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

The solid to smecticG (SmG) phase transition in a Schiff base liquid crystalline compound, terepthal-bis-heptylaniline (TB7A), is monitored in situ by temperature-dependent Raman microspectroscopy, using the band of a C-H in-plane bending mode as a marker. Contrary to the earlier report of a sudden wavenumber shift, the in situ measurement shows very clearly that a new Raman band at similar to 1160 cm(-1) appears at the Crystal II -> SmG transition. The dynamics of this phase transition is discussed in terms of a triple well potential below 210 K and a double well potential above 210 K. The phase transition essentially takes place as a result of intra-molecular rotation about the long molecular axis. The optimization energy at various fixed dihedral angles, (-C-C-C=N-) are calculated using density functional theory (DFT) at the B3LYP/6-31G* level of theory. The relative energy at each dihedral angle is calculated relative to optimization energy obtained without any constraints and plotted as a function of dihedral angle (Phi) between the adjacent phenyl ring planes, which also shows a double well potential at room temperature. Copyright (C) 2009 John Wiley & Sons, Ltd.