A mesogen is a compound that displays liquid crystal properties.[1][2] 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.[1] This liquid crystalline state (LC) is called the mesophase and occurs between the crystalline solid (Cr) state and the isotropic liquid (Iso) state at distinct temperature ranges.[2]
Mesophase.
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.[2] The rigid components align mesogen moieties in one direction and have distinctive shapes that are typically found in the form of rod or disk shapes.[2] The flexible segments provide mesogens with mobility because they are usually made up of alkyl chains, which hinder crystallization to a certain degree.[2] The combination of rigid and flexible chains induce structural alignment and fluidity between liquid crystal moieties.[2]
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.[1][2] The nematic phase leads to long range orientational order and short range positional order of mesogens.[1][2] 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.[1][2]
Examplesedit
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.[2]
Thermotropic mesogens are liquid crystals that are induced by temperature[1] and there are two classical types, which include discotic mesogens and calamitic mesogens.[3]
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.[1][2]
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 °C (crystal) and 122 °C (isotropic liquid phase).[2]
Calamitic mesogens contain a rod-shaped rigid core and tend to organize in distinctive layers, forming lamellar or smectic liquid crystal phases (Sm) of long range positional order.[1][2] 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.[3]
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 °C (crystal) and 62 °C (isotropic liquid phase) temperature range.[2]
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".[3] The rigid units of these phases pack in a way so that the highest density and polar order are achieved, typically with the apex of the bent rod pointing in one direction.[2] 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 ferroelectric switching.[2] 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 antiferroelectic switching.[2]
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.[2]
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
2D lattices of columnar mesogens; Colhex, Colsqu, Colrec and Colob stand for hexagonal, square, rectangular and oblique.[2]
Lamellar (layer) organization of bent-rod calamitic mesogens, including the smectic PF and PA phases[2]
A bent-rod (calamitic) mesogen, where the antiferroelectric smectic C phase exists between 145 °C (crystal) and 162 °C (isotropic liquid).[2][3]
^ abcdefghijklmnopqrstuvTschierske, Carsten (2011). "Fluorinated Liquid Crystals: Design of Soft Nanostructures and Increased Complexity of Self-Assembly by Perfluorinated Segments". Liquid Crystals. Topics in Current Chemistry. Vol. 318. Berlin, Heidelberg: Springer. pp. 1–108. doi:10.1007/128_2011_267. ISBN978-3-642-27590-6. PMID 22089090.
^ abcdDierking, I. (2001). "Crystallisation of a bent-core liquid crystal mesogen". Physica B. 304 (1–4): 51–59. Bibcode:2001PhyB..304...51D. doi:10.1016/S0921-4526(01)00549-X.
January 01, 1970
mesogen, mesogen, compound, that, displays, liquid, crystal, properties, described, disordered, solids, ordered, liquids, because, they, arise, from, unique, state, matter, that, exhibits, both, solid, liquid, like, properties, called, liquid, crystalline, sta. A mesogen is a compound that displays liquid crystal properties 1 2 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 1 This liquid crystalline state LC is called the mesophase and occurs between the crystalline solid Cr state and the isotropic liquid Iso state at distinct temperature ranges 2 Mesophase 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 2 The rigid components align mesogen moieties in one direction and have distinctive shapes that are typically found in the form of rod or disk shapes 2 The flexible segments provide mesogens with mobility because they are usually made up of alkyl chains which hinder crystallization to a certain degree 2 The combination of rigid and flexible chains induce structural alignment and fluidity between liquid crystal moieties 2 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 1 2 The nematic phase leads to long range orientational order and short range positional order of mesogens 1 2 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 1 2 Examples editFigure 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 2 Thermotropic mesogens are liquid crystals that are induced by temperature 1 and there are two classical types which include discotic mesogens and calamitic mesogens 3 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 1 2 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 C crystal and 122 C isotropic liquid phase 2 Calamitic mesogens contain a rod shaped rigid core and tend to organize in distinctive layers forming lamellar or smectic liquid crystal phases Sm of long range positional order 1 2 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 3 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 C crystal and 62 C isotropic liquid phase temperature range 2 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 3 The rigid units of these phases pack in a way so that the highest density and polar order are achieved typically with the apex of the bent rod pointing in one direction 2 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 ferroelectric switching 2 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 antiferroelectic switching 2 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 2 nbsp Calamitic and discotic mesogens based on benzyl cyanide and triphenylene respectively 2 nbsp 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 nbsp 2D lattices of columnar mesogens Colhex Colsqu Colrec and Colob stand for hexagonal square rectangular and oblique 2 nbsp Lamellar layer organization of bent rod calamitic mesogens including the smectic PF and PA phases 2 nbsp A bent rod calamitic mesogen where the antiferroelectric smectic C phase exists between 145 C crystal and 162 C isotropic liquid 2 3 References edit a b c d e f g h Mingos D M P 1999 Structure and Bonding Springer Preface p 7 ISBN 978 3662147160 a b c d e f g h i j k l m n o p q r s t u v Tschierske Carsten 2011 Fluorinated Liquid Crystals Design of Soft Nanostructures and Increased Complexity of Self Assembly by Perfluorinated Segments Liquid Crystals Topics in Current Chemistry Vol 318 Berlin Heidelberg Springer pp 1 108 doi 10 1007 128 2011 267 ISBN 978 3 642 27590 6 PMID 22089090 a b c d Dierking I 2001 Crystallisation of a bent core liquid crystal mesogen Physica B 304 1 4 51 59 Bibcode 2001PhyB 304 51D doi 10 1016 S0921 4526 01 00549 X Retrieved from https en wikipedia org w index php title Mesogen amp oldid 1198652406, wikipedia, wiki, book, books, library,
