Raman investigation of the low‐temperature phase transition in a liquid crystalline system: terephthalidine‐bis‐heptylaniline (TB7A)

Abstract

Raman spectra of terephthalidine-bis-heptylaniline (TB7A) were recorded in the region 1125-1225 cm −1 from room temperature to 10 K at 18 different temperatures. The splitting of the ∼1165 cm −1 band, and the variation of the ratio of integrated intensities of the original band at ∼1165 cm −1 to the new component at 1170/1171 cm −1 reveals a solid modification. The analysis of the temperature dependence of the linewidth was made using Rakov's relation. A logarithmic plot as a function of 1/T gives two straight lines with an abrupt discontinuity, which clearly exhibits a phase transition at about 210 K. The temperature dependence of the linewidth of the ∼1165 cm −1 band was used to characterize this transition. A relatively large variation (∼3 cm −1 ) of the linewidth on going from 250 to 10 K in TB7A having an odd (n = 7) number of carbon atoms in the alkyl tail compared with those observed (∼1 cm −1 ) in other homologues having an even (n = 2 or 10) number of carbon atoms in the alkyl tail is a clear demonstration of the even-odd effect.

Dynamics and mechanism of the Crystal II → smecticG phase transition in TB7A by a temperature‐dependent micro‐Raman study and DFT calculations

“…The accuracy of measurement of the temperature was ±0.1 K. The further details of the experimental set-up are given elsewhere. [4] Vacuum of the order of 10 −5 bar was achieved before the cooling of the sample was started. The Raman scattered signal was collected in a back scattering geometry using a microscope objective and focused onto the entrance slit of the double monochromator using a focusing lens.…”

“…[18,19] There are three prominent bands in the region under discussion, which have been shown to be sensitive to the Crystal II → SmG transformation. [2,4] These bands appear at ∼1165, ∼1170 and ∼1190 cm −1 . The temperature-induced wavenumber shifts ṽ with respect to the wavenumber position at 335 K of these modes are plotted as a function of temperature in Fig.…”

“…1), and only two components between 316 and 334 K. There is a crystal modification at 210 K, which was observed for the first time in a Raman study of this system. [4] At 335 K and above the β(C-H) mode exhibits a very different spectral feature. In earlier studies [2,3,5,10 -12] on TB7A and similar systems, the subtle changes in the spectral features close to the Crystal II → SmG transition (between 334 to 335 K for TB7A) could not be measured probably due to the experimental difficulty in reaching the precise transition temperature and low detection sensitivity.…”

Crystal → nematic phase transition in the liquid crystalline system 1‐isothiocyanato‐4‐(trans‐4‐propylcyclohexyl) benzene (3CHBT) probed by temperature‐dependent micro‐Raman study and DFT calculations

“…[1] Raman spectroscopy is a well-suited technique for understanding the molecular dynamics since any modification of inter-and intramolecular interaction as a consequence of any susceptible change in the surrounding modulates the polarizability of the active oscillator. The temperature-induced changes in spectral parameters of the marker bands which characterize a phase transition in crystalline or liquid crystalline materials help understand the changes in molecular structure and interaction with surrounding molecules nicely.…”

“…In addition, a careful study of both linewidth changes, which reflect variation in the dynamic processes and shifts in peak positions, as well as variations of integrated Raman intensities, which are basically the characteristic of static processes, help visualize the dynamics and molecular changes taking place at the transition point. [1,2] The compounds of 4-(trans-4 -n-alkylcyclohexyl) isothiocyanato benzene (nCHBT) series have been of interest for a long time. [3 -6] These are polar nematic materials characterized by low melting point and low viscosity (21 mPa·s).…”