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.     

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.     

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

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

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.     

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.     

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.     

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4?-[1,ω-alkanediylbis(thio)]bis-[1,1?-biphenyl]-4-carbonitriles (CBSnSCB), and 4?-({ω-[(4?-cyano[1,1?-biphenyl]-4-yl)oxy]alkyl}thio)[1,1?-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 3⁴-{6-[(4?-cyano[1,1?-biphenyl]-4-yl)thio]-hexyl}[1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitrile (CT6SCB), is also reported and shows enantiotropic N_TB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-N_TB transition corresponds to 4 longitudinal molecular distances.     

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

ABSTRACT

The twist-bend phase (N_TB) is most commonly observed in materials with a gross-bent shape: dimers; bent-cores; bent-oligomers. We had suggested previously that the bend-angle of such systems effectively dictates the relative thermal stability of the N_TB phase. However, our earlier paper relied on the use of a single energy-minimum conformer and so failed to capture any information about flexibility and conformational distribution. In the present work, we revisit our hypothesis and examine a second set of dimers with varying linking groups and spacer composition. We have improved on our earlier work by studying the conformational landscape of each material, allowing average bend-angles to be determined as well as the conformer distribution. We observe that the stability of the N_TB phase exhibits a strong dependence not only on the Boltzmann-weighted average bend-angle (rather than just a static conformer), but also on the distribution of conformers. To a lesser extent, the flexibility of the spacer appears important. Ultimately, this work satisfies both theoretical treatments and our initial experimental study and demonstrates the importance of molecular bend to the N_TB phase.     

Dependence of Mesomorphic Behaviour of Methylene‐Linked Dimers and the Stability of the NTB/NX Phase upon Choice of Mesogenic Units and Terminal Chain Length

Twelve symmetrical dimeric materials consisting of a nonamethylene (C9) spacer and either phenyl 4‐(4′‐alkylphenyl)benzoate, phenyl 4‐(4′‐alkylcyclohexyl)benzoate or phenyl 4‐(4′‐alkylbicyclohexyl)carboxylate mesogenic units were prepared and their mesogenic behaviour characterised by POM, DSC and XRD. All of the materials exhibited nematic phases with clearing points in excess of 200 °C. Four compounds were found to exhibit the twist‐bend nematic phase, with one material exhibiting a transition from the N_TB phase into an anticlinic smectic ‘X’ phase. Across all three series of compounds the length of terminal chain is seen to dictate, to some degree, the type of mesophase formed: shorter terminal chains favour nematic and N_TB mesophases, whereas longer terminal aliphatic chains were found to promote smectic phases.     

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.     

The twist-bend nematic phase: molecular insights from a generalised Maier–Saupe theory

ABSTRACT

Bent-shaped liquid crystal dimers exhibit a non-conventional nematic phase, which in the last few years has been actively investigated. A structural model of this phase has been proposed, which is characterised by a helical modulation with a periodicity of the order of few molecular lengths. Such a model, which is generally denoted as twist-bend nematic (N_TB), is consistent with various experimental evidences and is supported by theories and simulations. Here, we will examine in more detail the features and the implications of a generalised Maier–Saupe theory, which was recently proposed to explain the origin and structure of the N_TB phase. In particular we will analyse the relevant molecular order parameters, with special attention to the polar and biaxial ones, and we will discuss the concept of ‘tilted director’, which may give rise to some ambiguity. The phase behaviour will be described as a function of the bend angle between the mesogenic moieties of a dimer, which in the generalised Maier–Saupe theory is the key molecular feature. Extension of the theory to include fluctuations of the bend angle will allow us to examine the effect of the molecular flexibility on the phase diagram.     

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.     

New chiral liquid crystal monomers and cholesteric polyacrylates: synthesis and characterisation

The synthesis of new chiral monomers (M₁ ?M₃ ) based on menthol and the corresponding polyacrylates (P₁ ?P₃ ) is described. The chemical structures, formula and phase behaviour of the obtained monomers and polymers were characterised with FT-IR, ¹H-NMR, elemental analyses, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). The effect of the mesogenic core rigidity, spacer length and menthyl steric effect on the phase behaviour of M₁ ?M₃ and P₁ ?P₃ is discussed. The expected mesophase of the compounds based on menthol can be obtained by inserting a flexible spacer between the mesogenic core and the terminal groups. For the chiral monomers and polyacrylates, their corresponding melting temperature (T _m), glass transition temperature (T _g) and clearing temperature (T _i) increased with an increase of the mesogenic core rigidity; while the T _m, T _g and T _i decreased with increasing the spacer length. M₁ and P₁ showed no mesophase, while M₂ and M₃ all revealed a SmC* and cholesteric phases. P₂ and P₃ only showed a cholesteric phase.     

Molecular Flexibility and Bend in Semi-Rigid Liquid Crystals: Implications for the Heliconical Nematic Ground State

The N_TB phase phases possess a local helical structure with a pitch length of a few nanometers and is typically exhibited by materials consisting of two rigid mesogenic units linked by a flexible oligomethylene spacer of odd parity, giving a bent shape. We report the synthesis and characterisation of two novel dimeric liquid crystals, and perform a computational study on 10 cyanobiphenyl dimers with varying linking groups, generating a large library of conformers for each compound; this allows us to present molecular bend angles as probability weighted averages of many conformers, rather than use a single conformer. We validate conformer libraries by comparison of interproton distances with those obtained from solution-based 1D ¹H NOESY NMR, finding good agreement between experiment and computational work. Conversely, we find that using any single conformer fails to reproduce experimental interproton distances. We find the use of a single conformer significantly overestimates the molecular bend angle while also ignoring flexibility; in addition, we show that the average bend angle and flexibility are both linked to the relative stability of the N_TB phase.     

Mesogenic Unsymmetric dimers containing cholesteryl ester and tolane moieties

Among unsymmetric oligomesogens, chiral dimers formed by connecting a cholesteryl ester fragment with various aromatic mesogenic cores through a polymethylene spacer have been attracting much attention due to their remarkable thermal behaviour. In particular, dimers containing a diphenylacetylene segment having an alkoxy chain have shown interesting mesomorphic behaviour. In view of this a new series of unsymmetric dimers consisting of a diphenylacetylene moiety having an alkyl chain and a cholesteryl ester unit joined through a paraffinic spacer have been synthesized and their liquid crystalline properties characterized. The lengths of the central methylene spacer (C₃, C₄, C₅ and C₇) as well as that of the alkyl chain (n-butyl, n-pentyl, n-hexyl and n-heptyl) have been varied to establish structure–property relationships. These investigations have revealed that all the dimers exhibit smectic A, twist grain boundary and chiral neamtic (N*) phases with the exception of one of the dimers for which only the N* phase was observed. Some differences in the mesomorphic properties of the unsymmetric dimers containing odd or even parity methylene spacers have been observed. The majority of dimers having an even (C₄) parity paraffinic spacer show a blue phase while the dimers with odd (C₃, C₅ and C₇) parity spacers exhibit the chiral smectic (SmC*) phase. In some cases, the SmC* phase exists well below (?60°C) and above room temperature.     

The intercalated smectic A phase. The liquid crystal properties of the α-(4-cyanobiphenyl-4′-yloxy)-ω)-(4-alkyloxycinnamoate)alkanes

Abstract

A novel system of non-symmetric dimers containing 4-n-alkyloxy-substituted cinnamic acid and cyanobiphenyl groups has been studied. Two series were prepared: in one the flexible spacer was varied in length while the spacer was fixed. The spacer length has a profound influence on the nematic-isotropic transition temperature of these materials and a large odd-even effect is observed for the series. The terminal chain also plays a significant role in determining the liquid crystal phase behaviour: a smectic A phase is exhibited for the ethyl and propyl homologues, in addition to a nematic phase; this smectic phase vanishes for intermediate chain lengths but then reappears for the nonyl and decyl members of the series. X-ray diffraction has revealed the structure of the smectic A phase for the ethyl homologue to be intercalated, whereas that for the decyl compound is interdigitated. The existence of the intercalated smectic A phase has previously been explained in terms of a charge-transfer interaction between unlike mesogenic groups. However, for the non-symmetric liquid crystal dimers described here this specific interaction appears unlikely and we discuss, therefore, other possible mechanisms for the formation of intercalated smectic phases.     

Correlation of structure and phase behaviour for a series of modular,chiral liquid crystal diacrylates based on lactic acid

The mesogenic properties of a family of chiral liquid crystal (LC) diacrylates based on a 4-[4-(1R-methyl-2-hydroxyethoxy)phenyl]phenyl 4-hydroxybenzoate core were studied as a function of different tail lengths. In general, this family of LCs was found to exhibit a strong preference for adopting the chiral smectic A phase. Systematic variation of the alkyl spacer lengths on either side of the chiral core revealed that the onset of smectic A behaviour is highly sensitive to the length of the tail adjacent to the chiral unit. However, no correlation between phase transition temperatures and the length of the spacer on the other side of the core was observed. With a fixed spacer length on the chiral side of the core, systematic changes in the length of the other tail resulted in the formation of a monotropic smectic B phase and an increased tendency to supercool.     

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).     

Synthesis,structure, and properties of chiral liquid crystal monomers and polymers based on menthol

To study structure–mesomorphism relationships of the monomers and polymers based on menthol, four new chiral monomers ( M₁ – M₄ ) and the corresponding homopolymers ( P₁ – P₄ ) with menthyl group were synthesized. Their chemical structures, formula, phase behavior, and thermal stability were characterized by FTIR, ¹H NMR, ¹³C NMR, elemental analyses, differential scanning calorimetry, polarizing optical microscopy, X‐ray diffraction, and thermogravimetric analysis. The selective reflection of light was investigated with ultraviolet/visible spectrometer. The influence of the mesogenic core rigidity, spacer length, and menthyl steric effect on the mesomorphism of M₁ – M₄ and P₁ – P₄ was examined. By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, four target monomers and polymers could form the expected mesophase. Moreover, their melting temperature (T_m), glass transition temperature (T_g), clearing temperature (T_i), and mesophase range (ΔT) increased with increasing the mesogenic core rigidity; whereas the T_m and T_g decreased, T_i and ΔT increased with an increase of the spacer length. M₁ and M₂ showed monotropic and enantiotropic cholesteric phase, respectively, whereas M₃ and M₄ all revealed chiral smectic C (SmC*), cholesteric and cubic blue phases. In addition, with increasing temperature, the selective reflection of light shifted to the long wavelength region at the SmC* phase range and to the short wavelength region at the cholesteric range, respectively. P₁ and P₂ only showed a smectic A (SmA) phase, whereas P₃ and P₄ exhibited the SmC* and SmA phases. All the obtained polymers had very good thermal stability. © 2012 Wiley Periodicals, Inc. J. Polym. Sci. Part A: Polym Chem, 2012