Time-Resolved FT-IR Analysis of Electric Field-Induced Reorientation Dynamics in Nematic 4-Pentyl-(4-cyanophenyl)-Cyclohexane Liquid Crystals

The application of rapid-scan FT-IR time-resolved spectroscopy to the study of molecular reorientation dynamics in a nematic liquid crystal during the electric field-induced homogeneous-to-homeotropic transition is reported. The average orientation of bond segments in a homogeneously oriented 4-pentyl- (4-cyanophenyl)-cyclohexane liquid crystal is determined from static infrared dichroism measurements. The observed temperature dependence of the molecular orientational order parameter agrees with the prediction of the Maier-Saupe theory. Analysis of time-resolved dichroic spectra shows that the homogeneous-to-homeotropic transition requires an induction period of several milliseconds and is completed in 10-20 ms. The cyanophenyl group reorients as a unit in response to the applied electric field and the reorientation rate increases as temperature is increased.