Research at CMMPEMaterialsIntroduction to liquid crystals (page 3 of 4)

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4. Chemistry of Liquid Crystalline Molecules

In general, thermotropic liquid crystals have a similar molecular design; consisting of a rigid part (commonly derived from aromatic rings), a flexible region (accomplished by long hydrocarbon chain(s)) and a polar group(s). This is exemplified by 4’-hexyl-4-cyanobiphenyl (Figure X), where the anisotropic geometry allows for preferential alignment along one spatial direction.

Figure X: An illustration of the different regions of a typical liquid crystal molecule.

Aromatic units are often linked to one another via units containing multiple bonds, such as -(CH=N)-, -N=N-, -(CH=CH)n-, -CH=N-N=CH-, etc. because they restrict the freedom of rotation and rigidify at that portion of the molecule. These groups can conjugate with phenyl rings, enhancing the polarisability. This increases the molecular length and maintains the rigidity. (Depicted figure X illustrates further details)

Figure X: A detailed retro-dissection of a typical liquid crystal molecule.

Liquid crystalline molecules are generally further distinguished with respect to the molecular shape of the constituent molecules as illustrated in Figure X. In terms of broad classifications, there are three types of thermotropic liquid crystals based on their molecular shape;

• Calamitic (rod-like) – these from the vast majority of thermotropic mesogens,
• Discotic (disk-like) and
• Sanidic (brick- or lath-like).

Figure X: Schematic representation of the different types of liquid crystals.

Rod-shaped molecules have an elongated, anisotropic geometry, which allows for preferential alignment along one spatial direction. Examples include X – X.

X Cr 24.0 °C N 35.0 °C I

X Cr 40.0 °C N 108.0 °C I

X Cr 88 °C N 172 °C I

Discotic molecules are flat disc-like molecules consisting of a core of fused aromatic rings as exemplified in structures X and X. This allows for two dimensional columnar ordering. This class of mesogen was not known prior to 1977 when it was discovered independently by Chandrasekhar and Billard.

X Cr 98.2 °C ND 131.21 °C I

X Cr 67 °C Dho 260 °C I

The term “sanidic” is derived from the Greek word for “board”. The majority of examples of this type of liquid crystal are in the form of polymers, however non-polymeric examples, such as X, exist. In this case, the side chain on the central aromatic ring essentially enlarges the “breadth” of the molecule, greatly restricting rotation about the long axis, altering its behaviour from that of a rod-like molecule to that of a planar type unit.

X Cr 56.2 °C N 69.2 °C I

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