FT-IR Study of the Electric Field Modulation of Liquid Crystalline 5CB

Abstract

The dynamic IR spectra of electrically modulated liquid crystalline 5CB (4-pentyl-4'-cyanobiphenyl) are determined to be due to the director reorientation. In the absence of dc fields, the IR response is mainly at two times the modulation frequency. IR signals at the modulation frequency may be induced by application of additional dc fields. These signals at the modulation frequency are caused by the creation of polar orientation due to the static field. No difference in dynamic response has been found between the rigid aromatic core and the pentyl chain of 5CB. Expressions are given for the in-phase and in-quadrature signals at the modulation frequency and at twice its value. It is shown that the frequency-domain dynamic IR experiment mainly probes the transition range between the uniform planar to the homeotropic regions in the sample. The different probing of the sample by time-domain experiments is briefly discussed.

Effects of Temperature on the Alignment and Electrooptical Responses of a Nematic Nanoscale Liquid Crystalline Film

“…Much research has been conducted on the effects of surface properties on the alignment of liquid crystalline mesophases, ,, and the methods through which specific alignments can be induced, whether textured, − chemical, , or other, , are now well established. Even so, the physics of these influences (especially as regards to their impacts on electrooptical dynamics − ) remain incompletely understood, and there exist opportunities to exploit new approaches to alignment that might serve as the foundation of a vastly more energy-efficient display technology …”

Time resolved infrared spectroscopy of electro‐optic switching of 5CB

“…In the limit t R << 1/Γ, the dynamic response should become distinctly bimodal, exhibiting a driven reorientation in time t R to a non-equilibrium state of internal molecular distortion, which would relax on the much longer time scale 1/Γ. The absence of such effects in our data thus implies its absence in slower experiments, and, indeed, no evidence for internal relaxation is found in several slower experiments [2,20,21]. However, our data appear to be fundamentally inconsistent with earlier observations on 5CB and related molecules at much longer time scales, in which such relaxation is found [7,8,13,14].…”

Time-Resolved Fourier Transform Infrared Spectroscopic Imaging

“…50 It has been suggested that the relaxation behavior in different m olecular m oities may not be completely due to different relaxation processes. 22 Another study that employed NMR spectroscopy to examine reorientation effects concluded that the characteristic m olecular relaxation times are independent of the functional group used to determine the director orientation. 51 Due to considerable spectral overlap of the polymer and liquid crystal's vibrational modes, we were only able to determine the temporal response of a few selected m olecular moieties that could be unambiguously assigned to a chemical species.…”

“…Events in the reorientation and subsequent relaxation processes occur on time scales ranging, for example, from picoseconds for intra-molecular changes to milliseconds for the rotation of an entire LC m olecule. Both time-and frequency-domain experiments 22 have been widely applied in extracting responses of the LC director to variations in applied potential differences. 23 W hile numerous TR spectroscopic studies have analyzed the m acroscopic dynamic responses of low-molecularweight nematic LCs, 24 the responses of LC molecules in con ned geometries have been less studied.…”