Novel sulphur-containing banana-shaped liquid crystal molecules

In this preliminary report we present the mesomorphic properties and electro-optic investigations of three new 'banana-shaped' mesogens. The materials are structurally similar to Niori's the original bent-core materials but possess alkylthio substituents in place of alkoxy substituents. Microscopic investigations revealed an unusual 'spiral-domain texture', similar to that observed in the 'B₇' phase of related materials. Further investigations support our earlier observations that the materials display antiferroelectric switching in this phase, in two of the new materials studied. The nature of this phase, in comparison with the 'switchable' (B₂) and 'chiral-domain' (B₄) phases of the original materials, is discussed.     

Novel sulphur-containing banana-shaped liquid crystal molecules

In this preliminary report we present the mesomorphic properties and electro-optic investigations of three new 'banana-shaped' mesogens. The materials are structurally similar to Niori's the original bent-core materials but possess alkylthio substituents in place of alkoxy substituents. Microscopic investigations revealed an unusual 'spiral-domain texture', similar to that observed in the 'B₇' phase of related materials. Further investigations support our earlier observations that the materials display antiferroelectric switching in this phase, in two of the new materials studied. The nature of this phase, in comparison with the 'switchable' (B₂) and 'chiral-domain' (B₄) phases of the original materials, is discussed.     

STM studies on adsorbed liquid crystal on HOPG

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Antiferroelectric liquid crystal from a banana-shaped achiral molecule

A new banana-shaped achiral compound, 1,3-phenylene bis[4-(3-fluoro-4-n-octyloxyphenyliminomethyl)benzoate] (PBFOB) was synthesized and its antiferroelectric liquid crystallinity determined. The PBFOB was characterized by differential scanning calorimetry, X-ray diffractometry in the small and wide angle regions, and polarizing optical microscopy; its polarization and antiferroelectric properties were also investigated. The presence of a lateral fluoro-substituent in the banana-shaped achiral molecules containing a Schiff's base mesogen induced a decrease in melting temperature and formation of the switchable smectic B₂ phase in the melt. The spontaneous polarization for PBFOB was about 250 nC cm^-2 and the polarization of this phase switched on the reversal of an applied electric field.     

Antiferroelectric liquid crystal from a banana-shaped achiral molecule

A new banana-shaped achiral compound, 1,3-phenylene bis[4-(3-fluoro-4-n-octyloxyphenyliminomethyl)benzoate] (PBFOB) was synthesized and its antiferroelectric liquid crystallinity determined. The PBFOB was characterized by differential scanning calorimetry, X-ray diffractometry in the small and wide angle regions, and polarizing optical microscopy; its polarization and antiferroelectric properties were also investigated. The presence of a lateral fluoro-substituent in the banana-shaped achiral molecules containing a Schiff's base mesogen induced a decrease in melting temperature and formation of the switchable smectic B₂ phase in the melt. The spontaneous polarization for PBFOB was about 250 nC cm^-2 and the polarization of this phase switched on the reversal of an applied electric field.     

Effect of chemical structure on the liquid crystallinity of banana-shaped molecules

Banana-shaped molecules having two side wings attached to a bent core may exhibit liquid crystallinity. The most studied material is 8-OPIMB that comprises 1,3-dihydroxybenzene as a central core, a Schiff 's base moieties as the wing groups and octyloxy tail groups. To clarify the effect of chemical structure on the liquid crystallinity of such a molecule, we have prepared several banana-shaped molecules, with side wings and central cores different from those of 8-OPIMB and examined their liquid crystallinity, which is sensitive to change in chemical structure. Especially, changing the position of the carbonyl group of the ester function linking the central core to the wing and the position of the nitrogen atom in the Schiff 's base moiety caused a loss of liquid crystallinity. On the other hand, smectic liquid crystallinity was maintained for five new types of banana-shaped molecule with different central cores. Although all these smectic phases have liquid-like association of the molecules within the smectic layers, they showed unconventional smectic textures through the separation of spiral, fractal and germ textures from the isotropic melt. Moreover, a frustrated smectic phase and chiral smectic phases were found. Several possible smectic structures for those phases will be discussed.     

Effect of chemical structure on the liquid crystallinity of banana-shaped molecules

Banana-shaped molecules having two side wings attached to a bent core may exhibit liquid crystallinity. The most studied material is 8-OPIMB that comprises 1,3-dihydroxybenzene as a central core, a Schiff 's base moieties as the wing groups and octyloxy tail groups. To clarify the effect of chemical structure on the liquid crystallinity of such a molecule, we have prepared several banana-shaped molecules, with side wings and central cores different from those of 8-OPIMB and examined their liquid crystallinity, which is sensitive to change in chemical structure. Especially, changing the position of the carbonyl group of the ester function linking the central core to the wing and the position of the nitrogen atom in the Schiff 's base moiety caused a loss of liquid crystallinity. On the other hand, smectic liquid crystallinity was maintained for five new types of banana-shaped molecule with different central cores. Although all these smectic phases have liquid-like association of the molecules within the smectic layers, they showed unconventional smectic textures through the separation of spiral, fractal and germ textures from the isotropic melt. Moreover, a frustrated smectic phase and chiral smectic phases were found. Several possible smectic structures for those phases will be discussed.     

Helical filamentary growth in liquid crystals consisting of banana-shaped molecules

The formation of coils is common in nature when achiral symmetry breaking occurs. Here we describe spectacular examples of single, double and triple coils observed in smectic liquid crystal phases of achiral banana-shaped molecules. Such molecules form chiral smectic phases due to two symmetry-breaking instabilities: polar molecular packing, and molecular tilt. The appearance of helical filaments at the isotropic-smectic transition is therefore a direct indication of the achiral symmetry-breaking of the smectic structures. The number of observed left- and right-handed domains is equal, reflecting the achiral nature of the constituent molecules. Our studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed.     

Synthesis and liquid crystal properties of new banana-shaped cinnamoyl derivatives

Three new achiral five-ring banana-shaped homologous series have been synthesized and their liquid crystalline properties studied by polarizing microscopy, differential scanning calorimetry, and electro-otpical analysis. The compounds consist of two identical mesogenic structures of bis (alkoxycinnamoyloxybenzoates) linked on a different central ring (unsubstituted or methyl-substituted). The length of the terminal alkoxy groups was varied from heptyloxy to dodecyloxy; the hexadecyloxy group was also included in the study. It was found that the polymorphism of compounds can be strongly dependent upon molecular structure in this class of compound. In particular, substituents on the central ring may exert a significant effect on the type of mesophase observed.     

Helical filamentary growth in liquid crystals consisting of banana-shaped molecules

The formation of coils is common in nature when achiral symmetry breaking occurs. Here we describe spectacular examples of single, double and triple coils observed in smectic liquid crystal phases of achiral banana-shaped molecules. Such molecules form chiral smectic phases due to two symmetry-breaking instabilities: polar molecular packing, and molecular tilt. The appearance of helical filaments at the isotropic-smectic transition is therefore a direct indication of the achiral symmetry-breaking of the smectic structures. The number of observed left- and right-handed domains is equal, reflecting the achiral nature of the constituent molecules. Our studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed.     

Synthesis and liquid crystal properties of new banana-shaped cinnamoyl derivatives

Three new achiral five-ring banana-shaped homologous series have been synthesized and their liquid crystalline properties studied by polarizing microscopy, differential scanning calorimetry, and electro-otpical analysis. The compounds consist of two identical mesogenic structures of bis (alkoxycinnamoyloxybenzoates) linked on a different central ring (unsubstituted or methyl-substituted). The length of the terminal alkoxy groups was varied from heptyloxy to dodecyloxy; the hexadecyloxy group was also included in the study. It was found that the polymorphism of compounds can be strongly dependent upon molecular structure in this class of compound. In particular, substituents on the central ring may exert a significant effect on the type of mesophase observed.     

Liquid crystal phases of achiral banana-shaped molecules: a computer simulation study

The phase behaviour of achiral banana-shaped molecules was studied by computer simulation. The banana-shaped molecules were described by model intermolecular interactions based on the Gay-Berne potential. The characteristic molecular structure was considered by joining two calamitic Gay-Berne particles through a bond to form a biaxial molecule of point symmetry group C 2v with a suitable bending angle. The dependence on temperature of systems of N =1024 rigid banana-shaped molecules with bending angle ϕ=140° has been studied by means of Monte Carlo simulations in the isobaric-isothermal ensemble ( NpT ). On cooling an isotropic system, two phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. For the first time by computer simulation of a many-particle system of banana-shaped molecules, at low temperature an untilted smectic phase showing a global phase biaxiality and a spontaneous local polarization in the layers, i.e. a local polar arrangement of the steric dipoles, with an antiferroelectric-like superstructure could be proven, a phase structure which recently has been discovered experimentally. Additionally, at intermediate temperature a nematic-like phase has been proved, whereas close to the transition to the smectic phase hints of a spontaneous achiral symmetry breaking have been determined. Here, in the absence of a layered structure a helical superstructure has been formed. All phases have been characterized by visual representations of selected configurations, scalar and pseudoscalar correlation functions, and order parameters.     

Ferroelectric lyo-mesophase of banana-shaped molecules

Preliminary results are reported on the first ferroelectric 'lyo-mesophases' of banana-shaped molecules. We studied the effect of the non-polar solvents p-xylene and m-xylene on the macroscopic properties of banana-shaped molecules having a B₇ phase. It was observed that by adding only 15% of solvent the isotropic-B₇ transition temperature was lowered from 130°C to 70°C and the ferroelectric electro-optical switching range was extended from 1-2°C to more than 50°C, thus maintaining the switching even at room temperature. The xylene molecules act as spacers, maintaining larger separations between the banana-molecules, thus reducing the polar packing order. These observations provide unique opportunities for tuning the intermolecular interactions of banana-shaped molecules and modifying their macroscopic properties.     

Electro-disclinic effect in tilted smectic phases of banana-shaped liquid crystal materials

We give the first evidence that the director tilt angle can be reduced by electric fields in the tilted smectic phase of banana-shaped molecules. In these phases the value of polarization is determined by the molecular packing and no electro-clinic effect is expected. Our studies show that high electric fields eventually induce a meta-stable phase with zero director tilt. The tilted phase slowly recovers at low fields. We propose that the field-induced quenching of the layer fluctuations is responsible for the observed effects.     

Electro-disclinic effect in tilted smectic phases of banana-shaped liquid crystal materials

We give the first evidence that the director tilt angle can be reduced by electric fields in the tilted smectic phase of banana-shaped molecules. In these phases the value of polarization is determined by the molecular packing and no electro-clinic effect is expected. Our studies show that high electric fields eventually induce a meta-stable phase with zero director tilt. The tilted phase slowly recovers at low fields. We propose that the field-induced quenching of the layer fluctuations is responsible for the observed effects.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^?2 on reversal of an applied electric field.     

Synthesis and characterization of achiral banana-shaped liquid crystalline molecules containing bisnaphthyl moieties

Three analogous series of symmetric banana-shaped liquid crystalline molecules containing bisnaphthyl units were synthesized and characterized. The effects of linking groups (on the side wings) and lateral meta-fluoro substitutions (on the middle outer rings) on the mesogenic properties were examined. The type of mesophase depends on the lengths of terminal alkoxy chains. Thus, achiral molecules with shorter flexible chains (n = 8) exhibit a rectangular columnar (B₁) phase, while analogous derivatives with longer flexible chains (n = 12) display the B₂ phase. All lateral meta-fluoro substituted analogues (series II) possess the lowest isotropization temperatures and the narrowest mesophasic ranges of the B₁ and B₂ phases. The B₁ and B₂ phases were confirmed by X-ray diffraction, polarizing optical microscopy (POM) and electro-optical (EO) switching experiments. An electric field-induced transition from an antiferroelectric (tristable) state to a ferroelectric (bistable) state was observed in the EO measurements. Spontaneous polarization (by switching current response), tilt angle of chiral domains (by POM), and transmittance-voltage measurements of the B₂ phase in related compounds have been surveyed in this study.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^-2 on reversal of an applied electric field.     

Ferroelectric lyo-mesophase of banana-shaped molecules

Preliminary results are reported on the first ferroelectric 'lyo-mesophases' of banana-shaped molecules. We studied the effect of the non-polar solvents p-xylene and m-xylene on the macroscopic properties of banana-shaped molecules having a B₇ phase. It was observed that by adding only 15% of solvent the isotropic-B₇ transition temperature was lowered from 130°C to 70°C and the ferroelectric electro-optical switching range was extended from 1-2°C to more than 50°C, thus maintaining the switching even at room temperature. The xylene molecules act as spacers, maintaining larger separations between the banana-molecules, thus reducing the polar packing order. These observations provide unique opportunities for tuning the intermolecular interactions of banana-shaped molecules and modifying their macroscopic properties.