Halogen-bonded Liquid Crystals - Halogen Bonding: Fundamentals and Applications - page 161

It is the author's view that a chapter such as this needs some sort of con-
text and needs to be self-contained. From this point of view, it will begin with
a general introduction to liquid crystals themselves and will then introduce
hydrogen -bonded liquid crystals in order to provide some context. The intro-
duction to liquid crystals will not be referenced or illustrated heavily, so those
readers requiring more information are directed to reference [1].
2
An Introduction to Thermotropic Liquid Crystals
The liquid crystal state represents the fourth state of matter and exists be-
tween the solid and liquid states, which form its boundaries. The liquid
crystal state is reached from the solid state either by the action of temperature
( thermotropic liquid crystals) or of solvent ( lyotropic liquid crystals) and it is
the former that will be the subject of this chapter.
Being bordered by the solid and liquid states, the liquid crystal state has
some of the order of a solid, combined with the fluidity of a liquid. As such, it
is an anisotropic fluid and it is this anisotropy that has led to the widespread
application of liquid crystals.
Theideaofanisotropyinliquidcrystalsisanimportantoneandindeed,
it is shape anisotropy that distinguishes liquid-crystalline molecules (meso-
gens) from those that are not liquid crystalline. In general, mesogens have
one axis that is very different in dimensions to the other two, the most com-
mon examples being rod-shaped molecules (one long and two short axes)
and disc-like molecules (one short and two long axes). This shape anisotropy
leads to additional anisotropic dispersion forces between the molecules and
these are sufficiently strong to stabilise phases intermediate in order between
the solid and the liquid.
Thermotropic liquid crystals may be classified as either low molar mass
(i.e. non-polymeric), or high molar mass (i.e. polymeric) and within each of
these broad classifications, there are several sub-classifications.
2.1
Low Molar Mass Liquid Crystals
There are now three major shape classifications of low molar mass liquid
crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of
these is the most recent, and while examples of bent mesogens have been
known for some years, it is only since the mid-1990s that the area has at-
tracted widespread attention [2].
Calamitic liquid crystals generally contain two or more rings, at least
one flexible chain and often a small, polar function; in some cases, small,
lateral groups (normally fluorine) are attached. Some will contain metal