Synthesis of Columnar Liquid Crystals
The columnar phases of discotic liquid crystals (DLCs) have been well recognized as one-dimensional organic semiconductors due to their unique self-organising supramolecular architecture. Though more than 3,000 DLCs from about 60 different cores have been
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Synthesis of Columnar Liquid Crystals Sandeep Kumar
4.1 Introduction Thermotropic liquid crystals consist primarily of two main types, i.e., rod-like, commonly known as calamitic liquid crystals and disk-like, described as discotic liquid crystals. Recently discovered banana-shaped liquid crystals are basically rod-like systems in which two calamitic units are connected to each other via a central bridge in such a way that the overall molecular shape is banana-like or bow-like. The synthetic strategies used to prepare these materials are essentially same as that used to generate rod-like liquid crystals. More than 100,000 calamitic liquid crystals have so far been synthesized using classical as well as modern synthetic methods. Most of the calamitic liquid crystalline compounds consist of two or more ring structures, bonded together directly or via linking groups. They usually have terminal hydrocarbon chains and sometimes lateral substituents as well. The typical chemical structure of these molecules can be represented by the general template as shown in Fig. 4.1, where A and B are core units such as, benzene, naphthalene, biphenyl, etc., R and R0 are flexible moieties such as, normal and/or branched alkyl chains, M and N are generally small lateral substituents e.g., –Cl, –Br, –NO2 , –CH3 , –OCH3 , –CN, etc. Y is a linking group to the core units and X & Z are linking groups of terminal chains and core units. Clearly, several permutations and combinations are possible to generate new calamitic liquid crystals and consequently over hundred thousand calamitic liquid crystals have so far been synthesized. In most cases, well-known classical organic chemistry reactions are applied to produce these materials. Basic synthetic strategies towards the synthesis of calamitic liquid crystals have been extensively covered in chapters of the “Handbook of Liquid Crystals” [1] and, therefore, the chemistry of these S. Kumar () Soft Condensed Matter Group, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560 080, India e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 4, © Springer ScienceCBusiness Media Dordrecht 2013
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A
B
Fig. 4.1 General structural template for calamitic liquid crystals where A and B are core units R and R0 are commonly flexible chains, M and N are generally small lateral substituents and X, Y, Z are linking groups
linking group
O
O O
HN
O
O
X
CORE
O
flexible chains X
Y S O
Se
Fig. 4.2 General structural template for discotic liquid crystals
materials by giving specific examples is not covered in this chapter. However, some important organic reactions e.g., carbon-carbon bond formation reactions are presented in the synthesis of discotic liquid crystals section. From a semiconducting properties point of view, discotic liquid crystals forming columnar phases are of primary importance, see Chaps. 3, 8 and 9 [2–15]. Molecules forming columnar mes
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