Time-resolved infrared spectroscopy of nematic-liquid-crystal electro-optic switching.

Time resolved infrared (IR) vibrational absorption spectroscopy is used to probe the dynamics of electric field-induced reorientation of the biphenyl molecular core and alkyl tail subfragments of the nematic liquid crystal 5CB (4-pentyl-4-cyanobiphenyl). The planar to homeotropic transition, induced by application of an electric field step to high pre-tilt planar aligned cells, is studied for switching times ranging from 200 micros down to 8 micros, the latter a factor of 1000 times faster than any previous nematic IR study. The reorientation rates of the core and tail are found to be the same to within experimental uncertainty, and scale inversely with applied field squared, as expected for the balance of field and viscous torques. Thus any molecular conformation change during switching must relax on a shorter time scale. A simple model shows that these data eliminate the possibility of there being substantial differences between the reorientational dynamics of the tails and cores on the time scales longer than on the order of 10 micros.