Covalent dendritic ordering

As envisioned, covalent dendritic ordering can be realized by using secondary (embedded or latent) protection—deprotection schemes in concert with those already developed for dendrimer construction. For example, consider the preparation of a tetrahedrally-based dendrimer 31 possessing internal sites of attachment whereby three sites are protected and one is available for connection additional peripheral and internal functionalities are inert to the chosen attachment and deprotection conditions. Furthermore, consider the connection of two of these dendrimers to afford a bisdendrimer 32. Deprotection of the internal moieties (in no case N02 reduction) allows further dendrimer attachment, etc. (Scheme 9.4). 

Controlling the size, shape and ordering of synthetic organic materials at the macromolecular and supramolecular levels is an important objective in chemistry. Such control may be used to improve specific advanced material properties. Initial efforts to control dendrimer shapes involved the use of appropriately shaped core templates upon which to amplify dendritic shells to produce either dendrimer spheroids or cylinders (rods). The first examples of covalent dendrimer rods were reported by Tomalia et al. [43] and Schluter et al. [44], These examples involved the reiterative growth of dendritic shells around a preformed linear polymeric backbone or the polymerization of a dendronized monomer to produce cylinders possessing substantial aspect ratios (i.e. 15-100) as observed by TEM and AFM. These architectural copolymers consisting of linear random... 

All ordered and random dendritic networks that are constructed via covalent or non-covalent means result from positioning of one dendrimer relative to another. Thus, macroassembly positioning can be effected via at least one of three different methods of connectivity. These methods are geometrically rooted in dendritic chemistry. 

The cavities and voids inside dendritic structures are of great importance, particularly in the study of supramolecular systems. The nature of this void, how it is affected by dendrimer size, constitution and solvent are of supreme importance in relation to potential applications [15], It is therefore necessary to discover ways to characterize the microenvironment in the dendrimer. Many investigators have made use of functional probes in order to study dendritic microenvironments [56,57], These probes are either attached covalently to the dendrimer or introduced as guest species and the effects of solvent and dendrimer size on the microenvironment are then studied. 

Polytopic nitrogen ligands such as tripyridyl-triazine, non-covalently finked multi-pyridyl ligands via bipy-metal interactions or buUt up through dendritic cores, were used for the formation of higher-ordered architectures. Several examples shall be presented here to demonstrate the utility of these poly topic ligands for the assembly of metalloporphyrins. 

It is therefore of little surprise that attention has recently begun to focus on the potential of dendritic systems to assemble into nanoscale arrays which exhibit gel-phase materials behaviour. As outlined above, in order to generate tunable gel-phase materials, it is necessary to understand and modulate the molecular recognition event, i.e. the specific non-covalent interactions... 

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