Structural and conformational investigations in SmA and different SmC phases of new hockey stick-shaped compounds

New meta -substituted homologous three-ring mesogens, the 4-(3- n -decyloxyphenyliminomethyl) phenyl 4- n -alkyloxybenzoates, have been synthesized, which are non-linear due only to the attachment of one of the alkyloxy groups in a meta -position. The mesophases were studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diffraction, and electro-optical and dielectric measurements. Unusual phase behaviour was observed on varying the length of the terminal chain. The most interesting finding is the occurrence of two polymorphic tilted smectic phases designated as SmC 1 and SmC 2 . The existence of these phases was revealed by calorimetric studies and also from the pronounced difference in optical textures. It was shown from NMR measurements that the molecular orientation changes from a synclinic to an anticlinic arrangement in the SmC 1 to SmC 2 phase transition. It has also been shown, using NMR, that the SmC 1 →SmC 2 phase transition in these compounds is accompanied by a conformational change in the molecular fragment containing the aromatic ring with the meta -substituted terminal alkyloxy chain. This conformational change is linked to a change in the shape of the molecules and leads to a different packing of the molecules within the layers of the SmC 2 phase. From dielectric measurements an increase by a factor of two was detected in the molecular mobility at the transition into the low temperature SmC 2 phase. This finding supports a change in the packing as result of conformational changes.     

Isotropic cubic phase of 4-n-pentadecyloxy-3-nitrobiphenyl4-carboxylic acid (ANBC-15): DSC, microscopic and dynamic viscoelastic studies

The phase behaviour of 4-n-pentadecyloxy-3-nitrobiphenyl-4-carboxylic acid (ANBC-15) was investigated by differential scanning calorimetry, polarizing optical microscopy, and dynamic viscoelastic measurements. The phase sequence for the virgin sample of ANBC-15 is crystalsmectic C (SmC)-cubic (D)-smectic A (SmA)-'structured liquid' (I1)-isotropic liquid (I2) on first heating. It was found that the appearance of the D phase on the second heating depends on the top temperature of the first heating. It was also shown that the D phase is formed on heating when the preceding SmC phase is 'solid-like' from the viscoelastic point of view, i.e. storage modulus (G) loss modulus (G) at 62.8 rad sec -1, while the 'liquid-like' SmC phase is transformed directly into the SmA phase without showing the D phase on heating. The isothermal frequency scans at a temperature in the D phase showed the existence of a cross-over point, G G , with G G in the lower frequency side, suggesting a structural fluctuation of the D phase. These results are ascribed to the molecular structure of ANBC-15, which has a critical alkoxy chain length in ANBC series.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr 2 )-high temperature crystal (Cr 1 ) - cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30-40 MPa, in conjunction with the disappearance of the Cr 1 phase. The transition sequence changes to Cr 2 -SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (d T /d P ) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0 ±1.0°C for the SmC, Cub and Cr 1 phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

Thermal behaviour under pressure of the thermotropic cubic mesogen 4'- n -alkoxy-3'-nitrobiphenyl-4-carboxylic acids

The thermal behaviour of members of a homologous series which exhibits the optically isotropic cubic phase, the 4'- n -alkoxy-3'-nitrobiphenyl-4-carboxylic acids having alkoxy chains containing 16, 20 and 22 carbon atoms (referred to as ANBC-16, -20 and -22, respectively) was investigated under pressures up to 200-400 MPa by high pressure differential thermal analysis. In the phase diagram of ANBC-16 obtained on heating, a triple point was estimated at 54 ±1 MPa and 205 ±1°C for the SmC, Cub and SmA phases. It was found that the X phase is formed on cooling under all pressures, while appearing on heating at high pressures above about 54 MPa. Thus the X phase appears monotropically between the SmA and Cub phases in the low pressure region and enantiotropically between the SmA and SmC phases under higher pressures. It is strongly suggested that the X phase is a columnar mesophase. For ANBC-20 and -22, the cubic phase tends to be destabilized with increasing pressure. The temperature region of the cubic phase of ANBC-20 becomes narrower with increasing pressure and a triple point for the SmC, Cub and I phases is estimated to be at about 309 MPa. On the other hand, the cubic phase of ANBC-22 is still observed at the highest pressure examined.     

Ferroelectric liquid crystals with a thioester linking group positioned at the chiral tail

A homologous series of chiral thioesters, hexyl (2S)-2-[6-(4-alkanoyloxyphenyl)benzoyloxy-2'-naphthyl]thiopropionates, HnPBNTP (n=7-11) was synthesized, and the mesophases and electro-optical properties were studied. All the materials possess a ferroelectric SmC* phase, which was confirmed by the switching current behaviour. It was found that compounds with a shorter alkyl chain length (n=7-9) displayed SmA* and SmC* phases, while, compounds with a longer alkyl chain length (n=10 and 11), displayed the SmC* phase exclusively. Spontaneous polarization and apparent tilt angle were also measured. The maximum P_s values are in the range 15-26 nC cm^-2, and the maximum θ values are in the range 23°-28°.     

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′-(1,3-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.     

Inversion of the phase sequence between the cubic and smectic C phases under pressure

The phase behaviour of a thermotropic cubic mesogen of 1,2-bis(4′-n-tetradecyloxybenzoyl)hydrazine BABH-14 was studied under hydrostatic pressure using a polarising optical microscope equipped with a high-pressure optical cell, and the P–T phase diagram was constructed. BABH-14 shows the Cr–Cub–I transition sequence under atmospheric and lower pressures, but the Cub phase is replaced completely by the high-pressure SmC, SmC(hp), phase under higher pressures. There is a narrow intermediate-pressure region between the low- and high-pressure regions, in which the Cr–SmC(hp)–Cub–I phase sequence is recognised. The SmC(hp)–Cub transition line has a positive slope with pressure and there are two triple points: one is for the Cr, Cub and SmC(hp) phases and the other is for the I, Cub and SmC(hp) phases. Comparing the phase sequence of BABH-14 with those for BABH-8 and -10, the pressure-induced inversion of the phase sequence between the cubic and SmC phases occurs in the BABH-n homologous compounds. Another new phenomenon is the formation of the monotropic cubic phase on cooling in the intermediate- and high-pressure regions, and an intriguing phenomenon of the cubic phase appearing twice, i.e. I–Cub–SmC(hp)– Cub–Cr phase transition, occurs in the intermediate-pressure region.     

Phase behaviour of the thermotropic cubic mesogens 1,2-bis-(4-n-undecyl- and 4-n-dodecyl-oxybenzoyl)hydrazine under pressure

The phase transition behaviour of two optically isotropic, thermotropic cubic mesogens 1,2-bis-(4-n-undecyloxy- and 4-n-dodecyloxy-benzoyl)hydrazine, BABH(11) and BABH(12), was investigated under hydrostatic pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope equipped with a high pressure optical cell. It is found that for BABH(11) and BABH(12), a smectic C (SmC) phase is induced between the isotropic liquid (I) and the cubic (Cub) phases by applying pressures above 10-12 and 16-17 MPa, respectively. A sea-island texture consisting of bright sand-like sea regions (SmC phase) and areas of dark islands (Cub phase) appears in the mesophase under pressures up to 140 MPa, while the sand-like texture of the SmC phase is formed predominantly on cooling under pressure. These observations indicate the destabilization of the cubic phase with increasing pressure. The phase transition sequence of BABH(11) and BABH(12), Cr-Cub-I at atmospheric pressure, changes to Cr-Cub-SmC-I under intermediate pressures and would change to Cr-SmC-I under elevated pressure.     

Non-symmetric chiral isoflavone dimers: synthesis,characterisation and mesomorphic behaviour

A new series of non-symmetric chiral isoflavone-based liquid crystalline dimers, α-(2-methylbutyl-4′-(4″-phenyloxy)benzoate)-ω-(3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy)alkanes, with 3–12 carbon atoms in the alkyloxy spacer, have been synthesised. A pronounced odd–even effect for the phase transition temperatures upon varying the spacer length was observed. The short dimers exhibited monolayer smectic A (SmA) and smectic C (SmC*) phases while for longer homologues a chiral nematic (N*) phase was found. The temperature range of the nematic phase was broadened with elongation of the alkyl spacer. Stabilisation of the nematic phase resulted from competition between the monolayer and intercalated smectic structures. The SmA–SmC* phase transition was second order for all studied compounds with a cross over to the de Vries type behaviour for the shortest homologue.     

Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

The mesophase behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented. The nematogenic 4,6-dichloro-1,3-phenylene bis[4′-(10-undecen-1-yloxy)-1,1′-biphenyl-4-carboxylate] (I) was the banana-shaped component. As the calamitic compound ethyl 4′-(9-decen-1-yloxy)-1,1′-biphenyl-4-carboxylate (II), similar to one arm of the bent-core molecule, was used which exhibits smectic phases in a wide temperature range. A total of six mixtures with different compositions were prepared and studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction on non-oriented samples. In the mixtures, a nematic phase is not concomitant with smectic A phase, and the temperature range of both phases highly depends on the concentration of the comprising compounds. Lowered melting temperatures have been observed for all mixtures with respect to that of the pure compounds. Unforeseen finding is the induction of a monotropic SmC phase in mixtures with lowest concentration of the bent-core compound. Semi-empirical quantum-chemical calculations have also been performed. Based on the calculated molecular conformation, as well as on collected X-ray diffraction data, a model for a possible self-assembly of the banana-shaped and calamitic compounds is proposed.     

Synthesis and mesomorphic properties of some esters of trans-4-n-alkoxycinnamic and trans-4-n-alkoxy-alpha-methylcinnamic acids exhibiting ferroelectric and antiferroelectric phases

The synthesis and mesomorphic properties of several homologues of (S)-(+)-1-methylheptyl 4-[4(trans-4-n-alkoxycinnamoyloxy)benzoyloxy]benzoates (series I) and (S)-(+)-1-methylheptyl 4-[4-(trans-4-n-alkoxy-alpha-methylcinnamoyloxy)benzoyloxy]benzoates (series II) are described. The influence of the alpha -methyl group on the different mesophases and the clearing temperatures has been evaluated. Many homologues of both the series show a ferroelectric chiral smectic C (SmC*) phase while the antiferroelectric smectic C phase (SmC*A) is exhibited by compounds of series I. The mesophases have been identified by optical polarizing microscopy, differential scanning calorimetry and miscibility studies. The spontaneous polarization, the helical pitch and the tilt angle of two of the compounds have also been measured.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4-n-octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr₂)-high temperature crystal (Cr ₁)- cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30–40 MPa, in conjunction with the disappearance of the Cr₁ phase. The transition sequence changes to Cr₂-SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (dT/dP) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0±1.0°C for the SmC, Cub and Cr₁ phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Nematogenic and smectogenic liquid crystals from new heterocyclic isoflavone derivatives:Synthesis,characterization and X-ray diffraction studies

A new series of liquid crystals comprising eight heterocyclic isoflavone esters,7-alkanoyloxy-3-[4’-(3-methylbutyloxyphenyl)]- 4H-1-benzopyran-4-ones exhibiting enantiotropic nematic(N) and smectic C(SmC) phases were synthesized and investigated.The mesomorphic properties of all derivatives were investigated by means of differential calorimetry and polarized optical microscopy. Wide angle X-ray diffraction technique was employed to investigate the molecular packing associated with the intermolecular interaction as well as the correlation between the thermal behaviour of these compounds with their anisotropy properties within a mesophase.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

study of liquid crystalline N -[4-(4- n -alkoxybenzoyloxy)-2-hydroxybenzylidene]methylanilines

New homologous series of N -[4-(4- n -alkoxybenzoyloxy)-2-hydroxybenzylidene]methylanilines [ n AH m M( n =1-8/10; m =2: ortho , m =3: meta , m =4: para )] were synthesized. They exhibited a nematic phase except for 1AH3M. The temperature dependence of their Raman spectra was observed in the spectral range of 900-1700 cm -1 . In one group of n AH m M compounds, the Raman band at about 1360 cm -1 abruptly decreased in intensity and wavenumber when the crystalline solid-liquid crystal phase transition was approached. In another group, the corresponding band increased through the phase transition. The bands have been assigned to the coupling mode between the in-plane CCH deformational vibration and the ring-N stretching vibration. Such a behaviour can be explained by the molecular conformation with different twist angles of the aniline ring in relation to the Schiff 's base plane of the molecule. Some n AH m Ms exhibited photochromism.     

Electro-optic response of cubic liquid crystal compounds in Kerr cell geometry

We present the results of our investigations on the electro-optic response of the cubic phase liquid crystal compounds 1,2-bis-[4-n-octyloxy-benzoyl]-hydrazine (BABH8) and 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC16) in Kerr cell geometry. The AC electric field response in the BABH8 cubic phase was found to be as small as that of the isotropic phase, even though there was a response in the adjacent smectic C (SmC) phase. The response in the SmC phase means that the BABH8 molecule itself has an electric field coupling ability, but this ability is strongly inactivated in the cubic phase. This inactivity to the AC fields was also found in the cubic phase of ANBC16. This behaviour could be explained by the small structural unit size of the cubic phase.     

New ferroelectric and antiferroelectric liquid crystalline materials containing differing numbers of lactate units

A new series of chiral liquid crystalline materials having the same rigid core containing a keto group, and various numbers of ( S )-lactate groups in the chiral segment, has been synthesized and studied. All the compounds contain a branched terminal alkyl chain derived from ( S )-2-methylbutanol. The influence of the number of lactate units on mesogenic behaviour has been studied. All the compounds show the paraelectric SmA phase and the ferroelectric SmC * phase over rather wide temperature ranges. The antiferroelectric SmC * A phase was observed for the compound possessing two lactate groups.     

Synthesis and characterization of a chiral antiferroelectric series with new SmC A*-L transition

Two new chiral series with a tolane core: (S) 4-(1-methylheptyloxycarbonyl)phenyl (series IB) and (S) 4-(1-ethylheptyloxycarbonyl)phenyl (series IC) 4-alkanoyloxytolane-4'-carboxylates have been synthesized and characterized. All the compounds are mesomorphic, and most of them display the antiferroelectric SmC*A phase. The mesomorphic properties have been analysed by optical microscopy, DSC, helical pitch and electro-optical measurements and X-ray diffraction study. Series IB exhibits a very rich polymesomorphism with SmC*alpha, SmC*, SmC* F12, SmC* F11 and SmC*A phases, like the previously reported series IA, [(S) 4-(1-methylheptyloxycarbonyl)phenyl 4-alkyloxytolane-4-carboxylates]. Series IC is more interesting because it presents for the first time in antiferroelectric series the liquid-like phase (L phase) between the SmC*A and isotropic phases. Preliminary optical studies plead in favour of a helical short range structure for this new liquid-like L phase.     

Synthesis and characterization of a chiral antiferroelectric series with new SmC A*-L transition

Two new chiral series with a tolane core: (S) 4-(1-methylheptyloxycarbonyl)phenyl (series IB) and (S) 4-(1-ethylheptyloxycarbonyl)phenyl (series IC) 4-alkanoyloxytolane-4'-carboxylates have been synthesized and characterized. All the compounds are mesomorphic, and most of them display the antiferroelectric SmC*A phase. The mesomorphic properties have been analysed by optical microscopy, DSC, helical pitch and electro-optical measurements and X-ray diffraction study. Series IB exhibits a very rich polymesomorphism with SmC*alpha, SmC*, SmC* F12, SmC* F11 and SmC*A phases, like the previously reported series IA, [(S) 4-(1-methylheptyloxycarbonyl)phenyl 4-alkyloxytolane-4-carboxylates]. Series IC is more interesting because it presents for the first time in antiferroelectric series the liquid-like phase (L phase) between the SmC*A and isotropic phases. Preliminary optical studies plead in favour of a helical short range structure for this new liquid-like L phase.