Electric-Field-Induced Reorientation of a Ferroelectric Liquid Crystal

The orientation dynamics of a ferroelectric liquid crystal (FLC) with a naphthalene ring and without a carboxylate group near the stereocenter (FLC-2)...
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J. Phys. Chem. B 2003, 107, 4227-4236

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Electric-Field-Induced Reorientation of a Ferroelectric Liquid Crystal Molecule without a Carboxylate Group near the Stereocenter Studied by Time-Resolved Infrared Spectroscopy Combined with Normalized Sample-Sample Two-Dimensional Correlation Spectroscopy J. G. Zhao,† K. Tatani,† T. Yoshihara,‡ and Y. Ozaki*,† Department of Chemistry, School of Science and Technology, Kwansei-Gakuin UniVersity, Gakuen, Sanda, Hyogo 669-1337, Japan, and Display Laboratories, Fujitsu Laboratories Limited, Ohkubo, Akashi 674-0054, Japan ReceiVed: October 23, 2002; In Final Form: December 22, 2002

The orientation dynamics of a ferroelectric liquid crystal (FLC) with a naphthalene ring and without a carboxylate group near the stereocenter (FLC-2) during the electric-field-induced switching has been investigated by time-resolved infrared (IR) spectroscopy combined with normalized sample-sample twodimensional (2D) correlation spectroscopy. To reveal the orientation dynamics, we have proposed a normalized sample-sample 2D correlation spectroscopy that is calculated by normalizing a sample-sample 2D correlation spectrum over every column or row. An important advantage of the normalized sample-sample 2D correlation spectroscopy over sample-sample 2D correlation spectroscopy is that the excessive information is deleted and more precise information therefore can be extracted even from spectra with a rather low signal-to-noise ratio. The normalized sample-sample 2D correlation spectroscopy allows us to explore the differences in the dynamics of each segment more clearly than the usual sample-sample 2D correlation spectroscopy. Timeresolved IR spectra of FLC-2 in a planar-aligned cell were measured over two delay time ranges from 0 to 150 µs and from 200 to 350 µs at an interval of 10 µs with the polarization angle of 45.0° under a rectangular electric field of (3 V with a 2.5 kHz repetition rate in the smectic-C* (Sm-C*) phase at 115 °C. The normalized sample-sample correlation spectroscopy was applied to these time-resolved spectra. We explored the reorientation process of the alkyl chains, the core, and the carboxylate group of FLC-2 in the positive and negative half-periods of the electric-field-induced switching. It has been found from the present study that the reorientation track of the core in the positive half-period is different from that in the negative half-period, while the reorientation tracks of the CdO dipole moment and alkyl chains are very similar between the positive and negative half-periods. The present study has also revealed that the orientation profile of the core moiety is different from those of the alkyl chains and the CdO dipole moment, and the difference in the profile depends on the change of dc electric field from -3 to +3 V and from +3 to -3 V at an interval of 0.25V . It seems that the core moiety functions in the presence of memory properties. Furthermore, a tilt angle for FLC-2 with a naphthalene ring and without a carboxylate group near the stereocenter is smaller than other similar FLCs with a naphthalene ring and a carboxylate group near the stereocenter. It has been suggested from the analysis of the polarization-angle dependent spectra under the external dc electric field of two polarities that the carboxylate group near the stereocenter plays an important role in forming a large tilt angle and that the carboxylate group in the core moiety is responsible for the emergence of ferroelectricity.

Introduction The electricoptical effects of liquid crystals (LCs) in nematic and smectic phases have been widely studied from the points of basic science as well as practical applications.1-3 Liquid crystals in the smectic-A* (Sm-A*) and the surface-stabilized ferroelectric liquid crystals (SSFLCs) and antiferroelectric liquid crystals (AFLCs) show fast response times to the external electric field and have excellent electrooptical properties.4-9 The electroclinic effect was discovered first in the Sm-A* phase,7 and most of the subsequent investigations have been carried for the Sm-A* phase. However, it was found in 1989 that other phases also exhibit the electroclinic effects.9 The electric-fieldinduced reorientation of LC molecules in a nematic phase is * To whom correspondence should be addressed. Mailing address: Department of Chemistry, School of Science and Technology, KwanseiGakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan. E-mail: [email protected]. Fax: +81-795-65-9077. † Kwansei-Gakuin University. ‡ Fujitsu Laboratories Limited.

nonpolar (i.e., the director n cannot switch by changing the polarity of the external applied electric field).8 In contrast, the long axis of LC molecules in a smectic phase can switch between different states by changing the polarity of the external electric field.9 Thus, the electric-field-induced reorientation of LCs in the smectic phase is polar switching. Despite the great promise of FLCs, the detail mechanism of polar switching associated with a transverse dipole has not yet been fully elucidated because it has been difficult to obtain information about the orientation dynamics of different segments of FLCs separately. Polarized infrared (IR) and time-resolved IR spectroscopies have been used for the past decade to probe the orientation and reorientation dynamics of FLC molecules at submolecular level.10-34 Polarized IR spectroscopy has been employed as a powerful tool for obtaining information about the projection of the orientation of each vibrational mode in the plane orthogonal to the incident IR radiation, while time-resolved IR spectroscopy has been utilized to elucidate the dynamic information regarding the structural and orientational changes of molecular segments

10.1021/jp027295i CCC: $25.00 © 2003 American Chemical Society Published on Web 04/10/2003

4228 J. Phys. Chem. B, Vol. 107, No. 18, 2003 in transient systems.10-34 In our previous studies, the static orientation and dynamics of FLCs with a naphthalene ring (FLC-1 and FLC-3, see Figure 1) were investigated by the use of polarization angle-dependent IR and time-resolved IR spectra, respectively.30-34 We developed the theory on polarization angle-dependent IR absorption of FLCs to obtain the information about the orientations of functional groups.32 From the timeresolved IR spectroscopic study, we found that during the electric-field-induced switching the FLC-3 molecule not only rotates around the layer normal but it also revolves around its own long axis.33 In the present study, the theory on polarization angle-dependent IR absorption of the FLCs has been used to analyze the orientation of FLC-2 in the smectic-C* (Sm-C*) under a dc voltage of (3 V. It has been revealed that the orientation of the FLC-2 molecules is different from the orientations of the FLC-1 and FLC-3 molecules under the external dc electric field of both polarities. The orientations of molecular segments in the Sm-C* phase are symmetrical about the layer normal for FLC-1 and FLC-3, but those of molecular segments for FLC-2 in the Sm-C* phase are asymmetrical. It is generally accepted that in the Sm-C* phase a transverse permanent dipole located near the stereocenter is a key factor for the emergence of ferroelectricity.23 The main purpose of the present study is to explore the dynamical orientation of a FLC molecule with a naphthalene ring and without a carboxylate group near the stereocenter (FLC-2, Figure 1) during the electricfield-induced switching by measuring temporal absorption responses of selected IR bands. Although the structure of FLC-2 is similar to that of FLC-3, FLC-2 does not have a carboxylate group near the stereocenter. We have attempted to explore electric-field-dependent orientation behavior by polarized IR spectroscopy and the electric-field-induced reorientation of FLC-2 by time-resolved IR spectroscopy. It has been suggested that a carboxylate group near the stereocenter is responsible for forming a large tilt angle and that a carboxylate group near the stereocenter plays an essential role in the appearance of ferroelectricity. In recent years, there has been important progress in understanding the theoretical and methodological aspects of twodimensional (2D) correlation spectroscopy.35-37 Sˇ asˇic´ et al.35,37 have proposed two new possibilities of 2D correlation spectroscopy. One is sample-sample correlation spectroscopy,35 and the other is statistical 2D correlation spectroscopy.37 In samplesample correlation spectroscopy, generalized 2D correlation maps having sample axes are created instead of generating 2D maps with variable (wavenumber) axes.35 In the usual generalized 2D38,39 correlation spectroscopy (variable-variable correlation spectroscopy), the correlation between bands is discussed while in the sample-sample correlation spectroscopy one can discuss the concentration dynamics directly. In our previous paper,34 the sample-sample 2D correlation spectroscopy of the time-dependent IR spectra of FLC-3 provided new insight into the dynamics of the liquid crystalline molecule during the electric-field-induced switching. The sample-sample correlation spectroscopy suggests that the dynamics of the alkyl chain is very similar to that of the core and that the large CdO dipoles of the FLC-3 take less time to perform the switching process than the core. An important advantage of the sample-sample 2D correlation spectroscopy is that reliable results can be obtained even from spectra with a rather low signal-to-noise ratio. In the present study, we have proposed normalized samplesample 2D correlation spectroscopy, which is obtained by normalizing sample-sample 2D correlation spectra over every

Zhao et al. column or row, and applied it to time-resolved spectra to reveal the reorientation dynamics of FLC-2 during electric-field-induced switching. The normalized sample-sample 2D correlation spectroscopy enables one to investigate the differences in the dynamics of each segment more clearly than the usual samplesample 2D correlation spectroscopy and the conventional method that monitors changes in absorbances versus time. It has been shown from the normalized sample-sample 2D correlation spectroscopy that the reorientation track of the core moiety in the positive half-period of the electric-field-induced switching is different from that in the negative half-period, while the reorientation tracks of the CdO dipole moment and the alkyl chains are very similar between the positive and negative halfperiods. Theory and Background Two-dimensional correlation spectroscopy is a recently developed spectral analysis method, and we will give a brief description of this technique. The technique was first introduced by Noda40 in 1986 and later developed into the more applicable one, generalized 2D correlation spectroscopy, by the same author.38 Enhanced spectral resolution can be achieved by spreading the data over the second dimension, and detailed interand intramolecular interactions can be studied by 2D correlation spectroscopy. In recent years, the sample-sample 2D correlation spectroscopy were proposed by Sˇ asˇic´ et al.35,37 The approach offers the possibility of analyzing the concentration changes of the species as a function of the external perturbation and is particularly powerful in combination with variable-variable 2D correlation spectroscopy.36 In our previous paper, the sample-sample 2D correlation spectra were calculated for the time-resolved IR spectra of FLC3.34 The orientation dynamics of the alkyl chains, the CdO dipole moment, and the core moiety of FLC-3 were explored by analyzing synchronous sample-sample 2D correlation spectra. The plots of the slice spectra and the three-dimensional representations of the synchronous sample-sample 2D correlation spectra were employed to clearly visualize the information contained in the spectra. By the use of the plots and the threedimensional representations, the time-dependent dynamics of FLC-3 was clearly explored. The covariance matrix of the sample-sample 2D correlation of the time-resolved spectra of FLCs shows correlations between orientations of FLCs as in the case of the correlation Z between concentrations by the following equation:35

Z ) 1/(w - 1)MTM

(1)

where Z denotes the resulting square matrix of s × s (s is the number of samples) and M is the data matrix of w × s (w is the number of wavenumber points). In the case of the timeresolved spectra, each row of M corresponds to a time-resolved spectrum at a certain delay time and s corresponds to the delay time. The cross-product matrix formed by eq 1 provides a 2D pattern having the samples on both axes, and each point in the 2D map presents a correlation between the orientation of a given pair of samples, si and sj. Moreover, from the following equation

(MTM)T) MTM

(2)

ZT ) Z

(3)

we obtain

Equation 3 means that the orientational dynamics represented by ith column of Z is equal to that represented by its ith row.

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Figure 1. Chemical structure of FLC-1, FLC-2, and FLC-3 and the phase transition temperature of FLC-2 in the cooling process.

Moreover, the ith column of Z represents a correlation between the ith delay time and all delay times. This implies that every columns or rows contain common information about the orientational dynamics. However, the profiles of these columns or rows are different from each other because of the differences in the relative intensities of the time-resolved spectra. Thus, it is difficult to compare in more detail the difference in the two kind dynamics for the different intensities of time-resolved spectra. To extract the common information about the orientational dynamics as the delay times, we define each column or row of normalized covariance matrix Znorm as follows:

Zi-norm ) (Zij - Zi-min)/(Z i-max - Z i-min)

(4)

Zj-norm ) (Zij - Zj-min)/(Z j-max - Z j-min)

(5)

where i,j ) 0, 1, ..., s - 1, s. Zi-norm and Zj-norm are the ith column and the jth row of Znorm, repectively. Zij is the (i,j) element of Z. Zi-max and Zi-min are the maximum and the minimum of the ith column of Z. Zj-max and Zj-min are the maximum and minimum of the jth row of Z, respectively. If we calculate Znorm from eq 4, all the columns of Znorm present the orientational dynamics during the electric-field-induced switching. Similarly, we obtain the orientational dynamics during the electric-field-induced switching from all the rows of Znorm calculated from eq 5. The important advantages for Znorm over Z are that the excessive information is deleted and the reliable results can be obtained even for spectra with a rather low signal-to-noise ratio. The reason for the former advantage is that the norm of common vector (row or column of matrix is a vector) can be deleted from the columns or rows of Z. On the other hand, that for the latter advantage is as follows. Normalized samplesample 2D correlation spectroscopy with the cross-product calculation (MTM) of the experimental matrix is the same as sample-sample 2D correlation spectroscopy. The cross-product calculation possesses S/N ∼ w|xi|/2σ, while S/N ∼ |xi|/σ for each data point. The |xi| denotes the absolute value of each data point. The σ is the mean of variances for the experiment data. The limitation of the normalized sample-sample 2D correlation spectroscopy is that it needs two steps to exploit the dynamics for the changes of components in the system. At first,

it decides the kinds of components such as different phases. After that, it is used to analyze the dynamic of a particular component. All columns or rows of Znorm are very similar for monotonic increase or decrease in the dynamics. We can define the average normalized (AN) synchronous sample-sample correlation spectrum Zaverage as follows:

Zaverage ) 1/s

∑ Zi-norm

or

Zaverage ) 1/s

∑ Zj-norm

(6)

where Zaverage is a column or row matrix. Although Zaverage is not a two-dimensional matrix, it contains valuable information of Z. Furthermore, by using Zaverage, one can explore more easily the orientational dynamic of FLCs, because the profile of Zaverage is simple. In general, the sample-sample 2D correlation spectroscopy is a technique where the spectral intensity is plotted as a function of two independent sample variables.35 However, in the normalized sample-sample 2D correlation spectrum, the spectral intensity is plotted as a function of one independent sample variable. Experimental Section Figure 1 shows the structure and the phase transition sequence in the cooling processes of FLC-2. The synthesis of FLC-2 was reported in ref 41. The sample cell used consisted of two BaF2 plates coated with conducting layers of indium tin oxide (ITO) and polyimide rubbed in one direction. The thickness between the two plates was adjusted by a silicone spacer and has been determined to be 1.7 µm from the interference fringe pattern. The cell was filled with the molten sample by capillary action, heated to the isotropic phase, and then slowly cooled to a temperature in the Sm-C* phase. Cyclic temperature treatment was employed to obtain a good homogeneous alignment. Temperature was controlled to an accuracy of (0.05 °C with the aid of a METTLER FP80HT thermocontroller. The approximate size of the domains was in the range of several hundred micrometers.

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Zhao et al. TABLE 1: Dichroic Ratio D and Band Assignments for the Relevant Peaks in the Infrared Spectra of FLC-2 in the Sm-C* Phase

Figure 2. Polarization IR spectra of FLC-2 in the parallel (85°) and perpendicular (175°) polarization geometries.

The time-resolved IR measurements were made by use of a multichannel asynchronous time-resolving FT-IR system (JEOL model JIR-6500 FTIR) equipped with a JEOL IR-MAU 100 microattachment and a mercury-cadmium-tellurium detector. The details of this system were reported in refs 42 and 43. The measurement geometry was described in our previous papers.32,33 A wire grid polarizer was rotated about the propagation direction of the IR radiation at an interval of 5.0°. A rectangular electric field of (3 V and with a 2.5 kHz repetition rate was applied to the electrodes of the cell from a function generator (SONY AFG310). The spectra obtained were corrected for the base line using a nonlinear spline function, and bands arising from the water vapor in the spectrometer were subtracted. To separate the overlapping bands, the IR spectra were curve-fitted using the GRAMS program, and the areas and heights of the separated bands were used for further processing of the data. The normalized sample-sample 2D correlation spectra were calculated in MAT-LAB (version 6, The MathWorks, Inc., Nattick, MA). The original algorithm for the sample-sample 2D correlation spectra was described in ref 35. Results and Discussion Band Assignments and Measurements of Dichroic Ratio. Figure 2 shows polarization IR spectra of FLC-2 in the Sm-C* phase at the polarization angle of 85° and 175°. The direction of ω ) 85° is close to the direction of the molecular long axis of FLC-2 under the external dc electric field of -3V, and the direction of ω ) 175° is close to the perpendicular direction of the molecular long axis. From these spectra the dichroic ratio D, defined as the ratio of the absorbances for the parallel and perpendicular polarizations of the radiation, was calculated for the representative absorption bands. The dichroic ratios and assignments for the isolated and relevant bands are listed in Table 1. The high value of the dichroic ratio for many of the bands associated with the mesogen moiety shows that orientational order of FLC-2 in the Sm-C* phase is very high. The band assignments of the IR spectra were made on the basis of the comparison of the IR spectra of FLC-2 with those of FLC-1 and FLC-3 that have very similar structures (Figure 1).30,31 Figure 3 shows the absorbance versus the polarization angle for bands at 2931, 2859, 1735, 1603, and 1187 cm-1 in the Sm-C* phase at 115 °C under the external dc electric field of (3 V. The symbols represent measured absorbances, and the curves were obtained by applying a curve fitting by a leastsquares method to the data. The angle β is an angle of P0 (P0 is

wavenumber/cm-1

assignmenta

2958 2931 2871 2859 1735 1603 1523 1508 1494 1473 1388 1272 1244 1220 1187 1148 1116 1070 1014

CH3 asym st CH2 antisym CH3 sym st CH2 sym st CdO st (core) ring CdC st ring CdC st ring CdC st ring CdC st CH2 scissoring ring CdC st C-O-C antisym st C-O-C sym st C-O-C sym st

dichroic ratio (D) 0.87 0.76 0.96 0.74 0.74 9.00 10.4 7.70 7.90 4.41 11.78 8.32 8.74 9.21 10.69 6.78 8.71 3.74 7.33

a sym: symmetric. asym: asymmetric. antisym: antisymmtric. st: stretching.

Figure 3. The absorbance versus the polarization angle for some representative bands in the polarized IR spectra of FLC-2 in the SmC* phase at 115 °C under external dc electric fildes of +3 and -3 V.

the polarization direction of the incident radiation at the polarizer setting of 0°) with the direction of the molecular long axis.32 The angle β is called the orientation angle of the molecular long axis. To obtain the β values, a curve-fitting procedure was applied to the plots of the absorbance versus the polarization angle by a least-squares method for the typical bands. The values of β are obtained from the plots of the absorbance versus the polarization angle for some representative bands, and the values of β are listed in Table 2. It can be seen from Figure 3 that the angle β for some representative bands of FLC-2 varies by the change of the polarity of external dc electric field. The variations of β between the two polarities of external dc electric field of (3 V are similar to those for FLC-1 and FLC-3.31,32 The value of β under the external dc electric field of +3 V (β+3V) is different from that of β under the external dc electric field of -3 V (β-3V) (Table 2). As can be seen in Table 2, the difference (average angle) between β+3V and β-3V is 5.9° with the standard deviation of 0.6° for the bands at 1603, 1272, 1244, 1187, 1148, 1116, and 1070 cm-1 due to the core moiety of FLC-2. The angle of 5.9° is much smaller than that of 30° ( 5° for FLC131 and that of 34.0° ( 1.3° for FLC-3.32 These results suggest that a carboxylate group near the stereocenter plays an important role in forming a large tilt angle for FLC-1 and FLC-3 molecules

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TABLE 2: β for Infrared Bands of FLC-2 under External dc Electric Fields of (3 V wavenumber/cm-1

β +3Va

β-3Va

2958 2931 2871 2859 1735 1603 1272 1244 1220 1187 1148 1116 1070

104.2 97.3 114.9 95.9 91.7 93.1 94.0 92.3 94.1 93.8 94.7 93.3 97.2

94.9 89.7 104.5 89.9 87.4 87.6 88.3 87.3 88.4 88.1 88.5 87.9 90.3

a The β+3V and β-3V denote the orientation angles of the molecular long axis under external dc electric field of +3 and -3 V, respectively. Their angle precision is 0.1°.

Figure 5. (A) The synchronous sample-sample correlation spectrum in the 1650-1000 cm-1 region generated from the time-resolved IR spectra of FLC-2 shown in Figure 4. (B) The normalized synchronous sample-sample correlation spectrum in the 1650-1000 cm-1 region generated from the time-resolved IR spectra of FLC-2 shown in Figure 4.

Figure 4. Time-resolved IR spectra of FLC-2 in the Sm-C* phase at 115 °C over the delay time (A) from 0 to 150 µs and (B) from 200 to 350 µs with an interval of 10 µs, respectively.

with a naphthalene ring, and a carboxylate group near the stereocenter plays an essential role in the appearance of ferroelectricity for FLC-2. Time-Resolved Infrared Spectroscopy Combined with Normalized Sample-Sample Two-Dimensional Correlation Spectroscopy. Parts A and B of Figure 4 show time-resolved IR spectra measured under a rectangular electric field of (3 V and with a 2.5 kHz repetition rate for a polarization angle of 45°. These spectra were measured over two delay time ranges from 0 to 150 µs (Figure 4A) and from 200 to 350 µs (Figure 4B) at an interval of 10 µs. The delay times of 0 and 200 µs are starting points of the positive and negative half-periods of

the rectangular electric field of (3 V and with a 2.5 kHz repetition rate, respectively. It can be seen from Figure 4A,B that the absorbance in the 1650-1000 cm-1 region decreases in the positive half-period, while it increases in the negative half-period. This behavior of polar electric-field-induced switching of FLCs is very much different from that of the electricfield-induced reorientation of nematic LCs.1 It is not easy to compare in detail the reorientation of a FLC-2 molecule in the positive half-period with that in the negative period from these time-resolved IR spectra. Thus, we carried out normalized sample-sample 2D correlation analysis for the time-resolved IR spectra. Parts A and B of Figure 5 depict a sample-sample covariance matrix generated from the time-resolved IR spectra in the 16501000 cm-1 shown in Figure 4 and its normalized covariance matrix, respectively. They represent the reorientation process of the core in the positive and negative half-periods of the rectangular electric field of (3 V and with a 2.5 kHz repetition rate. Note that the covariances between the delay time range

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Figure 6. The slices extracted from the normalized synchronous sample-sample correlation spectrum shown in Figure 5B. The symbols denote the normalized covariance at the delay times.

from 0 to 150 µs denote the profile of the positive half-period and that those between the delay time range from 200 to 350 µs denote the profile of the negative half-period. From each delay time axis, it can be seen that the covariances decrease in the positive half-period and increase in the negative half-period in Figure 5A. The peak and valley positions in Figure 5A correspond to the points for the polarity changes from negative to positive and from positive to negative. These results disclose the polar reorientation of the core moiety during the electricfield-induced switching of FLC-2. In the delay time from 0 to 350 µs, there is only one occasion of polarity change from negative to positive and one from positive to negative. However, there are four peaks, (0, 0), (0, 350), (350, 0), and (350, 350) µs, in Figure 5A. The unnecessary information for the analysis of dyanmics is involved in the usual sample-sample 2D correlation spectra. Of note in Figure 5B is that the columns of Znorm are unchanged because we carried out the normalization using by eq 4 for these columns. The delay time at 0 µs represents the start of the positive half-period or the end of the negative half-period, while the delay time at 200 µs corresponds to the end of the positive half-period or the start of the negative half-period. From Figure 5B, we can obtain all information about the reorientation of FLC-2 during the electric-field-induced switching. Figure 6 exhibits slice spectra extracted from the normalized sample-sample covariance matrix shown in Figure 5B. This result is very similar to the plots of the normalized absorbance changes versus the delay times.28,29 However, the result from the normalized sample-sample 2D correlation spectroscopy is more reliable than the plots. The reason for this is that the normalized sample-sample 2D correlation spectroscopy inherits the advantage of the sample-sample 2D correlation spectroscopy for the analysis of the electric-field-induced switching of FLCs, while the plots of the normalized absorbance changes versus the delay times only use the peak value of bands. The important advantage of the sample-sample 2D correlation spectroscopy is that the reliable results can be obtained even for spectra with a rather low signal-to-noise ratio.35 Parts A and B of Figure 7 depict the sample-sample covariance matrix generated from the time-resolved spectra in the 1780-1680 cm-1 region (Figure 4) and its normalized covariance matrix, respectively. They reflect the reorientation

Figure 7. (A) The synchronous sample-sample correlation spectrum in the 1780-1680 cm-1 region generated from the time-resolved IR spectra of FLC-2 shown in Figure 4. (B) The normalized synchronous sample-sample correlation spectrum in the 1780-1680 cm-1 region from generated the time-resolved IR spectra of FLC-2 shown in Figure 4.

process of the CdO dipole moment of FLC-2 in the positive and negative half-periods of the rectangular electric field of (3V and with a 2.5 kHz repetition rate. Figure 8 exhibits the slice spectra of the normalized covariance matrix shown in Figure 7B. It can be seen from these figures that the covariances increase in the positive half-period and decrease in the negative half-period. The peak and valley positions for the CdO dipole moment are different from those of the core moiety (Figures 6 and 8). The similar differences in the peak and valley positions between the core moiety and the CdO dipole moment were observed for FLC-1 and FLC-3.31,33 Parts A and B of Figure 9 show the sample-sample covariance matrix generated from the time-resolved spectra in the 3000-2780 cm-1 region (Figure 4) and its normalized covariance matrix, respectively. The spectra in Figure 9 reveal that the dynamics of alkyl chains of FLC-2 is similar to that of the CdO dipole moment. Figure 10 shows the slice spectra of the normalized sample-sample covariance matrix shown in Figure 9B. Figures 9B and 10 show that the profile of one

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Figure 8. The slices obtained from the normalized synchronous sample-sample correlation spectrum shown in Figure 7B. The symbols denote the normalized covariance at the delay times.

column is similar to those of the other columns in the normalized covariance matrix. This result is in good agreement with the above results for the core moiety (Figures 5B and 6) and the CdO dipole moment (Figures 7B and 8). The very similar profiles of the normalized columns or rows in the normalized covariance matrices of the core moiety, alkyl chains, and the CdO dipole moment suggest that, by using the normalized sample-sample 2D correlation spectroscopy, one can explore the dynamics of FLCs during the electric-field-induced switching. To compare the dynamics among the core moiety, alkyl chains, and the CdO dipole moment, the AN-synchronous sample-sample correlation spectra Zaverage, in the 3000-2780, 1780-1680, and 1650-1000 cm-1 regions, were calculated by use of eq 6 and are shown in Figure 11. It is of note that the profiles of the alkyl chains and the CdO dipole moment are almost identical during the electric-field-induced switching. The profile for the core moiety is nearly opposite from the profiles of the alkyl chains and the CdO dipole moment. Thus, it is difficult to compare the differences in the dynamics between the core moiety and the alkyl chains or the CdO dipole moment. Another notable point in Figure 11 is that the core moiety responds instantaneously upon switching the electric field, while the alkyl chains and the CdO dipole moment require an induction period of about 10 µs (delay times from 0 to 10 µs) before responding to the electric field in the positive half-period. The core requires an induction period of about 10 µs (delay times from 200 to 210 µs) before responding to the electric field in the negative half-period. The dynamics of the alkyl chains are similar to those reported in the literature.29 The different acts of the ferroelectric torque and dielectric torque on the CdO dipole moment and core moiety result in the different the dynamics of the CdO dipole moment and the core moiety in the initial delay times on switching the electric field. Furthermore, the different orientations of the core moiety in the two surface-stabilized states may be the cause for the difference in the dynamics of the core moiety between the initial delay times in the positive and negative half-periods. To analyze the dynamic behavior of the core moiety, the Cd O dipole moment, and the alkyl chains in more detail, the normalized covariance of the core moiety was subtracted from 1. The obtained result is shown in Figure 12, together with the

Figure 9. (A) The synchronous sample-sample correlation spectrum in the 3000-2780 cm-1 region generated from the time-resolved IR spectra of FLC-2 shown in Figure 4. (B) The normalized synchronous sample-sample correlation spectrum in the 3000-2780 cm-1 region generated from the time-resolved IR spectra of FLC-2 shown in Figure 4.

normalized covariances of the CdO dipole moment and the alkyl chains. From Figure 12 it can be seen that the profile of core moiety encircles the profiles of the alkyl chains and the CdO dipole moment during the entire delay times. This suggests that the change rate of the former is slower than the latter two. Parts A and C of Figure 13 show AN-synchronous samplesample correlation covariances of the core moiety (1650-1000 cm-1), the CdO dipole moment (1780-1680 cm-1), and the alkyl chains (3000-2780 cm-1) versus the delay time for the positive and negative half-periods at a rectangular electric field of (3 V. Note that the reorientation track of the core in the positive half-period (from 0 to 150 µs) is different from that in the negative half-period (from 200 to 350 µs) during the electricfield-induced switching. In Figure 13, the coordinate of the delay time axis from 0 to 150 µs coincides with that from 200 to 350 µs with an interval of 10 µs. The profiles for the core responding to the switch of the electric field from the positive to negative and from the negative to positive (Figure 13(A)) are very similar

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Figure 10. The slices obtained from the normalized synchronous sample-sample correlation spectrum shown in Figure 9B. The symbols denote the normalized covariance at the delay times.

Figure 11. The AN-synchronous sample-sample correlation spectrum. The absolute errors of AN covariances are marked by error bars.

to the nonlinear response of FLCs to a change in the external dc electric field.2,3 It can be seen from parts B and C of Figure 13 that the reorientation tracks in the positive and negative halfperiods are very alike for the CdO dipole moment and the alkyl chains. Parts A and C of Figure 14 show normalized covariances of the core moiety (1650-1000 cm-1), the CdO dipole moment (1780-1680 cm-1), and the alkyl chains (3000-2780 cm-1) versus the dc electric field. The dc electric field changes from -3 to +3 V and from +3 to -3 V with an interval of 0.25 V. It is noted that the profiles for the core moiety are different (Figure 14A), while the profiles for the CdO dipole moment and the alkyl chains are alike (Figure 14B,C) during the dc electric field changes from -3 to +3 V and from +3 to -3 V. The tracks of the alkyl chains and the CdO dipole moment show the states of chaos in Figure 13B,C. The reason for this

Figure 12. The AN-synchronous sample-sample correlation spectrum for the core moiety, the CdO dipole moment, and the alkyl chains. The absolute errors of AN covariances are marked by error bars.

may be that the orientation depending on the dc electric field means a change of the state of confusion. Another possible reason is a rather low signal-to-noise ratio of the polarized IR spectra. Note that the profiles for the changes of the orientations depending on the dc electric field (Figure 14) are similar to those of the electric-field-induced the reorientation (Figure 13) for the core moiety, the CdO dipole moment, and the alkyl chains. These results demonstrate that the core moiety plays a key role in the presence of memory properties. Conclusions In the present study, the electric-field-induced switching of FLC-2 without a carboxylate group near the stereocenter has been explored by using polarized time-resolved IR spectroscopy combined with normalized sample-sample 2D correlation

Ferroelectric Liquid Crystal Molecule

J. Phys. Chem. B, Vol. 107, No. 18, 2003 4235

Figure 14. The slices of normalized sample-sample correlation spectrum (A) 1650-1000 cm-1, (B) 1780-1680 cm-1, and (C) 30002780 cm-1 regions under the dc electric field changes from -3 to +3 V and from +3 to -3 V with an interval of 0.25V. The symbols denote the normalized covariance for the dc voltages.

Figure 13. The AN-synchronous sample-sample correlation spectra in the positive and negative half-periods at a rectangular electric field of (3 V in the (A) 1650-1000 cm-1, (B) 1780-1680 cm-1, and (C) 3000-2780 cm-1 regions. The absolute errors of AN covariances are marked by error bars.

spectroscopy. It has been shown that the reorientation track of the core in the positive half-period is different from that in the negative half-period, while the reorientation tracks of the Cd O dipole moment and the alkyl chains are very much alike between the positive and negative half-periods.

The polarization angle-dependent IR spectra have revealed that a tilt angle for FLC-2 with a naphthalene ring and without a carboxylate group near the stereocenter is smaller than that for other similar FLCs with a naphthalene ring and a carboxylate group near the stereocenter. It has been suggested that the carboxylate group near the stereocenter is responsible for forming the large tilt angle and that the carboxylate group group near the stereocenter is essentially important for the emergence of the ferroelectricity. The orientational dyanmics of the core moiety is clearly different from those of the alkyl chains and CdO dipole moment on changing the dc electric field from -3 to +3 V and from +3 to -3 V with an interval of 0.25 V. It seems that the core

4236 J. Phys. Chem. B, Vol. 107, No. 18, 2003 moiety plays a key role in the presence of memory properties. The normalized and AN-synchronous sample-sample 2D correlation analysis has provided solid information about the profiles of the dynamic orientations of each segment during the electric-field-induced switching. The important advantage of the normalized and AN-synchronous sample-sample correlation spectroscopy is that the excessive information is deleted and the reliable results therefore can be obtained even for spectra with a rather low signal-to-noise ratio. Moreover, the normalized and AN-synchronous sample-sample correlation spectroscopy makes easier the detailed comparison of the differences in the two kind dynamics for the different intensities of the timeresolved spectra. References and Notes (1) Scheffer, T. J.; Nehring, J.; Coates, D.; Dijon, J. In Liquid Crystals: Application and Uses; Bahadur, B., Ed.; World Scientific: Singapore, 1990; Vol. 1, pp 232-356. (2) Clark, N. A.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899. (3) Lagerwall, S. T. Ferroelectric and Antiferroelectric Liquid Crystals; Wiley-VCH: Weiheim, 1999; p 169. (4) Patel, J. S.; Goodby, J. W. Opt. Eng. 1987, 26, 373. (5) Kocot, A.; Vij, J. K.; Perova, T. S. In AdVances in Liquid Crystals; Advances in Chemical Physics 113; Vij, J. K., Ed.; Wiley: New York, 2000; p 203. (6) Clark, N. A.; Lagerwall, S. T. Ferroelectrics 1984, 59, 25. (7) Garoff, S.; Meyer, R. B. Phys. ReV. Lett. 1977, 38, 848. (8) Chandani, A. D. L.; Hagiwara, T.; Suzuki, Y.; Ouchi, Y.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys., Part 2 1988, 27, L1265. (9) Li, Z.; Petschek, R. G.; Rosenblatt, C. Phys. ReV. Lett. 1989, 62, 1577. (10) Shilov, S. V.; Mu¨ller, M.; Kru¨erke, D.; Heppke, G.; Skupin, H.; Kremer, F. Phys. ReV. E 2002, 65, 021707-1. (11) Hide, F.; Clark, N. A.; Nito, K.; Yasuda, A.; Walba, D. M. Phys. ReV. Lett. 1995, 75, 2344. (12) Gregoriou, V. G.; Chao, J. L.; Toriumi, H.; Palmer, R. A. Chem. Phys. Lett. 1991, 179, 491. (13) Urano, T.; Hamaguchi, H. Chem. Phys. Lett. 1992, 195, 287. (14) Takano, T.; Yokoyama, T.; Toriumi, H. Appl. Spectrosc. 1993, 47, 1354. (15) Sasaki, H.; Ishibashi, M.; Tanaka, A.; Shibuya, N.; Hasegawa, R. Appl. Spectrosc. 1993, 47, 1390. (16) Shilov, S. V.; Okretic, S.; Siesler, H. W. Vib. Spectrosc. 1995, 9, 57. (17) Masutani, K.; Yokota, A.; Furukawa, Y.; Tasumi, M.; Yoshizawa, A. Appl. Spectrosc. 1993, 47, 1370. (18) Jang, W. G.; Park, C. S.; Clark, N. A. Phys. ReV. E 2000, 62, 5154.

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