Crystal structures smectic form

Similar lamellar structures are formed for l-alkyl-3-methylimida2olium cations with [PdCl4] when n > 12. As with the pyridinium systems, mesomorphic liquid crystal structures based on the smectic A structure are formed [24]. 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

Hartung and Rapthel [64] described the crystal structure of the mesogenic 2-methylthio-5-(4 -n-butyloxyphenyl)-pyrimidine which forms a monotropic smectic A phase. The chemical structure of this compound is presented in Fig. 5. The compound crystallises in the triclinic space group PI with two molecules per unit cell. The molecules adopt a fully stretched and nearly planar form. The pyrimidine ring is nearly planar. The dihedral angle between the phenyl and the pyrimidine rings is 22.7°. The molecules are arranged parallel to each other. 

In this section we will report on the crystal structure analyses of mesogenic 2,5-diphenyl pyrimidines. The crystal structure of 5-phenyl-2-(4 -n-butoxy-phenyl)-pyrimidine (5-PBuPP) and 2-phenyl-5-(4 -n-pentoxyphenyl)-pyrimi-dine (2-PPePP) were determined by Winter et al. [83, 84]. Compound 5-PBuPP forms a monotropic nematic phase, whereas compound 2-PPePP exhibits a smectic A mesophase within a wide temperature range. The chemical structure of the mesogenic 2,5-diphenyl pyrimidines is shown in Fig. 13. 

In this section, we will present the crystal structures of chiral mesogenic compounds exhibiting ferroelectric liquid crystalline phases which are listed in Table 18 [152-166]. Moreover, four compounds of the list show antiferroelectric properties and two compounds form only orthogonal smectic phases. The general chemical structures of the investigated chiral compounds are shown in Fig. 27. 

The mesogenic structure of a benzoic acid dimer has been introduced as a noncovalent cross-linker for polysiloxanes [79]. Polymer 57 exhibits a smectic C phase due to the dynamics of H-bonding. In contrast, mesomorphic order is locked in the solid state of poly[(4-acryloyl)benzoic acid] by polymerization in its mesophase [128]. No liquid-crystalline state is observed for this material because of the lack of flexibility of the structures. Main-chain-type polymeric liquid-crystal associates are formed from carboxyl-bifunctionalized aromatic compounds [129]. 

By wide-angle X-ray diffraction, four different crystal forms have been identified (3,4). These polymorphs are referred to as the a, y and "smectic" forms and their unit cell structures have been determined to be monoclinic, hexagonal, triclinic and pseudohexagonal, respectively. The relative amounts of these phases are very sensitive to the conditions of crystallization. In the process of slow cooling and isothermal crystallization, the iPP melt crystallizes into the monoclinic, hexagonal and more rarely triclinic lattice. Quenching the melt into the liquid nitrogen produces a "smectic" phase. 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

In the chiral crystal smectic mesophases the helix is in effect unwound by the crystal structure and the form chirality is suppressed. There are also other chiral mesophases of the smectic type (e.g., TGBA and S anti phases) which will be discussed later. 

It is important to note that also nonchiral molecules are capable of forming chiral mesophases. In particular, molecules with a bent core ( bananashaped molecules) can build polar, and even chiral liquid crystal structures [75]-[78]. Bent-core molecules form a variety of new phases (B1-B7, Table 1.3) which differ from the usual smectic and columnar phases (see also Chapter 8). As a consequence of the polar arrangement, antiferroelectric-like switching was observed in the B2 phase formed by bent-core molecules, and second harmonic generation was found in both the B2 phase and the B4 phase. The latter phase is probably a solid crystal. It consists of two domains showing selective reflection with opposite handedness. In the liquid crystalline B2 phase, the effective nonlinear susceptibility can be modulated by an external dc field [79] (Figure 1.15). 

Usual illustration of the liquid crystals that form nematic (b) and smectic (c) phases upon cooling from the isotropic liquid (a) and above the crystal structure (d). 

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