Transition metals complexed to ordered mesophases. XIII. Synthesis and mesomorphic properties of potentially ferroelectric Schiff's base palladium(II) complexes

A series of N-[4'-(dodecyloxy)resorcylidene]-4-(RO*)-anilines (HL_n n = 1, R*O = (-)-cis-myrtanyl; n = 2, R*O = (-)-menthyl; n = 3, R*O = S-(-)-β-citronellyl; n = 4, R*O = R-(-)-2-octyl) has been synthesized and the mesomorphic properties investigated. These ligands are able to chelate a metal atom; accordingly three different series of palladium complexes, [(Ln)Pd(Ln)], [(Ln)Pd(Azoxy-6)] and [(L₃)Pd(Ph-Py_n)] (where Azoxy-6 and PhPy_n are cyclopalladated 4,4'-dihexyloxyazoxybenzene and 5-substituted-2-(4-substituted-phenyl)pyrimidine, respectively) have been prepared. The mesogenic HL₃ and HL₄ ligands exhibit a S*_C phase, retained in [(L₃)Pd(L₃)], which changes to a cholesteric phase in [(L₄)Pd(L₄)] and becomes the more ordered S*_H phase in the [(Ln)Pd(Azoxy-6)] (n = 3,4) derivatives. In contrast, in the [(L₃)Pd(Ph-Py_n)] compounds the mesomorphic phase is a S_A phase.     

Side-chain liquid crystalline polysiloxanes containing a cyanohydrin chiral centre

A number of new ferroelectric side chain liquid crystal homopolymers derived from poly(hydromethylsiloxane), their copolymers with different mesogens and with poly(dimethylsiloxane) backbones have been prepared. The transition temperatures of the monomers, homo and copolymers have been determined. The homopolymers exhibit S*_C and N* phases, while copolymers show only a S*_C phase. A measured spontaneous polarization value of monomer, 9b, is larger than 500 nC cm^-2.     

The ferro-, ferri- and antiferro-electric properties of a series of novel 2- or 3-substituted-alkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)-benzoate esters

This paper reports the synthesis of six chiral liquid crystalline 2- or 3-substituted-alkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)benzoates, their mesophase assignments and the phase transition temperatures. All six materials display S*_A followed by S*_C phases on cooling from their isotropic liquids. The magnitudes of switched tilt angle, spontaneous polarization, helical twist sense (L.H.) and direction of polarization (P_s(-)) are given for the S*_C phases. Close inspection of the S*_C phases revealed that for the six compounds, two showed a ferrielectric (S*_Cγ) and an antiferroelectric (S*_A) subphase, two showed only a ferrielectric subphase and one showed only an antiferroelectric subphase. The field-dependent behaviour of the switched tilt angle of the ferrielectric and antiferroelectric phases of some of the compounds is also presented.     

Photomechanically induced phase transitions in ferroelectric liquid crystals

We demonstrate that the addition of small amounts of a novel azo-dye to a ferroelectric liquid crystal and illumination with low intensity (∼ 0.8 mW cm^-2) UV light can result in reversible, isothermal phase transitions and dramatic changes in the properties of the system. In particular we examine light induced transitions between the S*₁, and S*_C phases and the consequent photomechanical regulation of spontaneous polarization.     

Bistable and monostable polarized states of a liquid-crystalline ferroelectric in an electric field

A condition for obtaining bistable states in a liquid-crystalline ferroelectric (S*_C) has been found by computer simulation and analytical estimates; it depends on the value of the applied electric field, the magnitude of the polar contribution to anchoring energy and the material parameters of S*_C     

Optical activity studies in the pretransitional isotropic region of strongly chiral propiolate ester liquid crystals

Optical rotation and circular dichroism measurements performed in the pretransitional isotropic region above the chiral smectic phases TGBA*, TGBC*, S*_C, and S*_A exhibited by 1-alkylalkyl 4'-(4'-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates are reported. These results revealed a high degree of increasing chiral ordering on cooling in the isotropic phase, particularly in the temperature region where a broad DSC peak appears. However, the behaviour of the pretransitional optical activity was unlike that commonly observed for chiral nematic or blue phase compounds and did not follow a simple Landau-de Gennes temperature dependence. Characteristic features of the pretransitional optical activity may aid in understanding the local structure of chiral smectic phases. Electric fields did not seem to influence the results significantly, but strong electric field-induced optical rotation behaviour was demonstrated for a mixture constituted of the laterally fluorinated propiolate ester component and a nematic with a large positive dielectric anisotropy. These field-induced results could be fitted to expressions from the existing theory of pretransitional optical activity. Circular dichroism studies at low temperatures were performed for two compounds in dilute solution and were found to exhibit very large changes in their extinction coefficient at wavelengths in the ultraviolet region, which may indicate strong chiral interactions even in dilute isotropic solutions of these materials.     

The characteristic dielectric behaviour in ferroelectric liquid crystals at a phase transition and the contribution of the soft mode

The characteristic dielectric behaviour of ferroelectric liquid crystals with a large spontaneous polarization has been studied as functions of the D.C. bias field, frequency, cell thickness and applied pressure. Under the condition in which the contribution of the Goldstone mode is suppressed, a sharp peak in the temperature dependence of the dielectric constant is clearly observed at the transition between S_A and S*_C phases T_SC*sA. The relaxation of the soft mode is observed both in the S_A and S*_C phases by eliminating the contribution of the Goldstone mode under a D.C. bias field. Another relaxation is also observed in the S*_C phase around several kHz in addition to that of the soft mode and the Goldstone mode. The pressure effect on the soft mode was also studied.     

Circular dichroism in ferroelectric and antiferroelectric liquid crystals

Liquid crystal induced circular dichroism (LCICD) measurements were made to investigate the pretransitional phenomena in the S_A phase just above the S_A-S*_C and S_A-S*_CA phase transitions of both the first and the second order. The pretransitional LCICD in S_A was observed in the second order phase transition to S*_c and S*_cA, suggesting the existence of a dynamic helical structure in S_A. Such behaviour disappears when the transition is of the first order. It seems that the handedness of the dynamic helix in S_A is the same as that in S*_c even when the lower temperature phase is S*_cA. This is explained as a result of a dominant contribution of ferroelectric soft mode.     

Mesomorphic properties of some laterally fluorinated 4-alkyl,4'-oxoalkyl 1,1': 4',1'terphenyls

The liquid crystalline behaviour and nematic viscosities of a series of laterally fluorinated alkyl-oxoalkyl terphenyls are reported. The mesomorphic behaviour and nematic viscosity can be influenced by the position of the oxygen atom in the oxoalkyl chain as well as the position of lateral fluorination. Compounds have been synthesized that are suitable for use in ferroelectric smectic C (S*_C) mixtures; the switching properties of a four component mixture are also reported.     

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the SA to SC* (SC) transition

We investigated the correlation between orientational order and microscopic organization of the molecules in smectic A and chiral (racemic) smectic C phases by means of solid-state C-13 NMR, powder X-ray diffraction, and electrooptical measurements. The compounds under investigation are 4-((S)-2-methyloctanoyl)phenyl 4'-nonylbiphenyl-4-carboxylate ((S)-MONBIC) and its corresponding racemic compound ((S, R)-MONBIC). Static C-13 NMR indicates that: (1) the orientational angle of the tail with respect to the magnetic field decreases slightly both in the S_A and S*_C phases as decreasing temperature, and (2) the angle of the core with respect to the field decreases in the S_A phase but increases in the S*_C phase as decreasing temperature. Analysis of C-13 T₁ reveals that the dynamic molecular deformation for the core part can occur near the transition. We discuss the dynamic molecular deformation in comparison with the reorientation of the director at the S_A to S*_C transition. Based on the experimental results, we propose the structural model in which describes the microscopic organization of the molecules in the mesophases.     

Thickness dependence of chiral smectic C liquid crystal phase transitions

In this paper we consider the influence of solid boundaries on the transition temperatures of a chiral smectic C liquid crystal. Particular attention has been paid to the S*_C-S_A transition. A simple model to explain the thickness dependence of the S*_C-S_A transition is proposed. An experimental method to determine some elastic constants and the anchoring energy of ferroelectric liquid crystal molecules is demonstrated.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Structure-property relationships of 'diluted' ferroelectric polysiloxanes

A series of new ferroelectric copolysiloxanes with systematically varied comonomer content ('dilution') has been synthesized. Good planar alignment could be achieved for all copolysiloxanes and they were studied with respect to their mesomorphic and ferroelectric properties. Broad enantiotropic S*_C phases and spontaneous polarizations up to 286 nC cm^-2 are found. X-ray diffraction experiments show a linear increase of the smectic layer spacing by 'dilution'. This points to a microphase separated structure of mesogenic groups and siloxane chains. It is found that the fixation of a mesogen to homopolysiloxane leads to an increase of P_s, whereas the 'dilution' of the mesogens with dimethylsiloxane units decreases P_s again. Based on the microphase separated model, it can be shown that the decrease of P_s is not only due to the decrease of the vol % of mesogenic groups. The coupling between different mesogens mediated by the polymer chain, has additionally to be taken into consideration. A remarkable drop in the response times τ with decreasing mesogen content is confirmed and switching times less than 1 ms were measured.     

Experimental observation of multicritical points with TGB phases on a pure compound

High pressure studies for the n = 12 homologues of the 3-fluoro-4((R) or (S)-methylheptyloxy)-4'-(4-alkyloxy-3-fluorobenzoyloxy) tolanes series have been performed by thermobarometric analysis. The phase sequence presents two Twist Grain Boundary phases denoted TGB_A and TGB_C. The pressure-temperature phase diagram has been determined; it exhibits a S_C* phase, and an induced phase identified as S_A which does not exist under atmospheric pressure. The TGB_A temperature stability domain versus pressure first increases, then rises to a maximum and finally disappears. Such phase behaviour leads to an experimental observation of two multicritical points S_C*-S_A-TGB_A and S_A-TGB_A-N* for the pure compound under high pressures. This result confirms the existence of singular points previously predicted by the Renn-Lubensky theory.     

Re-entrant and induced mesophases: Mixed systems showing re-entrant TGBA and re-entrant cholesteric phases

We have investigated the behaviour of chiral carbohydrate-based trioxadecalines with a cyano group in mixed systems. The mixture of a trioxadecaline having a Ch phase and a trioxadecaline having a Ch, S_A and a S*_Cphase sequence induces a re-entrant TGB_A and Ch phase. Below the stabilized S*_C phase, a re-entrant Ch phase is observed. In the mixture of the cholesteric trioxadecaline with CCH7, itself only showing a nematic phase, a TGB_A phase and a broad range S_A phase are induced from two non-smectic compounds. Both mixtures show a blue phase with a pitch ranging from the UV to red depending on the molar fraction.     

Synthesis and mesomorphic properties of new compounds exhibiting TGBA and TGBC liquid crystalline phases

We synthesised a series of rod-like mesogens with a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free-standing films, by Grandjean-Cano texture and by NMR studies on a deuterium-labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20-30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in ²H-NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases.     

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.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4'-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8-12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

New TGBA series exhibiting a SC*SASA*N* phase sequence

A new series with a chiral tolane core has been synthesized. These materials belong to the optically active series: 3-fluoro-4-[(R) or (S)-1-methylheptyloxy]4'-(4'-alkyloxy-3'-fluorobenzoyloxy)tolanes (nFBTFO₁M₇). For the first time, the helical S_A* phase or TGB_A phase is found in all of the derivatives from heptyloxy to octadecyloxy. The S_A-S_A*-N* phase sequence is observed in several compounds with short chains and the S_C*-S_A-S_A*-N* phase sequence is obtained with the decyloxy derivative. The TGB_A phase has filament or cholesteric textures. The helical pitch of the TGB_A phase is short for the octadecyloxy derivative and is compared with that for different chain lengths as a function of temperature. The layer spacing in the TGB_A phase shows that it is incommensurate with the fully elongated molecular length.     

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.