New chiral imines based on S-α-phenylethylamine and S-α-benzylethylamine in induced cholesteric and smectic mesophases: II. Behavior of chiral dopants in ester liquid-crystal systems

The temperature dependences of the pitch of induced helical structures in the ester liquid-crystal systems containing hydroxyphenyl benzoate derivatives and chiral N-aroyloxybenzylidene-S-α-phenylethylamines or N-aroyloxybenzylidene-S-α-benzylethylamines have been measured. The twisting power of chiral dopants is calculated and the temperature dependence of the helical pitch in the induced cholesteric (N ^*) and smectic C ^* mesophases is quantitatively characterized. The parameters A = dT _is/dC that characterize the effect of dopants on the thermal stability of the N ^* phases are determined from the concentration dependences of the cholesteric-isotropic transition temperature T _ts for the n-hexyloxyphenyl-n-butylbenzoate-chiral dopant systems. The results obtained are compared with the data for the 4-n-pentyl-4′-cyanobiphenyl (5CB)-based cholesteric systems induced by the chiral dopants under consideration.     

Synthesis and Liquid Crystal Behaviour of Further 4,4″-Disubstituted 2'-Fluoro-1,1':4',1”-Terphenyls

Further to earlier work on the liquid crystal properties of fluoro-1,1':4',1”-terphenyls¹ we have now extended this series to include terminally fluoro-and cyano-substituted 2'-fluoro-1,1':4',1″-terphenyls, chiral 2'-fluoro-1,1':4',1″-terphenyls, and esters derived from 2'-fluoro-1,1':4',1”-terphenyls and incorporating the alkylcyclobutyl group. The preparations and transition temperatures for these series of compounds are presented and their transition temperatures and mesophase types are discussed. An interesting result from this work was the appearance of an S^* _C phase for one chiral homologue and of S _c phases for the esters incorporating the alkylcyclobutyl group. The S _c formation is attributed to the presence of the fluoro-substituent.

Keywords: terphenyls, fluoroterphenyls, smectic C phases, cyclobutyl esters     

New Mesogenic Compounds Exhibiting a Thermotropic Optically Isotropic Phase

Abstract

The only two series of carboxylic acids, 3-nitro- or 3-cyano-4-alkoxybiphenyl-4′-carboxylic acid, are known to exhibit the optically isotropic phase (S_D). Now we synthesised 4-(branched alkoxy)-anilinebenzylidene-4′-carboxylic acid series, all being racemic, and investigated their mesophase properties by means of DSC, optical microscopy and X-ray measurement. As a result, some branched azomethine derivatives were found to form a S_D phase. We also synthesised cinnamic acid derivatives with branched alkoxy tail that were turned out to exhibit either S_D phase or hexagonal columnar phase. We tried biphenyl or azobenzene core analogues which were found to produce a highly ordered smectic phase or no mesophase, respectively. At this stage, we suppose it plays an important role that the branch is introduced into the terminal chain and a certain dipole moment is located within the mesogenic core. These factors may allow the peculiar molecular packing that causes an optically isotropic property.     

The Existence of SA 1, SA d,SA 2 and S∼C Smectic Phases in the New Series 4-Alkoxyphenyl-4′- [4″-Cyanobenzyloxv] Benzoates

A homologous series of 4 - alkoxyphenyl-4′-[4″-cyano-benzyloxy] benzoates have been synthesized. Within this series three smectic A phases : S_{A 1}, S_{A d} and S_{A 2} were identified by mean of small angle X-Ray scattering. Furthermore three derivatives exhibit the novel smectic phase made of ribbons S～_C.     

Synthesis and Mesomorphic Characteristics of Fluoroaniline Derivatives with Different Lateral Groups

Two new homologous series of liquid crystals viz. 4-(4'-n-alkoxybenzoyloxy)-2-chlorophenylazo-4”-fluorobenzenes(I) and 4-(4'-n-alkoxybenzoyloxy)-2-methylphenyl azo-4”-fluorobenzenes(II) with terminal fluoro, lateral chloro(I) and methyl(II) group and central ester and azo linkages are synthesized and their mesomorphic properties are studied. Both the series are similar in molecular structure with the difference in their lateral substitutions; series I has chloro group and series II has methyl group as laterally substituted groups. All the twelve homologues of each of the series are mesogenic in nature. Series I shows nematic mesophase from the first C₁ to the last C₁₆ derivative synthesized; smectic mesophase is exhibited by last two viz. C₁₄ and C₁₆ derivative, where as all the members from C₁ to C₁₆ of series II only show nematic mesophase. The nematic mesophase shows marble texture and the smectic mesophase shows Schlirene texture of the Smectic C variety. Both the series are compared with structurally related series.     

Bifurcated-tail chiral fluorinated organosiloxane liquid crystalline materials

Abstract

The synthesis and characterization of a series of chiral fluorinated low molar mass (bifurcated tail) organosiloxane materials is presented. The mesogenic moiety is similar to that in the TSiKN65F mesogen reported by Naciri et al., which displays a de Vries-type S_A^* phase. The one parameter varied across the series reported herein is the length of the alkyl chain linking the mesogen moiety to the bifurcated siloxane tail.     

Dielectric relaxation in nematic,smectic A and C phases in the MHz region

The dielectric properties of four 4-n-alkyloxyphenyl 4-n-alkyloxybenzoates with nematic, smectic C and partly additional S_A phases were investigated. In 2 substances for ϵ′ reproducible values also in the smectic phases resulted allowing to conclude on a good orientation in the samples. In all 4 substances the dielectric relaxation in the MHz region was measured and the respective activation energies were determined. The molecular mobility in the S_C phases is higher than in the S_A phases, however comparable with that of the nematic phases. In all 3 phase types a single relaxation mechanism exists, which is explained as rotation of the molecules around a short axis.     

Liquid-Crystalline Solvents as Mechanistic Probes. 10. Dynamics of Intramolecular Quenching of Pyrenyl Fluorescence in the Liquid-Crystalline and Isotropic Phases of a Cholesteric Solvent1

Abstract

The effect of cholesteric order in a 59.5/15.6/24.9 (w/w/w) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride (CM) on the intramolecular fluorescence quenching of l,3-bis(l-pyrenyl) propane (P3P) has been explored. A comparison with fluorescence quenching of N,N-dimethyl-4-[3-(l-pyrenyl)propyl]aniline (P3D) in CM is made. From the Arrhenius activation parameters for quenching in the cholesteric and isotropic phases, it is concluded that the motions which take the ground state conformers of P3P to their quenching transition state are nearly impervious to macroscopic CM mesophase order: in the cholesteric phase, E'_a = 10.5±0.4 kcal mol^?1 and ΔS^? = 1 ± 1 eu; in the isotropic phase, E'_a = 10.0 ± 0.2 kcal mol^?1 and ΔS^? = 0 ± 0.5 eu. An explanation of these results is advanced.     

Mesomorphism dependence of a terminal or lateral alkoxy group

A novel homologous series of thermotropic mesomorphs has been synthesized and studied with a view to understanding and establishing the effect of molecular structure on mesomorphic properties with reference to rigidity and flexibility of the homologues series: RO-C₆H₄-CH?CH-CO-C₆H₄-OC₁₈H₃₇(n). The novel homologous series comprises 13 novel homologues (C₁ to C₁₈), from which 11 homologues are enantiotropically mesogenic (C₃ to C₁₈). Smectogenic mesophase commences from C₇ homologue, and nematogenic mesophase is exhibited by C₃ to C₁₈ homologues in enantiotropic manner. Thus, C₇ to C₁₈ homologues are enantiotropically smectogenic plus nematogenic. Textures of smectic phase are of the type A or C, and that of nematic phase are threaded or Schlieren as confirmed through an optical polarizing microscope equipped with a heating stage. Transition curve of a phase diagram behaved in a normal manner except N-I transition curve, which shows minor deviating trend (C₁₂ to C₁₈) from expected normal behavior. An odd-even effect is exhibited by N-I and Sm-N transition curves. Analytical, spectral, and thermal data confirm the molecular structures of novel homologues. Thermal stabilities for smectic and nematic are 52.7°C and 66.6°C, respectively, whose total mesophase lengths range from 12.0°C to 28.0°C. Thus, novel series of chalcones is a low melting series whose mesogenic transition temperatures vary between 38.0 and 86.0°C.     

Investigation of a Twisted Phase between the Cholesteric and the Smectic A Phase

In a ternary mixture of two liquid crystals and a non-mesomorphic chiral dopant an intermediate state with an unusual texture was found between the cholesteric and the S_A phase which could be identified as a separate twisted mesophase. The reflection wavelength of this mesophase does not change with temperature whereas the transmission of unpolarized light decreases from 50 to 20%. The experimental findings point to the existence of a twisted grain boundary phase.     

Dielectric and pyroelectric studies of an antiferroelectric liquid crystal

Polymorphism and polar properties of an antiferroelectric (R)-2-methylheptyloxycarbonylphenyl-4-[(4-decyloxy-3-fluoro)benzoyloxy]benzoate liquid crystal are studied. The phases are identified, and the phase transition points are determined. Dielectric constant, dielectric losses, and pyroelectric properties are studied for the orthogonal smectic SmA and tilted smectic, SmC _α ^* , SmC ^*, SmC _γ ^* , and SmC _A ^* phases. The temperature dependence of spontaneous polarization is measured by the repolarization current technique and integration of the pyroelectric constant.     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Pitch Behavior of the Cholesteric Helix at the Phase Transition Point Cholesteric/Smectic A up to 2500 Bars. Study of Some Cholesteryl n-Alkanoates

Experimental evidence is given, for the first time, for a finite pitch of the cholesteric helix at the phase transition point cholesteric/smectic A (Ch/S_A). This finite pitch changes into an infinite one at a given point on the phase equilibrium line Ch/S_A. Therefore this point is called the pitch infinity point (PIP).

With increasing n-alkyl chain length of the investigated cholesteryl n-alkanoates the pressure coordinate of the PIP is shifted to higher values up to a chain length of 13 C atoms. For the alkanoates with 13,15 and 17 C atoms the same PIP pressure of about 1000 bars was determined.

The Ch/S_A phase equilibrium line of the cholesteryl esters could at best be followed up to 2600 bars. The respective transition enthalpies were measured at normal pressure and the corresponding changes in volume calculated by means of the Clausius-Clapeyron equation.     

Comparing investigation of the smectic C and A phases of the di-n-alkyl 4,4′-azoxycinnamates (II) X-ray studies

The paper continues the investigation of the smectic C and A phases of the title compounds which has been done in Part I by DSC calorimetry. Here the layer spacings in the S_C and S_A phases are presented which exhibit a remarkable different temperature dependence in the S_C phases within the homologous series. A tendency of a change from a second order S_C/S_A transition to a first order transition has been found. The layer spacings of the S_A phase show a chain length dependent difference to the lengths of the molecules which may be explained by an increasing amount of gauche conformations in the higher homologues.     

Dependence of Mesomorphism on Molecular Rigidity with Reference to Lateral Substitution and Central Bridge

A novel mesogenic ester homologous series is synthesized and studied with a view to understanding and establishing the effect of laterally substituted -OCH₃ on mesomorphic behavior. The series consists of twelve members. C₁ to C₄ members are nonmesogenic, C₆ to C₁₂ are smectogenic in addition to nematogenic, and C₁₄ to C₁₆ are only smectogenic. The textures of smectic and nematic phases are A or C type and threaded or Schlieren, respectively. The transition curves in a phase diagram exhibit an odd–even effect and behave in normal manner except for the C₁₄ and C₁₆ derivatives in Sm-I transitions. Thermometric data were determined by an optical polarizing microscope equipped with a heating stage. Average thermal stability for smectic and nematic are 107.7°C and 121.0°C, respectively. Smectogenic and nematogenic mesophase length ranges from 11.0 to 44.0°C and 12.0°C to 39.0°C, respectively. The mesomorphic properties of present series are compared with structurally similar other known series. Thus, present series is predominantly smectogenic and partly nematogenic of middle ordered melting type.     

The Smectic A-Smectic A Transition in a Homologous Series of 4-(4-Alkoxyphenoxycarbonyl)phenyl 5-Cyano-2-furancarboxylates

The thermal properties for a homologous series of 4-(4-alkoxyphenoxycarbonyl)phenyl 5-cyano-2-furancarboxylates have been examined. The smectic A phase commences from the hexyloxy homolog, and the octyloxy, nonyloxy and decyloxy homologs show two kinds of smectic A phases and experience the S_A-S_A transition. The lower phase which shows a typical fan shaped texture is assigned to a smectic A with a bilayer arrangement of the molecules (S_A2). The higher phase which shows a typical fan shaped texture on the cooling stage and a broken fan one on the heating stage, is assigned to a smectic A with a partially bilayer arrangement of the molecules (S_Ad). The S_A-S_A transitions could be easily detected by both differential scanning calorimeter and optical microscopic method.     

Dielectric behaviour of the smectic I phase

The dielectric behaviour of 4-n-butyl-4′-n-dodecyloxyazobenzene was investigated in the smectic A, C and I phases from 10² to 3 ṁ 10⁶ Hz. In all phases a dielectric relaxation process could be found which is connected with the reorientation of the molecules around their short molecular axes. At the phase transition S_C/S_I a step of one decade was observed in the relaxation frequencies.     

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K _θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P _s and the data on K _θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ₁(T) for the smectic C* phase. The curves γ₁(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.     

High Pressure Studies on Mesomorphic and Polymesomorphic Transitions

The paper presents the following results: (1) pressure induced mesomorphism in p-p'-n-butylazobenzene and biphenyl p-ethyl benzoate. (2) the monotropic nematic-isotropic transition in trans-p-n-propoxy-x-methyl cyanophenyl cinnamate becoming enantiotropic under pressure, (3) suppression of the cholesteric mesophase in cholesteryl nonanoate and the location of a solid-cholesteric-isotropic triple point at ～ 2.85 kbar. confirming an earlier observation of Keyes. Weston and Daniels on the same compound, (4) a measurement of dT/dp for the solid-smectic A, smectic A-cholesteric and cholesteric-isotropic transitions in cholesteryl myristate and a comparison with the values evaluated from the Clausius-Clapeyron equation using the known latent heat and volumetric data.     

Reentrant nematic phase with the new pentamorphism n sa sc nre sare in a mixture

Three homologues of the new series 4 - cyanobenzylidene - 4′ - [4″ - alkoxybenzoyloxy] anilinie are presented. One of them exhibits a reentrant nematic and a reentrant smectic A phases. A diagram of state with 4 - cyanobenzylidene - 4′ - [4″ - decyloxybenzoyloxy] aniline and 4 - [4″ - decyloxybenzoyloxy] benzylidene - 4′ - cyanoaniline has been studies. In the system studies, the following new phase sequence was found with decreasing temperature N S_A S_C N S_A.