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J. Phys. Chem. B 2000, 104, 2846-2852
Polarized Infrared Spectroscopic Study on Hindered Rotation around the Molecular Axis in the Smectic-C* Phase of a Ferroelectric Liquid Crystal with a Naphthalene Ring. Application of Two-Dimensional Correlation Spectroscopy to Polarization Angle-Dependent Spectral Variations Yoshihisa Nagasaki,† Toshiaki Yoshihara,‡ and Yukihiro Ozaki*,† Department of Chemistry, School of Science, Kwansei-Gakuin UniVersity, Nishinomiya 662-8501, Japan, and Display Laboratories, Fujitsu Laboratories Limited, Ohkubo, Akashi 674-0054, Japan ReceiVed: NoVember 5, 1999; In Final Form: January 16, 2000
Polarization angle-dependent infrared spectra of a ferroelectric liquid crystal with a naphthalene ring in the smectic-C* phase were measured at 60 °C under dc voltage of 40 V with positive and negative polarity for investigating the relative orientation of the alkyl chain, mesogen, and chiral segments. The obtained polarization angle-dependent infrared spectra were analyzed by two-dimensional (2D) correlation spectroscopy. The 2D correlation analysis clearly detects slight phase differences in the polarization angle dependence which are hardly recognized by ordinary plots of intensity changes in infrared bands. It has been revealed from the 2D correlation analysis that not only the two carbonyl groups, but also other parts of molecule, carry out hindered rotations about the long molecular axis. The 2D correlation analysis also allows us to separate a CH3 asymmetric stretching band due to the chiral methyl group from those arising from other methyl groups. The CdO stretching bands due to the carbonyl group in the chiral part are also separated by 2D correlation spectroscopy into two bands, which may arise from the rotational isomerism around the O-C axis of the chiral part.
Introduction Ferroelectric liquid crystals (FLCs) have recently received keen interest from the point of view of basic science as well as practical applications because FLCs show fast response and excellent electrooptical properties.1-6 Despite the great promise of FLCs, the detailed mechanism of electric-field-induced reorientation of different segments of FLCs is not yet fully elucidated because of the complexity of their structure and dynamics. In general, molecules forming LCs have a variety of conformational and orientational states with equilibrium population of several possible molecular conformations. Such structural flexibility strongly influences the physical and electrooptical properties of LCs. In the cases of FLCs and antiferroelectric liquid crystals (AFLCs), the conformation and orientation of the chiral alkyl chain relative to the average long molecular axis play key roles in determining their dynamics. During the past decade polarized infrared spectroscopy and time-resolved infrared spectroscopy have been applied as powerful tools for investigating the conformation and dynamics of LCs.7-29 By using infrared spectroscopy, one can monitor orientational and conformational changes at molecular segmental level. We have been investigating dynamics of electric-fieldinduced reorientation and segmental mobility in the smecticC* (Sm-C*) phase of a variety of FLCs by means of polarized infrared spectroscopy and time-resolved infrared spectroscopy with the asynchronous Fourier transform technique. For example, in our recent paper, we reported a time-dependent response of different molecular segments during dynamical switching in * To whom all correspondence should be sent. Mailing address: Department of Chemistry, School of Science, Kwansei-Gakuin University, Nishinomiya 662-8501, Japan Fax: +81-798-51-0914. E-mail: ozaki@ kwansei.ac.jp. † Department of Chemistry. ‡ Display Laboratory.
the electric-field-induced ferroelectric phase of a chiral, smectic AFLC.29 From measurements of temporal response of absorption changes in infrared bands over a range of polarizer orientations at different time delays, we demonstrated that the core responds instantaneously on switching the electric field, while the chiral and achiral alkyl chains require an induction period of ∼15 µs before responding to the electric field.29 In the present study we have investigated a new FLC with a naphthalene ring which has a bookshelf layer structure for a particular alignment of the film in the Sm-C* phase.30 Polarized infrared spectroscopy has been used in the present study to explore molecular structure and alignment of FLCs because it yields valuable information about polarization angle dependence for each functional group.28,29 The analysis of the polarization angle dependences of infrared band intensities is not always easy partly because some of bands may overlap each other and partly because some of bands may show very similar polarization angle dependences. To analyze the polarization angle dependences in more detail, we have applied two-dimensional (2D) correlation spectroscopy.31 2D correlation spectroscopy is a technique where the spectral intensity is plotted as a function of two independent spectral variables.31 This technique often enables one to obtain additional useful information, which is not readily available from the conventional one-dimensional spectrum, and sort out complex or overlapped spectral features by spreading peaks along the second dimension.31 The 2D spectra indicate clear differentiation between the origins of infrared signals, e.g., those from the molecular vibrations of the side group and those from the main chain backbone methylene groups. In a typical 2D experiment, a series of perturbation-induced dynamic spectra are collected in some sequential order.32-41 In many cases, dynamic spectra are collected as a transient function of time. However, it is also possible to detect dynamic spectra as a function of the
10.1021/jp9939266 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/07/2000
Smectic-C* Phase of Ferroelectric Liquid Crystal
Figure 1. Structure of FLC-1 and FLC-2 and the phase transition temperatures of FLC-1
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Figure 2. Schematic arrangement for the polarized infrared measurements. I and II are the projections of the directions of the molecular long axis in the plane of the cell window, YZ (surface-stabilized states of the FLC), θ is the angle between the projection of the molecular long axis and the projection of the smectic layer normal, E is the direction of the applied electric field, X is the direction of the infrared light.
quantitative measure of the imposed physical effect itself, such as temperature, pressure, concentration, stress, electric field, etc. In the present case, the perturbation is not imposed upon a sample itself, and polarization angle is used as a perturbation. The polarization angle dependences of all band intensities should obey simple sinusoidal forms if the transmittance or absorptivity (absorptivity ) 1 - transmittance) is used. Thus, we have selected the absorptivity for the analysis of polarization angle dependence. Experimental Section Sample. The chemical structure along with the phase transition temperatures of the investigated chiral ferroelectric liquid crystal, FLC-1, are shown in Figure 1, where the structure of FLC-2 is also shown. FLC-2 was employed to help band assignments of FLC-1. The synthesis of these LCs was reported in ref 30. The sample cell consisted of two BaF2 plates coated with conducting layer of indium tin oxide (ITO) and polyimide rubbed in one direction. The thickness between the two plates, as determined from the interference fringe pattern, was adjusted to be 1.7 µm with silicone spacers. The cell was filled from the melted sample by capillary action, heated to the isotropic phase, and then slowly cooled to a temperature in the Sm-C* phase. Temperature was controlled to an accuracy of (0.05 °C with the aid of METTLER FP80HT. The approximate size of the monodomain was in a several hundred micrometer range. Infrared Spectroscopy. Polarized infrared spectra were measured with a JEOL JIR-6500 FT-IR spectrometer equipped with a JEOL IR-MAU100 microattachment and a mercurycadmium-telluriam (MCT) detector. Figure 2 illustrates the measurement geometries schematically. A wire grid polarizer was rotated about the axis parallel to the propagation direction of the radiation. The two surface-stabilized states are indicated by arrows I and II. The polarized infrared spectra were measured under dc applied voltages of 40 V of both polarities at 60 °C. The polarization angle ω is taken as zero when the polarization direction of the incident infrared radiation coincides with the rubbing direction. Two-Dimensional Correlation Analysis. The synchronous and asynchronous spectra were calculated based upon a new algolithm recently developed by Noda42 used in a software program named 2D Pocha composed by D. Adachi (Kwansei Gakuin University).
Figure 3. A. Infrared spectra of FLC-1 and FLC-2. B. An enlargement of the CdO stretching band region of the spectra shown in Figure 3A.
Results Band Assignments and Measurements of Dichroic Ratio. Figure 3A shows infrared spectra of FLC-1 and FLC-2, respectively. The infrared spectrum of FLC-2 was measured in order to distinguish the band assignments of two CdO stretching modes of FLC-1. An enlargement of the CdO stretching band region is presented in Figure 3B. Note that FLC-2 yields one symmetrical band at 1736 cm-1 while FLC-1 gives two bands at 1736 and 1721 cm-1. Thus, we assign the bands at 1736 and 1721 cm-1 to the CdO stretching modes of the core part and
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Figure 4. Polarization infrared spectra of FLC-1 at 60 °C in the parallel (ω ) 15°) and perpendicular (ω ) -75°) polarization geometries.
TABLE 1: Dichroic Ratio (D) and Vibrational Band Assignments for the Relevant Peaks in the Infrared Spectra of FLC-1 in the Sm-C* Phase wavenumber/cm-1 a
D (A|/A⊥)
2960 (m) 2928 (s) 2874 (m) 2856 (m) 1736 (s, sh) 1721 (s) 1606 (m) 1510 (m) 1475 (m) 1274 (m) 1257 (m) 1192 (m) 1170 (m) 1150 (m) 1096 (m) 1065 (m)
0.8 0.6 0.8 0.5 0.8 0.6 9.5 6.8 3.2 6.3 8.0 9.0 9.8 6.5 5.8 2.7
assignmentb CH3 asym. st. CH2 antisym. st. CH3 sym. st. CH2 sym. st. CdO st. (core) CdO st. (chiral) ring CdC st. ring CdC st. C-O-C antisym. st. C-O-C antisym. st. ring CH def.
Figure 5. Polarization angle dependence of polarized infrared spectra of FLC-1 in the Sm-C* monodomain at 60 °C under dc electric field of +40 V.
C-O-C sym. st. C-O-C sym. st.
a w, weak; m, medium; s, strong; sh, shoulder. b sym, symmetric; asym, asymmetric; antisym, antisymmtric; st, stretching; def, deformation.
the part near the chiral carbon atom, respectively. Band assignments for the relevant peaks in the infrared spectrum of FLC-1 are summarized in Table 1. Figure 4 shows the polarized infrared spectra as a function of polarization angle ω for a monodomain of FLC-1 in the SmC* phase at ω ) 15° and -75°. The direction of ω ) 15° is that of molecular long axis under dc voltage of +40 V, and that of ω ) -75° is perpendicular to that of ω ) 15°. From these spectra, the dichroic ratio D, defined as the ratio of the absorbances for the parallel and perpendicular polarizations of light, for the absorption bands was calculated. The dichroic ratios for the isolated and relevant bands are also listed in Table 1. The high value of the dichroic ratios for some of the bands associated with the mesogen moiety shows that the degree of orientational order of FLC-1 in the Sm-C* phase is very high. Figure 5 exhibits polarization-angle dependent infrared spectra of the sample in the Sm-C* phase measured at 60 °C under dc voltage of +40 V. Figure 6 shows normalized absorptivity
Figure 6. Normalized absorptivity versus polarization angle for some representative bands in the polarized infrared spectra of FLC-1 in the Sm-C* monodomoain at 60 °C under dc electric field of +40 V.
(absorptivity ) 1 - transmittance) versus polarization angle for the bands at 2928, 1736, 1721, 1606, 1192, 1170, and 1150 cm-1. To calculate the absorptivity of overlapped bands at 1736 and 1721 cm-1, we carried out curve fitting. Of note in Figure 6 is that the two CdO stretching bands, particularly the band at 1736 cm-1, show a polarization angle dependence clearly different from the other bands. However, it seems that other five bands show intensity changes with the same phase or totally reverse phase. The observations in Figure 6 suggest that motion
Smectic-C* Phase of Ferroelectric Liquid Crystal
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Figure 7. (A) Synchronous and (B) asynchronous 2D infrared correlation spectra in the 1780-1550 cm-1 region generated from the polarization angle (between -90°and 90°) -dependent polarized spectral variations of FLC-1 in the Sm-C* monodomain at 60 °C under dc electric field of +40 V.
Figure 8. (A) Synchronous and (B) asynchronous 2D infrared correlation spectra in the 1310-1150 cm-1 region generated from the polarization angle (between -90°and 90°) -dependent polarized spectral variations of FLC-1 in the Sm-C* monodomain at 60 °C under dc electric field of +40 V.
of the two carbonyl groups is strongly hindered. Since the carbonyl groups have large polarization, it is likely that they are subjected to the hindered rotation. Figure 6 does not give us any information about the rotation of other segments of FLC1. Thus, we carried out 2D correlation analysis for the series of polarization-angle dependent infrared spectra. 2D Correlation Spectroscopy. Figure 7 shows (A) synchronous and (B) asynchronous 2D correlation spectra in the 17801550 cm-1 region, generated from the polarization angle dependent infrared spectra of FLC-1. Note that one strong autopeak is clearly observed at 1606 cm-1. This peak corresponds to the stretching modes of the benzene and naphthalene rings. This suggests that the band due to the ring stretching modes changes significantly with the polarization angle. There
are negative cross-peaks at (1736 and 1721 vs 1606) cm-1, indicating that the two CdO stretching bands and the ring stretching band show the polarization angle dependent intensity changes in the reverse direction. In the asynchronous spectrum are observed three cross-peaks at (1608 vs 1736, 1728 and 1715) cm-1. This is clear evidence for the existence of three CdO stretching bands. The splitting of the band near 1721 cm-1 due to the carbonyl group in the chiral part may be ascribed to the rotational isomerism around the O-C bonds; there may be two conformers around the O-C bond. The sign of the cross-peaks indicates that the phase of the intensity change in lower frequency components of the two CdO stretching bands is delayed compared with that of the high-frequency component. In addition, the sign of cross-peaks
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Figure 9. Synchronous (A) and asynchronous (B) 2D infrared correlation spectra between the 1780-1550 cm-1 and 1310-1150 cm-1 regions generated from the polarization angle (between -90°and 90°) -dependent polarized spectral variations of FLC-1 in the Sm-C* monodomain at 60 °C under dc electric field of +40 V.
Figure 10. (A) Synchronous and (B) asynchronous 2D infrared correlation spectra between the 1780-1550 cm-1 and 3000-2800 cm-1 regions generated from the polarization angle (between -90°and 90°) -dependent polarized spectral variations of FLC-1 in the Sm-C* monodomain at 60 °C under dc electric field of +40 V.
between the CdO stretching bands and the ring stretching band suggests that the phase of the band intensity change is delayed in the order of bands at 1736, 1606, and 1728 and 1715 cm-1. This conclusion is in good agreement with the observation in Figure 6. Another notable point in Figure 7B is that there are cross-peaks near 1600 cm-1. Probably, the ring stretching bands of the benzene and naphthalene rings appear separately in the cross-peaks. The existence of the cross-peaks in the asynchronous spectrum suggests that the directions of the transition moments of the ring stretching modes of the benzene and naphthalene rings are different and that the rotations of the two rings are hindered. Since each ring forms a π-electron resonance system with the neighboring carbonyl group, it seems very likely
that the rotation of both phenyl and naphthalene rings are affected by the hindered rotation of each carbonyl group. Figure 8 shows (A) synchronous and (B) asynchronous 2D correlation spectra in the 1310-1130 cm-1 region. In this region bands due to the C-O-C antisymmetric and symmetric stretching modes and in-plane stretching modes of both rings are expected to appear. It seems from the synchronous spectrum that all the bands in this region show nearly in phase polarization angle dependence. However, the asynchronous spectrum in Figure 8B reveals that the phase of intensity change of the band at 1192 cm-1 is ahead compared with those of the bands at 1170 and 1150 cm-1 because there are cross-peaks (1192 vs 1170 and 1150 cm-1) and the sign of the cross-peaks above
Smectic-C* Phase of Ferroelectric Liquid Crystal the diagonal line is positive. In Figure 6 three bands at 1192, 1170, and 1150 cm-1 show almost the same intensity changes with the polarization angle. However, the 2D correlation analysis clearly reveals that the band at 1192 cm-1 has slightly different polarization angle dependence from the rest. Thus, this is a good example showing the potential of 2D correlation spectroscopy in detecting a slight difference in response to perturbation. Synchronous and asynchronous 2D correlation spectra between the 1780-1550 cm-1 and 1310-1150 cm-1 regions are shown in Figure 9A,B, respectively. Figure 10A,B depict those between the 3000-2800 cm-1 and 1780-1550 cm-1 regions. It can be seen from Figures 9B and 10B that there are crosspeaks between the band at 1736 cm-1 and most of the bands in the 1310-1150 and 3000-2800 cm-1 regions and between the bands at 1728 and 1715 cm-1 and most of the bands in the 1310-1150 and 3000-2800 cm-1 regions and that the crosspeaks relating to the band at 1736 cm-1 and those relating to the bands at 1728 and 1715 cm-1 show always different signs. These observations are in good agreement with the results in Figure 6. In Figure 9B, the band at 1192 cm-1 again shows different behavior from those at 1170 and 1150 cm-1. A negative crosspeak appears between the bands at 1606 and 1192 cm-1, indicating that there is a phase difference in the polarization angle-dependent intensity variations between the bands at 1606 and 1192 cm-1. The asynchronous spectrum in Figure 10B clearly shows that the band at 1606 cm-1 due to the ring stretching modes of the two aromatic rings does not always undergo complete out of phase intensity change with the band at 2928 cm-1 due to the antisymmetric CH2 stretching mode of the alkyl chain. There may be slight deviation from the complete out of phase intensity change, although Figure 6 suggests the out of phase change. Another notable point in Figure 10B is that the band at 2945 cm-1 has a positive cross-peak with the band at 1606 cm-1 while the band at 2965 cm-1 shares a negative cross-peak with the same band. In the original spectrum it seems that only one band assigned to the asymmetric CH3 stretching band appears at 2960 cm-1. The asynchronous spectrum demonstrates a powerful deconvolution ability. Probably, the two bands at 2965 and 2945 cm-1 are due to the asymmetric CH3 stretching modes of different CH3 groups of FLC-1 whose intensities show different polarization angle dependence. The two methyl bands at 2965 and 2945 cm-1 show different signs in the cross-peaks at (2965, 1606) and (2945, 1606) cm-1, showing that the two bands have completely different polarization angle dependences. Probably, the band at 2945 cm-1 is assigned to the chiral CH3 groups because it is likely that only this band, whose transition moment is in the direction of the short molecular axis, appears separately. A band due to the antisymmetric CH2 stretching modes also splits into two at 2918 and 2928 cm-1 in the asynchronous spectrum (Figure 10B). The low frequency component probably corresponds to the CH2 antisymmetric stretching mode of alkyl chain with trans-zigzag conformation while the high-frequency component may be ascribed to the alkyl part having significant gauche structures. Conclusions This paper has demonstrated the potential of 2D correlation spectroscopy in detection of slight differences in the polarization angle dependent intensity variations. 2D correlation spectroscopy has been employed extensively in the past to analyze timedependent infrared spectral changes of LCs. In almost all these cases time-dependent evolution and subsequent relaxation of
J. Phys. Chem. B, Vol. 104, No. 13, 2000 2851 the spectral signals arising from various excited constituents of the system as a consequence of the applied electric field perturbation have been investigated. This is the first time that the polarization angle has ever been used as the perturbation variable of 2D correlation spectroscopy. This study has demonstrated a new possibility of 2D correlation spectroscopy in LC research. Three important conclusions can be reached from the present study. First, not only the two carbonyl groups but also the benzene and naphthalene rings undergo a hindered rotation around the molecular long axis, although the effects of the rotations are smaller for the two aromatic rings. Second, there are two rotational isomers around the O-C bond in FLC-1. Third, the chiral CH3 group shows an asymmetric stretching mode at 2945 cm-1, separately from those due to other CH3 groups. References and Notes (1) Clark, N. A.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899. (2) Matsumoto, S.; Maruyama, A.; Hatho, H.; Kinoshita, Y.; Harai, H.; Ishikawa, M.; Kamagani, S. Ferroelectrics 1988, 85, 235. (3) Lagerwall, S. T.; Clark, N. A.; Dijon, J.; Clerc, J. F. Ferroelectrics 1989, 94, 3. (4) Dijon, J. in Liquid Crystals, Application and Uses; Bahadur, B., Ed.; World Scientific: Singapore, 1990; Vol. 1, p 305. (5) Walba, D. M. Ferroelectric Liquid Crystals. In AdVances in the Synthesis and ReactiVity of Solids; Lallouk, T. E., Ed.; JAI Press Ltd: London 1991; Vol. 1, p 173. (6) . Goodby, J. W.; Blinc, R.; Clark, N. A.; Lagerwall, S. T.; Osipov, M. A.; Pikin, S. A.; Sakurai, T.; Yoshino, K.; Zeks, B. Ferroelectric Liquid Crystals. Principles, Properties and Applications; Gordon and Breach: Philadelphia, 1991. (7) Toriumi, H.; Sugisawa, H.; Watanabe, H. Jpn. J. Appl. Phys. 1988, 27, L935 (Part 2). (8) Gregoriou, V. G.; Chao, J. L.; Toriumi, H.; Palmer, R. A. Chem. Phys. Lett. 1991, 179, 491. (9) Sugisawa, H.; Toriumi, H.; Watanabe, H. Mo1. Cryst. Liq. Cryst. 1992, 214, 11. (10) Urano T.; Hamaguchi, H.; Chem. Phys. Lett. 1992, 195, 287. (11) Takano, T.; Yokoyama, T.; Toriumi, H. Appl. Spectrosc. 1993, 47, 1354. (12) Sasaki, H.; Ishibashi, M.; Tanaka, A.; Shibuya, N.; Hasegawa, R. Appl. Spectrosc. 1993, 47, 1390. (13) Urano T.; Hamaguchi, H. Appl. Spectrosc. 1993, 47, 2108. (14) Shilov, S. V.; Okretic, S.; Siesler, H. W. Vib. Spectrosc. 1995, 9, 57. (15) Masutani, K.; Sugisawa, H.; Yokota, A.; Furukawa, Y.; Tasumi, M. Appl. Spectrosc. 1992, 46, 560. (16) Masutani, K.; Yokota, A.; Furukawa, Y.; Tasumi, M.; Yoshizawa, A. Appl. Spectrosc. 1993, 47, 1370. (17) Czarnecki, M.; Katayama, N.; Ozaki, Y.; Satoh, M.; Yoshio, K.; Watanabe, T.; Yanagi, T. Appl. Spectrosc. 1993, 47, 1382. (18) Czarnecki, M.; Katayama, N.; Satoh, M.; Watanabe, T.; Ozaki, Y. J. Phys. Chem. 1995, 99, 14101. (19) Katayama, N.; Czarnecki, M. A.; Ozaki, Y.; Murashiro, K.; Kikuchi, M.; Saito, S.; Demus, D. Ferroelectrics 1993, 147, 441. (20) Katayama, N.; Sato, T.; Ozaki, Y.; Murashiro, K.; Kikuchi, M.; Saito, S.; Demus, D.; Yuzawa, T.; Hamaguchi, H. Appl. Spectrosc. 1995, 49, 977. (21) Shilov, S. V.; Okretic, S.; Siesler, H. W.; Zentel, R.; Oge, T. Macromolecules 1995, 16, 125. (22) Czarnecki, M. A.; Okretic, S.; Siesler, H. W. J. Phys. Chem. B 1997, 101, 374. (23) Shilov, S. V.; Okretic, S.; Siesler, H. W.; Czarnecki, M. A. Appl. Spectrosc. ReV. 1996, 31, 82. (24) Hide, F.; Clark, N. A.; Nito, K.; Yasuda, A.; Walba, D. M. Phys. ReV. Lett. 1995, 75, 2344. (25) Kim, K. H.; Ishikawa, K.; Takezoe, H.; Fukuda, A. Phys. ReV. E 1995, 51, 2166. (26) Jin, B.; Ling, Z.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.; Fukuda, A.; Kakimoto, M.; Kitazume, T. Phys. ReV. E 1996, 53, 4295. (27) Kim, K. H.; Miyachi, K.; Ishikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys., Part 1, 1994, 33, 5850. (28) Verma, L.; Zhao, B.; Jiung, S. M.; Shen, J. C.; Ozaki, Y. Phys. ReV. E 1997, 56, 3053. (29) Verma, A. L.; Zhao, B.; Terauchi, H.; Ozaki, Y. Phys. ReV. E 1999, 7, 1868.
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