The Liquid Crystal & Advanced Materials Group

About Liquid Crystals

The Liquid-crystalline State - Molecular Shape and Structure and Liquid Crystals - Thermotropic Liquid Crystals - Lyotropic Liquid Crystals - Plastic Crystals - Structures of Thermotropic Liquid Crystals - Structures of Smectic Liquid Crystals

Molecular Shape and Structure and Liquid Crystals

The type of liquid crystal phase formed by a mesomorphic material is essentially dependent on the molecular properties of the substance. A primary factor in the formation of liquid crystal phases is the overall, or gross, molecular shape of the compound. Three separate species can be easily identified where the molecules have the following rotational volumes; spheroid, ellipsoid and discoid. Spheroid mesomorphic materials generally give plastic crystals. Ellipsoid or rod-like molecules give rise to calamitic liquid crystals, these include nematic (N, nematos, Greek for thread-like), smectic liquid crystals, and anisotropic plastic crystals (S, Smectos, Greek for soap-like). Discoid materials produce nematic-discotic and columnar discotic liquid crystals (D, disk-like).

Molecules with combinations of these shapes can also be mesomorphic. For instance molecules that possess disc- and rod-like shapes can exhibit both calamitic and discotic phases, such materials are called phasmidic because of their molecular shapes, similarly molecular structures that combine features of both discs and spheres can have bowl-ilke shapes and can produce bowlic or pyramidal mesophases.

Three other variants also exist that are essentially specialized examples of the above groups; these are polymer liquid crystals which can exhibit both ellipsoid or discoid properties (see section on Polymer Liquid Crystals), chiral liquid crystals which can show ellipsoid (blue phases, cholesteric, and chiral smectic phases) and discoid (chiral discotic) properties, and metallo-mesogens which possess metal atoms in their otherwise organic based structures.

Clearly not all molecules that possess these gross shapes exhibit liquid-crystalline behaviour and therefore some other molecular factors which influence phase formation have to be taken into account. One molecular constraint that greatly influences phase propagation is the dichotomous nature of the molecular structure of most mesogens. Generally, the structure of a mesomorphic compound consists of two distinct moieties that have dissimilar chemical properties. For example, a single molecular entity can possess a hybrid structure that can have one of the following pairs of molecular combinations; aliphatic-aromatic, dipolar-non-polar, hydrophobic-hydrophilic, flexible-rigid, or hydrocarbon-fluorocarbon. These moieties must also be of a substantial size, or have a strong enough effect, that they give the molecule a dual personality. In this way, the two different portions of the molecular structure can locally interact with similar regions of neighboring molecules causing a form of internal phase separation. This produces a phase structure where the interactions of the two moieties are localized in certain ways so that they produce areas with strong interactions and regions with weak interactions.

Molecules that possess more than two indentifiable, separate structural regions are called polyphilic. As with dichotomous structures, polyphilic compounds must be able to match like-chemical regions when they pack together in order to exhibit mesomorphic behavior. The special case where a material that has a structure that combines hydrophilic and hydrophobic regions is described as being amphiphilic in nature. Amphiphiles tend to exhibit lyotropic mesophases, however, certain materials, see later, can show both thermotropic and lyotropic phases and are said to exhibit amphitropic properties.

Another factor that influences liquid-crystalline phase formation is the relative strengths of the interactions of the dissimilar structural moieties with those of analogous regions of neighbouring molecules. If the stronger interactions dominate over the weaker ones or vice versa, then the tendency to form liquid crystal mesophases is markedly reduced; ie, there needs to be some form of balance between these interactions so that they are both allowed to operate, but in a locally restricted manner. For example, (S)-2-methylbutyl 4'-n-undecyloxybenzoyloxybenzoate exhibits well-defined smectic phases, whereas the analogous amide (S)-2-methylbutyl 4'-n-undecyloxybenzoyloxybenzamide does not exhibit any liquid crystal properties at all.

This is because the hydrogen-bonding interaction in the amide is so strong that the melt point of the solid is increased thus negating the liquid crystal properties. Consequently, the strength and ansiotropy of the molecular interactions are important for mesophase formation.