Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4'-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4'-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0-23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.     

Orientational properties of liquid-crystalline 4-n-alkyloxybenzilidene-4'-fluoroanilines

The orientational order of 4-n-hexyloxybenzilidene-4'-fluoroaniline, (FAB-OC6) and its 4-n-heptyloxy analogue, (FAB-OC7), studied by means of ²H NMR and ¹⁹F NMR is reported. The entire temperature range of the liquid-crystalline phases (S_B, S_A and N for FAB-OC6, S_B and S_A for FAB-OC7) have been investigated. The results are discussed in relation to previous X-ray diffraction measurements. The chemical shift tensor components of the fluorine bond, evaluated from the ¹⁹F NMR spectra are compared with previous results.     

FTIR of liquid crystals. Organization and phase transitions in binary mixtures of nonylcyanobiphenyl and phenyl-cyclohexanecarboxylate

The FTIR spectra of the isotropic and mesomorphic N, S_Ad, S_A1, S_B and S_G phases of 4-n-nonyl-4'-cyanobiphenyl (9CB), 4-n-pentylphenyl-trans-4'-n-pentylcyclohexane-1-carboxylate (5H5) and their binary mixtures have been recorded and analysed. Changes in absorbance related to spontaneous reorientation of the molecules at the phase transitions have been observed and assigned to the orientation of the transition dipoles of typical vibrations. The results are discussed in terms of the phase diagram obtained by D.S.C. and by optical microscopy.     

Liquid crystalline properties of dissymmetric molecules VI. The effect of alkyl chain length on molecular arrangement in the smectic A phase in three-aromatic-ring systems with two ester groups

The effect of alkyl and alkoxy chain lengths on the layer structures of smectic A and C phases has been examined by X-ray diffraction measurements on three isomeric systems: 4-alkoxyphenyl and 4-n-alkylphenyl 4-[(4-octyloxyphenyl)carbonyloxy]benzoates (1); 4-octyloxyphenyl 4-[4-(octyloxyphenyl)carbonyloxy]benzoates (2); 4-octyloxyphenyl, 4-alkoxyphenyl and 4-n-alkylphenyl terephthalates (3); and p-phenylene 4-octyloxybenzoates, 4-alkoxybenzoate and 4-n-alkylbenzoate (4). Although all the derivatives exhibit smectic A and/or C phases having a monolayer arrangement of the molecules, the layer spacings are considerably affected by alternation of the ester linkages. The layer spacings for the homologues of 1 are a little shorter than the calculated molecular lengths, while those for 2 agree with the calculated molecular lengths. The layer spacings for 3 show a notable even-odd alternation in the higher homologues. The results are discussed in terms of a subtle change in the molecular structures due to replacement of the ester groups.     

NMR study of molecular dynamics in a mixture of two polar liquid crystals (CBOOA and DOBCA)

A mixture containing 70 per cent 4-cyanobenzylidene-4'-n-octylaniline (CBOOA) and 30 per cent 4-n-dodeciloxybenzylidene-4'-cyanoaniline (DOBCA) has been studied by optical microscopy, differential scanning calorimetry, X-ray diffraction and, primarily, by NMR spectroscopy. The smectic phase of the mixture is a partial bilayer smectic A phase (S_Ad), in which about one half of the molecules are associated in pairs through their polar cyano end groups. The molecular dynamics in the smectic A_d phase of the mixture was studied by proton spin-lattice relaxation. Self-diffusion and rotations/reorientations were found to be essential relaxation mechanisms while the contribution of the order director fluctuations seems to be very small in all frequency regions. The correlation times associated with the molecular rotational motion around the short molecular axis, τ_S, are of the order 10^-10 s and the ratio τ_S/τ_L ≈6.2. The translational diffusion coefficients D∥ are of the order 10^-11m²s^-1. The possible contribution to the relaxation rate of an additional relaxation mechanism associated with the dissociation and recombination of molecules in dimers within the smectic layers is also analysed.     

Liquid crystalline phases of perfluorinated compounds with segregated sublayers

The liquid crystalline polymorphism (S_A, S_B, CrE) and the structures of the phases of N-n-perfluoralkylethyl-(4-phenylbenzylidene)imines are described. It is shown that the smectic layer consists of two sublayers in to which the incompatible moieties of the compounds segregate. It is proved that the perfluoralkyl chains remain disordered until crystallization occurs, whereas with decreasing temperature, the aromatic parts are ordered in types of S_B and CrE structure.     

Synthesis and transition temperatures of novel fluorinated chiral liquid crystals containing 1,4-tetrafluorophenylene units

To continue the search for novel series of fluorinated ferroelectric liquid crystals, an additional two series of 4-[(S)-2-methylbutoxy]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] benzoates (C) and 4-(n-alkoxy)phenyl 4-[(4-(S)-2'-methylbutoxy-2,3,5,6-tetrafluorophenyl)] benzoates (D) have been synthesized. Polarizing microscopic textural observations and DSC measurements of the phase transitions of these novel compounds showed that compounds C were liquid crystals with a chiral nematic (N) phase and a monotropic chiral smectic C phase (S_c), and compounds D exhibited a chiral nematic (N) phase.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d₁-4'-heptylaniline and 4-n-pentyloxybenzylidene-4'-heptylaniline-2,3,5,6-d₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

The synthesis and liquid crystal properties of some 4-n-nonylphenyl esters of 3-(4'-n-alkoxybiphenylyl)- and 3-(4'-n-alkoxybiphenylyl)-3-methyl-propanoic acids and their laterally fluorinated analogues

A number of 4-n-nonylphenyl esters and mono- and di-fluorinated 4-n-nonylphenyl esters derived from 3-(4'-n-alkoxybiphenylyl)propanoic acids (II) have been synthesized and their thermotropic liquid crystal properties assessed with a view to obtaining tilted smectic phases for possible use in ferroelectric display devices. Many of these compounds exhibit wide temperature range S_C and S_I/F phases, but none gave the ideal phase sequence S_C-S_A-N-I mainly because the structure of these compounds was not conducive to the formation of the nematic phase. To try to alleviate this problem a series of 4-n-nonylphenyl esters based on 3-(4'-n-alkoxybiphenylyl)-3-methylpropanoic acid was prepared, where a lateral methyl group was incorporated in the β-position of the -CH₂CH₂CO₂- linkage. Incorporation of the lateral methyl group encouraged the formation of a nematic phase at the expense of both the smectic A phase and tilted smectic phases.     

X-ray study of the highly ordered smectic phases of N-pentyl-N'-(p-pentyloxyphenyl)piperazine

The structures of the highly ordered liquid crystalline smectic phases of N-pentyl-N'-(p-pentyloxyphenyl)piperazine are identified using X-ray diffraction methods. For this compound a phase sequence hexagonal smectic B (S_B)-orthofrhombic crystalline smectic H (C_H)-monoclinic crystalline smectic H (C_H) is observed for the first time. The changes in structural symmetry at the phase transitions are discussed.     

Molecular dynamics of a ferroelectric smectogen in its smectic phases by means of 2H NMR spectroscopy

In this work the dynamic behaviour of the ferroelectric liquid crystal (-)-(S )-[4-(2-methylbutyloxycarbonyl)phenyl] 4-n-heptylbiphenylcarboxylate (MBHB) in its smectic A (SmA), unwound chiral smectic C (uSmC*) and chiral smectic C (SmC*) phases has been studied by means of ²H NMR spectroscopy. Zeeman (T _1Z) and quadrupolar (T _1Q) spin-lattice relaxation times have been analysed to extract dynamic parameters (diffusion coefficients and activation energies). The small step rotation diffusion model in the uniaxial approximation has been used to describe overall spinning and tumbling motions, and the strong collision model to describe the internal reorientations of the aromatic fragment. Relaxation data in the SmC* phase have been analysed by using a theoretical approach. The dynamic features obtained in the smectic phases of this mesogen are here presented and discussed in comparison with the results obtained in other ferroelectric liquid crystals, focusing on the fast regime of motions.     

Unsymmetrical dimers possessing a cholesteryl ester moiety and a difluoro-substituted biphenyl core: synthesis and mesomorphic behaviour

Some new unsymmetrical dimers consisting of a cholesteryl ester moiety, covalently linked to either a 4'-(2,3-difluoro-4-n-octyloxy) biphenyloxy or a 4'-(2,3-difluoro-4-n-decyloxy) biphenyloxy through odd-even parity paraffinic central spacers, have been synthesized and investigated for their mesomorphic behaviour. Except for one, all the dimers exhibit enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases. Five of the eight unsymmetrical dimers synthesized show a chiral smectic C (SmC*) phase. Interestingly in some of the compounds the SmC* exists over a wide temperature range including room temperature. Among the eight compounds, a dimer having a C₇ central paraffinic spacer and a C₈ alkoxy terminal chain shows an enantiotropic twist grain boundary with SmC* blocks (TGB_C*) phase. It appears that the variation in the length of the spacer has a remarkable influence on the phase transition temperatures as well as on the mesomorphic behaviour.     

A new smectic phase between SE and SB in 4,4'-dipentylbiphenyl

A new smectic phase existing between S_E and S_B in the non-polar compound 4,4'-dipentylbiphenyl was observed (DPB). It has been termed smectic E' [1]. Enthalpies and phase transition temperatures for DPB were measured and characteristic textures of all three smectic phases are presented.     

Novel fluorinated liquid crystals. II. The synthesis and phase transitions of a novel type of ferroelectric liquid crystals containing 1,4-tetrafluorophenylene moiety

4-[(S)-2-Methylbutoxycarbonyl]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl]benzoates have been prepared from the starting material 1-pentafluorophenyl-2-trimethylsilylacetylene. Polarizing microscope textural observation and DSC measurements of the phase transitions of these novel compounds showed that they were liquid crystals with chiral smectic C phase (S*_C), smectic A(S_A) and cholesteric (Ch) phases. The effects of the alkoxy chain length on the transition temperatures and enthalpies were also studied.     

2-(4-Alkylphenyl)-5-(alkenyloxy)pyrimidines: Synthesis, liquid crystal transition temperatures and some physical properties

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is the high melting points of the trans-substituted materials and the low melting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.     

Dimerization of polar smectics A1 with a terminal cyano group in the matrix of smectics A1 of lower polarity

Binary mixtures have been studied of polar compounds with smectic A₁ phases belonging to the following series: 4-cyanobiphenyl 4-yl 4'-n-alkylbiphenyl-4-carboxylate (CBnAB) and 4-(trans-4-n-decylcyclohexyl) benzoates (10XPCHB) with X = F, NCS, COCH₃. In these systems a strong destablization of the smectic A₁ phase is observed on the side of the CBnAB component and an enhancement of the smectic A₁ phase on the side of 10XPCHB. It is shown that the anomalous behaviour of the mixtures of two A₁ smectics with a similar layer spacing is due to the dimerization of CBnAB in the matrix of 10XPCHB which is of lower polarity.     

Possible smectic A ordering in a droplet of 4-n-octyloxybenzoic acid liquid crystal

Our microtextural as well as depolarized light scattering analyses of 4-n-octyloxybenzoic acid liquid crystal droplets provide evidence for a possible smectic A ordering induced by the free surface. This appears near the bulk smectic C-nematic transition at T_SCN. A simple mean field analysis, based on a Landau-de Gennes-Benguigui free energy functional with an additional surface parameter K_s, allows, in principle, the possibility for a smectic A surface phase at temperatures slightly above T_SCN.     

Influence of an electric field on phase transitions in ferroelectric liquid crystals

The transition temperatures between various smectic liquid crystal phases in chiral (ferroelectric) compounds have been determined as a function of an applied DC electric field. We find that S_G-S_A and S_G-S_C transitions show a field-induced temperature shift proportional to the applied field strength similar to the already known behaviour of first order S_C-S_A transitions. In contrast, for the nonferroelectric S_B-S_A and S_E-S_B transitions no effect of the electric field is observed. The field-induced temperature shift is compared with the temperature shift occurring in chiral-racemic systems of the same compounds (see Bahr, Ch., Heppke, G., and Sabaschus, B., 1991, Liq. Crystals, 9, 31). Using Landau theory we derive a simple relation between these two quantities.     

A study of the induced helical pitch in a reentrant nematic mixture

The temperature dependence of the induced helical pitch is reported for cholesteric and reentrant cholesteric phases of liquid crystal systems comprising 4-n-hexyloxy and 4-n-octyloxy-4'-cyanobiphenyl with a non-mesogenic optically active dopant. It was found that on adding small quantities of the dopant that the temperature range of the S_A phase is narrowed and subsequently disappears, while short range smectic fluctuations persist, influencing the helical twisting features. Critical index values were determined from the temperature dependence of the pitch.     

Mesomorphic properties of a homologous series of chiral liquid crystals containing the α-chloroester group

The mesomorphic properties of the N*, S*_c and higher ordered smectic phases have been investigated for a homologous series of 4-[(S)-2-chlor-3-methylbutanoyloxy]-4'-(4-n-alkyloxy-benzoyloxy)biphenyls. They have been characterized by optical texture observation, differential scanning calorimetry (DSC), small angle X-ray scattering and electro-optic measurements. The compounds exhibit a strongly twisted cholesteric phase and smectic phases with large spontaneous polarization and tilt angle values. One or two higher ordered, monotropic smectic phases were found which significantly differ in their rotational viscosity.