Alexander J. Seed

High Birefringence Isothiocyanato Tolane Liquid Crystals

The phase transition temperatures, birefringence and visco-elastic coefficient of several high birefringence isothiocyanato tolane compounds were evaluated. The polarizability of these compounds was calculated by the Austin Model 1 (AM1) and Modified Neglect of Diatomic Overlap (MNDO) methods. Using these compounds, we have formulated a eutectic mixture exhibiting a wide nematic range and high figure of merit.

Synthesis of self-organizing mesogenic materials containing a sulfur-based five-membered heterocyclic core

The majority of low molar mass calamitic mesogenic systems in the literature contain linear cores based on the 1,4-disubstituted phenyl ring. Heterocyclic cores such as thiophene impart unique physical properties as their slightly bent structure leads to features including a reduced packing ability (generally giving rise to lower melting points than their phenyl counterparts), a medium to strong lateral dipole, high anisotropy of the polarizability, low viscosity etc. This critical review describes the most recent synthetic methodology that has been used to prepare thiophene, 1,3-thiazole, and 1,3,4-thiadiazole-based mesogenic materials.

Synthesis, transition temperatures, and opticalproperties of various 2,6-disubstituted naphthalenes and related 1-benzothiopheneswith butylsulfanyl and cyano or isothiocyanato terminal groups

Fourteen compounds based on 2,6-disubstituted naphthalenes or related 1-benzothiophene moieties with butylsulfanyl and cyano or isothiocyanato terminal groups have been synthesised. The transition temperatures of the compounds and their refractive indices have been determined and the derived values for the optical anisotropies, polarisabilities and order parameters have been calculated. With one exception (which also shows a smectic A phase), all the compounds with naphthyl and phenyl groups are solely nematogenic; for these compounds the naphthyl unit gives an average increase in TN–I value and melting point of 72 and 20°C respectively compared to the values for the compounds with a phenyl in place of the naphthyl unit. The incorporation of a 2,5-thiophene unit in place of phenyl lowers TN–I and when it is part of a 1-benzothiophene unit the depression is even greater. The naphthalene compounds increase the values of optical anisotropy by approximately 0.04, compared to the phenyl systems, mainly because they increase the refractive index of the e-ray (n∥ values); compound 12 has an exceptionally high optical anisotropy (Δn value) of 0.54.

Ring fluorinated thiophenes: applications to liquid crystal synthesis

Abstract Ring fluorinated thiophenes were synthesized via a Balz–Schiemann fluorination approach and were successfully employed in the synthesis of liquid crystals using regioselective electrophilic bromination and regioselective Suzuki coupling chemistry.

Synthesis, optical anisotropies, polarisabilities and order parameters of 4-cyanophenyl and 4-isothiocyanatophenyl 4'-butylsulfanylbenzoates with oxygen and sulfur substitution in the ester linkage

The optical anisotropies, polarisabilities, refractive indices and order parameters of 4-cyanophenyl and 4-isothiocyanatophenyl 4′-butylsulfanylbenzoates with oxy- and thin-substitution in all possible combinations in the ester link have been measured and the results are discussed in terms of the position and number of sulfur atoms in the ester linkage. The S-aryl thio- and dithio-benzoates have greater polarisabilities than the corresponding O-aryl esters; the differences between O-aryl and S-aryl thiobenzoates are discussed in terms of the electronegativity and structural changes caused by substitution with sulfur.

Synthesis and mesomorphic properties of 1,1-difluoroalkyl-substituted biphenylthienyl and terphenyl liquid crystals. A comparative study of mesomorphic behavior relative to alkyl, alkoxy and alkanoyl analogs

A variety of biphenylthienyl and terphenyl-based liquid crystalline materials were prepared that incorporate a 1,1-difluoropentyl terminal group. The mesogenic properties of these compounds were compared and contrasted with analogs where the CF2 moiety was replaced by a -CH2-, -O- or -CO- group. The 1,1-difluoroalkyl group is unique in its behavior and tends to promote orthogonal smectic phase behavior.

New bent core mesogens with exceptionally high clearing points

A homologous series of novel two-ring alkoxythiophene-containing liquid crystals has been synthesized using a recently developed cyclization methodology to construct the heterocyclic core. The new heterocyclic unit is expected to confer high negative dielectric anisotropy and high dielectric biaxiality when incorporated into suitable mesogenic structures. The materials reported here possess nematic phases, with the longer chain homologues displaying smectic mesomorphism. In all cases the clearing points are significantly enhanced when compared with other two-ring thiophene-based mesogens. The synthesis, mesomorphic behaviour and stability are discussed, and the mesomorphic behaviour is compared with the analogous 4-alkoxy-4′-cyanobiphenyl derivatives synthesized by Gray.

Synthesis and stability of 2-(1,1-difluoroalkyl) thiophenes and related 1,1-difluoroalkyl benzenes: fluorinated building blocks for liquid crystal synthesis

Abstract The synthesis of a series of 2-(1,1-difluoroalkyl) thiophenes, including some biphenylthienyl liquid crystalline materials, was examined using a variety of fluorination approaches. For comparison purposes, a series of 1,1-difluoroalkyl benzene analogs were also prepared. The direct fluorodeoxygenation of alkyl thienyl ketones and alkyl phenyl ketones using various aminofluorosulfuranes proceeded in only moderate to poor yields. In contrast, fluorodesulfurization of the corresponding 1,3-dithiolanes using NOBF4/PPHF cleanly afforded the desired 2-(1,1-difluoroalkyl) thiophenes (and analogous 1,1-difluoroalkyl benzenes) in high yields. Fluorodesulfurization of 2-alkyl-2-thienyl-1,3-dithiolanes using NBS (or DBH)/PPHF was complicated by competing ring and/or side chain bromination pathways. These problems were avoided when using NIS/PPHF. Although the various 1,1-difluoroalkyl arene products were sensitive to hydrolytic decomposition on prolonged exposure to silica, the purified products proved quite stable and were well suited for use as building blocks for liquid crystal synthesis.

Synthesis of some 2,4- and 2,5-disubstituted thiophene systems and the effect of the pattern of substitution on the refractive indices, optical anisotropies, polarisabilities and order parameters in comparison with those of the parent biphenyl and dithienyl systems

The refractive indices, optical anisotropies, polarisabilities and order parameters of a number of phenylthiophenes and the parent dithienyl and biphenyl compounds with butylsulfanyl and cyano substituents have been measured. The presence of a thiophene ring leads to: (i) increased values of n∥ and n⊥ with the latter values normally being increased more so that the optical anisotropies are typically reduced, and (ii) increased molecular polarisabilities with the 2,5substituted thiophene compounds having higher values than the 2,4-substituted compounds. The dithienyl compound has the lowest order parameter and the highest molecular polarisability. The results are discussed in terms of the structural differences of the compounds, including differences in the pattern of substitution in the thiophene ring and in linearity, inter-annular torsion angle and the distribution of the permanent dipoles.

Synthesis, transition temperatures and optical anisotropy of some isothiocyanato-substituted biphenyls

A range of alkyl-, alkoxy- and alkylsulfanyl-substituted biphenyls with terminal isothiocyanato substituents have been prepared and their suitability as dopants in nematic mixtures of high optical anisotropy evaluated. Palladium-catalysed cross-coupling reactions involving arylboronic acids have been used to synthesize the biphenyl core unit, followed by functional-group interconversions to give the desired isothiocyanato-substituted materials. The –NCS terminal group is in conjugation with the biphenyl core and this combination was expected to provide materials of high optical anisotropy. Mesogenic and optical properties have been determined and are discussed.