Volume 51, Number 4, 1997 APPLIED SPECTROSCOPY 4870003-7028 / 97 / 5104-0487$2.00 / 0q 1997 Society for Applied Spectroscopy

Asynchronous Time-Resolved FT-IR Study of the DynamicalBehavior of Ferroelectric Liquid Crystal with a Tolane Ring

N. KATAYAMA,* M. A. CZARNECKI, M. SATOH, T. WATANABE, and Y. OZAKI*Department of Chemistry, Kitasato University, Kitasato, Sagamihara 228, Japan (N.K.); Institute of Chemistry, University ofWrocøaw, F. Joliot-Curie 14, 50-383 Wrocøaw, Poland (M.A.C.); Advanced Materials Laboratory, Kansai Research Institute,Shimogyo-ku, Kyoto 600, Japan (M.S.); Sanyo Chemical Industries Ltd., Higashiyama-ku, Kyoto 605, Japan (T.W.); and Depart-ment of Chemistry, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan (Y.O.)

Transient infrared spectra of a ferroelectric liquid crystal with atolane ring, (S)-4-methylhexyl-4-[4-(decyloxy)phenylethynyl]-2-¯ uo-robenzate, in the smectic C* phase have been measured under var-ious temperatures and voltages by use of an asynchronous time-resolved FT-IR method. The effects of temperature and voltage onthe rate and tilt angle of the electric ® eld-induced reorientation ofthe molecule have been studied. The absolute values of the observedintensity changes and their sign during the switching can be ex-plained by the static properties of the sample. The spectra obtainedunder the different experimental conditions suggest that the tem-perature and applied voltage alter the tilt angle and angular velocityof reorientation of the liquid crystal, respectively. Dominant infra-red bands show very similar time-dependent intensity changes un-der various conditions, indicating that the whole molecule reorientssimultaneously irrespective of temperature and the applied ® eldstrength as though the molecule were a rigid rod.

Index Headings: FT-IR; Time-resolved spectroscopy; Liquid crystal;Reorientation.

INTRODUCTION

Liquid crystals (LCs) have many attractive features forvarious kinds of devices, such as display systems inwhich their ability to reorientate in an electric ® eld isutilized.1 In particular, a ferroelectric smectic C* liquidcrystal (FLC) device has recently received keen interestas a promising candidate for large-area, high-resolution,and fast-switching displays.1 In spite of this capability,however, the mechanism of the electric ® eld-induced re-orientation of FLC is still not fully understood. The in-vestigations of the effects of the conditions surroundingthe FLC molecules are crucially important for the under-standing of the behavior of FLCs in external electric andmagnetic ® elds. Time-resolved infrared spectroscopy isvery suitable for the studies of the dynamics of electric® eld-induced switching of LCs; it provides informationabout the motion of the whole LC molecules as well asdetails on the time dependence of the reorientation ofparticular molecular segments.2± 14

This paper is concerned with a study of the dynamicalbehavior of (S)-4-methylhexyl-4-[4-(decyloxy)phenyl-ethynyl]-2-¯ uorobenzate (MDOPEFB; Fig. 1) under var-ious experimental conditions in the chiral smectic C (Sc*)phase by time-resolved polarization FT-IR spectroscopy(TR/FT-IR). There is great interest in the properties ofthis compound because it forms the Sc* phase at roomtemperature and shows interesting properties on a graph-ite surface.15 We carried out a time-resolved infrared

Received 31 July 1996; accepted 18 December 1996.* Authors to whom correspondence should be sent.

study of MDOPEFB at a ® xed temperature (25 8 C) andvoltage ( 6 20 V) by using an asynchronous time-resolvedFT-IR method,8 which does not require any modi® cationsin either the hardware or the software of the existingspectrophotometer.7 In the present study, particular em-phasis has been focused on the effects of the temperatureand voltage of the electric ® eld applied on the mechanismof the reorientation of the FLC molecule.

EXPERIMENTAL

The sample of MDOPEFB was synthesized at SanyoChemical Industries, Ltd.16 An asynchronous time-re-solved FT-IR method was employed for the measure-ments of transient infrared spectra. The spectra were re-corded at a 4-cm2 1 resolution on a JEOL JIR 6500 FT-IRspectrophotometer equipped with a micro-attachment(JEOL IR-MAU 110) and an MCT detector. A boxcarintegrator (Stanford Research Systems; SR 250) with thegate width of 10 m s was applied. The experiment wascarried out at a range of delay times from 0 to 500 m swith intervals of 50 m s. One hundred scans were accu-mulated to ensure an acceptable signal-to-noise ratio. Thereorientation of the sample was induced by rectangularelectric pulses of 6 5± 28 V and 1.0-kHz frequency arisingfrom a function generator (Kenwood; FG-273) and home-made linear ampli® er. For construction of the LC cell,BaF2 windows covered with a thin layer of transparentconductive material (ITO) were used; they served as elec-trodes. The surface of the windows was treated with apoly(vinyl alcohol) (PVA) ® lm and rubbed in one direc-tion in order to induce uniform orientation of the FLCmolecule. The windows were separated by poly(ethyleneterephthalate) (PET) spacers, and the pathlength calcu-lated from the interference fringes of the empty cell wasfound to be 3.2 m m.

A top view of the LC cell with respect to the polarizedinfrared radiation is shown in Fig. 2. The axis of a po-larizer was ® xed at 45 8 to the rubbing direction in orderto obtain the largest intensity changes. The measurementswere performed in the temperature range of 14± 32 8 C,where the sample exists in the Sc* phase. The cell wasput into a temperature control unit consisting of an OM-RON E5T thermocontroller and Peltier element. This sys-tem guaranteed a temperature control and stability of 60.1 8 C. The sample inserted into the cell by capillaryaction was heated up to the isotropic liquid state and thenslowly ( ; 1.0 K min2 1) cooled down.

The FT-Raman spectrum was recorded on a JEOL

488 Volume 51, Number 4, 1997

FIG. 1. (A) The molecular structure of (S)-4-methylhexyl-4-[4-(decy-loxy)phenyl ethyl]-2-¯ uorobenzate (MDOPEFB). (B) Phase transitionsequence of MDOPEFB.

FIG. 2. A top view of the reorientation of LC molecule.

FIG. 3. FT-IR and FT-Raman spectra of MDOPEFB in bulk states(KBr disk and powder, respectively).

JRS-FT 6500N FT-Raman spectrophotometer equippedwith an InGaAs detector.

RESULTS AND DISCUSSION

Band Assignments and their Transition Moments inan Infrared Spectrum of MDOPEFB. Figure 3 showsFT-IR and FT-Raman spectra of MDOPEFB in the solidstate. The band assignments were made in our previousstudy by an analysis of the infrared and Raman polariza-tion spectra.8 The polarization spectra indicate that thedegree of orientational order of the sample in the LCphase is very high. With the application of an infraredmicro-attachment, we could localize one particular well-oriented domain and then restrict the observation to thisdomain only. The intensity changes for most of the bandsin the time-resolved spectra were clear and easily ob-servable.11

The molecule of MDOPEFB consists of a rigid coreand two hydrocarbon chains. All infrared bands observedare assigned to vibrational modes of the core or chains,so that the reorientation of each part can be investigatedseparately. With regard to the response of the bands tothe electric ® eld, they may be divided into two types:8

type 1, giving upwards peaks, represents an increase inthe intensity during the reorientation, while type 2, whichgives downwards peaks, shows a decrease in the intensityduring change of the orientation. If the molecular axesare assumed to be predominantly parallel to the rubbingdirection (without an electric ® eld), one can conclude thatthe modes which have their transition moments in theplane of the core belong to type 1, and those with tran-sition moments perpendicular to the plane of the corebelong to type 2. A schematic illustration for the reori-entation of the FLC molecule with respect to the polar-ized infrared radiation is shown in Fig. 2.

Features at 2922 and 2848 cm2 1 are assigned to CH2

antisymmetric and symmetric stretching modes of the hy-drocarbon chains, respectively, which have transition mo-ments perpendicular to the long axis of the molecule(type 2). A band due to a C[ C stretching mode of thecore part appears at 2208 cm2 1 (type 1), while bands at1613 and 1514 cm2 1 are assigned to the ring stretchingmodes of the tolane ring (type 1). An intense band at1281 cm2 1 is due to a C± O± C antisymmetric stretchingmode of the ether-functionality14 (type 1). The directions

of the transition moments of these modes are parallel tothe long axis of the core part of the molecule.

Dependence of the Rate of the Reorientation ofMDOPEFB on the Applied Voltage. Figures 4A and4B show TR/FT-IR spectra of the FLC measured at 308 C and 6 9 and 6 28 V, respectively. The delay timeranges from 0 to 500 m s, every 50 m s. Each spectrumrepresents the absorbance change from the spectrum ofzero time delay, taken as a reference. As the time increas-es, the intensities of the bands due to the vibrations ofthe hydrocarbon chains decrease, while those of the corepart increase, indicating that the molecule reorients froma large angle to a small one with respect to the directionof polarized infrared radiation. The absorption bands inthe spectra recorded at the higher voltage are consider-ably more intense in comparison with those ones record-ed at the lower voltage. The time-dependent variations ofthe absorbance change ( D A) of the band at 1286 cm2 1,assignable to the C± O± C antisymmetric stretching mode,are illustrated for the applied voltage of 5 to 28 V in Fig.5. It is noted that the reorientation under low voltages ( ,6 12 V) is not complete within 500 m s. Moreover, therate of the reorientation in the time delay region of 0 ± 50m s depends upon the ® eld strength. On the other hand,the intensity of the delta absorbance ( D A) at 500 m s isvery similar at higher voltages ( . 20 V), indicating thatthe electric ® eld strength only slightly alters the tilt angleof the reorientation. Although the maxima of the absor-bance changes cannot be determined at the low voltagesbecause of the experimental setup limitation, a similar tiltangle of the reorientation may be expected in this caseas well. The intensities of other bands change similarlyto that of this band, indicating that the whole molecule

APPLIED SPECTROSCOPY 489

FIG. 4. TR/FT-IR spectra of MDOPEFB measured at 30 8 C and 6 9V (A) and 6 28 V (B) for delay times ranging from 0 to 500 m s, every50 m s.

FIG. 5. Time-dependencies of D A for the band at 1286 cm2 1 measuredat 30 8 C and 6 5± 28 V.

FIG. 6. Time-dependencies of D A for the band at 1286 cm2 1 measuredat 14± 30 8 C and 6 20 V.

reorients simultaneously, irrespective of the magnitude ofvoltage of the electric ® eld.

Temperature Dependence of the Rate of the Reori-entation of MDOPEFB. Figure 6 shows time-dependentvariations of the absorbance change of the band at 1286cm2 1 in the temperature range from 14 to 30 8 C. Thechange of reorientation starts immediately after the elec-tric ® eld is applied. The rate of the increase of D A in thetime range of 0± 50 m s changes only a little with tem-perature, while the intensity at 500-m s delay time dependsupon the temperature. At elevated temperatures, the in-tensity change of each band decreases. It seems likelythat the tilt angle of the reorientation of the moleculedecreases at higher temperatures. Thus, it may be con-cluded that the temperature does not in¯ uence the angularvelocity of reorientation of FLC molecules, while the tiltangle is altered by the temperature. The absorbancechanges below 25 8 C indicate that the FLC molecule doesnot complete the reorientation within 500 m s, since thetilt angle is widely extended.

In Fig. 7A and 7B the time-dependencies of the ab-sorbance changes ( D A) of four major bands measured at14 and 30 8 C, respectively, are shown. The absorptionbands due to the vibrations of both the core part and thehydrocarbon chains of MDOPEFB are clearly observableeven for the ® rst 50-m s time period. This result suggeststhat, in principle, the molecule reorients as a rigid unit,contrary to the expectation that the core is more sensitive

to the electric ® eld than the hydrocarbon chains andneeds a shorter period of time for the reorientation.2

When an external electric ® eld is applied to a LC cell,the dominant mechanism is an interaction of the ® eldwith the anisotropy of the dielectric tensor of the samplein the LC molecules.1,2 The anisotropy of the dielectrictensor is mainly due to the polar properties of the core.According to this theory, the rigid core of the moleculeshould start the reorientation ® rst, and then the hydro-carbon chains should follow the core. The results of thepresent investigation, however, show that the moleculesof the FLC reorient as a whole as if they were rigid rods.For both temperatures, the rates of the intensity changesof these bands are similar to each other, indicating thatthe FLC molecules reorient as rigid units irrespective ofthe temperature.

Conspicuous behavior of intensity reduction in theC5 O stretching band is observed in the time-resolvedspectra. Similar results were noted previously also for4-decyloxy-3 9 -¯ uoro-4 9 -[(S)-2-methyloctyloxycarbon-yl]tolane (DOFMOOCT)14 and (S)-4-(2-methyloctanoyl)-4 9 -biphenyl 3-chloro-4-octyloxybenzoate.17 These results

490 Volume 51, Number 4, 1997

FIG. 7. Time-dependencies of D A for the major bands measured at 148 C (A) and 30 8 C (B). The voltage of 6 20 V was employed.

may be explained by considering that the C5 O group ishindered from rotating with respect to the molecular longaxis14 and reorients within an angle at which the quantityof the projection component to the polarization axis doesnot change.

In the present study MDOPEFB shows an identical

trend in the reorientation behavior of tilt angle to that ofDOMHOCT and DOFMOOCT reported previously.14

The angular velocity of reorientation of this sample, how-ever, is not altered by temperature. One of the possibleexplanations of this phenomenon is the occurrence ofnonlinear effects at the higher voltages. Simultaneous re-orientation of entire FLC molecule is commonly ob-served for all three FLCs investigated.

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