Time-resolved infrared spectroscopy of molecule/binding site reorientation during ferroelectric liquid crystal electro-optic switching

Polarized Fourier transform infrared (IR) absorption is used to probe molecular conformation in a ferroelectric liquid crystal during the large-scale collective reorientation induced by external applied electric field. Spectra of planar-aligned cells of the ferroelectric liquid crystal (FLC) W314 ((S)-4(')-(decyloxy)4-[(1-methylheptyl)oxy]-2-nitrophenyl-[1, 1'-biphenyl]-4-carboxylic acid ester) are measured as functions of IR polarizer orientation and time following the reversal of the electric field applied to the FLC. The time evolution of the dichroism of the absorbance due to the specific molecular vibration modes, particularly from the biphenyl core and alkyl tail, is observed. Static IR dichroism experiments show a W314 IR dichroism structure in which the principal axis of the dielectric tensor from molecular core vibrations are tilted further from the smectic layer normal than those of the tail. This structure indicates that the effective binding site in which the molecules are confined in the Sm-C phase has, on average, a "zig-zag" shape. The dynamic experiments show that this zig-zag binding site structure is rigidly maintained while the molecular axis rotates about the layer normal during field-induced switching.