B. Gogoi

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.

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.

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.

Study of dielectric properties and the molecular dynamics using Raman spectroscopy in pure and nanoparticle doped liquid crystal compound, 6O.4

ABSTRACT A comparative study on the dielectric properties (in 100 Hz to 40 MHz frequency range) of a pure and silver nano particle dispersed (0.25% and 0.50% by weight) Liquid crystal compound, 6O.4 and the resultant changes at the molecular level using Laser Raman Spectroscopy is reported. The dielectric anisotropy of the two samples was found to be negative but the magnitude of the anisotropy slightly decreased due to the dispersion of the nanoparticles and the relaxation frequency in nematic phase was found to increase with the temperature and the concentration of nanoparticles. Raman spectroscopy studies on pure and SNP dispersed 6O.4 confirms the modification in molecular polarizability in SNP dispersed sample.

INVESTIGATION OF RARE PHASE TRANSITIONS BY DENSITY MEASUREMENTS

Abstract We report here the study of nature of few rare phase transitions viz., Isotropic ‒ Sm C, Sm C ‒ Sm F and Srn A ‒ Sm F transition in two homologues of α, ω-bis(4-n-alkylanilinebenzylid- ine-4′-oxy)alkane series using density measurements. Compound 10.040.10 exhibits Iso- tropic ‒ Sm C, Sm C ‒ Sm F and Sm F ‒ Crystal phase transitions. Whereas 10.0100.10 exhibits Isotropic ‒ Sm A, Sm A ‒ Sm F and Sm F ‒ Crystal transitions. Variation of density with temperature across the phase transitions Isotropic ‒ Sm A, Isotropic ‒ Sm C, Sm C ‒ Sm F and Sm A ‒ Sm F show a jump in density confirming that all these transitions are first order in nature. However, the magnitude of the density jump is smaller than expected at some of these transitions. The calculated pressure dependence of transition temperature is also reported.

Dielectric properties of a strongly polar nematic liquid crystal compound doped with gold nanoparticles

ABSTRACT This study focuses on the electrical characteristics of a strongly polar nematic liquid crystal, Hexyloxy-cyanobiphenyl (6OCB), doped with a low concentration (2% by weight) of citrate buffer stabilised gold nanoparticles (GNPs) at low frequencies between 20 Hz and 35 MHz. The doped samples have lower values of nematic–isotropic transition temperature, permittivity (both parallel and perpendicular to the field direction) and dielectric anisotropy; however, relaxation time and activation energy were increased. The observed results could be explained on the basis of weakly anisotropic nature of GNPs and a local rearrangement of liquid crystal molecules surrounding the nanoparticles. Moreover, a complimentary suggestion on a possible change in the dipole–dipole correlation is made to explain the difference in changes (qualitative and quantitative) observed for permittivity of the host nematic liquid crystal doped with GNP. Temperature dependent dielectric relaxation studies indicate an increase in viscosity and potential barrier; and hence a change in strength of inter-molecular and intra-molecular interactions is suggested. Graphical Abstract

Temperature-dependent vibrational spectroscopic studies of pure and gold nanoparticles dispersed 4-n-Hexyloxy-4’-cyanobiphenyls

ABSTRACT Raman spectra of pure and 2 wt.% gold nanoparticles (GNPs) dispersed liquid crystalline compound 4-n-Hexyloxy-4ʹ- cyanobiphenyls (6OCB) has been recorded as a function of temperature from room temperature (solid crystal) to 80°C (isotropic liquid) in the spectral region of 500–2500 cm−1. The variation of Raman spectral parameters (peak positions and line width) with temperature is used to explain the changes in molecular alignment and its effect on inter-/intra-molecular interactions at crystal-Nematic (K-N) transition. To understand the change in molecular structure during phase transition and on account of dispersion of gold nanoparticles in pure liquid crystal more precisely, two spectral regions 1000–1500 cm−1 and 1500–2400 cm−1 have been selected separately. From the detailed study, it is concluded that increased orientational/vibrational freedom of the molecules as well as delocalisation of electron clouds results in the spectral anomalies at K-N transition. The geometrical structure of 6OCB was optimised using density functional theory (DFT) and theoretical Raman spectra have been obtained for comparison with experimental spectra. The tentative assignment of vibrational modes observed in our region of study was calculated based on potential energy distribution (PED) using vibrational energy distribution analysis (VEDA) calculation. Graphical Abstract

Dielectric Study on the Alignment or Orientation of N(4-n-heptyloxy benzylidene)4´-n-butylaniline and the Anomalous Dielectric Behavior in the Smectic G Phase

ABSTRACT The present article reports on the alignment or orientation of a particular compound N(4-n-heptyloxy benzylidene)4´-n-butylaniline through dielectric study. The effect of using two different alignment layers (for homogenous alignment) and doping silver nanoparticles (0.25% by weight) on the behavior in dielectric property with varying temperature and biasing electric field was studied in terms of liquid crystal alignment or orientation using the mechanism proposed by earlier workers. The study includes the anomalous dielectric behavior in SmG phase by presenting a technique for the analysis of the absorption process, which exists only in SmG phase at fixed frequency while varying temperature, is also presented.

SM A ‒ SM C TRICRITICAL POINT IN TBBA AND TBDA MIXTURES

Abstract Terephtal-bis-p-n-decylaniline (TBDA) exhibit a weakly first order Sm A‒Sm C Phase Transition Whereas TBBA, a lower homologue of the same series, exhibit a second order Sm A‒Sm C transition. In order to investigate the tricritical point (TCP) of Sm A‒Sm C transition, a systematic study of binary mixtures of TBBA + TBDA are undertaken. DSC and polarizing thermal microscopy studies infer the TCP of the Sm A‒Sm C Transition at the weight percent of 80.05 TBDA in TBBA. We report here a systematic study of the approach of TCP by DSC and density measurements as a function of temperature in few compositions of TBBA + TBDA on both sides of the tricritical point.