Liquid Crystals

Chapter 1.1.1 - Chemical Structures

Figure 1.1 shows the basic structures of the most commonly occurring liquid crystal
molecules. They are aromatic, and, if they contain benzene rings, they are often
referred to as benzene derivatives. In general, aromatic liquid crystal molecules such
as those shown in Figure 1.1 comprise a side chain R, two or more aromatic rings A
and A', connected by linkage groups X and Y, and at the other end connected to a terminal
group R'.

Examples of side-chain and terminal groups are alkyl (CnH2n+1), alkoxy (CnH2n+1O),
and others such as acyloxyl, alkylcarbonate, alkoxycarbonyl, and the nitro and cyano
groups. The Xs of the linkage groups are simple bonds or groups such as stilbene

(-CH=CH-), ester (
), tolane (-C≡C-), azoxy (-N=N-), Schiff base
(-CH=N0), acetylene (-C≡C-), and diacetylene (-C≡C-C≡C-). The names
of liquid crystals are often fashioned after the linkage group (e.g., Schiff-base liquid
crystal).

Figues 1.1 - 1.4

There are quite a number of aromatic rings. These include saturated cyclohexane
or unsaturated phenyl, biphenyl, and terphenyl in various combinations.

The majority of liquid crystals are benzene derivatives mentioned previously. The
rest include heterocyclics, organometallics, sterols, and some organic salts or fatty
acids. Their typical structures are shown in Figures 1.2–1.4.

Heterocyclic liquid crystals are similar in structure to benzene derivatives, with
one or more of the benzene rings replaced by a pyridine, pyrimidine, or other similar
groups. Cholesterol derivatives are the most common chemical compounds that
exhibit the cholesteric (or chiral nematic) phase of liquid crystals. Organometallic

Figure 1.5. Molecular structure of pentylcyanobiphenyl (5CB).

compounds are special in that they contain metallic atoms and possess interesting
dynamical and magneto-optical properties.4

All the physical and optical properties of liquid crystals are governed by the properties
of these constituent groups and how they are chemically synthesized together.
Dielectric constants, elastic constants, viscosities, absorption spectra, transition temperatures,
existence of mesophases, anisotropies, and optical nonlinearities are all
consequences of how these molecules are engineered. Since these molecules are
quite large and anisotropic, and therefore very complex, it is practically impossible
to treat all the possible variations in the molecular architecture and the resulting
changes in the physical properties. Nevertheless, there are some generally applicable
observations on the dependence of the physical properties on the molecular constituents.
These will be highlighted in the appropriate sections.

The chemical stability of liquid crystals depends very much on the central linkage
group. Schiff-base liquid crystals are usually quite unstable. Ester, azo, and azoxy
compounds are more stable, but are also quite susceptible to moisture, temperature
change, and ultraviolet (UV) radiation. Compounds without a central linkage group
are among the most stable liquid crystals ever synthesized. The most widely studied
one is pentylcyanobiphenyl (5CB), whose structure is shown in Figure 1.5. Other
compounds such as pyrimide and phenylcyclohexane are also quite stable.

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