Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACT

A series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, ¹H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase.     

Non-symmetric liquid crystal dimers based on 1,3,4-oxadiazole derivatives: synthesis,photoluminescence and liquid crystal behaviour

A series of non-symmetric liquid crystal compounds consisting of two different semi-rigid anisometric cores, namely 1,3,4-oxidiazole and biphenyl units, and two short terminal groups, have been synthesised in good yield. It has been shown by polarising optical microscopy and differential scanning calorimetry that all these compounds display liquid crystalline behaviour, with nematic and/or smectic A mesophases. The nature of the mesophases is dependent on the electronic properties of the terminal groups. In methylene chloride solution all the compounds displayed a room temperature emission with λ_max at 358–396 nm and quantum yields of 0.29–0.56. The effect of the terminal groups on the mesomorphic and photoluminescent properties is briefly discussed in the context of their electronic characteristics.     

Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

ABSTRACT

New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (T_SmC*-SmA*/T_SmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan’s molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.     

1,2-Propanediol-linked chiral symmetric and non-symmetric liquid crystal dimers containing trifluoromethyl

Four symmetric and non-symmetric chiral liquid crystal dimers containing trifluoromethyl groups, termed as TFBA-PD-TFBA, UEBBA-PD-TFBA, UEBA-PD-TFBA and UEA-PD-TFBA, respectively, have been synthesised and characterised. UEA-PD-TFBA exhibited chiral nematic phase, whereas the other three dimers displayed chiral smectic A phase. X-ray diffraction (XRD) has revealed the structure of the smectic A phase for TFBA-PD-TFBA to be intercalated, whereas that for UEBBA-PD-TFBA and UEBA-PD-TFBA to be monolayer and interdigitated, respectively. In addition, the weaker peak corresponding to a shorter layer spacing was observed for UEBBA-PD-TFBA and UEBA-PD-TFBA. Considering the results of XRD measurements and computer simulations, the structural model corresponding to the shorter layer spacing is assigned as horseshoe-like shape. The absence of smectic behaviour for UEA-PD-TFBA reveals that the weaker aromatic–aromatic interactions cannot stabilise the smectic A phase.     

Methylene-linked liquid crystal dimers

A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized. The nematic-isotropic transition temperature, T _NI, and the associated entropy change, ΔS _NI/R, are sensitive to the chemical nature of this link. Specifically, T _NI falls on replacing ether links with methylene links for both odd and even members although this reduction is more pronounced for odd members. In comparison, ΔS _NI/R increases on changing ether links for methylene links for even dimers, but decreases for odd-membered dimers. These observations are completely in accord with the predictions of a model developed by Luckhurst and co-workers in which the difference between the ether-linked and methylenelinked dimers rests exclusively in their shapes. Furthermore, the highly non-linear pentamethylenelinked dimers show a greater tendency to exhibit smectic behaviour; this is interpreted in terms of molecular packing giving rise to an alternating smectic phase.     

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.     

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.     

Synthesis and properties of non-symmetric liquid crystal dimers containing a cholesteryl moiety

A series of non-symmetric liquid crystal dimers having cholesteryl and 4-trans-(4-n-alkylcyclohexyl)phenoxy groups were synthesized by condensation of cholesteryl ω-bromoalkanoates with appropriate 4-trans-(4-n-alkylcyclohexyl)phenols. The structures and thermal phase behaviour of the dimers were characterized using IR, ¹H NMR and mass spectroscopy, elemental analysis, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction measurements. Their thermal phase behaviour is significantly different to that of other cholesterol-based liquid crystal dimers. All of these liquid crystal dimers exhibit low phase transition temperatures. The relationships between their properties and chemical structures of these new dimers are discussed.     

Enhancement of flexoelectric ratio of nematic liquid crystal by doping calamitic ferroelectric liquid crystal

We measured the flexoelectric ratio e* of a nematic liquid crystal (NLC) doped with a calamitic ferroelectric liquid crystal (FLC). We doped two kinds of commercial FLC into the pure NLC at a weight concentration of 5 wt%. The absolute value of the flexoelectric ratio was increased up to 49% compared to the pure NLC. The greater transverse dipole moment and the elastic constant of FLC are thought to be related to the increase of the flexoelectric ratio.     

Non-symmetric ether-linked liquid crystalline dimers with a highly polar end group

A new class of non-symmetric dimeric compounds derived from 4-cyano-4′-hydroxybiphenyl in which two rigid parts are connected via flexible spacers have been designed and synthesised. These materials possess trialkoxy chains attached at one end of the molecule, while the other end consists of a biphenyl moiety terminated with the highly polar cyano group. The molecular structures of these dimers have been confirmed by elemental analysis and spectroscopic data and their phase behaviour has been characterised by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Almost all of the synthesised materials exhibit liquid crystalline properties depending on the number of carbon atoms in the terminal chains, where all short chains derivatives form nematic phases and depending on the length of the internal spacer long terminal chains homologues display crystalline or unidentified smectic phase.     

Nematic-isotropic interfacial tension anisotropy of a calamitic lyotropic liquid crystal

In a planar oriented sample of a calamitic nematic lyotropic system (mixture of sodium lauryl sulphate/water/decanol), isotropic pretransitional domains appear at the nematic to isotropic transition. The domains are oblong in shape with the long axis along the orientational direction. We show experimental evidence that this oblong shape is determined by the nematic-isotropic interfacial tension anisotropy. Two uniparametric models of simple angular dependences for the interfacial tension are tested. Using the differential system obtained from the Young-Laplace condition at the nematic-isotropic interface, the domain shape can be numerically calculated for each value of the interfacial tension anisotropy. By processing the values of the transmitted light through both an isolated isotropic domain and its surrounding nematic zone, we obtain the anisotropy of the interfacial tension as the main fitting parameter. An estimation of the ratio of the extreme values for the interfacial tension is given.     

Observation of intercalated smectic phases in symmetric liquid crystal dimers containing hydrazide groups

Dimeric hydrazide derivatives with nitro, phenyl, and methyl terminal subsistents were synthesized. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and wide angle X-ray diffraction. Interestingly, intercalated smectic phases were observed in these symmetric liquid crystal dimers. The effect of the substituents and the length of the spacer on the mesophase is discussed, confirming that intermolecular hydrogen bonding between the hydrazide groups was the driving force for the formation of the intercalated structures.     

Synthesis and properties of new non-symmetric liquid crystal dimers containing mandelic acid and cyano group

Four non-symmetric dimers containing mandelic acid as the chiral core have been synthesised, termed as QBMA-B, QBMA-BCN, QBMA-BBCN and QBBMA-BBCN, respectively. Chemical structures and liquid crystal (LC) properties of the dimers were characterised by FTIR, ¹H-NMR, differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and polarised optical microscopy (POM). The results indicated that the rigidity and conformation of the molecules of the dimers played important effects on their mesophase properties. QBMA-B did not display any mesophase, QBMA-BCN and QBMA-BBCN exhibited nematic (N) phases, while QBBMA-BBCN displayed cholesteric (ch) phase, which indicated that chiral mandelic acid could induce cholesteric phase, but some attention should be paid to the molecular conformation to obtain cholesteric phase. For the four dimers, melting temperature (T_m) increased first and then decreased, inferring that molecular conformation played a more important effect besides molecular weight and rigidity.     

Synthesis and mesomorphic evaluation of new calamitic liquid crystals containing benzothiazole core

The design and synthesis of new calamitic benzothiazole-based liquid crystals,2-[4-（4-alkyloxybenzoyloxy）-phenyl]ben-zothiazoles are presented.The target compound was characterized using spectroscopic techniques,such as FT-IR,NMR(¹H and ¹³C),microanalysis and EI-MS.The liquid crystalline behaviours of these compounds were thoroughly examined by differential scanning calorimetry and polarizing optical microscope techniques.These materials exhibited enantiotropic nematic phase with high thermal stability（>168℃）.Smectic A phase starts to emerge as monotropic（metastable） phase from C10 member and changes into enantiotropic（stable） phase from C12 and persists up to C16 members.     

Effect of pressure on liquid crystal dimers

We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic–isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic–isotropic phase boundary in the pressure–temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius–Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd–even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds.     

Effect of pressure on liquid crystal dimers

We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic-isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic-isotropic phase boundary in the pressure-temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius-Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd-even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds.     

Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation

We report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e., smectic A, nematic and isotropic. These quantities have then been used to compute diffusion coefficients. In addition, we have calculated viscosities in the nematic and isotropic phases. Despite its crude nature, the model is capable of providing a faithful reproduction of many features of the transport behavior observed in real liquid-crystalline materials. The simulation results have been compared with the predictions of the modified affine transformation theory, finding only a fair agreement.     

Azobenzene-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two new sets of non-symmetric liquid crystal dimers is reported, the 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexanes (CB6OABX) and 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yloxy)pentanes (CBO5OABX). The terminal substituents are methyl, methoxy, butyl, butyloxy, nitrile and nitro. All the CB6OABX dimers exhibit twist-bend nematic (N_TB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the N_TB phase. The transitional behaviour of the non-symmetric dimers is compared to that of the corresponding symmetric dimers, the 1,5-bis(4-substitutedazobenzene-4′-yloxy)pentanes (XABO5OABX) and either 1,7-bis(4-cyanobiphenyl-4′-yl)heptane or 1,5-bis(4-cyanobiphenyl-4′-yloxy)pentane. The XABO5OABX dimers all show a nematic phase and in addition, the butyl homologue exhibits a smectic A phase. The difference in transitional behaviour between the CB6OABX and CBO5OABX dimers is attributed to the difference in their molecular shapes arising from different bond angles between the para axis of the cyanobiphenyl unit and the first bond in the spacer. Specifically, the all-trans conformation of a CBO5OABX dimer is more linear than that of the corresponding CB6OABX dimer. Differences within each set of dimers are attributed to changes in the average molecular shape and the strength of the mixed mesogen interaction on varying the terminal group. Crystal structures are reported for CB6OABOMe, CBO5OABNO₂ and MeOABO5OABOMe.     

Distinctive dielectric properties of nematic liquid crystal dimers

ABSTRACT

We provide an overview of the effect of the molecular structure on the dielectric properties of dimers exhibiting nematic and twist-bend nematic phases with special focus on how the conformational distribution changes are reflected by the dielectric behaviour. Nematic dimers show distinctive dielectric properties which differ from those of archetypical nematic liquid crystals, as for example, unusual temperature dependence of the static permittivity or dielectric spectra characterised by two low-frequency relaxation processes with correlated strengths. The interpretation of such characteristic behaviour requires that account is taken of the effect of molecular flexibility on the energetically favoured molecular shapes. The anisotropic nematic interactions greatly influence the conformational distribution. Dielectric behaviour can be used to track those conformational changes due to dependence of the averaged molecular dipole moment on the averaged molecular shape. Results for a number of dimers are compared and analysed on the basis of the influence of details of the molecular structure, using a recently developed theory for the dielectric properties of dimers.     

Triphenylene-based discotic liquid crystal dimers, oligomers and polymers

Discotic liquid crystals are unique nanostructures with remarkable electronic and optoelectronic properties. Triphenylene derivatives play a major role in the research on discotic liquid crystals. Following recent reviews of the chemistry of triphenylene-based monomeric liquid crystals, this article now reviews the chemistry and physical properties of triphenylene-based discotic dimeric, oligomeric and polymeric liquid crystals.