Applications of Raman spectroscopy

The C1-C8 and C10 members of the homologous series of terephthal-bis-n-alkylaniline (including TBBA) have been prepared by the condensation of 4-alkylanilines with terephthaldehyde. Synthesis of the anilines was achieved through a Friedel-Crafts acylation of acetanilide followed by a Wolff-Kishner reduction. Interphase transitions between solid, mesomorphic and isotropic phases were studied by hot-stage microscopy and DTA. The nematic phase was observed in the C1-C7 homologs, the smectic A and C phases in the C3-C10 compounds and the smectic H(B) phase in the C2-C10 homologs. An additional smectic phase with a mosaic texture was observed in the C5 homolog and some unexplained anomalies were found in the phase transition behavior of the C2 and C3 compounds. A study of the textures of the smectic C phases as a function of temperature indicated that all these phases have a temperature dependent tilt angle. Relationships between structure and mesomorphic properties as well as with thermodynamic data ...

The Preparation and Mesomorphic Properties of the Homologous Series of Terephthal - bis- n -Alkylaniline

An infrared study of the different phases of liquid crystalline CBOOA (N-p-cyanobenzilidene p-n-octyloxyaniline) has been made in the range 30-3100 cm−1. From a study of the -C≡N vibration in aligned samples, the absolute orientational order parameter, S, is determined in the smectic A and nematic phases. The variation of S is compared with the relative values determined earlier from diamagnetic anisotropy, NMR and birefringence studies. The farinfrared spectrum of the solid (Phase I) exhibits a mode whose intensity decreases gradually as the crystal-smectic A transition is approached. Also, a band observed at 296 cm−1 in the solid (Phase I) practically disappears in the fluid phases. These results are discussed in terms of possible pretransition effects and intramolecular changes characteristic of crystal-mesophase transitions. Dichroic effects are evident in the far-infrared absorption of the liquid crystalline phases and these are considered in relation to the influence of molecular structure ...

Infrared Spectroscopic Study of Orientational Order and Phase Transformations in Liquid Crystalline CBOOA

Raman spectra of the Schiff 's base liquid crystalline compound 5O5, N-(4- n-pentyloxybenzylidene)-4'-n-pentylaniline, have been recorded as a function of temperature from 22 to 80°C in the 1140-1220 cm-1 and 1550-1640 cm-1 spectral regions. From careful deconvolution of the spectral features using Lorentzian profiles, precise values of peak positions, integrated intensities and linewidths of some selected Raman bands were obtained. The variations of the Raman spectral parameters with temperature are discussed in terms of changes in the molecular alignment and its effect on intra-/inter-molecular interactions at the Cr-G, G-SmF, SmF-SmC and SmA-N phase transitions. From a detailed study, it is inferred that the increased orientational/vibrational freedom of the alkyl chains, as well as the delocalization of the electron clouds, is responsible for the spectral anomalies at the Cr-G transition. Loss of positional ordering and twist around the -C6H4-N= bond takes place at the SmF-SmC transition. In the SmA-N...

Dynamics of phase transitions in a liquid crystal probed by Raman spectroscopy

Measurements of rotational viscosity in the various liquid crystalline phases of TBBA (terephthal-bisbutylaniline) are reported. The quantity measured is the viscous energy dissipation in the liquid crystal as it oscillates in a magnetic field. This dissipation varies greatly in the different smectic phases. It is small in the $A$ and $B$ phases, but large and strongly temperature dependent in the $C$ phase. This behavior is explained in terms of the structure of these phases. The smectic $A$ to smectic $C$ transition is of second order, and the dissipation increases dramatically at the transition point. The critical behavior is found to fit mean-field theory, rather than a predicted heliumlike behavior.

Rotational viscosity in the smectic phases of terephthal-bis-butylaniline (TBBA)

Raman investigation on the lattice modes (8-200 ${\mathrm{cm}}^{\ensuremath{-}1}$) of several smectic phases in terephtal-bis-butylaniline (TBBA) reveals significant spectral differences. Softening of the lowest lattice 19-${\mathrm{cm}}^{\ensuremath{-}1}$ mode, observed very near to the solid-smectic-$B$ phase transition, is correlated with the gradual release of the smectic inter-layer glide motion. The Raman spectra add new insight into the ordering within the layers of the metastable smectic phases and also indicate the existence of molecular dynamic rotations in the smectic-$B$ phase.

E. Wiener-Avnear

Papers

Raman study on several smectic phases in terephtal- bis -butylaniline (TBBA)