Study of the freedericksz transition in 5CB using time-resolved Raman microscopy

The study of the kinetics of electric field-induced reorientation of liquid crystals is a well-established field. However, the microscopic mechanism of reorientation at a molecular level is not well understood. A means of studying the time course of reorientation is time-resolved Raman spectroscopy(1). Each Raman peak relates to a different region of the molecule, thus by investigating the transient change in intensity of a given peak, information about the reorientation process of distinct molecular fragments can be gained. The method therefore has an advantage over methods such as polarised optical microscopy, which study the time response of the director only. Raman spectroscopy has been used previously by this group to study the switching dynamics of 5CB(1,2) immediately after the application of the electric field. This work extends the investigation to include the field-free relaxation of the molecules to their equilibrium configuration. A comparison is made between Raman bands associated with the core of the molecule, and those associated with the flexible alkyl chains. We also present the relative change of Raman intensity as a function of applied voltage.