A homologous series of U‐shaped dimeric liquid crystals in which two mesogenic groups are connected via catechol has been prepared and the effects of terminal alkyl chains, alkyl spacers and core structures on the transition properties investigated by means of optical microscopy and differential scanning calorimetry. The phase sequence exhibits a pronounced odd–even effect as the length and parity of the spacers is varied, in which the even members favour the nematic and smectic C phases, whereas the odd members favour the nematic and smectic A phases. We discuss the transition behaviour of the U‐shaped compounds in terms of molecular shape.
We have prepared novel symmetric and non-symmetric dimeric liquid crystals possessing a phenylpyrimidine moiety and investigated the transition behaviour. We discuss effect of the spacer length and the core structure on the transition behaviour. The present system is thought to be a model to investigate molecular packing in SmA and SmC phases of 2-ring phenylpyrimidine liquid crystals. Results of binary mixture systems between the dimeric materials indicate that intermolecular interaction organized by antiparallel packing of the phenylpyrimidine group increases the stability of the SmC phase.
The structure and dynamics in an optically isotropic liquid crystalline phase exhibited in a dichiral compound with fluorines have been investigated using X-ray, 19F-NMR and dielectric measurements. Results in the dielectric measurement and 19F-NMR measurements clearly show that molecular motion in this phase is just like in the isotropic phase as observed by texture. However, several X-ray diffraction peaks observed in a small angle range indicate akind of cubic phase (Cub). Based on the X-ray diffraction patterns together with the existence of circular dichroism, the phase is assigned to a chiral cubic structure belonging to crystallographic spacegroupI432. Moreover, the exotic phase with blue colored and foggy texture was found in the lower temperature range of isotropic liquid. X-ray diffraction in this phase clearly shows a diffraction ring corresponding to a layer structure, indicating that this blue-colored phase is assignable to a smectic blue phase (BPsm). It was also found that the layer correlation length is remarkably long compared with the previously reported BPsm phases. We want to emphasize that the present material is quite unique in the sense that both cubic phases, BPsm and Cub, appear. The influence of cell thickness and incubation time on the emergence of the smectic C* phase is also discussed based on dielectric measurements and AC calorimetry.
We prepared an asymmetric liquid crystal dimer possessing a terminal hydroxyl group, 2-{4-{7-[4-(4-cyanophenyl)phenyloxy]heptyloxy}phenyl}-5-(6-hydroxyhexyloxy)pyrimidine, and investigated the phase transition behaviour using polarising optical microscopy and differential scanning calorimetry. The compound exhibited an enantiotropic nematic phase. On cooling, two distinct domains were formed during the nematic-to-crystal phase transition. During heating, one domain melted at 126.4°C to the N phase, whereas the other domain changed to the N phase at 144.6°C. Therefore, the crystal and nematic phases coexisted at a temperature of 18.2 K. The coexistence behaviour characteristics depend on the cooling rate. We discuss the manner in which intermolecular interactions affect the phase transition behaviour.
Abstract The effects of a second branched alkyl chain, lateral substitution, and double chiral centres on the phase transition and spontaneous polarization of the ferroelectric liquid crystal having a 2-methylalkanoyl group have been investigated. The introduction of another branched alkyl chain away from the 2-methylalkanoyl group causes a sharp SC∗-SA transition peak and also enhances the ferroelectric properties in the SC∗ phase. Since the order within the layers is liquid-like in the SC∗ phase, the alkyl chain branching away from both the chiral centre and a polar group affects the overall motion of the molecule in the SC∗ phase. In the system of a compound with double chiral centres, the existence of the chiral centre in the 2-methylalkanoyl group affects the phase transition temperatures and the magnitude of the spontaneous polarization in the SC∗ phase. On the other hand, the existence of the chiral centre in the 2-methylbutyl group only affects the stability of a more highly ordered smectic phase appearing below the SC∗ phase.
We have designed novel liquid crystal oligomers in which three mesogenic units are connected via 3,5‐dihydroxybenzoic acid. 4‐Cyanobiphenyl‐4′‐yl 3,5‐bis{6‐[4‐(5‐octylpyrimidin‐2‐yl)phenyloxy]hexyloxy}benzoate and 4‐(5‐octylpyrimidine‐2‐yl)phenyl 3,5‐bis{6‐[4‐(5‐octylpyrimidine‐2‐yl)phenyloxy]hexyloxy}benzoate have been prepared, and their physical properties investigated by optical microscopy and differential scanning calorimetry. The Y‐shaped liquid crystal oligomers were found to show a direct phase transition from isotropic liquid to anticlinic SmC (SmCanti) phase.
We reported that a dichiral liquid crystal 2-{4-[(R)-2-fluorohexyloxy]phenyl}-5-4-[(S)-2-fluoro-2-methyldecanoy]phenyl}pyrimidine (1) upon cooling exhibits an endothermic transition from a chiral smectic C phase to an optically isotropic phase (IsoX). We have investigated transition behaviour of a binary mixture between 1 and the corresponding monochiral liquid crystal without the IsoX phase, 2-[4-[hexyloxy]phenyl}-5-4-{[(S)-2-fluoro-2-methylnonanoyl]phenyl}pyrimidine (2). In the phase diagram clear hysteresis was observed in appearance of the IsoX phase between heating and cooling.
We prepared a homologous series of novel liquid crystal trimers, 1,4‐bis(4‐{ω‐[4‐(5‐dodecylpyrimidine‐2‐yl)phenyloxy]alkyloxyphenyl}carbonyloxy)benzene, I‐(m, n), and investigated their physical properties using optical microscopy, differential scanning calorimetry and X‐ray diffraction. All the compounds exhibited enantiotropic nematic (N) and smectic C (SmC) phases. Pronounced odd–even effects were apparent for the transition properties on varying the spacer length. X‐ray diffraction measurements suggest that the SmC phase has a monolayer structure in which the molecular long axis is tilted to the layer normal. Furthermore, formation of defect‐lines was observed for the SmC phases.
A novel amphiphilic compound (1) comprising a semi-perfluorinated alkyl chain and a 2,3-difluoro-1,4-diphenylbenzene core has been prepared and its physical properties investigated using optical microscopy, differential scanning calorimetry and X-ray diffraction. The compound exhibited SmA and SmC phases. The maximum layer contraction in the SmC phase was found to be 2.0% at T – T AC = –40 K, and the optical tilt angle was 21.8°. The electro-optical properties of a mixture of the compound with a ferroelectric liquid crystal were investigated, and it was found that the chiral mixture exhibited fast electro-optical switching, with a response time of 10 μs in the chiral SmA phase, in spite of the absence of the SmC* phase.
We synthesized novel liquid crystal tetramers possessing cyanobiphenyl and phenylpyrimidine moieties, and investigated their phase transition behavior. The liquid crystal tetramers exhibited nematic (N) and smectic A (SmA) phases. X-ray diffraction measurements suggest that the SmA phase has an intercalated structure. We discuss the transition behavior of the newly designed tetramers in terms of their molecular shape.
