S. Radhika

Structure–property correlation of a hockey stick-shaped compound exhibiting N-SmA-SmCa phase transitions

We report the measurement of birefringence, dielectric constant, splay, bend elastic constants and rotational viscosity in the nematic phase (N) of a hockey stick-shaped compound exhibiting smectic-A (SmA) and anticlinic smectic-C (SmCa) phase transitions at lower temperature. It is found that the physical properties are significantly different than calamitics and partially similar to the bent-core liquid crystals. It exhibits the positive dielectric and the negative elastic anisotropy. The optical and the thermal measurements show that all the transitions are first order. The rotational viscosity is comparatively higher than calamitic liquid crystals and exhibits weak pre-transitional divergence. The temperature dependent static dielectric measurements indicate antiparallel correlation of both the longitudinal and the transverse components of dipole moments in both the smectic phases. The parallel component of the dielectric constant exhibits a single Debye type relaxation in all the phases. The activation energy in the N, SmA and SmCa phases are comparatively larger than that of the respective phases of calamitic liquid crystals and can partially be attributed to the higher rotational viscosity of the hockey stick-shaped compound.

Randomly polarised smectic A phase exhibited by bent-core molecules: experimental and theoretical studies

We report experimental studies on a newly synthesised bent-core liquid crystal exhibiting partial bilayer SmAdPR and SmAdPA phases. Though the SmAdPR phase appears to be optically uniaxial as that of the SmA phase, the observed distinctive properties of the SmAdPR phase are (a) a current peak associated with a polarisation reorientation process under the application of a triangular wave voltage, (b) large variation of the effective birefringence of the planarly oriented sample with the applied in-plane electric field and (c) the presence of a strong dielectric mode. We have developed a simple theoretical model to describe the observed properties of the SmAPR phase and the transition from the SmAPR to the SmAPA phase. We propose that the above characteristic properties of the SmAPR phase arise due to the strong coupling between the transverse polarisation and the transverse polar order of the bend directions of the bent-core molecules in the layers.

Anticlinic smectic C phase in new and novel five-ring hockey stick-shaped compounds

The synthesis of a new homologous series of novel five-ring compounds composed of hockey stick-shaped molecules derived from 4-hydroxybiphenyl-3-carboxylic acid which exhibit an interesting sequence of phase transitions is presented. Uniaxial nematic and smectic A phases, anticlinic smectic C phase and an unidentified smectic phase at a lower temperature than the latter have been observed. This is perhaps the first example of hockey stick-shaped compounds exhibiting anticlinic smectic C phase directly from the isotropic phase. Polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction measurements and electro-optical switching studies were carried out to identify the mesophases.

Occurrence of a transition between two polar orthogonal smectic phases in a new homologous series of novel bent-core compounds

Two new homologous series of isomeric achiral five-ring bent-core compounds containing 3,4-disubstituted biphenyl moiety have been synthesised and the mesophases that they exhibit characterised. Compounds of series A are obtained from 4-hydroxybiphenyl-3-carboxylic acid while those in series B are prepared by reversing the position of the hydroxyl and carboxylic groups. This change seems to have a profound effect on the mesophase properties. Interestingly, for compounds of series A containing the long terminal chain, a transition between two polar orthogonal smectic phases has been observed. The observed mesophases have been characterised using a combination of polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical measurements. This is the first example of a transition between two partial bilayer polar orthogonal smectic phases. In addition, the effect of a fluoro substituent ortho to the strongly polar terminal cyano group on the mesophase has also been examined.

Two distinct modulated layer structures of an asymmetric bent-shape smectic liquid crystal

We present chemical synthesis, polarising optical microscopy, electric current, and small-angle X-ray scattering measurements on a strongly anisotropic bent-shape liquid crystal material. We find that it has two layer-undulated ferroelectric phases, M1 and M2. In the higher temperature M2 phase the undulation amplitude and period increases on cooling, similar to other published materials. It can be identified with a tilted modulated phase. In the M1 phase the molecular plane is not tilted, and – in sharp contrast with all prior observations – the modulation amplitude and period decrease on cooling. These observations can be explained with a ‘leaning’ director structure, where the leaning angle is decreasing on cooling.

Strain glass analogue in a bent-core liquid crystal exhibiting the dark conglomerate phase

Vitrified states formed from the dark conglomerate liquid crystal (DCLC) phase, by two members of a homologous series made of bent-core molecules derived from 2,7-dihydroxynaphthalene under different cooling procedures, have been characterised using a variety of experimental techniques, such as optical microscopy, differential scanning calorimetry, X-ray diffraction, electro-optics, dielectric spectroscopy and atomic force microscopy. The experimental results show that though the vitrified state formed on gradual cooling of the sample from the DC phase has some glassy features, it is fragile and undergoes crystallisation over time. On the other hand, direct quenching of the DC phase to room temperature results in stronger glass formation. We propose that the vitrified state obtained on progressive cooling of the DC phase may have some analogy to strain glass and that the DC phase itself may be rather like unfrozen strain glass.

Novel zigzag-shaped compounds exhibiting apolar columnar mesophases with oblique and rectangular lattices

The design, synthesis and characterisation of some compounds whose constituent molecules have a completely new shape have been carried out. A total of eight different series/part of series of these zigzag-shaped compounds that are either symmetrical or unsymmetrical were synthesised. Only two mesophases were observed, and on the basis of polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies, these have been characterised as columnar phases with a rectangular or an oblique lattice. Perhaps, these represent the first examples of a rigid zigzag-shaped compound exhibiting a mesophase.

Apolar novel mesogenic symmetric dimers composed of five-ring bent-core monomeric units

The synthesis and mesomorphic properties of parts of six different series of symmetric dimers composed of five-ring bent-core monomeric units have been investigated. By design, all the dimers are apolar as all the dipoles of the monomers cancel with each other. The mesomorphic properties have been investigated (1) as a function of the length of the terminal chain for a fixed spacer unit, and (2) as a function of the length of the spacer unit for a fixed terminal chain length. Interestingly, two orthogonal lamellar phases have been observed in many of these dimers which differ from one another only in the layer spacing. The chemical structure of the dimers has been determined using a combination of spectroscopic techniques and elemental analysis. The mesophases are characterised using complementary methods such as polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical measurements.

The SmCPA phase in five‐ring bent‐core compounds derived from 5‐methoxyisophthalic acid

The synthesis and mesophase characterization of a homologous series of five‐ring bent‐core compounds derived from 5‐methoxyisophthalic acid are described. Most of the compounds exhibit a polar antiferroelectric smectic C phase. Replacement of the terminal n‐alkoxy chains by n‐alkyl carboxylate groups, not only destabilizes the formation of mesophases but induces a calamitic mesophase. However, extension of the arms of the bent‐core molecule by a phenyl moiety stabilizes the switchable phase. The mesophases were investigated using a combination of polarizing optical microscopy, differential scanning calorimetry, X‐ray diffraction and electro‐optical methods.

Polar columnar and lamellar mesophases in homologous bent-core compounds derived from methyl 3,5-dihydroxybenzoate

The synthesis and mesomorphic properties of the first three homologous series of symmetrical seven-ring bent-core compounds derived from methyl 3,5-dihydroxybenzoate have been described. In all the three series the lower homologues show an intercalated smectic (B6) phase. The middle homologues exhibit a polar columnar phase that shows antiferroelectric characteristics, while the higher homologues exhibit a switchable smectic phase with chiral conglomerates (SmCPAF). More interestingly, in one of the homologues, coexistence of a switchable columnar phase and a switchable lamellar phase has been observed. The mesophases were characterised using classical techniques of polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies.