Crystal-smectic G transformation investigated by temperature-dependent Raman study

We report on a temperature-dependent Raman study of terephthalidine-bis-butylaniline (TBBA) and terephthalidine-bis-heptylaniline (TB7A) in the temperature range 10-406 K and in the wavenumber regions 1120-1240 cm-1 and 1500-1700 cm-1. The variations of peak positions and linewidths of several Raman marker bands with temperature, which clearly show crystal-SmG transition at approximately 334 K, have been used to discuss the dynamics of this phase transition. The temperature-dependent Raman study revealed that the increased vibration of the long alkyl tail and the rotation around the long molecular axis are the two primary phenomena responsible for the crystal-SmG transition. Density functional theory (DFT) at the B3LPY/6-31G(d) level was employed to obtain the optimized geometry and the harmonic vibrational wavenumbers of TBBA. The eigenvectors of the modes corresponding to these marker bands were also calculated.