The influence of the liquid crystalline core geometry on the mesogenicity of novel chiral 2-(4-substituted-phenoxy)propanonitriles

Abstract

The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense.     

Blue phase stability of n-OCB homologue chiral nematic liquid crystal mixtures

Blue phase (BP) stability of a chiral nematic liquid crystal (LC) mixture is dependent upon chemical structure as well as physical properties. In this study, the blue phase temperature range dependent on alkyl chain length was investigated in order to evaluate the relationship between blue phase stability and the molecular structures of four kinds of 4-n-alkyloxy-4'-cyanobiphenyl (n-OCB) homologue chiral nematic LC mixtures composed of rod-like nematic LCs. It was confirmed that the blue phase temperature range was strongly dependent upon the molecular parity, K ₃₃/K ₁₁ and the helical twist power of the n-OCB homologues chiral nematic LC mixtures.     

The effect of the position of the methyl group on the helical twisting powers of aldol condensation products of methyl cyclohexanones

The four aldol condensation products of methylcyclohexanone isomers and 4-(4-hexyloxybenzoyloxy)benzaldehyde have been prepared. These chiral products were either obtained with high enantiomeric excess or the enantiomers were separated by means of chiral HPLC. In all cases only the E-isomer was obtained. UV irradiation was used to isomerize the E-isomers to the Z-isomers. Comparison of the helical twisting powers (HTP) of the E-isomers revealed that the derivative in which the methyl group is directly next to the double bond exhibits the highest value. Although the other three E-isomers have lower HTP values, they show large HTP changes and reversal of the helical twist sense upon irradiation. Of these compounds, the one derived from the commercially available (R)-3-methylcyclohexanone is an interesting candidate for use as an alternative to compounds derived from menthone for realizing photochemically induced twist changes in twisted nematic and cholesteric materials.     

On the parity in helical twisting power of Ru(III) 1,3-diketonates of C2 symmetry in nematic liquid crystals

It is demonstrated that the sign of helical twisting power (HTP) of an enantiomeric Ru(III) complex of type [Ru(acac)(2)L] can be switched by choosing L from either L(per) or L(para), which is elongated either perpendicular or parallel to the C(2) symmetry axis, and four states become available in combination with DeltaLambda-chirality of the metal center. Complexes 1-n, in which 4,4'-dialkoxylated dibenzoylmethanate ligands are used as L(per), and 2 having L(para) = 3-(4'-decyloxyphenyl)pentane-2,4-dionate ligand were prepared for this purpose. They were optically resolved into the enantiomers by means of a clay column chromatography, and their performance as chiral dopants was evaluated in nematic liquid crystals including a room-temperature system, N-methoxybenzylidene-4-n-butylaniline (MBBA), which allowed facile measurements of the helical pitch lengths and CD spectra in the induced chiral nematic states. The induced CD signals have provided a clear evidence for the helical inversion between the two structure types, 1 and 2, of the same chirality. The twisting power of these six-coordinate metal complexes and their structure versus twist sense correlations are interpreted by the shape model. Intrinsically high HTP of Delta-[Ru(acac)(2)L(per)] has also allowed for observation of the pitch band due to the selective reflection in the visible wavelength range at the doping level of 2 mol % in MBBA.     

The effect of the position of the methyl group on the helical twisting powers of aldol condensation products of methyl cyclohexanones

The four aldol condensation products of methylcyclohexanone isomers and 4-(4-hexyloxybenzoyloxy)benzaldehyde have been prepared. These chiral products were either obtained with high enantiomeric excess or the enantiomers were separated by means of chiral HPLC. In all cases only the E-isomer was obtained. UV irradiation was used to isomerize the E-isomers to the Z-isomers. Comparison of the helical twisting powers (HTP) of the E-isomers revealed that the derivative in which the methyl group is directly next to the double bond exhibits the highest value. Although the other three E-isomers have lower HTP values, they show large HTP changes and reversal of the helical twist sense upon irradiation. Of these compounds, the one derived from the commercially available (R)-3-methylcyclohexanone is an interesting candidate for use as an alternative to compounds derived from menthone for realizing photochemically induced twist changes in twisted nematic and cholesteric materials.     

Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.     

Modification of the pitch of chiral nematic liquid crystals by means of photoisomerization of chiral dopants

New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the non-irradiated regions. It is expected that this approach may be useful in the preparation of dual domain TN cells with a reduced viewing angle dependence.     

Synthesis and characterization of non‐symmetric chiral dimers

A new class of cholesterol‐based non‐symmetric dimers have been synthesized and characterized. They comprise O‐alkylated cinnamic acid and pro‐mesogenic cholesterol segments interlinked covalently through a methylene spacer varying in its length and parity. All the dimers and some of the key precursors have been studied for their phase behaviour. All the intermediates show mesomorphism. Importantly, the thermal properties of dimers are found to be critically dependent on the parity of the flexible spacer. The dimers with odd‐parity spacer display chiral nematic and/or twist grain boundary phases. In contrast, the dimers with even‐parity spacer are either crystalline or exhibit metastable chiral nematic and/or twist grain boundary phases with the exception of one compound for which two unknown mesophases have been observed. The odd–even effect was found to be indistinct for selective reflection wavelengths of the chiral nematic phase. For some dimers, a variation in the pitch of the chiral nematic phase as a function of temperature was observed. Cyclic voltammetry experiments revealed the electrochemical properties of a representative liquid crystal dimer.     

Fourth-order nematic elasticity and modulated nematic phases: a poor man’s approach

We propose an extension of Frank-Oseen’s elastic energy for bulk nematic liquid crystals which is based on the hypothesis that the fundamental deformations allowed in nematic liquid crystals are splay, twist and bend. The extended elastic energy is a fourth-order form in the fundamental deformations. The existence of bulk spontaneous modulated or deformed nematic liquid crystal ground states is investigated. The analysis is limited to bulk nematic liquid crystals in the absence of limiting surfaces and/or external fields. The non deformed ground state is stable only when Frank-Oseen’s elastic constants are positive. In case where at least one of them is negative, the ground state becomes deformed. The analysis of the stability of the deformed states in the space of the elastic parameters allows to characterise different types of deformed nematic phases. Some of them are new nematic phases, for instance a twist – splay nematic phase is predicted. Inequalities between second-order elastic constants which govern the stability of the twist–bend, splay–bend, and splay–twist states are obtained. Their stability in respect to triple splay–bend–twist deformations is investigated.     

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.     

A simple method to measure the twist elastic constant of a nematic liquid crystal

We demonstrate a simple method for measuring the twist elastic constant (K₂₂) of a nematic liquid crystal (LC). By adding some chiral dopant to an LC host, the LC directors rotate 180° in a homogeneous cell, which is known as 180° super-twisted nematic (STN) cell. By preparing two such STN cells with different chiral concentrations and measuring their Fréedericksz threshold voltages, we can obtain the K₂₂ and helical twisting power simultaneously. In the whole process, there is no need to measure the pitch length. Our obtained K₂₂ values agree well with those reported by using other methods.     

Solution of a molecular statistical model for chiral nematic liquid crystals

A molecular statistical model for the chiral nematic phase of liquid crystals is investigated. The model is treated in the molecular field approximation. The resulting set of coupled integral equations for the order parameters and the pitch determining equation are solved numerically. The model hamiltonian consists of a nematic and a twist producing term. If only the nematic term is present, the model is known to have a first order phase transition. The model containing only the twist producing term is shown to exhibit a second order phase transition. The order parameters and wave length are presented for three ratios of the coupling constants in the case that both interactions are present.     

Improved nematic mesophase stability of benzoxazole-liquid crystals via modification of inter-ring twist angle of biphenyl unit

A series of nematic heterocyclic liquid crystals, 2-(2?-fluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(2)PBx) bearing hydrogen (nPF(2)PBH), methyl (nPF(2)PBM) and nitro (nPF(2)PBN) terminal groups, respectively, are developped and investigated. The results show that the nematic mesophase stability is obviously improved via a slightly increase in the inter-ring twist angle between the planes of two phenyl rings in the biphenyl unit, where modification of the twist angle can be achieved by adjusting the position of the lateral monofluorine substituent from the outer to the inter ring of the biphenyl mesogenic core unit. Meanwhile, the inter-ring lateral fluorine substituent results in a decrease in both melting and clearing points but only a nematic mesophase with an acceptably wide mesophase range for nPF(2)PBx, and thus a potential application in liquid crystal display mixtures is expected, especially for the compounds nPF(2)PBM and nPF(2)PBN. The reduced π–π interaction/conjugation caused by the increased twist angle is considered as one of the possible reasons for the formation of a nematic mesophase.     

Textures of homologous 4-n-alkyloxybenzoic acids: spontaneous chirality and surface memory

By using polarizing microscopy analysis we have found that several achiral homologues of the 4-n-alkyloxybenzoic acids, displaying only the nematic phase, exhibit the optical properties of a chiral liquid crystal system. These acids possess a mesophase due to the formation of dimers via hydrogen bonding. The microtextural analysis was carried out in the temperature ranges of the isotropic, nematic and crystal phases. The nucleation of a chiral texture in small domains emerging on cooling in the isotropic phase was observed. These small domains are characterized by a conoscopic cross which presents an azimuth of 45° with respect to the polarizer axis, contrary to the usual nematic drops, for which the conoscopic cross is not rotated. On further cooling, these domains coalesce in the nematic phase close to the clearing point, thus building large chiral monodomains. Such coalesced droplets exhibit very thin stripe lines, as in the case of pure cholesterics with a tilted helix axis. Moreover, left- and right-handed chiral domains were observed, combined in regions partially separated by 'oily streaks', also typical of pure cholesterics. On cooling, the chiral nematic (N*) phase transformed through a pronounced texture transition into a normal nematic phase. However, the small chiral grains that formed from the isotropic phase are retained close to the surface, acting as 'memorizing centres'. With suitable boundary conditions, they can provide a macroscopic twist driven by the surface. Moreover, a twisted smectic B not present in the bulk phase diagram was found and interpreted as induced by the surface. Also in the crystal phase a strong memorization of the chiral N* texture was observed.     

The use of bowed reverse twist disclination lines for the measurement of long pitch lengths in chiral nematic liquid crystals

A novel method is described for the measurement of long pitch lengths in chiral nematic liquid crystals using the bowing of reverse twist disclination lines in 90° twisted nematic devices. The method has been used to measure pitch lengths of up to 50 mm, significantly longer than previously possible using existing methods.     

The use of bowed reverse twist disclination lines for the measurement of long pitch lengths in chiral nematic liquid crystals†

A novel method is described for the measurement of long pitch lengths in chiral nematic liquid crystals using the bowing of reverse twist disclination lines in 90° twisted nematic devices. The method has been used to measure pitch lengths of up to 50 mm, significantly longer than previously possible using existing methods.     

A Nanohelicoidal Nematic Liquid Crystal Formed by a Non‐Linear Duplexed Hexamer

The twist‐bend modulated nematic liquid‐crystal phase exhibits formation of a nanometre‐scale helical pitch in a fluid and spontaneous breaking of mirror symmetry, leading to a quasi‐fluid state composed of chiral domains despite being composed of achiral materials. This phase was only observed for materials with two or more mesogenic units, the manner of attachment between which is always linear. Non‐linear oligomers with a H‐shaped hexamesogen are now found to exhibit both nematic and twist‐bend modulated nematic phases. This shatters the assumption that a linear sequence of mesogenic units is a prerequisite for this phase, and points to this state of matter being exhibited by a wider range of self‐assembling structures than was previously envisaged. These results support the double helix model of the TB phase as opposed to the simple heliconical model. This new class of materials could act as low‐molecular‐weight surrogates for cross‐linked liquid‐crystalline elastomers.     

Coexistence of nematic and chiral nematic phases of an achiral liquid crystal trimer possessing an octafluorobiphenyl unit

We prepared some achiral flexible liquid crystal oligomers possessing an octafluorobiphenyl unit and investigated their phase transition properties using polarising optical microscopy and differential scanning calorimetry. All the compounds showed nematic phases. The trimer with even-numbered spacers was found to exhibit coexistence of the nematic regions and the chiral nematic regions with opposite twist senses of a sample contained in a homogeneous aligned cell during a cooling process, whilst that with odd-numbered spacers did not. We discuss how the even-numbered trimer produces the helical state.     

Anomalies in the twist elastic behaviour of mixtures of calamitic and bent-core liquid crystals

ABSTRACT

The splay, twist and bend elastic constants (K₁₁, K₂₂ and K₃₃) have been measured as a function of temperature in bent-core/calamitic mixtures based on three different calamitic materials (5CB, 8CB and ZLI1132) and two bent-core dopants. The behaviour of the splay and bend constants are as expected; a reduction in K₃₃ of ~20%, in line with predictions from mixing rules and other observations. Interestingly, no change is seen in the splay constant, K₁₁ of the calamitic hosts. Surprisingly though, the twist elastic constant exhibits a reduction of 30 – 40% in all mixtures across the nematic range, an effect not previously reported and much larger than mixing rules can explain. The elastic behaviour is universal in our mixtures. We explain part of the reduction in the twist deformation by considering the influence of the chiral conformer fluctuations of the bent-core molecules on the twist elastic constants of the mixtures. However, the dramatic reduction can only be fully explained by also including contributions from chiral conformer fluctuations of the calamitic host, a form of chiral amplification.     

Photochemically induced cholesteric-nematic transition in liquid crystals

By doping a nematic liquid crystal with dissymmetric chiral compounds it has been shown that the twist of the resulting cholesteric structure is proportional to the solute concentration. Using as doping agent a photolabile chiral compound a cholesteric-nematic transition has been obtained under UV irradiation.