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.     

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.     

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.     

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.     

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

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4?-[1,ω-alkanediylbis(thio)]bis-[1,1?-biphenyl]-4-carbonitriles (CBSnSCB), and 4?-({ω-[(4?-cyano[1,1?-biphenyl]-4-yl)oxy]alkyl}thio)[1,1?-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 3⁴-{6-[(4?-cyano[1,1?-biphenyl]-4-yl)thio]-hexyl}[1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitrile (CT6SCB), is also reported and shows enantiotropic N_TB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-N_TB transition corresponds to 4 longitudinal molecular distances.     

Non-symmetric calamitic liquid crystal dimers containing troponoid and benzenoid units

Five types of non-symmetric calamitic dimers were synthesized to investigate the effect of the core structure and length of the spacer on mesomorphic properties. Two non-symmetric dimers containing a troponoid and benzenoid unit showed smectic A and C phases whereas the corresponding benzenoid dimers showed no mesophase. Non-symmetric dimers with a three-ring system showed smectic A and C phases with higher transition temperatures than the two-ring system. We propose packing models for these non-symmetric dimers by considering the direction of the dipole moments of the ring structures and microsegregation between the polar units and the non-polar chains.     

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.     

Synthesis and mesomorphism of diacetylene-bridged triphenylene discotic liquid crystal dimers

Connecting two discotic mesogens via a spacer not only stabilizes the columnar mesophase but also leads to the formation of glass columnar phase, and therefore improves the physical properties of discotic liquid crystals as organic semiconductor. Here, we report the synthesis of eight diacetylene-bridged triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)4(OMe)O2C-C8H16-C≡≡ C-]2, 3(n), (n = 4-8), [C18H6(OC6H13)5O2C-C8H16-C≡≡ C-]2, 6 and [C18H6(OC6H13)5O-(CH2)m-C≡≡ C-]2, 8(m), (m = 1, 3) by Eglinto...     

Oxadiazole-based unsymmetrical chiral liquid crystal dimers: synthesis and mesomorphic properties

The synthesis and liquid crystalline properties of novel chiral Schiff's base dimers containing the 1,3,4-oxadiazole ring are reported. The length of the terminal S-alkyl chain has been varied. All the compounds synthesised were thermally stable and exhibited enantiotropic mesomorphism, showing either SmC*–SmA–TGB–N*–BP or SmC*–SmA phase sequence.     

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.     

Spontaneously modulated chiral nematic structures of flexible bent-core liquid crystal dimers

ABSTRACT

We report the induction of spontaneously undulated chiral nematic structures of liquid crystal (LC) dimers with rigid aromatic molecular arms linked by flexible chains with an odd number of carbons. When a small amount of chiral dopants (CD) are added to the dimers, we find the formation of different stripe textures on cooling 4–10 μm films in the nematic phase. The temperature where the stripes form depends on the film thickness and the direction of the stripes depends on the CD concentrations. We show that the experimentally observed stripes are due to undulation instabilities that spontaneously form as a result of the anomalously small bend elastic constant that prefers director bend instead of twist deformation, the opposite of the situation in usual cholesteric LCs.     

Anomalies of periodicity in TGB structures in new liquid crystal dimers

Non-symmetric liquid crystal dimers consist of two different mesogenic units linked through a polymethylene flexible spacer. Our previous studies have shown that dimers containing a cholesteryl moiety as one of the mesogenic groups and a Schiff 's base unit as the second, exhibit a rich polymorphism and that several types of smectic packing are obtained depending on the molecular parameters: specifically, a smectic periodicity similar to the molecular length and an intercalated structure with a smectic parameter lower than half the molecular length can be obtained. The competition between these two incommensurate lengths can induce two-dimensional phases and/or an incommensurate smectic phase in which the two smectic periodicities coexist over a long range. Small modifications of the molecular structure can significantly influence the phase sequence. Here we have replaced the Schiff 's base by a tolan unit and the terminal alkyl chains by alkoxy chains. As a result, anomalies of periodicity are also observed in this new dimeric series, but they occur mainly in TGB structures.     

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.     

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.     

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.     

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.     

Symmetrical and asymmetrical liquid crystal dimers: synthesis,characterisation and mesomorphic behaviour

ABSTRACT

A series of new asymmetrical liquid crystal dimers, R_nO–S2O–OR_n (n = 2–10), linked by an ethylene spacer having carbothiol – COS – and carboxyl – COO – linkages to the core centre of the molecule and bearing different lengths of terminal alkoxy chains were synthesised in order to study the effects of the length of the terminal alkoxy chains on mesomorphic properties. As well as, five symmetrical and asymmetrical dimers linked by butylene spacers bearing different linkages to the core and various terminal chains were also synthesized in order to study the effect of the nature of the spacer and terminal groups. The structures of the synthesised dimers were confirmed by physico-chemical techniques, i.e. FTIR, NMR and mass spectra. Differential scanning calorimetry and polarising optical microscopy verified the liquid crystal behaviour transition temperatures. The isotropic transition temperatures of the dimers R_nO–S2O–OR_n (n = 2–10) decreased with increasing length of alkoxy chain. Structural effects on the mesomorphic and physical properties were investigated in terms of alteration of carboxylate and thioester groups linking the spacer. The mesomorphic investigation reveals that all the dimers formed an enantiotropic Nematic phase except for dimer HO2SH which is not a liquid crystal.     

Synthesis and characterization of novel chiral dimers exhibiting highly frustrated liquid crystal phases

Two novel series of optically active dimers comprising cholesterol and biphenyl-4-yl 4-(n-alkoxy)benzoate cores interlinked though either an odd-parity/even-parity spacer have been prepared and characterized. They stabilize an extremely complex, frustrated liquid crystalline state viz., the twist grain boundary (TGB) phase with chiral smectic C structure, denoted as TGBC^∗ phase, over a wide (50–110 °C) temperature range. Notably, the dimers with an odd-parity spacer show an additional frustrated liquid crystal phase namely, the blue phase (BP). The presence of such frustrated phases suggests that the synthesized dimers are characterized by high enantiomeric excess and strong molecular chirality. Thus, 12 new optically active, nonsymmetric dimers reported herein constitute new examples of rarely found strongly chiral, optically pure dimers showing frustrated liquid crystal phase over an adequately wide thermal range.