Banani Das

New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases

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 (SmCs) and the anticlinic SmCa 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 SmCs–SmCa 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.

Structural Investigations of a Non Calamitic Shaped Liquid Crystalline Compound Showing Unusual Phases

X-ray diffraction, birefringence, density and static dielectric permittivity measurements were performed on a calamitic hockey stick shaped mesogen. In addition to SmA phase, this compound exhibits two tilted mesophases – the SmC, synclinic and the SmCa, anticlinic phase. From x-ray diffraction and birefringence data, the orientational order parameters have been determined. The order parameters values drop at the SmC – SmCa phase boundary. Possible cause for the drop in the order parameter values has been discussed. Interestingly, it has been found that the dielectric anisotropy of this compound changes from positive to negative values in the SmCa phase.

Optical, dielectric and visco-elastic properties of a few hockey stick-shaped liquid crystals with a lateral methyl group

Measurements of birefringence (Δn), dielectric permittivity (e‖, e⊥), 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 SmCa–N transition is found to be of first order in nature while the SmCs–N transition is either second order or weakly first order. All the mesogens exhibit a temperature dependent inversion in the static dielectric anisotropy (Δe = e‖ − e⊥) from positive to negative values on entering the SmCa 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 (Ea) 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.

Rotational viscosity measurements of bent-core nematogens

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 (Δe) is observed in the nematic phase, while the higher homologue (1/10) exhibits negative Δe 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.

Comparison of the orientational order parameters determined from X-ray diffraction and 13C NMR studies of a hockey stick-shaped compound

Orientational order parameters (OOPs) have been determined in the Smectic A, Smectic C1 and Smectic C2 phases of a hockey stick-shaped compound from X-ray diffraction measurements and compared with NMR measurements. While the OOP values from NMR measurements abruptly drop at the Smectic C1 to Smectic C2 transition, those determined from X-ray diffraction measurements decreases sharply at the Smectic A to Smectic C1 transition. This drop in the OOP values from X-ray measurements has been explained using a model where the molecular director is randomly oriented in a cone about the smectic layer normal. OOPs from X-ray diffraction and NMR measurements have been successfully fitted with McMillans theory, after necessary correction to the potential, incorporating tilt angles, for three homologues with n = 10, 11 and 12. The temperature dependences of the optical birefringence, dielectric anisotropy and OOP values determined from birefringence measurements of compound 11 are also reported.

Determination of the orientational order parameter of the homologous series of 4-cyanophenyl 4-alkylbenzoate (n.CN) by different methods

The orientational order parameters of a homologous series of 4-cyanophenyl 4-alkylbenzoates have been determined at different temperatures from (13)C-NMR, x-ray diffraction, optical birefringence, high resolution density and diamagnetic susceptibility anisotropy measurements. To determine the temperature dependence of the orientational order parameter from the (13)C chemical shift values, the two-dimensional polarization inversion spin exchange at the magic angle (PISEMA) method was also used for the measurement of (13)C-(1)H dipolar couplings at different sites in the oriented phase. The nematic order parameters determined from each of these methods have been compared. Apart from a slight shift in their values their trends with temperature are very similar. The differences among the results obtained by these five different methods have been discussed. From the high resolution density data, the values of the critical exponents near the T(N-I) transition are found to lie between the Ising model and tricritical behaviour.

Mesomorphic, optical, dielectric, elastic and viscous properties of multi-component isothiocyanato mixtures

Four nematic mixtures A, B, C and D, comprising alkylphenylethylphenyl isothiocyanates, alkylcyclohexylphenylisothiocyanates, biphenyl and terphenyl derivatives, respectively, were prepared and their physical properties investigated, including birefringence, static permittivity, dielectric anisotropy, splay elastic constant and rotational and bulk viscosity. All the mixtures exhibited a broad nematic range (∼100°C), moderate birefringence (Δn ∼0.23–0.26), positive dielectric anisotropy (Δε ∼ 8–10) and low rotational viscosity. The mixtures showed improved performance in birefringence and viscosity properties in comparison with the widely used commercial mixture E7. As far as display performance is concerned, mixture D was found to be the most valuable. A comprehensive study of the usefulness of the biphenyl and terphenyl derivatives as dopants in mixtures has also been conducted with a view to improving their physical parameters to give faster electro-optical response times in liquid crystal display devices. Application of these mixtures is envisaged in high-speed photonics and liquid crystal display devices.

Determination of the orientational order parameter of a binary mixture showing an induced smectic Ad phase from magnetic susceptibility measurements

The diamagnetic susceptibility anisotropy (Δχ) measurement of a binary mixture comprising of a strongly polar mesogen (CPPCC) and a weakly polar mesogen (ME6O.5) showing an induced smectic A(d) phase is reported here. Assuming an axially symmetric molecule, the temperature dependence of the orientational order parameter ⟨P(2)⟩ has been investigated from the anisotropy of the susceptibility at different temperatures throughout the entire composition range. The results are compared with x-ray and optical birefringence measurements along with the mean-field theory of the smectic A phase. The maximum in the stability of the smectic A(d) phase (at x(CPPCC) = 0.33) corresponds to the minimum in the order parameter values. The order of the smectic to nematic phase transition has also been discussed.

Dielectric and visco-elastic properties of laterally fluorinated liquid crystal single compounds and their mixture

The physical properties of three laterally fluorinated liquid crystalline compounds with negative dielectric anisotropy have been studied from static dielectric permittivity, optical birefringence, bend elastic constant, relaxation time and rotational viscosity measurements. Such negative dielectric anisotropy materials find use as components of mixtures for application in vertically aligned mode liquid crystal displays. Moreover, the physical properties of one phenyl cyclohexane compound with positive dielectric anisotropy have also been studied. A five-component mixture comprising these four mesogens and a non-mesogenic component has been formulated and its physical properties have been thoroughly investigated. An attempt has been made to strike a balance between the optical birefringence of the mixture to adjust the cell gap and the dielectric anisotropy and threshold voltage (Vth ) to ensure low driving voltages. The pretilt angle effect on the threshold voltage and the relaxation time has also been studied. At T = 20°C, the response time decreases to 22% and 41% for the mixture for 2° and 5° pretilt as compared to zero pretilt. On the other hand, at the same temperature the Vth values are decreased by 5% and 9%, respectively.

Physical Properties of Three Liquid Crystals with Negative Dielectric Anisotropy from X-Ray Diffraction and Optical Birefringence Measurements

In this work, we report the results of x-ray diffraction, density and birefringence measurements on three fluorinated liquid crystalline compounds having negative dielectric anisotropy. Small angle x-ray diffraction data from aligned samples have been analyzed to determine the orientational order parameter as a function of temperature. The order parameter values were also determined from birefringence measurements and the results were compared with mean field theory. Structural parameters like intermolecular distance and layer thickness (in smectic B phase), apparent molecular length (in nematic phase) have been determined and the order of the phase transition in these compounds has been discussed.