Christopher J. Booth

Chirality and Frustration in Ordered Fluids

Abstract Chirality in liquid crystal systems is a complex and sometimes difficult concept to understand and appreciate. In this article we review some of the reduced symmetry aspects of calamitic liquid crystal phases with reference to point symmetry, space symmetry, and helicity. We utilize these concepts in the discussion of new results obtained on Twist Grain Boundary phases, inversions in chiral dependent properties such as the helicity in cholesteric and smectic C* phases and the spontaneous polarization in ferroelectric liquid crystals, and antiferroelectric behaviour in low molar mass systems. Our observations suggest that many novel effects found in chiral liquid crystals are a result of conflicts between the effects of reduced symmetry and the desire to form normal liquid-crystalline structures, i.e., a form of frustration exists.

Chirality in liquid crystals. The remarkable phenylpropiolates

Abstract The biphenylyl esters of the 4-n-alkoxyphenylpropiolic acids are a unique family of liquid-crystalline materials. In particular, when the biphenyl moiety of the compounds carries a chiral end-group, many optically active mesophases are created which exhibit unusual structures and physical properties. For instance, when the chiral group attached to the biphenyl moiety is 1-methylheptyl then Abrikosov, twist grain boundary smectic A* and antiferroelectric smectic C* phases are observed. The wide variety of chiral phases and electrochiral properties exhibited by this family of materials makes them ideal candidates for exploring chirality in the liquid-crystalline state. These investigations allow us to contrast and compare chirality dependent phenomena in liquid crystals, thereby producing a broader view of the concept of chirality in organized fluids than is traditionally presented.

Achiral swallow-tailed materials with ‘antiferroelectric-like’ structure and their potential use in antiferroelectric mixtures

Abstract Achiral ‘swallow-tailed’ liquid crystalline materials are known to give alternating-tilt smectic C phases (SCalt) which have structural similarities to the chiral antiferroelectric phases denoted as S∗CA. This paper describes the synthesis and characterization of three achiral branchedalkyl 4-(4′-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates. Optical microscopy and differential scanning calorimetry confirm that these materials show SCalt and overlying SA phases. The compounds were investigated as potential hosts which could be doped with a chiral ferroelectric liquid crystal so as to provide a viable- antiferroelectric mixture. These studies (microscopy and differential scanning calorimetry), to characterize the properties of the mixtures, show that antiferroelectric phases are induced. However, switching studies show that the antiferroelectric phases are extremely stable, a property which is almost certainly a consequence of the length of the lateral branching groups (ethyl, propyl and butyl).

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

Abstract 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∗C γ) 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.

The Synthesis and Transition Temperatures of Novel Low Molar Mass Cholesteric Materials Derived from (R—(4-Hydroxyphenoxy)propanoic Acid

Abstract A comprehensive series of (R—(4-substituted-phenoxy)propanoates and (R—(4-substituted-phenoxy)propanonitriles have been prepared. A wide variety of 4-substituents and ester functions have been examined to determine how the position of the chiral centre affects the cholesteric phase formation in these classes of materials; the synthesis of these novel materials, their transition temperatures and a procedure for assessing their optical purity are described and discussed. Mesogenicity is significantly depressed if the chiral centre is placed centrally within the molecule.

Effect of the position of lateral fluoro substituents on the phase behaviour and ferroelectric properties of chiral 1-methylheptyl 4′-[(2- or 3-fluoro-4-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-carboxylates

The syntheses of four chiral laterally fluoro-substituted propiolate esters are described, along with transition temperatures, ferroelectric properties, phase diagrams and related data. The position of the fluoro-substituent was found to influence dramatically the formation of twist grain boundary (TGB A* and TGB C*) phases as well as the magnitudes of spontaneous polarization and optical tilt angle in the ferroelectric smectic C* mesophases. Differential scanning calorimetric studies revealed the presence of a diffuse liquid-liquid transition above the clearing point in both of the 3-fluoro enantiomers, but not in the racemate. Circular dichroism and optical rotation measurements, carried out over a temperature range in which the diffuse peak occurs, appear to confirm the presence of a degree of chiral organization within the isotropic liquid. It is suggested that this phenomenon may be due to the presence of cybotactic groups or a network of entangled screw dislocations occurring close to the clearing point.

Highly twisting enantiomeric radial multiyne dopants for discotic liquid-crystalline systems

The synthesis and dopant properties of both the (R,R,R,R,R)- and (S,S,S,S,S)-hexadecyloxy{pentakis [4-(1-methylheptyloxy)-phenylethynyl]} benzene enantiomers are described. Although the materials were found not to be liquid-crystalline, it was found possible to induce left [for the (R,R,R,R,R)-enantiomer] and right handed [for the (S,S,S,S,S)-enantiomer] chiral discotic nematic phases (ND*) as well as being only the second instance of the observation of discotic blue phases (BPDIII and BPDII) in binary mixtures with the hexayne host, hexakis(4-nonylphenylethynyl)benzene; a phase diagram is presented. The helical pitch and helical twisting power of the dopants was measured by use of the modified Cano method, and revealed far higher twisting powers than previously reported pentaynes. In addition the temperature dependent and angular dependent selective reflection spectra are reported, along with average refractive index approximations, which enable the pitch length of the chiral discotic nematic phase (ND*) to be estimated to be of the order of 0.36 to 0.35 µm.

The influence of the liquid crystalline core geometry on the mesogenicity of novel chiral 2-(4-substituted-phenoxy)propanonitriles

Abstract The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense.

The Influence of Lateral Fluoro-Substituents on TGB Phases in Chiral Propiolates

Abstract The transition temperatures and associated enthalpies of chiral and racemic 1-methylheptyl 4′-[(2- or 3-fluoro-4-alkoxyphenyl)propioloyl-oxy]biphenyl-4-carboxylates are reported along with phase diagrams for binary mixtures of (R)- and (S)-isomers. The position of the fluoro substituent was found to influence the formation of twist grain boundary phases both in the enantiomers and in binary mixtures of enantiomers. DSC studies revealed that the enantiomeric 3-fluoro compounds possessed a diffuse liquid-liquid transition above the clearing point, which did not arise for the racemates or for the 2-fluoro analogues. Circular dichroism and optical rotation studies carried out over the temperature range of the diffuse liquid phase, confirm that a form of cybotactic, chiral organisation in the isotropic liquid is responsible for this phenomenon.

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

Abstract 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 ...