Molecular crystal substrates crystalline phases

Crystollization of Secondary Crystalline Phases on Molecular Crystal Substrates. 

Liquid crystals possess physical properties which lie somewhere between those of solids and liquids cf. Section 3.1 and [725]. The rigidity which is present in a solid matrix is absent in liquid crystals, thus permitting molecular motion as well as conformational flexibility of the dissolved solute molecules. At the same time, due to the order in the liquid-crystalline phase, the randomness in motion and conformational flexibility of the dissolved solute molecules is to some extent restricted. If the structures of the solute and solvent molecules are compatible, then solute molecules can be incorporated into the liquid-crystalline phase without disrupting its order. Thus, the reactivity of substrate molecules incorporated into liquid crystals without destroying their order should be different from that in isotropic solvents. Apart from the first report on the influence of liquid crystals on chemical reactions by Svedberg in 1916 [726], the use of liquid crystals as... 

If, on the other hand, there is a stronger interaction between the substrate and the evaporated molecules than between the molecules themselves, then the molecules will tend to cover the substrate optimally, i.e. completely. Covalent bonds can also form between the molecules and the substrate, leading to changes in the molecular stmcture. One then refers to epitaxy, when the lattice parameter of the (single-crystal) substrate is commensurable with those which can exist in the crystalline phases formed by the evaporated molecules. Examples of epitaxy are shown in Figs. 2.23 and 1.9. 

It has been successfully demonstrated by the authors that macro cyclic heterocyclophanes 1-3 exhibit enzyme-like or rdceptor-like functions with remarkable substrate specificity in the solution phase Heterocyclophane 1 is of considerable interest, since 1 forms a molecular cavity inclusion crystal which is to be differentiated from the lattice (cavity) inclusion and an interesting guest-dependent polymorphism and plane chirality in the crystalline phase have been observed K... 

Epitaxy is defined as the growth of one crystalline phase (the guest crystal, here the polymer) on the surface of a crystal of another phase (the host crystal, here the substrate) in one or more strictly defined crystallographic orientations. The interactions imply a structure analogy between the two species in their contact planes, either in two, or sometimes only one direction. The details of the molecular interactions may be difficult to reach, and, as a consequence, the epitaxy is often defined in terms of a geometrical concordance of matching unit-cell dimensions the disregistries should remain < 10%. 

But what about defects The TFS-films certainly will possess defects arising from the fact that they were transferred at elevated temperature, but then cooled down to the crystalline bulk phase. Since the thermal expansion coefficients of the substrate and the film should be different, the formation of some defects, even besides the occurrence of grain boundaries due to crystallization, is to be expected. In fact cracks have been observed between very smooth areas from which molecular resolution could be obtained (57). However, these films appear perfectly transparent to the naked eye and macroscopic defects (besides dust particles) are not even visible under an optical microscope. LB-films of the same material are also initially transparent but already show some defects visible to the naked eye. After rearrangement to the monolayer phase, the defects become even more pronounced. Nevertheless, comparison of the rearranged LB-films with LB-films of... 

Another concept in synthesis is epitaxy. Epitaxy is the continuation of the crystal orientation of the monocrystalline substrate in the deposited crystalline product, which may be the same compound as the substrate or a different solid that has the same crystal orientation as the monocrystalline solid. Epitaxial layers are essential for microlithography in the electronic industry carefully formed epitaxial layers do not have localized electronic interface states, which are deleterious for the functioning of the device. The process conditions for epitaxy by molecular beam epitaxy (MBE) are very low process pressure, comparatively high temperatures, and a low growth rate. MBE is a form of CVD, which was described in Chapter 6. Liquid phase epitaxy (LPE) is a form of growth of single crystals from a melt. 

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