P. R. Alapati

Laser Raman Study of Terepthalylidene-bis-p-n-Decylaniline (TBDA)

Abstract The Raman spectra of Terepthalylidene-bis-p-n-declyaniline (TBDA), have been recorded as a function of temperature in the temperature range from 55.0°C (solid) to 199.0°C (isotropic liquid) in the 1100–1200cm−1 and 1520–1650cm−1 regions. The precise values of peak positions, integrated intensities and linewidths of some selected Raman bands have been obtained by deconvoluting the bands. The changes in the molecular alignment and its effect on inter/intra molecular interactions at two particular phase transitions viz., solid-smectic G (K-SG ) and smectic I-Smectic C (SI-Sc ) have been discussed on the basis of variation in the Raman spectral parameters with temperature. From detailed considerations, it is inferred that the increased orientational/vibrational freedom of long alkyl chain on the core is responsible for major spectral changes at the K-SG transition while the changes in the intramolecular interactions of the core part with increased fluidity, is responsible for spectral anomalies at th...

Optical studies of a liquid crystalline compound 6O.6

A detailed optical study for a Schiffs base liquid crystalline nO.m compound (6O.6) is reported in this article. The phase transition temperatures for the compound were investigated using a polarising microscopy and differential scanning calorimetry (DSC). Refractive index studies were conducted using the thin prism method with the 633 nm line from a He–Ne laser. The order parameters of the compound were calculated from the refractive index data. Order parameters calculated by two methods, namely, direct extrapolation method and modified Vuks’ method were found to be in close agreement. The normalised polarisabilities for extraordinary and ordinary rays and their ratio for the sample were also calculated. The order parameters are found to be reasonably high and the trend shown by normalised polarisabilities substantiates the correctness of our results. From the methods we adopted, the order parameters and the normalised polarisabilities could be determined directly from the refractive index measurements without doing the density studies. Both these methods are found to be highly suitable for the calculation of order parameters of liquid crystalline compounds having high clearing temperature where density studies are difficult and hazardous.

A comparative laser Raman study on TB4A, TB7A and TB10A

The basic structural differences between three thermotropic liquid crystals TB4A, TB7A and TB10A have been interpreted by analysing their Raman spectra. Three spectral regions, 925-1025, 1275-1475 and 1525-1650 cm-1, have been chosen in this context to incorporate their structural dissimilarity. The induced planarity of the backbone, as an effect of increasing chain length, has been studied by observing the changes in the respective Raman spectra of the three compounds. A tentative assignment of all the modes observed in the region 400-1700cm-1 is made in this context. The molecular conformations of the three compounds have been predicted in their solution states, and are compared with the molecular environment that exists in their liquid crystalline states. Structural disorder at the solid-SmG transition is discussed and the changes are incorporated systematically.

Laser Raman studies on compounds 7.O4O.7 and 7.O5O.7

Laser Raman studies on two compounds of the α,ω-bis(4-alkylanilinebenzylidene -4′-oxy)alkane series were carried out in the spectral regions 1140–1220 and 1550–1650 cm-1 as a function of temperature. Compounds 7.O4O.7 and 7.O5O.7, exhibit SmA and SmF phases. The structural differences between these liquid crystal dimers, having either an odd or an even number of carbon atoms in the spacer, are remarkably existent in the room temperature Raman spectra. The results are rationalized on the basis of the tendency of these compounds to exhibit bent shapes, and how this manifests in the odd-even effect at the molecular level assuming a semi-rigid core region of the dimeric molecule. It is found that the behaviour of the odd spacer dimer (7.O5O.7) agrees with the molecular model, whereas the even spacer dimer (7.O4O.7) behaves in a similar fashion to monomeric compounds such as the nO.m. compounds. In both cases, dynamic changes around the C=N bond have a profound effect both on the molecular shape in the different phases and on phase behaviour.

Solvent dependent frequency shift and Raman noncoincidence effect of SO stretching mode of Dimethyl sulfoxide in liquid binary mixtures

The isotropic and anisotropic Raman peak frequencies of S=O stretching mode of Dimethyl sulfoxide (DMSO) have been discussed in different chemical and isotopic solvent molecules using different mechanisms. The shifting of peak frequency in further dilution of DMSO with solvent molecule is observed for all solvents. Transition dipole - transition dipole interaction and hydrogen bonding may play a major role in shifting of peak frequencies. The non-coincidence effect (NCE) of DMSO was determined for all the solvents and compared with four theoretical models such as McHales model, Mirones modification of McHales model, Logans model and Onsager-Fröhlich dielectric continuum model respectively. Most of the theoretical models are largely consistent with our experimental data.

On the dipole–dipole correlation and dielectric anisotropy of some N(4-N-alkyloxy benzylidene)4′-N-alkylaniline compounds doped with silver nanoparticles

ABSTRACT Dielectric study of the compounds N(4-n-heptyloxy benzylidene)4′-n-butylaniline and N(4-n-heptyloxy benzylidene)4′-n-hexylaniline at low frequency region (1 kHz–1 MHz) is performed using LCR meter (Agilent 4284 A). A qualitative interpretation in terms of dipole–dipole correlation for both the parallel and the perpendicular components of the dipole moment is given. Further study on the change in the dielectric permittivity and the dielectric anisotropy is made by doping the two compounds separately with dodecanethiol capped silver nanoparticles (0.25% by weight). The molecular positional ordering and relative smectic layer distance influences the change of degree (or type) of dipole–dipole pairing.

Negative Birefringence in the Higher Homologs of the 5O.m Series of Liquid Crystals

A detailed study of the different parameters of the higher homologs of the 5O.m (m = 14, 16) series of liquid-crystalline compounds is reported. These are interdigitated compounds with unsymmetrical alkyl chain length. The compounds have a unique nature, unlike the other members of the nO.m series. The molecular structure reported in this article is not purely uniaxial; it has a bending tendency. In this article, we report that both the compounds exhibit negative birefringence. For the optical study, the refractive indices, ne and no, of the sample are measured by the thin-prism technique, using a He-Ne laser beam of wavelength 633 nm. A four-parameter model was used for fitting the experimental results. From the experimentally measured refractive indices, it is possible to compare different parameters with those of the theoretical models.

Phase Transition Studies of 6.O12O.6 and 7.O6O.7 Using Density Measurements

The symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, show a rich variety of smectic polymorphism. We report here the nature of different transitions exhibited by two of these dimers using density measurements as a function of temperature. The compound 6.O12O.6 exhibits only nematic phase; while 7.O6O.7 exhibits smectic A and smectic F phases. All the phase transitions studied viz., isotropic – nematic, isotropic – smectic A, and smectic A – smectic F are confirmed to be of first order nature. The calculated density jumps, thermal expansion coefficient data and pressure dependence of transition temperatures are analysed in the light of available literature data.

Density Studies on Two Homologues of α,ω-Bis(4-n-Alkylanilinebenzylidine-4′-Oxy) Alkanes

Abstract α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes exhibit a rich smectic polymorphism. We have studied the variation of density as a function of temperature across the phase transitions in two compounds of the series viz., 7.O4O.7 and 7.O5O.7. Both of these compounds exhibit a rare phase transition of Sm A - Sm F. The results confirmed first order nature of the transitions of Isotropic - Sm A and Sm A - Sm F, but with a smaller jump than expected in density as compared to monomers across the transitions. The estimated pressure dependence of transition temperature is also reported. The results are discussed in the light of available literature data.

A comparative study of the properties of the liquid crystalline compounds 5O.5 and 5O.6 in solid, solution and thin film forms by laser Raman spectroscopy

Raman spectroscopic studies of the liquid crystalline compounds 5O.5 and 5O.6 revealed that the two aromatic rings in the core are in different environments with distorted electron cloud distributions, and the behaviour of each of these rings is independent of the other. Increased chain length results in the development of strain on the molecules. During the SmF-SmB phase transition there is a slight transfer of charge, along with an increase in the freedom of the individual molecules. Our experiments also reveal that the molecules are strained in the crystalline state and adopt some preferred orientations in free-standing films. There is also a tendency towards the elongation of the molecules along the C=N- linkage in solution for 5O.6. Raman spectra of 5O.6 in free-standing films have shown dramatic changes compared with the spectra recorded in the bulk.