Structural Perfection

There is a very short list of organic semiconductors with reported thin-film field-effect mobilities greater than 1 cm2 V-1 s h These include pentacene, sexithio-phene [5a], and anthradithiophene [17]. If we extend this list to include single crystal and n-type materials, we can add perylene [18], rubrene [19], copper phtha-locyanine (CuPc) [20], tetracyanoquinodimethane (TCNQ) [21], and dithiophene-tetrathiofulvalene (DT-TTF) [22] - still a short list. 

What is interesting about all of these materials is the overlying similarity in their 

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The first ladder polymer with a high degree of structural perfection was reported in 1960 and was prepared by the equilibrium condensation of phenyltrichlorosilane Figure 29.19). 

Using a mononuclear tetramine a polycondensation very similar to that used with BBB can occur which leads to a ladder polymer of high structural perfection (BBL) Figure 29.20). 

This Topics volume also shows that in the development of dendrimer chemistry there is still a need for efficient synthetic methods ensuring multiple high-yield conversion and leading to more or less pure (monodisperse, structurally perfect) dendritic molecules. Large libraries of imperfect substances are often formed at higher generations, only differing in small structural details. 

In 1978 we described a synthetic methodology as a repeating-step principle , which led us to the first cascade molecules , today known as dendritic molecules [17]. We recognized then that a synthetic pathway, which allows consecutive repetition, implies the advantage of likewise reactants and reaction conditions and is suited for the building of more or less structure perfect highly branched molecules, particularly of polyamines (Fig. 4). 

In order to enhance the understanding of the properties in polymers, iterative pathways have been chosen for the synthesis of structurally perfect molecules. Data obtained from the analysis of precisely defined oligomers and polymers may relate chain length and conformation to physical, electronic and optical properties. Statistical polymerization processes are not suitable as they yield polydisperse material. 

The advantage of iterative strategies is based on the specific preparation of well defined structures and structurally perfect spacers of nanometer scale. This stepwise approach yields monodisperse material in contrast to other statistical routes. The use of the same reactants and the conversion of the same functional groups facilitates the synthetic effort compared with non-iterative methods. 

A unit, or perfect, dislocation is defined by a Burgers vector which regenerates the structure perfectly after passage along the slip plane. The dislocations defined above with respect to a simple cubic structure are perfect dislocations. Clearly, then, a unit dislocation is defined in terms of the crystal structure of the host crystal. Thus, there is no definition of a unit dislocation that applies across all structures, unlike the definitions of point defects, which generally can be given in terms of any structure. 

Dendritic polymers, the fourth major architectural class of macromolecules, can be divided into three subclasses. These subclasses may be visualized according to the degree of structural perfection attained, namely (1) hyperbranched polymers (statistical structures, Chapter 7), (2) dendrigraft polymers (semi-controlled structures, reviewed in this chapter) and (3) dendrimers (controlled structures, Chapter 1). 

To characterize dendrimers, analytical methods used in synthetic organic chemistry as well as in macromolecular chemistry can be applied. Mass spectrometry and NMR spectroscopy are especially useful tools to estimate purity and structural perfection. To get an idea of the size of dendrimers, direct visualization methods such as atomic force microscopy (AFM) and transmission electron microscopy (TEM), or indirect methods such as size exclusion chromatography (SEC) or viscosimetry, are valuable. Computer aided simulation also became a very useful tool not only for the simulation of the geometry of a distinct molecule, but also for the estimation of the dynamics in a dendritic system, especially concerning mobility, shape-persistence, and end-group disposition. 

Over the past 40 years a great deal of research has been done on the nucleation and growth of fibers for industrial uses. The predominant industrial fiber is a silica-based glass. Whiskers, with a high degree of internal structural perfection, have been produced under a variety of special conditions from an extraordinarily wide range of compounds. 

Purification of Silicon. Chemical purity plays an equally important role in the bulk of materials as on the surface. To approach the goal of absolute structural perfection and chemical purity, semiconductor Si is purified by the distillation of trichlorosilane [i0025-78-2], SiHCl, followed by chemical vapor deposition (CVD) of bulk polycrystalline silicon. 

The polydispersity of dendritic molecules, expressed in the form of their poly-dispersity index (PDI), is directly related to their structural perfection. The PDI is a measure of molecular weight distribution. 

If the polydispersity index (PDI) has a value of unity, the substance is designated as monodisperse. Monodispersity is considered to be a property of the casca-danes (defect-free dendritic molecules cf. Section 1.4) and almost perfect dendrimers. Since these compounds are synthesised via an iterative approach, monodispersity has so far generally been limited to lower generations. Should it prove possible to repeatedly remove all reactants and by-products of the individual synthetic steps during the construction of a dendrimer, then structurally perfect dendrimers will result. 

One disadvantage of this synthetic methodology is seen in the exponentially increasing number of functional terminal groups (KxMn see Section 1.2), since they cannot always be made to react quantitatively and thus give rise to structural defects. Such defects cannot always be avoided, even on addition of large excesses of reactants. Moreover, purification and separation of structurally perfect from defective dendrimers are problematic because the compounds have very similar properties. 

While not so structurally perfect (see Sections 1.3 and 1.4) as their dendritic relatives, these hyperbranched dendritic compounds (see Section 2.7) can be produced faster and at less expense, making them especially interesting for the materials sector. The various commercially available monomers (Fig. 4.51) offer great structural versatility. 

When assessing the purity of dendrimers it should also be noted that NMR spectroscopic methods approach their limit of detection at contamination levels of ca. 5%. Additional chromatographic method such as gel permeation chromatography (GPC, SEC Section 7.1.2) or mass spectrometric methods (MALDI-MS, ESI-MS), as presented in Section 7.4, should also be employed in verification of structural perfection and purity of dendrimers. 

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