S. Haldar

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmCA*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmCA* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmCA* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm−2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmCA*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol−1 in the SmC* phase.

X‐ray diffraction and optical birefringence studies on four nematogenic difluorobenzene derivatives

The results of optical microscopy, DSC, X‐ray diffraction and optical birefringence studies on four nematogenic difluorobenzene derivatives are presented. The thermal stability of the compounds increases substantially as greater flexibility is introduced into the core structure. Average intermolecular distances, obtained from X‐ray data, are found to increase when a comparatively rigid planar phenyl ring is substituted by a flexible non‐planar cyclohexyl ring in the core structure. From the values of apparent molecular lengths in the nematic phase, a short range antiferroelectric type of association is found to exist between neighbouring molecules. Density values suggest that the molecular packing is less efficient with increasing flexibility either in chain‐ or in core‐structure. All the refractive indices exhibit normal dispersive behaviour, and a positive optical anisotropy is observed in all the compounds which decreases quickly with temperature. Temperature dependences of orientational order parameters, determined from both X‐ray and optical birefringence studies, are presented. Variations of different physical properties with order parameters are discussed.

Influence of Molecular Core Structure and Chain Length on the Physical Properties of Nematogenic Fluorobenzene Derivatives

The influence of molecular core structure and chain length on various physical properties of nine nematogenic fluorobenzene derivatives obtained by optical polarizing microscopy, differential scanning calorimetry, X-ray diffraction, and optical birefringence techniques have been studied. It is observed that the temperature range of the nematic phase decreases when a cyclohexyl ring is replaced by a phenyl ring and when the number of fluorine atoms in the benzene group is increased. A change of some properties was noticed in difluoro (ff) derivatives compared to monofluoro (f) and trifluoro (fff) derivatives. The average intermolecular distance is found to increase systematically from mono to trifluoro derivatives. Effective molecular length (l) is found to be higher than the most extended length of the molecules, suggesting the presence of anti-parallel-type dimers. l is also found to be less in ff derivatives than in f and fff derivatives, indicating that the overlap between dimerized molecules in ff derivatives is more than in f and fff derivatives. The optical anisotropy (Δn) is found to be highest in an ff system compared to f and fff systems in 5ccp and 5cpp series, but the opposite behavior is observed in 3ccp series. 5ccp-f, having the lowest Δn and nematic phase over a large temperature, is expected to be useful for high-information-content display devices. The density of fff derivatives is found to be more than in ff derivatives in all systems, which is also true in the crystalline state of a 3ccp system. The orientational order parameters obtained from polarizability data are found to be slightly more, whereas the X-ray order parameters are found to be close to the mean field values in all the systems. A decreasing trend of is observed as one moves from f ⇒ fff ⇒ ff system.

Induction and Enhancement of Ferroelectric Smectic C* Phase in Multi-Component Room Temperature Mixtures

Six multi-component room-temperature ferroelectric liquid crystal (FLC) mixtures have been formulated and investigated using optical polarizing microscopy, dielectric spectroscopy as well as electro-optic methods. In all the mixtures smectic C* phase is either induced or enhanced. Effect of various dopants on the occurrence and thermal stability of different phases has been discussed. Spontaneous polarizations, measured as a function of temperature by reversal current method, are found to vary between 13–62 nC/cm2 at room temperature in all the mixtures. Different types of dielectric absorption behavior were observed in planar geometry of the FLC cells. Critical frequencies of Goldstone and soft modes have been determined by frequency dependent dielectric study. Rigidity of the core structure, nature of chirality and extent of fluorination of the constituent molecules are found to have pronounced effect on the collective mode relaxation behavior of the mixtures.

X-Ray Structural Analysis in the Crystalline Phase of a Nematogenic Fluoro-Phenyl Compound

Crystal and molecular structures of a nematogenic compound 4′-(3,4-difluoro phenyl)-4-propyl-bicyclohexyl (3ccp-ff) has been determined by direct methods using single crystal X-ray diffraction data. The compound (C21 H30 F2) crystallizes in the monoclinic system with the space group P21/n and Z = 4. The unit cell parameters are a = 16.293(15) Å, b = 6.278(7) Å, c = 18.3064) Å and β – 96.846)°. The two cyclohexyl rings are almost coplanar, so also are the phenyl ring and the propyl chain. The cyclohexyl groups are in chair conformation. Imbricated mode of packing of the molecules is found in the crystalline state which is precursor to the nematic phase structure. Several close van der Waals interactions are found to exist between the neighbouring molecules. Short-range antiparallel type of association is found to exist between the neighbouring molecules in crystalline as well as in nematic phase.

Novel synthetic route to liquid crystalline 4,4′‐bis(n‐alkoxy)azoxybenzenes: spectral characterisation, mesogenic behaviour and crystal structure of two new members

A simple synthetic method has been developed for the preparation of long‐chain 4,4′‐bis(n‐alkoxy)azoxybenzenes by reductive coupling of 4‐n‐alkoxynitrobenzenes using zinc powder and ammonium chloride in aqueous ethanol medium at ambient temperature. The new method was employed to synthesise known members (n = 1–12) of the 4,4′‐bis(n‐alkoxy)azoxybenzene (C n H2n+1OPhN(O) = NPhOC n H2n+1) series and also two hitherto unknown members (n = 13–14) of the series. The new compounds were characterised using spectral (IR, UV–visible, 1H NMR and FAB‐MS) data. The mesogenic behaviour of both compounds was studied by polarising optical microscopy, differential scanning calorimetry and small‐angle X‐ray diffraction techniques. The crystal structure of 4,4′‐bis(n‐tetradecyloxy)azoxybenzene was determined using single‐crystal X‐ray diffraction data. The packing of the molecules in the crystalline state is found to be a precursor to the smectic C phase structure.

Effect of molecular conformation on the mesogenic properties of a partially fluorinated nematogenic compound investigated by X-ray diffraction and dielectric measurements

A bicyclohexyl trifluorophenyl compound has been studied in both the crystalline and the nematic phases by X-ray diffraction and dielectric methods. Difference in molecular geometry, conformation, packing, density and orientation of molecular dipole moments explains substantial increase of melting point and decrease of nematic range in the compound compared to structurally very closely related systems. The facts that the apparent length of molecules is about 1.35 times more than the most extended molecular length and the average value of the dielectric constant is about 6% less than the extrapolated isotropic phase values give conclusive evidence of existence of short-range antiparallel order in fluorinated compounds like cyanobiphenyls and isothiocyanates supporting our previous conclusion (Haldar S, Mandal PK, Prathap SJ, Guru Row TN, Haase W. X-ray studies of the crystalline and nematic phases of 4′-(3,4,5-trifluorophenyl)-4-propylbicyclohexyl. Liq Cryst. 2008;35:1307–1312). About 5 V dc bias is sufficient for switching the orientation of the molecules from homogeneous to homeotropic configuration. Dielectric anisotropy and splay elastic constant exhibit similar decreasing trend with temperature. Only one strong Debye-type dielectric absorption process, connected with reorientations of the molecules around the short axis, is observed. Dielectric spectra fitted to modified Cole-Cole function revealed that the relaxation frequency increases from 2.6 MHz to 4 MHz exhibiting Arrhenius-type behaviour. Activation energy of the thermally activated relaxation process is about 32.4 kJ/mole.

Structural Analysis by X-Ray Diffraction of a Polar Mesogen 4-Cyanobiphenyl-4'-heptylbiphenyl Carboxylate

Crystal and molecular structure of a compound 4-cyanobiphenyl-4′-heptylbiphenyl carboxylate (7CBB), which exhibit both monolayer smectic A and nematic phases, have been determined by direct methods using single crystal X-ray diffraction data. The structure is monoclinic with the space group P21/c and Z = 4. The unit cell parameters are a = 16.9550(5) Å, b = 5.5912(18) Å, c = 27.5390(9) Å, α = 90.000°, β = 93.986(6)°, and γ = 90.000°. Packing of the molecules is found to be precursor to SmC phase, although SmA1 phase is observed on melting. Several strong van der Waals interactions are observed in the core part of the neighboring molecular pairs. Crystal to mesophase transition is probably of reconstitutive nature. Geometry, packing, and nature of crystal-mesophase transition are compared to those in 6CBB.

Influence of fluorination and chain length on the static dielectric properties of nematogenic bi-cyclohexyl phenyl derivatives

ABSTRACT Effects of fluorination and chain length of six bi-cyclohexyl phenyl compounds (3ccp-f, 3ccp-ff, 3ccp-fff, 5ccp-f, 5ccp-ff, 5ccp-fff) on their physical properties have been investigated. All the compounds exhibit nematic phase over a wide range of temperatures. Molecular mechanics calculation reveals that dipole moments of the molecules increases (1.93D to 3.37D) with fluorination in both propyl- and pentyl-based systems; increment in f to ff derivatives is more than in ff to fff derivatives. However, no change in dipole moment is observed with increased chain length. In the nematic phase, average value of dielectric constant (ϵavg) is less than the extrapolated values of ϵiso in isotropic phase. Increased fluorination also decreases splay elastic constant, and thus faster response is expected in triply fluorinated compounds.

Molecular Structure and Packing Analysis of Two Nematogenic Fluoro-Phenyl Compounds in the Crystalline Phase

The crystal and molecular structures of two nematogenic compounds 4′-(3, 4-difluoro-phenyl)-4-propyl-bicyclohexyl(3ccp-ff) and 4′-(3,4,5-trifluoro-phenyl)-4-propyl-bicyclohexyl(3ccp-fff) have been determined by direct methods using single crystal X-ray diffraction data. The compounds (C21 H30 F2) and (C21H29F3) crystallize in the monoclinic system with the space group P21/n and Z = 4 and in the triclinic system with the space group \( {\text{P}}\overline{1} \) and Z = 2 respectively. The fluorine atoms are in the plane of phenyl ring. Both the cyclohexyl groups are found to be in chair conformation. Parallel imbricated mode of packing of the molecules are found in the crystalline state which is precursor to the nematic phase. Several van der Waals interactions are observed between the neighbouring molecules. Results of crystal structure have been compared with that obtained from molecular modelling.