Arie Koudijs

Odd-even effects in the thermotropic and optical properties of three series chiral twin liquid crystals.

Abstract Three series of novel chiral twin liquid crystals consisting of a cholesteryl and a 4′-cyanobiphenyl-4-yloxy group (III) or a 4-cyanophenoxy group (IV) and of a dihydro-cholesteryl and a 4′-cyanobiphenyl-4-yloxy group (V) connected by an alkanoate spacer with a varying number (1–7 and 10) of methylene units were synthesized and their mesogenic properties investigated. Strong odd-even effects were observed as a function of spacer length for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phase. The compounds with an even number of methylene units have a smaller pitch than the compounds with an odd number of methylene units. Replacement of the cholesteryl group by a dihydrocholesteryl group results in an larger pitch, whereas replacement of the 4′-cyanobiphenyl-4-yloxy group by a 4-cyanophenoxy group gives a smaller pitch.

Cholesterol-containing liquid crystal dimers with ether linkages between the spacer and mesogenic units

Three series of chiral liquid crystalline dimers were investigated, having a cholesteryl and a cyanobiphenylyl, butoxybiphenylyl or hexyloxybiphenylyl group connected to a variable alkyl spacer through ether linkages. Their properties were compared with those of the corresponding ester derivatives. The phase behaviour of compounds with ether and ester linkages is comparable, showing N* and SmA phases. The melting points of the compounds with ether linkages are in the same range as those of the ester compounds, but the liquid crystal transition temperatures are lower. The smectic layer spacings and smectic ordering properties are also similar. The cyanobiphenylyl compounds have an interdigitated SmA layer structure, which shows a small odd–even effect with spacer parity. The alkoxybiphenylyl compounds have a monolayer SmA phase for short spacers and an intercalated SmA phase for longer spacers. The selective reflection wavelengths of the chiral nematic phase of the ether compounds are lower than those of the corresponding ester compounds. The transition from N* to interdigitated or monolayer SmA is accompanied by a strong increase in the selective reflection wavelength, indicative of an intermediate TGB phase. This is absent for the transition from N* to intercalated SmA.

Influence of spacer and terminal group lengths on the smectic ordering of cholesterol-containing dimer liquid crystals

The smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 times the length of the molecule. Three series of alkoxybiphenylylcontaining dimers exhibit two other smectic modifications. For short spacers, a smectic layer spacing is observed that is about the same length as the molecule and for long spacers the smectic layer spacing is about half the length of the molecule. In the latter cases the entropy change at the SmA-N* transition is clearly larger than for the compounds with the other smectic modifications. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd number of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.

Liquid crystalline properties of salicylaldimine-based dimers: influence of terminal alkyl chain length and central part

The synthesis and thermotropic properties of four homologous series of salicylaldimine-based dimer liquid crystals are reported. Two 4-(4-alkoxy-2-hydroxybenzylideneamino)benzoyloxy groups are connected to a central part consisting of a 1,3-phenylene, 1,5-pentylene, 2,2-dimethyl-1,5-pentylene or 3,3-dimethyl-1,5-pentylene unit. The terminal alkoxy chains have been varied from 4 to 16 carbon atoms in length. All the compounds exhibit liquid crystalline phases whose behaviour depends on the nature of the central part and the length of the alkoxy terminal chains. All compounds of the series with the central phenyl part exhibit enantiotropic B-phases, and the sequence B6–B1–B2 on increasing terminal chain length was observed. Replacement of the phenyl group with a pentyl central group partly suppresses the formation of B-phases. The longer homologues of this series show the B1 phase, while the shorter exhibit an intercalated SmCc mesophase. The introduction of methyl substituents to the pentyl spacer causes the melting points to fall dramatically and the formation of B-phases is totally suppressed. The compounds with the long tails show intercalated SmAc phases and those with short tails show intercalated SmCc phases.

Dimer liquid crystals with bent mesogenic units

Three series of dimer liquid crystals containing an aromatic and a cholesteryl mesogenic unit were synthesized and investigated The compounds within the series differ in the length of the spacer connecting the mesogenic groups. Two of the three series contain an aromatic mesogenic group that is connected to the spacer at a meta-position instead of the para-position. Due to the presence of this bent mesogenic group, the odd-even effect in the transitional properties that is normally observed for dimer liquid crystals is reversed as compared with that of the third series, which contains a para-substituted aromatic mesogenic group. The reversed odd-even effect is found for the isotropization temperatures, the associated enthalpy changes and also for the optical properties.

Odd-even effects in the thermotropic and optical properties of chiral triplet liquid crystals

Abstract A series of chiral triplet liquid crystalline compounds with a central biphenyl group to which two dihydrocholesteryl groups are attached via two flexible alkanoate spacers has been prepared and investigated. As a function of spacer length, these compounds show strong odd-even effects for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phases. Asymmetrical compounds with one odd and one even number of methylene groups in their spacers have properties intermediate between those of symmetrical compounds with two odd or two even spacers.

Asymmetric banana-shaped liquid crystals with two different terminal alkoxy chains

Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4′-biphenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4′-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B1 or B2 phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B2 phase was antiferroelectric.

Non‐symmetric bent‐core mesogens with one terminal vinyl group

Two series of non‐symmetric banana‐shaped compounds, both with one alkyl and one alkenyl terminal tail, have been synthesized and studied. Both series were compared with the corresponding series with two saturated terminal alkyl tails. All the compounds have a bent central 1,3‐phenylene bis(4‐benzoyloxy)benzoate core; their mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, X‐ray diffraction and switching current response experiments. In all four series one of the terminal tails is varied from OC8H17 to OC16H33. The other terminal tails are OC11H23, O(CH2)9CH = CH2, OC10H21 and O(CH2)8CH = CH2. The short‐tailed compounds show monotropic or enantiotropic B1 phases and the long‐tailed compounds the B2 phase. The introduction of one terminal vinyl group slightly lowers the transition temperatures. The introduction of a second terminal vinyl group further suppresses the liquid crystalline properties. All compounds with B2 phases have layer spacings that suggest a tilt of ∼45° of the bent molecules in the layers, and their switching behaviour is antiferroelectric.

Banana-shaped side chain liquid crystalline siloxanes

Eight banana‐shaped side chain liquid crystalline oligomers and polymers have been synthesized by hydrosilylation of vinyl‐terminated bent‐core mesogens with trimethylsilyl‐terminated siloxanes. The synthesized oligomers and polymers, and their olefinic precursors, were investigated by polarizing optical microscopy (POM), differential scanning calorimetry, X‐ray diffraction (XRD), electro‐optical experiments and Maldi‐Tof. The short‐tailed olefins form a Colr mesophase, whereas those with longer chains exhibit the SmCPA mesophase. All the oligomers and polymers studied show liquid crystalline properties and do not crystallize upon cooling. Most oligomers with around four repeating siloxane units, show a lamellar (layer) structure and antiferroelectric switching properties, the SmCPA phase. XRD shows that the layer spacings are hardly influenced by the length of the terminal tails. The oligomer prepared from the smallest olefinic precursor, having the shortest alkyl tail, shows an XRD pattern reminiscent of a columnar phase, although POM displays domains of opposite chirality, and no switching behaviour could be detected. The polymers with around 35 repeating siloxane units are liquid crystalline, but due to their high viscosity a thorough characterization of the liquid crystalline phases was impossible.

Liquid crystalline dimers with bent-core mesogenic units

Two series of dimers, in which two identical aromatic bent core mesogenic units are connected via an organosiloxane or an alkylene spacer, have been investigated. The dimers with a spacer consisting of a trisiloxane central group and relatively long alkylene groups show an intercalated tilted smectic structure. The layer spacings appear to be very weakly dependent on the terminal chain lengths. In these dimers the smectic phase is stabilized for the compounds with a short terminal chain. Dimers with an aliphatic alkylene spacer are liquid crystalline only when the spacer is relatively short and the terminal chains are long. For these dimers a monolayer tilted smectic phase is observed.