Symmetric Liquid Crystal Dimers Research Articles

Synthesis, mesophase behavior, and computational studies of dimers composed of three-ring rod-shaped ester-imine linkage monomeric unit

New symmetrical liquid crystal dimers composed of three-ring ester imine rod-shaped monomers with flexible alkyl chains of different parity were synthesized. The thermal stability of the newly synthesized compounds was determined using thermogravimetric analysis. The phase transition and mesogenic behavior of the compounds were explored using polarising optical microscopy and differential scanning calorimetry. Dimers with odd parity spacers exhibited the phase sequence Iso-N-Nx-SmX-Cr. The Nx phase transition was observed by polarising optical microscopy. This transition was distinct from that of the uniaxial nematic phase. The Nx phase was absent in the dimer with even parity spacers. A density functional theory study was carried out to investigate the stable configuration and influence of the solvent on the structural parameters, chemical reactivity parameters, and excited state properties of the molecules.

Exploring the nematic to twist-bend nematic phase transition: calorimetric and optical measurements

ABSTRACT We investigate the nematic (N) to twist-bend nematic (NTB) phase transition in binary mixtures of a symmetric liquid crystal dimer (CB7CB) and a calamitic bi-phenyl mesogen (7OCB). As the molar concentration of 7OCB increases, both the Iso–N and N–NTB transition temperatures decrease gradually, consequently, the width of the nematic phase above the NTB phase can suitably be varied from ~12 K to 46 K. Notably, as the nematic range expands, there is a remarkable reduction in transitional entropy, eventually approaching negligible or zero values. Additionally, the heat capacity peak and the phase shift disappear when transitional entropy reaches zero. This behaviour diverges from typical N–SmA and N–SmC phase transitions, where transitional entropy decreases near the tricritical point but the heat capacity peak persists. Our findings indicate a finite positive δ(∆n) across all concentrations at the N–NTB phase transition, suggesting a gradually weakening first-order nature. This contrasts with theoretical predictions of a second-order N–NTB phase transition, as our experimental data consistently supports a first-order nature. The self-consistent mean-field Landau Model successfully validates our experimental data, establishing the universal first-order nature of the N–NTB phase transition, with experimental entropy values in good agreement with theoretical prediction.

Effects of polar end group, flexible spacer length and molecular conformation on properties of the liquid crystal dimers

ABSTRACT Two series of symmetric liquid crystal dimers, namely 1F and 3F, have been synthesised by varying the length of the flexible odd spacer. The melting temperatures and clear points of the 3F series are higher than those of the 1F series. The 1F series only exhibits nematic phase, while the 3F series displays both smectic A and nematic phases. As the length of the flexible spacer increases, the dominance of the smectic phase becomes more pronounced. This phenomenon may be attributed to the combined effect by the polar end group CF3 and the long flexible spacer on the molecular conformation.

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.

Dielectric Study of Liquid Crystal Dimers: Probing the Orientational Order and Molecular Interactions in Nematic and Twist-Bend Nematic Phases.

Dielectric spectroscopy in frequencies that range from 10 Hz to 1 GHz has been used to study the molecular orientational dynamics of the two types of dimers that form the twist-bend nematic phase over a wide range of temperatures for both nematic and twist-bend nematic phases. The symmetrical and asymmetrical liquid crystal dimers with the cyanobiphenyl mesogenic groups were investigated. The results were analyzed within the framework of the molecular theory of dielectric permittivity for nematogens. The two molecular processes can be assigned to the reorientation of the monomeric unit: the high frequency one to the precessional rotation of the longitudinal components of the cyanobiphenyl groups (CN) and the second (low frequency) to the end-over-end rotation of the CN dipole around the short molecular axis. The precession mode, which is determined by the local order and is almost unaffected by the phase transition from the nematic to the twist-bend phase, while the end-over-end rotation clearly slowed down at the transition, as it is affected by the growth of the intermolecular interactions. The latter corresponds well to the fact revealed by IR spectroscopy about the longitudinal correlation of the molecular dipoles.

The effects of alkylthio chains on the properties of symmetric liquid crystal dimers

The properties of four series of dimers are discussed to investigate the effect of thioether links on their mesogenic behaviour.

Comparative dielectric and thermally stimulated-depolarization-current studies of the liquid crystal dimers 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane and heptane and a binary mixture between them, close to the glass transition.

We have performed dielectric spectroscopy and thermally stimulated-depolarization-current experiments to study the molecular dynamics of the twist-bend nematic phase close to the glass transition of two members of the 1″,7'-bis(4-cyanobiphenyl-4'-yl)alkane homologous series (CBnCB): the liquid crystal (LC) dimers CB9CB and CB7CB, as well as a binary mixture of both. By doping CB9CB with a small quantity of CB7CB, the crystallization is inhibited when cooling the sample down, while the bulk properties of CB9CB are retained and we can investigate the supercooled behavior close to the glass transition. The study reveals that the inter- and intramolecular interactions of the mixture are similar to those of pure CB9CB and confirms that there is a single glass transition in symmetric LC dimers.

Phase transitions in a high magnetic field of an odd, symmetric liquid crystal dimer having two nematic phases, N_{U} and N_{TB}, studied by NMR spectroscopy.

Both ^{1}H and ^{13}C NMR spectra have been obtained in a static magnetic field of 23.5 T on a bent-shaped dimer molecule, 1^{''},7^{''}-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB), which shows the sequence of liquid crystal phases twist-bend nematic, N_{TB}, and uniaxial nematic, N_{U}, before entering the isotropic phase. The ^{1}H spectra are used to locate the temperature at which the sample melts to form a twist-bend nematic, T_{CrN_{TB}}, and then T_{N_{U}I} when the isotropic phase is entered, both in a magnetic field of 23.5 T, and to compare these with those measured at the Earth's field. The differences between these transition temperatures are found to be zero within the error in their measurement, in stark contrast to previous measurements by Salili et al. [Phys. Rev. Lett. 116, 217801 (2016)10.1103/PhysRevLett.116.217801]. In the isotropic phase in the presence of the field the sample exists in a paranematic phase in which the molecules of CB9CB are partially ordered. The ^{1}H and ^{13}C NMR spectra in the paranematic phase are used to measure the critical temperature T* below which this phase is unstable. The spectra are also used to study the structure, molecular orientational order, and distribution of molecular conformations in the paranematic phase.

Birefringence and photoluminescence properties of diphenylacetylene-based liquid crystal dimers

We herein report phase transitions, mesomorphism, birefringence behavior and photoluminescence properties of symmetric liquid crystal (LC) dimers based on diphenylacetylene or tolane.

Fluorinated chiral nematic liquid crystal dimers based on (S)-1-phenylethane-1,2-diol

ABSTRACTA series of symmetric liquid crystal (LC) dimers with the same chiral core (S)-1-phenylethane-1,2-diol ((S)-PE) have been synthesised, termed TBDA-(S)-PE, 3F3B-(S)-PE, 3F2B-(S)-PE, 1F3B-(S)-PE, 1F2B-(S)-PE, respectively. Chemical structures and LC properties of the five symmetric LC dimers were characterised by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectrometer (1H NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and polarised optical microscopy (POM). TBDA-(S)-PE displayed enantiotropic chiral smectic A (SmA*) phase while 3F3B-(S)-PE, 3F2B-(S)-PE, 1F2B-(S)-PE all displayed enantiotropic chiral nematic (N*) phase and 1F3B-(S)-PE displayed monotropic N* phase. The results indicated that the removal of the flexible spacers between the rigid mesogenic arm and the chiral core facilitated the formation of the N* phase. When the rigid mesogenic units are connected to the chiral core directly, the structure of the terminal fluorine group and the rigidity of the mesogenic unit played certain influence on the thermal properties of the LC dimers, but did not change the type of mesomorphic phase. Compared to 3F3B-(S)-PE and 1F3B-(S)-PE, 3F2B-(S)-PE and 1F2B-(S)-PE displayed wider LC ranges, respectively, suggesting molecular regularity had greater influence on LC-isotropic (I) transition temperature.

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

ABSTRACTA series of new asymmetrical liquid crystal dimers, RnO–S2O–ORn (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 RnO–S2O–ORn (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.

The influence of flexible spacer and the chirality of the core on the formation of chiral nematic phase of symmetric dimers containing trifluoromethyl terminal

ABSTRACTSeven symmetric liquid crystal (LC) dimers containing different chiral cores and LC arms have been synthesised, termed EPDA-(R,S)PD, TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD, TFBA-(R)PD, TFBDA-IB and TFBA-IB, respectively. TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD and TFBDA-IB displayed chiral smectic A (SmA*) phase, while EPDA-(R,S)PD, TFBA-(R)PD and TFBA-IB exhibited chiral nematic (N*) phase. The effects of flexible spacer, structural type of LC arms and the chirality of the cores on the thermal properties of the dimers and the formation of N* phase have been studied. The results indicated that the chiral core was prone to induce the N* phase in LC dimer that contained nematic arms although the chirality of the core is very weak, while for the smectic LC arms containing CF3 terminal, the removal of the flexible spacer between chiral core and rigid LC units was conducive to the formation of N* phase. For example, TFBDA-(R)PD and TFBDA-IB displayed SmA* phase, while TFBA-(R)PD and TFBA-IB exhibited N* phase. However TFBA-(R,S)PD did not display N* phase, which reminded us that the chirality of the core and the conformation of the dimer also played an influence in the formation of the LC phase.

Synthesis and mesomorphic behavior of symmetrical chiral liquid crystal dimers incorporating ester linked biphenyl- naphthyl cores and terminal vinyl group

ABSTRACTA series of symmetrical chiral, liquid crystal dimeric molecules possessing ester- linked, biphenyl-naphthyl cores with varied spacer lengths and terminal vinyl groups have been synthesized using Naproxen as the synthetic precursor. The synthesized symmetrical chiral dimers were characterized by 1H NMR spectroscopy, and their liquid crystalline behavior was confirmed by DSC and HOPM studies. Structural effects on the mesomorphic and physicochemical properties were investigated in terms of variation of chiral chain length. The synthesized dimeric compounds exhibited SmX*, SmC*, SmA*, N*, BPI*, and BPII* mesophase sequences. An odd-even effect was observed in the dimers and the duration of the mesophase decreased with increasing spacer length. The synthesized vinyl substituted liquid crystalline dimers are particularly useful in understanding liquid crystal polymorphism and act as model compounds for liquid crystal polymers.

Twist-bend nematics, liquid crystal dimers, structure–property relations

ABSTRACTOne of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (NTB) and determine its properties. During our earlier hunt for the NTB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the NTB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the 2H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.

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.

Similarities and differences between molecular order in the nematic and twist-bend nematic phases of a symmetric liquid crystal dimer.

The order parameter, Szz, where z is the para axis of the difluoroterphenyl groups in DTC5C9, has been obtained from chemical shift anisotropies measured by (13)C - {(1)H} NMR experiments at temperatures throughout the nematic, NU, and twist-bend nematic, NTB, phases shown by this compound. The order parameter temperature profiles are unusual in having a maximum value in the NU phase and then decreasing until the NTB phase is reached. There is a small discontinuity (∼2%) in Szz at TNNTB and then a gradual decrease until a new phase appears. This behaviour is interpreted as revealing a temperature-dependent tilting of local directors in both phases away from the applied magnetic field direction. In the enantiomorphic twist-bend phase this tilt is consistent with the structure of the phase as a helical arrangement of local directors, whilst in the high-temperature non-chiral nematic the tilt must involve a non-chiral arrangement. It is proposed that in both phases the tilting of directors has a common origin in the bent shape of the molecules.

Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases

We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.

Preparation and Mesomorphic Characterization of Supramolecular Hydrogen-Bonded Dimer Liquid Crystals

Two new series of hydrogen bonding liquid crystal dimers were prepared and their self-assemble behavior and structure–property relationships were studied through intermolecular hydrogen bond formation. The first series of symmetric liquid crystal dimers have two terminal alkoxy chains and the second have four terminal alkoxy chains. Their structures were elucidated using spectroscopic techniques, elemental analysis, and powdered X-ray diffraction. The mesophases and their corresponding transition temperatures were identified by differential scanning calorimetric measurements and polarized optical microscope.

Synthesis and Mesomorphic Properties of New Methylene-Linked Linear Symmetrical Liquid Crystal Dimers

A series of methylene-linked symmetrical liquid crystal dimers were synthesized. Their structures were confirmed by physicochemical techniques. Differential scanning calorimetry and polarizing optical microscopy verified the liquid crystal behaviors and their transition temperatures. The thermodynamic data for phase transition, melting transition (Tm), and isotropic transition temperatures (Ti) of the dimers were decreased with increasing the length of spacer. Both Tm and Ti and enthalpy change for isotropic transition (δHi) for dimers with even numbers of methylene units were higher than those with odd numbers. Dimers AC-OnO-AC (n = 2–10) formed enantiotropic nematic phase only, while the unsymmetrical dimer formed a smectic phase.

A Comparison of the Conformational Distributions of the Achiral Symmetric Liquid Crystal Dimer CB7CB in the Achiral Nematic and Chiral Twist-Bend Nematic Phases

The sets of residual dipolar couplings between carbon and hydrogen nuclei obtained from the proton-encoded (13)C 2D NMR experiment are used to investigate the conformational changes which occur when the achiral symmetric liquid crystal dimer CB7CB changes from the achiral nematic to the chiral twist-bend nematic phase. It is found that these changes are a consequence of the chirality of the twist-bend nematic phase, rather than being the driving force for the stability of this phase.