Bent-core Mesogens Research Articles

1,2,4‐Oxadiazole‐Based Bent‐Core Liquid Crystals with Cybotactic Nematic Phases

Several series of bent-core mesogens derived from 3,5-diphenyl-1,2,4-oxadiazole with or without lateral groups and with different length terminal chains at both ends, and polycatenar molecules with three to six alkoxy chains are synthesized and their mesomorphic behaviour is investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction (XRD), dielectric, electro-optical and second-harmonic generation (SHG) experiments. Most compounds exhibit broad regions of skewed cybotactic nematic (NcybC ) and tilted smectic (SmC) phases with a strong tilt of the aromatic cores (up to 63°), but non-tilted SmA and NcybA phases are also observed for a compound that has only one terminal chain. The XRD patterns of the nematic phases of most of the compounds investigated indicate a 2D periodicity with short correlation length in the magnetically aligned samples. This is of importance for the general interpretation of the small-angle XRD splitting patterns typically observed for aligned samples of bent-core nematic phases. In most nematic phases one current peak is observed in the half period of an applied electric field, though no coherent signal is found in the SHG experiments. Based on additional electro-optical and dielectric results, the nematic phases are considered to be cybotactic nematic phases with local polar order, and show a dielectric reorientation of the polar domains. Only chiral nematic phases (NcybC *), but not blue phases, are obtained for compounds with one or two chiral (3S)-3,7-dimethyloctyloxy tail(s).

Cybotactic nematic phase in main-chain polyesters with bent-core mesogens

New six main-chain polyesters containing bent-core mesogens with various central units (Ar = 1,2- or 1,3-phenylene and 1,6-, 1,7-, 2,3- or 2,7-naphthylene) were synthesized. All of the polymers were soluble in a mixture of phenol/p-chlorophenol/TCE (25/40/35 w/w/w). The inherent viscosities were in the range of 0.18–0.38 dL/g. Despite the existence of bent-core mesogens eliciting a remarkable diminution of molecular packing, polymer with Ar = 2,7-naphthylene formed a smectic phase, and polymers with both Ar = 1,2-phenylene and 2,3-naphthylene formed nematic phases. Moreover, most of the polymers were distinctly semi-crystalline. Herein, we report our findings regarding the first detection of cybotactic groups from the nematic phases of main-chain polyesters with bent-core mesogens.

Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen.

The chiral, heliconical (twist-bend) nematic ground state is reported in an achiral, rigid, bent-core mesogen (UD68). Similar to the nematic twist-bend (N(TB)) phase observed in bent molecular dimers, the N(TB) phase of UD68 forms macroscopic, smecticlike focal-conic textures and exhibits nanoscale, periodic modulation with no associated modulation of the electron density, i.e., without a detectable lamellar x-ray reflection peak. The N(TB) helical pitch is p(TB) ∼ 14 nm. When an electric field is applied normal to the helix axis, a weak electroclinic effect is observed, revealing 50-μm-scale left- and right-handed domains in a chiral conglomerate.

Electro-optic and molecular relaxation behaviour of fluoro substituted achiral unsymmetrical four-ring bent-core mesogen

Electro-optic and molecular relaxation studies in a new achiral unsymmetrical four-ring bent-core mesogen are presented in this study. The compound consists of a laterally fluoro-substituted 3,4′-disubstituted biphenyl unit with an ester linkage between the two phenyl rings as the central unit. The mesomorphic properties were characterised by polarising optical microscopy and differential scanning calorimetry. The compound exhibited enantiotropic B7–Bx phase sequence. The electro-optic studies revealed a bistable switching from planar to a homeotropic state under an electric field in B7 phase. Further memory effect was observed between planar and homeotropic textures after the removal of electric field in B7 phase. Temperature dependent permittivity studies as a function of frequency revealed that the molecular relaxation increases with the increasing temperature in B7 phase than in Bx phase.

Development of polar order and tilt in lamellar liquid crystalline phases of a bent-core mesogen

A new bent-core mesogen combining a 4-cyanoresorcinol unit with two terephthalate based rod-like wings and terminated by two long alkyl chains, was synthesized and investigated by DSC, XRD, optical, electrooptical and dielectric methods. A series of liquid crystalline phases in the unique sequence SmA-SmA(P)-SmCPR-(M1/SmCPα)-SmCsPA-SmCPA-SmCaPA, mainly distinguished by the degree and mode of correlation of tilt and polar order, was observed. The development of polar order is associated with the emergence of a small tilt (<10°). With decreasing temperature the tilt changes from random (SmA) via synclinic to anticlinic, while the coherence length of the polar domains grows. This small tilt gives rise to an only weak layer coupling which is in competition with the polar coupling and this leads to new modes of self assembly in lamellar phases of bent-core mesogens, among them the SmCPR and the SmCPα phases. The SmCPR phase is an only slightly tilted biaxial smectic phase with randomized polar order and the SmCPα phase is a slightly tilted and antiferroelectric switching, but uniaxial smectic phase. For this phase a regular change of the in-plane polarization vector between the layers by an angle between >0° and <90° is proposed.

Dark conglomerate phases of azobenzene derived bent-core mesogens - relationships between the molecular structure and mirror symmetry breaking in soft matter.

New 4-bromoresorcinol based bent-core molecules with peripheral fluoro substituted azobenzene wings have been synthesized and the liquid crystalline self-assembly was investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM), electro-optic studies and X-ray diffraction (XRD). A new type of optically isotropic mesophase composed of chiral domains with opposite handedness (dark conglomerate phases, DC phases) is observed, which for some homologues with medium alkyl chain length is stable down to ambient temperature. It is proposed that these DC phases are formed by helical twisted nano-domains of limited size and composed of the crystallized aromatic cores which are separated by the disordered alkyl chains. This structure is distinct from the previously known soft helical nano-filament phases (HNF phases, B4 phases) formed by extended crystalline nano-filaments and also distinct from the fluid sponge phases composed of deformed fluid layers. Comparison with related bent-core molecules having H, F, Cl, I, CH3 and CN groups in the 4-position at the resorcinol core, either with or without additional peripheral fluorines, provided information about the effects of these substituents on the tendency to form DC phases. Based on these relationships and by comparison with the minimum energy conformations obtained by DFT calculations a hypothesis is provided for the formation of DC phases depending on the molecular structure.

A structure–property relationship study of bent-core mesogens with pyridine as the central unit

The synthesis and characterization of pyridine bent-core liquid crystals are reported. Some compounds exhibit B1 and B7 mesophases. Lower and wider mesophase ranges are obtained by decreasing the polarity of linkers between the pyridine ring and the inner aromatic rings, and increasing the length of terminal chains.

The cybotactic nematic phase of bent-core mesogens: state of the art and future developments.

The molecular clustering observed in the fluid nematic phase of nonlinear liquid crystal molecules underlies exaggerated field effects that portend unique technological advances in next-generation liquid crystal displays. However, the detailed nature of the molecular organization within the clusters and the temporal and spatial persistence of the organization remain unclear. Herein we review the evolution of structural studies of this unique nematic phase. The mounting experimental evidence points to a converging picture of the microscopic nature of this relatively new class of liquid crystals.

4-Methylresorcinol based bent-core liquid crystals with azobenzene wings – a new class of compounds with dark conglomerate phases

Stochastic achiral symmetry breaking in soft matter systems, leading to conglomerates of macroscopically chiral domains (so-called dark conglomerate = DC phases) is of contemporary interest from a fundamental scientific point of view as well as for numerous potential applications in chirality sensing and non-centrosymmetric materials. Herein we report the synthesis and investigation of first azobenzene containing bent-core mesogens derived from 4-methylresorcinol forming DC phases with a new structure, distinct from the known fluid sponge-like distorted smectic phases as well as from the helical nano-filament phases (HNF phases, B4 phases). The effects of chain length and other structural modifications on achiral symmetry breaking were investigated. Homologues with relatively short alkyl chains form achiral intercalated lamellar LC phases (B6 phases), but on increasing the chains, these are replaced by the chiral and optically isotropic DC phases. Compounds with the longest alkyl chains form low birefringent crystalline conglomerates which represent less distorted versions of the optically isotropic DC-phases. Introducing additional peripheral substituents at both outer rings removes the DC phases. The DC phases were also removed and replaced by modulated smectic phases if the azo groups were replaced by ester units, showing that azo groups favour DC phase formation with new nanostructures, distinct from the previously known types.

Synthesis and properties of azomethine polymers with bent-core mesogens

Three azomethine polymers with bent-core mesogens in their main chains were synthesized via polycondensation, and the effect of the central bent-core structure (Ar=1,2-phenylene, 2,3-naphthylene, or 2,7-naphthylene) was studied. Although polymers contain bent-shaped mesogens in their main chains, no polymer showed good solubility in common organic solvents but all polymers exhibited liquid crystalline and semi crystalline properties. Two of the three polymers showed thermal degradation before melting due to the high melting point. The polymer with 2,3-naphthylene had a 229°C melting point. The optical texture of the smectic phase could be observed using quenched samples from the melt state. During heating, the smectic layer structure could be detected via X-ray diffraction.

The biaxial nematic phase of oxadiazole biphenol mesogens

Herein we review the attributes of the cluster biaxial nematic exhibited by bent-core mesogens derived from the oxadiazole biphenol mesogenic core. We present an array of static 2H NMR spectra as well as 2D powder spectra generated by rotating the nematic phase of directly labelled mesogens. Analysis of these motionally averaged NMR observations requires the nematic phase to have monoclinic symmetry. X-ray diffraction data, in particular the effects of electric and magnetic field effects, are also consonant with the cluster picture of this biaxial nematic phase.

4‐Cyanoresorcinol‐Based Bent‐Core Mesogens with Azobenzene Wings: Emergence of Sterically Stabilized Polar Order in Liquid Crystalline Phases

A new series of azobenzene containing bent‐core molecules incorporating 4‐cyanoresorcinol as the central core unit exhibiting cybotactic nematic, rectangular, columnar, and different types of tilted smectic (SmC) phases are synthesized. The mesophase behavior and phase structures are characterized in bulk and freely suspended films using a variety of experimental techniques. Depending on the chain length and temperature a series of different mesophases is observed in these compounds, ranging from cybotactic nematic via paraelectric SmC phases, polarization randomized SmCsPR phases to ferroelectric and antiferroelectric SmC phases, associated with increasing size and correlation length of the polar domains. Spontaneous formation of chiral domains is observed in the paraelectric SmC and the SmCsPR phases and discussed in relation with superstructural chirality, bend elastic constants, and surface effects.

Laterally aryl-substituted bent-core mesogens

Laterally aryl-substituted bent-core mesogens, prepared from 2-methyl-isophthalic acid, were studied. Optical polarising microscopy gave hints for nematic and smectic A phases which were supported by X-ray measurements. Two-dimensional patterns of samples aligned in the magnetic field prove clusters of the smectic C type within the nematic phases. The layer spacings point to intercalated structures because they are significantly shorter than the molecular lengths derived from molecular models. The shape of the molecules seems to be neither rod-like, nor really bent, nor discotic due to the extension of the bending angle caused by the lateral methyl group in a bay position and the different cross-sections of different molecular parts.

Optimizing the SmCP phase bent-core mesogen characteristics by introducing dimeric liquid crystals

This article demonstrates a method to simultaneously enhance the electro-optical properties and decrease the mesophase temperature of MD-H bent-core liquid crystals by introducing organosiloxane dimeric liquid crystals, C11Si2C11 and C11Si3C11. The differential scanning calorimetry, spontaneous polarization, and switching time were systematically studied. It has been shown that the MD-H bent-core liquid crystals doped with 20% dimeric liquid crystals have lower mesogen temperatures, faster switching times, and lower rotational viscosities.

Mesogenic properties of two 1,3,4-oxadiazole-based bent-core geometrical isomers

Two new symmetrical oxadiazole-based bent-core mesogens bearing cis and trans geometrical stereoisomer tails, geranyloxy and neryloxy, have been synthesised and the effect of the cis–trans geometric configuration of the terminal chains on the thermal profile and mesomorphic behaviour were investigated using differential scanning calorimetry and polarised optical microscopy. The trans–trans stereoisomer compound 4g with terminal geranyloxy chains exhibited a nematic phase characterised by the schlieren texture, which contained both 2- and 4-brush defects and remarkably no smectic mesophases. The cis–cis stereoisomer neryloxy derivative 4n has a much lower melting temperature but did not exhibit mesomorphism. In an attempt to further lower the melting point of the nematic mesophase, a binary mixture study was carried out between the geranyloxy and neryloxy 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole derivatives. The mesomorphism is retained up to 50% ratio, with a cross-over reached at 70% of the geranyloxy trans–trans stereoisomer 4g, resulting in monotropic behaviour of the mixture. Once neryloxy cis–cis stereoisomer homologue 4n is in excess, the mesophase is completely suppressed.

A Liquid Crystalline Phase with Uniform Tilt, Local Polar Order and Capability of Symmetry Breaking

A new liquid crystalline (LC) phase with uniform tilt, local polar order and capability of symmetry breaking is found for a bent-core mesogen combining a 4-cyanoresorcinol unit with two azobenzene wings. The combination of local polar order and long range synclinic tilt in this SmC(s)P(R) phase leads, under special conditions, to macroscopic domains with opposite chirality, though the molecules themselves are achiral.

Fluorescent unsymmetrical four-ring bent-core mesogens: 2D modulated phases

Based on the chemical incompatibility between the aromatic mesogenic cores and the flexible end aliphatic (methylene units) chains, which can lead to the formation of polarization modulated layered undulated phases, four-ring bent-core compounds have been designed and synthesized. The designed compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding, highly fluorescent and exhibit 2D-polarization modulated layer undulated (PMLU) smectic phases. The phase transition temperatures have been confirmed by differential scanning calorimetry and the phases are confirmed by polarizing optical microscopy and X-ray diffraction studies. For compounds 1-10-11 and 1-15-11 the peaks are indexed to tilted columnar lattice (Colob), while for 2-12-11 the peaks are indexed to the rectangular lattice belonging to the PMLU family The influence of the direction of ester linking group played an important role in the mesophase as well as the evaluated parameters of the dipole moment, the bending angle and the asymmetry parameter by spin-restricted DFT calculations using the B3LYP exchange-correlation functional and the 6-311G(d,p) basis set.

A comparison of short-range molecular order in bent-core and rod-like nematic liquid crystals

Thermotropic liquid crystals exhibit strongly temperature-dependent positional correlations among molecules in the nematic phase above the transition to a smectic-A or C phase. However, even in the absence of a lower temperature smectic phase, nematics composed of reduced symmetry molecules (bent-core mesogens, for example) reveal small angle X-ray scattering (SAXS) patterns with features similar to those associated with pretransitional smectic order. We report on a quantitative analysis and comparison of two-dimensional SAXS data on a bent-core nematic compound (lacking a smectic phase) and a rod-like nematic (exhibiting a nematic to smectic-C transition). This analysis demonstrates that a nanostructure based on distinct, smectic-C ordered molecular clusters provides a more accurate account of the diffraction data from the bent-core nematic than the standard models used to describe fluctuating smectic order in rod-like nematics. The average dimensions of the clusters are weakly temperature dependent, while the interlayer spacing and layer tilt angle with respect to the average molecular long axis are temperature independent. Our results may help to explain interesting and unusual macroscopic properties recently discovered in the fluid phases of bent-core molecules.

Synthesis and Surface Characterizations of New Banana-Shaped Liquid Crystal 4-Bromo-1,3-phenylene-bis[4-[4′-(10- undecenyloxy)-benzoyloxy]]benzoate

A new banana-shaped liquid crystal compound, i.e., the bent-core mesogen, 4-Bromo-1,3-phenylene-bis[4-[4′-(10-undecenyloxy)-benzoyloxy]]benzoate (BPUBB), has been synthesized and characterized. Phase transition temperatures of BPUBB were determined by polarizing microscopy and differential scanning calorimetry. The inverse gas chromatography method was utilized to obtain the surface characterization of BPUBB. This compound was used as a stationary phase in the chromatographic column and nine solutes with a different chemical nature were used as eluents. The dispersive component of the surface free energy, γ D S , of studied adsorbent surface was estimated using retention times of different nonpolar organics in the infinite dilution region. The specific free energy of adsorption, ΔGS A , the enthalpy of adsorption, ΔHS A , and the entropy of adsorption, ΔSS A , of solvents on liquid crystal were determined. The values of the ΔHS A were correlated with both the donor and the acceptor numbers of the probes to quantify the acid KA and the basic KD parameters of the liquid crystal surface. The values obtained for the KA and KD parameters indicated a basic character for the surface of the BPUBB.

Novel columnar–calamitic phase sequences in a binary system of bent-core and rod-like mesogens

We report here X-ray, electrical switching and dielectric measurements on a binary system comprising achiral bent-core and chiral rod-like components. While the pure bent-core compound exhibits a single mesophase, namely the B2 phase, the rod-like compound shows smectic A and smectic C* mesophases. A particular mixture, studied in detail here, presents a novel sequence of three columnar type B1 phases, smectic A–B1OPAF1–B1OPAF2–B1TPAF–smectic C* phase, as the sample is cooled, where the subscripts O, T, P, F and AF indicate orthogonal, tilted, polar, ferroelectric and antiferroelectric properties, respectively, with the B1 phases being of the B1rev type. In the electrical switching measurements a twin peak profile – characteristic of an antiferroelectric structure – is seen for the B1OPAF phases, but with no textural change except on a transient scale. On the other hand the B1TPAF phase, which also shows antiferroelectric-type switching, exhibits clear changes between the field-off and field-on states, as well as for the two signs of the field. This phase also possesses a three-fold higher value of polarization than the smectic C* phase, indicating the stronger influence of the polar ordering. Dielectric studies show the presence of a soft mode of relaxation in the vicinity of the B1OPAF2–B1TPAF transition, with the relaxation frequency of the mode exhibiting behavior similar to that seen for the smectic A–smectic C* phase transition. Mean field coefficients determining the soft mode behavior as well as the thermal variation of the tilt angle have been determined.