This critical review begins with a brief, but essential, introduction to the special nature of liquid crystal materials, their peculiar properties, and their commercial applications, followed by an introductory insight into the remarkable nature of the fluoro substituent, and its fascinating influence on the properties of organic compounds. However, the main focus of the review is to discuss the enormous amount of exciting research on fluorinated liquid crystals that has been reported. The small size of the fluoro substituent enables its incorporation into all types of liquid crystal, including calamitic, discotic, banana, lyotropic, and polymers, without ruining the liquid crystalline nature of the material. However the fluoro substituent is larger than hydrogen, and hence causes a significant steric effect, which combined with the high polarity, confers many fascinating, and often remarkable, modifications to melting point, mesophase morphology and transition temperatures, and the many other very important physical properties, such as dielectric anisotropy, optical anisotropy, and visco-elastic properties. There are many different positions within a liquid crystal structure where a fluoro substituent can be located, including (i) a terminal position, (ii) within a terminal chain, as a semi-fluorinated or as a perfluorinated chain, or as one fluoro substituent at a chiral centre, (iii) as part of a linking group, and (iv) a lateral position in the core section. Such variety enables the interesting and advantageous tailoring of properties, both for the fundamental purposes of establishing structure–property relationships, and for materials targeted towards commercially-successful liquid crystal display applications.

Fluorinated liquid crystals – properties and applications

An intriguing problem in condensed matter physics is understanding the glass transition, in particular the dynamics in the equilibrium liquid close to vitrification Recent advances have been made by using hydrostatic pressure as an experimental variable These results are reviewed, with an emphasis in the insight provided into the mechanisms underlying the relaxation properties of glass-forming liquids and polymers

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R03/pdf

Supercooled dynamics of glass-forming liquids and polymers under hydrostatic pressure

Use of Lawesson's reagent in organic syntheses.

An overview over the current state of research in the field of biaxial nematic liquid crystalline materials is given. After a short introduction, providing some general aspects and summarizing the classical approaches, the main part focuses on recent developments of new concepts for designing biaxial nematics. First, the major theoretical concepts are outlined, including the classification to different symmetries, the importance of cooperativity and cluster formation for the development of biaxial order and the conditions for the establishment of field induced and spontaneous biaxiality in nematic phases. These new concepts also require the re-evaluation of the tools used for the identification of phase biaxiality, which are discussed briefly. In the second part, recent progress in the design of potential biaxial nematic materials, especially focussing on bent-core molecules with nematic phases, is reported and, finally, comparisons with phase biaxiality as observed in smectic liquid crystals are made.

Biaxial nematic phases

Polymer as an important component of blends and composites with liquid crystals

Methods for preparing optically active hydroxyesters: 1-hydroxy-4-(1-methylheptyloxycarbonyl)benzene and 4-hydroxy-4′-(1-methylheptyloxy-carbonyl)biphenyl, 4-(1-methylheptyloxycarbonyl)phenyl 4′-hydroxybiphenyl-4-carboxylate and 4′-(1-methylheptyloxy carbonyl)biphenyl-4-yl 4-hydroxybenzoate have been elaborated. These compounds were used as intermediates for preparing liquid crystalline homologous series of di- and triesters exhibiting tilted smectic phases CA* and C*. The phase transitions have been measured by DSC and thermomicroscopic methods and the smectic phases have been identified by microscopic textures and miscibility studies.

New Antiferroelectric Compounds Containing Partially Fluorinated Terminal Chains. Synthesis and Mesomorphic Properties

Results of the dielectric studies of nine members of the nBT (4-n-alkyl-4'-thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented. The dependence of the static permittivity ∊s in the isotropic phase, the longitudinal relaxation times τis and τ∥, and the activation enthalpies ΔH is and ΔH ∥ on the number of carbon atoms in the alkyl chain n are analysed. A considerable increase in the retardation factor g= τ∥/τis with decreasing n is observed. The results are compared with those obtained for similar two-ring homologous series. The parameters characterizing the molecular rotations around the short axis in the E phase (τ∥ and ΔH ∥) indicate a hardening of this solid-like phase with shortening of the alkyl chain.

Dielectric studies of the 4-n-alkyl-4'-thiocyanatobiphenyl (nBT) homologous series (n=2-10) in the isotropic and E phases

The longitudinal relaxation time τ of a series of alkyl-isothiocyanato-biphenyls (nBT) liquid crystals in the smectic E phase was measured as a function of temperature T and pressure P using dielectric spectroscopy. This relaxation time was found to become essentially constant, independent of T and P, at both the clearing point and the lower temperature crystalline transition. τ(T,P) could also be superposed as a function of the product TVγ, where V is the specific volume and γ is a material constant. It then follows from the invariance of the relaxation time at the transition that the exponent γ superposing τ(T,V) can be identified with the thermodynamic ratio Γ=−∂log(Tc)∕∂log(Vc), where the subscript c denotes the value at the phase transition. Analysis of literature data on other liquid crystals shows that they likewise exhibit a constant τ at their phase transitions. Thus, there is a surprising relationship between the thermodynamic conditions defining the stability limits of a liquid crystalline phas...

Thermodynamic scaling and the characteristic relaxation time at the phase transition of liquid crystals.

The mesogenic properties of three structurally similar compounds containing 1,10-dicarbadecaborane, 1,12-dicarbadodecaborane (carboranes), or bicyclo[2.2.2]octane are investigated in pure states and binary mixtures. The carborane-containing compounds (BC10 and BC12) exhibit nematic and monotropic smectic C transitions at lower temperatures than the analogous bicyclo[2.2.2]octane compound (BO1). The thermal stabilities of the smectic phases for BO1 are generally reduced by addition of the carborane analogues except for the smectic C phase, the stability of which is enhanced for equimolar concentrations. The effects exerted by the carborane mesogens on mesophase stability are consistent with their relative molecular sizes.

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Effects of Carborane-Containing Liquid Crystals on the Stability of Smectic Phases

Orthoconic Antiferroelectric Liq. Cryst. (OAFLC) have been extensively studied due to their unique electrooptical properties [1,2]. Practical implementation of this kind of liquid crystalline medium experiences many obstacles including a number of parasitic effects deteriorating the electrooptical performance. In search for working OAFLC a mixture with the helical pitch longer than the typical cell gap was formulated. Using this mixture near perfect optical uniformity and excellent dark state at the zero electric field applied was obtained. The physical and structural properties of mixture are presented and their parameters are discussed. The electrooptical performance is presented and discussed.

Krzysztof Czupryński

Papers

New Antiferroelectric Compounds Containing Partially Fluorinated Terminal Chains. Synthesis and Mesomorphic Properties

Dielectric studies of the 4-n-alkyl-4'-thiocyanatobiphenyl (nBT) homologous series (n=2-10) in the isotropic and E phases

Thermodynamic scaling and the characteristic relaxation time at the phase transition of liquid crystals.

Effects of Carborane-Containing Liquid Crystals on the Stability of Smectic Phases

Long Pitch Orthoconic Antiferroelectric Binary Mixture for Display Applications