Pradip Kumar Mandal

X-Ray Studies on the Mesogen 4′-n-Pentyloxy-4-Biphenylcarbonitrile(5OCB) in the Solid Crystalline State

Abstract The crystal and molecular structure of nematogen 4′-n-Pentyloxy-4-Biphenylcarbonitrile has been determined by direct methods. The crystals belong to the monoclinic system with space group P21/n. a = 21.378(5), b = 5.695(3), c = 12.789(2) A, β = 106.074(17)° with four molecules per unit cell. Least-squares refinement leads to R = 0.050 (Rw = 0.038) for 1773 observed reflections. The molecules are in their most extended conformation, the rigid part lies parallel to a-axis while the chain part is inclined to it. Rigid part is also highly planar and is almost parallel to ab-plane. The molecules are stacked along c-axis. The molecules associate in pairs about the centre of inversion. In the unit cell the adjacent molecules are antiparallel to each other. The structure is of a common type for nematogens and could transform to the nematic state by means of a simple displacive transition.

X-ray diffraction, optical birefringence, dielectric and phase transition properties of the long homologous series of nematogens 4-(trans -4′- n -alkylcyclohexyl) isothiocyanatobenzenes

The temperature dependence of density, refractive indices, apparent molecular length and intermolecular distance of 11CHBT, are reported, together with the static dielectric permittivities, ε∥, ε┴, ε iso of 11CHBT and 12CHBT; the effect of chain length on various physical properties of the CHBT series is also discussed. All the temperatures T m, T NI and T NCr show an odd-even (OE) effect, but transition enthalpies ΔH m and ΔH c show no such behaviour. T m, T NI and T NCr and ΔH m are found to increase with chain length, while ΔH c shows a maxima at n = 6. At the common reduced temperature T* = 0.98 TNI, the density of the compounds are found to decrease with increasing chain length while the polarizability parameters αe, αo and α increase. However, the volume expansion coefficient, polarizability anisotropy and normalized polarizability anisotropy show a distinct OE effect. Similarly, n e decreases smoothly with chain length but n o and Δn show an OE effect like ε┴ and Δε. The ratio ln ⟨P 2⟩ /ln ⟨ P 4⟩ remains closer to Faber theory than MS theory. Effective dipole moments are found to be approximately constant in the series; antiparallel association is observed in all the members. Kirkwood correlation factors g┴ and Δg are found to exhibit an OE effect like ε∥ and ε∥, but g∥ and ε∥ show no such effect. The effects of substitution in the core and end polar group on these properties are also discussed.

Optical microscopy, DSC and dielectric relaxation spectroscopy studies on a partially fluorinated ferroelectric liquid crystalline compound MHPO(13F)BC

Phase transitions in a partially fluorinated compound, MHPO(13F)BC, have been studied by optical microscopy, calorimetry and miscibility methods. Spontaneous polarization (PS), helical pitch measurements, and frequency-domain dielectric study, have also been carried out. These studies indicate the presence of an intermediate alpha sub-phase between a ferroelectric and a paraelectric phase. Both the Goldstone-mode (GM) and the soft-mode (SM) relaxation processes have been observed, and by fitting the spectra to the Cole–Cole function, values of dielectric strengths, critical frequencies and dispersion parameters have been determined. Anomalies have been observed in the temperature dependence of PS and inverse dielectric strength, which are interpreted as a signature of a phase between the SmC* and SmA* phases. Microscopic patterns, helical pitch measurements, and miscibility studies give further evidence in support of this inference.

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.

Crystal and Molecular Structure of the Nematogenic Compound 4-Cyanophenyl-4′-n-Heptylbenzoate (CPHB)

Abstract It is now well established that for a proper understanding and interpretation of several physical properties of liquid crystalline phases, a knowledge of the molecular arrangement in the crystalline state is very useful. A preliminary survey of the present knowledge of the solid-mesophase relationships has been given by Bryan1. The classical view is that in typical nematogen crystals the long narrow molecules lie more or less parallel and are interleaved giving what was described by Bernal and Crowfoot2 as an imbricated packing and that the transformation from the solid to the nematic phase is characterised by the breakdown of the positional order of the molecules but not of the orientational order. Leadbetter3 has however pointed out that although this is true of at least the majority of the small number of cases so far known, it is a generalisation which must at this stage be treated with caution. Hence the determination of the crystal and molecular structure of the title nematogenic compound w...

An X-Ray Study of Cyanophenylpyrimidines Part III. Crystal Structure of 5-(trans-4- Heptylcyclohexyl)-2-(4-Cyanophenyl) Pyrimidine

Abstract The compound C24H31N3 at higher temperatures shows a smectic liquid crystal phase followed by a nematic phase. The crystals belong to space group P21/c, with a = 13.0422(6), b = 17.7464(11) and c = 9.5950(5) A, β = 107.170(1)°, Z = 4. The structure has been solved by the direct method program system SIMPEL83 and refined to an R-value of 0.041. Molecules are packed in associated pairs running parallel to the a axis. An X-ray study at higher temperatures shows that the smectic phase is also built ap from similar pairs of molecules.

The molecular and crystal structure of a polar mesogen 4-cyanobiphenyl-4′-hexylbiphenyl carboxylate

The crystal structure of the compound 4-cyanobiphenyl-4′-hexylbiphenyl carboxylate (6CBB), which exhibits both monolayer smectic A and nematic phases, has been determined by direct methods using single crystal X-ray diffraction data. The structure is triclinic with the space group P-1 and Z=2. The unit cell parameters are a=9.3511(7) Å, b=11.2456(7) Å, c=13.1417(6) Å, α=85.872(4)°, β=76.258(5)° and γ=70.697(5)°. The molecule is found to be slightly bow-shaped although the alkyl chain is in all-trans conformation. The phenyl rings in 6CBB are non-coplanar. The packing of the molecules in the crystalline state is found to be a precursor to the smectic A phase structure. Comparison of crystal structures and packing of the four-ring 6CBB with those of the two-ring nCB or nOCB compounds has been made to explain the observed phase behaviour.

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.

Optical microscopy, DSC and X‐ray diffraction studies in binary mixtures of 4‐pentyloxy‐4′‐cyanobiphenyl with three 4,4′‐di(alkoxy)azoxybenzenes

A large number of binary mixtures of 4‐pentyloxy‐4′‐cyanobiphenyl (5OCB) and 4,4′‐di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6 and 7 have been studied by optical microscopy, DSC and X‐ray diffraction methods. Over a wide concentration range the smectic A (SmA) phase is induced and the nematic (N) phase of the parent compounds is absent. Instead of the tilted smectic C phase of 7OAB an orthogonal SmA phase is observed when only 12% of the host molecules are replaced by 5OCB molecules. Moreover, in some mixtures the thermal stability of the induced SmA phase is found to be more than 1.6 times the stability of the mesophses in the pure compounds. The N–isotropic or Sm–I transition temperatures slowly increases with increasing concentrations, reaching a maximum at x 5OCB = 0.50 and then falling quite rapidly. In all the mixtures the enthalpy changes at the N–I transitions follow the simple additivity rule. In no mixture, except mixture C2, could the SmA–N transition be detected by DSC, although in all the mixtures the ratio T NA/T NI is found to be more than 0.95, which is in contradiction to McMillans theory. Average intermolecular distances are found to have a minimum value near the equimolar concentration. From the concentration dependence of the smectic layer spacings it also appears that in all the mixtures the smectic Ad phase evolves from the smectic A1 phase with increasing concentrations.

X-ray scattering studies on two nematogenic p-alkylphenyl-2-chloro-4-(p-alkylbenzoyloxy)-benzoates and their mixtures

Abstract The compounds, p-n-pentylphenyl-2-chloro-4-(p-n-pentylbenzoyloxy)-benzoate and p-n-octyiphenyl-2-chloro-4-(p-n-heptylbenzoyloxy)-benzoate show a nematic phase with supercooling effect below room temperature. Marble textures are observed under a crossed polarizing microscope; however, near the N-I transition schlieren textures are observed. Mixtures of three different mole fractions of them form a nematic phase at well below room temperature. Small-angle X-ray scattering studies on all the magnetically aligned samples have been done in their entire mesomorphic ranges. X-ray diffraction patterns show that in each case the mesophases skewed cybotactic nematic. The average intermolecular distance is found to be ∼ 5.10 Å and the interplanar spacing of cybotactic groups is found to be less than the stereo model length of the molecules in all trans-configurations. Temperature dependence of these parameters is discussed. Orientational order parameters, ⟨P 2⟩ and ⟨P 4⟩. for the Pure compounds and their eutectic mixture are calculated and compared with Maier-Saupe theoretical values. ⟨P 2⟩ and ⟨P 4⟩ values of the mixture are found to lie between those of the individual components.