Isa Nishiyama

Optical isotropy and iridescence in a smectic ‘blue phase’

When liquid crystal molecules are chiral, the twisted structure competes with spatially uniform liquid crystalline orders, resulting in a variety of modulated liquid crystal phases, such as the cholesteric blue phase, twist grain boundary and smectic blue phases. Here we report a liquid crystal smectic blue phase (SmBPiso), formed from a two-component mixture containing a chiral monomer and a ‘twin’ containing two repeat units of the first molecule connected by a linear hydrocarbon spacer. The phase exhibits the simultaneous presence of finite local-order parameters of helices and smectic layers, without any discontinuity on a mesoscopic length scale. The anomalous softening of elasticity due to a strong reduction in entropy caused by mixing the monomer and the twin permits the seamless coexistence of these two competing liquid crystal orders. The new phase spontaneously exhibits an optically isotropic but uniformly iridescent colour and automatically acquires spherical symmetry, so that the associated photonic band gap maintains the same symmetry despite the local liquid crystalline order. We expect a range of unusual optical transmission properties based on this three-dimensional isotropic structure, and complete tunability due to the intrinsic softness and responsiveness of the liquid crystalline order against external fields.

New Ferroelectric Liquid Crystals Having 2-Fluoro-2-Methyl Alkanoyloxy Group

Abstract We have prepared ferroelectric liquid crystals with newly designed chiral part, 2-fluoro-2-methyl alkanoyloxy group. The physical properties of the ferroelectric liquid crystals having this chiral part are compared with those of other ferroelectric liquid crystals having 2-fluoro alkanoyloxy group, 2-trifluoromethyl alkanoyloxy group, 2-fluoro-2-trifluoromethyl alkanoyloxy group and 2-methyl alkanoyloxy group. The ferroelectric liquid crystals having 2-fluoro-2-methyl alkanoyloxy group show large spontaneous polarization, compared with other fluorinated ferroelectric liquid crystals. We have investigated the effect of the core structures on the spontaneous polarization. Introduction of a pyrimidine ring in the core part extremely enhances spontaneous polarization, over 400 nC/cm2. Moreover, we have also investigated the effect of chiral dopants of the compound having this chiral part.

Twist inversion in a cholesteric material containing a single chiral centre

A novel liquid-crystalline material, S-2-chloropropyl 4′-(4″-n-nonyloxyphenylpropioloyloxy)biphenyl-4-carboxylate (902C13T), which undergoes an inversion of helical-twist sense in the cholesteric phase, was prepared and its physical properties were investigated. The behaviour of the pitch and the twist sense of the helical structure in the cholesteric phase are reported. It is suggested that the helix inverts because of changes in the populations of conformational isomers, and that these changes are temperature dependent. An equation that was originally formulated to account for an inversion in the sign of the spontaneous polarization (Ps) in certain smectic C* materials may be adapted for the modelling of helix inversions.

Antiferroelectric Chiral Smectic C Materials Having a 2-Alkyl Alkanoyl Group in the Chiral Part

Novel antiferroelectric chiral smectic C (SmCA*) materials are presented. Four homologous compounds having a 2-alkyl alkanoyl group in the chiral part are investigated. Three of them have SmCA* and show very short helical pitch in SmC*. In contrast, the other one has no SmCA* and exhibits rather long pitch in SmC*. This result suggests that the length of the pitch relates closely to occurrence of SmCA*. The introduction of an ethyl group into the chiral carbon instead of a methyl group increases the stability of SmCA* in contrast with decreasing that of SmA and SmC*. All four compounds show the third state even in SmC* by application of an AC field. Microscope observation indicates that the third state in SmC* is different from that in SmCA*.

Ferrielectric and antiferroelectric chiral twin liquid crystals showing a stable chiral nematic phase

A homologous series of chiral twin liquid crystals possessing identical chiral moieties at both peripheral ends, i.e. optically active α,ω -bis{4-[(4′-(1-methylheptyloxycarbonyl)-4-biphenylyl)oxycarbony]phenoxy}alkanes, has been prepared and their liquid crystalline properties investigated. The homologues preferred to show the ferrielectric and/or antiferroelectric phase rather than the ferroelectric phase. With ascending central spacer length, the temperature range of the ferrielectric phase became narrow and eventually disappeared for the dodecyl homologue, suggesting that the coupling in motion and/or direction between two mesogenic parts of each twin molecule has an important effect on the stabilization of the ferrielectric phase. The octyl and dodecyl homologues showed a wide temperature range chiral nematic phase (ca. 10°C or more), so that these compounds were found to be the first examples showing antiferroelectric and/or ferrielectric phases with a broad temperature range of the chiral nematic phase. X-ray diffraction studies showed that the homologues tend to form a mono-layered structure. The formation of a relatively well defined layered structure was also indicated, which is considered to be important for generating anticlinic ordering in the antiferroelectric and ferrielectric phases. An isotropic-isotropic transition characterized by the emergence of a broad diffuse DSC peak was observed for the even-membered homologues.

Unusual smectic phases organized by novel λ-shaped mesogenic molecules

A homologous series of novel λ-shaped mesogenic compounds in which three mesogenic groups are connected to 3, 4-dihydroxybenzoic acid has been prepared and their physical properties investigated by means of optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. The λ-shaped compounds studied showed unusual smectic phases. 4-Cyanobiphenyl-4′-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}benzoate was found to realize an incommensurate SmA (SmAinc) phase. A “uniform planarly” aligned sample of 2-(4-octylphenyl)pyrimidin-5-yl 3,4-bis{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy} benzoate showed a Schlieren-like texture in the smectic phase. We discuss structural effects of the λ-shaped compounds on molecular organization in liquid-crystalline phases.

A symmetric chiral liquid-crystalline twin exhibiting stable ferrielectric and antiferroelectric phases and a chirality-induced isotropic–isotropic liquid transition

In this article we report the first example of a pair of symmetrically substituted chiral twin enantiomers that exhibit stable wide temperature range ferrielectric and antiferroelectric phases, and which have large tilt angles of over 45°. In addition, both of the enantiomers exhibited an isotropic–isotropic liquid transition just above the transition to the liquid crystalline state.

A non-chiral swallow-tailed liquid crystal exhibiting a smectic C phase that has an antiferroelectric structure

Antiferroelectric smectic liquid-crystalline materials have recently been shown to be of use in display devices where they show tristable switching behaviour. Most liquid-crystalline materials that exhibit antiferroelectric properties are, however, highly chiral and therefore have helical macrostructures. Unfortunately, this helical ordering is undesirable in the construction of display devices. In this article we report on the liquid-crystalline properties of a non-chiral material which apparently exhibits a smectic phase that has antiferroelectric-like layer ordering. As the material is non-chiral, it does not possess a helical macrostructure, and therefore could be of value as a suitable host for antiferroelectric mixtures. We suggest that the formation of an antiferroelectric structure is due to the presence of dimeric molecular species. This possibility allows us to speculate on other structures for mesophases that might be found in chiral liquid-crystalline systems.

C-13 NMR and X-Ray Investigations of Phase Transitions in an Antiferroelectric Liquid Crystal

We have investigated molecular motion and ordering in the smectic phases of a nonoriented antiferroelectric liquid crystal by means of C-13 NMR and powder X-ray diffraction. The marked line broadenings for core carbons observed by C-13 NMR indicate the existence of the characteristic molecular motion for the molecular core in the antiferroelectric chiral smectic C (SmC*A) phase. In the smectic phases appearing during the cooling and heating processes, local and overall molecular motions of each molecule observed by C-13 NMR are discussed in comparison with long-range order inside the layer observed by X-ray diffraction.

Synthesis and physical properties of α‐(4‐cyanobiphenyl‐4′‐yloxy)‐ω‐[4‐(5‐alkylpyrimidine‐2‐yl)phenyloxy]alkanes

We prepared a homologous series of dimeric liquid crystals, α‐(4‐cyanobiphenyl‐4′‐yloxy)‐ω‐[4‐(5‐alkylpyrimidine‐2‐yl)phenyloxy]alkanes (mPYnOCB), in which cyanobiphenyl and phenylpyrimidine moieties are connected via a flexible central spacer. We investigated the physical properties of mPYnOCB and of binary mixtures between the dimeric compounds. The interdigitated anticlinic smectic C (SmCanti) phase was found to be induced for some binary mixtures between the dimers with a different odd‐numbered spacer. Furthermore, a striped pattern was observed in the SmCanti phase of a homeotropically aligned sample. The striped pattern was propagated when the temperature of the sample was varied. We discuss the correlation between the structure of the SmCanti phase and the appearance of the striped pattern.