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Chiral Correlation between Low-Birefringent Phases with Twist Grain Boundary-like Helix and Highly Birefringent Phases with Layer Chirality as Elucidated from Circular Dichroism Observations Seng Kue Lee,* Lu Shi, Masatoshi Tokita, and Junji Watanabe Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552 ReceiVed: January 12, 2008; ReVised Manuscript ReceiVed: March 14, 2008; In Final Form: March 19, 2008
The bent-shaped molecule with a central naphthalene core, 2,7-naphthalene-bis[4-(4-dodecylphenyliminomethyl)]benzoate, forms the low-birefringent B2 (LB-B2) phase with the twist grain boundary (TGB)-like helical structure and the low-birefringent B4 (LB-B4) phase in order of decreasing temperature. By applying the electric field, the LB-B2 phase is altered to the highly birefringent B2 (HB-B2) phase because of unwinding of the TGB-like helix. The HB-B2 phase is transformed to the HB-B4 phase without a loss of birefringence on cooling. These four phases show characteristic circular dichroism spectra, showing the consistent correlation through the transformation between these phases. The source of the chirality in the achiral system and the correlation in the chirality between these phases are discussed. Introduction Chirality is an inherent property in chiral molecular systems and one of the most attractive subjects in physicochemical research. In the liquid crystalline field, the chirality is also an interesting property, since there are many chiral superstructures formed. In all these cases, the chirality is produced from chiral molecules. However, recent studies show that some mesophases of bent-shaped molecules also exhibit the chiral phase, although the molecules themselves are achiral.1,2 Among several phases so far identified, the smectic B2 phase is particularly interesting with respect to its unique chirality as well as the polarity.1–3 The chirality in the B2 phase is generated from symmetry breaking due to the tilting of the molecule to the layer normal.3 It is called the layer chirality. On the basis of the layer chirality and polarity, the B2 phase has been described as being able to possess four distinct types of structures: two homochiral (SmCSPF and SmCAPA) and two antichiral or racemic (SmCSPA and SmCAPF). The B4 phase is also interesting from a chirality viewpoint.4 It forms the specific helical superstructure, such as a twist grain boundary (TGB) structure, with the helical axis parallel to the layers. Because the molecules are not titled from the layer normal in the B4 phase, the conformational chirality induced in molecules has been proposed as a chiral source for the TGBlike helix.4–6 Recently, an unusually low-birefringent B2 phase with chiral domains has been reported,6–19 which was considered to possess the TGB-like helical structure as the B4 phase.4,18 This type of B2 phase shows antiferroelectric switching between SmCAPA and SmCSPF states after the low-birefringent domains are altered to the highly birefringent ones characteristic of a conventional B2 phase.11,12,18,19 One of the typical examples is observed in the following achiral bent-shaped molecule with a central naphthalene core, 2,7-naphthalene-bis[4-(4-dodecylphenyliminomethyl)]benzoate (N(2,7)-12-PIMB).5,19 The chiral correlation between the * Corresponding author.
TGB-like helix of the low-birefringent B2 phase and the layer chirality of the highly birefringent field-induced B2 phase has been examined.19 The result indicates that the helical sense of the TGB-like helix is memorized as the layer chirality in the field-induced homochiral SmCAPA phase. In this report, we examine in detail the chiral correlation between these two B2 subphases, low-birefringent and highly birefringent B2 phases, by means of circular dichroism (CD), which is the most effective probe for the interpretation of optical activities. We also investigate the relationship between the layer chirality of the homochiral B2 phase and the conformational chirality of the B4 phase. Experimental The detailed synthetic method for N(2,7)-12-PIMB is described elsewhere.6 The texture observation was made under a polarizing optical microscope (an Olympus BX50). The electric field was applied using a high-speed voltage amplifier (FLC Electronics, F20A) connected to a function generator (NF Electronic Instruments, WF1945A). The sample was sandwiched between two glass plates with a transparent indium tin oxide electrode. Neither polymer coating nor rubbing was made on the substrate surface. The cell gap was controlled at 7.5 µm using polyethylene terephthalate films. CD spectra were collected using a JASCO J-720WI circular dichroism spectrometer. The temperature for all the measurements was regulated within an accuracy of 1 °C by using a Metter FP-90 hot stage. Results N(2,7)-12-PIMB shows an Iso-B2-B4 transition on decreasing temperature, as shown above. The B2 phase appears with a very
10.1021/jp8002849 CCC: $40.75 2008 American Chemical Society Published on Web 05/10/2008
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Figure 1. Photomicrographs observed for (a) LB-B2 with two types of optically active domains with opposite chiral senses, which initially appears on cooling the isotropic melt; (b) LB-B4 formed on cooling LB-B2; (c) HB-B2 obtained by applying the electric field for LB-B2; and (d) HB-B4 formed on cooling HB-B2.
low birefringence from the isotropic melt on cooling. This lowbirefringent B2 phase is characterized by showing two types of optically active domains with opposite chiral sense, which are distinguished by clockwise and counterclockwise rotations of the polarizer from the cross-polarization position (see Figure 1a). Simultaneous exhibition of the clear CD effect as shown in later details is also characteristic of this B2 phase. The lowbirefringent and chiral domains are considered to be produced by the helical TGB-like structure, and the CD effect is explained as the selective reflection of circularly polarized light due to the helical structure.4 The X-ray pattern of the B2 phase shows that molecules are packed into the layer with a liquidlike nature and tilted with a tilt angle of about 20°. This low-birefringent B2 phase is called here LB-B2. On further cooling to the B4 zone, the low-birefringent chiral domains are retained as shown in Figure 1b, but the layer spacing, which is nearly equal to the molecular length,19 is increased. Thus, in the B4 phase, the molecules lie perpendicular to the layer, although the TGBlike helical structure is sustained. This B4 state is called LBB4. The low-birefringent domain texture in LB-B2 is altered to the highly birefringent fan-shaped texture on an application of an external electric field (compare parts a and c of Figure 1). Unwinding of the TGB-like helical structure is attributable to this texture change. In the fan-shaped texture, the extinction brushes are parallel to the layer normal in off-state of field while they rotate oppositely between the two homochiral domains in on-state of field,19 showing the antiferroelectric switching between the SmCAPA and SmCSPF states. The highly birefringent B2 phase, so-called HB-B2, stably exists after the field is stopped in the lower B2 temperature region, although it is transformed to the LB-B2 state when the B2 temperature is
Figure 2. Schematic presentation of the relationship among LB-B2, HB-B2, LB-B4, and LB-B4 states.
raised above 210 °C. On cooling HB-B2 to the B4 zone, the fan-shaped texture changes to the broken one, but it is still highly birefringent (Figure 1d). This B4 phase is called HB-B4. In Figure 2, the relationship among theLB-B2, HB-B2, LB-B4, and HB-B4 states is schematically presented. It is of interest to note that the chiral domain in LB-B2 is completely retained after the field-induced transformation to HBB2. This can be clearly detected when HB-B2 goes back to LB-B2 on heating up to 210 °C. The recovered low-birefringent chiral domains are completely the same as those of the initial LB-B2 (compare parts a and e of Figure 1), showing that the chilarity in LB-B2 is memorized on the field-induced transformation to HB-B2. This memory effect can also be observed, even though the HB-B2 has experienced HB-B4. To clarify the correlation of chirality between these phases, CD measurements were performed. Here, the same position was probed during a cycling between the phases. Parts a and d of
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Figure 3. CD spectra observed for (a) LB-B2, (b) LB-B4, (c) HB-B2, and (d) HB-B4.
Figure 4. (a) CD spectra observed on the transformation of LB-B2 to LB-B4. CD spectra are collected at 5 °C intervals in the temperature region of 180 and 135 °C. (b) CD intensities at 420 nm plotted against the temperature.
Figure 3 show typical CD spectra observed for four states: LBB2, LB-B4, HB-B2, and HB-B4. First, on cooling, the isotropic melt to the LB-B2 phase, clear CD peak is observed, with an apparent maximum peak around 420 nm (Figure 3a). It should be noted that two chiral domains with opposite chirality equally appear, since the system is achiral. Hence, the CD intensity, if totally averaged over the whole area, is canceled to zero.4,6 However, there is some imbalance in the chiral domains in the local area so that the positive or negative CD peak is observed with an appreciably high intensity when the beam size is as small as 2 mm, as in the present case. All of the CD data exhibited here are collected for the area where the LB-B2 phase shows the positive CD peak, as in Figure 3a. Figure 4a shows the CD spectra observed on cooling LB-B2 to LB-B4. On entering the LB-B4 phase, as found here, the sense and shape of the CD are not changed. This is reasonable, since there is no morphological change that takes place on this transformation (refer to parts a and b of Figure 1). From Figure 4b, which shows the temperature dependence of the CD intensity
at 420 nm, the intensity is found to increase significantly on the transformation to the LB-B4 phase. This may be due to the change of birefringence, which is caused by the change of molecular association within a layer from the tilted one to the perpendicular one. The typical CD spectrum of the LB-B4 phase is given in Figure 3b. Upon the electric field-induced transformation of LB-B2 to HB-B2, a dramatic change on the CD effect takes place; the direction of the CD peak is altered from the positive to the negative, as found in Figure 3c. This reversal of direction is observed independently on the direction or intensity of the CD of the initially prepared LB-B2. The resulting CD spectrum in HB-B2 is relatively sharp, with its maximum peak at 380 nm, whereas that of LB-B2 at 420 nm is widely spread over the long-wavelength region. Figure 5a shows the CD spectra observed on cooling HB-B2 to HB-B4. One notices that the CD peak at 380 nm completely disappears in HB-B4. This is not artificial, because on heating HB-B4 back to HB-B2, the initial negative CD peak is
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Figure 5. (a) CD spectra observed on the transformation of HB-B2 to HB-B4. CD spectra are collected at 0.5 °C intervals in the temperature region of 166 and 150 °C. (b) CD intensities at 380 nm collected on heating and cooling cycle plotted against the temperature. Open and closed circles stand for the data taken with decreasing and increasing CD intensity, respectively.
completely recovered. Appearance and disappearance of the CD signal are reversibly observed on temperature cycles between HB-B2 and HB-B4, as found in Figure 5b, which shows the temperature dependence of the CD intensities at 380 nm. Finally, it should be stated that when HB-B2 is transformed to LB-B2 on heating up to 210 °C, the negative CD peak is altered to the positive CD peak, which was initially observed for LB-B2. This chiral memory corresponds to that observed from the microscopic textures. Discussion In general, the distinct CD effects result from two structural features. One is the selective reflection of circularly polarized light due to a helical superstructure,20 and the other is the LCICD as the extrinsic circular dichroism, which is induced on the absorption wavelength of molecules as a consequence of an unsymmetric array of the molecules in the helical LC field.21 The reflection CD band is extremely large, but the LCICD is also of high intensity, 102-103 higher than that of the intrinsic CD in chiral compounds.21 The significant CD effect observed in LB-B2 and LB-B4 is attributable to a selective reflection of circularly polarized light, which results from a helical twisting structure similar to the TGB-like structure with layer blocks twisted from each other along the layer.4 Probably, the reflection band peak may be located at a certain wavelength shorter than 300 nm, and the edge of the reflection band may be observed in the present wavelength region of 350-700 nm. Here, a question arises about why the HB-B2 shows the distinct CD peak irrespective of the unwinding of the helical structure by the electric field. On this point, two groups have reported that the homochiral B2 phase shows the unusual optical activities based on the layer chirality, which is produced by tilting of the molecules within a layer. Ortega et al.,13 modeling SmCAPA with a locally chiral gyration tensol with optical axes rotating with a pitch of two layers, showed possible huge optical rotation for light propagating perpendicularly to the helical axis. Recently, Hough and Clark et al.22,23 presented a new model based on the layer chirality without assuming a helix. This succeeded in demonstrating optical rotations as large as 1000 times the molecular optical activity. In this model, the chiral layer structure is constructed to be composed of four sublayers: two uniaxial sublayers with optic axes along each arm of a bentcore molecule and two isotropic sublayers modeling the aliphatic tails. It is possible that such two-optical active structures induce the LCICD on the absorption band as the conventional macro-
helical structure of a cholesteric or chiral SmC phase.21 In fact, the CD peak position of 380 nm in HB-B2 corresponds to the peak position in the absorption band due to the Schiff’s base moiety. It is known that there are two types of the homochiral subphases of B2, SmCAPA and SmCSPF and two types of the racemic subphases of B2, SmCAPF, and SmCSPA. In homochiral phases, all of the layers have the same layer chirality, whereas in racemic phases, the layers with different chirality are alternately piled up.3 Hence, in the racemic phase, the LCICD effect would be compensated between neighboring layers. These characteristics of the B2 phases have been confirmed experimentally by Niwano et al.24 They prepared three B2 subphases in the same molecular system and found that no CD signal is detected for the racemic subphase, whereas the strong CD signal is observed for the homochiral ones. Disappearance of the CD effect on cooling HB-B2 to HBB4 can be explained on the same basis as the layer chirality: in the B4 phase, the molecules are not tilted to the layer, and then the layer chirality due to symmetry breaking vanishes. On this transformation, of interest is that the negative CD is completely recovered on heating back to HB-B2 (refer to Figure 5b). This result indicates that the layer chirality of HB-B2 can be conserved even after the transformation to HB-B4 without the layer chirality. We propose here that the layer chirality of the B2 phase, that is, the tilt direction of the molecules to the layer, is memorized as a conformation chirality in the B4 phase.24 In other words, the molecules with the conformational chirality in HB-B4 choose the tilt direction on going back to HB-B2, giving rise to a correspondence between the conformational chirality and layer chirality. Such a chiral conformation in the B4 phase, in fact, has been confirmed by NMR and IR measurements.25–27 Although the intrinsic CD effect might be expected from the chiral conformation, it is known to be appreciably smaller than that of the LCICD due to the helical or chiral packing structure.21 This is the reason why no appreciable CD effect is detected in HB-B4. Conclusion We have demonstrated the relationship in chirality between the four characteristic phases, LB-B2, HB-B2, LB-B4, and HBB4, by investigating CD spectra as well as optical microscopic textures in the bent-shaped molecule with a central naphthalene core, N(2,7)-12-PIMB. LB-B2 shows clear but broad CD with a peak maximum at 420 nm. On cooling to LB-B4, an essentially similar CD spectrum is observed. Upon application of an electric field to transform the LB-B2 to HB-B2, the CD characteristic
6766 J. Phys. Chem. B, Vol. 112, No. 22, 2008 of the TGB-like structure disappears, and in its place, a relatively sharp CD at 380 nm appears with the opposite sign. On cooling to HB-B4, this CD peak at 380 nm completely disappears. The former CD effect in LB-B2 and LB-B4 arises from the selective reflection of circularly polarized light due to a TGB-like helix, whereas the latter CD is well-explained as the LCICD induced by the layer chirality, which is caused by the tilting of molecules to the layer. Of importance is that such CD effects are reversibly observed on applying the electric field as well as on a heating and cooling cycle between the phases. This means that the initially formed chirality of the TGB-like helix in LB-B2 can be preserved as the layer chirality in the field-induced HB-B2 state and that the layer chirality in HB-B2 is memorized as the conformational chirality in HB-B4. These results lead to the conclusion that the conformational chirality exists as a common chiral source on the transformation between subphases. 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.; Takezoe, H.; Watanabe, J. Jpn. J. Appl. Phys. 2001, 40, 3277. (5) Thisayukta, J.; Niwano, H.; Takezoe, H.; Watanabe, J. J. Am. Chem. Soc. 2002, 124, 3354. (6) Thisayukta, J.; Nakayama, Y.; Kawauchi, S.; Takezoe, H.; Watanabe, J. J. Am. Chem. Soc. 2000, 122, 7441. (7) Dantlgraber, G.; Eremin, A.; Diele, S.; Hauser, A.; Kresse, H.; Pelzl, G.; Tschierske, C. Angew. Chem., Int. Ed. 2002, 41, 2408. (8) Pelzl, G.; Eremin, A.; Diele, S.; Krese, H.; Weissflog, W. J. Mater. Chem. 2002, 12, 2591. (9) Huang, M. Y. M.; Pedreira, A. M.; Martins, O. G.; Figueiredo Neto, A. M.; Ja´kli, A. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2002, 66, 031708.
Lee et al. (10) Shreenivasa Murthy, H. N.; Sadashiva, B. K. Liq. Cryst. 2002, 29, 1223. (11) Eremin, A.; Diele, S.; Pelzl, G.; Weissflog, W. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top 2003, 67, 020702. (12) Etxebarria, J.; Folcia, C. L.; Ortega, J.; Ros, M. B. Phys. ReV E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top 2003, 67, 042702. (13) Ortega, J.; Folcia, C. L.; Etxebarria, J.; Gimeno, N.; Ros, M. B. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top 2003, 68, 011707. (14) 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. (15) Pyc, P.; Mieczkowski, J.; Pociecha, D.; Gorecka, E.; Donnion, B.; Guillon, D. J. Mater. Chem. 2004, 14, 2374. (16) Jakli, A.; Huang, Y.-M.; Fodor-Csorba, K.; Vajda, A.; Galli, G.; Diele, S.; Pelzl, G. AdV. Mater. 2003, 15, 1606. (17) Liao, G.; Stojadinovic, S.; Pelzl, G.; Weissflog, W.; Sprunt, S.; Jakli, A. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top 2005, 72, 021710. (18) Kang, S.; Saito, Y.; Watanabe, N.; Tokita, M.; Takanishi, Y.; Takezoe, H.; Watanabe, J. J. Phys. Chem. B 2006, 110, 5205. (19) Lee, S. K.; Shi, L.; Tokita, M.; Takezoe, H.; Watanabe, J. J. Phys. Chem. B 2007, 111, 8698. (20) de Vries, H. Acta Crystallogr. 1951, 4, 219. (21) Saeva, F. D. Liquid Crystals: The Fourth State of Matter; Marcel Dekker, Inc.: New York, 1979. (22) Hough, L. E.; Clark, N. A. Phys. ReV. Lett. 2005, 95, 107802. (23) Hough, L. E.; Zhu, C.; Nakata, M.; Chattham, N.; Dantlgraber, G.; Tschierske, C.; Clark, N. A. Phys. ReV. Lett. 2007, 98, 037802. (24) Niwano, H.; Nakata, M.; Thisayukta, J.; Link, D.; Takezoe, H.; Watanabe, J. J. Phys. Chem. B 2004, 108, 14889. (25) Kurosu, H.; Kawasaki, M.; Hirose, M.; Yamada, M.; Kang, S.; Thisayukta, J.; Sone, M.; Takezoe, H.; Watanabe, J. J. Phys. Chem. A 2004, 108, 4674. (26) Zennyoji, M.; Takanishi, Y.; Ishikawa, K.; Thisayukta, J.; Watanabe, J.; Takezoe, H. Mol. Cryst. Liq. Cryst. 2001, 366, 693. (27) Choi, S.-W.; Kawauchi, S.; Tanaka, S.; Watanabe, J.; Takezoe, H. Chem. Lett. 2007, 36, 1018.
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