Nitrogen-containing fused heterocycles as the structural fragments in calamitic liquid crystals

This review examines in some detail the effect of introducing nitrogen-containing fused heterocycles into the molecular core of calamitic liquid crystals on their physico-chemical properties. The results of this study are compared with those obtained for other molecular fragments, and are rationalized in terms of existent theories.     

Germanium liquid crystals

Liquid-crystalline compounds containing germanium atoms were synthesised and assessed for liquid-crystalline properties. These new compounds generally possess smectic C phases, and many also possess nematic, smectic A and higher order smectic phases. The germanium-containing liquid crystals were incorporated into smectic C mixtures. These mixtures tend to exhibit little change in smectic C*?layer thickness over temperature. This characteristic is associated with de Vries smectic A materials, but measurements show that, although they have high smectic C stability, the materials' smectic cone angles are small. Measurement of smectic cone angle versus temperature of an exemplar material and its analogues containing carbon and silicon in place of the germanium, all show small cone angles which fall smoothly and extrapolate to zero as the smectic C*?to smectic A transition is approached. These measurements largely explain the observed small layer changes and establish that the materials are not first-order de Vries materials. They must be located elsewhere along the de Vries-orthogonal continuum of smectic A phases.     

Oligomeric rod-disc nematic liquid crystals

Six new oligomeric nematic liquid crystals are reported consisting of a triphenylene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers.     

The effect of chiral doping in achiral smectic liquid crystals on the de Vries characteristics: smectic layer thickness,electro-optics and birefringence

An addition of chiral dopant to two achiral smectic liquid crystals from a homologous series, by varying weight percentages with known low values of layer shrinkage, leads to chiral smectic-C* phase with a finite value of the spontaneous polarisation. The electro-optical response arising from changes in the induced apparent tilt angle brought about by a weak electric field in the SmA* phase gives rise to power law dependency on the reduced temperature. The critical exponent γ of the power law depends on the dopant concentration but its value is found to be greater than the typical value of 1.32. This implies that the short-range correlation extends from two dimensions to three dimensions in these materials in the SmA phase. The layer thickness of smectic layers in the guest–host system remains unaltered up to the 15 wt % addition of the chiral dopant to two achiral smectics. The system thus retains the low layer shrinkage of the achiral smectic as evidenced by measurements of the layer thickness from X-ray scattering and thickness measurements from optical interferometry. Results on the optical birefringence and the apparent tilt angle lead us to the conclusion of having successfully obtained chiral smectic materials for devices with de Vries characteristics by chiral doping.     

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%.     

From colloids in liquid crystals to colloidal liquid crystals

ABSTRACT

Liquid crystals in combination with nanoparticles are a fascinating topic of research, because of the wealth of aspects and questions to study. These range from simple effects of nanoparticles on phase transitions and phase diagrams, to the tuning of physical properties, adding of novel functionalities, all the way to the formation of spontaneous order by nanoparticles themselves and the possibilities that templating has for future materials design and applications. This article intends to provide a flavour of the multiplicity, variety and diversity that these thermotropic and lyotropic systems have to offer in the area of materials development, which we believe will become increasingly important, especially for switchable non-display applications and nanotechnology. It is not intended to provide a conclusive overview, which would be a presumptuous attempt considering the limited space available, but rather to place our own work into a wider context and to point out some more recent developments and trends in liquid crystal – nanoparticle dispersions.     

From monomolecular films to bulk liquid crystals

Molecules forming thermotropic liquid crystal bulk phases, were investigated at the air-water interface. With nematic and smectic substances, compression of the film resulted in an increase of the lateral pressure, provided the molecules had an amphiphilic character. Two types of π-A-diagrams were found for areas below a threshold value: (i) A single coexistence region (constant pressure at different areas) is obtained for molecules forming nematic liquid crystal bulk phases. The original monomolecular film grew in the third dimension to form a nematic liquid crystal at the interface. (ii) The π-A-diagram showed several coexistence regions with constant pressures if the molecules had the capacity to form smectic bulk phases. The “collapses” were at areas in the relation 1 : 1/2 : 1/3 etc. suggesting that smectic layers have been formed. The aqueous subphase is found to make a large contribution to the latent heat. The initial layer (together with the subphase) is found to be of lower symmetry than the upper layer(s). The transition is first order at low temperature (< 30°C) and second order at high temperatures. Evidences are given for a smectic Shubnikov phase where the layer-to-layer material flow is concentrated in flux lines.     

From tectons to luminescent supramolecular ionic liquid crystals

New phosphorescent and room-temperature liquid-crystalline materials were obtained by combining dicyanometallate anions with dicationic bisamidinium based tectons bearing four peripheral lipophilic pyrogallate moieties.     

From mushroom alcohol to liquid crystals: a useful platform molecule

The syntheses and liquid crystal properties of two novel esters derived from 4-(4-(decyloxy)phenyl)thiophene-2-carboxylic acid and either (±)-oct-1-en-3-yl 4?-hydroxybiphenyl-4-carboxylate or (S)-(+)-oct-1-en-3-yl 4?-hydroxybiphenyl-4-carboxylate are reported. Within the synthesis of the (S)-(+)-oct-1-en-3-yl 4?-hydroxybiphenyl-4-carboxylate, mushroom alcohol, a natural source of chiral oct-1-en-3-ol and a platform molecule, was employed. The phases present within these compounds have been characterised by thermal optical polarising microscopy and differential scanning calorimetry and assigned as SmA, SmC and SmC Alt for the racemic compound and; SmA, SmC* and SmC*_A for the enantiomerically pure compound. This is first reported occurrence of a liquid crystalline ester derived from mushroom alcohol, and potential platform molecule, exhibiting SmA*, SmC* and SmC*_A phases.     

From smectic to columnar phase of polypedal liquid crystals based on tetrathiafulvalene/1,3-dithiol-2-thione and cholesterol

Bipedal 1,3-dithiole-2-thiones attached two cholesteryl through a ω-thioalkanoyloxy spacer of varying length were synthesized. The bipedals were easily transformed to the appropriate tetrapedal tetrathiafulvalene derivatives by self-coupling reaction in net triethyl phosphite. All the synthesized compounds exhibit mesogenic phases in a wide temperature region, no crystallization but vitrifying to form glassy mesogens during cooling from the isotropic melt. The liquid crystals with shorter spacer (n=2-6) exhibited only a smectic A phase and those with the longest spacer (n=7) exhibited only a hexagonal columnar. In these series, the molecular packing depended on the length of spacers.     

Oligomeric tetrathiafulvalenes: a new route toward conjugated TTF dimers and trimers

The synthesis of bis-TTF 1 and tris-TTF 2, linked by an ethenyl spacer, is described using new useful Wittig-type reagents with the introduction of phosphonate functionality on 1,3-dithiole and TTF frameworks. Electrochemical and spectroscopic studies show intramolecular electronic interactions between conjugated TTF units.     

Relaxation,de Vries behavior,and layer contraction in chiral liquid crystal by dielectric spectroscopy

Low-frequency (LF) (20 Hz to 10 MHz) field response investigations are reported in TSiKN65F series-based chiral liquid crystal compound, C-10, viz., 1-[3?-nitro-4?-S-(2-decyloxy)phenyl]phenyl-4-(1,1,1,3,5,5,5-heptamethyltrisiloxydecyloxy)-3-Fluorobenzoate. Molecular frame with 10-methylene units and a chiral center on one end is connected to the core with 3-phenyl rings. Other end of core connected to biforked trisiloxy chain to promote de Vries phase. Isotropic-smectic-A and smectic-A-smectic-C*_deVr transitions are determined by capacitance study. DC field reversal and polarized optical microscopy textures confirmed occurrence of SmC*_deVr phase below SmA. Hysteresis is observed in SmC*_deVr. Tilt order parameter growth estimated in SmC*_deVr phase through β equal to 0.673 agrees with mean field prediction. Relaxation behavior in kilo Hertz region is explained through longitudinal dipole moment μ_l reorientation. Dynamics studied through LF dielectric relaxation infers dominance of order director fluctuations at few kilohertz. Higher activation energy in de Vries phase infers greater degrees of freedom and additional constraints. Loss ε″(T) exhibited anomalous trend at SmA-to-SmC*_deVr interface. Temperature variation of tilt θ(T) is estimated from loss. Loss in de Vries phase gets suppressed by field to infer collective response. Curie–Weiss behavior by γ-exponent ~0.03 infers weaker ferroelectric response in SmC*_deVr phase. Loss variation ε″(T) addressed by perturbed bookshelf model reveals marginal layer contraction by <1% in SmC*_deVr phase. Estimates of layer shrinkage through R- and f-parameters in SmC*_deVr phase confirm the de Vries behavior.     

H-shaped liquid crystals inducing nematic order in the isotropic liquid

ABSTRACT

We prepared a homologous series of H-shaped liquid crystals I-n and investigated their phase transition properties using optical microscopy and differential scanning calorimetry. All the compounds exhibited a nematic phase at room temperature. The phase transition behaviour is explained in terms of molecular shape anisotropy. Furthermore, those compounds were found to exhibit electro-optical switching in the isotropic liquid in the vicinity of the nematic–isotropic liquid transition, indicating that the microscopic nematic order with a certain coherence length of the molecules exists in the optically isotropic temperature range.     

From rods to knots using cyclohexane in smectic liquid crystals

The effects of using cyclohexane as a bulky terminal group or as a linkage between multiple mesogens using 1,4-cyclohexane dicarboxylic acid and 1,3,5-cyclohexane tricarboxylic acid has been investigated. We demonstrate that the diversity of mesomorphism is reduced as the complexity of the molecule increases such that twist grain boundary phases, chiral nematic, ferroelectric and antiferroelectric behaviour are observed in the mono compounds but only ferroelectric behaviour is observed for the trimeric compounds. The electro-optical behaviour is discussed with a focus on the ferroelectric behaviour and the nature of the twist grain boundary phase.     

Synthesis and photoswitching properties of bent-shaped liquid crystals containing azobenzene monomers

Three novel bent-shaped monomers, namely 1,3-phenylene bis-{4-[4-(n-allyloxyalkyloxy)phenylazo]benzoate} 5a–c, containing azobenzene as side arms, resorcinol as central units and terminal double bonds as polymerisable functional groups were synthesised and characterised. The mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements and it was found that all three compounds display SmA_intercal mesophases. These bent-shaped molecules exhibit strong photoisomerisation behaviour in solutions in which trans to cis isomerisation takes about 50 seconds whereas the reverse process takes almost 31 hours.     

High-pressure phase transitions in liquid crystals

Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

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Zusammenfassung In einem p-V-T-kontrollierten Scanning Kalorimeter wurden zur Untersuchung der Phasenumwandlungen in Flüssigkeitskristallen simultane Messungen der Geschwindigkeit der Wärmeentwicklung und der Veränderungen von mechanischen Größen von Umwandlungen, wie z.B. von Volumen oder Druck durchgeführt. In diesem Gerät kann die Phasenumwandlung durch eine kontrollierte Veränderung von Druck, Volumen oder Temperatur unter isothermen, isobaren oder isochoren Bedingungen ausgelöst werden. Vorliegende Untersuchungen wurden an 4-n-Pentyl-phenylthiol-4-dezyloxybenzoat durchgeführt, welches im Flüssigkristallzustand eine nematische und drei smektische Phasen aufweist.

     

New chiral liquid crystal monomers and cholesteric polyacrylates: synthesis and characterisation

The synthesis of new chiral monomers (M₁ ?M₃ ) based on menthol and the corresponding polyacrylates (P₁ ?P₃ ) is described. The chemical structures, formula and phase behaviour of the obtained monomers and polymers were characterised with FT-IR, ¹H-NMR, elemental analyses, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). The effect of the mesogenic core rigidity, spacer length and menthyl steric effect on the phase behaviour of M₁ ?M₃ and P₁ ?P₃ is discussed. The expected mesophase of the compounds based on menthol can be obtained by inserting a flexible spacer between the mesogenic core and the terminal groups. For the chiral monomers and polyacrylates, their corresponding melting temperature (T _m), glass transition temperature (T _g) and clearing temperature (T _i) increased with an increase of the mesogenic core rigidity; while the T _m, T _g and T _i decreased with increasing the spacer length. M₁ and P₁ showed no mesophase, while M₂ and M₃ all revealed a SmC* and cholesteric phases. P₂ and P₃ only showed a cholesteric phase.