Heat capacities and phase transitions of the antiferroelectric liquid crystals MHPOBC and MHPOCBC

Heat capacities of the antiferroelectric liquid crystals 4-(1-methylheptyloxycarbonyl)phenyl4-octyloxybiphenyl-4-carboxylate (MHPOBC) and 4-(1-methylheptyloxycarbonyl)phenyl4-octylcarboxybiphenyl-4-carboxylate (MHPOCBC), have been measured with an adiabatic calorimeter between 350 and 460 K. MHPOBC showed three smectic subphases (ferrielectric C*, ferroelectric C* and a fancy phase C*) between antiferroelectric smectic C* and paraelectric gamma alpha smectic A, while MHPOCBC exhibited only one subphase (smectic C*). These phases are clearly discriminated by the existence of phase transitions. The enthalpies and entropies gained at the respective phase transitions were very small. A much larger phase transition from smectic A to isotropic liquid was also observed in both compounds. A alpha     

Phase effect on formation of intermediates in the pulse radiolysis of 4-cyano-4'- octylbiphenyl liquid crystals

It is shown that the reaction intermediates were formed by pulse radiolysis of 4-cyano-4'- octylbiphenyl liquid crystals. The remarkable phase effect was observed in the pulse radiolysis of the liquid crystals. The optical absorption spectrum obtained in the smectic A phase of 4-cyano-4'-octylbiphenyl had no absorption maximum in the visible region, but had an absorption increasing toward the shorter wavelengths below 400 nm, while a broad absorption band with a maximum around 550 nm was observed in the nematic phase. In the isotropic liquid phase an absorption band with a maximum at 480 nm was observed, but it was observed neither in the smectic A phase nor in the nematic phase. The absorption below 400 nm was observed in the liquid phase as well as in the smectic A phase. The assignment of the absorptions observed for each phase is discussed. Then it is suggested that the structural properties could be related to the formation and behavior of reaction intermediates.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4'-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral-racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic-smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Synthesis and mesomorphic properties of 4-(1,1,7-trihydroperfluoroheptyloxycarbonyl)phenyl 4'-n-alkoxybiphenyl-4-carboxylate

A family of 4-(1,1,7-trihydroperfluoroheptyloxycarbonyl)phenyl 4'-n-alkoxybiphenyl-4-car- boxylates has been synthesized. The compounds show the smectic A phase while the mesogens with intermediate terminal alkyl chain ( n 7-12) also exhibit a broad smectic C phase.     

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.     

Analysis of phase transitions by photothermal probe beam deflection

A recent photothermal technique has been used to study the smectic A-nematic and nematic-isotropic phase transitions in 4-n-octyl-4prime;-cyanobiphenyl and in binary mixtures of 4-n-octyl- and 4-n-octyloxy-4prime;-cyanobiphenyl. A numerical analysis shows that both amplitude and phase of the photothermal signal are sensitive to the change of certain thermal parameters for the sample.     

Blue phases and twist grain boundary phases (TGBA and TGBC) in a series of fluoro-substituted chiral tolane derivatives

A series of fluoro-substituted tolane derivatives: ( R )-1-methylheptyl 3-fluoro-4-(3-fluoro-4- n alkoxybenzoyloxy)tolane-4-carboxylates is reported. Some members of this series exhibit the phase sequence: Cr-SmC*-TGBC-TGBA-BPI-BPII-BPIII-I. The blue phases, the TGBA and TGBC phases and the SmC* phase were characterized in detail by microscopic observation, differential scanning calorimetry, helical pitch measurements, X-ray structural analysis and electro-optical study. The blue phases directly next to the TGBA phase were shown to be a new type of blue phase exhibiting smectic ordering. A commensurate TGBC phase with constant number of slabs per pitch was observed.     

Mesogens containing p-polyfluoro-m-nitrobenzoate and 2,3-difluorotolane groups

A series of 4-(4-n-heptoxy-2,3-difluorophenylethynyl)phenyl 3-nitro-4-polyfluoroalkoxybenzoate liquid crystals was synthesized and their phase transition behaviours were studied by optical polarizing microscopy and DSC. The results show that the mesogens containing long polyfluoroalkoxy chain tend to exhibit the smectic A phase.     

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.     

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.     

Induction of smectic mesomorphism by mixing a non-liquid-crystalline methacrylate with a nematic liquid-crystalline compound having a cyano-containing mesogen

The structure and phase diagram of a binary mixture of a non-liquid-crystalline, mono-functional (meth)acrylate monomer and a liquid-crystalline compound having a cyano group-containing mesogen were examined. The monomers had mesogenic units such as biphenylene and phenyl benzoate, but did not show liquid-crystallinity. The liquid-crystalline compounds possessed a cyanobiphenyl or a cyanophenyl benzoate mesogenic unit. The liquid-crystalline 4-cyanophenyl 4'-n-octyloxybenzoate did not show a smectic A phase, while the binary mixture of the 4-butoxy-4'-(ω-methacryloyloxyhexyloxy)biphenyl monomer with 4-cyanophenyl 4'-n-octyloxybenzoate showed the induction of a smectic A phase in a nearly equimolar composition range. However, the binary mixture of the monomer, containing a phenyl benzoate group, and the liquid-crystalline compound did not induce a smectic 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.     

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.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

Abstract

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

The synthesis of 1-biphenyl-4-alkyl-[1,2,3]-triazoles and their mesomorphic behaviour

A series of 1-biphenyl-4-alkyl-[1,2,3]-triazole-based liquid crystal compounds were synthesised via a fast and efficient microwave mediated ‘Click Reaction’ and their phase behaviour is presented. Most of the 1-(3-fluorobiphenyl)-4-alkyl-[1,2,3]-triazole compounds exhibited a smectic A phase with relatively low transition temperatures compared to previously examined aryl-triazole-based mesogens. Small angle X-ray scattering measurement indicated the existence of deep smectic phase for some cases. Structural variations gave rise to different phase behaviour in these triazole-based compounds.     

Analysis of phase transitions by photothermal probe beam deflection

A recent photothermal technique has been used to study the smectic A-nematic and nematic-isotropic phase transitions in 4-n-octyl-4prime;-cyanobiphenyl and in binary mixtures of 4-n-octyl- and 4-n-octyloxy-4prime;-cyanobiphenyl. A numerical analysis shows that both amplitude and phase of the photothermal signal are sensitive to the change of certain thermal parameters for the sample.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures V. Mixtures of bimolecular smectics and linear mesogens

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).     

Laterally fluorinated phenyl biphenylcarboxylates; versatile components for ferroelectric smectic C mixtures

Twenty-six fluoro-substituted 4-n-alkoxyphenyl 4'-n-alkyi-and 4'-n-alkoxybiphenyl-4-carboxylates are reported. The number and position of the fluoro groups influences the liquid crystal phase sequence and has a dramatic effect on the liquid crystal transition temperatures. Some compounds have low melting points and a very wide smectic C phase, whereas other compounds exhibit a very wide smectic A phase. Compounds containing a 2,3-difluorophenyl group provide useful materials which have a large negative dielectric anisotropy.     

Morphological studies of a hydrogen-bonded LC polymer obtained by photopolymerization in LC solvents

Anisotropic morphologies and the phase behaviour of a hydrogen-bonded LC polymer obtained by photopolymerization in two kinds of LC solvent are discussed. The hydrogen-bonded LC monomer, 4-(6-acryloyloxyhexyloxy) benzoic acid (A6OBA), was photopolymerized in 4-cyano-4′-hexyloxybiphenyl (6OCB) and in 4-cyano-4′-undecyloxybiphenyl (11OCB), which show a nematic phase and a smectic A phase, respectively. After photo-polymerization, the LC media were removed by extraction and the pure polymer was observed by scanning electron microscopy. SEM images showed that the polymer possessed fibrous morphology with a fibre diameter of a few micrometers, based on polymerization-induced phase separation. The overall geometries reflected typical LC characteristics such as schlieren and focal-conic fan textures. It was found that the hydrogen bond between benzoic acid groups in the monomer was rigid enough to fix the anisotropic phase-separated structure forming during the early stage of phase separation; however, it could not permanently maintain the fibre structure due to dissociation at elevated temperature. X-ray measurements revealed that a well developed layer structure of the hydrogen-bonded mesogen existed in the polymer obtained from the smectic phase of 11OCB, whereas a polymer layer structure could develop only partially from the nematic phase of 6OCB.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures V. Mixtures of bimolecular smectics and linear mesogens

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).