Crossover temperature (TCO) and temperature gradient of refractive indices and length to breadth ratio of 4O.m mesomorphic compounds – an optical study

Refractive indices as a function of temperature are measured in a number of liquid crystals which belong to the N-(p-n-butoxy benzylidene)-p-n alkyl anilines, 4O.m compounds of the famous Schiff’s base nO.m liquid crystal compounds with m = 4 to 10 and 12. The temperature gradient of refractive indices, dn_e/dT and dn_o/dT, of these compounds are estimated. It is well known that in the case of dn_o/dT, there exists a crossover temperature, T_CO, which exhibits an odd–even effect with the alkyl chain length as expected. Further, using the birefringence data, the length to breadth ratio, k, for these compounds is estimated, which shows not exactly even–odd effect but irregular change with the chain number. The results are discussed with the body of the data.     

Role of dielectric medium on benzylidene aniline: A computational analysis

A computational analysis of ordering in N-(p-n-ethoxy benzylidene)-p-n-butyl aniline (2O.4) was performed based on quantum mechanics and intermolecular forces. The atomic charge and dipole moment at atomic centre were evaluated using the all valance electron CNDO/2 method. The modified Rayleigh-Schrodinger perturbation theory and multicentre—multipole expansion method were employed to evaluate long-range intermolecular interaction, while a 6-exp potential function was assumed for short-range interactions. The total interaction energy values obtained in these computations were used as input for calculating the probability of each configuration in a noninteracting and nonmesogenic solvent (i.e., benzene) at room temperature (300 K) using the Maxwell-Boltzmann formula. The molecular parameter of 2O.4, including the total energy, binding energy, and total dipole moment were compared with N (p-n-butoxy benzylidene)-p-n-ethyl aniline (4O.2). The present article offer theoretical support to the experimental “observations, as well as a new and interesting way of looking at liquid crystalline molecule in a dielectric medium.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

Multispectral correlations technique for finding phase transition temperatures in 7O.Om series

Transition temperatures in N-(p-n-heptyloxybenzylidene)-p-n-alkyloxy anilines, 7O.Om liquid crystalline compounds are computed from optical textures using Multispectral Correlations Technique in image processing. The statistical parameters like mean, standard deviation and entropy of textures as a function of temperature are computed from the image processing technique. These results show abrupt changes as a function of temperature, indicating the phase transition of the 7O.Om samples. The MATLAB simulation results are in good agreement with DSC results and the proposed methodology is one of the techniques in giving reliable and suitable technique to identify transition temperatures at low cost.     

Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

As a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A exhibited by DLCs, α,ω-bis-(4-n-alkylaniline benzylidene-4?-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities α_e and α_o are obtained for the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. α_e and α_o calculated in this way are used to obtain Δα. The polarisability anisotropy in the perfect order (absolute K) is calculated semi-empirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence δn = (n_e-n_o) and this value of S is compared with those obtained from field models.     

Liquid Crystals: A Bibliography for 1975

Abstract

The static dielectric constants of biphenyl-4-p-n-alkoxybenzoates have been measured as functions of temperature. The dielectric anisotropy (δ?) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (T _NI). The dispersion of ?_∥ in the frequency range of 0.1-15 MHz has been studied for four successive members of biphenyl-4-p-n-alkoxybenzoates (heptyl to decyl derivatives) as well as of trans-p-n-alkoxy-α-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (f _R) and the frequency of dielectric isotropy (f _o) evaluated at a common relative temperature show an alternation similar to T _NI. For any given compound the Arrhenius plots for f _R and f _o give the same value of the activation energy (W). In the case of the biphenyl esters the vaue of W is the same for the seventh and eighth members, but increases for the higher members which exhibit a smectic phase at lower temperatures. On the contrary for cinnamic acid esters W is found to be the same for all homologues irrespective of whether the nematic phase is preceded by a smectic phase or not.     

Dielectric Anisotropy and the Order Parameter of HXBBA

Abstract

Measurements of dielectric anisotropy (Δ?), refractive indices (n _e , n _o), birefringence (Δn) and density (ρ) have been made in the nematic and smectic phases of N(-p-hexyloxybenzylidene)-p-butylaniline (HXBBA). The results indicate that the various transitions are of the first order type except smectic B-smectic G, which may be a second order transition. The order parameter S has been determined using the isotropic internal field model (Vuks approach) and the anisotropic internal field model (Neugebauer's approach) and both the values agree fairly well. The dielectric anisotropy (Δ?) increases strongly in the smectic phases whle S increases only slowly. It is interpreted by an increase of dipole-dipole correlations.     

Measurement of the An isotropic Refractive Indices of Polybenzylglutamate Liquid Crystals. Molecular Factors and Dispersion

The ordinary and extraordinary refractive indices have been measured for lyotropic liquid crystal solutions of poly-γ-benzyl-L-glutamate. The onset of the liquid crystal formation is noted as an abrupt jump in the sample birefringence in the biphasic region. The concentration dependence is otherwise unremarkable. The polymer is found to add a small, anisotropic increment to the refractivity of the supporting solvent. Both n _o and n _e can thus be measured in an ordinary Abbé refractometer for the low refractive solvents common to these liquid crystal solutions. The optical birefringence does not depend significantly on polymer molecular weight. Data is also included for the wavelength dependence (dispersion) of n _o, n _e, and Δn.     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

Effects of Nematic Compounds with High Clearing Points on the Multiplexing Performance of Twisted Nematic Displays

The addition of nematic compounds with high clearing points (N_h) to multiplexable nematic liquid crystals decreases the temperature dependence of the threshold voltage of twisted nematic displays. This effect depends on the dielectric anisotropy of the nematic liquid crystals containing N_h, as well as on the increase in T _c. In the case of 4-n-butylphenyl 4'-(4”-butyibenzoyloxy) benzoate, the low threshold voltage was exceptionally compatible with a small temperature dependence of threshold voltage, and this result was attributed to the large temperature dependence of the dielectric anisotropy in comparison with that of nematic liquid crystals containing other N_h's.     

Truxene Derivatives

Eight hexa-n-alkanoyloxy truxenes are presented. These new disc-like liquid crystals exhibit both columnar and N _D nematic phases. The N _D nematic phase is the first low temperature one. All these compounds show an inverted nematic-columnar sequence: the fluid N _D nematic phase is observed while heating after the crystalline phase below one or two viscous columnar phases and shows a phenomenon quite similar to the reentrant nematic behaviour.     

The Internal Field Parameters for Liquid Crystals

This paper presents calculations of the internal field constants γ_e for four homologous liquid crystal materials in their nematic and crystalline phases. We find that for the nematic phase, the γ_e ～ S ₂ curve is nearly a straight line for each compound. When one extrapolates the straight line to S ₂ = 1, the corresponding γ_e value is nearly equal to γ∥, the crystalline internal field constant. The birefringence data used are those reported by Somashekar et al.⁴ The four compounds studied are:

(1) p(p'-ethoxyphenylazo)phenyl valerate;

(2) p(p'-ethoxyphenylazo)phenyl hexanote;

(3) p(p'-ethoxyphenylazo)phenyl heptanoate;

(4) p(p'-ethoxyphenylazo)phenyl undecylenate.     

Birefringence and Order Parameter of Some Alkyl and Alkoxycyanobiphenyl Liquid Crystals

The refractive indices and densities of six liquid crystals—two alkylcyanobiphenyls, three alkoxycyanobiphenyls and one mixture of two alkylcyanobiphenyls have been reported as a function of temperature. The effective polarizabilities α_eand α_o in the nematic phase calculated using the methods due to Neugebauer, Vuks and Saupe and Maier, are found to be appreciably different, though the order parameter S evaluated with those values of (α_e - α_o) are in good agreement. The preference of one method over the other two is discussed.     

A simple optical method of measuring the tilt bias angle of liquid crystals

A simple optical method based on refractive indices measurement for determining the tilt bias angle of liquid crystals is described. The required refractive indices n_o, n_e and n_θ are measured by (i) Abbe's refractometer and by (ii) optical wedge method. The tilt bias angle by this technique can be determined to accuracy of ± 0.2° in the high tilt angle range (20° to 40°). This method is rather simple and quick and does not require any external magnetic field and/or knowledge of other physical constant of liquid crystals, as are necessary in some of the earlier reported methods.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Inter and Intramolecular Contributions to the Proton Dipolar and Rotating Frame Spin-Lattice Relaxation Times in the Nematic Phase of the Liquid Crystal 5CB-d 11

An isotopic dilution study of the proton dipolar and rotating frame spin-lattice relaxation times (T ₁ D and T ₁ p) in the nematic liquid crystal 4-cyano-4′-n-pentyl-d ₁₁-biphenyl (5CB-d ₁₁) at 13.8 MHz over the nematic phase temperature range was used to separate the intermolecular and intramolecular contributions to the ring proton relaxation rates. This is the first such study to be reported. Combined with our earlier proton study of inter/intra Zeeman spin-lattice relaxation (T ₁) in 5CB-d ₁₁, these data permit a critical study of the soft-mode dynamics in the system. A qualitative explanation of the T ₁ D> results is presented, but a complete explanation of the T ₁ p results is not possible with existing theory.     

Influence of TiO2 and ZnO nanoparticles on orientational order parameter of LC compounds – An optical study

In the present paper ZnO and TiO₂ nano particles are dispersed in Cyano-biphenyl liquid crystalline compounds (n?=?7, 12). The thermal polarizing microscopy and differential scanning calorimetry techniques are employed to measure the transition temperatures. The Nematic transition temperatures are decreased by 2.34?°C and 1.53?°C in Heptylbiphenyl, 1.07?°C and 1.12?°C in dodecylbiphenyl compounds due to the dispersion of nanoparticles. The refractive indices and Newton’s rings methods are exploited to measure birefringence at different temperature in nematic phase. The orientational order parameters are estimated by finding the birefringence in perfect order. The orientational order parameter is increased in nanoparticles dispersed liquid crystals when compare to pure liquid crystalline compounds.     

Efficient and practical method for the synthesis of hydrophobic azines as liquid crystalline materials

A series of hydrophobic symmetrical azines: 1,2-bis[4-(n-alkoxy)benzylidene]hydrazine (where, n-alkoxy: O(CH₂)_nH, n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 16, or 18) have been prepared following an efficient and practical method. These compounds have been synthesized via the condensation reaction of hydrazine hydrate and appropriately 4-(n-alkoxy) benzaldehydes in acidic medium under ambient conditions. The prepared organic compounds have been characterized and their structures were elucidated depending upon micro-elemental analysis and spectral data (IR, UV-Vis, ¹HNMR, ¹³C{¹H}NMR, 2D ¹H-¹H-cozy, 2D ¹H-¹³C-HSQC, and mass spectra). Liquid crystalline behavior of the prepared compounds was studied using polarized light optical microscopy and differential scanning calorimetry techniques. This study revealed that all the compounds displayed enantiotropic liquid crystal properties, exhibiting smectic and nematic mesophases.     

Synthesis,redox properties,and X-ray diffraction structure of 2,3-<Emphasis Type="Italic">bis</Emphasis>(ethylthio)-<Emphasis Type="Italic">N</Emphasis>-(<Emphasis Type="Italic">p</Emphasis>-MeOC<Subscript>6</Subscript>H<Subscript>4</Subscript>)maleimide

Refluxing 2,3-dichloromaleic anhydride with p-anisidine in benzene with water removal gives the condensation product 2,3-dichloro-N-(p-MeOC₆H₄)maleimide (1) 75% yield. This new maleimide compound reacts with added ethanethiol in the presence of Et₃N or DBU to furnish the bidentate sulfide ligand 2,3-bis(ethylthio)-N-(p-MeOC₆H₄)maleimide (2) in 85% yield. Each product has been characterized in solution by IR, NMR, and UV-vis spectroscopies, and the solid-state structure of 2,3-bis(ethylthio)-N-(p-MeOC₆H₄)maleimide was unequivocally established by single-crystal X-ray diffraction analysis. 2,3-bis(Ethylthio)-N-(p-MeOC₆H₄)maleimide crystallizes in the monoclinic space group C2/c, a = 20.035(3)Å, b = 9.188(1)Å, c = 16.887(2)Å, = 93.696(2)°, V = 3102.3(8)ų, Z = 8, and D_calcd = 1.385 mg/m³; R = 0.0268, R_w = 0.0676 for 2025 reflections with I > 2 (I). The nature of the LUMO in 2,3-bis(ethylthio)-N-(p-MeOC₆H₄)maleimide (2) has been determined by extended Hückel molecular orbital calculations, and these data are discussed relative to the cyclic voltammetry results and other structurally relevant compounds prepared in our labs.