Dendrimer components

Upon immersion in acidic water (pH = 1) these unimolecular inverted micelles form vesicles in which the dendrimer component has a highly distorted conformation with an axial ratio of 1 8 for the highest generations. The amphiphilic dendrimers within the aggregates in solution are thought to have a flattened shape similar to those proposed for dendrimers at the air-water interface (section 16.2.1). 

The initiator core (CD) may be as small as an atom or as large as a molecule. It may be homogeneous with the other dendrimer components or it may contain special design features (e.g., metal atoms, chromophores, etc.) that differentiate it from the dendrimer interior thus making it heterogeneous. 

This ever-growing field continues to flourish. New supramolecular dendrimer complexes having a central [Ru(2,2 -bipy)3]2+ core and containing 12 or 24 naphthyl units seem to have a very efficient antenna effect in transmitting TJV absorption to the centre while protecting the luminescent core from quenching by dioxygen.39 For these complexes one oxidation and three one-electron reduction steps can be identified in the central core and other redox processes can be found in the outer dendrimer components.40... 

Thus, removal of the dendrimer component needs to be performed on supported DMNs to render active metal sites accessible for catalyzing gas-phase reactions. ... 

Dendrimer thermolysis, i.e., the removal of the dendrimer component via exposure to high temperatures in oxidative, reducing, or neutral gas-phase environments, is the technique that has been so far exclusively employed for this task. Other potential low-temperature removal methods, such as chemical leaching of the dendrimCT or treatment under plasma conditions, have been suggested but not demonstrated experimentally. 

Characterization of the supported metal nanoparticles following the removal of the dendrimer component has been performed in our group using carbon monoxide as a probe molecule. FTIR measurements conducted with Pt4oG40H/Si02 indicate that CO adsorption can be maximized following oxidation at 425°C for 1 h and subsequent reduction at 200°C for 1 Additionally, this treatment did not affect the mean Pt particle size, whereas exposure to nonoptimal conditions (e.g., oxidation at 425°C followed by reduction at 400°C) can result in substantial sintering. 

Dendrimer chemistry has taught us that these molecules create a nano-sized closed space that, presumably, is the origin of the specific physical properties of this class of materials. As the next stage of dendrimer chemistry, a macromolecule capable of creating such a space inside its molecule is proposed. To create the nano-sized space, porphyrin is considered to be the best candidate for the component molecules, because it has versatile properties associated with its expanded 7i-electron system and the incorporated metal. The resultant multi-detectable properties of porphyrin, that is, a number of its properties are detectable by many physical methods, may reveal the function of the nanometer-sized space. 

Rotaxane dendrimers with dendron units attached to the rod component Rotaxane dendrimers with dendron units attached to the ring component Rotaxane dendrimers with dendron units attached to both the ring and rod components... 

Dendrimers with (pseudo)rotaxane-decorated periphery (Pseudo)rotaxane-terminated dendrimers with covalently-attached rod components at the periphery... 

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. 

Fig.1. Rotaxane dendrimers from crown ether type macrocycle ring, bipyridinimn derivatives rod component and Frechet-type dendron stoppers...

As described earlier, Type II pseudorotaxane dendrimers have pseudorotaxane-like features at the periphery of dendrimers. Depending on whether the terminal units of the dendrimers serve as rod components or ring components, they can be further classified as Type II-A and Type II-B pseudorotaxane dendrimers, respectively. 

Pseudo)rotaxane-Terminated Dendrimers with Covaientiy-Attached Rod Components at the Periphery Type ii-A... 

As described earlier, we classify dendritic poiyrotaxanes in which rotaxane building units grow like a dendrimer, as Type III rotaxane dendrimers. Depending on whether ring components are located on the branches or at the branching points. Type III rotaxane dendrimers are further classified as 111-A and 111-B, respectively. 

In this review, we tried to cover all the supramolecular species that maybe classified as rotaxane dendrimers. We classified them by their structures - where in dendrimer rotaxane-hke features are introduced. Several different types of macrocycles have been employed as a ring component in the templated synthesis of rotaxane dendrimers. While the synthesis of Type I and II rotaxanes dendrimers is relatively straightforward, that of well-defined Type III rotaxane dendrimers, particularly those of second and higher generations, is still challenging. 

Also, from the dendrimer point of view, the introduction of mechanical bonds to dendrimers has an enormous potential to alter the properties of dendrimers in a controlled way. For example, in the synthesis of a Type II rotaxane dendrimers, the wheel components are introduced to the terminal groups of the dendrimers. This can improve the solubility of dendrimer in organic and/or aqueous media due to the formation of complexes soluble in such solvents. 

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. 

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