Structure of trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) in the isotropic and nematic phases: a computer simulation study

We have studied the single particle structural properties of the nematogen trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) by molecular dynamics simulation using realistic atom-atom potentials. On going from the isotropic phase at 390 K to the nematic phase at 350 K, the molecules become significantly longer and thinner, as indicated by the equivalent molecular moment-of-inertia spheroid and the distribution of trans and gauche bonds. This change is only partly accounted for by the lowering of the temperature, there being a significant quenching effect due to the change in the molecular environment. This quenching effect is also apparent in the distribution of molecular shapes seen in molecular width-breadth contour maps. In the nematic phase, at 350K, the distributions of alkyl tail bond orientations with respect to the director show a pronounced odd-even effect, with peaks in the distributions occurring alternately parallel to, and at an angle to, the director.     

Density studies on various smectic liquid crystals

Density measurements as a function of temperature for four homologues of the 5-n-alkyl-2-(4-n-alkyloxy-phenyl)-pyrimidines (PYP nOm) which exhibit nematic, smectic A and smectic C phases are reported. Furthermore 1-butyl-c-4-(4'-octyl-biphenyl-4-yl)-r-cyclo-hexan- carbonitrile (NCB84) is studied; this has additionally a smectic G phase. From these data the thermal expansion coefficients are calculated. Comparing PYP 907 and PYP 709, differing in their exchanged alkyl chains, we observe in the smectic A and the smectic C phase a distinctly lower density for PYP 709 whereas their densities nearly agree in the isotropic phase. The pyrimidines PYP 709 and PYP 808 exhibit a continuous volume change on crossing the smectic A-smectic C transition which differs dramatically from PYP 909 which shows a small volume jump. Furthermore a binary mixture of PYP 708 and PYP 706 is analysed which shows only a nematic and a smectic C phase. The associated phase transition is probably first order revealing nearly no pretransitional behaviour. The smectic A-smectic C transition of NCB84 seems to be second order exhibiting a continuous change of volume across the transition whereas the smectic C-smectic G transition shows a volume discontinuity and is first order. In order to induce ferroelectric smectic C* phases all smectic C materials were doped with a chiral pyrimidine dopant. Astonishingly the thermal expansion coefficient across the smectic A-smectic C* transition is influenced by the dopant in a very different way.     

Intercalated smectic a phases in banana-shaped liquid crystals with carbonate end groups.

Novel, symmetrical, bent-core mesogens, namely substituted 1,3-phenylenebis{4'-[(ethoxycarbonyl)oxy]-1,1'-biphenyl}-4-carboxylates, are prepared with carbonate groups at both ends of the molecules. The mesophase properties are investigated for different electron-donating and -withdrawing substituents connected to the central ring at different positions. These compounds exhibit nematic and/or smectic A intercalated (SmA(int)) mesophases. Electro-optical studies show a very significant electric-field-induced biaxiality that is fast (<1 ms) and large (0.03) associated with a high birefringence (0.33 at 500 nm, 10 V mum(-1) field) for the unsubstituted analogue. For one substance this effect is observable from above 100 degrees C down to room temperature. Such electric-field-induced biaxiality may find practical applications in fast electro-optic modulation devices.     

Surface tensions of smectic liquid crystals

We determine surface tensions σ of smectic liquid crystals from the curvature pressure of smectic films. A new technique is introduced for the comparison of surface tensions of different smectic materials. The method is based on the relation of curvatures of smectic films drawn on communicating vessels. The measurement of the temperature dependence of σ reveals anomalies in the vicinity of phase transitions to low temperature smectic modifications. This anomalous slope dσ/dT can be related to the surface excess entropy of the material in the corresponding temperature range. The surface tension values determined for a number of mesogens fit well into the classification proposed by Mach et al.     

Calorimetric study of the relationship between molecular structure and liquid-crystallinity of rod-like mesogens I. Heat capacities and phase transitions of 4'-propylbiphenyl-4-carbonitrile and trans, trans-4'-propylbicyclohexyl-4-carbonitrile

The heat capacities of the rod-like compounds 4'-propylbiphenyl-4-carbonitrile (3-BBCN) and trans, trans-4'-propylbicyclohexyl-4-carbonitrile (3-CCCN) have been measured with an adiabatic calorimeter between 12 and 383 K. 3-BBCN is not mesogenic and melts into an isotropic liquid at 338·77 K, whereas 3-CCCN is mesogenic and its melting and clearing points are 330·73 K and 353·80 K, respectively. The enthalpy and entropy of fusion of 3-BBCN are 22·7 kJ mol^-1 and 67·0 J K^-1 mol^-1, respectively, while those of 3-CCCN are 27·0 kJ mol^-1 and 81·7 J K^-1 mol^-1, respectively. The enthalpy and entropy gain due to the nematicisotropic transition of 3-CCCN are 1·8 kJ mol^-1 and 5·0 J K^-1 mol^-1. 3-CCCN shows a mesomorphic transition from a smectic to the nematic state at 329·62 K, which occurs as a metastable state, its transition enthalpy and entropy are 5·4 kJ mol^-1 and 16·4 J K^-1 mol^-1, respectively. The temperature dependence of the molar entropies of both compounds shows that molecular arrangement in the crystal is more ordered in 3-CCCN than in 3-BBCN. This fact may be related to the stability of mesophases. Finally, Eidenschink's theoretical model for the nematicisotropic transition has been applied to 3-CCCN. As far as the present mesogen is concerned, the transition enthalpy estimated according to this model agrees well with the observed value.     

Synthesis, properties and crystal structure of chiral semiperfluorinated liquid crystals with ferro and anticlinic smectic phases

A new chiral and semiperfluorinated series with ferro and anticlinic properties has been synthesized and characterized. The mesomorphic behaviour has been established on the grounds of both microscopic observations and DSC measurements. The non-chiral intermediate ethyl 4-semiperfluorinated alkyloxybenzoates exhibit SmA phases, unusual for compounds with a single phenyl ring. The final derivatives display SmA, SmC* and in several cases SmC*A phases. The longer fluorinated chains favour the SmA and SmC* phases at the expense of the SmC*A phase. Electro-optical measurements were carried out with the classical SSFLC geometry. The spontaneous polarization and tilt angle at saturation are higher than those of the hydrogenous homologues, around 140 nCcm-2 at 40 degrees C. One compound of the series, the 4,4,5,5,6,6,7,7,8,8,8-nonafluoroheptyloxy derivative, C36H35O7F9, Mx=750.6 g mol-1, crystallizes in the triclinic system, space group P1 with four independent molecules in the asymmetric unit (Z=4). The molecules are arranged in a head-to-tail fashion with two molecules oriented in the same direction and the two others in the opposite direction. They give rise to sheets with a smectic C-like arrangement. The final reliability factors were R=0.117 and wR=0.134; the goodness of fit was S=1.366.     

Synthesis,properties and crystal structure of chiral semiperfluorinated liquid crystals with ferro and anticlinic smectic phases

A new chiral and semiperfluorinated series with ferro and anticlinic properties has been synthesized and characterized. The mesomorphic behaviour has been established on the grounds of both microscopic observations and DSC measurements. The non-chiral intermediate ethyl 4-semiperfluorinated alkyloxybenzoates exhibit SmA phases, unusual for compounds with a single phenyl ring. The final derivatives display SmA, SmC* and in several cases SmC*A phases. The longer fluorinated chains favour the SmA and SmC* phases at the expense of the SmC*A phase. Electro-optical measurements were carried out with the classical SSFLC geometry. The spontaneous polarization and tilt angle at saturation are higher than those of the hydrogenous homologues, around 140 nCcm-2 at 40 degrees C. One compound of the series, the 4,4,5,5,6,6,7,7,8,8,8-nonafluoroheptyloxy derivative, C36H35O7F9, Mx=750.6 g mol-1, crystallizes in the triclinic system, space group P1 with four independent molecules in the asymmetric unit (Z=4). The molecules are arranged in a head-to-tail fashion with two molecules oriented in the same direction and the two others in the opposite direction. They give rise to sheets with a smectic C-like arrangement. The final reliability factors were R=0.117 and wR=0.134; the goodness of fit was S=1.366.     

反-4-(反-4'-正丙基环已基)环已醇的合成与表征

环己烷类液晶因具有高度的稳定性,较宽的相列相温区而受到人们的青睐。反-4-(反-4’-正丙基环己基)环己醇(trans-3HHE)是一种合成环己烷类液晶的重要中间体,它可通过酶选择性催化和顺式构型异构化二种方法合成。文献报道顺式构型异构化法是：4-(反-4’-正丙基环己基)苯酚以铑／碳或铑／氧化铝为催化剂,在酸性介质条件下进行加氢反应,     

Induction of ionic smectic C phases: a systematic study of alkyl-linked guanidinium-based liquid crystals

A series of ionic liquid crystals with an alkoxy biphenyl unit tethered via an alkyl spacer to a guanidinium head group were synthesised and the mesomorphic properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction (XRD; WAXS and SAXS). Whereas all symmetrical guanidinium chlorides with the same chain lengths in alkyl tail and spacer displayed enantiotropic SmA₂ phases, monotropic SmC₂ phases with 1–2 K temperature range were only formed for chain lengths ≥ C₁₀. Shifting the calamitic core more closely to the ionic head group by decreasing the tether length and simultaneously increasing the terminal alkyl chain improved the stability of both SmA and SmC phases considerably and led to enantiotropic SmC phases for the guanidinium chloride with C₁₄ alkyl tail and C₆ spacer. An even more pronounced effect was detected during anion exchange. Bromide, iodide, hexafluorophosphate, thiocyanate and triflate suppressed any SmC phase, whereas tetrafluoroborate behaved similar to chloride maintaining the SmC phase. However, acetate stabilised the SmC phase at the expense of the SmA phase. Based on temperature-dependant XRD measurements, a bilayer structure was proposed.     

Dielectric studies of liquid crystals under high pressure III. Low frequency relaxation process in 4-(trans-4'-n-hexylcyclohexyl)isothiocyanato benzene

Dielectric studies of 4-(trans-4'-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) were performed in the pressure range 0·1-150 MPa, the frequency range 1 kHz-13 MHz and the temperature range 295-325 K. The temperature and pressure dependencies of the static permittivity ε_0∥ and of the relaxation time τ_∥ are analysed and compared with the analogous data obtained recently for 4-n-pentyl-4'-cyanobiphenyl (5CB) (Parts I and II of this series). Marked differences in the dielectric properties of the nematic phases of the two substances are observed. They are interpreted as a result of varying degrees of molecular association in particular compounds. It is concluded that in the nematic phase of 6CHBT dipole-dipole correlations do not exist or are very weak, whereas for 5CB they are easily broken by a relatively low pressure.     

Optical studies on two tilted smectic phases of meta-substituted three-ring liquid crystal compounds

Null transmission ellipsometry and depolarized reflected light microscopy have been performed on freestanding films of two achiral meta-substituted three-ring compounds. These compounds exhibit two different tilted smectic phases, SmC₁ and SmC₂. Our studies confirmed previous results obtained from another achiral meta-substituted three-ring compound that the SmC₁ and SmC₂ phases are SmC and SmC_A, respectively.     

Molecular theory of smectic C liquid crystals

We generalize an earlier density functional theory of liquid crystals by Mederos and Sullivan. The original theory took account of anisotropic hard core interactions, and for suitable intermolecular interactions predicted nematic and smectic A phases as well as isotropic liquid and vapour phases. In this generalization we also take into account quadrupolar or dipole induced dipole interactions. The modified theory now also predicts the existence of a smectic C phase.     

Polar smectic phases in quasi-bent-core liquid crystals with hockey-stick-shaped mesogen: synthesis and crossover mesomorphism

ABSTRACT

New hockey-stick-shaped liquid crystals (HSLCs) have been designed and synthesised. By extreme shortening of one ‘arm’ in the HSLCs, we have constructed the pronounced overall hockey-stick-shaped molecular conformation of these materials, and called the pseudo-rod-like (PRL) molecules with hockey-stick-shaped mesogen. The chemical structures were identified using spectroscopy and elemental analysis. The mesomorphic behaviour was investigated by polarising optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffractometry (XRD), and via electro-optical experiments. As a result, in spite of the PRL molecules, the various mesomorphic polymorphism, the interdigitated bi-layered structure and macroscopic polarity were found. Similar to up-to-date HSLCs, the quasi-bent-core LCs showed a transition from the synclinic SmC phase (SmC_s) to anticlinic SmC (SmC_a) phase, and exhibited SmA-type (N_CybA) and SmC-type (N_CybC) cybotactic nematic phases. At the same time, similar to practical rod-like LCs, the PRL molecules showed good surface alignment behaviour under planar boundary conditions.     

X-ray studies of the layer thickness in smectic phases

X-ray investigations of nine smectogenic substances exhibiting the smectic A_d, A₁ and crystalline E phases were performed at various temperatures. X-ray patterns yielded the layer thickness d (A_d, A₁ phases) and orthorhombic unit cell parameters (E phase). The layer thickness of the A_d phase in 4'-n-alkyl-4-cyanobiphenyls (nCBs) has different temperature coefficients for shorter (n = 8-10) and longer (n = 12-14) members, which is explained as resulting from two competing effects: a weakening with temperature of the intermolecular association energy that favours an increase in d, and the increasing number of conformers which reduces the molecular length. A small anisotropy of the thermal expansivity in the smectic phases was found by comparing the linear quantity d(T) with the linearized bulk characteristic of the system, V^-3(T), where V = 1/ρ is the specific volume, ρ is the density. Differences between the slopes of the two quantities are less in the case of the A₁ phase of two nDBTs (5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxanes). The present X-ray data and recent results of studies of the low frequency relaxation process in these compounds (under atmospheric as well as elevated pressures) give a consistent picture of molecular reorientations around the short axes in the smectic phases.     

Nematic,smectic and columnar phases of main-chain liquid crystal polyethers

A number of interesting thermotropic mesophases and transitions occur in main-chain polyethers and copolyethers based on aliphatic spacers and various semi-flexible mesogens of the general 1-(4'-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane (RBPE) or 1-(4'-hydroxybiphenyl-4-yl)-2-(4-hydroxyphenyl)propane (TPP) type. The unusual effects described here are (i) nematic-nematic transition, (ii) smectic-B (crystal-B) phase and its shear-induced orientation, (iii) calamitic columnar phase, and (iv) nematic-columnar phase sequence.     

Stereochemistry of the [4 + 2] cycloadditions of trans,trans- and cis,trans-2,4-hexadiene to C(60)

The [4 + 2] cycloaddition of trans,trans-2,4-hexadiene with C(60) proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct 1a. However, when cis,trans-2,4-hexadiene reacts with C(60), isomerization of the cis,trans to the thermodynamically more stable trans,trans isomer occurs. Subsequently, the cis,trans diene isomerized to the trans,trans isomer and cycloadds to C(60), to form adduct 1a. When the reaction is carried out at higher temperatures, the formation of cycloadduct 1b is also obtained. This result is consistent with a concerted cycloaddition of cis,trans-2,4-hexadiene with C(60), which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.     

Internal molecular disorder in the nematic and smectic phases of thermotropic liquid crystals studied by NMR SPDE experiments

The recently developed NMR SPDE experiment is shown to provide a new and particularly convenient technique for probing the conformational dynamics of mesogens in thermotropic liquid crystals. Measurements have been made in the nematic and smectic phases of the 4,4′-di-n-alkoxyazoxybenzenes. It is shown for the first time that the internal disorder of the alkyl end chains is intimately related to the molecular organization of these mesophases.     

Dynamics of the smectic layer reorientation of ferroelectric liquid crystals

Detailed experimental results of a systematic investigation of the dynamics of the in-plane smectic layer reorientation in SmC* ferroelectric liquid crystals on application of different types of asymmetric electric fields are reported. The reversible reorientation of smectic layers is characterized as a function of field asymmetry, electric field amplitude, frequency, cell gap and temperature. On the basis of the observed behaviour we discuss a phenomenological interpretation of the smectic layer reorientation in terms of dominant influences director switching, convection due to ionic motion and liquid crystal substrate interactions which limit the rotation to the amount of twice the tilt angle.