Dendrimers - Molecular Trees

Fullerenes and carbon nanotubes are formed spontaneously based on the restricted rules of carbon linkage. Therefore, their fundamental frameworks are not easily redesigned. Superstructures where we are free to design their sizes and shapes are sometimes more attractive prospects. In this section, the dendrimer family is introduced, as examples of artificially controllable superstructures. The word dendrimer contains the root dendr- , which means tree, an accurate reflection of the structures of dendrimers - molecular trees. The first dendrimer was proposed by Tomalia. After several groups had 

When we do not care much about obtaining a product with precise size and structure, branched monomers can be condensed randomly. The polymeric materials obtained in this way are called hyperbranched polymers (Fig. 3.10). 

Dendrimers have various useful properties. The number of branches increases with the step munber (the dendrimer generation). The branches are crowded at the outer surface while the inner part of the dendrimer has more empty space. Therefore, the dendrimer can behave like a capsule. Size-matched functional guest molecules become entrapped in this nanometer-scale capsule. 

The dendrimer shown in Fig. 3.11 has imide groups that can trap tin ions site-specifically. Interestingly, the tin ions bind to this dendrimer in a stepwise fashion according to the electron density gradient. 

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 

---

Dendrimers - Molecular Trees Dendrimers have systematic branching structures and they are built in a stepwise manner. Supermolecules from this family can be used to shield functional groups and to collect energy. 

The synthesis and study of dendrimers is a relatively new branch of macro-molecular chemistry. It began in 1985 with the publication of two landmark papers (D.A. Tomalia, H. Baker, J. Dewald, J.M. Hall, G. Kallos, R. Martin and J. Ryder, Polym. J., 1985,17,117-132 and G.R. Newkome, Z. Yao, G.R. Baker and V.K. Gupta, J. Org. Chem., 1985, 50, 2003-2004), and has grown to become a very vibrant research field. The word dendrimer comes from the Greek word dendra, meaning tree, and was applied to these compounds by Tomalia et al. in their very first paper. Newkome s team, by contrast, called their molecules arborols from the Latin word arbor, which also means a tree. The term cascade molecule has also been used, but the word dendrimer is the one that is used most widely throughout the literature, and is also used in the present chapter. 

Recent developments in the design of dendritic molecules has provided both new methodology and molecular architecture which are structure controlled macromolecules, globular-shaped, dendritic-branched tree-like structures with nanoscscale dimensions [1], Dendrimers generally consist of a focal core, many building blocks (monomer units) and a mathematically defined number of ex-... 

A useful toy theoretical model which captures the essential features of self-entangled dendritic polymers is the monodisperse Cayley tree in which each chain segment branches with a fixed functionality z at each of its ends, except those at the extremity of the molecule (see Eig. 13). Smaller versions of these structures, too low in molecular weight to be entangled, have been synthesised and are usually referred-to as dendrimers [47]. 

---