J. Phys. Chem. 1995,99, 14101-14107
14101
Dynamical Behavior of Surface-Stabilized States of Ferroelectric Liquid Crystals in Electric Field Studied by Time-Resolved FT-IR Spectroscopy Miroslaw A. Czarnecki Institute of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
Norihisa Katayama Department of Chemistry, School of Science, Kitasato University, Kitasato, Sagamihara 228, Japan
Masahiro Satoh Advanced Materials Laboratory, Kansai Research Institute, Chudoji Minami-machi, Shimogyo-ku, Kyoto 600, Japan
Tetsuya Watanabe Sanyo Chemical Industries Ltd., Higashiyama-ku, Kyoto 605, Japan
Yukihiro Ozaki* Department of Chemistry, School of Science, Kwansei Gakuin University, 1-1 -155 Uegahara, Nishinomiya 662, Japan Received: March 28, 1995; In Final Form: July 5, 1995@
Time-resolved FT-IR studies of two liquid-crystals (4-decyloxy-4'-{ (s)-2-methyl hexyloxycarbony1)tolane and 4-decyloxy-3'-fluoro-4'-((s)-2-methyloctyloxycarbonyl}tolane) in S,*phase were undertaken. The effects of temperature and voltage on the rate of electric field-induced reorientation of the molecules were studied. According to the theoretical predictions, the electric field does not affect the response time, whereas the influence of the temperature is more complex and cannot be described by the existing theoretical approach. The absolute value of the observed intensity changes (and their sign) during the switching can be explained by the static properties of the samples. A marked intensity reduction of the C=O stretching band in the time-resolved spectra (as compared with the static ones) may be due to the existence of two different conformations of the group in relation to the core. A substituent in the ring (ortho position) directly attached to the carbonyl group splits the C=O stretching band into a doublet in the infrared spectra.
Introduction The ferroelectric liquid crystals (FLC) are the most recent members of the family of ferroelectric materials, and the first ones with fluid properties.' Since theoretical prediction and experimental confirmation of ferroelectricity in DOBAMBC (pdecyloxybenzylidene-p'-amino-2-methylbutylcinnamate) by Meyer et al.,233various kinds of FLCs have been synthesized. An FLC in the S,* phase placed between two surfaces (cell windows) may interact with them and form two stable states (surface-stabilized FLC). This happens when the gap between the windows is thin enough (d < 10 pm).' By changing the polarity of the applied electric field, one can switch between these two states. FLCs constitute the basis for a new branch of material science and technology. A range of different electrooptic switches and devices is presently under development. The potential advantage of FLCs for various applications is evident; the microsecond response speed is very fast, compared to what had earlier been available in liquid crystals, and with a much larger change of refractive index (An > O.l).' Generally, the faster the switching speed, the greater the variety of applications of the material. On the other hand, the complex nature of FLCs has generated obstacles for the practical
* Author to whom all correspondenceshould be addressed. FAX: +81798-5 1-0914. Abstract published in Aduunce ACS Absrrucrs, September 1, 1995. @
0022-365419512099-14101$09.00/0
applications of these potentially powerful materials. In the relatively short time of their investigation, many of their fundamental aspects have not yet been studied in reasonable depth. A number of important phenomena are still not fully understood. One of them is an exact mechanism of electric field-induced switching. The real switching mechanism in FLCs at a molecular level is very complicated. The molecules in the S,* phase are often depicted as having zigzag shapes. However, it should be remembered that the molecules in these phases are in a dynamical state and undergo reorientational and conformational changes at a rapid rate.' The dynamical properties of FLC depend on temperature, voltage, cell preparation, and so on. An understanding of the behavior of FLC in extemal electric (and magnetic) fields is crucially important for their applications as optical displays. In recent years significant progress in the studies of dynamics of electric field-induced switching of liquid crystals has been made owing to the application of timerresolved infrared The method seems to be particularly relevant to studies of the time course of reorientation because it provides information at the molecular level. Hence, one can monitor independently the movement of particular functional groups of the molecule. As yet, greater attention has been paid for studies of nematic liquid In fact, most of the studies are concemed with one compound, 5CB (4-pentyl-4'-cyanobiphenyl).4,5,7387'0However, in the last couple of years, one can observe 0 1995 American Chemical Society
Czamecki et al.
14102 J. Phys. Chem., Vol. 99, No. 38, 1995
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