Template superstructure

Certain azacrown-appended cholesterol derivatives can form unique vesicular or lamellar structures in the absence and the presence of metal salts in aqueous solution.187-190 These superstructures created from the azacrown-appended cholesterol derivatives are useful as a template for the transcription into the silica structure. Azacrown-appended cholesterol gelator creates the novel multilayered spherical structure in acetic acid, whereas this gelator results in the fluffy globular aggregates in the presence of Pd(N03)2.191... 

X-ray structure analysis showed that each of the cavities in the chabazite-type host structure contains two disordered SDA molecules. The lack of a superstructure suggests a random layered arrangement of the template molecules (Figure 2) similar to the crystal structure of the related compound CAL-1 [5],... 

Further studies by the same authors have demonstrated that PFj acts a hydrogen bond-acceptor template in the assembly of several other interwoven structures. In an extensive study aimed at using a combination of hydrogenbonding motifs to self-assemble pseudorotaxanes into more complex structures it was discovered that PFj assists on the organization of the components that yield the final superstructure [74]. Particularly, it was found that this anion dictates the orientation of the two carboxylic acid groups of the [3]pseudorotaxanes 58 and 59 (see Schemes 27 and 28) when these groups are co-directional with respect to each other the formation of discrete hydrogen-bonded dimers is observed. 

The last example we would like to discuss is a lattice of holes formed in stoichiometric hexagonal (h) BN double layers on Rh(lll), see Fig. 5(c) and [99]. The lattice is composed of holes in the BN-bilayer with a diameter of 24 2 A, and an average distance of 32 2 A. The holes in the upper layer are offset with respect to the smaller holes in the lower layer. We note that well-ordered superstructures with a large period have already been observed some time ago by means of LEED for borazine adsorption onto Re(0001) [102], while borazine adsorption onto other close-packed metal surfaces, such as Pt(lll), Pd(lll), and Ni(lll), leads to the self-limiting growth of commensurate ABN monolayers [103,104]. For BN/Rh(lll) it is not clear at present whether the Rh(lll) substrate is exposed at the bottom of the holes. If this was the case the surface would not only be periodic in morphology but also in chemistry, and therefore would constitute a very useful template for the growth of ordered superlattices of metals, semiconductors, and molecules. 

The synthesis of interlocked molecules has become commonplace over the past 25 years with the gradual development of a number of highly facile template methods for their construction. What were once laboratory curiosities have now taken a prominent place in the broad field of supramolecular chemistry, especially regarding their uses and further potential as molecular switches and machines [1], We present here an overview of the main synthetic approaches to these molecules, with a focus on methods in which macrocyclization reactions result in interlocked products. The analysis is by no means meant to be comprehensive or exhaustive in detail, but rather to convey the variety and utility of the selected synthetic strategies in generating abiotic rotaxane and catenane superstructures. 

One of the first examples of mesoscopic-macroscopic two-dimensional ordering within a structure involved a bacterial superstructure formed from the co-aligned multicellular filaments of Bacillus subtilis that was used to template macroporous fibers of either amorphous or ordered mesoporous silica [82], The interfilament space was mineralized with mesoporous silica and, following removal of the organic, a macroporous framework with 0.5 pm wide channels remained. Mesoporous silica channel walls in this hierarchical structure were curved and approximately 100 nm in thickness. Dense, amorphous walls were obtained by replacing the surfactant-silicate synthesis mixture with a silica sol solution. The difference in the mode of formation between porous and non-porous wall structures was explained in terms of assembly from close-packed mesoporous silica coated bacterial filaments in the former compared to consolidation of silica nanoparticles within interfilament voids in the latter. 

Mann[170] showed how a bacterial superstructure, consisting of a thread of coaligned multicellular filaments of Bacillus subtilis, can be used as template to make macroporous... 

After having exploited the use of reconstructed metal surfaces and vicinal surfaces as templates we will now turn to metal films. Since it has been shown that the nanopatterns of the above mentioned surfaces are in many cases excellent templates for overlayer growth the same can be expected for nanostructured metal films. Indeed a number of such systems have been investigated in terms of their potential use as templates. As the first example we refer to the homoepitaxial growth of Ag on the reconstructed 2 ML thick Ag film on Pt(l 11) (see Fig. 10). Already in 1995 Brune et al. were able to show that further Ag deposition at 100 K on this specific surface leads to the ordered growth of Ag islands [169,170]. Later it was reasoned that the ordering occurs due to the confined nucleation of adatoms within the superstructure cells of the periodic surface dislocation network [171]. The same effect is also present for the deposition of mass select Ag7 clusters [172] and Fe film growth on 2 ML Cu on Pt(l 1 1) [170]. 

This work demonstrates the influence of kinetics of growth, which can be used to form various surface phase and to control the silicide formation. Magnesium silicide at room temperature and its strained ((2/3) 3x(2/3) 3)-7 30° surface phase can serve as useful templates to form novel superstructures. 

PANI tower-shaped hierarchical nanostructures were prepared by a limited hydrothermal reaction [432]. The growth process is quite different from that of traditional inorganic layer-like superstructures. The conductivity of the superstructures was about 10 to 10 S cm. Fabrication of spherical and cubic PANI shells with hierarchical nanostructures, by using Mn02 hollow hierarchical nanostructures with different morphologies as reactive templates in a controlled manner, has also been reported [433]. 

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