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Chiral liquid crystals-art and science Dr. M. Manickam School of Chemistry The University of Birmingham CHM3T1 Lecture-5
Outline of Lecture Introduction Structure-Property Relations Synthesis of Chiral liquid crystals Strategies and Methods Final comments
Learning Objectives After completing this lecture you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods. Understand helical order in chiral nematic or cholesteric phases Understand what you mean chiral nematic phase Understand what you mean chiral smectic phase Importants fo chiral nematic and smectic phases in devices systems General structure of chiral materials Physical properties of chiral molceules. Understand ferroelectric, antiferroelectric and ferrielecric structures Understand achiral ferroelectric liquid crystals Chiral discotic and chiral polymeric mesogens
Nomenclature Cr or C or K: crystalline phase N*: chiral nematic phase or cholesteric phase S*: chirl smectic phase n: director P s : spontaneous polarisation P: polarisation T g : glass-transition temperature I: isotropic N D *: chiral nematic discotic phase B: banana phase (B 1 to B 7 ) TGBA*: twist-grain boundary phase
Introduction An object or a molecule that is not superimposable on its mirror image is termed chiral. For example, a glove is chiral but a coffee cup is achiral. However, the term chiral is usually employed in chemistry to denote that a molecular structure is asymmetric. Chirality is generated by there being four different structural moieties attached to a tetrahedral, sp 3 carbon atom (often described as a chiral centre). Four such different units can be arranged in two ways, generating two compounds (enantiomers) that are mirror images of each other. One stereochemical configuration is termed S and the other enantiomer is termed R. For example, 2-octanol is chiral and is commonly used in the synthesis of chiral liquid crystals; one isomer is (S)-2-octanol and the other isomer is (R)-2-octanol. (S)-2-octanol (R)-2-octanol
Enantiomers Both enantiomers are identical except for the way in which the structural moieties are arranged in space. The different arrangement of functional groups in space is responsible for optical activity, i.e., an enantiomer will rotate the of plane polarised light; Its mirror image will produce the same magnitude of rotation but in the opposite direction; a 1:1 mixture of enantiomers is termed racemic and will not generate a rotation of plane polarised light. The chiral liquid crystalline molecules organise into an asymmetric, chiral structure that takes the form of a helix. The helical structure is handed; for example, one enantiomer will generated a left-handed helix and the other enantimer will produce a right-handed helix.
Cholesteryl Benzoate The very first thermotropic liquid crystalline material (cholesteryl benzoate) discovered in 1888 by the Austrian botanist Reinitzer, exhibits what is now known as the chiral nematic (N*) phase. C 146 N* 178 I Cholesterol, has an asymmetric molecular structure and it is chiral and optically active. Cholesteryl benzoate has 8 chiral centres giving a total of 256 stereisomers; however, only one is produced in nature Historically, the chiral nematic phase was called the cholesteric phase because the first materials exhibiting this phase were in cholesterol derivatives Cholesteryl benzoate
Physical Phenomenon of The Helical Structures Attractive physical phenomenon connected to these helical structures is that when plane polarised light interacts with them, its plane of polarisation will be rotated in the same direction of the helix. When the pitch of the helix corresponds to a wavelength in the visible region of the spectrum ( nm) the chiral mesophase is coloured. Additionally, the pitch of the helix is temperature-sensitive and, therefore, colour changes result as the temperature varies. Furthermore, helical structures can be unwound by the application of an electric field, which drives the reorientation of the molecular axis along the field direction.
Helical Order in Chiral Nematic Phases The chief of a mesophase formed by chiral mesogens with a predominant concentration of one enantiomer, is the organisation of the molecules into bulk helical structures. The helical ordering can be defined as left- or right-handed by the rotation of the helical pitch
Helical Pitch The pitch is the distance for one full director rotation If the molecules that form a liquid crystals phase are chiral (lack of inversion symmetry), then chiral phases exist in place of certain non-chiral phases. In calamitic liquid crystals, the nematic phase is replaced by the chiral nematic phase, in which the director rotates in helical fashion about an axis perpendicular to the director. The structure of the chiral nematic phase. The views represent imaginary slices through the Structure and do not imply any type of layered structure The pitch of a nematic phase is the distance along the helix over which the director rotates by 360 o Chiral Nematic Phase
Nematic phase The least ordered mesophase (the closest to the isotropic liquid state) is the nematic phase, where the molecules have only an orientational order. The molecular long axis points on average in one favoured direction referred to an the director. The classical examples of LC displaying a nematic mesophase in the cynobiphenyl Cartoon representation of N phase. The molecules are oriented on average, in the same direction referred to as the director, with on positional ordering with respect to each other
Chiral Nematic or Cholesteric Phase The simplest chiral mesophase is the chiral nematic where the local molecular ordering is similar to that of the nematic phase (only orientation order), and additionally the molecules pack to form helical macrostructures in the direction perpendicular to the director. The helicity depends on the absolute configuration (enantiomer R or S) of the molecules. (a)Helical structure of the chiral nematic phase; (b) The director lies in the xy plane, perpendicular to the direction of the helix (z), and rotates in the plane that defines the helical structure.
Chiral Smectic Phases Chiral smectics are also found. There are many different types of smectic liquid crystals. The form of chirality of all of these chiral smectic mesophases takes the form of a helical structure Types of smectic liquid crystal phases (S C *, S I *, and S F *) Crystal smectic mesophases (J*, G*, and K*, H*) which could generate form chirality as a direct result of the molecular chirality of the constituent molecules In the chiral crystal smectic mesophases the helix is in effect unwound by the crystal structure and the form chirality is suppressed. There are also other chiral mesophases of the smectic type (e.g., TGBA* and S C * anti phases)
Smectic phases The next level of organisation is classified as smectic (S), where in addition to the orientational order the molecules possess positional order, such that the molecules organise in layered structures. The S phase has many subclasses, which are illustrated below. Cartoon representation of (a) the S A phases, and (b) the S C phase
Chiral Smectic Phase The chiral smectic C phase is by far the most important phase. The tilted director rotates layer to layer forming a helical structure. These systems can be surface stabilised in this case the helix may be decreased by liquid crystal in cell. This means the material is effectively trapped between two glass plates. Once the helix is suppressed and the directors in each layer are forced to lie in the plane of the glass plates, this creates spontaneous polarisation within each layer because of the chiral nature of the molecules. This is the basis of ferroelectric display devices. These ferroelectric liquid crystal displays and antiferroelectric liquid crystal displays operate by the application of an electric field which couples with the spontaneous polarisation and switches the director in the layers.
Chiral Smectic Phase Helical macrostructure of the chiral smectic C (S C *) phase; (b) chiral molecule represented in its layer plane (xy) with its polarisation (P) due to the inherent asymmetry. The layers precess around the normal (z) to the layers, forming a helical macrostructure.
A General Structure of Chiral Materials for Ferroelectric Mixture Core Chiral Chain Terminal Chain Common terminal chain units Common units found within the core Common chiral centre unit combinations
Transition temperatures Cinnamate Ester Schiff’s Bases Compound (a) photochemically unstable, relatively high viscosity, very low Ps, It not suitable for use in devices
Ferroelectric Host: Physical Properties A low melting point (below room temperature) A wide S C range with no underlying ordered smectic phases A cooling phase sequence of I-N-S A -S C A low viscosity A tilt angle (θ) of A low to moderate optical anisotropy A negative dielectric anisotropy A high dielectric biaxiality A high chemical and photochemical stability Chiral Dopant: Physical properties A reasonably high spontanous polarisation (P S ) A long nematic pitch Good solubility and compatibility beween host and dopant Problem: Chiral materials are often difficult and expensive to synthesise
Ferroelectric LCs Schematic representation of the helical macroscopic superstructure of Sc* unwound to the ferroelectric state, when an external electric is applied. P represents the polarisation On reversal of the polarity of the applied field, the precession of the molecules around the normal to the layer (z) allows a very fast switching driven by the interaction of P and the electric field
Ferroelectric LCs The switching time in ferroelectric materials is very fast, relative to the nematic materials in the twisted nematic displays, because the energy required for the molecular reorientation is small. In fact, the polar chiral molecules simply rotate about the normal to the layers (z) in the so-called cone-like fashion mode, driven by the tendency of P to align with the external field. The cone-like fashion mode is represented in the figure where the chiral molecule is symbolised by a “fish”. This powerful macroscopic property of net polarisation is combined with the processability of fluids in ferroelectric LCs, which, therefore, represent a highly attractive class of materials for technological display application. Cone-like fashion switching by precession of the director about the normal to the layers (z). The chiral molecule is symbolised by a “fish” representation, where the “eye” represents the direction of P (black forwards, and white backwards)
Ferroelectric LCs Generally a material is ferroelectric when it possesses spontaneous polarisation (P s ), which confers to the materials the property of being switched between two states of polarisation, by reversing the direction of an applied electric field. Solids such as NaNO 2, Li 2 Ge 7 O 15, and (CH 3 NH 3 ) 5 Bi 2 Br 11 can be ferroelectric as well as chiral LCs, as they allow appropriate symmetry-breaking elements in order to generate P s. S C* phase can be driven towards a ferroelectric state, by applying an external electric field. Each single polarised layer reorients in such a way to position the direction of polarisation P with the electric field. As a consequence, the S C* helix is unwound and all the layers will be oriented in the same direction, driven by the interaction with the electric field (Figure-6). The overall result is a polarised phase (ferroelectric), which has a net dipole alignment along the electric field and can be readily further switched between the opposite states of polarisation (P 1 and P 2 ), by reversing the applied electric field (Figure-7)
Antiferroelectric LCs Antiferroelectric liquid crystals are similar to ferroelectric liquid crystals, although the molecules tilt in an opposite sense in alternating layers. In consequence, the layer-by-layer polarization points in opposite directions. These materials are just beginning to find their way into devices, as they are fast, and devices can be made “bistable”. The chevrons represent the banana-shaped molecules The block arrows represent the polarisation P of the layer Ferroelectric Antiferroelectric Ferrielectric phase
Ferrielectric Structure The ferrielectric chiral smectic C (S C * freei) phase also has an alternating tilted structure expcept that the alternation is not symmetrical and more ‘layers’are tilted in one direction than the other. According, the ferrielectric phase generates a P S which depends upon the degree of alternation of tilt direction. Ferroelectric AntiferroelectricFerrielectric The chevrons represent the Banana-shaped Molecules The block arrows represent the polarisation P of the layer
Achiral Ferroelectric LCs The presence of a chiral centre in liquid crystalline molecules is not necessary the only method to introduce an element of asymmetry in a LC phase. Symmetry can also be broken by achiral molecules, such as bent molecules (banana-or bow-shaped) or bowl –shaped molecules, which lead to a net symmetry-breaking and chiral bulk structure. Banana-shaped molecules Bowl-shaped molecules Both bent and bowl-shaped molecules can pack in a form an isotropic fashion, generating sequences of sheets or columns with a polar axis. Thus, there will be sheets or columns either pointing all in the same direction or in an anti-parallel fashion. In the first case, the arrangement of the molecules will be parallel and have an overall noncentrosymmetric structure, hence the phase is ferroelectric.
Achiral Ferroelectric LCs In the second arrangement, the molecules will result in either a symmetric antiferroelectric structure, if the same number of layers are pointing in the two opposite directions, or a noncentrosymmetric ferrielectric structure, if one direction is preferred to the other Ferroelectric phase Anti-Ferroelectric phase Ferrielectric phase The chevrons represent the Banana-shaped Molecules The block arrows represent the polarisation P of the layer
Banana – Shaped LCs Banana shaped LCs are generally composed of three molecular units: An angular central core (commonly 1,3-disubstituted benzene ring) to make the bend and two linear rod like units (often containing Schiff’s base) Terminal chains such as alkoxy groups Banana phases are numbered according to their chronological discovery, from B 1 to the most recent B 7 In 1996, the first observation of ferroelecitricty B 2 exhibited by achiral banana-shaped mesogens caught the attention of the whole LCs community, opening worldwide intense research and discussion on this new and exciting field of LCs, most of all towards the exploitation of novel types of LCs-based technological devices. Bend unit Linear rod like unit Terminal chain
Banana – Shaped LCs The main feature of molecules such as is their symmetry C 2v in this case and therefore its polarisation in the direction of the C 2 axis. Subsequently interchangeable ferroelectric states may be induced. C 2V Banana-shaped LCs have a much faster switching time, as these types of LCs can reorientate in an electric field, not by a 90 o turn, but, by precessing around an axis to realign themselves (figure- 10c). This process does not require as much energy and can occur much faster.
B 2 Phase The most investigated banana mesophase is the B 2 phase shown by the series of previous compound. The interesting feature of the B 2 phase is that it is a layered phase with a C 2v symmetry and spontaneous polarisation P s in the direction of the C 2 (two-fold) symmetry axis. Therefore, a switchable ferroelectric state can be induced. X-ray, NMR, and electro-optical studies have led to a detailed structural model of the B 2 phase and its ferroelectric switching mode.
Chiral Discotic Phases Discotic system can be made chiral by incorporating a chiral unit into one or more of the periperal units that surround the discotic core This compound exhibits solely a chiral nematic discotic phase (N D *) phase because the steric effect of the branched chains at the chiral centre disrupt the ability of the molecules to pack in columns The liquid crystal tendency depends critically on the type of chiral peripheral chain C N D * I
Chiral Polymeric Mesogens Liquid crystal polymers are usually made chiral in the same manner as for low molar mass liquid crystals, i.e., by incorporating a chiral moiety within the structure (usually as part of the terminal chain because of simplicity). Chiral liquid crystal polymers are commonly designed and synthesised to exhibit the chiral smectic C (S C *) phase because of their potential use as non-linear optical materials or as pyroelectric detectors. Such ferroelectric polymers have a spontaneous polarisation and can be switched in the same manner as the analogous low molar mass materials. However, polymers are extremely viscous and switching times are quite long. However, ferroelectric switching is much less affected by polymerisation than nematic switching. Accordingly, ferroelectric polymers will probably find use in ferroelectric displays (e.g., storage type displays) g 56 S F * 80 S C * X 148 S C * Y 197 S A 216 I Typical ferroelectric liquid crystals poly(acrylate with simple chiral terminal chiral chain Several chiral phases Two S C * phases
Final Comments Liquid crystalline materials are fascinating fluids that are widely recognised for their use in devices. Chiral liquid crystalline materials are even more fascinating and have an array of special properties enabling them to be used in new technological application (thermochromics and ferroelectric displays). In some case chiral liquid crystalline materials generate intriguing new liquid crystalline phase structures, for example, the antiferoelectric phase which has tremendous potential in a fast-switching display device. Other phase structure generated by chiral materials remain a curiosity without applications (e.g., blue phase and twist grain boundary phases). Chirality in liquid crystals is currently the subject of intense research; however, the topic is still in its infancy. As synthetic routes to novel and more pure chiral materials become available, then a wider range of chiral liquid crystals will be generated. Such chiral liquid crystals are expected to become technologically important in the coming years.