Dendritic architecture

The name dendrimers, which has meanwhile largely displaced the original designation of cascade molecules, is derived from the Greek words dendron and meros, and is meant to underscore the tree-like branched structure of this class of compounds (see Section 1.1). 

Fractals are mathematically defined self-similar structures (Fig. 1.11) [26]. The scaffold of cascade or dendritic molecules is fractal if the atoms are considered to be points and the bonds to be strictly one-dimensional lines. Self-similarity 

The intermediate result serves as the starting point for each iteration. (The individual images were produced in colour, but could only be presented in modified form here) 

If a three-dimensional body such as a tetrahedron is substituted for the two-dimensional triangle, the branching sites he on the surface of an imaginary sphere. This accounts for the shell-like structure of dendrimers. 

Owing to their self-similar (fractal) structure, the number of terminal groups of a dendrimer of any generation can be calculated with the aid of the following equation  

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Poly(aryl ether) branches of generation 1 to 3 have been appended to a pho-totautomerizable quinoHne core to investigate the effect of dendritic architecture on the excited state intramolecular proton transfer [45]. The changes observed in the absorption and emission spectra on increasing dendrimer generation indicate that the dendritic branches affect the planarity of the core and therefore the efficiency of the excited state intramolecular proton transfer and of the related fluorescence processes. 

The structural design of nerve cells is a striking example of dendritic architecture, which acts as a signal transduction system. Neurons are known to send out a series of long specialized processes that will either receive electrical signals (dendrites) or transmit these electrical signals (axons) to their target cells (Fig. 5.40). 

FIGURE 5.40 Schematic of the dendritic architecture of a neuron. The electrical signal is transferred in a convergent manner from the dendrites toward the axon, where it diverges to the synaptic terminals. 

P. Antoni, D. Nystrom, C. J. Hawker, A. Hult, and M. Malkoch, A chemose-lective approach for the accelerated synthesis of well-defined dendritic architectures, Chem. Commun. (2007) 2249-2251. 

By the development of these concepts leading to the efficient synthesis of dendritic architectures,151 a new impetus has been given in organic and inorganic chemistry which is leading to the production of new materials. Possible applications will be dis-... 

Employing ways (1), (2), and (3) a broad range of dendritic architectures can be envisaged. It has enabled many research groups to expand their special interests into this particular macromolecular... 

As described earlier in this book, the dendritic architecture is perhaps one of the most pervasive topologies observed at the macro and micro-dimensional length scales (i.e. jum-m). At the nanoscale (molecular) level there are relatively few natural examples of this architecture. Most notable are probably the glycogen and amylopectin hyperbranched structures that Nature uses for energy storage. 

Another exciting development has been the emerging role that dendritic architecture is playing in the production of commodity polymers. A recent report... 

Figure 1.18 Comparison of properties for (I) linear and (IV) dendritic architecture...

Figure 1.23 Intermediary of (III) branched and (IV) dendritic architecture in the conversion of (I) linear thermoplastics to (II) crosslinked thermoset polymers. Intermediary of (IVb) dendrigrafts and (IVc) dendrimers in the formation of megamers...

In order to explore the properties that may be obtained by hybridizing the linear and dendritic architectural states, both diblock and triblock copolymer... 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

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