Mesomorphic compounds

At least one terminal chain, such as an alkyl (R) or alkoxy chain (RO) is attached to the molecular core. The second group may be a similar group or a small substituent such as a halogen atom (F, Cl, Br, I) or a cyano (CN), isothio-cyanato (NCS), dimethylamino (N(CH3)2) or nitro (NO2) group or a fluorinated group such as trifluoromethyl (CF3), trifluoromethoxy (CF3O), difluoro-methoxy (CF2HO), etc. Compounds with two short terminal alkyl or alkoxy chains tend to exhibit a nematic phase. Mesomorphic compounds with short... 

It is worth mentioning the very recent report of new calamitic liquid crystals containing a metal-metal bond, based on the stable [Ru2(CO)4(/t -t/ -02CR)2L2] sawhorse unit, 72, [134]. This class of metallomesogens represents a rare example of mesomorphic compounds containing covalent bonds between two metal atoms, and are of considerable interest due their structure and electronic properties, and also to their potential catalytic reactivity [135]. 

In the case of thermotropic liquid crystals, a liquid-crystalline phase is obtained by heating a solid mesomorphic compound. At the melting point (Tm), the thermal motion of the molecules has increased to such an extent that the material passes from the crystalline phase to the liquid-crystalline phase. A mesomorphic compound that exists in the glass state will enter the liquid-crystalline phase at the glass-transition temperature (Tg). On further heating, the orientational order of the molecules is lost as well. The LC transforms into an isotropic, clear liquid at the clearing point (or isotropization point T. Many materials are liquid-crystalline at room temperature. Several types of liquid-crystalline phases can occur between the solid state and the isotropic liquid state. Sometimes decomposition of the material occurs before the... 

The mesophases of calamitic molecules mentioned above have a well-defined thermod3mamic order when a hypothetical calamitic mesomorphic compound exhibiting almost all types of mesophases (enantiotropic with respect to the melting point and disregarding the various chiral phases) is heated, the mesophases will occur in a certain defined sequence ... 

A number of different smectic states exist among the low molecular weight mesomorphic compounds. The molecules, for example, might be normal to the planes or tilted relative to the normal. The different states have been given various letter designations (Smectic A, B, C etc.) and deVries (2) has classified the x-ray patterns obtained from the various types of smectic (and also nematic) states. 

Most known mesomorphic compounds are aromatic systems of the following general shape... 

Most of the above mesomorphic compounds are not suited as solvents for polar molecules such as amino acids. The most polar liquid crystalline solvents of type (I) are provided by derivatives of benzoic acids, such as 4-n-octyloxy benzoic acid... 

Many mesomorphic compounds undercool readily and may therefore form rigid... 

Mesogen (mesomorphic compound) - a compound that under suitable conditions of temperature, pressure and concentration can exist as a mesophase. 

As discussed in Sec. 2.1, the prototype of the rod-like liquid crystal molecule (1) consists of a rigid core substituted with terminal flexible substituents, and eventually small lateral substituents. In the last few years in particular many mesomorphic compounds that do not correspond to this formula have been synthesized. Such materials may be called unconventional liquid crystalline compounds [1,2]. 

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