Structure dendritic

Another area of interest in recent years is that of dendritic structures (dendrimers) to which P compounds have made a significant contribution. Following the discovery in 1990 of dendrimers based on phosphonium derivatives, various phosphorus compounds have now been obtained. These contain P groups which act as initiator cores or as subsequent branching points (12.254), (8.356) [102-106]. 

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Fig. 6.8. Most metals solidify with a dendritic structure. It is hard to see dendrites growing in metals but they con be seen very easily in transparent organic compounds like camphene which - because they have spherical molecules - solidify just like metals.

The scaling arguments given here for two-dimensional growth patterns can be extended formally in a straightforward fashion to three dimensions. For dendritic structures this seems to be perfectly permissible since the basic growth laws are rather similar in two and three dimensions [117,118] ... 

In the second part of the 20th century, the tantalum capacitor industry became a major consumer of tantalum powder. Electrochemically produced tantalum powder, which is characterized by an inconsistent dendrite structure, does not meet the requirements of the tantalum capacitor industry and thus has never been used for this purpose. This is the reason that current production of tantalum powder is performed by sodium reduction of potassium fluorotantalate from molten systems that also contain alkali metal halides. The development of electronics that require smaller sizes and higher capacitances drove the tantalum powder industry to the production of purer and finer powder providing a higher specific charge — CV per gram. This trend initiated the vigorous and rapid development of a sodium reduction process. 

Fig. 14. SEM images of cuprous oxide nanostructures (A) 100 x, (B) 1,000 x, and (C) Schematic illustration of the dendrite structure formation process.

Figure 2. Photographs of cellular and dendritic structures in a thin-film solidification experiment of an organic alloy (succinonitrile-acetone) reported by Ref. 6.

Metal-acetylide complexes including metal-poly(yne) polymers often show unique properties [21-23]. Thus, metal-acetylide dendrimers are of interest because amplification of the functionality due to metal-acetylide units based on three-dimensional assembly with a regular dendritic structure is expected. 

Dendrimers are complex but well-defined chemical compounds, with a treelike structure, a high degree of order, and the possibility of containing selected chemical units in predetermined sites of their structure [4]. Dendrimer chemistry is a rapidly expanding field for both basic and applicative reasons [5]. From a topological viewpoint, dendrimers contain three different regions core, branches, and surface. Luminescent units can be incorporated in different regions of a dendritic structure and can also be noncovalently hosted in the cavities of a dendrimer or associated at the dendrimer surface as schematically shown in Fig. 1 [6]. 

Quantitative analysis of the results obtained has shown that a single eosin guest is sufficient to completely quench the fluorescence of any excited dansyl unit of the hosting dendrimer. Fluorescence lifetime measurements indicated that the dye molecules can occupy two different sites (or two families of substantially different sites) in the interior of the dendritic structure. 

Remarkable concentration of the photon energy absorbed by aromatic dendritic scaffold toward the focal point (herein called antenna effects ) is one of the most distinct features of the dendritic macromolecules. Poly(benzyl ether) (PBE) dendron [1] has been studied most extensively as an efficient antenna dendron. Careful studies have revealed the crucial role of the symmetric dendritic structure for the antenna effect. 

This manuscript describes the dendritic macromolecules for optical and optoelectronic apph-cations, particularly stimulated emission, laser emission, and nonlinear optics. Dendrimers have been designed and synthesized for these applications based on simple concepts. A coreshell structure, through the encapsulation of active imits by dendritic branches, or a cone-shaped structure, through the step-by-step reactions of active imits, can provide particular benefits for the optical high-gain media and nonlinear optical materials. It also described experimental results that support the methods presented for designing and fabricating functionalized dendrimers for optoelectronic applications, and theoretical results that reveal the intermolecular electronic effect of the dendritic structure. 

NLO materials (16 and 17) (Fig. 13) have been obtained from polyurethanes by the incorporation of sidechains with boron chromophores.37 The dihydroxy ligand of an azobenzene ligand containing a dimesityl boron acceptor was reacted in a polycondensation fashion with the diisocyanate groups of the polyurethanes to yield the desired polymers. Halogen displacement and transmetallation reactions have been utilized in the development of extended ir-conjugated systems of tri-9-anthrylborane with dendritic structures.38 In one (18) (Fig. 14) of the novel compounds, three identical... 

Dendrimers containing metal complexes in the branches. In these compounds (Fig. lc), metal complexes may play the role of connectors along the branches of a dendritic structure as in the case of (tpy)Ru(tpy)2+ (tpy=2,2 6, 2"-ter-pyridine) [6], or may be attached to specific sites as in the case of cobalt carbonyls [7]. 

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