A new series of liquid crystalline dimers containing a cholesteryl group were synthesised, exhibiting multiple mesophases including chiral smectic phases (SmC* and SmA*), twist grain boundary phases ...(TGBA*), and chiral nematic phase (N*). The chemical structure and liquid crystal properties of the dimers were characterised by FTIR,
1
H-NMR, DSC, POM and XRD. It was in evidence that the lengths of the terminal tail and the flexible spacer were the decisive factors of the mesophase behaviours.
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•A new series of liquid crystalline dimers containing a cholesteryl group were synthesized.•An unknown smectic phase (SmX*) was analyzed and found sharing some similar liquid crystal ...properties with SmC*.•Re-entrant phenomenon occurs, which is rarely reported for pure matters of dimeric liquid crystals.•NL* phase is further analyzed, based on a few previous researches.
A new series of liquid crystalline dimers containing a cholesteryl group were synthesized, exhibiting multiple mesophases including chiral smectic phases (SmX*, SmC* and SmA*), twist grain boundary phases (TGBA* and TGBC*), and chiral nematic phase (N*). Re-entrant phenomenon occurs, which is rarely reported for pure matters of dimeric liquid crystals. The chemical structure and liquid crystal properties of the dimers were characterized by FTIR, 1H NMR, DSC, POM and XRD. It was in evidence that the length and the parity of the spacer and the length of the terminal tail were the decisive factors of the mesophase behaviours.
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
o
C. 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.
Chirality induces structural frustration in liquid crystal systems, producing various kinds of chiral frustrated phases, for example, twist grain boundary (TGB) phases, blue phases (BPs) and dark ...conglomerate (DC) phases. Almost all molecules exhibiting these frustrated phases have a rigid shape. Especially, a bent-core unit is regarded as a key structure for BPs and DC phases. This paper describes that some flexible liquid crystal oligomers being far from a rigid bent-core molecule stabilise these phases. The LC oligomers have a supermolecular structure in which mesogenic units are connected via flexible spacers. By designing intermolecular interactions, they can exhibit various molecular packing structures in the liquid-crystalline phases as follows: chiral dimers inducing TGB phases, U-shaped and T-shaped oligomers stabilising BPs and achiral liquid crystal trimers exhibiting DC phases. I discuss how the designed liquid crystal oligomers produce the chiral frustrated phases.
We report on the stabilisation of the liquid-crystalline, twist-grain boundary A (TGB
A
) phase in mixtures of a chiral liquid crystal and surface-functionalised spherical Au nanoparticles (NPs) of ...10 nm diameter. The results, obtained by calorimetric, optical, small-angle X-ray and plasmon resonance measurements, demonstrate that a TGB
A
phase, which is metastable for the pure liquid crystal host, can be effectively stabilised for a 3 K range in the presence of NPs. Moreover, the role of NPs size on the TGB
A
stabilisation is briefly discussed.
Frustration is a powerful mechanism in condensed matter systems, driving both order and complexity. In smectics, the frustration between macroscopic chirality and equally spaced layers generates ...textures characterized by a proliferation of defects. In this article, we study several different ground states of the chiral Landau–de Gennes free energy for a smectic liquid crystal. The standard theory finds the twist grain boundary (TGB) phase to be the ground state for chiral type II smectics. However, for very highly chiral systems, the hierarchical helical nanofilament phase can form and is stable over the TGB.
One of the most frequently utilized liquid crystalline (LC) materials is a rod-like (calamitic) compound 4-cyano-4′-pentylbiphenyl (5-CB). The main objective of this work is to enhance its ...functionality by introducing a photoresponsive diazenyl spacer in the aromatic core and replace the non-chiral pentyl chain with various chiral alkyl carboxylate units. The mesomorphic properties of the prepared materials have been studied using polarizing optical microscopy and differential scanning calorimetry. It has been found that materials with an extended aromatic system possess the liquid crystalline behavior. The studied LC materials have shown mesophases at lower temperatures than previously reported analogous substances. Furthermore, one of them exhibits a chiral orthogonal frustrated twist grain boundary smectic phase, which has not been previously observed for this structural type of materials. We also investigated photoresponse of the mesophases under illumination with UV-light (365 nm) using a polarizing optical microscope. A non-conventional photoresponse of the prepared materials in a crystalline phase is presented and discussed.
We report on the appearance and stabilisation of twist-grain-boundary-A
(TGB
) phase in confined geometry samples of the liquid-crystalline compound CE8, by means of polarised optical microscopy. The ...chiral liquid crystal (LC) compound was chosen such as to undergo a cholesteric-smectic-A
transition with temperature (type-I smectic), and to be close to the triple point between the cholesteric, TGB
, and smectic-A
phases. We used several types of anchoring conditions and various effective anchoring strengths. The results reveal that (i) the TGB
phase exists as a metastable phase with strong fluctuations in the bulk, and (ii) it can be stabilised by the sample boundaries for adequate anchoring conditions. This means that we probably observed a type-I to type-II smectic transition induced by the substrate. Some dependence of the stabilisation on the surface-anchoring conditions and on the cell thickness is reported.
Several series of chlorine-substituted rod-like liquid crystalline compounds have been synthesised containing a different number of lactate units in the chiral chain. Sequences of phases and phase ...transition temperatures were determined by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction studies. Depending on the molecule structure, the materials possess a rich variety of mesophases, i.e. the paraelectric chiral smectic A (SmA*), ferroelectric chiral smectic C (SmC*) and antiferroelectric smectic C (SmC
A
*) phases. Additionally, some compounds with one and two lactate groups exhibit the frustrated antiferroelectric twist grain boundary (TGB
A
) and smectic Q (SmQ) phases, respectively. The existence of the SmQ phase ordered in two directions was confirmed by X-ray diffraction on a non-oriented sample. The effects of the non-chiral chain length and number of the chiral lactate groups on the mesomorphic properties, spontaneous polarisation, spontaneous tilt angle and complex permittivity have been studied and discussed.
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