Synthesis and properties of new materials with banana-shaped achiral cores and chiral end groups

New chiral bent-core mesogens, derivatives of 1,3-phenylene bis[4-(alkanyloxyphenyliminomethyl)benzoate], were synthesized with variation of a substituent (X=F, Cl); their antiferroelectric properties are described. The mesomorphic and switching properties were characterized by differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X-ray diffractometry in the small and wide angle regions. The presence of chiral tails at the terminals of side wings in the bent-core molecules induced a decrease in transition temperature and formation of the switchable SmC* phase in the melt. In addition, the introduction of a lateral halogen substituent in the Schiff's base moiety prevented the regular stacking of the molecules, resulting in the formation of very complex optical textures. The smectic phase with F-substituted PBFDOB and Cl-substituted PBCDOB showed layer spacings of 39 and 38.5?Å, respectively, corresponding to the end-to-end distance of molecules with a bent conformation. Significantly, the smectic phases of PBFDOB and PBCDOB exhibited a period of 179.5 and 131?Å, respectively, compatible with a helical structure with periodicity about 4.6 and 3.4 times the layer spacings.     

Bent Molecules with a 60° Central Core Angle that Form B7 and B2 Phases

Small‐angle bent‐core liquid‐crystalline (LC) molecules based on a 1,2‐bis(phenylethynyl)benzene central core have been synthesized that form banana smectic phases with a ferroelectric B7–antiferroelectric B2 phase sequence upon cooling. The formation of polar, switchable ferro‐/antiferroelectric banana phases indicates that, despite the low core bend angle of approximately 60°, banana smectic phases are still formed with the bend direction parallel to the layer. This study offers significant evidence that shows bent‐core molecules with a 60° bend angle can form the well‐known B2 and B7 banana phases. Consequently, it may lead to the preparation of a wide variety of novel bent molecules with low bend angles that spontaneously form an LC phase with both polarization and chirality.     

Design of advanced multicomponent ferroelectric liquid crystalline mixtures with submicrometre helical pitch

New functional multicomponent ferroelectric liquid crystalline mixtures have been designed while searching smart self-assembling materials with submicrometre periodicity of the helical structure responding definite demands for application in electro-optic devices and photonics that exploit the deformed helix ferroelectric effect. The resulting designed mixtures possess the paraelectric smectic A* and the tilted ferroelectric smectic C* phases over a very broad temperature range down below the room temperature. The mesomorphic, electro-optic and dielectric properties have been studied and discussed. The presence of a very stable enantiotropic ferroelectric smectic phase exhibiting almost temperature independent helical pitch within 150–250 nm range and reasonably high values of the tilt angle might allow these mixtures to be applicable for practical purposes.     

Ferroelectric switching in a novel bent-shaped mesogen having two non-mesogenic units linked by an alkylene spacer

We report the synthesis, X-ray diffraction results and electro-optical switching measurements for a bent-shaped mesogen having two non-mesogenic units linked by an alkylene spacer. The novelty of the molecular structure lies in the carbonyl group of the ester linkage being directly attached to the spacer unit, unlike for banana-shaped molecules reported so far, in which it is one oxygen atom away from the spacer or the central aromatic unit. The compound shows two mesophases: the high temperature mesophase is a tilted smectic phase showing ferroelectric switching characteristics; the low temperature phase is more highly ordered with textural features similar to that of the B₃ banana phase.     

AFM investigations of a sample showing the B2 phase

Atomic force microscopy (AFM) images reveal that the liquid crystalline B₂ phase has no helical superstructure, in contrast to the B₇ phase. This is demonstrated on comparing the AFM images of focal-conic domains in the smectic A, cholesteric, B₂ and B₇ phases. The B₂ and B₇ phases are phases formed by banana-shaped molecules. The results point to the existence of ferroelectric clusters with a random size distribution which also cause the low frequency dielectric relaxation.     

A bent-shape liquid crystal compound with antiferroelectric triclinic-monoclinic phase transition

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric ‘smectic banana’ phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C₁) symmetry, than that of the LT phase (monoclinic, C₂). It is also shown that electric fields can induce transitions between different ‘smectic banana’ phases.     

A new class of bent‐shaped mesogens exhibiting unusual mesophase behaviour

Examples of a new class of bent‐core compounds are presented, the central fragment of which consists of a benzoyl derivative of a secondary cyclic amine. The mesophase behaviour has been studied by differential scanning calorimetry, polarizing optical microscopy, X‐ray diffraction and electro‐optical measurements. It was found that the five‐ring compounds form smectic CP or B₁ phases which are typical for bent‐core mesogens. The six‐ring compounds exhibit, in some cases, only conventional non‐polar smectic phases; in other cases non‐polar smectic phases as well as banana phases are seen. Of particular interest is the occurrence of a switchable uniaxial semectic A‐like phase with an antiferroelectric structure.     

Bent-core mesogens with biphenyl moieties: observation of a B7 to B4 phase transition

Four new bent-core mesogens containing biphenyl moieties are reported. All these, except the first homologue, exhibit B₇ and B₄ banana phases, both of which are known to have a helical structure. The B₇ to B₄ phase transition is very rare, this being perhaps only the second report of the observation of such a phase sequence. The first homologue exhibits only one mesophase X₁ with textural features reminiscent of the B₁ phase. All the compounds synthesized are characterized by spectral data. The mesophases exhibited by these compounds are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray studies.     

Banana‐shaped molecules with 4,6‐dichlorinated central core: effect of lateral substituents and terminal chains on the formation of antiferroelectric smectic mesophases

Six banana‐shaped compounds with a central core based on a 4,6‐dichloro‐1,3‐phenylene group were synthesized by varying the terminal chains (R = OC₁₀H₂₁ or OC₁₁H₂₁) and the lateral substituents (X = H, F or Cl). Their mesophases were characterized by a combination of differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X‐ray diffractometry. Mesomorphic properties of the banana‐shaped mesogens with an olefinic group (R = OC₁₁H₂₁) as a terminal chain are sensitive to lateral halogen substituents as much as those of the analogues with a saturated group (R = OC₁₀H₂₁). The compounds with X = F showed an antiferroelectric switchable smectic phase, which has been designated a B₂ phase. The compounds without a lateral halogen substituent only formed a nematic phase, while the compounds with X = Cl did not exhibit a mesophase in the melt.     

First observation of double-twisted helical structure in a banana-shaped achiral molecule

The double-twisted helical structure was confirmed for the first time for the smectic phase of a banana-shaped molecular system from observation of the microscopic pattern. Optically polarized microscopic textures and phase transitions of 1,3-phenylene bis\[4-(3-chloro-4- n-octyloxyphenyliminomethyl) benzoate (PBCOB) and related compounds are described. The chloro-substituent introduced into the 3-position of the Schiff's base moiety of the banana-shaped molecule destabilized the smectic phase and enhanced new mesomorphic properties. The mesophases exhibited by the octyloxy homologue of the chloro-substituted compound were of the smectic phases, SmX1 and SmX2. The mixing behaviour between chloro-substituted and non-chloro-substituted compounds resulted in the appearance of a wide miscibility gap and enlargement of the SmX1 phase which can be seen in a lateral chloro-substituted compound.     

First observation of double-twisted helical structure in a banana-shaped achiral molecule

The double-twisted helical structure was confirmed for the first time for the smectic phase of a banana-shaped molecular system from observation of the microscopic pattern. Optically polarized microscopic textures and phase transitions of 1,3-phenylene bis\[4-(3-chloro-4- n-octyloxyphenyliminomethyl) benzoate (PBCOB) and related compounds are described. The chloro-substituent introduced into the 3-position of the Schiff's base moiety of the banana-shaped molecule destabilized the smectic phase and enhanced new mesomorphic properties. The mesophases exhibited by the octyloxy homologue of the chloro-substituted compound were of the smectic phases, SmX1 and SmX2. The mixing behaviour between chloro-substituted and non-chloro-substituted compounds resulted in the appearance of a wide miscibility gap and enlargement of the SmX1 phase which can be seen in a lateral chloro-substituted compound.     

Aggregation-driven,re-entrant isotropic phase in a smectic liquid crystal material

An azobenzene-core chiral mesogen designed for a photoactive ferroelectric liquid crystal system with switchable polarisation displays a highly unusual phase sequence, with a re-entrant, optically isotropic, fluid phase found below smectic phases in mixtures with high enantiomeric purity. The re-entrant isotropic phase is found on the basis of X-ray scattering and freeze-fracture transmission electron microscopy experiments not to be a cubic or other highly ordered phase but instead a translationally disordered liquid. The material also forms a gel under a wide range of concentrations in 50:50 ethanol/chloroform solutions. Ultraviolet/visible and infrared spectroscopy and quantum chemistry calculations suggest that the primary unit in the re-entrant isotropic and gel phases is a dimer composed of molecules crossed by about 90°, which hinders the formation of crystal phases and forms tubules of helical aggregates in the gel phase.     

Ferroelectrically Switching Helical Columnar Assembly Comprising Cisoid Conformers of a 1,2,3‐Triazole‐based Liquid Crystal

The 1,2,3‐triazole molecule, which is a product of click chemistry, possesses a high dipole moment and can be a useful polar motif for ferroelectric columnar liquid crystal (LC) materials—though it has not been used to date. Herein, we report the helical assembly and ferroelectric switching properties of a columnar liquid crystal comprising a naphthalene core and 1,2,3‐triazolyl linkages. The molecule assembles into a double‐stranded helical columnar LC structure (Col_hel). The X‐ray simulations of cisoid and transoid columnar models suggest that the helical assembly comprises cisoid conformers with a non‐zero dipole moment. The helical columns in the Col_hel phase are aligned homeotropically under an electric field. The ferroelectric switching of the axial polarization can be observed in the temperature range of 105–115 °C in the Col_hel phase, wherein the triazolyl hydrogen bonding along the column axis is weakened. The ferroelectric switching event is attributed to the rotation of the polar triazolyl units in response to the electric field.     

Powder diffraction data and mesomorphic properties of 4-butyloxyphenyl 4′-decyloxybenzoate

Unit cell parameters calculated from X-ray powder diffraction data are presented for the crystalline phase of a liquid crystal 4-butyloxyphenyl 4′-decyloxybenzoate: a = 23.098 (4) Å, b = 5.974 (6) Å, c = 12.357 (10) Å, β = 121.5283 (788)°, unit-cell volume V = 1453.56 ų. Temperature dependent X-ray diffraction data confirmed the existence of smectic A and smectic C mesophases and a more ordered, tilted crystalline smectic phase. Possibility of existence of previously reported smectic B phase as well as another crystalline phase was refuted.     

Dielectric and electro-optical properties of Mn12-acetate and ferroelectric liquid crystal composite

The impact of single-molecule magnet (SMM), Mn₁₂-acetate, on the dielectric properties of ferroelectric liquid crystal (FLC) has been investigated by dielectric spectroscopy and electro-optical techniques in a dual electrode sample cell (DESC). The temperature-dependent dielectric studies on Mn₁₂-acetate/FLC composite have revealed the enhancement in the ferroelectric (smectic C*) to paraelectric (smectic A*) phase transition temperature by 3.5°C. The relaxation process corresponding to Goldstone mode in the ferroelectric phase of the composite is found to be slower compared to pure FLC sample. The electrical response for an input triangular wave shows the existence of one extra polarisation peak in Mn₁₂-acetate/FLC composite which is ascribed to the induced dipole moment in Mn₁₂-acetate molecule. The electro-optical texture of Mn₁₂-acetate/FLC composite revealed that the incorporation of SMM in FLC significantly improve the memory effect.     

Thermal‐ and Photo‐Induced Phase‐Transition Behaviors of a Tapered Dendritic Liquid Crystal with Photochromic Azobenzene Mesogens and a Bicyclic Chiral Center

A ribbon‐shaped chiral liquid crystalline (LC) dendrimer with photochromic azobenzene mesogens and an isosorbide chiral center (abbreviated as AZ₃DLC) was successfully synthesized and its major phase transitions were studied by using differential scanning calorimetry (DSC) and linear polarized optical microscopy (POM). Its ordered structures at different temperatures were further identified through structure‐sensitive diffraction techniques. Based on the experimental results, it was found that the AZ₃DLC molecule exhibited the low‐ordered chiral smectic (Sm*) LC phase with 6.31 nm periodicity at a high‐temperature phase region. AZ₃DLC showed the reversible photoisomerization in both organic solvents and nematic (N) LC media. As a chiral‐inducing agent, it exhibited a good solubility, a high helical‐twisting power, and a large change in the helical‐twisting power due to its photochemical isomerization in the commercially available N LC hosts. Therefore, we were able to reversibly “remote‐control” the colors in the whole visible region by finely tuning the helical pitch of the spontaneously formed helical superstructures.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.     

Experimental studies on the B7 phase of a banana-shaped achiral mesogen

We report optical, X-ray, polarization and response time measurements on a new 'bananas-haped' mesogen. The material has a salicylaldimine segment in the linear part of the core, but is otherwise structurally similar to the mesogen first reported to show a banana mesophase. Textural and X-ray studies show that this mesophase has all the characteristics of the recently discovered B₇ phase. It is observed that the voltage required to obtain electrical switching is quite low compared with values for the few other materials exhibiting the B₇ phase.     

Chiral butadiene-tricarbonyliron liquid crystal complexes; racemates and enantiomers

Mesogenic butadiene-tricarbonyliron complexes 6 and 7 have been obtained in optically active forms and they exhibit columnar, cholesteric, smectic A and smectic C properties. Both smectic mesophases can potentially be used in switchable bistable electro-optic devices and all the mesogenic enantiomers can act as dopants by inducing ferroelectric properties when dissolved in a non-chiral smectic A or C phase.     

New switchable smectic phases in banana-shaped compounds

A new series of achiral compounds with banana-shaped molecules ('E _n ') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (E₇-E₁₀) give switchable smectic mesophases. Nevertheless their textures clearly differ from those of the B2 mesophase. In addition, the E₁₀ compound displays a smectic polymorphism involving a new bilayered structure.