The electro-optical properties of orthoconic antiferroelectric liquid crystals of some analogues of (S)‑MHPOBC ((S)‑4‑(1‑methylheptyloxy)carbonylphenyl 4'-octyloxy-4-biphenylcarboxylate) have been investigated and discussed in terms of the structure property correlations of liquid crystalline molecules. Polarisation field reversal technique has been used for their characterisation. In this study the occurrence of such a high value of spontaneous polarisation (P_s) for the investigated compound is especially noteworthy as they comprise a bulky chiral unit, zigzag ester linkage and polar fluorinated terminal. Lengthening of oligomethylene spacer in the terminal chain and placement of fluorine atom in the molecular core structure leads to reduction in the value of P_s. The P_s data suggest that the nature of the chiral smectic C (SmC*) to smectic A (SmA) phase transition is of second order and the critical exponent β is in the range 0.19–0.34. The ‘V-shaped’ mode or thresholdless switching in such high-polarisation ferroelectric liquid crystals has also been discussed.

Measurements of birefringence (Δn), dielectric permittivity (ε_‖, ε_⊥), elastic moduli (K₁₁, K₃₃) and rotational viscosity (γ₁) have been carried out on the nematic (N) phase of a few hockey stick-shaped compounds 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-alkyloxycinnamates with a lateral methyl group inserted between the m-alkyloxy chain and the azomethine connecting group. Interestingly, a dual characteristic (i.e. partially calamitic like and partially bent-core like) is revealed in the N phase of these compounds. The SmC_a–N transition is found to be of first order in nature while the SmC_s–N transition is either second order or weakly first order. All the mesogens exhibit a temperature dependent inversion in the static dielectric anisotropy (Δε = ε_‖ − ε_⊥) from positive to negative values on entering the SmC_a phase. Remarkably in the entire nematic range, the bend elastic modulus (K₃₃) is substantially lower than the corresponding splay modulus (K₁₁). The rotational viscosity coefficient (γ₁) as obtained by extracting the relaxation time (τ₀) values from two precise, independent probing methods, viz. the capacitive decay technique and the optical phase-decay-time measurement method, are slightly higher than those of many known calamitic systems. Moreover, the activation energy (E_a) calculated from the viscosity data is found to be considerably higher in the nematic phase than those obtained for conventional calamitics. The observed behaviours are accounted for by considering the intriguing shape-determined inter-molecular interactions and molecular associations appearing in the mesophase.

For a newly designed binary liquid crystalline system we report the induction of two stable twist grain boundary (TGB) phases, namely the orthogonal paraelectric TGB smectic A* (TGBA*) and tilted ferroelectric TGB smectic C* (TGBC*) phases, within a quite broad temperature range. The designed binary mixtures consist of a highly tilted chiral ferroelectric compound, (S)(+)-4′-(ω-alkanoyloxy)alkoxybiphenyl-4-yl-4-(1-methylheptyloxy) benzoates (2H6R) and an achiral hockey stick-shaped compound (H-22.5). Measurements of optical transmission intensity, static permittivity (ε_‖, ε_⊥), spontaneous polarization (P_s), torsional bulk viscosity (η) and dielectric spectroscopy have been carried out for all the mixtures and the pure chiral compound. Possible outcomes and related transitional phenomena have been discussed from the viewpoint of perturbation introduced by the achiral hockey stick-shaped molecules. Dielectric studies reveal a sign inversion (positive to negative) of the dielectric anisotropy for all the mixtures including the pure compound. By increasing the concentration of achiral hockey stick-shaped compound, it is possible to increase the temperature range of the positive dielectric anisotropy. On the other hand, the measured values of spontaneous polarization in the TGBC* phase for the mixtures are considerably lower than that of the SmC* phase for the compound 2H6R; conversely, the response time and effective torsional bulk viscosity for the mixtures possess a higher value than that of the SmC* phase of the pure 2H6R compound. Additionally, the frequency-dependent dielectric spectroscopy measurement reveals that the relaxation frequency and dielectric strength of the Goldstone mode in the TGBC* phase slightly decreases compared to the SmC* phase of the pure compound due to the addition of the H-22.5 dopant.

New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases – the synclinic smectic C (SmC_s) and the anticlinic SmC_a phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro–optical and dielectric spectroscopy measurements. The SmC_s–SmC_a phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small ‘soft’ ferroelectric clusters responsible for a low frequency absorption range.

In the field of soft matter research, the characteristic behavior of both nematic–isotropic ($N-I$) and smectic-$A\mathrm{nematic}(\mathrm{Sm}\text{−}AN)$ phase transitions has gained considerable attention due to their several attractive features. In this work, a high-resolution measurement of optical birefringence $\left(\Delta n\right)$ has been performed to probe the critical behavior at the $N-I$ and $\mathrm{Sm}\text{−}AN$ phase transitions in a binary system comprising the rodlike octylcyanobiphenyl and a laterally methyl substituted hockey-stick-shaped mesogen, 4-(3-$n$-decyloxy-2-methyl-phenyliminomethyl)phenyl 4-$n$-dodecyloxycinnamate. For the investigated mixtures, the critical exponent $\beta$ related to the limiting behavior of the nematic order parameter close to the $N-I$ phase transition has come out to be in good conformity with the tricritical hypothesis. Moreover, the yielded effective critical exponents (${\alpha }^{\prime }$, ${\beta }^{\prime }$, ${\gamma }^{\prime }$) characterizing the critical fluctuation near the $\mathrm{Sm}\text{−}AN$ phase transition have appeared to be nonuniversal in nature. With increasing hockey-stick-shaped dopant concentration, the $\mathrm{Sm}\text{−}AN$ phase transition demonstrates a strong tendency to be driven towards a first-order nature. Such a behavior has been accounted for by considering a modification of the effective intermolecular interactions and hence the related coupling between the nematic and smectic order parameters, caused by the introduction of the angular mesogenic molecules.

Two simple but precise probing methods have been utilized to investigate the influence of structure, properties and molecular association on the director response time (τ₀) and hence on the rotational viscosity (γ₁) in the nematic phase of three 4-cyanoresorcinol bent-core derivatives (1/7, 1/9 and 1/10) having different terminal chain lengths. The results obtained from the two methods are in good agreement with a small deviation of about 10–15%. The γ₁ values are more than an order of magnitude higher compared to the conventional calamitic nematics. Measurements of birefringence, dielectric anisotropy, splay and bend elastic constants and relaxation time of these compounds have also been carried out. For the lower homologues (1/7, 1/9) an inversion in dielectric anisotropy (Δε) is observed in the nematic phase, while the higher homologue (1/10) exhibits negative Δε throughout the entire mesomorphic range. The splay elastic constant (K₁₁) is found to be comparatively greater than the bend elastic constant (K₃₃); however, on approaching the low temperature CybC phase (a phase comprising elongated cybotactic clusters) K₃₃ exhibits a small pretransitional divergence. For all three compounds the magnitude of the rotational viscosity coefficient (γ₁) is found to exhibit a strong enhancement on approaching the CybC phase, while in the nematic phase activation energies are found to be nearly an order of magnitude higher than the normal calamitics. The observed behaviours are explained by considering the strong dipolar correlation of the bent-core molecules and the existence of the short range cybotactic clusters, their gradual growth and uniform alignment under the presence of strong surface anchoring.