8698
2007, 111, 8698-8701 Published on Web 06/29/2007
Chirality Transfer between Weakly Birefringent and Electric-Field-Induced Highly Birefringent B2 Phases in a Bent-Core Mesogen Seng Kue Lee,* Lu Shi, Masatoshi Tokita, Hideo Takezoe, and Junji Watanabe Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 ReceiVed: May 20, 2007; In Final Form: June 17, 2007
Electric-field-induced transition was observed for the weakly birefringent chiral B2 phase, which is formed from the banana molecule based on the naphthalene bent core. This phase is considered to possess the twisted grain boundary (TGB)-like helical structure. When an electric field is applied, the TGB-like helix unwinds. The resulting large domain of the SmCAPA phase shows the high birefringence and simultaneously the antiferroelectric switching between SmCAPA and SmCSPF states. Through this field-induced transformation, two interesting features are obtained. First, the initially formed chiral domains are preserved even after the field-induced transformation to the unwound SmCAPA phase. This indicates the close correlation between the TGB-like helix and the layer chirality in such a way that the helical sense of the TGB-like helix is memorized as the layer chirality of the homochiral SmCAPA phase. Second, there is a critical temperature, above which the helicoidal structure is stable against the electric field. There is a competition between winding into a TGB-like structure and unwinding due to the electric field, and at higher temperatures, the helicoidal power is too strong to surpass the effect of the electric field.
Introduction Since the discovery of unique polarity and chirality in bentshaped molecules, their unconventional mesomorphic properties have been extensively studied and have opened a new era for polar order and chiral superstructures in soft materials.1,2 Among several phases so far identified, the B2 phase is particularly interesting. In this phase, the bent-shaped molecules are tilted to the smectic layer, resulting in the layer chirality as well as the polarity. On the basis of the layer chirality and polarity, the B2 phase has been described to be able to possess four distinct types of structures, two homochiral (SmCSPF and SmCAPA) and two antichiral (SmCSPA and SmCAPF).3 The antiferroelectric state is generally the ground state. More often, the observed B2 structure is synclinic antiferroelectric (SmCSPA), while anticlinic antiferroelectric (SmCAPA) order is less frequently observed. Recently, the unusual B2 phase forming the weakly birefringent and chiral domains has been reported.4-17 The domain texture looks like the B4 phase.18 However, from the X-ray diffraction pattern, there is no reason to distinguish it from the normally observed B2 phase with high birefringence. It exhibits the layer reflection, showing the tilting of molecules to the layer, and the broad outer reflection, showing the liquid-like packing of molecules within a layer. Its first example was the highesttemperature mesophase (called Sm1) formed from the naphthalene based-banana molecules,4 and then, several reports were made.5-17 More recently, it was also reported that this type of B2 phase shows antiferroelectric response after the weakly birefringent domains are altered to the highly birefringent domains characteristic of the conventional B2 phase.10,11,17 Thus, the weakly birefringent and chiral structure is one of the typical 10.1021/jp0739014 CCC: $37.00
domain structures in the B2 phase and has been attributed to the helical twisted grain boundary (TGB)-like structure.17,18 In this Letter, we investigate the field-induced switching behavior of the unusual B2 phase formed from the bent-shaped molecule with a central naphthalene core, N(2,7)-12-PIMB,
Experimental Section N(2,7)-12-PIMB was synthesized by following the methods in the previous paper.4 The calorimetric behavior was investigated with a Perkin-Elmer DSC-7 calorimeter at a scanning rate of 10 °C min-1. Optical microscopic observations were made using a polarizing microscope (an Olympus BX50). X-ray diffraction (XRD) photographs were taken at different temperatures by using Ni-filtered Cu KR radiation (a Rigaku RU-200 BH). Polarization reversal current was observed using a highspeed amplifier (FLC Electronics, F20A) connected to a function generator (NF Electronic Instruments, WF1945A). Here, the sample was sandwiched between two glass plates with a transparent indium tin oxide (ITO) electrode. The cell gap was controlled to be 8 µm using polyethylene terephthalate films. The temperature for all of the measurements was regulated within an accuracy of 1 °C by using a Metter FP-90 hot stage. Results and Discussion N(2,7)-12-PIMB shows the following Iso-B2-B4 transition upon decreasing temperature: Iso (216.3 °C/ ∆H ) 17.1 kJ © 2007 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 111, No. 30, 2007 8699
Figure 1. Photomicrographs in the B2 phase. Upon cooling from the isotropic melt, the B2 phase appears as small fractal nuclei in (a). Those coalesce to each other in (b) and finally form two types of chiral domain with opposite optical rotations in (c). Here, the uniform background observed in (a) and (b) is the isotropic phase. Upon cooling to the B4 phase, domain texture is not altered at all, but birefringence is slightly increased. The textures were observed by rotating the polarizer by 15° from the cross-polarized position to detect the development of two types of chiral domains. One of chiral domains is bright, and another is dark. The brightness and darkness are altered to each other by an opposite rotation of the polarizer.
Figure 2. Photomicrographs observed upon the field-induced transition between the weakly birefringent chiral phase and the highly birefringent phase at 170 °C. Immediately after the field (70 V for cell gap of 8 µm) is applied for the weakly birefringent phase in (a), the highly birefringent grainy textures of the SmCAPA phase appear as in (b). The grainy textures finally develop to the well-defined fan textures in (c) when the field is continuously applied. The transmittance light is strengthened to give a good contrast for (a). The inset indicates the relative positions of polarizer and analyzer.
mol-1) B2 (162.6 °C/ ∆H ) 18.7 kJ mol-1) B4 (mp 172.1 °C). Upon cooling from the isotropic melt, the B2 phase exhibits unusual textures, as shown in Figure 1. Several small fractal nuclei initially appear from the isotropic liquid phase (see Figure 1a) and then gradually combine to large domains with a very low birefringence (Figure 1b). The textures finally obtained consist of two types of optically active domains (Figure 1c), the brightness of which is interchanged by the positive and negative rotations of the polarizer against the analyzer. This means that these two domains have optical rotation with the opposite signs. In addition, the circular dichroism (CD) spectra are observed with corresponding signs. X-ray pattern shows the outer broad reflection with a spacing of 4.5 Å and the inner layer reflection with a spacing of 44.9 Å. Thus, molecules are packed into a layer with liquid-like nature, as in a conventional B2 phase. Upon cooling to the B4 phase, domain texture is not altered at all, but birefringence is slightly increased. The spacing of the layer reflection is 47.8 Å, which corresponds to 48.0 Å, the molecular length calculated for most extended chain. Thus, the molecules in the B4 phase lie perpendicularly to the layer, while those in the B2 phase are tilted by 20° to the layer normal. The relatively high birefringence in the B4 phase is due to this rearrangement of molecules within a layer.12 The unusual B2 phase shows the switching behavior against an external electric field, as has been reported.10,11,17 Upon application of an electric field, the weakly briefringent chiral domains are altered to the highly birefringent but grainy domains, as shown in Figure 2a and b, and then, the clear switching behavior is observed. The brightness of the grainy domains may be due to the unwinding of the TGB-like structure; the size of each domain becomes larger than the wavelength. When the field is continuously applied, the grainy domains develop to the well-defined fan texture, as in Figure 2c. In this fan texture, we can detect that the light extinction directions
Figure 3. Polarization reversal current in the B2 phase at 170 °C under the application of a 320 Vpp triangular wave voltage of 12.5 Hz. The cell gap is 8 µm, and the spontaneous polarization value is 9.97 nC/ cm2.
are along the polarizer-analyzer axes in the field-off state, while they are inclined to these axes in the field-on state (see later Figure 5). Thus, we know the field-induced transformation of anticlinic to synclinic alignment of molecules. Inclination angle is around 20°, which coincides with the tilt angle determined using the X-ray data. Moreover, by applying a triangular wave electric field, two switching current peaks showing the antiferroelectric response are observed in a half cycle for both the grainy and well-developed fan-like texture, as shown in Figure 3. The integral of current peaks increases as the birefringent area grows, meaning that the antiferroelectric switching response could be attained after a textural transformation. Thus, the observed field-induced switching indicates that the unusual B2 phase possesses the homochiral SmCAPA structure as a ground state and that the antiferroelectric switching takes place between SmCAPA and SmCSPF states.
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Figure 4. Electric-field-induced transition behavior of the B2 phase observed as a function of the temperature and field strength. The data are collected with the increase of field strength at constant temperature. The cell gap is 8 µm. No response is observed in zone (a), and the clear transition from the weakly birefringent chiral domain to the highly birefringent one is detected in zone (c). Two types of domains coexist in the intermediate zone (b).
It should be noted here that there are critical field strengths depending on temperature for the switching behavior. As found in Figure 4, the higher the temperature, the larger the critical electric strength. In a higher temperature region above 195 °C, chiral domains do not switch at all by applying an electric field (in zone a of Figure 4), probably because of the strong helical twisting power to sustain the TGB-like structure. In a lower temperature region, on the contrary, those easily transform to the highly birefringent domains even at a low field strength (zone c). Two types of domains coexist in an intermediate temperature region (zone b). No electric response is observed in the B4 phase. Here, a simple question arises on how the weakly birefringent chiral domains are re-formed after the transformation to the
Letters highly birefringent phase. To clarify this point, we observed the change of the optical microscope textures with a temperature variation between 170 and 195 °C at a constant field of 70 V (refer to Figure 4). Figure 5a shows the original texture observed for the weakly birefringent chiral B2 phase at 195 °C. At this temperature, the weakly birefringent chiral phase is stably formed even if the high electric field is applied as stated above. Upon cooling to 170 °C, the highly birefringent grainy textures appear and develop into the fan-shaped textures when the field is continuously applied (see Figure 5b). Upon heating again to 195 °C, it transforms again to the weakly birefringent chiral B2 phase (see Figure 5c). Comparing Figure 5a and c, we notice that, by this transformation, upon a cooling and heating cycle, the chiral domains are restored, in other words, the chirality is preserved after a conversion to the highly birefringent domains by external electric field. Further, we know that the extinction direction rotates oppositely between the fan-shaped textures developing from the domains with opposite chiral senses (see the texture within the circles in Figure 5b and d). This means that the helical sense of the TGB-like helix in weakly birefringent domains is memorized as the sign of layer chirality even after the transformation to the highly birefringent B2 domain. Conclusion In summery, the unusual B2 phase with low birefringence and chirality has essentially the homochiral SmCAPA structure as a ground state; however, it is segmented to the small size of domains. The X-ray pattern by a microbeam with a beam size of 3 × 4 µm2 does not show any orientation for this phase; therefore, the size of each domain is markedly less than this size. Such a fine domain may be smaller than the wavelength, leading to the low birefringence or, sometimes, an apparently isotropic phase.12 All of these characteristics as well as the exhibition of optical rotatory dispersion (ORD) and CD can be explained by the TGB-like helix; small blocks are twisted with respect to each other to make a super helical structure.17,18 Some papers report that the segmentation into fine domains randomly takes place.12,14,16 However, there is no reason for this. When
Figure 5. Photomicrographs observed through the transition between a weakly birefringent chiral domain at 195 °C and a fan-shaped domain at 170 °C. Here, a field of 70 V is continuously applied, while the temperature is varied between 195 and 170 °C. Upon cooling to 195 °C from the isotropic melt, the chiral domain texture appears in (a), and upon further cooling to 170 °C, it is altered to the fan-shaped texture in (b). Upon heating again to 195 °C, the chiral domain texture is re-formed in (c). From a comparison of (a) and (c), it can be found that the chirality of the weakly birefringent phase is preserved even after the transformation to the highly birefringent B2 phase. Part (d) shows the fan-shaped texture observed upon the off-state of the field. In a comparison with (b), it can be found that the extinction direction rotates oppositely between the two domain zones initially formed with opposite chirality in (a).
Letters the field is applied, the TGB-like helix unwinds. The resulting large domain of the SmCAPA phase shows the high birefringence and, simultaneously, the antiferroelectric switching between the SmCAPA and SmCSPF states. Through this experimental observation, two important facts are obtained. First, the initially formed chiral domains are preserved even after the field-induced transformation to the unwound SmCAPA phase. This indicates the close correlation between the TGB-like helix and the layer chirality such that the helical sense of the TGB-like helix is memorized as the layer chirality of the homochiral smectic phase. Such a memory effect has been observed upon a transformation between the B2 and B4 phases.19 Second, at the higher temperatures above 195 °C, the helicoidal structure is stable against the field. This can be caused by a competition between winding into a TGB-like structure and unwinding due to the electric field; at higher temperatures, the helicoidal power may be too strong to surpass the effect of the electric field. References and Notes (1) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater. Chem. 1996, 6, 1231. (2) Sekine, T.; Niori, T.; Sone, M.; Watanabe, J.; Choi, S. W.; Takanishi, Y.; Takezoe, H. Jpn. J. Appl. Phys. 1997, 36, 6455. (3) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924. (4) Thisayukta, J.; Nakayama, Y.; Kawauchi, S.; Takezoe, H.; Watanabe, J. J. Am. Chem. Soc. 2000, 122, 7441.
J. Phys. Chem. B, Vol. 111, No. 30, 2007 8701 (5) Dantlgraber, G.; Eremin, A.; Diele, S.; Hauser, A.; Kresse, H.; Pelzl, G.; Tschierske, C. Angew. Chem., Int. Ed. 2002, 41, 2408. (6) Pelzl, G.; Eremin, A.; Diele, S.; Krese, H.; Weissflog, W. J. Mater. Chem. 2002, 12, 2591. (7) Huang, M. Y. M.; Pedreira, A. M.; Martins, O. G.; Figueiredo Neto, A. M.; Ja´kli, A. Phys. ReV. E 2002, 66, 031708. (8) Shreenivasa Murthy, H. N.; Sadashiva, B. K. Liq. Cryst. 2002, 29, 1223. (9) Fodor-Csorba, K.; Vajda, A.; Galli, G.; Jakli, A.; Demus, D.; Holly, S.; Gacs-Baitz, E. Macromol. Chem. Phys. 2002, 203, 1556. (10) Eremin, A.; Diele, S.; Pelzl, G.; Weissflog, W. Phys. ReV. E 2003, 67, 020702. (11) Etxebarria, J.; Folcia, C. L.; Ortega, J.; Ros, M. B. Phys. ReV. E 2003, 67, 042702. (12) Ortega, J.; Folcia, C. L.; Etxebarria, J.; Gimeno, N.; Ros, M. B. Phys. ReV. E 2003, 68, 011707. (13) Weissflog, W.; Sokolowski, S.; Dehne, H.; Das, B.; Grande, S.; Schroder, M. W.; Eremin, A.; Diele, S.; Pelzl, G.; Kresse, H. Liq. Cryst. 2004, 31, 923. (14) Pyc, P.; Mieczkowski, J.; Pociecha, D.; Gorecka, E.; Donnion, B.; Guillon, D. J. Mater. Chem. 2004, 14, 2374. (15) Jakli, A.; Huang, Y.-M.; Fodor-Csorba, K.; Vajda, A.; Galli, G.; Diele, S.; Pelzl, G. AdV. Mater. 2003, 15, 1606. (16) Liao, G.; Stojadinovic, S.; Pelzl, G.; Weissflog, W.; Sprunt, S.; Jakli, A. Phys. ReV. E 2005, 72, 021710. (17) Kang, S.; Saito, Y.; Watanabe, N.; Tokita, M.; Takanishi, Y.; Takezoe, H.; Watanabe, J. J. Phys. Chem. B 2006, 110, 5205. (18) Thisayukta, J.; Takezoe, H.; Watanabe, J. Jpn. J. Appl. Phys. 2001, 40, 3277. (19) Niwano, H.; Nakata, M.; Thisayukta, J.; Link, D.; Takezoe, H.; Watanabe, J. J Phys. Chem. B 2004, 108, 14889.
