Non-symmetric Liquid Crystal Dimers Research Articles

The influence of the side-arms structure on the properties of the liquid crystal dimers centred on mandelic acid

ABSTRACT With mandelic acid (MA) as the chiral core, two series of non-symmetric liquid crystal (LC) dimers have been synthesised by changing the rigidity of the side-arms, namely the DBM-nC2B series and the DBBM-nC2B series. In the DBM-nC2B series, one side-arm is the same, while the length of the flexible tail of the other side-arm increases from 0 to 4. The same is true of the DBBM-nC2B series. Chemical structures and LC properties of the two series LC dimers were characterised by FT-IR, 1H NMR, 13C NMR, DSC, TGA and POM. The results show that the DBM-nC2B series LC dimers display the nematic phase (N), and the DBBM-nC2B series LC dimers exhibit the chiral nematic phase (N*). It shows that the side-arm should have sufficient rigidity for MA to induce the N* phase. In the same series, when the length of the flexible tails of the side-arm increases from 0 to 4, it does not change the mesophase, while it has some effect on the melting temperature (Tm), the clearing temperature (TN(*)I), and mesophase range of the dimers. The molecular length-width ratio (l/w) and dipole moment (μ) were used to explain the variation of Tm and (TN(*)I) of LC dimer.

The influence of the imine bond direction on the phase behaviour of symmetric and non-symmetric liquid crystal dimers

The synthesis and characterisation of two series of liquid crystal dimers are reported: the symmetric 1,5-bis(4′-oxyanilinebenzylidene-4-alkane)pentanes (m-O5O-m), and the non-symmetric 1-(4-cyanobiphenyl-4′-yloxy)-5-(4′-oxyanilinebenzylidene-4-alkane)pentanes (CBO5O-m). In the acronyms for each series m refers to the terminal chain length. The m-O5O-m series showed monotropic nematic phases for m = 1-7. Smectic behaviour emerged at m = 7 and nematic behaviour extinguished at m = 8. For the CBO5O-m series, an enantiotropic nematic phase is seen for m = 1-10. In addition, m = 1 shows a twist-bend nematic phase, and m = 9 and 10 an interdigitated smectic A phase. These series are compared to the corresponding dimers in which the imine link in the benzylideneaniline fragments is reversed, the m.O5O.m and CBO5O.m series. For the symmetric dimers, reversing the imine link has a small effect on the nematic-isotropic transition temperatures, TNI, but a much larger effect on the smectic-isotropic transition temperatures, TSmI. In both cases, the m.OnO.m series shows the higher values. The values of TNI are also slightly higher for the CBO5O.m series than the CBO5O-m series. Surprisingly, the values of TSmI are higher for the CBO5O-m than for the CBO5O.m series. These differences are discussed in terms of the changes in shape and electronic distributions arising from reversing the imine link.

Synthesis and Mesomorphic Properties of New Non-symmetric Liquid Crystal Dimers

Abstract: New non-symmetric liquid crystal dimers, N-(4-(n-(4-(benzothiazol-2-yl)phenoxy) alkyloxy) benzylidene)-2,4-difluoroaniline (DSFCn), containing two mesogenic core units of benzothiazole and benzylideneaniline were reported. The core system was connected by a flexible alkyl spacer, -(CH2)n-(where n =2-6, 8, 10, and 12). The spectroscopic techniques were employed to confirm the molecular structure of the title compounds. Differential scanning calorimetry and polarizing optical microscopy were used to study the mesomorphic properties of the synthesized compounds. Observation under crossed polarized light reveals that compounds (n = 4, 5, 6, 8, 10, 12) are purely nematogens. The nematic phase in the dimers are caused by the presence of the lateral fluorine atoms at the benzylideneaniline core. Mesophase thermal stability has been suppressed significantly by the fluorine atom at the lateral position which increased the molecular breadth and subsequently, results in weaker overall lateral intermolecular attraction. The structure-property relationships of the dimers were inferred through comparison of thermal properties of the present dimers with those of the earlier reported analogous that do not possess any lateral substituents.

Synthesis and mesomorphic properties of non-symmetric liquid crystal dimers stabilized by benzothiazole and substituted aromatic imines

Synthesis and mesomorphic properties of non-symmetric liquid crystal dimers stabilized by benzothiazole and substituted aromatic imines

Cholesterol-based nonsymmetric dimers comprising phenyl 4-(benzoyloxy)benzoate core: the occurrence of frustrated phases

ABSTRACT The synthesis and mesomorphism of sixteen new nonsymmetric liquid crystal dimers formed by covalently attaching cholesterol with a three-ring mesogenic core, namely, phenyl 4-(benzoyloxy)benzoate, via an ω-oxyalkanoyl spacer are described. The lengths of the terminal chain attached to the three-ring core, as well as those of the ω-oxyalkanoyl spacer, have been varied; accordingly, they have been grouped into four sets. They display an enantiotropic chiral nematic (N*) behaviour with the isotropization temperature crossing over 200 oC. Four dimers having an odd-parity spacer were found to additionally show blue phase-I/II (BP-I/II), and the smectic A (SmA) phase. Twelve dimers with an even-membered spacer also stabilise either chiral smectic C (SmC*) or twist grain boundary (TGB) phase having SmC* slabs (TGBC* phase). The dimers with longer spacer and terminal tail favour the formation of the TGBC* phase. Thus, the lengths of both the terminal tail and the central spacer are seen to direct the type of mesophase and liquid crystal phase sequences to be formed. X-ray diffraction measurements have been carried out on representative samples to corroborate the structure of the mesophases. The N* phase has been probed with the aid of CD technique to figure out the handedness of the helicoidal structure.

The effect of molecular structure on mesophase behaviour of non-symmetric liquid crystal dimers containing mandelic acid and fluorine group

ABSTRACT A series of non-symmetric dimers with the same chiral core, mandelic acid (MA), have been synthesised by varying the terminal group and the rigidity of the molecules, termed QBBMA-3F3BT, QBMA-3F3BT, QBMA-3F2BT, 3F3BMA-BBCN, 3F2BMA-BBCN and 3F1BMA-BBCN. Chemical structures and liquid crystal (LC) properties of the dimers were characterised by Fourier transform infrared spectroscopy, 1H NMR, polarised optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric analysis (TG). The six dimers displayed different optical activity (in THF). QBBMA-3F3BT and QBMA-3F3BT displayed enantiotropic chiral smectic A (SmA*) phase. QBMA-3F2BT displayed N phase. The Tm of QBMA-3F3BT is the highest among QBBMA-3F3BT, QBMA-3F3BT and QBMA-3F2BT, which indicated that molecular conformation played an important role on Tm besides molecular weight and molecular rigidity. 3F3BMA-BBCN and 3F2BMA-BBCN displayed chiral nematic (N*), which indicated that the mesogenic arm BBCN is beneficial to the formation of N* phase. The LC range of 3F2BMA-BBCN is wider, which may be due to that the similar length of the two LC arms is advantageous to the interaction between C≡N group and other molecules so as to stabilise the LC phase.

The influence of terminal chain on the formation of chiral nematic phase of non-symmetric liquid crystal dimers containing fluorinated units

In this study, we have synthesized four fluorine liquid crystals (LCs), termed FA-PD-UBA, FA-PD-PBA, FA-PD-ABA and FA-PD-BA. Through the research on thermal properties and mesophase type of the dimers, we can draw a conclusion that the non-symmetric structure type with flexible terminal chain of appropriate length, may help the dimers to form cholesteric fluorinated liquid crystals.

Structure-property relationships in azobenzene-based twist-bend nematogens

ABSTRACTThe synthesis and characterisation of a new set of azobenzene-based non-symmetric liquid crystal dimers, the 1-(4-substitutedazobenzene-4ʹ-yloxy)-6-(4-methoxybiphenyl-4ʹ-yl)hexanes (MeOB6OABX), that exhibit the twist-bend nematic phase, NTB, is described. The terminal substituents are methyl, methoxy, ethyl, butyl, butoxy, and nitrile. All six dimers exhibit both the NTB and conventional nematic, N, phases. The identification of the NTB phase was performed using polarised light microscopy and confirmed for binary mixtures with a standard twist-bend nematogen 1,7-bis-4-(4ʹ-cyanobiphenyl) heptane (CB7CB). The transitional behaviour of the MeOB6OABX dimers is compared with that of the corresponding ether-linked 1-(4-substitutedazobenzene-4ʹ-yloxy)-6-(4-methoxybiphenyl-4ʹ-yloxy)pentanes, MeOBO5OABX, all of which exhibit a conventional nematic phase. In addition, the nitrile-substituted MeOBO5OABCN shows the NTB phase. The behaviour of these non-symmetric dimers is also compared to that of the corresponding symmetric dimers. Differences in the transitional properties between these sets of new materials are accounted for in terms of not only molecular shape but also other factors including the strength of the mixed mesogen interaction.

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

ABSTRACTA 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 new non-symmetric liquid crystal dimers containing mandelic acid and cyano group

ABSTRACTFour 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 (Tm) increased first and then decreased, inferring that molecular conformation played a more important effect besides molecular weight and rigidity.

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

ABSTRACTThe 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 (NTB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the NTB 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.

Influence of internal flexibility on the double glass transition in a series of odd non-symmetric liquid crystal dimers characterised by dielectric measurements

ABSTRACTDielectric measurements (thermally stimulated depolarisation currents and broadband dielectric spectroscopy) have been performed near the glass transition to study the glass transition on the odd non-symmetric liquid crystal (LC) dimers of the series α-(4-cyanobiphenyl-4'-oxy)-ω-(1-pyreniminebenzylidene-4'-oxy) alkanes (CBOnO.Py) with n ranging from 3 to 9. A previous study [S. Diez-Berart et al., Materials 8 (2015) 3334] carried out in CBO11O.Py showed the presence of two glass transition temperatures, attributed to different molecular motions of the terminal groups. The study performed allows us to analyse the molecular dynamics in the rest of the series and determine the role played by the flexible spacer. Parallel and perpendicular orientations of the molecular director with regards to the probe electric field have been investigated. The low and intermediate observed relaxations are explained in the framework of Stocchero’s theoretical model [M. Stocchero, J. Chem. Phys. 121 (2004) 8079] for the dielectric behaviour of non-symmetric LCs dimers, as independent end-over-end rotations of each terminal semi-rigid unit. As the length of the spacer chain in the series of compounds decreases, the different relaxations become progressively more coupled at the glass transition. Numerical simulations of the calorimetric response from the obtained kinetic parameters show good agreement with experimental behaviour.

Structure–property relationships in twist-bend nematogens: the influence of terminal groups

ABSTRACTThe synthesis and characterisation of a range of non-symmetric liquid crystal dimers designed to exhibit the twist-bend nematic phase is reported. Beginning with 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl) hexane, each nitrile group is exchanged systematically for a methoxy group. The behaviour of these dimers is interpreted in terms of their bent shape being the predominant driving force for the formation of the twist-bend nematic phase, and the small differences between the twist-bend nematic–nematic transition temperatures are attributed to the differences between the interaction strength parameters of the mesogenic units. The 4-alkyloxyphenyl 4-[6-(4′-cyanobiphenyl-4-yl)hexyloxy]benzoates with ethyl, butyl, hexyl and octyl chains show the twist-bend nematic phase, whereas the corresponding 4-alkyloxyphenyl 4-[5-(4′-cyanobiphenyl-4-yloxy)pentyloxy]benzoates do not. This difference in behaviour is attributed to the more bent structure of the former. Increasing the terminal chain length initially decreases the twist-bend nematic–nematic transition temperature and this suggests that the chain disrupts the interactions between the mesogenic units. Subsequent increases in chain length have a smaller effect suggesting that the chain can be accommodated within an intercalated arrangement. The transitional behaviour of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-butyloxybiphenyl-4′-yl) hexane is compared to that of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-((S)-2-methyl)butyloxybiphenyl-4′-yl) hexane, and it is shown that chain branching strongly destabilises the twist-bend nematic phase. This is attributed to difficulties associated with packing the dimers.

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

ABSTRACTFour 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.

Understanding the twist-bend nematic phase: the characterisation of 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl)hexane (CB6OCB) and comparison with CB7CB.

The synthesis and characterisation of the nonsymmetric liquid crystal dimer, 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl)hexane (CB6OCB) is reported. An enantiotropic nematic (N)-twist-bend nematic (NTB) phase transition is observed at 109 °C and a nematic-isotropic phase transition at 153 °C. The NTB phase assignment has been confirmed using polarised light microscopy, freeze fracture transmission electron microscopy (FFTEM), (2)H-NMR spectroscopy, and X-ray diffraction. The effective molecular length in both the NTB and N phases indicates a locally intercalated arrangement of the molecules, and the helicoidal pitch length in the NTB phase is estimated to be 8.9 nm. The surface anchoring properties of CB6OCB on a number of aligning layers is reported. A Landau model is applied to describe high-resolution heat capacity measurements in the vicinity of the NTB-N phase transition. Both the theory and heat capacity measurements agree with a very weak first-order phase transition. A complementary extended molecular field theory was found to be in suggestive accord with the (2)H-NMR studies of CB6OCB-d2, and those already known for CB7CB-d4. These include the reduced transition temperature, TNTBN/TNI, the order parameter of the mesogenic arms in the N phase close to the NTB-N transition, and the order parameter with respect to the helix axis which is related to the conical angle for the NTB phase.

Two Glass Transitions Associated to Different Dynamic Disorders in the Nematic Glassy State of a Non-Symmetric Liquid Crystal Dimer Dopped with g-Alumina Nanoparticles

In the present work, the nematic glassy state of the non-symmetric LC dimer α-(4-cyanobiphenyl-4′-yloxy)-ω-(1-pyrenimine-benzylidene-4′-oxy) undecane is studied by means of calorimetric and dielectric measurements. The most striking result of the work is the presence of two different glass transition temperatures: one due to the freezing of the flip-flop motions of the bulkier unit of the dimer and the other, at a lower temperature, related to the freezing of the flip-flop and precessional motions of the cyanobiphenyl unit. This result shows the fact that glass transition is the consequence of the freezing of one or more coupled dynamic disorders and not of the disordered phase itself. In order to avoid crystallization when the bulk sample is cooled down, the LC dimer has been confined via the dispersion of γ-alumina nanoparticles, in several concentrations.

Synthesis, characterization and molecular organization for induced smectic phase of triazole ring in non-symmetric liquid crystalline dimer

New non-symmetric liquid crystal dimers containing cholesterol, triazole and biphenylene segments have been synthesized via click reaction. Increase in the flexible spacers length affects the mesophase formation. The homologues with short and medium alkyl spacer exhibit SmA* and SmC* phases whilst the analogue with the longest spacer favors the formation of enantiotropic SmA and N* phases. The XRD studies reveal the emergence of the SmA* and SmC* phases that can be associated with the monolayer ordering of these dimers. Further investigation through a comparison study shows that the triazole ring affects the mesophase formation wherein apart from the molecular dipole the subtle electrostatic interaction and van der Waal's forces enhance the SmC* phase.

Synthesis and phase transition behaviours of new non-symmetric liquid crystal dimers

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-4-nitro-benzylideneimine) containing benzothiazole and benzylideneaniline as the mesogenic core units connected by a flexible alkyl spacer, -(CH2)n-, with n ranging from 2 to 6, 8, 10 and 12 have been prepared. Their molecular structures were proposed via spectroscopic techniques. Mesomorphic properties were studied using differential scanning calorimetry and polarising optical microscopy. The results showed that the clearing temperatures decreased upon lengthening of the alkyl spacers. Whilst even member with the shortest alkyl spacer (n = 2) exhibited nematic phase, the rest of even members (n = 4, 6, 8, 10 and 12) possessed smectic and nematic properties. Nematic phase range gradually decreased from n = 2 to n = 12. Odd members are belonged to non-mesogenic compounds except for n = 5 which exhibited monotropic (metastable) nematic phase.

Overall Dielectric Study on the Odd Members of a Highly Nonsymmetric Pyrene-Based Series of Liquid Crystal Dimers

Broadband dielectric spectroscopy (10(3) Hz-1.8 × 10(9) Hz) has been performed on the odd nonsymmetric liquid crystal dimers of the series α-(4-cyanobiphenyl-4'-oxy)-ω-(1-pyreniminebenzylidene-4'-oxy) alkanes (CBOnO.Py) with n ranging from 3 to 11, as a function of temperature. A previous thermal behavior study through heat capacity measurements has been made. Dielectric measurements enable us to obtain information about the molecular dynamics in the nematic mesophase as well as in the isotropic phase. Two orientations (parallel and perpendicular) of the molecular director with regard to the probe electric field have been investigated. In the nematic mesophase, the dielectric anisotropy is revealed to be positive for all studied compounds. Measurements of the parallel component of the dielectric permittivity are well-explained by means of the molecular theory of dielectric relaxation in nematic dimers (Stocchero, M.; Ferrarini, A.; Moro, G. J.; Dunmur, D. A.; Luckhurst, G. R. J. Chem. Phys., 2004, 121(16), 8079). The dimer is seen as a mixture of cis and trans conformers, and the model allows us to estimate their relative populations at each temperature. The main molecular motions are interpreted by the model as independent end-overend rotations of each terminal semirigid unit of the dimer.

Synthesis and Characterization of Nonsymmetric Liquid Crystal Dimer Containing Biphenyl and Azobenzene Moiety

Calamitic liquid crystalline dimer containing azobenzene moiety and a decyloxy biphenyl linked by flexible spacers {4-[7-(4′-decyloxy-biphenyl-4-yloxy)-alkyloxy]-phenyl}-(4-decyl-phenyl)-diazene has been synthesized and characterized by spectroscopic methods. The transition temperatures and phase behaviours were studied by Differential Scanning Calorimeter (DSC) and Polarizing Optical Microscope (POM). The synthesized compounds exhibited enantiotropic liquid crystal phase with higher spacer display nematic and smectic C phases while lower spacer shows nematic and smectic A phases.