Laterally substituted liquid crystals

Fig. 2. The generic molecular structure of calamitic liquid crystals illustrating the semi-rigid core fragments, the positions of the end-groups (C and A), linking groups (B) and, possibly, laterally substituted groups (L)...

FIGURE 17. Top Lateral substituted biphenyls, bottom chain branched cinnamoyl liquid crystals... 

Fig. 19 Gold nanoparticles 13 and 14 capped with a mixed monolayer of hexanethiol (13) or dedecanethiol (14) and a lateral substituted nematic liquid crystal thiol attached in a side-on fashion (thiol ratio 1 1) [540, 541]...

In this case, lateral substitution plays an important role in weakening hydrogen bonding and significantly depresses the melting temperature of the compound. As a result, the potential of this type of rod-like molecule to form liquid crystal phases is revealed. In Table 3.3 are shown the liquid crystalline property of a few of the 2-bromo-l,4-bis(4-alkoxybenzamido)benzene. 

It is also interesting to mention that the compound (3.11) is the first published example that has lateral but no terminal substitutions and yet forms a liquid crystal phase. By and large, the studies on substitution effect have shown that while the terminal substitutions are favorable to the thermal stability and the formation of a liquid crystal phase, the lateral substitutions are not. The larger the lateral substitution, the more harm it can do to the thermal stability of the liquid crystalline phase. A typical and very convincing example for this idea is given by homologues of the well known para-substituted azoxyanisole (3.12) ... 

Porphyrins substituted with a guanine side chain interact in aprotic solvents or liquid crystals with cytosine-substituted quinones or another porphyrin to form a 1 1 lateral heterodimeric system (Scheme 6.6.2). Such pairs showed photoinduced charge separations with lifetimes of a few microseconds (Sessler et al, 1995). 

Cho and Lim compared the effect of a variety of lateral substituents on the thermal behavior of peripherally octasubstituted, metal-free phthalocyanines and their copper complexes. Despite being an interesting study, the characterization of the mesophases was tentatively made by optical microscopy and thus some doubts concerning mesophase identification still persist. The results of this study are gathered in Table 8 along with some metal-free and copper complexes discussed above to allow comparison. The free base with a chiral chain exhibited a texture resembling that of the cholesteric phase, whereas that of the copper complex was not identified. Compounds substituted with chiral chains were room-temperature liquid crystals, whatever the length and number of asymmetric carbon atoms, and the columns described a helical twist. ... 

Results were also described from studies which introduced lateral substituents into these complexes and in one paper the effect of introducing lateral alkanoate groups was reported (113). " The use of lateral alkanoates had been described previously by Maitlis and co-workers in palladium complexes of 4-alkoxystilbazoles (Section 7.9.14.3.6(iv)) and had been shown to induce the formation of liquid crystal nematic phases. In the present situation where the complex was polycatenar, it was not clear whether the chains would act to increase the volume of the core of the polycatenar mesogen or simply as extra chains, so rendering the complex more classically discotic. In fact, the answer was neither and at all chain lengths (n and m) and degrees of substitution (R = H or OC H2 +i) where mesomorphism was observed, a nematic phase was seen at or near room temperature. This behavior was somewhat puzzling and is so far without explanation. 

Small molecule liquid crystals suffer a lowering of the mesophase-isotropic transition temperature upon lateral substitution, i,e. replacement of hydrogen on an aromatic ring. As can be seen from the compounds below the same trend is observed for both polymer and the chemically analogous small molecule. It is known that for small molecule mesogens the transition temperature, governed by the relation Tni - A H/AS, decreases upon lateral substitution primarily due to an entropic effect. The... 

Wittmann JC, Lotz B (1985) Polymer decoration the orientation of polymer folds as revealed by the crystallization of polymer vapors. J Polym Sci, Polym Phys Ed 23 205-211 Wunderlich B, Grebo wig J (1984) Thermotropic mesophases and mesophase transitions of linear, flexible macromolecules. Adv Polym Sci 60/61 1-59 Zhou QF, Li HM, Feng XD (1987) Synthesis of liquid-crystalline polyacrylates with laterally substituted mesogens. Macromolecules 20 233-234... 

As described in Chapter 3, lateral substitution is often employed in liquid crystal systems in order that their mesomorphic and physical properties can be tailored to the intended application. The synthesis of materials with lateral substituents must be approached with... 

Kelly, S. M. Schad, H. Some novel nematic liquid crystals of negative dielectric anisotropy. Mol. Cryst. Liq. Cryst. 1984,110, 239-261 Gray, G. W. Kelly, S. M. Laterally substituted 4-n-alkylphenyl 4-alkylbi-cyclo[2.2.2]octane-l-carboxylates. Mol. Cryst. Liq. Cryst. 1981, 75, 109-119. Gray, G. W. Influence of composition and structure on the liquid crystals formedby non-amphiphilic systems, in Liquid Crystals and Plastic Crystals, G. W. Gray and P. A. Winsor, Eds. Wiley Sons, New York, 1974, Vol. 1 p. 125-135. 

As discussed in Sec. 2.1, the prototype of the rod-like liquid crystal molecule (1) consists of a rigid core substituted with terminal flexible substituents, and eventually small lateral substituents. In the last few years in particular many mesomorphic compounds that do not correspond to this formula have been synthesized. Such materials may be called unconventional liquid crystalline compounds [1,2]. 

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