Smectic phase homologous series

Mandal et al. [89-91] investigated the crystal structures of three members of the homologous series of 5-(4 -n-alkylcyclohexyl)-2-(4"-cyanophenyl)-pyrimi-dines. The crystal structures of the ethyl (ECCPP), pentyl (PCCPP), and heptyl (HCCPP) compounds were determined. The chemical structure of the compounds is presented in Fig. 15. The two lower homologues possess only a nematic phase, while the heptyl compound has a smectic phase in addition to a nematic phase. 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

This density profile shows a wide maximum in its middle (z = d/2) which represents the region of mesogenic cores. Another secondary maximum (z = 0) is observed in between the sublayers of mesogenic cores. This secondary maximum arises from the backbones which have a relatively large electron density because they have silicon atoms. Therefore, the backbones appear squeezed between the sublayers of mesogenic cores so that the structure of this SmA phase may be described in terms of microphase separation . This is the situation depicted in Fig. 9a. Moreover, Fig. 10 also shows that adding aliphatic parts to the macromolecule helps the backbone to resist the confinement. Nevertheless, one should not conclude that all the SmA phases of LCPs can be described in this way. For instance, in the same homologous series, polymer P4 j shows only one order of smectic reflection so that p(z) is sinusoidal and the backbones are not confined at all (Fig. 9b). 

The narrower a range of the nematic phase in a homological series of different compounds (as an example see Fig. 6.1 la) the stronger are first order features of the N-SmA transition. In some sense, the SmA phase feels the proximity of the isotropic phase. In other words, we may say that, in the isotropic phase, there are traces of both nematic and smectic A short-range order. 

Sadashiva, B. K. Reddy, R. A. Pratibha, R. Madhusudana, N. V. Biaxial smectic A phase in homologous series of compounds composed of highly polar unsymmetrically substituted bent-core molecules. J. Mater. Chem. 2002,12, 943-950. 

It has been shown [34] that molecules containing a bent rigid core, so-caUed banana molecules, exhibit spontaneous polarization an chirality in their smectic LC phases in spite of the fact that the molecule itself is achiral. The banana molecules in a 1.3-benzene bis [4- (4- -alkoxyphenyliminomethyl) benzoate] (P -0-PIMB) homologous series, as illustrated in Fig. 1C, have a various characteristic physical properties in the LC phases, which are classified and designated as B-phases [35]. It has been reported by... 

To date, four homologous series of carborane and one series of tricarbollide mesogens have been investigated. Series 38,40, 64 were investigated up to n = 10, series 66L to n = 18, and diesters 59D were prepared to n = 22. In all series of carborane derivatives no smectic phases were detected. For instance, diester 59D[22] exhibits only a nematic phase, while in the hydrocarbon analogs the smectic phases onset at n = 5 and completely replace the nematic phase at n = 12. 

In many homologous series the lower members are nematic, the medium members nematic and smectic, and the higher members are smectic only, eventually occurring in several modifications (Figure 6). Of course, there are also series where smectic phases occur in even the lowest member, and in a few series even in the highest members only nematic phases exist. Quite exceptionally, in a very few series the lower members are smectic and only at somewhat high-... 

Lateral substituents can be in different positions of the basic molecule [195], and lateral aromatic substituents can have several substituents themselves [191, 192] or they can be alicyclic [191]. In the so-called A-shaped mesogens [196], which occur in nematic and smectic A phases, the aromatic lateral substituents, as in older examples, are bound by carboxylic groups. Matsuna-ga etal. [20] synthesized 1,2-benzene derivatives and 2,3-naphthalene derivatives, which may be considered as compounds with ring-containing lateral substituents, attached at a terminal benzene ring. Fig. 12 presents the transition temperatures of a homologous series of this type. The first compounds of this U-shaped type were already synthesized by Vorlander and Apel [ 16]. Recent investigations by Attard et al. [187] in U-shaped compounds proved the existence of bilayers in the different smectic phases. 

The silver complexes reported by Bruce et al. [289] show quite unusual polymorphism. In addition to conventional nematic and smectic phases, in several members of the series cubic phases have been observed (Figure 18). These cubic phases, in contrast to the case in other thermotrophic homologous series, exist even in members having relatively short alkyl chains. 

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