Arborols

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. 

Fig. 20. Self-assembling arborols reported by Newkome and Jorgenson... 

Dendrimers, or arborols, or cascade, or cauliflower, or starburst polymers, were first synthesized in the early 1980s [3,4]. In 1985 Tomalia et al. [5] and Newkome et al. [6] presented the first papers dealing with dendrimers. A multitude of dendrimers have been presented in the literature ranging from polyami-doamine [7,8],poly(propylene imine) [9,10], aromatic polyethers [11-13] and polyesters [14, 15], aUphatic polyethers [16] and polyesters [17], polyalkane [18-19], polyphenylene [20], polysilane [21] to phosphorus [22] dendrimers. Combinations of different monomers as well as architectural modifications have also been presented. For example, chirality has been incorporated in dendrimers [23,24]. Copolymers of linear blocks with dendrimer segments (dendrons) [25-27] and block-copolymers of different dendrons have been described [28]. 

Arborol Dendrimers. The spherical topology of dendrimers resembles the size and shape of the Hartley model (1936) of a micellar aggregate formed by surfactant molecules (Fig. 11.2 Tomalia et al. 1990). Micellar stmctuies have a dynamic, spherical structure possessing a close-packed, solvent-incompatible core surrounded by an open, solvent-compatible layer. 

Figure 11.3 Chemical structures of the (a) arborol dendrimer and (b) water-soluble, allhydrocarbon dendrimer.

In contrast, neutral hydroxy-terminated dendrimers generally exhibit hydrodynamic volumes that are invariant with pH (Young et al. 1994). The stmcture of carboxyl-terminated arborol dendrimers has been extensively characterized by... 

Figure 11.29 (a) Structure of Newkome s two-directional arboral molecule, (b) Negatively stained transmission electron micrograph of the arborol gel. (c) Schematic representation of the formation of a long fibrous rod structure via orthogonal stacking of the arboral molecules. Reprinted from Newkome et al. (1990). Copyright 1990 American Chemical Society. 

Newkome GR, Yao Z, Baker GR, Gupta VK. Micelles. Part 1. Cascade molecules a new approach to micelles. A [27]-arborol. J Org Chem 1985 50 2003-2004. 

In this context, much attention is currently devoted to the preparation of highly branched tree-like species, variously called cascade molecules, arborols, or dendrimers. The reasons why such compounds are interesting from a fundamental viewpoint and promising for a variety of applications have been reviewed and highlighted by several authors. Many of the dendrimers obtained so far are organic in nature. This review describes a novel family of dendrimers containing metal ions, prepared in our laboratories over the last few years. 

Figure 23. Structure of a compound made by a metal-complex core and appended organic arborols/ ...

An alternative approach to the construction of a light-harvesting antenna system is to synthesize organic arborols containing a metal chelating site. With these... 

J0rgensen et al. have synthesized the bis(arborol)-TTF derivative 46 (presumed to be a mixture of 4,4 - and 4,5 -isomers, with respect to the position of substituents... 

Astruc and co-workers were the first to report divergent a[ roaches to the construction of organometallic molecular trees. A representative example of their exciting synthetic strategy for the synthesis of multimetallic arborols is illustrated in Scheme 8. The growing step is based on the fact that the n-com-plexation of aromatic compounds by electron-withdrawing cationic organo-transi-... 

Polymers have provided an alternative approach to self assembly. Particularly exciting are the recently developed hyperbranched dendrimers [383] and arborols [384] (32), both of which can be synthesized in a relatively simple step-wise manner. Pairs of arborols, connected with proper-length spacers, have been found to self assemble to form gels and long fibrous rods similar to those formed from chiral aldonamides [384],... 

Another approach to controlled molecular morphogenesis is provided by the generation of globular molecules such as the starburst dendrimers and the arborols , based on highly branched structures formed via cascade processes and growing from a central core [7.55-7.60]. Most such molecular scaffolding hase been produced by repetitive processes the use of sequences of different reactions is expected to give access to an even richer variety of non-repetitive branched architectures. 

Numerous metallomacrocycles self-assemble from their components giving species possessing various structures, for instance triangular shape [9.30,9.31] containing a cavity [9.32], which may include a guest molecule [9.33] square [9.34,9.35] or star like [9.36-9.39] shapes wheel-shaped or toroidal hexameric [9.40], octameric [9.41] or decameric [9.42,9.43] structures square [9.44,9.45] rectangular [9.46,9.47] or bent [9.48] boxes into which substrate molecules may bind [9.49,9.50] adaman-tanoid shape [9.51,9.52] with cation inclusion [9.51c], formally related to that of the spheroidal macrotricycle 21 catenane type [9.53]. Coordination species of dendri-mer or arborol nature have been constructed [7.61, 8.27, 9.54]. 

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