Discotic nematic liquid crystals: science and technology

Abstract

The nematic phase of discotic liquid crystals, although rarely observed, has made very significant progress over the past three decades since their discovery. It has made its way from a mere scientific curiosity to application in commodities. The negative birefringence films formed by polymerized nematic discotic liquid crystals have been commercialized as compensation films to enlarge the viewing angle and enhance the contrast ratio of commonly used twisted nematic liquid-crystal displays. High strength and high performance carbon fibers for industrial applications have been obtained from the carbonaceous mesophase and a liquid-crystal display device with wide and symmetrical viewing angle has been demonstrated by using discotic nematic liquid crystals. Discotic films with patterned colours have been obtained from cholesteric lyo-mesophases of discotic liquid crystals. Various molecular architectures have been designed and synthesized to exhibit the discotic nematic phase over a wide range of temperature. This critical review focuses on the synthesis and physical properties of these fascinating materials. It deals with the structure of various nematic phases, different discotic cores exhibiting the nematic phase, novel designing and transition temperature engineering principles, alignment and physical properties, and finally the application of discotic nematic LCs as the active switching component and as optical compensation films for widening the viewing angle and contrast ratio of liquid-crystal display devices (98 references).

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Hari Krishna Bisoyi

Hari Krishna Bisoyi was born at Lokanathpur, India in 1981 and currently he is finishing his PhD in the Soft Condensed Matter Group, Raman Research Institute, Bangalore, India working under the guidance of Prof. Sandeep Kumar. He has obtained his BSc (2001) and MSc (2003) in Chemistry from Berhampur University, Orissa. His research work involves design, synthesis, functionalization and characterization of liquid crystals and other self-organizing soft and nano π-conjugated functional materials using supramolecular and green chemistry approach. He has published about a dozen research papers.

Sandeep Kumar

Sandeep Kumar received his PhD in 1986 under Professor A. B. Ray and than worked with Dr Sukh Dev at MRC, Vadodara. After postdocs in Israel, USA and Germany, he returned to India in 1995 to the Centre for Liquid Crystal Research, Bangalore. In 2002, he moved to the Raman Research Institute. He has already published 130 research papers and patents. The Royal Society of Chemistry awarded him a journals grant for international authors in 2001 for his significant publications in RSC journals. He was awarded the inaugural LG Philips Display Mid-Career Award by the International Liquid Crystal Society in 2008.

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1. Introduction

Liquid crystals (LCs) are unique functional soft materials which possess both order and mobility ranging from the molecular to the macroscopic level. Liquid crystals are accepted as the fourth state of matter after solids, liquids and gases. Liquid crystals form a state of matter intermediate between the solid and the liquid state. For this reason, they are referred to as intermediate phases or mesophases. This is a true thermodynamic stable state of matter. The constituents of the mesophase are called mesogens. The mesogens can be organic (forming thermotropic and lyotropic phases), inorganic (metal oxides forming lyotropic phases) or organometallic (metallomesogens), etc. LCs are important in material science as well as in the life sciences. Important applications of thermotropic LCs (the mesophase is obtained by varying temperature) are electrooptic displays, temperature sensors and selective reflecting pigments. Lyotropic LC (mesophase is obtained by varying concentration and/or temperature) systems are incorporated in cleaning processes, and are important in cosmetic industries. They are used as templates for the preparation of mesoporous materials and serve as model systems for biomembranes. LCs are important in living matter, for example, biological membranes, DNA-based lyotropic mesophases, etc. LCs can potentially be used as new functional materials for electron, ion, molecular transporting, sensory, catalytic, optical and bio-active materials. LCs are extremely diverse since they range from DNA to high strength synthetic polymers such as Kevlar (used for bullet-proof vests, protective clothing, high performance composites for aircraft and automotive industries) and from small organic molecules such as alkyl and alkoxycyanobiphenyls used in liquid-crystal displays (LCDs) to self-assembling amphiphilic soap molecules. Natural silk is spun by the silk worm Bombyx mori from an aqueous nematic liquid-crystalline phase of fibroin; processing polymeric materials from the LC phase can improve their material properties. Recently their biomedical applications such as in controlled drug delivery, protein binding, phospholipid labeling, and in microbe detection have been demonstrated. Apart from material science and bio-science, LCs are now playing significant roles in nanoscience and nanotechnology such as the synthesis of nanomaterials using LCs as templates, the design of LC nanomaterials, alignment and self assembly of nanomaterials using LC phases and so on. Owing to their dynamic nature, photochemically, thermally or mechanically induced structure changes of liquid crystals can be used for the construction of stimuli-responsive materials. Although LCs have diverse applications such as temperature sensing, solvents in chemical reactions, in chromatography, in spectroscopy, in holography, etc., they are primarily known for their extensive exploitation in electrooptical display devices such as watches, calculators, telephones, personal organizers, laptop computers, flat panel televisions, etc.

Nematic liquid crystals are amazing anisotropic ordered fluids possessing only orientational order and have been playing a very significant role to understand soft condensed matter basic science and technology. They are primarily known for their extensive application in liquid-crystal display (LCD) devices. Due to their high fluidity (low viscosity) the effect of electric and magnetic fields are substantial on nematic liquid crystals. They can be easily aligned and reoriented by application of relatively small electric and magnetic fields which is the basis for their practical use in LCDs. The nematic mesophase is the most diverse mesophase occurring in calamitic (rod-like) molecules but not common in discotic (disc-like) molecules. It occurs in both thermotropic and lyotropic liquid crystals and appears in monomeric and polymeric liquid crystals. The constituents of the nematic phase can vary from organic and inorganic to organometallics. Nematic liquid crystals of chiral compounds exhibit helical superstructures and exhibit the phenomenon of selective reflection which is the basis of temperature sensors (medical thermometers). Moreover, the chiral nematic or cholesteric phase represents the architecture of fundamental biomolecules such as DNA, proteins and polysaccharides which have revealed the importance of chiral helical architectures in the development of essential functions in living systems. Nematic liquid crystals can be enantiotropic (mesophase occurs both on heating and cooling) or monotropic (mesophase occurs while on cooling only) in nature and can be optically uniaxial (possess only one optic axis) or biaxial (possess two orthogonal optic axes) and display fascinating optical textures under polarizing microscope. The term nematic is derived from the Greek word for thread i.e. nematos and relates to the thread-like texture formed by disclination between ordered regions visible in the optical microscope. Most of the nematic liquid crystals are uniaxial owing to the free rotation of the molecules whereas some thermotropic and lyotropic nematic LCs exhibit phase biaxiality because of restricted molecular rotation in the mesophase. The applications of nematics are not confined to only display applications but also they are very important in non-display applications. High strength fibers such as Kevlar, natural silk and high performance carbon fibers are processed from the nematic phase, and recently optical compensation discotic films for widening the viewing angle and to increase contrast ratio of LCDs, have also been prepared from nematic liquid-crystal phases.

Recently, liquid crystals formed by disc-shaped molecules have attracted growing interest because the supramolecular assembly of their columnar phases is of fundamental importance, not only as models for the study of energy and charge migration in self-organized systems, but also as functional materials for device applications such as one-dimensional conductors, photoconductors, light emitting diodes, photovoltaic solar cells, field effect transistors, gas sensors, etc. The functional capabilities of these materials are due to their easy processability, spontaneous alignment between electrodes and self-healing of defects owing to their dynamic nature. Most of the discotic liquid crystals (DLCs) form columnar mesophases probably due to strong π–π interaction among the aromatic cores.¹ There are various types of columnar phases depending on the degree of order in the molecular stacking, orientation of the molecules along the columnar axis, the dynamics of the molecules within the columns, and the two-dimensional lattice symmetry of the columnar packing. Very few of the DLCs exhibit a less ordered nematic phase.² This critical review focuses on the rarely found discotic nematic liquid crystals. It deals with the structure of various nematic phases, different discotic cores exhibiting the nematic phase, novel designing and transition temperature engineering principles, alignment and physical properties, and finally the application of discotic nematic LCs as active switching components and as optical compensation films for widening the viewing angle and contrast ratio of LCD devices.

2. Structure of the nematic phases of DLCs

This section is devoted to the detailed structures of various nematic phases of discotic compounds, their experimental identification and characterization by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction studies. The nematic phases of disc-like molecules can be sub-divided into four types: (i) discotic nematic (N_D), (ii) chiral nematic (N*_D), (iii) columnar nematic (N_Col) and (iv) nematic lateral (N_L). The structure of these nematic phases are shown in Fig. 1.

Fig. 1 Structure of different nematic phases formed by discotic liquid-crystal compounds: (a) discotic nematic (N_D), (b) chiral discotic nematic (N*_D), (c) columnar nematic (N_col) and (d) nematic lateral (N_L).

The discotic nematic (N_D) phase is the least ordered, least viscous and most symmetric mesophase among the other nematic phases. In a discotic nematic mesophase, the discotic molecules possess full translational and rotational freedom around their short molecular axis (disc normal) but on an average the short molecular axes are oriented in a preferred direction called the director n. In other words, the short molecular axes of the molecules orient more or less parallel to each other, while their centers of mass are isotropically distributed in the nematic phase. N_D has the same symmetry as that of calamitic nematic but are not miscible with each other and hence phase separation occurs owing to the fundamental structural differences. Like chiral calamitic or cholesteric phases the chiral discotic nematic phase N*_D also exists. The mesophase occurs in mixtures of discotic nematic and mesomorphic or non-mesomorphic chiral dopants as well as in pure chiral discotic molecules.² The helical structure of the chiral discotic nematic phase is shown in Fig. 1. The columnar nematic phase (N_Col) is characterized by a columnar stacking of the molecules. However these columns do not form two-dimensional lattice structures. They display a positional short-range order and an orientational long-range order. The columns behave like supramolecular rods and can be regarded as building blocks of the N_Col phase instead of single molecules. Recently another nematic phase has been reported, where the disc-shaped molecules aggregate into large superstructures, and these supramolecular aggregates show a nematic arrangement. The phase is referred to as the nematic lateral phase (N_L) due to the strong lateral interactions.^3–5

All the nematic discotic phases are fluid and exhibit typical schlieren textures (Fig. 2(a)) with two and four brush defects and marbled textures similar to that of a nematic phase of calamitic molecules. Similar to the textures of cholesteric phases of calamitic molecules, oily streaks, fingerprint and polygonal textures are observed for N_D* phases. Almost all the nematic columnar and nematic lateral phases are exhibited by charge transfer (CT) interactions and these show deep colours under the microscope and show viscosities higher than normally observed for discotic nematic phases. Transition enthalpies of different phase transitions depend on the amount of order present in the system. Since the discotic nematic phase is least ordered, the N_D–I transition enthalpy is usually <1 kJ mol⁻¹ whereas the isotropic transition enthalpies of nematic columnar and nematic lateral phases are 1–3 kJ mol⁻¹ and about 5 kJ mol⁻¹, respectively.^2,4,5

Fig. 2 Typical polarizing optical microscopic Schlieren texture (a) and X-ray diffraction pattern of an aligned sample (b) of discotic nematic liquid crystals.

The X-ray diffraction profile of a N_D phase resembles with that of an isotropic phase. The wide-angle diffraction peak is related to the lateral distance between the cores, while the small angle diffraction peak is attributed to the diameter of the core. In an aligned sample of the nematic phase the small-angle reflections are normal to the reflections in the wide angle region (Fig. 2(b)). Owing to the negative diamagnetic anisotropy of discotic nematic LCs the director of the discotic nematic phase will always be orthogonal to the applied magnetic field. X-Ray patterns of a columnar nematic phase show relatively sharp reflections in the wide-angle regime that correspond to the regular stacking of the discotic molecules on top of each other. The reflections in the small-angle regime are rather diffuse and broadly related to the liquid-like arrangement of the columns. Therefore the columns are oriented more or less parallel and have only short-range positional order, which is characteristic for a nematic mesophase. In the N_L phase, relatively sharp reflections are found in both the small- and wide-angle region of the diffraction pattern. The correlation length is a measure of spatial order in terms of the molecular dimensions. The correlation length shows a low value for the N_D phase, in the N_Col phase the correlation length increases in the columnar direction, and in the N_L phase it increases in both the columnar and lateral directions. XRD is a powerful technique to distinguish between the nematic phases.

The columnar nematic phase was first observed,⁶ one decade after the discotic nematic phase,⁷ in mixtures of triphenylene based amorphous polymers with non-mesomorphic electron acceptors (2,4,7-trinitrofluorenone (TNF) and 2,4,7-trinitrofluoren-9-ylidenemalonodinitrile (TNF-CN)) of different acceptor strength because of charge transfer (CT) interactions. Columnar nematic phases can be obtained by both inter- and intra-molecular CT interactions with substituted and unsubstituted multi-nitrofluorenones. The various electron acceptors (1–5) used for CT complexation are shown in Fig. 3. These electron acceptors can induce, modify, stabilize as well as destabilize mesophases by the combination of CT and steric interactions. Recently the columnar nematic phase has been observed in pure single component multiyne trimers⁸ and side-chain polymers^9,10 in contrast to the CT complexes of multiynes and electron acceptors, which provokes one to consider it a frustrated columnar phase, since the presence of the polymer backbone and a central unit in trimers do not allow a two-dimensional (columnar) lattice to build up. Discotic polymers combine two characteristic features, the tendency of the disc-shaped moieties to self organize and the tendency of the flexible backbone to destroy such a structure formation which leads to frustrated mesophases such as nematic columnar and nematic lateral in multiyne polymers. In the superstructures of the N_L and N_Col phases, the local organization can be very high, but the long-range positional ordering is very low, i.e. nematic. It should be noted that the columnar nematic and nematic lateral phases are yet to be observed in pure single component monomeric discotic compounds. Interestingly novel phase transition sequences such as nematic columnar to discotic nematic (nematic–nematic) and nematic lateral to discotic nematic raises the question if these are real phase transitions in the thermodynamic sense. Since there is no change in the phase symmetry and optical texture of the system upon phase transition, it is believed that it is the result of disintegration of molecular aggregates into single molecules i.e. supramolecular nematic to nematic transitions, but DSC and XRD analysis of these phases prompts one to consider it a phase transition.

Fig. 3 Different kinds of electron acceptor molecules 1–5 used for charge transfer interaction with discotic molecules to induce and/or stabilize columnar nematic phases.

3. Different discotic cores exhibiting nematic phases

To date more than 50 discotic cores are known to exhibit mesomorphism.¹ However, most of them exhibit columnar mesomorphism, only a very few of them display a discotic nematic phase such as some benzene, naphthalene, triphenylene, truxene and phenylacetylene derivatives, and recently some disc–rod hybrids. Radial and cyclic multiynes, and hexabenzoates are the most common derivatives which exhibit discotic nematic phases. Following are the different discotic cores which on suitable substitution exhibit the nematic liquid-crystal phase.

3.1 Benzene-based discotic nematic liquid crystals

Benzene derivatives, as we know, are the first discotic compounds in which discotic columnar mesomorphism was unequivocally observed in pure single component compounds. Hitherto, benzene-based radial multiynes i.e. hexakis- and pentakis-(phenylethynyl) benzene derivatives are the most widely synthesized and studied materials to exhibit solely the discotic nematic LC phase, that is, they are truly representatives of nematic discogens.^11,12 These were the first nematogens which displayed the columnar nematic and N*_D phase in pure compounds and the chiral compounds exhibit selective reflection and temperature induced helix inversion.¹³ The whole spectrum of nematic phases exhibited by discotic liquid crystals are displayed by these compounds and, more interestingly, nematic–nematic phase transitions were exhibited by some side-chain polymers based on multiynes. In binary mixtures, owing to their high electron richness, with substituted and unsubstituted non-mesomorphic multinitro fluorenone electron acceptors 1–4 of different strength, multiyne derivatives form charge-transfer complexes and the molecular organization changes from discotic nematic to columnar nematic and moreover, a novel nematic phase called the nematic lateral (N_L) phase is also induced and stabilized by CT interactions. N_D phases which are stable well above and below room temperature have been obtained by transition temperature engineering and have been demonstrated in a LCD device as the active switching component with wide and symmetrical viewing angle characteristics.^14,15 Relatively easy availability of terminally mono-functionalized multiyne monomers have paved the way to explore and exploit discotic nematic liquid-crystalline dimers, trimers, tetramers and side-chain polymers from a structure–property relationship point of view.¹⁶ Initially the synthesis and mesomorphism studies of radial multiynes were carried out by Praefcke et al. who reported various hexakis(phenylethynyl)benzenes (6, 7), pentakis(phenylethynyl)phenyl ethers (8) and pentayne discotic dimers (9, 10), with different spacer length and peripheral substitution.^11,12,17 In some of the monomeric and dimeric compounds, optical biaxiality was speculated but later it was experimentally clarified that they do not possess any optical biaxiality. They were able to induce columnar nematic phases in mesomorphic and non-mesomorphic benzene-based radial mutliynes with the help of flat electron acceptors by CT interaction. Subsequently Praefcke et al. designed and synthesized various chiral multiynes,¹³ some of which (11) exhibit solely the chiral discotic nematic phase at relatively low temperature and others (12) are non-mesomorphic but can act as potential chiral dopants.¹⁸

Recently Hsu et al. have reported several stable, low-temperature discotic nematic liquid crystals of hexakis(phenylethynyl)benzene (13, 14, 15) incorporating laterally substituted side arms.¹⁹ Various physical properties of some of the multiynes have been evaluated in the nematic phase.^20,21 They exhibit negative optical and dielectric anisotropies, the bend (K₁₁) and splay (K₃₃) elastic constants are of the same order of magnitude as for other discotic nematic compounds and calamitic nematic compounds, K₁₁ is larger than K₃₃, i.e.K₁₁/K₃₃ > 1, and their viscosity is higher than for calamitic nematic LCs. An optically controlled electro-optic effect was discovered in the nematic phase which could be of interest for optical data processing, i.e. writing and reproducing optical images.²² Aligned chiral nematic LCs with negative dielectric anisotropy exhibit electric field dependent selective reflection, that is, they can potentially act as optical modulators and electrically tunable cholesteric mirrors.²³

To prepare room-temperature N_D LCs we focused our attention on discotic multiynes (hexa- and penta-alkynylbenzene based molecules), primarily because a number of compounds of this class are known to show stable N_D phases and therefore some minor structural modifications may lead to materials having room-temperature discotic nematic phases. The use of branched chains in LCs often reduces melting and isotropic temperatures. The mesophase range is generally widened, but the type of mesophase formed is not affected by the introduction of branching in many cases. The decrease in the transition temperature could be due to the disorder introduced by branched chains and stereoheterogeneity. This methodology has been applied to reduce melting and isotropic temperatures of several DLCs forming columnar mesophases. From the reported thermal data of alkyl- and alkoxy-substituted hexaalkynylbenzene derivatives it is very clear that when the peripheral alkyl chains are attached to the phenyl ring in the hexaalkynyl benzene derivatives via an oxygen atom, the melting and clearing points are higher compared to when the alkyl chains are directly attached to the ring. Therefore it was logically anticipated that the replacement of normal alkyl chains by branched alkyl chains connected directly to the phenyl ring would reduce the melting point of alkynyl benzene derivatives and thus stabilize the mesophase. Compound 16 was designed on this basis, and it was indeed found to exhibit the nematic phase at ambient temperature, and has been successfully employed in LC display devices exhibiting wide and symmetrical viewing angle characteristics.^14,24

It has been observed in the case of several triphenylene-based discotic nematic LCs that a lateral methyl group substitution reduces the transition temperatures significantly. Accordingly, we designed and synthesized several new hexa- and penta-alkynyl benzene based molecules with a combination of normal and branched alkyl chains to look for room-temperature discotic nematic phases and to understand the structure–property relationship. Out of several molecules synthesized (17–19), two pentaalkynyl benzene derivatives (19) displayed a stable N_D phase at room temperature.²⁵

Mono-functionalized discotic liquid crystals are important precursors of LC dimers, oligomers, dendrimers and polymers. The introduction of mono-functionalized discotic nematic pentaynes (20) in the mid 1990s by Janietz et al. opened up new avenues for discotic nematic oligomers, multipodes and side-chain polymers.¹⁶ Accordingly a variety of discotic dimers (21, 22) differing in the peripheral substituents and nature of the spacers have been prepared and their mesophase behavior has been compared with the corresponding monomer analogues (23).²⁶ The nature of the spacer varies from alkyl chain to azacrown ether via amide, oligo(ethylene oxide), perfluoroalkyl and siloxanes, etc. All the spacers seem to stabilize the N_D phase in the dimers and the perfluoro spacer additionally induces cybotactic smectic ordering in the nematic phase owing to microphase segregation. Interestingly, the discotic liquid-crystalline trimers (24)⁸ containing benzene as the central core show only the columnar nematic phase in the pure compounds whereas the tetramer 25 containing a siloxane core²⁷ exhibits discotic nematic phase. The pentayne 20 bearing a terminal polar functional group on the alkoxy chain behaves as an amphiphile and is capable of forming stable edge-on oriented monolayers at the air/water interface with two-dimensional nematic ordering as well as highly organized Langmuir–Blodgett (LB) multilayers. Photophysical properties such as absorption and photo-luminescence of functionalized multiynes have been studied in the mesophase and in solution in order to see the effect of molecular symmetry and was found to depend on molecular organization rather than on the molecular symmetry.²⁸ A photoactive triple mesogen (26) showing a nematic columnar phase has been demonstrated to act as potential optical storage material.²⁹

A variety of discotic nematic liquid-crystal side chain polymers (27–32) have been prepared and investigated by Kouwer et al.^3–5 by reacting functionalized monomers with a reactive polymer backbone. The attachment of mesogens to a reactive poly(acryloyl chloride) followed by quenching offers a very versatile route for synthesizing many structurally similar polymers and enables to manipulate many molecular parameters such as the lateral substituents on the mesogen, the nature and length of the spacer, degree of substitution of the mesogen on the polymers and the nature of the quenching agent. Another way to manipulate the polymers is by inter- and intra-molecular CT interactions since it also offers a simple and versatile approach to obtain a broad variety of LC phases starting from a limited number of materials. Accordingly polymers were prepared and their mesomorphism was investigated. For the first time a discotic nematic LC phase was observed in polymers and surprisingly, the polymer 27 exhibits a nematic columnar to nematic discotic phase transition.⁹ Other polymers (28) with different mesogen concentrations were prepared and it was found that at high mesogen concentration the polymers exhibit both columnar nematic and discotic nematic phases, however, with lower mesogen concentration the discotic nematic mesophase disappears and the columnar nematic phase prevails.¹⁰ A macromolecular electron-acceptor 5 was prepared and it was observed that this acceptor induces and stabilizes a nematic lateral phase in the functionalized discotic monomers. Later the nematic lateral phase was also observed when various electron acceptors were added to a non-liquid-crystalline discotic polymer 30. Interestingly, the nematic lateral phase transforms to discotic nematic phase at higher temperatures in some mixtures. The donor/acceptor copolymer 31 has been observed to stabilize the columnar nematic phase like the physical mixture of the donor and acceptor monomers. Recently discotic side chain polymers (32) with different linking groups to the mesogenic rigid core have been prepared and some of them are found to exhibit the discotic nematic phase.³⁰

There are several C₃ symmetric star-shaped discotic liquid crystals based on a benzene central core which exhibit the discotic nematic phase. Compounds containing oxadiazole (33),³¹ stilbinoid (34, 35)³² and hydrazone-based (36)³³ side-arms exhibit discotic nematic phase behavior. Compounds such as 37 containing thiophene groups around the central benzene core also exhibit the N_D phase.³⁴ There are various hydrogen-bonded supramolecular (38)³⁵ and organometallic (39)³⁶ discotic compounds which exhibit the N_D phase whereas there is only one triester derivative of benzene (40) which exhibits a discotic nematic phase.³⁷

3.2 Naphthalene-based discotic nematic liquid crystals

Unlike benzene, naphthalene-based discotic nematic liquid crystals are much less in number and only some of the radial multiynes (41, 42) of naphthalene exhibit the discotic nematic phase.^2,38 However the naphthalene-based radial multiyne discotic nematic liquid crystals are interesting because, unlike the benzene and triphenylene radial multiynes, they show the inverse nematic phase, such as the truxene derivatives i.e. the nematic phase appears below the highly ordered columnar phases. Compound 42 shows the normal sequence but the compound starts decomposing at higher temperature. The physical properties of certain naphthalene-based discotic nematic liquid crystals have been measured and it is found that they possess negative dielectric anisotropy, as expected from the molecular structure, but the splay elastic constant is greater than the bend elastic constant i.e.K₁₁/K₃₃ < 1, which has been attributed to the presence of short-range cybotactic order in these compounds, since the nematic phase appears below the columnar phases.^20,21

3.3 Triphenylene derivatives

Triphenylene derivatives are probably the most widely synthesized and studied materials in the family of discotic liquid crystals. Among the various triphenylene derivatives, hexaalkyl or alkoxy benzoates of triphenylene are especially very interesting and are the most widely studied materials exhibiting the discotic nematic mesophase along with other highly ordered columnar phases.³⁹ Hexabenzoates of triphenylene were the first discotic compounds to exhibit the discotic nematic phase in pure discotic compounds and they can also exhibit the chiral nematic phase in combination with mesomorphic and non-mesomorphic chiral discotic compounds as well as in the pure compounds by the substitution of chiral peripheral alkyl chains.⁷ Noteworthy is the fact that hexabenzoates of triphenylene were the first discotic liquid-crystalline compounds for which practical commercial application has been achieved.⁴⁰ Polymerizable hexabenzoates of triphenylene have been polymerized in the discotic nematic phase, which is being used as optical compensation films to widen the viewing angle characteristics of thin film transistor-liquid crystal display (TFT-LCD) devices. Hexabenzoates of triphenylene not only exhibit thermotropic nematic liquid crystals but also lyotropic nematic phases when dissolved in a suitable solvent, i.e. they act as amphotropic materials.^41,42 Furthermore, owing to its tolerance, various small and bulky groups have been substituted in the peripheral phenyl rings to study the effect on mesomorphism of hexabenzoates of triphenylene. Among the hexa-alkoxybenzoates of triphenylene (43) the lower members show only the nematic phase while the higher members show both columnar rectangular and nematic phases.³⁹ Unfortunately the temperature range of their enantiotropic N_D phases are very high, above the columnar phase. However, the transition temperatures decrease with increase in the terminal chain length. Very few members of the hexaalkylbenzoates of triphenylene (44) exhibit the fluid nematic phase and the temperature range of the nematic phase is short. It is well known for calamitic materials that lateral substitution in the core and/or branching in the terminal peripheral aliphatic chains can reduce melting points and also radically affect phase morphology. To investigate the effect of lateral substitutions on the mesomorphism of hexaalkoxybenzoates of triphenylene various substituents such as fluorine, methyl, ethyl, isopropyl and even bulky tert-butyl groups have been substituted in the peripheral phenyl rings. When methyl (45), ethyl (46), isopropyl (47) and tert-butyl (48) groups are substituted either ortho or meta to the ester linkage group, suppression of the columnar phase was observed for both the series.^43–45 A general increase in the clearing temperature was observed for the meta-substituted series when the size of the lateral substituents is increased but a depression of the clearing points was found for the ortho-substituted series. This demonstrates that thickening the molecular disc weakens the face-to-face interaction, thereby suppressing the columnar phase, whereas the nematic phase stability remains relatively unaffected. Similarly when two methyl groups are substituted as in 49, meta to the ester group, in the peripheral phenyl rings, the compounds were found to display hexagonal columnar and discotic nematic mesophase, whereas the analogous compounds with substitution ortho to the ester group exhibited only a nematic phase. The lateral steric interactions of the methyl groups in ortho positions force the peripheral phenyl rings out of the plane of triphenylene core thereby producing only the nematic phase. The use of a lateral methyl substitution as part of the terminal alkoxy chain reduces the melting point and nematic phase stability of the compound 50.⁴⁴ The chiral analogous of this compound exhibit a chiral nematic phase.⁴¹ Recently, stimuli-responsive discotic nematic liquid crystals 51 have been prepared by replacing the terminal alkyl chains of triphenylene hexabenzoates by oligo(ethylene oxide) chains.⁴⁶ These compounds exhibit room-temperature nematic phase over a wide range of temperature. Interestingly the nematic phase of these compounds can be transformed to columnar phase triggered by alkali-metal ions. There has been an enormous amount of exciting research on fluorinated liquid crystals. The small size of the fluoro substituents enable their incorporation into all types of liquid crystals, including calamitic, discotic, banana, lyotropic, as well as polymers, without ruining the liquid-crystalline nature of the material. However the fluoro substituent is larger than hydrogen, and hence causes a significant steric effect, which combined with the high polarity confers many fascinating and often remarkable modifications to melting point, mesophase morphology, transition temperatures, and many other important physical properties, such as dielectric anisotropy, optical anisotropy and viscoelastic properties. Accordingly, various fluorinated hexaalkoxy benzoates of triphenylene (52) have been investigated. Only one compound with a single fluorine substitution on the phenyl ring ortho to the ester group exhibits nematic mesomorphism. However, when more fluorine atoms are substituted on the phenyl ring the nematic phase is eliminated and in some compounds the rectangular columnar phase is replaced by columnar hexagonal phases. The drastic change in the mesomorphic behavior could be due to several interactions such as fluorophilic/fluorophobic and quadrupolar/dipolar interactions.⁴⁷

Polymerizable hexabenzoates of triphenylene (53–55) are the revolutionary materials which has made the less abundant discotic nematic liquid-crystalline materials more ubiquitous—in a world with more LCDs than people—by acting as the compensation films for widening the viewing angle.⁴⁰ Among the above polymerizable compounds the acryloyl derivative 53 is compatible for photopolymerization speed and thermal stability. Photopolymerizable monofunctionalised 56 and difunctionalised 57 monomers have been synthesized and found to exhibit only the discotic nematic phase.⁴⁸ Monofunctionalized monomer forms a side chain polymer whereas a mixture of isomers of difunctionalised monomer leads to a densely crosslinked colorless network film with negative birefringence. Fast charge carrier mobility (10⁻³ cm² V⁻¹ s⁻¹, hole mobility) has been observed in polymer films obtained by photo-polymerization of triphenylene mesogen isomers 58.⁴⁹ Furthermore oblique optic axis discotic negative birefringence phase compensation films have been obtained from 59 and 53 on a specifically designed rubbing-aligned polyimide layer surface for widening the viewing angle of LCD devices.^50,51

While it is often easy to design DLCs forming columnar phases with basic structural features, flat or nearly flat aromatic cores surrounded by plural flexible side chains, it is rather difficult to design molecules that may form the N_D phase. This could be a reason why only a comparatively small number of nematic DLCs are known so far. The majority of DLCs form columnar phases probably due to the significant π–π interactions of the polyaromatic cores. In order to get the nematic phase, sufficient steric hindrance around the core has to be introduced so that the rigid molecules may stay in a more or less parallel position, having only orientational order. We anticipated that linking two columnar discotic molecules via a short rigid spacer might create some steric hindrance due to overlapping or interdigitation of aliphatic side chains and a weak distortion of the overall planarity of the molecule. This may reduce strong π–π interaction and therefore, is likely to show a discotic nematic phase. Accordingly dimers 60 were designed to verify this idea. As predicted, all the dimers 60 form discotic nematic phases over a wide range of temperature.⁵² It should be noted that triphenylene dimers possessing flexible alkyl spacers exhibit well defined columnar phases, however when two discotic nematogens are attached to each other via flexible alkyl spacers they furnish a discotic nematic phase. Replacing one of the alkoxy chains by a thioalkoxy chain does not change the nature of the mesophase but leads to a slightly lower isotropic transition temperature.⁵³ From the synthetic point of view these derivatives (61) are rare examples of discotic liquid crystals exhibiting discotic nematic phase and possess three different kinds of substituents around the triphenylene core. Discotic dimers (62), obtained by linking two discotic units via a mercury atom exhibit a metastable N_D phase.⁵⁴ This is one of the rare examples of discotic metallomesogens exhibiting a discotic nematic phase albeit monotropic.

Other triphenylene-based discotic nematogens include radial multiyne hydrocarbons (63),⁵⁵ lightly substituted triesters (64),⁵⁶ the donor–acceptor triple mesogen 65,⁵⁷ and the tricarbonyl chromium complex of triphenylene (66).⁵⁸ The radial multiyne and the tricarbonyl chromium complex of triphenylene exhibit enantiotropic nematic mesophases, however the triesters display monotropic nematic phases whereas the donor–acceptor triple mesogen containing photoactive azobenzene as the bridging group shows a columnar nematic phase owing to charge transfer (CT) interaction.

Immediately after the observation of the discotic nematic phase in triphenylene hexabenzoates, attempts were made to align the nematic phase between two coated glass plates treated for homeotropic, planar and twisted alignment. Electro-optic effects as well as hydrodynamic instabilities were studied in a hope of understanding and applying these negative birefringence materials in display devices. Other physical properties of the discotic nematic phase of triphenylene-derivatives such as birefringence, magnetic susceptibility anisotropy, conductivities, dielectric and elastic constants, viscosity coefficients and order parameter have been measured and their temperature dependence have been studied.⁵⁹ It follows from the above studies that the discotic nematic phases possess negative birefringence, magnetic susceptibility anisotropy, positive dielectric and conductivity anisotropy. The viscosity coefficient is one order of magnitude higher than for calamitic nematic phases, the orientational order parameter and the elastic constants are of the same order of magnitude as the calamitic nematic phases, however, the ratio of splay elastic constant to bend elastic constant i.e.K₁₁/K₃₃ is higher than unity, which is in contrast to the calamitic nematic phases. Recently surface and temperature dependence of the pretilt angle of discotic nematic liquid crystals have been evaluated.⁶⁰

3.4 Truxene derivatives

Truxene is the second discotic core to exhibit the discotic nematic phase. Hexaalkanoates and benzoates of truxene and its two hetero-analogues are also well known discotic liquid crystals each with very interesting polymorphism including the N_D inverse phase, mostly below columnar phases and reentrant nematic phase behavior owing to the relatively large discotic core. However, the hexaethers of truxene exhibited a single columnar phase which was attributed to low steric hindrance of six ether linkages which allows a strong cohesion between cores. Discotic liquid crystals derived from the truxene core are interesting as they display a variety of phases. They can act as electroluminescent materials, organic semiconductors, non-linear optical materials, hole transport materials, etc. Truxene hexaalkanoates 67 show columnar and nematic phases, while higher chain length members displayed enantiotropic nematic phases; the nematic phase was monotropic in nature in the shorter chains and the nematic phase is an inverted nematic phase appearing below the columnar phase.⁶¹ The hexabenzoates (68) of truxene displayed normal phase sequence and exhibit a re-entrant nematic phase.⁶² The first three derivatives exhibited only a nematic phase, other derivatives display both columnar and nematic phases. Unlike triphenylene hexabenzoates, truxene benzoates have not been much explored probably because of synthetic difficulties. The inverted nematic–columnar sequence in truxene derivatives may correspond to conformational modification of the flexible alkyl chains of the truxene core. Presumably, the alkyl chains are folding back towards the central core at lower temperature thereby disrupting the columnar packing and at higher temperature the chains adopt a radial all-trans conformation that allows discotic cores to stack in columns. Hexalkanoates of trioxatruxene derivatives (69) exhibit nematic phases and are the first discotic compounds to exhibit the columnar oblique lattice. The nematic phase appears in a inverted sequence and these compounds display the full sequence of polymorphism i.e. Cr, N, Col_ob, Col_r, Col_h and isotropic phase. Trioxatruxene derivatives exhibit only the marbled texture in the nematic phase.⁶³ The corresponding trithiatruxene derivatives also exhibit the fluid nematic phase. The hexaalkanoates 70 exhibit both nematic and columnar phase whereas the hexabenzoates 71 exhibit solely the nematic phase over a broad temperature range.⁶⁴ The only complication with them lies in their relatively long and very tedious chemical synthesis. The thermal stability of the N_D phase follows the order truxene > trithiatruxene > trioxatruxene and could be because of the consequence of relative steric hindrance of CH₂, S and O. There are no reports of either chiral nematic or columnar nematic phases in truxene-based discotic liquid crystals. Benzotrisfuran 72 is a related heterocycle which has been prepared and found to give a discotic nematic phase above its melting point, however it decomposes rather quickly at higher temperatures which excludes further detailed investigation.⁶⁵

The dielectric constants, splay bend elastic constants and optical anisotropy of some truxene derivatives have been measured in the nematic phase. The dielectric anisotropy is positive, the optical anisotropy, as expected, is negative and the elastic constants are of the same order of magnitude for nematic phases of rod like molecules. The splay elastic constant K₁₁ is larger than the bend elastic constant K₃₃, i.e.K₁₁/K₃₃ > 1 which is in contrast to what is commonly observed in fluids of rod-like mesogens and this behavior is in agreement with the behavior predicted by mean-field theory.⁶⁶ However when the nematic phase appears in between two columnar phases, the bend elastic constant exceeds the splay elastic constant, i.e.K₁₁/K₃₃ < 1, probably as a consequence of columnar short-range order in the nematic phase.⁶⁷ Temperature dependence of the orientational diffusivities has been measured in the nematic phase and diffusivities are found to be 100 times weaker than the rod-like nematics, which can be attributed to higher viscosity of the discotic nematic phase.

3.5 Phenylacetylene macrocycles

Discotic liquid crystals based on macrocyclic compounds are particularly interesting because of their potential to self-organize into supramolecular nanochannels that could be processed into macroscopically aligned nanotubules. This possibility has prompted to search for DLCs based on phenyl acetylene macrocycles (PAMs) otherwise known as shape-persistent macrocycles. The relative rigidity and large internal diameter of PAMs make them candidates for mesogens of tubular liquid-crystal phases. Unlike hexacyclenes, azacrowns and crown ether derivatives in which the macrocycle can collapse due to its inherent flexibility, columnar phases based on PAMs should produce materials with well defined and non-collapsable internal channels. It may be possible to tailor the transport properties of oriented thin films of such materials over a broad spectrum by functionalization to the endo positions. Interestingly, certain PAMs are able to exhibit the fluid discotic nematic mesophase.⁶⁸ The hexaesters of PAMs, owing to their strong π–π interactions, do not furnish any nematic phase, however the mixed ether–ester derivatives 73 exhibit a monotropic nematic phase followed by a columnar phase. On the other hand the hexaethers 74 and hexaalkanoates 75 of the PAMs exhibit stable discotic nematic phases over a wide range of temperature and display schlieren textures. Recently novel shape-persistent macrocycles (76, 77) have been reported to exhibit exclusively the discotic nematic phase. Notable is the fact that these LC macrocycles have a molecular topology that is opposite to all the discotics described so far i.e. a rigid ring acts as a framework for flexible side groups pointing to the inside (rigid periphery and flexible core). The inverted molecular topology of these macrocycles has been revealed by single-crystal X-ray analysis of different structural isomers.⁶⁹ It is very clear from the analysis that the flexible alkyl chains should be placed in the adaptable positions of the macrocycles so that they can fill their interior by their own alkyl chains. If the alkyl chains are positioned at the extra-annular non-adaptable positions then their back folding is unfavorable for enthalpic and entropic reasons and no mesomorphism is observed. In contrast when the alkyl chains are positioned at the intra-annular non-adaptable positions they fill the interior and exhibit discotic nematic mesophases. Replacing phenyl groups by the polycyclic aromatic backbone as in 78 decreases the size of the cavity but it still exhibits a nematic phase with intraannular flexible alkyl chains.⁷⁰ Very recently a triangular ortho-phenylene ethynylene (o-PE) macrocycle 79 with oligo(ethylene oxide) side chains has been reported to exhibit the discotic nematic mesophase.⁷¹ It is interesting to note that this is one of the most compact shape-persistent macrocycles known and can act as the structural unit of graphyne.

3.6 Disc–rod hybrids exhibiting nematic phases

The most successful commercial application of liquid crystals is in LCD devices for information display. The uniaxial calamitic nematic LCs are the active switching components of present LCD technology, whereas the uniaxial discotic nematic counterparts act as optical compensation films to widen the viewing angle and to increase the contrast ratio of LCDs. The biaxial nematic phase has been rather elusive but it has been envisaged that the use of biaxial nematics could result in LCDs with faster refresh rates and dramatically lower power consumption. The biaxial nematic phase was first predicted in 1970 and is characterized by two directors, which are orthogonal to each other. The biaxial nematic phase has been observed in lyotropic and recently in thermotropic systems. Theoretical studies have shown that mixtures of rods and discs can exhibit the biaxial nematic phase, however in practice, the physical mixture of rods and discs phase separate into two uniaxial nematic phases. To overcome this difficulty, the search for biaxial nematic phases has prompted the synthesis of dimers consisting of one calamitic and discotic mesogenic units covalently attached to one another either terminally or laterally via flexible alkyl spacers (80–82).^72,73 Though these compounds do not furnish the biaxial nematic phase, they act as potential shape-amphiphiles i.e. they are miscible with both rod-like and disc-like liquid crystals. It has been observed that rods and discs are completely miscible in the nematic phase when they are chemically linked to one another. Optically they exhibit both schlieren and marbled textures of classical nematic liquid crystals. Subsequently, two and three rods were attached to one side of a disc via flexible alkyl spacers 83–87, to study the miscibility of disc and rods. Like the chiral nematic phase of discotics, chiral nematic phases of disc–rod systems have been achieved by introducing chiral substituents to disc–rod systems such as 84. When discotic nematogens were attached to calamitic smectogens they exhibit the nematic phase at higher temperature, but interestingly at lower temperature they exhibit non-conventional smectic phases with rods and discs phase segregating to alternating layers.⁷⁴ Recently, six rod-like molecules has been attached to a central discotic core. Compounds such as 88⁷⁵ and 89⁷⁶ exhibit the discotic nematic phase whereas compound 90⁷⁷ shows both nematic and smectic mesomorphism.

4. Carbonaceous mesophases

In the mid 1960s it was observed that there is a mesophase transformation at high temperatures during the carbonization of certain graphitizable organic materials such as petroleum and coal tar pitches and this mesophase with characteristic nematic texture is denoted as carbonaceous mesophase.⁷⁸ The thermodynamic phase that describes the carbonaceous mesophase is the discotic nematic liquid-crystal state. The carbonaceous mesophase is a complex multicomponent system composed of large disc-like polyaromatic molecules having a wide range of molecular weights rather than well defined organic molecules. It has a transient existence, its life time being limited by its hardening into semicoke. Carbonaceous mesophase is the only known naturally occurring discotic liquid crystal. However, the carbonaceous mesophase is not well understood as many other LC systems. Its complex composition and chemical instability have discouraged systematic investigation of its key fundamental properties. Although the carbonaceous mesophase was discovered in 1965, the synthesis of the first discotic pure compound exhibiting LC behavior did not occur until 1977, with the first pure nematic discotics following in 1979. The optical texture of carbonaceous mesophase identifies it as a discotic nematic phase, but an analogous phase has never been demonstrated with any pure compound truly representative of the constituents of pitch. In practice, carbonaceous mesophase typically forms upon heating, in contrast to the most LC systems, which forms ordered configurations upon cooling the isotropic liquid. Carbonaceous mesophases are opaque, a practical difference that precludes many optical device applications involving transmitted light. However, carbonaceous mesophases are used in the industrial manufacturing of high performance carbon fibers, carbon foams, carbon fiber–carbon mesophase composites and carbon nanotube–carbon mesophase nanocomposites.^79,80 This relatively new carbon fiber is more competitive than the conventional fibers made from acrylic precursors in several application areas. Their extremely high thermal conductivity combined with their relative low density make mesophase pitch based fibers attractive. There are a variety of potential applications for these new fibers in chemical sensing, adsorption, thermal protection material and electronic devices such as battery electrodes, because they should have a high density of surface active sites and easy access to interlayer spacers, etc.

5. Nematic lyo-mesophases from disc-shaped molecules

As has already been mentioned, lyotropic nematic liquid crystals are important from their technological point of view since high strength fibers such as Kevlar and natural silk are prepared from lyotropic nematic phases. Thermotropic mesomorphic and non-mesomorphic discotic compounds can also self assemble in solutions to exhibit lyotropic nematic mesomorphism i.e. anisotropic solutions can be obtained by dissolving discotic liquid crystals in suitable organic solvents. However, discotic nematic lyo-mesophase forming compounds are limited in number as compared to the thermotropic discotic nematic compounds. A polymerizable triphenylene-based discotic nematic monomer (59) shows a room-temperature nematic phase when dissolved in suitable organic solvents at an appropriate concentration (xylene, alkylbenzenes and 1,3-dichlorobenzene, conc. >60%).⁴¹ In the case of discotic cholesteric monomer 91, the cholesteric phase is observed at room temperature in xylene at a concentration of 55 wt%. The lyotropic cholesteric phase exhibits selective reflection, like the thermotropic cholesteric phase, and this is also temperature dependent. The mixture of both monomers (59 and 91) prepared in one solvent also exhibits a cholesteric lyo-mesophase. The lyotropic phase of the mixture can be aligned and photopolymerized in the mesophase to obtain thin films with patterned colors. The selective reflection of the thin films so obtained is temperature independent owing to the formation of a cross-linked network. Interestingly, another chiral triphenylene-based compound (92) has been found to form a cholesteric columnar nematic mesophase in dodecane solution at about 25% concentration.⁴² The helical pitch of the phase depends on temperature and concentration which has been studied by the selective reflection technique. Though there are no reports of phthalocyanines forming thermotropic nematic phases, probably due to strong π–π interactions, certain octasubstituted phthalocyanines 93 have been observed to exhibit a columnar nematic mesophase in hexadecane solution.⁸¹ A hexakis(phenylethynyl)benzene derivative 94 with chiral alanine pendant groups forms a lyotropic helical nematic columnar phase in hexane because of the intermolecular hydrogen bonding in hexane but does not do so in H-bond forming solvent.⁸² Indanthrone disulfonate 95, a non-mesomorphic discotic compound, forms a columnar nematic liquid-crystal phase in water. A thin crystal film (TCF) polarizer (E-polarizer) has been developed by processing from the lyotropic nematic phase of indathrone disulfonate. It has been demonstrated that the thin crystal film E-polarizer can be used to increase the contrast ratio and viewing angle of vertically aligned liquid-crystal display devices (VA-LCD) in combination with an O-polarizer and negative birefringent plate (Fig. 4).⁸³ Carbon nanotubes and nanoribbons have also been obtained from the columnar nematic lyo-mesphase of indanthrone disulfonate by template directed liquid-crystal assembly and subsequent covalent capture.⁸⁴ Certain halogen- and thiocyanato-bridged tetranuclear palladium complexes 96 which exhibit thermotropic columnar phases, also form lyotropic nematic phases in chloroform or pentadecane, rendering these metallomesogens amphotropic.³⁶ Other discoid amphiphiles which exhibit lyotropic nematic phase behavior include ethyleneoxy side chain substituted triphenylenes 97 and 98⁸⁵ and chromonic liquid crystals of drugs, dyes and nucleic acids.^86,87 Several ethyleneoxy side chain substituted triphenylenes have been designed and synthesized but only 97 and 98 are found to exhibit a columnar nematic phase in aqueous solution over a wide concentration range by stacking of amphiphilic triphenylenes.⁸⁵ However, both by increasing and decreasing the number of ethyleneoxy groups in the side chains no mesophase formation was observed in these amphiphilic triphenylenes. If the hydrophilic ethyleneoxy chains are too short the compounds are not water soluble whereas if the chains are made too long the compounds become very water soluble and no mesophases are observed. So it is essential to maintain a balance between the hydrophobic and hydrophilic properties of such molecules to exhibit lyo-mesomorphism.

Fig. 4 Schematic drawing of an improved vertically aligned (VA)-LCD with a negative compensator (c-plate) and the use of a combination of an E-type polarizer and an O-type polarizer. The ellipse in the VA-LC cell represents the index ellipsoid of the rod-like liquid-crystal medium, while the ellipse in the c-plate represents the index ellipsoid of the negative birefringent medium. Adapted from ref. 83.

Another interesting class of lyotropic liquid crystals which exhibit nematic phase behavior are chromonic liquid crystals (chromonics).^86,87 Chromonic liquid crystals are formed by self-organization of aromatic compounds with ionic or hydrophilic groups in aqueous solutions. Chromonic liquid crystals represent an intersection of two very active fields of research, supramolecular assembly and ordered complex fluids. In the chromonic liquid-crystal phase, an aqueous solution of a dye, drug or nucleic acid assembles to form aggregates which are anisotropic in shape. If the concentration of these aggregates is high enough and the shape of the aggregates is anisotropic enough, a nematic liquid-crystal phase (the chromonic N phase) forms in which the aggregate axes possess a preferred direction as the aggregates diffuse, break up and reform. At higher concentration and/or lower temperature the aggregates form a hexagonal array known as the chromonic M phase. The aggregates can be of monomolecular or multimolecular wide stacks and the lengths of the stacks is polydisperse in nature. The chiral nematic phase can be obtained by helical stacking of chiral chromonic mesogens or by doping a chromonic N phase with chiral dopants. Under a polarizing optical microscope, the chromonic nematic phase exhibits nematic droplets and Schlieren texture whereas characteristic cholesteric and fingerprint textures are shown by the high-pitch chiral N phase. The most widely studied chromonic system to date is disodium chromoglycate (DSCG) (99), an anti-asthmatic drug sold under the trade name Intal, followed by Sunset Yellow (100), a food colouring azo-dye. Disodium chromoglycate 99 first forms an isotropic solution at room temperature then a nematic (N) phase between ∼10 and 20 wt% and a hexagonal (columnar) mesophase (M) at higher concentration (>20 wt%). Sunset Yellow 100 also forms the nematic and hexagonal phase in water but at much higher concentrations than DSCG. Similar behavior is shown by numerous other systems including cyanine and azo-dyes.^86,87 Though their potential as optical compensators, biosensors, patterned dyes and as other functional materials have been demonstrated, the structure–property relationships of chromonic liquid crystals are not understood to the same extent as for amphiphile-based lyotropic liquid crystals. A much better fundamental understanding of the molecular mechanism underlying the aggregation is required for the full potential of chromonics to be developed.

6. Alignment technology of discotic nematic liquid crystals

The orientational control of liquid crystals is highly required not only to comprehend the correlation of their orientational order with the anisotropic properties but also to optimize performances of molecular devices using LC systems. There have been extensive reports on the orientational control of calamitic liquid crystals by mechanical rubbing and external field effects as well as photo-alignment technique, leading to their versatile applications such as in display devices. In contrast, the alignment technology for discotic liquid crystals is not well developed in general, and alignment of discotic nematic LCs, in particular. Recently there have been various efforts on ordered and organized alignment of discotic columnar phases owing to their possible application in semiconducting devices such as field effect transistors, organic photovoltaic and organic light emitting diodes, etc. In most cases the discotic molecules adopt a natural face-on arrangement on the substrate and grow as vertical columnar stacks, and they can find application in sandwich devices such as in solar cells and light emitting devices. Further, a number of techniques have recently been developed to grow columnar stacks lying parallel to the substrate surface with molecules standing edge-on, an orientation that could be useful for field effect transistors. Discotic molecules can form uniaxial alignments lying horizontal on different types of substrates by using appropriate techniques. These techniques include zone casting, friction transferred PTFE templates, Langmuir–Blodgett films, self-assembled monolayers, stationary nozzles onto a moving substrate (the drawing method) and more recently external field effects and lasers. Soon after the realization of the discotic nematic phase in pure discotic compounds, attempts have been made to align them horizontally and vertically between two coated glass plates to evaluate various physical parameters significant for display applications. Homeotropic or homogeneous alignments are obtained, respectively, with a surfactant layer such as mellitic acid, or oblique evaporation of silicon monoxide.⁸⁸ Freshly cleaved surfaces of crystals such as apophyllite and muscovite mica which are known to enforce alignment of discotic columnar phases can also be used to obtain homeotropic alignment of discotic nematic phases. Polarized infrared irradiation technique has been used for alignment control of columnar phases,⁸⁹ however, this technique is not adequate for the alignment control of nematic phases as the nematic phase shows dynamic turbulence of the texture and goes back to the original alignment. The discotic nematic mesophase shows a strong tendency for homeotropic alignment between BaF₂ substrates. Alignment behavior of discotic nematic LCs on self assembled monolayers (SAM) of alkanethiols and asymmetrical alkyl disulfides have been studied by Monobe et al.⁹⁰ The N_D phase shows typical Schlieren texture without any preferred alignment on gold and alkanethiol SAMs while a larger area of planar alignment was observed on asymmetrical alkyl disulfide SAMs when thin films are used. However nematic textures were observed for thicker films probably as a result of hybrid alignment of discotic molecules. The alignment behavior of the N_D phase has also been studied on polyimide and cetyltrimethylammonium bromide (CTAB) coated substrates.⁹¹ The orientational behavior of the nematic phase on substrates coated with a polyimide and CTAB was investigated by POM and average order parameter was evaluated by an infrared dichroic method. The discotic nematic mesophase exhibits a homeotropic alignment on a polyimide film and a tilted or planar homogeneous alignment on a CTAB-coated substrate. The order parameter is higher on a polyimide film than on a CTAB-coated substrate. In the case of the polyimide film the molecular core is uniformly parallel to the substrate while on CTAB the discotic core is perpendicular to the substrate surface. Recently, Ichimura et al. have reported a ‘command surface’ effect in which nematic liquid crystals homeotropically aligned on an azobenzene monolayer film change their alignment homogeneously by trans–cis photoisomerization.⁹² Polymethacrylate films containing photoactive p-cyanoazobenzene groups have been used for the orientational control of DLCs. Conoscopic observations revealed that a film of polymer irradiated with linearly polarized light from the surface normal induces solely homeotropic alignment with an orientational director of DLCs perpendicular to the surface plane. Oblique irradiation of the polymer film by non-polarized light induces tilted alignment of the discotic molecules in the nematic phase or hybrid alignment containing a distribution of the DLC director. An average pretilt angle of 70° from the substrate plane was estimated by birefringence measurement. This is a novel procedure to control the alignment of DLC materials based on the surface assisted photo-alignment technique.

After theoretical optimization and successful practical demonstration of the utilization of hybrid aligned and photo-polymerized discotic nematic films in commercial TFT-LCDs, deliberate attempts have been made in the Fuji photo film laboratory to achieve all different kinds of alignment of discotic nematic phases which are retained upon photo-polymerization and can be used as optical compensation films for any kind of LCD modes to widen the viewing angle and increase the contrast ratio of devices.^40,93 In the hybrid alignment thin film, which is very effective in expanding the viewing angle, the molecules in the neighborhood of the orientation film adopt horizontal orientation whereas the molecules are tilted near the free interface i.e. the hybrid alignment structure is a mixture of splay and bend deformations. The obtained thin films with appropriate alignment are durable and do not interfere with the transmittance and image quality of the display. Fuji have established hybrid, horizontal, vertical and vertical-twisted alignment of discotic nematic liquid crystals on web-coated thin films with the help of their newly developed alignment promoters that deposit toward the air surface during web coating, the alignment layer that promotes desired alignment of LCs from the substrate side, and small amounts of strong chiral agent (Fig. 5).

Fig. 5 Various alignment structures of discotic nematic liquid crystals: (a) hybrid, (b) homeotropic, (c) planar, (d) vertical twisted; and the alignment promoters used: (e) the horizontal alignment promoter from the air interface side, (f) vertical alignment promoter from the substrate side and (g) vertical orientation promoter from the free air interface. Adapted from ref. 40.

7. Physical properties of the discotic nematic liquid crystals

The discotic nematic liquid crystals were discovered during the time when its calamitic counterpart was revolutionizing the LCD technology. Quite naturally, it immediately caught the attention for the evaluation of various display parameters and development of novel alignment techniques, then, being a new probable candidate for LCD devices. The physical properties (elastic constants K₁₁, K₃₃, rotational viscosity and dielectric anisotropy) of the liquid crystals are the most important factors to be concerned with because the electrooptical characteristics of the LCDs such as the driving voltage, contrast ratio, and response time strongly depend on the physical properties of the anisotropic fluids. All the above mentioned physical properties have been measured for the discotic nematic phase of different kinds of materials and even a LCD device has been demonstrated with discotic nematic liquid crystals which possesses wide and symmetrical viewing angle characteristics. Of course the device has slower response time compared to that of conventional TN devices. It is understood that LCD industries are always on the look-out for LC materials with lower rotational viscosity and hence shorter response times. Discotic liquid crystals can not compete with their rod like counterparts in terms of electro-optic performance. As far as the physical properties are concerned there are striking similarities and distinct differences between the calamitic and discotic nematic phases. The values of elastic constants for the discotic and calamitic systems are of the same order of magnitude whereas the viscosity of the N_D phase is much higher than the calamitic nematic, and the optical anisotropy is negative, so also the diamagnetic anisotropy and the magnitude of optical anisotropy is generally lower for discotics than calamitics, but good enough for optical compensation. The dielectric anisotropy of both calamitic and discotic nematic phases can be either positive or negative depending on the detailed molecular structure. Scalar physical properties such as orientational order parameter and nematic to isotropic transition enthalpies are of the same order of magnitude for both calamitic and discotic nematic phases. There are very few reports on theoretical studies of the discotic nematic phase, though it has been predicted that discotic nematic materials can form a stable ferroelectric phase,⁹⁴ the elastic constants are very sensitive to packing density and the short range orientational order has a strong influence on the thermodynamic properties close to the N_D–I transition. However, due to technological importance of mesophase carbon fibers, more theoretical studies have been carried out on the carbonaceous mesophase to gain knowledge on how to optimize and control mesophase fiber textures; because of strong structure–property correlations, such knowledge is essential for their industrial production, product property optimization, and to reduce the post-treatment process cost.⁹⁵

8. Applications of discotic nematic LCs

8.1 LCDs using discotic nematic materials

We have prepared a novel LC display device employing discotic nematic materials. The device exhibits many improvements over a conventional TN display device using a calamitic nematic material.^15,24 The device is simple to fabricate, has excellent viewing angle characteristics showing a wide and symmetrical viewing angle profile (Fig. 6) and has much less difference in the pixel capacitance between the ON and OFF states resulting in a reduced cross talk problem. Initially we employed a high temperature nematic material and later we demonstrated the LCD device with a novel room-temperature discotic nematic material as the active switching material. The response time of the device was slower than for conventional TN devices due to the high viscosity of the N_D material and to overcome this problem we have doped, in small concentrations, a long-chain alkane compound to the parent room-temperature discotic nematic material. Systematic studies show that both the switch ON and switch OFF response times show a significant decrease, i.e. the device switches faster, in the case of the mixtures. It should be mentioned that wide and symmetrical viewing angle characteristics remain unaffected by the addition of the dopant material. However both the switch ON and OFF response times are still an order of magnitude slower compared to that of conventional TN devices. Considering the fact that these response times are not very different from those for the STN displays, the achievement of symmetric and wide viewing angle characteristics with a simple fabrication process makes this device quite interesting. To make them attractive enough to be considered for commercial applications, new discotic nematic materials with faster response times have to be developed, or alternatively novel dopants should be sought. In this regard single-walled carbon nanotubes seem to be promising candidates as they can be added as dopants to existing room-temperature discotic nematic liquid crystals to improve the device performance since they have been successfully demonstrated in conventional calamitic display devices to dramatically reduce the rotational viscosity and increase the response times.

Fig. 6 LCD device configuration in the off state (a) and in the on state (b) and the measured iso-contrast ratio (CR) plot (c) using discotic nematic liquid crystals. Note that the LCD exhibits wide and symmetrical viewing angle characteristics. Reproduced from ref. 15 with permission by The Society for Information Display.

8.2 Optical compensation films for LCDs

Twisted nematic (TN) and super twisted nematic (STN) liquid-crystal displays have been the dominating information display technologies since their invention. Simple twisted nematic displays were directly-addressed followed by multiplex-addressing for complex information display, however, the LC molecules can not respond fast enough to such addressing and hence the contrast of the display suffers greatly. STN displays followed in the mid-1980s to address the above mentioned problem but suffered from the same limitations along with inherent generation of interference colours. However, the above problems were solved by the development of active matrix (AM) addressing of the twisted nematic device through the use of thin film transistors (TFTs). Such technology allows the advantages of multiplex-addressing with no loss of contrast but this was complex and expensive. Hence cheaper and easily constructed multiplex-addressed TN and STN devices were dominating in most of the applications in the early 1990s. TN active matrix TFT technology developed steadily, eventually became much cheaper and much more reliable and consistent and is invaluable in satisfying the needs of small portable devices, such as personal organizers, cameras, mobile telephone, laptop computers, desktop monitors and some small televisions. However twisted nematic displays suffer from two major problems: one is the narrow-viewing-angle characteristics and the slow optical response speed which are severe limitations for large-area television and fast-moving graphics displays. To improve the viewing angle characteristics and response speed, other LCD formats such as in-plane switching (IPS) and multi-domain vertical alignment (VA) LCDs were introduced into the market. However, these are not very cost effective for smaller displays. When there was intense cost competition among the various LCD modes, a negative birefringence optical compensation film was introduced by Fuji photo film laboratory to widen the viewing angle characteristics and to increase the contrast ratio of TN TFT-LCDs owing to the advantages of high light transmittance, good process margin and cost effectiveness of the TN mode—the optical compensation film is simply a film made from a hybrid alignment of discotic nematic liquid crystals by photo-polymerization. The poor viewing angle performance of LCDs is caused by various factors such as the optical anisotropy of liquid crystals, the off-axis light leakage from crossed polarizers and light scattering on the surface of polarizers and colour filters, etc. The discotic optical compensation film widens the viewing angle by compensating the positive optical anisotropy of the calamitic liquid crystals by the negative optical anisotropy of discotic liquid crystals, since for a positive uniaxial medium, a negative uniaxial medium compensates the birefringence.^40,93

In the TN-LCD the rod-shaped liquid crystals orient horizontally on the orientation film of the electrode substrate when the voltage is not applied and tend to orient vertically to the substrate when voltage is applied, and so it should have a bright off-state and black on-state. However, the liquid crystals in contact with the surface of the orientation film interact strongly with it and stay almost horizontal and only gradually incline and change to vertical orientation in the thickness direction when voltage is applied. As a result a hybrid alignment region exists in the close vicinity of the alignment layer. When linearly polarized light transmits through this so-called hybrid region of the rod-like liquid crystals in the on-state of TN-LCD it gets elliptically polarized. Because of the fact that an elliptically polarized light can not be completely extinguished by a linear polarizer regardless of the nature of the polarizer, light leakage occurs and hence contrast ratio suffers. Moreover, owing to the positive birefringence of the rod-like LCs light leakages occur at oblique viewing directions. The narrowness of the viewing angle is considered to be an unacceptable aspect of TN-LCD performance. From the optical anisotropy point of view, rod-like and disc-like liquid crystals are in antisymmetric relationship, i.e. they have opposite optical anisotropy. Thus it was logically envisaged that the optical anisotropy of hybrid oriented rod-like molecules can be cancelled out by the optical anisotropy of disc-like molecules similarly oriented, that is, their combination will be optically equivalent to the isotropic space (Fig. 7). Then linearly polarized light passing through the combination will not suffer from any distortion and hence there will not be any viewing angle dependence. This compensation configuration minimizes the light leakage by the birefringence of the TN liquid-crystal layer, leading to high contrast ratio in all viewing angle and to wide-viewing angle characteristics. The discotic compensation film is based on this compensation idea and succeeded in giving a wide viewing angle (Fig. 8). The recent technological development of the optical compensation film for optically compensated bend mode is promising for the development of a fast optical response speed and wide viewing angle LCD-TVs.

Fig. 7 Schematic model of how the refractive index of TN-LCD is made isotropic by combining the average refractive index ellipsoid matching the hybrid orientation of a rod-shaped liquid crystal in the neighborhood of the two orientation films on the top and bottom in the liquid-crystal cell with the compensating refractive index ellipsoid of a discotic liquid crystal. Adapted from ref. 40.

Fig. 8 Measured iso-CR plots for TN-LCDs without (a) and with (b) the discotic optical compensation films. Clearly there is a remarkable widening of the viewing angle characteristics of the TN-LCD with the negative optical compensation film. Reproduced from ref. 93 with permission from IEEE (copyright @ 2005 IEEE).

Being motivated by the excellent performance of the optical compensating discotic film, many other possibilities were also speculated on the film to improve device performance and some of the possibilities were theoretically simulated and experimentally demonstrated. Compensation of both normally white and normally black modes was optimized for wide viewing cone, gray scale stability and an achromatic dark state.⁹⁶ The use of a single discotic film with twist alignment has been proposed for low voltage, high contrast ratio and wide viewing cone.⁹⁷ Furthermore the use of a single discotic film in a TN-LCD having reduced twist angle has also been proposed to reduce the cost of using two compensation films.⁹⁸

Summary and outlook

In spite of their rarity, discotic nematic liquid crystals have made many significant advances in a short span of time. Many discotic nematic compounds have been designed and synthesized, and various physical properties have been evaluated. The physical properties of discotic nematic liquid crystals have striking similarities and distinct differences when compared with their calamitic counterparts. The negative birefringence optical compensation films have actually found commercial application in the multibillion dollar liquid-crystal display industry. High strength and high performance carbon fibers have been obtained from carbonaceous mesophase and used for industrial applications. The control over its alignment has been well developed and the performance of the N_D phase in liquid-crystal display devices has been successfully demonstrated. This is only the beginning as many more novel discotic nematic LCs and their applications will be realized as the field grows.

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