The individual molecules that comprise a liquid crystal are commonly referred to as mesogens. These molecules tend to be 'rod like' or anisotropic structures, with one axis appreciably longer than the other axis. However, in reality, LC phases have been prepared using mesogen of a wide variety of shapes, including ring structures, banana, hockey stick, and T shaped molecules. Even DNA adopts a LC phase. While theoretically, any thermotropic or lyotropic LC can and have been used to prepare an LCE, we discuss some of the characteristics and practical considerations of mesogens used for LCE.

Three representative mesogen structures are shown above, with the flexible spacer groups highlighted blue,and the mesogen's core green.
The top structure 'acrylate derivative side chain mesogen' is a 5 OCB deriviative with a terminal acrylate group used to make a side chain LCE used in Prof. Kenji Urayama's group from Kyoto University. The acrylate group can react with another acrylate group in the presence of an initiator.
The middle 'vinyl derivative side chain mesogen' is from Prof. Heino Finkelmann's research group at Freiburg University. Here, the terminal double bond,(=-R) also known as a vinyl or olefin functional group reacts with the Si-H group of the siloxane polymer to form a side chain LC polymer.
The 'vinyl derivative main chain mesogen' at the bottom of the image has two reactive vinyl groups. Main chain LCE have the individual mesogens connected end to end using a di-methylhydro-siloxane linker, with a ring shaped crosslinker molecule connecting the polymerized main chain mesogens.

A mesogen used for an LCE contains three distinct components:

  1. A reactive group that takes part in the polymerization reaction
  2. A spacer unit that separates the core of the mesogen from the polymer.
  3. A core component, usually composed of cyclic structures such as benzene, cyclohexane, or heterocyclic rings

Illustrative chemical structures are shown in the image to the right.  The different functional groups of the mesogen are colored in the following manner: polymerization group (blue), flexible spacer group (red), LC core (green). Please click on the image to expand.

Chemical structures of the acrylate and vinyl functional groups.

The structure of the acrylate and vinyl functional groups that participate in polymerization aspect of LCE are shown above. Polymerization runs through different reaction mechanism for the different functional groups.
Acrylate groups react with neighboring acrylate groups to establish a polymer network as discussed on the previous page. Mesogens with acrylate groups on both ends function as cross-linkers
Vinyl groups react with the Si-H bond of a methyl siloxane polymer. Very short and long chain methyl-hydrosiloxane polymers are used to prepare either main chain or side chain LCE respectively.
Some of the advantages of the acrylate system are precisely controllable preparation conditions and relatively fast material preparation. Using acrylate based mesogen, materials may be prepared in glass cells, drawn as fibers, or printed using an inkjet printer. However, materials response may be more limited with acrylate materials. Advantages of siloxane polymer are large stimuli response and high yield.

The flexible spacer connects the LC mesogen with the linear polymer network. The above illustration shows a side chain siloxane LCE material. Here the siloxane polymer is colored blue, with the chemical structure of the flexible spacer in black connected to the accordingly named 'mesogen'. The odd-even effect leading to oblate prolate conformation is also illustrated in the cartoon. On the left, two different 6 atoms spacers are shown, 5 carbons and 1 oxygen (top) and 4 carbons with 2 oxygen atoms (bottom). This results in an oblate conformation, with the LC mesogen oriented approximately perpendicular to the linear polymer backbone. The right side cartoon shows a five atoms spacer, (4 carbon, 1 oxygen) with a prolate conformation, having the LC mesogen directed approximately parallel to the linear polymer backbone.
With increasing spacer length, the odd even effect becomes much less pronounced.

The flexible spacer is comprised of the atoms between the polymer backbone and mesogen core. Typically composed of methylene (CH2) groups and oxygen atoms, the spacer influences both the LC phase transition temperature and the orientation in the LCE.
With increasing length, the melting temperature (Tm) tends to decrease, while the clearing temperature (Tc) tends to increase. With an increasing number atoms, LC polymer networks exhibit and increasing tendency toward crystallization the LC mesogen tends to enter the crystalline phase. However, the core structure also strongly influences crystallization, so their are no set rules on the number of atoms before crystallization occurs.
The number of atoms in the spacer also influences the orientation of the mesogen in a phenomenon known as odd-even effect. Side chain LCE with an even number of atoms in the flexible spacer have the mesogen are directed perpendicular to the backbone in what is known as the oblate orientation, while an odd number of spacer atoms have the LC molecules directed parallel to the polymer backbone.

Chemical structures of typical mesogen core for rod shaped thermotropic liquid crystal mesogens. These are meant to present basic common structures. In reality, many variations can and have been synthesized with different functional group or atoms attached the side of or between the ring structures. The alkyl and alkoxy chains can vary in length from a 1 to 2 carbon atoms to more than 10 carbon atoms

The structure of the mesogen's core differs considerably of thermotropic and lyotropic LC. Thermotropic LC usually have an anisotropic shape, and are based upon various ring structures such as cyclohexane, benzene, and biphenol derivatives thermotropic mesogens. As the number of rings increased, the isotropic phase transition temperature tends to increases with the caveat that this depends strongly on the nature of the ring and spacer.
Lyoptropic mesogens are amphiphilic molecules, having both a hydrophilic and hydrophobic component with the hydrophobic moiety facing the exterior of the molecule to access the solvent.