Editing Mesogen

[[File:Figure A.png|upright=2|thumb|alt=description of mesophase.|Mesophase.]]

A '''mesogen''' is a compound that displays [[liquid crystal]] properties.<ref name="Mingos, 1999" /><ref name="Tschierske, 2012" /> Mesogens can be described as disordered solids or ordered liquids because they arise from a unique state of matter that exhibits both solid- and liquid-like properties called the liquid crystalline state.<ref name="Mingos, 1999" /> This liquid crystalline state (LC) is called the [[mesophase]] and occurs between the [[Crystal|crystalline solid]] (Cr) state and the isotropic liquid (Iso) state at distinct temperature ranges.<ref name="Tschierske, 2012" />

The liquid crystal properties arise because mesogenic compounds are composed of rigid and flexible parts, which help characterize the order and mobility of its structure.<ref name="Tschierske, 2012" /> The rigid components align mesogen [[Moiety (chemistry)|moieties]]  in one direction and have distinctive shapes that are typically found in the form of rod or disk shapes.<ref name="Tschierske, 2012" /> The flexible segments provide mesogens with mobility because they are usually made up of alkyl chains, which hinder crystallization to a certain degree.<ref name="Tschierske, 2012" /> The combination of rigid and flexible chains induce [[structural alignment]] and fluidity between liquid crystal moieties.<ref name="Tschierske, 2012" />

In doing so, varying degrees of order and mobility within mesogens results in different types of liquid crystal phases, Figure 1. The [[nematic]] phase (N) is the least ordered and most fluid liquid crystalline state or mesophase that is based on the rigid core of mesogen moieties.<ref name="Mingos, 1999" /><ref name="Tschierske, 2012" /> The nematic phase leads to long range orientational order and short range positional order of mesogens.<ref name="Mingos, 1999" /><ref name="Tschierske, 2012" /> The [[smectic]] (Sm) and columnar (Col) phases are more ordered and less fluid than their nematic phases and demonstrate long range orientational order of rod-shaped and disk-shaped rigid cores, respectively.<ref name="Mingos, 1999" /><ref name="Tschierske, 2012" />

==Examples==
[https://s-media-cache-ak0.pinimg.com/originals/37/bb/03/37bb03ee01f18a1bdbd5976dc176481f.png Figure 1] – Organization of rod-like and disk-like rigid cores in liquid crystal phases of mesogens, where Iso is the isotropic liquid state; N is the nematic phase of the liquid crystal state; SmA is the smectic A phase; SmC is the smectic C phase; and Col is the columnar phase.<ref name="Tschierske, 2012" />

'''Thermotropic mesogens''' are liquid crystals that are induced by temperature<ref name="Mingos, 1999" /> and there are two classical types, which include [[liquid crystal|discotic mesogens]] and [[liquid crystal|calamitic mesogens]].<ref name="Dierking, 2001"/>

'''Discotic mesogens''' contain a disk-shaped rigid core and tend to organize in columns, forming columnar liquid crystal phases (Col) of long range positional order.<ref name="Mingos, 1999" /><ref name="Tschierske, 2012" />

An example of a discotic mesogen type rigid core is a [[triphenylene]] based disk molecule, where the hexagonal columnar liquid crystal phase exists between 66&nbsp;°C (crystal) and 122&nbsp;°C (isotropic liquid phase).<ref name="Tschierske, 2012" />

'''Calamitic mesogens''' contain a rod-shaped rigid core and tend to organize in distinctive layers, forming [[lamella (materials)|lamellar]] or smectic liquid crystal phases (Sm) of long range positional order.<ref name="Mingos, 1999" /><ref name="Tschierske, 2012" /> Low-order smectic phases include smectic A (SmA) and smectic C (SmC) phases, while higher ordered smectic phases include smectic B, I, F, G and H (SmB/I/F/G/H) phases.<ref name="Dierking, 2001" />

An example of a calamitic mesogen type rigid core is a [[benzyl cyanide]] based rod molecule, where the smectic A liquid crystal phase exists between the 60&nbsp;°C (crystal) and 62&nbsp;°C (isotropic liquid phase) temperature range.<ref name="Tschierske, 2012" />

'''Bent-rod mesogens''' are special calamitic mesogens that contain a nonlinear rod-shaped or bent- rod shaped rigid core and organize to form "banana-phases".<ref name="Dierking, 2001" /> The rigid units of these phases pack in a way so that the highest density and polar order are achieved, typically with the [[Apex (geometry)|apex]] of the bent rod pointing in one direction.<ref name="Tschierske, 2012" /> When a layer of bent-rods points in the same polar direction as its adjacent layers the lamellar organization is known as the smectic PF (SmPF) phase, where the F subscript indicates [[Ferroelectricity|ferroelectric]] switching.<ref name="Tschierske, 2012" /> Smectic PA (SmPA) is the term given to a layer of bent-rods that points in the opposite polar direction as its neighbouring layers, where A stands for [[Antiferroelectricity|antiferroelectic]] switching.<ref name="Tschierske, 2012" />

Other variations of bent-rod liquid crystal phases include: antiferroelectric/ferroelectric smectic C (SmCPA/SmCPF) phases and antiferroelectric/ferroelectric smectic A (SmAPA/SmAPF) phases, which have distinctive tilt and orthogonal modes of lamellar organization.<ref name="Tschierske, 2012" />

<gallery mode=packed widths=250px heights=250px>
File:Mesogen_illustration 2.png|alt= examples of mesogenic structures.|Calamitic and discotic mesogens based on [[benzyl cyanide]] and [[triphenylene]], respectively.<ref name="Tschierske, 2012" />
File:LiquidCrystal-MesogenOrder-SmecticPhases.jpg|Lamellar (layer) organization of low order calamitic mesogens. The smectic A phase (left) has molecules organized into layers. In the smectic C phase (right), the molecules are tilted inside the layers
File:2D columnar mesogens.png|2D lattices of columnar mesogens; Col<sub>hex</sub>, Col<sub>squ</sub>, Col<sub>rec</sub> and Col<sub>ob</sub> stand for hexagonal, square, rectangular and oblique.<ref name="Tschierske, 2012" />
</gallery>
<gallery mode=packed>
File:Packing of banana mesogens.png|Lamellar (layer) organization of bent-rod calamitic mesogens, including the smectic PF and PA phases<ref name="Tschierske, 2012" />
File:Banana-mesogen.png|A bent-rod (calamitic) mesogen, where the antiferroelectric smectic C phase exists between 145&nbsp;°C (crystal) and 162&nbsp;°C (isotropic liquid).<ref name="Tschierske, 2012" /><ref name="Dierking, 2001" />
</gallery>

==References==
{{Reflist |refs=
<ref name="Mingos, 1999">[[Mingos, D. M. P.]] (1999) ''Structure and Bonding''. Springer. Preface, p. 7. {{ISBN|978-3662147160}}</ref>
<ref name="Tschierske, 2012">{{cite book | doi=10.1007/128_2011_267 | chapter=Fluorinated Liquid Crystals: Design of Soft Nanostructures and Increased Complexity of Self-Assembly by Perfluorinated Segments | title=Liquid Crystals | series=Topics in Current Chemistry | year=2011 | last1=Tschierske | first1=Carsten | volume=318 | pages=1–108 | pmid=22089090 | isbn=978-3-642-27590-6 |publisher=Springer |location=Berlin, Heidelberg}}</ref>
<ref name="Dierking, 2001">{{cite journal|doi=10.1016/S0921-4526(01)00549-X|author=Dierking, I. |journal=Physica B|year=2001|volume=304|pages= 51–59|title=Crystallisation of a bent-core liquid crystal mesogen|issue=1–4 |bibcode=2001PhyB..304...51D }}</ref>}}

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[[Category:Liquid crystals]]