Multichannel Time-Resolved Vibrational Spectroscopy of the Field-Induced Reorientation Dynamics of Liquid Crystal Thin Films

• Main Authors: Wen-Tse Shih, 施文慈
• Other Authors: Jung Y. Huang
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/04916832485913336294

博士 === 國立交通大學 === 光電工程系所 === 93 === The field-induced molecular reorientation dynamics of a nematic twisted pi-cell and surface stabilized ferroelectric liquid crystals (SSFLC) had been studied with time-resolved polarized Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. We constructed a multichannel time resolving system to perform the FTIR measurements and employed two dimensional (2D) correlation techniques to analyze the obtained IR spectra. The different information obtained from the 2D correlation technique and 2D hetero correlation technique was elucidated. The effects of the alignments and orientations of functional groups in an anisotropic film on the 2D correlation were also discussed. The alignment of the LC molecules in a nematic twisted pi-cell was revealed. The results suggest that the molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the electro-optical response were found to be 6 ms and 1.6 ms, respectively. In the switching dynamic process, the LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field induced reorientation process. A neat ferroelectric liquid crystal CMHCB in an SSFLC cell was studied. The molecular conformations and alignments of CMHCB in the K phase and SmC* phase were studied. The 2D IR correlations clearly exhibit Goldstone mode in the SmC* phase. The phase transition-induced re-arrangement was found to be dependent on the molecular fragments. We demonstrated that the 2D IR correlation technique can reveal the field-induced collective reorientation of the molecular segments in the CMHCB. The reorientation process starts from the chiral part with an intramolecular motion, which proceeds in less than 10 micor sec. In addition, we found an asymmetry in the response of the CMHCB to electric field polarities, indicating that the clockwise–anticlockwise symmetry about the cone axis is broken. The molecular motion of an SSFLC mixture during a field-induced reorientation process was studied. The 2D IR correlation technique clearly reveals that the thermal fluctuations in the azimuthal angle of the FLC director about a tilt cone in the SmC* phase with unwound structure are suppressed by an applied electric field. In addition, an asymmetry in the response of the surface stabilized FLC mixture to positive and negative driving pulses was also revealed in the 2D IR correlation analysis, indicating that similar to CMHCB, the clockwise– anticlockwise symmetry about the cone axis is broken at the molecular level in the FLC mixture. In a transient situation, the field-induced reorientation process was found to start with an intramolecular motion, which proceeds in about 10 micro sec. The intramolecular motion then propagates from the fragments attached to a single molecule to the ones belonging to different molecular coordinates. The molecular segments of C–O–C, C=C, and C–H stretch modes were found to rotate about a tilted cone with different angular speeds in a correlated but non rigid way. The combination of multichannel time-resolved vibrational spectroscopy and 2D correlation spectroscopy demonstrated in this thesis shall be useful for the studies of field-induced transient molecular deformations and alignment, submolecular dynamics and molecular-environment interactions.