The synthesis and liquid crystal transition temperatures of some weakly polar nematic methyl (E)-[trans-4-substituted-cyclohexyl]-allyl ethers

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain of a wide variety of liquid crystalline cyclohexane derivatives to produce a variety of new methyl (E)-allyl ethers. The melting points and tendency to form smectic mesophases are often low, while nearly all of the compounds prepared exhibit a nematic phase. Thus, even two-ring derivatives can exhibit nematic phases over a wide temperature range (≤80°C), sometimes starting below room temperature (T_m≈10°C). Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. Thus many of the new methyl (E)-allyl ethers exhibit nematic phases over a wider temperature range than the corresponding materials with an unsubstituted alkyl chain attached to the cyclohexyl ring. The new compounds are easily prepared from known starting materials. Many intermediates are themselves liquid crystalline. This allows investigation of the relationship between liquid crystal transition temperatures and the nature of the terminally substituted alkyl chain (for example, incorporating C=C, OH, CO₂C₂H₅ and OCH₃ groups).     

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.     

Synthesis and mesomorphic properties of 4-alkylamino-4'-substituted diphenyldiacetylenes

Various substituted aminodiphenyldiacetylenes of the type with X = C₃H₇, C₅H₁₁, F or NO₂ and R, R' = H, CH₃-C₆H₁₃ were synthesized and their mesomorphic properties determined. Semi-empirical and ab initio quantum chemical calculations using AM1, 421G and 631G* suggested that the amino group would increase the dielectric anisotropy and optical birefringence as compared to the alkyl chain. Mesomorphic properties were found to be poor with the maximum nematic phase range being 44.8°C and many of the compounds having no nematic phase. Both melting temperatures and enthalpies for those having nematic phases were too high to form good eutectic mixtures.     

Cholesteric liquid crystalline thermosets: synthesis, structure and properties of ChLCTs/precursor polymers

The synthesis of six liquid crystalline monomers M₁-M₆ and three series of side chain cholesteric liquid crystalline polymers P₁-P₃ is described. The chemical structures of the monomers were characterized by FTIR and ¹H NMR spectroscopy. The structure-property relationships of M₁-M₆ and P₁-P₃ are discussed. Their phase behaviour and optical properties were investigated by differential scanning calorimetry and polarizing optical microscopy. The monomers M₁-M₃ exhibited cholesteric phases; M₄-M₆ showed nematic or smectic phases. The polymer series P₁-P₃ showed cholesteric phases. Experimental results demonstrated that the selective reflection of the cholesteric monomers and homopolymers shifted to the shorter wavelength region (blue shift) with increasing length of the flexible spacer. The selective reflection of the copolymers shifted to the longer wavelength region (red shift) with increasing content of nematic units.     

Synthesis and thermal properties of dimeric alkoxy-substituted tolans

Four homologous series of dimeric tolans of the general formula RC₆H₄C≏C₆H₄O(CH₂)_m OC₆H₄C'CC₆H₄R (m'6-10; R'H, OC₆H₁₃, OC₁₀H₂₁ and OC₁₄H₂₉), denoted further as TOmOT or nOTOmOTOn, according to the number of aliphatic carbon atoms, have been synthesized. Dimers with terminal chains exhibited liquid crystalline behaviour. Crystal-crystal transitions were also observed. Resulting from the optical, thermal and miscibility studies, the following mesophases have been identified: nematic, smectics A and B, and the tilted smectics C and (probably) F or I. In the 6OTOmOTO6 series, a phase with a strong tendency to self-alignment occurred between the nematic and smectic A phases. This phase resembled the nematic in its viscoelastic properties, but also showed focal-conic textures. A transition between this 'intermediate phase' and the nematic was detected only from microscopic observations. On the contrary, the 'intermediate phase'-smectic A transition was also detected by DSC (δH ∼ 1kJ mol^-1). Both tilted (S_c and S_F/I) and non-tilted (S_A and S_B) smectics were observed in the 10OTOmOTO10 series, while only tilted smectic phases were identified in the dimers with longer terminal substituents (14OTOmOTO14). Double melting behaviour was found in 14OTO8OTO14 and 14OTO10OTO14. An alternation of the transition temperatures and enthalpies with the odd-even alternation of the lengths of bridging groups was clearly observed. A correlation between total enthalpies of transition and solubilities of the dimeric tolans is stated.     

X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines

We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4'-methoxyazoxybenzene two types of layering were found. One corresponds to the fluctuations of the smectic density wave with a monolayer wavevector q₁, and the other is due to the partial bilayer fluctuations with the incommensurate wavevector q₂ ≠ q₁/2. The temperature dependences of the X-ray scattering intensity and the longitudinal correlation length for both types of layering in the nematic phase are presented. The critical behaviour in the vicinity of the smectic A-nematic phase transition occurs for a fluctuation mode, either q₁ or q₂, depending on the position on the liquid crystal phase diagram. The influence of the molecular structure of cyano-substituted pyridines on the formation of layered structures of different types is also discussed.     

Adiabatic calorimetry of the metallomesogen purple cobalt stearate Co(O2CC17H35)2

The heat capacity of the metallomesogen purple cobalt stearate Co(O₂CC₁₇H₃₅)₂ has been measured by adiabatic calorimetry at temperatures between 16 and 420 K. This compound exhibits two crystalline phases (low temperature Cr2 and high temperature Cr1 phases), mesophase (M phase), and isotropic liquid (I phase). A third crystalline phase Cr3, which is entirely metastable with respect to all the others, is suggested by DSC studies. The Cr2-to-Cr1, Cr1-to-M, and M-to-I phase transitions occurred at 362.1, 380.9, and 400.4 K, respectively. The enthalpy and entropy gains at these phase transitions were determined. The mesophase is either smectic A or nematic.     

Mesomorphic properties of orthopalladated complexes of laterally substituted azobenzene derivatives, 2-alkoxycarbonyl-4-(4-ethoxyphenylazo)phenyl 4-methoxybenzoates

Homologous series of 2-alkoxycarbonyl-4-(4-ethoxyphenylazo)phenyl 4-methoxybenzoates (L) and their orthopalladated μ-chloro dimers (Pd₂Cl₂L₂) have been synthesized and their thermotropic phase transition behaviour has been characterized. The ligands, L, are three-ring nematogens with their central rings substituted in the 2-position by n-alkyl chains of varied length. The thermal stability of the nematic phases decreases rapidly with increasing side chain length and then levels off as the series is ascended. In every case the mesomorphic phase appears only over a small temperature range (less than 20° C). The orthopalladated species also proved to be nematogenic. Melting and clearing points rise on the average by 87° C and 126° C, respectively, and thus the temperature ranges of the nematic phases are enlarged considerably upon metallation. In relation to the observed stabilization of the nematic phases, particularly of higher homologues, a lath-like molecular structure, in which the side chains of the ligands fill up the clefts of the core part, has been deduced for a Pd2Cl2L2 homologue from its solution ¹H-NMR spectrum.     

The Effect Of The Position And Configuration Of Carbon-Carbon Double Bonds On The Mesomorphism Of Thermotropic, Non-Amphiphilic Liquid Crystals

The influence on their mesomorphic behaviour of introducing a carbon-carbon double bond into the chain, central linkage and alicyclic rings in the core of nematic and smectic liquid crystals (LCs) is discussed. Mesogens incorporating a trans-carbon-carbon double bond conjugated with an aromatic ring exhibit high mesophase-isotropic transition temperatures (T_c). However, they are photo-sensitive and can convert to the non-linear, non-mesogenic cis-isomers under the action of light. Non-conjugated double bonds in the terminal chain of mesogens can also lead to higher nematic and smectic C transition temperatures than those of the corresponding materials without a double bond, although the effect is not nearly as great. The position and trans-cis-configuration (E/Z) of the double bond are seen to be decisive. The combination of a hetero-atom (dipole effect) and the added rigidity imposed by the carbon-carbon double bond (steric effect) with a trans-configuration (E) in the terminal alkyl chain attached to the core of a liquid crystal molecule can give rise to a broad nematic phase. The double bond in the terminal chain of nematogens advantageously modifies the elastic constant ratios, as well as other properties of relevance to LCDs, especially for supertwisted TN-LCDs. The double bond in a central linkage gives rise to a broad spectrum of effects, sometimes suppressing undesired smectic phases and widening the nematic phase temperature range. Non-conjugated double bonds in the molecular core in the form of cyclohexene rings generally lead to lower transition temperatures, although smectic phases are sometimes suppressed and a nematic phase is observed. A conjugated double bond in a cyclohexene ring gives rise to a slightly higher T_NI. The effect on the transition temperatures of the double bond in steroid systems is complex.     

Metastable phases in kinetics of phase transformations in the frozen liquid crystal 4-ethoxybenzylidene-4'-n-butylaniline

The nematogen 4-ethoxybenzylidene-4'-n-butylaniline gives by fast cooling a frozen phase called C₁ different from a glassy nematic state. The X-ray diffraction spectra of a non-aligned sample and a sample aligned by a magnetic field show that the C₁ phase is a monolayer smectic phase: molecules are inclined to the normal of the smectic planes by an angle of 35° ± 5°. On reheating we obtain metastable phases more and more ordered; those phases C₂ and C₃ are crystalline. The kinetics for the metastable phases correspond to a nucleation growth process of the same type (n = 2) for the two transformations C₁→C₂→C₂. If we assume a thermal process the growth is monodimensional.     

Phase diagrams of binary mixtures of biaxial nematogens

A mean field theory is used to describe nematic phases of binary mixtures of biaxial molecules. Using a general pseudopotential consistent with the D_2h symmetry of the constituent particles, the theory is used to calculate the elements of the order tensors necessary to describe the orientational order in binary mixtures in both uniaxial and biaxial nematic phases. For a single component, the model only requires one parameter, r₂, a ratio of anisotropic interaction strengths, to predict the temperature dependence of the four order parameters. The temperature dependence of the orientational distribution functions is illustrated for both rod-like and plate-like molecules. For binary mixtures, three anisotropic interaction strengths, r₁, r₂, and r₃, are needed to calculate the order parameters of both components as a function of concentration and temperature. The free energy is evaluated to predict the phase stability of the mixture. By systematically varying the anisotropic interaction strengths, temperature-concentration phase diagrams for a variety of molecular shapes are presented. The theoretical predictions suggest that binary mixtures of molecules with highly asymmetric shapes will display stable biaxial nematic phases.     

Liquid crystalline phase transitions in hexakis(4-(4'-heptyloxy)biphenoxy)cyclotriphosphazene

Hexakis(4-(4'-heptyloxy)biphenoxy)cyclotriphosphazene (HHCP) was synthesized from hexachlorocyclotriphosphazene and 4-heptyloxy-4'-hydroxybiphenyl. The mesogenicity of HHCP was studied by DSC, FTIR spectroscopy and polarizing microscopy. Enantiotropic smectic C and nematic phases were observed between 450 and 455 K and 455 and 456 K, respectively, on heating, and between 456 and 455 K (nematic) and 455 and 440 K (smectic C) on cooling from the isotropic liquid phase. The introduction of the heptyloxybiphenoxy groups as side chains into cyclotriphosphazene has generated the liquid crystalline phase. FTIR spectroscopy showed that the P=N and P-O-(C) stretching vibrations converted to lower frequencies from 1224 to 1210 cm^-1 and from 920 to 910cm^-1, respectively, at the crystalline (C)-S_c phase transition. This result suggests that the state of the cyclotriphosphazene ring dramatically changes near the C-S_c phase transition.     

Liquid crystalline properties of and intramolecular hydrogen bonding in 4-methyl-2'-hydroxy-4'-alkoxyazobenzenes

A homologous series of 4-methyl-2'-hydroxy-4'-alkoxyazobenzenes with chain lengths from 1 to 12 carbon atoms was synthesized and the temperatures of melting, freezing and transition from nematic to isotropic phase were determined. The main thermodynamic characteristics for those transitions were estimated. The liquid crystalline properties were compared with those of homologous series without OH groups and show a marked extension of the nematic phase for the OH substituted compounds. The spectroscopic manifestations of the hydrogenbonded OH ... N chelate ring are discussed, based on a comparison of experimental and Density Functional Theory (DFT) calculated frequencies and IR intensities. The evolution of the IR absorption in the range 1540-1660 cm^-1, ascribed to the OH bending vibrations, is presented on going from the dilute CCl₄ solution through the isotropic and nematic phases to the solid state.     

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.     

Dielectric and elastic properties of a nematic phase induced by charge transfer interaction in a binary system containing disc-shaped molecules

The dielectric constants and the elastic coefficients for splay (K₁) and bend (K₃) of the charge transfer induced nematic (N_c) phase of tridecyl pentakis(phenylethynyl)phenyl ether (1) doped with different amounts of 2,4,7-trinitrofluorenone (2) were determined by studying the electric field induced bend deformation using the capacitance method. A negative dielectric anisotropy was observed. For the bend elastic constant K₃ values up to 22 × 10^-12 N are found which are one order of magnitude higher than the respective values of discotic nematic (N_D) phases. Values of 0·6-0·8 are obtained for the ratio K₁/K₃; these show a minimum for the equimolar complex.     

Oscillations in thin nematic layers

In the present paper a thin nematic liquid crystal layer between two identical boundary surfaces (solid walls or free surfaces in the case of a freely suspended film) is considered. In a mean field approximation it is shown that the interference between the boundary surface-induced smectic density waves results in oscillations of the free energy of the nematic layer and disjoining pressure acting on the boundary surfaces. Theoretical dependence of disjoining pressure on the nematic layer thickness is in qualitative agreement with experiment. Also we have considered a thin film of polar nematic in which in addition to an ordinary monolayer smectic A phase (S_A1) with the layer thickness d equal to the molecular length l the partial bilayer smectic A phase (S_Ad) occurs. It is shown that the variation of the distance between the boundary surfaces can result in the oscillatory S_A1-A_Ad phase transitions in this nematic film     

The synthesis and liquid crystalline behaviour of alkoxy-substituted derivatives of 1,4-bis(phenylethynyl)benzene

Despite the prevalence of organised 1,4-bis(phenylethynyl)benzene derivatives in molecular electronics, the interest in the photophysics of these systems and the common occurrence of phenylethynyl moeties in molecules that exhibit liquid crystalline phases, the phase behaviour of simple alkoxy-substituted 1,4-bis(phenylethynyl)benzene derivatives has not yet been described. Two series of 1,4-bis(phenylethynyl)benzene derivatives, i.e. 1-[(4'-alkoxy)phenylethynyl]-4-(phenylethynyl)benzenes (5a-5f) and methyl 4-[(4'-alkoxy)phenylethynyl-4'-(phenylethynyl)] benzoates (18a-18f) [alkoxy = n-C₄H₉ (a), n-C₆H₁₃ (b), n-C₉H₁₉ (c), n-C₁₂H₂₅ (d), n-C₁₄H₂₉ (e), n-C₁₆H₃₃ (f)] have been prepared and characterised. Both series have good chemical stability at temperatures up to 210°C, the derivatives featuring the methyl ester head-group (18a-18f) offering rather higher melting points and generally stabilising a more diverse range of mesophases at higher temperatures than those found for the simpler compounds (5a-5f). Smectic phases are stabilised by the longer alkoxy substituents, whereas for short and intermediate chain lengths of the simpler system (5a-5c) nematic phases dominate. Diffraction analysis was used to identify the SmB_hex phase in (5d-5f) that is stable within a temperature range of approximately 120-140°C. The relationships between the organisation of molecules within these moderate temperature liquid crystalline phases and other self-organised states (e.g. Langmuir-Blodgett films) remain to be explored.     

Detailed study of a two component smectic 4TPB-8OCB system with a nematic gap. Phase diagram and viscosity study

The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4'-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η₁, η₂, η₃ and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.     

Broadband dielectric studies of weakly polar and non-polar liquid crystals

Complex dielectric permittivities, for two orientations of the director n, parallel (E || n) and perpendicular (E ⊥ n) to the probing electric field E, of the weakly polar liquid crystals (LCs) 4,4'-dihexylazoxybenzene (D6AOB) and 4,4'-diheptylazoxybenzene (D7AOB) as well as the non-polar LC diheptylazobenzene (D7AB) have been measured in the frequency range 75 kHz to 1 GHz. The measurements were performed in the nematic, smectic and isotropic phases of the LCs. The dielectric anisotropies Δε (=ε_||-ε_⊥) obtained from the values of dielectric permittivities at 100 kHz in the nematic phase were found to increase with decreasing temperature. However, for the DnAOBs, the Δε values are somewhat smaller than that for D7AB which does not have a permanent dipole moment. In the nematic phase two molecular relaxation processes were observed for both DnAOBs in each of the orientations—parallel and perpendicular. The four processes merge into two separate processes in the isotropic phase. For D7AB no orientational relaxations were observed in the experimental frequency range.     

The heat capacity of the doubly reentrant mesogen: 4-cyanobenzoyloxy-4'-octylbenzoyloxy-p-phenylene

Heat capacity of a 20 g sample of the mesogen 4-cyanobenzoyloxy-4'-octylbenzoyloxy-p-phenylene was measured in the temperature range 380 to 510 K by adiabatic calorimetry. The C_p versus T data is interpreted in terms of thermodynamically equivalent S_A1 and S_Ad phases whose Gibbs potential surface intersects the thermodynamically equivalent reentrant nematic and nematic Gibbs potential surface in such a way as to define the first order doubly reentrant phase sequence: S_A1-N-S_Ad-N. The data do not allow any precise estimates of transition enthalpies.