3. Dendritic Architectures
Some degree of success had been accomplished in the functionalisation of dendrimers for particular purposes. Boron clusters have been introduced[1] into the internal regions of one dendrimer, and a polyether dendrimer was reacted in order to modulate the polarity of the surface groups.[2] Perhaps the most striking example of the post-synthesis modification of a dendrimer is the demonstration of the 'dendritic box' (Figure 4) by Meijer.[3] It was found that a 'shell' could be formed on the surface of a poly(propylene imine) dendrimer by reacting the terminal amines with bulky amino acids. This shell was used for the size-specific entrapment of guest molecules within the dendritic structure, and could later be removed to release the guest.
Although the functionalisation of ready-made dendrimers has some attractions, it could be considered a little clumsy. If accurate control over molecular structure is desirable, then a more versatile principle for the synthesis of involved architectures is needed. In particular, one might aim to incorporate all the aspects of the target material into the dendrimer synthesis.
3.1. Application-Directed Building Bocks
Dendrimers are constructed from branching units and a core. In the design of an application-based dendrimer, certain moieties are required within the structure. The two goals of dendrimer construction and incorporation of functionality can be attacked simultaneously by the use of 'application-based monomers'[4] or functional cores.[5] There are many examples of dendrimers whose constitutions have been designed with a purpose in mind. Porphyrin cores have been introduced into dendrimers by both convergent[6],[7],[8] and divergent[9] routes. These structures have been used to examine the effect of the dendritic microenvironment on the selectivity of catalysis by the porphyrins and on the electrochemical behaviour of the porphyrin. A porphyrin-cored third generation dendrimer was synthesised by Diederich[65] as a model for cytochrome C (Figure 5). The microenvironmental effect of the electron-rich dendrons caused reduction potentials to move by up to -300 mV. Furthermore, irreversible multielectron transfers involving the dendritic branches cause the reduction waves to merge and lose definition. Functional cores have also been used to study the effect of microenvironment on chirality[10] and solvatochromic effects.[11]
The design of dendritic monomers with the applications of the final structures in mind has been the inspiration for a wide variety of building blocks, ranging from simple chiral units[12,13] to liquid crystalline moieties[14] as well as large aza-crown containing units49 for metal complexation. Balzani[35] demonstrated the synthesis of dendritic polynuclear metal complexes with ruthenium atoms acting as branching points. The luminescence and redox properties of the dendrimers were studied in an investigation of the dendrimer as a potential light harvesting[15] or directional energy transfer unit. In another dendrimer,[16] phosphorus atoms were imaginatively built into the structure such that triphosphine ligands were present as a product of any two adjacent generations (Figure 6). Palladium complexes of these dendrimers were prepared for the investigation of catalysis. Their activity towards the electrochemical reduction of CO2 was not found to be enhanced by any dendritic microenvironmental effects.
Application-oriented groups can also be built in as surface groups at the beginning of convergent syntheses or at the end of a divergent scheme. Chemically interesting groups such as tetrathiafulvalene[17] (TTF), cholesterol,[18] tryptophan[19] and various saccharides[52,20,21] have been successfully incorporated onto the surface of dendrimers, as have complexes of ruthenium[22] and nickel.[23] A particularly unexpected result reflecting the unusual behaviour of dendrimers comes from the coating of the dendrimer surface with chiral amino acid residues.[24] In some cases, the optical rotation of large surface-coated dendrimers is found to be extremely small. It is postulated that this effect arises from the immobility of the chiral units as a consequence of severe surface crowding.
3.2. The Exact Positioning of Building Blocks
The potential of dendrimers in the fields of host-guest chemistry and nanotechnology[25] relies on the subtle engineering of their architectures to pre-defined designs. In order to synthesise structures to order, methodology must allow the selective incorporation of functionality within the dendritic structure. In a series of publications, Fréchet has shown how the convergent approach can be utilised in the control of surface[26] and internal[27] functionality. By the reaction of a wedge with a large excess of monomer or with a monoprotected monomer, a half-reacted branching unit can be generated. Another wedge can be attached to the other branch in a further step, giving a branching unit with different functionality on each branch (Figure 7). This technique has been used successfully in the synthesis of mono-surface functionalised and internally functionalised dendrimers. A similar method has been used by Meijer[28] to attach four different wedges to a pentaerythritol-derived core. The chiral product that results has still to be resolved into its enantiomerically pure forms.
If one is to have ultimate control over the construction of dendrimers, not only must the methodology for synthesis be highly developed, but a variety of compatible building blocks has also to be identified. The linking of many compatible subunits into highly defined structures is one of the great strengths that living systems have developed. At present, lack of compatibility83 has been the biggest stumbling block to the production of a range of 'segment-block'[29] and 'layer-block'[30] architectures.
3.3. Dendrimers as Macroscopic Building Blocks
Dendritic oligomers may be used as building blocks for the construction of even larger assemblies. Many of the research groups interested in dendrimers approach the subject from the viewpoint of a polymer chemist. This background has provided the inspiration for the synthesis of various pendant,[31] star,[32] and other[33] copolymers (Figure 8).
The polyether wedges developed by Fréchet have found wide use as components of larger systems. Apart from the surface and core units that have been used in conjunction with these dendrons, rotaxanes[34] and even C60[35] have benefited from the attachment of dendritic moieties. The Fréchet wedges found particularly interesting use when Zimmerman[36] demonstrated the self-assembly[37] of a hexameric aggregate of dendrons terminated with isophthalic acid moieties at their focal points (Figure 9). This aggregate was stable to GPC analysis and can be predicted by molecular modelling.
Self-assembly comes into play again when considering the aggregation of dumbbell-shaped molecules synthesised by Newkome.[18,38] Molecules composed of two hydrophilic wedges attached to either end of hydrophobic spacers were found to form rod-shaped assemblies in polar solvents, with diameters of 36-40 Ĺ. It is proposed that the spacers stack on top of each other, with the hydrophilic branches forming a sheath around them.