Dendrimer guest molecule

Dendrimers appear to have interiors that are, to all intents and purposes, empty and they, therefore, are able to accommodate guest molecules and also nanoparficles. Early theoretical work suggested that dendrimers develop in concentric shells, and enclose a considerable amount of empty space. More recent theoretical studies have suggested that they may not be as much free space as first thought, and this has been confirmed by X-ray diffraction studies. NMR has shown that there is a reasonable free volume within dendrimers though there is some experimental evidence that the amount of free volume varies with the thermodynamic quality of the solvent. This, in turn. 

The cavities in dendrimers are not permanent, but can be redistrubuted as the branches, which can possess considerable degrees of flexibility, move about. The inclusion of guest molecules within a dendrimer may occur as a result of movements in the branches, which allow temporary inclusion of the guest molecule within the dendrimer structure. In solution, it is assumed that molecules of solvent are able to move with ease through the branches of dendrimers, hopping between such temporary cavities with little or no hindrance. When the solvent is removed, the dendrimer may collapse to a distinctly reduced volume. 

Dendrimers can be designed to have a hydrophobic interior and a hydrophilic periphery. This gives them properties that are similar to those of conventional surfactants, and they can solubilize hydrophobic substances such as pyridine in aqueous solution by including them as guest molecules. They are therefore effectively mimolecular micelles. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

In particular, rotaxane dendrimers capable of reversible binding of ring and rod components, such as Type II, pseudorotaxane-terminated dendrimers, can be reversibly controlled by external stimuli, such as the solvent composition, temperature, and pH, to change their structure and properties. This has profound implications in diverse applications, for instance in the controlled drug release. A trapped guest molecule within a closed dendrimeric host system can be unleashed in a controlled manner by manipulating these external factors. In the type III-B rotaxane dendrimers, external stimuli can result in perturbations of the interlocked mechanical bonds. This behavior can be gainfully exploited to construct controlled molecular machines. 

Recently, Astruc et al. [189] reported novel amido-ferrocene dendrimers (e.g., 91) which were shown to act as supramolecular redox sensors for the recognition of small inorganic ions (Fig. 41). It was further observed that as the den-drimer generation number increased the sensitivity to the guest molecules also increased as observed by cyclic voltammetry experiments. 

Dendrimers can be used to effectively coat and passivate fluorescent quantum dots to make biocompatible surfaces for coupling proteins or other biomolecules. In addition, the ability of dendrimers to contain guest molecules within their three-dimensional structure also has led to the creation of dendrimer-metal nanoclusters having fluorescent properties. In both applications, dendrimers are used to envelop metal or semiconductor nanoparticles that possess fluorescent properties useful for biological detection. 

Dendrimer interior functional groups and cavities can retain guest molecules selectively, depending on the nature of the guest and the dendritic endoreceptors, the cavity size, the structure, and the chemical composition of the peripheric groups. Two main methods are known for the synthesis of metal nanoparticles inside dendrimers. The first method consists of the direct reduction of dendrimer-encapsulated metal ions (Scheme 9.4) the second method corresponds to the displacement of less-noble metal clusters with more noble elements [54]. 

Finally, do dendrimers change greatly in size when placed in different solvents For applications as size standards or molecular probes dendrimers possessing a relatively fixed size would be preferable. For applications using the release of stored guest molecules, however, it would be preferable to open and close dendrimers by designing appropriate container release strategies . 

Goddard and Tomalia [1] investigated the encapsulation of 2,4-dich-lorophenoxyacetic acid and acetylsalicylic acid into methyl ester-terminated PAM AM dendrimers by measuring the 13C spin-lattice relaxation times (7 ) of the guest molecules. In the presence of dendrimer, the 7 values of these guest... 

In the preceding section, we reviewed the non-covalent dynamic encapsulation of guest probe molecules within dendrimer interiors. The second case, Scheme 2, involves the physical encapsulation of guest molecules wherein, guest molecules are locked inside dendritic containers (so-called dendritic boxes). This concept was originally proposed by Tomalia et al and referred to as unimolecular encapsulation [2]. More recent and well characterized examples have now been demonstrated by Meijer and co-workers [11-15]. 

The encapsulation of small guest molecules was achieved by constructing the dendrimer shell in the presence of the guest molecules followed by extensive dialysis to remove free guest molecules in solution [11]. A variety of probe molecules were applied, including Rose Bengal, 7,7,8,8,-tetracyano-quino-dimethane (TCNQ) and 3-carboxy-PROXYL. UV-Vis, fluorescence and EPR... 

Figure 16.6 Schematic representation of the energy transfer process between OPV-poly(propylene imine) dendrimer and a dye guest molecule [84]...

The possibilities for encapsulating guest molecules in dendritic hosts were first proposed by Maciejewski in 1982 [143], In 1990, Tomalia presented evidence for unimolecular encapsulation of guest molecules in dendrimers and pointed out that it was one of the possible future research areas in dendrimer chemistry [164],... 

Figure 16.14 To investigate hydrogen bonding in dendrimers Zimmerman and Moore [187] prepared dendritic wedges (A-B = CH20 or C=C) functionalized with anthypyridine units at the focal point that function as hosts for benzamidinium guest molecules...

The field in which the interaction between the dendritic host and the guest molecule(s) can be classified as electrostatic is very elaborate and therefore the focus in this section will be on the interaction of organic acids with dendrimers. 

In contrast to dendrimers built up from aliphatic chains, polyphenylene dendrimer micelles possess shape and size persistent cavities due to their rigid scaffold which strongly depends on the type of dendrimer. In this case a selective incorporation of guest molecules, e.g., fluorescent dyes, should be possible, dependent on the size of the guest molecule and the cavity of the host. The non-covalent uptake of dyes with an appropriate size thus allows the investigation of their interactions within the dendritic micelle. In our case we made the second-generation polyphenylene dendrimer 48, which bears 16 carboxy-functions at the periphery, by starting from a tetrahedral core and an appropriately... 

---