] vs G relationship, and whether this behavior could be a characteristic fingerprint property of this type of macromolecular architecture, was questioned recently [19, 23], In fact, so was the linearity of the dependence of dendrimer molecular radii on M1/3 [19], so that whether this may be the beginning of yet another controversy, remains to be seen. Perhaps the future may bring an interesting debate on these subjects, but until new data become available, one should refrain from drawing premature conclusions because the exciting architectural beauty of idealized dendrimer structures has already proven itself to easily tempt the most astonishing hypotheses that may not be readily substantiated by reality. 

The above-mentioned results and drawbacks made futile any effort to grow larger generations of the dendrimer series 5 and 6 in order to get super high-spin macromolecules. Nevertheless, the outstanding structural characteristics and physicochemical properties of the first generations of both dendrimer series, 5 (G = 1) and 6 (G = 1), make both modest dendrimers worthy of detailed physicochemical studies. 

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. 

Specific functional units can be immobiUzed at the center of a dendrimer. In the example depicted in Fig. 3.12, a porphyrin imit is immobihzed in a dendrimer (which is called a dendrimer porphyrin). Because the porphyrin unit is buried deep in the dendrimer structure, the dendrimer porphyrin is a good model of a heme protein. The environment of the dendrimer can be evaluated via the spectral characteristics of the central porphyrin. If the size of the dendrimer is large enough, the adsorption spectrum of the porphyrin shows that it is basically independent of surrounding solvent molecules the central porphyrin is shielded by the dendrimer cage. The structural mobility of the inner part of the dendrimer porphyrin has been evaluated via NMR... 

These structural characteristics mean that the dendrimer porphyrin can be used to mimic the function of the heme protein - its ability to bind to oxygen. A dendrimer porphyrin with an Fe(II) ion can stably trap oxygen via coordination with imidazole ligands. The oxygen was reversibly trapped within the dendrimer, and it can be released when the oxygen in the surrounding solvent was removed. The dendrimer sphere shields the porphyrin part from the outer environment. Therefore, side effects such as irreversible oxidation of the porphyrin by water and dimerization of the oxygen-bound porphyrins can be suppressed. 

In this chapter, details are presented of the synthetic access to star-like and dendrimer-like polymers, and of their relevant structural characteristics. Due to limited space, an exhaustive review of the state of the art of the subject is impossible here rather, the aim is simply to highlight the merits, drawbacks, and possibilities for each synthetic approach. 

Dendrimers are polymers forming a rigid star-like branched structure. Monomer units of this starshaped structure could vary widely from amino acids to polyesters, which also determines its characteristics. In addition breakthrough synthetic techniques such as lego (Brauge et al. 2001) and click (Wu et al. 2004) chemistries have advanced both the efficiency and innovation of possible dendrimer structures. Different combinations of the core, monomer units, and surface functionality have resulted in the emergence of more than hundreds of dendrimers. Table 85.1 lists major classes of dendrimers based on their chemical structure and also their use in drug delivery applications. 

Each step in dendrimer synthesis occurs independent of the other steps therefore, a dendrimer can take on the characteristics defined by the chemical properties of the monomers used to construct it. Dendrimers thus can have almost limitless properties depending on the methods and materials used for their synthesis. Characteristics can include hydrophilic or hydrophobic regions, the presence of functional groups or reactive groups, metal chelating properties, core/shell dissimilarity, electrical conductivity, hemispherical divergence, biospecific affinity, photoactivity, or the dendrimers can be selectively cleavable at particular points within their structure. 

Will these emerging megameric structures of poly(dendrimers) represent a new class of macromolecular architecture with unique properties and characteristics ... 

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