Apolar Bimesogens and the Incidence of the Twist–Bend Nematic Phase

The nematic twist–bend phase (N_TB) was, until recently, only observed for polar mesogenic dimers, trimers or bent‐core compounds. In this article, we report a comprehensive study on novel apolar materials that also exhibit N_TB phases. The N_TB phase was observed for materials containing phenyl, cyclohexyl or bicyclooctyl rings in their rigid‐core units. However, for materials with long (>C7) terminal chains or mesogenic core units comprising three ring units, the N_TB phase was not observed and instead the materials exhibited smectic phases. One compound was found to exhibit a transition from the N_TB phase to an anticlinic smectic C phase; this is the first example of this polymorphism. Incorporation of lateral substitution with respect to the central core unit led to reductions in transition temperatures; however, the N_TB phase was still found to occur. Conversely, utilising branched terminal groups rendered the materials non‐mesogenic. Overall, it appears that it is the gross molecular topology that drives the incidence of the N_TB phase rather than simple dipolar considerations. Furthermore, dimers lacking any polar groups, which were prepared to test this hypothesis, were found to be non mesogenic, indicating that at the extremes of polarity these effects can dominate over topology.     

Effect of the linking unit on the twist-bend nematic phase in liquid crystal dimers: a comparative study of two homologous series of methylene- and ether-linked dimers

ABSTRACT

Two series of bimesogens with phenyl benzoate mesogenic units were prepared: one series having a heptamethylene spacer and the other a pentamethylene spacer with two ether-linking groups. These materials were prepared to provide experimental backing to the widely held hypothesis that methylene-linked bimesogens are more likely to exhibit the twist-bend nematic mesophase than their ether-linked counterparts. Several of the methylene-linked materials exhibited nematic and N_TB mesophases, whereas the analogous ether-linked materials gave only nematic phases albeit with significantly higher clearing points. Virtual N–N_TB transition temperatures for both methylene- and ether-linked bimesogens were extrapolated by constructing binary phase diagrams with the well-studied twist-bend material CB9CB. Contrary to our expectations these virtual transition temperatures were in most cases higher for the ether-linked bimesogens than in the analogous methylene compounds, this runs counter to reported theories and hypotheses that the incorporation of ether-linking groups should serve to destabilise the N_TB phase.     

A Twist‐Bend Nematic (NTB) Phase of Chiral Materials

New chiral dimers consisting of a rod‐like and cholesterol mesogenic units are reported to form a chiral twist‐bend nematic phase (N_TB*) with heliconical structure. The compressibility of the N_TB phase made of bent dimers was found to be as large as in smectic phases, which is consistent with the nanoperiodic structure of the N_TB phase. The atomic force microscopy observations in chiral bent dimers revealed a periodicity of about 50 nm, which is significantly larger than the one reported previously for non‐chiral compounds (ca. 10 nm).     

Ether- and Thioether-Linked Naphthalene-Based Liquid-Crystal Dimers: Influence of Chalcogen Linkage and Mesogenic-Arm Symmetry on the Incidence and Stability of the Twist–Bend Nematic Phase

The twist–bend nematic (N_TB) phase with a heliconical nanostructure of the local director generating symmetry breaking by achiral bent-shaped molecules is a hot topic of current liquid-crystal science. As opposed to the most common methylene-linked dimers, this study demonstrates chalcogen ether- and/or thioether-linked 6-(4-cyanophenyl)-2-naphthyl-based liquid-crystal dimers with symmetric and asymmetric π-conjugated mesogenic-arm structures that exhibit the N_TB phase. Although the symmetric bis(ether)-linked dimer exhibits only the conventional nematic (N) phase, the asymmetric bis(ether)-linked dimer can form the N_TB phase. All thioether-linked dimers form the N_TB phase, wherein the dimers with asymmetric arms vitrify in the N_TB phase on cooling to room temperature. The phase transitions are discussed in terms of the chalcogen linkage combination, mesogenic-arm symmetry, and spacer length. It is revealed that thioether-linked dimers based on asymmetric π-conjugated mesogenic arms with terminal cyano groups are highly beneficial for the realization of materials that form a wide range of N_TB phases and glassy N_TB states at room temperature.     

Formation of periodic zigzag patterns in the twist-bend nematic liquid crystal phase by surface treatment

ABSTRACT

Zigzag patterns were successfully generated in the twist-bend nematic (N_TB) phase of 1-(4-cyanobiphenyl-4′-yl)-6-(4-cyanobiphenyl-4′-yloxy)hexane (CB6OCB) via simple surface treatment. A detailed microscopy study using polarised optical microscopy and fluorescence confocal polarising microscopy was performed to observe the director arrangement in the zigzags, where distinctive periodic patterns were found to be aligned perpendicular to the rubbing direction. These patterns originate from the structural instability and generation of splay deformation with focal conic domain-like structures that are typically found in smectic phases, revealing that the N_TB phase has physical properties similar to those of the smectic phase. Observation of these unusual zigzag patterns in the N_TB phase opens an avenue for use of this phase in potential applications such as optical modulators and gratings.     

Twist-bend nematics,liquid crystal dimers,structure–property relations

ABSTRACT

One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (N_TB) and determine its properties. During our earlier hunt for the N_TB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the N_TB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the ²H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.     

On the twist-bend nematic phase formed directly from the isotropic phase

The intriguing twist-bend nematic (N_TB) phase is formed, primarily, by liquid crystal dimers having odd spacers. Typically, the phase is preceded by a nematic (N) phase via a weak first-order transition. Our aim is to obtain dimers where the N_TB phase is formed directly from the isotropic (I) phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)–N–I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory, since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9 and 11 methylene groups are found to possess the conventional phase sequence N_TB–N–I; surprisingly, for the propane spacer, the N_TB phase is formed directly from the I phase. The properties of these dimers have been studied with care to ensure that the identification of the N_TB phase is reliable.     

Azobenzene-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two new sets of non-symmetric liquid crystal dimers is reported, the 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexanes (CB6OABX) and 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yloxy)pentanes (CBO5OABX). The terminal substituents are methyl, methoxy, butyl, butyloxy, nitrile and nitro. All the CB6OABX dimers exhibit twist-bend nematic (N_TB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the N_TB phase. The transitional behaviour of the non-symmetric dimers is compared to that of the corresponding symmetric dimers, the 1,5-bis(4-substitutedazobenzene-4′-yloxy)pentanes (XABO5OABX) and either 1,7-bis(4-cyanobiphenyl-4′-yl)heptane or 1,5-bis(4-cyanobiphenyl-4′-yloxy)pentane. The XABO5OABX dimers all show a nematic phase and in addition, the butyl homologue exhibits a smectic A phase. The difference in transitional behaviour between the CB6OABX and CBO5OABX dimers is attributed to the difference in their molecular shapes arising from different bond angles between the para axis of the cyanobiphenyl unit and the first bond in the spacer. Specifically, the all-trans conformation of a CBO5OABX dimer is more linear than that of the corresponding CB6OABX dimer. Differences within each set of dimers are attributed to changes in the average molecular shape and the strength of the mixed mesogen interaction on varying the terminal group. Crystal structures are reported for CB6OABOMe, CBO5OABNO₂ and MeOABO5OABOMe.     

A Liquid Crystalline Oligomer Exhibiting Nematic and Twist‐Bend Nematic Mesophases

The twist‐bend nematic phase (N_TB) has been described as the structural link between the untilted uniaxial nematic phase (N) and the helical chiral nematic phase (N*). The N_TB phase exhibits phenomena of fundamental importance to science, that is, 1) the spontaneous formation of a helical pitch on the nanometer scale in a fluid and 2) the spontaneous breaking of mirror symmetry, leading to the emergence of chiral domains in an achiral system. In this Communication, we present a study on T4₉ [bis(4‐(9‐(4‐((4‐cyanobenzoyl)oxy)phenyl)nonyl)phenyl) 4,4′‐(nonane‐1,9‐diyl)dibenzoate], a liquid‐crystalline oligomer exhibiting the twist‐bend nematic phase, which has a molecular length that is of comparable dimensions to the sub‐10 nm pitch determined for CB9CB, and provide new insights into the differentiation between the nano‐ and macro‐science for self‐assembling supermolecular systems.     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side‐chain liquid crystalline (LC) polysiloxanes and elastomers ( IP ‐ VIP ) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one‐step hydrosilylation reaction with poly(methylhydrogeno)siloxane, a fluorine‐containing LC monomer 4′‐undec‐10‐enoyloxy‐biphenyl‐4‐yl 4‐fluoro‐benzoate and a crosslinking LC monomer 4′‐(4‐allyloxy‐benzoxy)‐biphenyl‐4‐yl 4‐allyloxy‐benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, ¹H‐NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T_d) were greater than 250°C for all the polymers, and the weight of residue near 600°C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP , IIP , IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T_g) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase–isotropic phase transition temperature (T_i) and smectic A–nematic mesophase transition temperature (T_S‐N) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Side-chain liquid crystalline polymers with silphenylene-siloxane main chains. III. Synthesis and characterization of polymers with phenyl benzoate mesogenic groups

Side-chain liquid crystalline (SCLC) silphenylene-siloxane polymers with a phenyl benzoate mesogenic group and polymethylene spacers were prepared and characterized, and their properties were compared with those of equivalent SCLC polymers, SCLCPs, with a biphenyl mesogenic group. With identical spacers and terminal substituents, the melting temperatures of the former were much lower, but the isotropization temperatures were lowered to a lesser extent, than those of the latter, and, consequently, a more thermally stable nematic phase was obtained for the former. Both types of SCLCPs formed nematic phases, while polymethylsiloxanes with the same side-chain mesogens exhibited smectic phases with wider temperature ranges. The lower thermal stability of the mesophases in the silphenylene-siloxane SCLCPs compared to those of the SCLC polymethylsiloxanes can be attributed to both the rigidity of the backbone and the greater separation of the side-chains along the main chains of the former.     

Cybotactic nematic phases with wide ranges in photoresponsive polycatenars

ABSTRACT

We report on the synthesis and phase behaviour of new light-responsive polycatenars. The new materials represent tetracatenar molecules with four terminal chains which are substituted at 2,3,4-position in one benzene ring at one terminus and at the other terminus only one chain is present. The liquid-crystalline behaviour of the prepared materials was characterised by differential scanning calorimetry (DSC), polarised optical microscope (POM) and X-ray diffraction (XRD). We investigated the effect of changing the type of one terminal chain, while keeping the remaining three chains fixed on the mesomorphism of these materials. All of the tetracatenars exhibit nematic and smectic C phases. By investigating the nematic phase with XRD it was found that it represents nematic phase with cybotactic clusters of the SmC type (N_CybC). Moreover, these nematic phases exist over wide temperature ranges for all compounds. Additionally, the photosensitivity of these polycatenars was studied upon light irradiation.     

Chiral palisade side-chain liquid crystal copolymers: effect of comonomer end group on the nature of the liquid crystalline

Three series of copolymers were prepared from 4-[2-(S)-methyl-3-acryloxy)-4′-methoxy phenyl] benzoate, as the common chiral monomer, and three nonchiral 4-alkyloxy phenyl-4′-(6-acryloxy hexyloxy) benzoates with alkyloxy tail groups containing seven to nine atoms. All of the copolymers exhibited liquid crystalline (LC) behavior over the entire composition ranges studied. It was demonstrated that by changing the length of the alkyloxy unit, significant differences could be induced in the LC phases observed. When the alkyloxy unit was seven atoms long only chiral nematic (N*) phases were detected, whereas lengthening the alkyloxy unit to eight and nine atoms led to the formation of smectic A (S_A) and chiral smectic C (S*_C ) phases in addition to the N* phase. Films of these materials exhibited selective reflection in the visible region as one would expect from the presence of N* and S*_C phases. © 1998 John Wiley & Sons, Ltd.     

Odd-even effects in the phase transition behaviour of novel U-shaped liquid crystals

A homologous series of U-shaped dimeric liquid crystals in which two mesogenic groups are connected via catechol has been prepared and the effects of terminal alkyl chains, alkyl spacers and core structures on the transition properties investigated by means of optical microscopy and differential scanning calorimetry. The phase sequence exhibits a pronounced odd-even effect as the length and parity of the spacers is varied, in which the even members favour the nematic and smectic C phases, whereas the odd members favour the nematic and smectic A phases. We discuss the transition behaviour of the U-shaped compounds in terms of molecular shape.     

Reentrant phenomenon in cyanoaryl alkylthioxybenzoates and cyanobenzoyloxyphenyl alkylthiobenzoates

Abstract

Three new series of cyanoaryl 4-alkylthioxybenzoates have been synthesized and characterized. The 4-cyanophenyl series displays only transient mesomorphic properties while the 6-cyano-2-naphthyl and 4-cyanobiphenyl series exhibit the classical nematic and smectic A phases; a reentrant nematic phase is also found in the latter. Two other new series in which the position of the sulphur atom in the central rigid core has been varied were also prepared: the 4–[4′-cyanobenzoyloxy]-phenyl 4-alkylthiobenzoates and 4–[4′-alkylbenzoyloxy]phenyl 4-cyanothiobenzoates. As in the analogous benzoate series, the reentrant phenomenon with the sequence C, S_Are, N_re, S_A, N, I is observed with the nonyl derivatives. Comparisons between the alkoxybenzoates and the alkylthioxybenzoates are given.     

Structural assignment and molecular order of three-ring mesogen by 13C NMR spectroscopy in mesophase

A homologous series of rod-like molecules with three phenyl rings in the core and terminal alkoxy chains are synthesised from mesogenic two-ring aldehyde by coupling with non-mesogenic 4-alkoxy anilines. The mesophase properties are evaluated with hot-stage optical polarising microscopy and differential scanning calorimetry, and accordingly, all the molecules exhibited enantiotropic nematic and smectic C phases along with monotropic low-temperature smectic phases. For a representative homologue, the existence of smectic C phase is further confirmed by noticing a sharp reflection at small angle region in powder X-ray diffraction which varies with change in temperature. The main focus of the investigation, however, is the clear demonstration of chemical shift assignment of static ¹³C NMR of a representative three-phenyl ring mesogen in smectic C phase. In this novel approach, the static ¹³C NMR spectral data of synthetic mesogenic intermediate namely two-ring aldehyde are utilised for the chemical shift assignment of three-ring mesogens. Further, the orientational order parameter of two-ring aldehyde in smectic A and three-ring mesogen in smectic C phase is carried out by measuring the ¹³C-¹H dipolar couplings by 2D separated local field spectroscopy.     

Smectic-like bâtonnets in nematic/twist-bend nematic biphasic samples

ABSTRACT

We have studied a mixture of the twist-bend nemogenic dimer CB7CB with rod-like nematic molecules, which exhibits nematic (N) and twist-bend nematic (N_TB) phases and a very large biphasic coexistence range. At the N-N_TB transition, we observe the nucleation of highly anisometric N_TB droplets which are very similar to the classic smectic A (SmA) bâtonnets. These observations confirm the recently proposed close analogy between the N_TB and SmA phases, on the basis of their identical macroscopic symmetry. As for their smectic analogues, the N_TB bâtonnets are fluid in two dimensions; they easily merge when brought into contact and they are solid-like in that they did not flow along their optic axis. The observed fluidity and low viscosity show that the N_TB phase is indeed a nematic phase, i.e. an anisotropic fluid, rather than a soft crystal or glassy state. Unlike their smectic analogues, the N_TB bâtonnets have almost perfect symmetry of revolution and the axis of the N_TB helix is uniformly aligned parallel to the long axis of the bâtonnet. The large aspect ratio of the bâtonnets, typically ≈ 10–30, indicates a very strong anisotropy of the N-N_TB interfacial energy, W₂/W₀ ≈ 200–2000, and suggests that the anchoring energy differs from the usual Rapini–Papoular form.     

Molecular structure and the twist-bend nematic phase: the role of terminal chains

ABSTRACT

The synthesis and characterisation of two homologous series of non-symmetric dimers are reported, the 1-(4-methoxybiphenyl-4?-yl)-6-(4-alkylanilinebenzylidene-4?-oxy)hexanes (MeOB6O.m, m = 1–10) and 1-(4-methoxybiphenyl-4?-yl)-6-(4-alkyloxyanilinebenzylidene-4?-oxy)hexanes (MeOB6O.Om, m = 1–9). All 10 members of the MeOB6O.m series exhibit the conventional nematic phase. At lower temperatures, the members with m = 1–7 formed the twist-bend nematic phase, N_TB, whereas for m = 8–10 smectic behaviour replaced the N_TB phase. All nine members of the MeOB6O.Om series also show the conventional nematic phase and for m = 1–3, a strongly monotropic N_TB phase is also observed. The alkyloxy terminated dimers show the higher values of T_NI and T_{NTB N} . For both series, the values of T_NI and T_{NTB N} show a modest alternation and in the same sense as m is increased. These observations suggest that the spatial uniformity of molecular curvature is important in driving the formation of the N_TB phase. The observation of smectic behaviour is attributed to the molecular inhomogeneity arising from the long terminal alkyl chain driving microphase separation. The transitional behaviour of these series is compared to those of the corresponding cyanobiphenyl-based series and overarching observations discussed.