DOI: 10.1002/cphc.201402578
Communications
Photoresponsive Stripe Pattern in Achiral Azobenzene Liquid Crystals Kunihiko Okano,*[a] Satoshi Aya,[b] Fumito Araoka,[c] Haruki Obara,[d] Shiori Sato,[a] Takashi Yamashita,[d] Hideo Takezoe,[b] and Kouichi Asakura[a] A periodic stripe pattern is found in the nematic phase close to the smectic phase of photoresponsive achiral liquid-crystalline compounds. The origin of the stripe patterns can be ascribed to an extremely large bent elastic constant K33. In addition, we succeeded in controlling the pattern by the following two methods: 1) the stripe disappears by a trans–cis photoisomerization upon UV light irradiation and reappears upon light termination, and 2) the stripe pattern is stabilized over the whole nematic phase, at approximately 10 8C, by polymerization of the compounds.
nematic (N) LCs, that is, periodic stripe patterns corresponding to half the helical pitch. The latter example is electrically modulated structures such as a Williams domain.[3] However, even non-chiral LCs without any external stimuli sometimes exhibit stripe patterns. For instance, we often observe stripe texture, so-called transition bar, at the transition from the N phase to the smectic (Sm) phase.[4] A few other examples have also been reported in some particular surface conditions, as we will describe later.[5] In this Communication, we add another example of stripe patterns. Azo molecules are important for constructing photoresponsive functional organizations, particularly in LCs. Actually, they are used in many photoresponsive materials systems such as azo molecular layers for inducing LC orientation change,[6] photoresponsive azopolymer films,[7] and so on. As photoisomerization from trans to cis in azobenzene molecules embedded in LCs disturbs LC orientation and decreases the order parameter, phase transitions could be achieved to lower-ordered states,[8] for example, from N to isotropic (Iso) phases. Moreover, the photoinduced decrease in the order parameter was applied to spatial light modulators by using ferroelectric and antiferroelectric LCs.[9] Thus, azo molecules always bring new functions and phenomena in molecular systems. Herein, we report that a homologous series of azobenzene derivatives, shown in Figure 1, exhibit a photoresponsive stripe pattern in the nematic LC phase. In addition, we investigated the effect of polymer stabilization on the stability of the stripe. We also show a photochemical change of the stripe through photoisomerization of the azobenzene moiety. The compounds used in this study (Figure 1 a) were synthesized by using a modified synthetic procedure described in a previous report[10] (see the Supporting Information). For the characterization of LC properties, the measurements of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) were performed (see the Supporting Information). As shown in the DSC results (Figure 1 b), the isotropization temperature decreases with increasing length of the terminal oligooxyethylene moiety. In some compounds, several Sm phases emerge below the N phase. Although we could not identify all of the Sm phases, XRD measurements indicate SmA for 1 and SmC for 2, 3, and 4 for the Sm phase just below the N phase.[11] We found that all compounds showed the stripe pattern over a narrow temperature range of 1–2 8C in the N phase in the vicinity of the phase transition to the Sm phase, which are indicated by arrows in the DSC charts.
Liquid crystals (LCs) possess fluidity and long-range directional order, which make LCs attractive as functional materials. From a chemistry point of view, LC phases provide a possibility to find characteristic properties of the self-assembly of functional molecules in a bottom-up approach.[1] Moreover, a knowledge of the relationship between chemical structures and molecular orders in LC phases aids the understanding of the self-assembly process in supramolecular chemistry. Such structures at the molecular level often result in characteristic macroscopic order in LCs, leading to anisotropy in many cases. Owing to the anisotropy in fluidic systems, LCs exhibit a variety of characteristic textures under crossed polarizers. For the display application of LCs, uniform appearance without such characteristic textures is indispensable. However, LC textures often provide physical insights into the orientation structure of the director, which, in turn, gives important information about the physical properties of LCs. One example is stripe pattern, originated from intrinsic and extrinsic periodic structures. An example of the former is a “finger-printed texture”[2] observed in chiral
[a] Dr. K. Okano, S. Sato, Prof. Dr. K. Asakura Department of Applied Chemistry, Keio University 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 (Japan) E-mail: [email protected] [b] Dr. S. Aya, Prof. Dr. H. Takezoe Department of Organic and Polymeric Materials Tokyo Institute of Technology O-okayama, Meguro, Tokyo, 152-8552 (Japan) [c] Dr. F. Araoka Physicohemical Soft-Matter Research Unit RIKEN Center for Emergent Matter Science (CEMS) 2-1 Hiroshima, Wako,Saitama 351-0198 (Japan) [d] H. Obara, Prof. Dr. T. Yamashita Department of Pure and Applied Chemistry 2461 Yamazaki, Noda-shi, Chiba 278-8510 (Japan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402578.
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Figure 1. a) Molecular structures of azobenzene compounds 1–4. b) DSC curves of 1–4. Arrows indicate the N phase temperature, where the stripe pattern appears.
We now turn to discuss the driving force for the emergence of the stripe pattern. As mentioned above, similar stripe patterns have been observed in the N phase close to the Sm phase in achiral rod-like molecules without applying any external fields.[5] There are many theoretical works on periodic pattern formation based on surface elasticity, K13 and K24.[12] However, most experimental observations were made under an external field such as electric and magnetic fields.[13] Pergamenshchik et al.[5b] have proposed a mechanism of pattern formation by considering the divergent increase of K33 and K22, but not of K11, when approaching the Sm phase in the N phase. In the present example, we have already reported extremely large K33 values in one of the present compounds, 3.[14] We also found recently that all homologues (1–4) have smectic cybotactic clusters over the whole N phase, which is the origin of the large K33 values. By taking into account the experimental re-
Figure 2 shows an example of the stripe pattern. The sample is a drop of compound 4 with a free surface on a bare glass substrate. If we use sandwich cells with bare glass surfaces or non-treated polymer surfaces, the stripe is always observed in homologous compounds 1–4 (see Figure S1). The period of the stripe depends on the cell thickness; the period increased almost in proportion to the square root of the cell thickness. When cells with unidirectionally rubbed polymers such as polyimide were used, we could not observe any stripes (Figure S2 a). In addition, when we used homeotropically oriented cells, the cells were fully dark in the N phase (Figure S2 b). Upon transition to the SmC phase, a slight brightness was observed because of the tilt of molecules with respect to the smectic layer, and the stripe pattern emerged within a very narrow temperature range just above the transition to the SmC phase. ChemPhysChem 2015, 16, 95 – 98
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Figure 3. POM textures, indicating a texture change from the stripe texture to a schlieren texture upon irradiation of UV light (365 nm, 5.0 mW cm 2). Photoirradiation was performed in 4 at 153 8C. See also the Supporting Information movie.
In low-molecular-weight compounds, the temperature range of the stripe LC phase is much narrower. It is known that a blue phase, which is a frustrated LC phase, appears in a narrow temperature range (ca. 1 8C) between a chiral N phase and an Iso LC phase. Previously, Kikuchi et al. demonstrated the polymer-stabilized blue phase with a broad temperature range of more than 60 8C.[16] This result inspired us to try to design and synthesize the polymerized azobenzene derivative of 5 (Figure 4 a) for the stabilization of the stripes. In
Figure 2. POM textures of the stripe pattern in 4. Sample prepared by slow evaporation of chloroform solution. The image was taken at 153 8C.
sults mentioned above; that is, 1) the emergence of the stripe pattern in the vicinity of the N–Sm transition and 2) the square root dependence of the stripe period on cell thickness, we suggest the same mechanism as in Ref. [5b], that is, large elastic anisotropy for the spontaneous emergence of the stripe pattern, although some differences in the detailed experimental conditions exist. The observation when using a homeotropic cell (see above) suggests that, in addition to a high anisotropy of elastic constants, at least a slight molecular tilt from the surface is necessary for the stripes to form, as suggested in Ref. [5b] and shown in Figure S2 b. To support the mechanism based on large elastic anisotropy, we observed textures in the vicinity of the N–Sm transition in analogous compounds, in which the azo linkage of 3 is replaced by an ester or a tolan linkage.[15] Both compounds showed no stripes. We also confirmed through XRD measurements that the ester and tolan compounds showed no cybotactic clusters and all azo homologues 1–4 showed cybotactic clusters.[15] In these results, the ester and tolan compounds have normal K33 values and normal elastic anisotropy.[15] Next, we investigated the photoresponsive behavior of the stripe pattern induced by trans–cis photoisomerization of azobenzenes. It is well known that cis-azobenzene destabilizes the phase, owing to the decrease in the order parameter, and sometimes a photoinduced phase transition occurs, for instance, from the N to the Iso phase.[8] To induce a trans–cis photoisomerization, we used UV light at 365 nm of 5.0 mW cm 2. Upon UV irradiation to the N phase of 4 with a stripe pattern, the periodic pattern disappeared immediately, and then changed to a schlieren texture (Figure 3, see also the Supporting Information movie). This means that the stripe pattern is in a more ordered state and this order is disturbed by photoisomerized cis state to form a schlieren texture. After the illumination ceased, the schlieren texture reverted back to the stripe pattern. This process was repeatable. The result indicates that the state with the stripe pattern is not metastable, but is thermodynamically stable in a certain temperature range. ChemPhysChem 2015, 16, 95 – 98
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Figure 4. a) Chemical structure of polymerized azobenzene of 5. b) POM texture of 5 prepared by slow evaporation from chloroform and annealing at 210 8C for 5 h. c) DSC curve for the compound (recording rate: 10 8C min 1).
one of the low-molecular-weight azobenzenes, the temperature range in which the stripe pattern can be seen is narrow. In the next trial, we designed and synthesized a polymerized azobenzene derivative that had previously been successful in the polymer-stabilization of unstable phases that can be seen in the blue phases, which are frustrated LC phases and appear 97
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Communications in a narrow temperature range (ca. 1 8C) between the chiral N phase and the Iso phase. Kikuchi et al. showed that the blue phase is stabilized over a broad temperature range of more than 60 8C through polymerization.[16] By using this concept, we succeeded in expanding the temperature range, showing the stripe pattern (Figure 4 b) over the whole N phase between 215 and 206 8C in a cooling process (Figure 4 c). In summary, we synthesized a homologous series of achiral azobenzene compounds and found a stripe pattern over a narrow temperature range (1–2 8C) in the N phase of all of the compounds in the vicinity of the N–Sm phase transition. These compounds possess cybotactic Sm clusters in the whole N phase, and thus have large elastic anisotropy, and K33/K11 100, which could be the origin of the stripe texture. This interpretation is supported by the fact that the stripe pattern is not observed in analogous compounds with a tolan or an ester group instead of the azo group, which show no cybotactic clusters and have normal K33/K11 values. We also found that the stripe pattern is erased by UV light irradiation to show a schlieren texture and it reappears after UV light termination. In addition, we succeeded in stabilizing the stripe texture through polymerization over the whole N phase. In this study, we proposed a concept of construction of periodic structures in the N phase with cybotacticity, destabilization by UV light, and polymer stabilization, which will give a novel insight into self-assembling systems.
Keywords: cybotactic cluster · liquid crystals · nematic phase · photochemistry · stripe pattern [1] C. Tschierske, Angew. Chem. Int. Ed. 2013, 52, 8828; Angew. Chem. 2013, 125, 8992. [2] K. Akagi, Chem. Rev. 2009, 109, 5354. [3] R. Williams, J. Chem. Phys. 1963, 39, 384. [4] I. Dierking, Textures of Liquid Crystals, WILEY-VCH Verlag GmbH & CO.KGaA, Weinheim, 2003. [5] a) I. Lelidis, G. Barbero, Europhys. Lett. 2003, 61, 646; b) V. M. Pergamenshchik, I. Lelidis, V. A. Uzunova, Phys. Rev. E 2008, 77, 041703. [6] a) K. Ichimura, Chem. Rev. 2000, 100, 1847; b) W. M. Gibbons, P. J. Shannon, S.-T. Sun, J. Swetlin, Nature 1991, 351, 49. [7] a) T. Ikeda, J. Mater. Chem. J. Mater. Chem 2003, 13, 2037; b) A. Natansohn, P. Rochon, Chem. Rev. 2002, 102, 4139. [8] a) S. G. Odulov, Yu. A. Reznikov, M. S. Soskin, A. I. Khizhnyak, Sov. Phys. JETP 1983, 58, 1154; b) T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873. [9] a) T. Ikeda, T. Sasaki, K. Ichimura, Nature 1993, 361, 428; b) T. Moriyama, J. Kajita, Y. Takanishi, K. Ishikawa, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 1993, 32, L589. [10] a) K. Okano, Y. Mikami, T. Yamashita, Adv. Funct. Mater. 2009, 19, 3804; b) K. Okano, Y. Mikami, M. Hidaka, T. Yamashita, Macromolecules 2011, 44, 5605. [11] The XRD data of the Sm-N phase transition of 1and 2 will be shown in the future manuscript, those of 3 were discussed in detail in Ref. [15]. [12] a) V. M. Pergamenshchik, Phys. Rev. E 2000, 61, 3936; b) G. Barbero, V. M. Pergamenshchik, Phys. Rev. E 2002, 66, 051706. [13] a) F. Lonberg, R. B. Meyer, Phys. Rev. Lett. 1985, 55, 718; b) P. Cludis, S. Torza, J. Appl. Phys. 1975, 46, 584. [14] S. Aya, H. Obara, D. Pochiecha, F. Araoka, K. Okano, K. Ishikawa, E. Gorecka, T. Yamashita, H. Takezoe, Adv. Mater. 2014, 26, 1918. [15] S. Aya, S. Ogino, Y. Hayashi, K. Okano, D. Pochiecha, K. V. Le, F. Araoka, S. Kawauchi, E. Gorecka, N. Vaupotic, H. Takezoe, K. Ishikawa, submitted. [16] H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, T. Kajiyama, Nat. Mater. 2002, 1, 64.
Acknowledgements We thank Rigaku for the XRD and DSC measurements.
ChemPhysChem 2015, 16, 95 – 98
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Photoresponsive Stripe Pattern in Achiral Azobenzene Liquid Crystals Kunihiko Okano,*[a] Satoshi Aya,[b] Fumito Araoka,[c] Haruki Obara,[d] Shiori Sato,[a] Takashi Yamashita,[d] Hideo Takezoe,[b] and Kouichi Asakura[a] A periodic stripe pattern is found in the nematic phase close to the smectic phase of photoresponsive achiral liquid-crystalline compounds. The origin of the stripe patterns can be ascribed to an extremely large bent elastic constant K33. In addition, we succeeded in controlling the pattern by the following two methods: 1) the stripe disappears by a trans–cis photoisomerization upon UV light irradiation and reappears upon light termination, and 2) the stripe pattern is stabilized over the whole nematic phase, at approximately 10 8C, by polymerization of the compounds.
nematic (N) LCs, that is, periodic stripe patterns corresponding to half the helical pitch. The latter example is electrically modulated structures such as a Williams domain.[3] However, even non-chiral LCs without any external stimuli sometimes exhibit stripe patterns. For instance, we often observe stripe texture, so-called transition bar, at the transition from the N phase to the smectic (Sm) phase.[4] A few other examples have also been reported in some particular surface conditions, as we will describe later.[5] In this Communication, we add another example of stripe patterns. Azo molecules are important for constructing photoresponsive functional organizations, particularly in LCs. Actually, they are used in many photoresponsive materials systems such as azo molecular layers for inducing LC orientation change,[6] photoresponsive azopolymer films,[7] and so on. As photoisomerization from trans to cis in azobenzene molecules embedded in LCs disturbs LC orientation and decreases the order parameter, phase transitions could be achieved to lower-ordered states,[8] for example, from N to isotropic (Iso) phases. Moreover, the photoinduced decrease in the order parameter was applied to spatial light modulators by using ferroelectric and antiferroelectric LCs.[9] Thus, azo molecules always bring new functions and phenomena in molecular systems. Herein, we report that a homologous series of azobenzene derivatives, shown in Figure 1, exhibit a photoresponsive stripe pattern in the nematic LC phase. In addition, we investigated the effect of polymer stabilization on the stability of the stripe. We also show a photochemical change of the stripe through photoisomerization of the azobenzene moiety. The compounds used in this study (Figure 1 a) were synthesized by using a modified synthetic procedure described in a previous report[10] (see the Supporting Information). For the characterization of LC properties, the measurements of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) were performed (see the Supporting Information). As shown in the DSC results (Figure 1 b), the isotropization temperature decreases with increasing length of the terminal oligooxyethylene moiety. In some compounds, several Sm phases emerge below the N phase. Although we could not identify all of the Sm phases, XRD measurements indicate SmA for 1 and SmC for 2, 3, and 4 for the Sm phase just below the N phase.[11] We found that all compounds showed the stripe pattern over a narrow temperature range of 1–2 8C in the N phase in the vicinity of the phase transition to the Sm phase, which are indicated by arrows in the DSC charts.
Liquid crystals (LCs) possess fluidity and long-range directional order, which make LCs attractive as functional materials. From a chemistry point of view, LC phases provide a possibility to find characteristic properties of the self-assembly of functional molecules in a bottom-up approach.[1] Moreover, a knowledge of the relationship between chemical structures and molecular orders in LC phases aids the understanding of the self-assembly process in supramolecular chemistry. Such structures at the molecular level often result in characteristic macroscopic order in LCs, leading to anisotropy in many cases. Owing to the anisotropy in fluidic systems, LCs exhibit a variety of characteristic textures under crossed polarizers. For the display application of LCs, uniform appearance without such characteristic textures is indispensable. However, LC textures often provide physical insights into the orientation structure of the director, which, in turn, gives important information about the physical properties of LCs. One example is stripe pattern, originated from intrinsic and extrinsic periodic structures. An example of the former is a “finger-printed texture”[2] observed in chiral
[a] Dr. K. Okano, S. Sato, Prof. Dr. K. Asakura Department of Applied Chemistry, Keio University 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 (Japan) E-mail: [email protected] [b] Dr. S. Aya, Prof. Dr. H. Takezoe Department of Organic and Polymeric Materials Tokyo Institute of Technology O-okayama, Meguro, Tokyo, 152-8552 (Japan) [c] Dr. F. Araoka Physicohemical Soft-Matter Research Unit RIKEN Center for Emergent Matter Science (CEMS) 2-1 Hiroshima, Wako,Saitama 351-0198 (Japan) [d] H. Obara, Prof. Dr. T. Yamashita Department of Pure and Applied Chemistry 2461 Yamazaki, Noda-shi, Chiba 278-8510 (Japan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402578.
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Figure 1. a) Molecular structures of azobenzene compounds 1–4. b) DSC curves of 1–4. Arrows indicate the N phase temperature, where the stripe pattern appears.
We now turn to discuss the driving force for the emergence of the stripe pattern. As mentioned above, similar stripe patterns have been observed in the N phase close to the Sm phase in achiral rod-like molecules without applying any external fields.[5] There are many theoretical works on periodic pattern formation based on surface elasticity, K13 and K24.[12] However, most experimental observations were made under an external field such as electric and magnetic fields.[13] Pergamenshchik et al.[5b] have proposed a mechanism of pattern formation by considering the divergent increase of K33 and K22, but not of K11, when approaching the Sm phase in the N phase. In the present example, we have already reported extremely large K33 values in one of the present compounds, 3.[14] We also found recently that all homologues (1–4) have smectic cybotactic clusters over the whole N phase, which is the origin of the large K33 values. By taking into account the experimental re-
Figure 2 shows an example of the stripe pattern. The sample is a drop of compound 4 with a free surface on a bare glass substrate. If we use sandwich cells with bare glass surfaces or non-treated polymer surfaces, the stripe is always observed in homologous compounds 1–4 (see Figure S1). The period of the stripe depends on the cell thickness; the period increased almost in proportion to the square root of the cell thickness. When cells with unidirectionally rubbed polymers such as polyimide were used, we could not observe any stripes (Figure S2 a). In addition, when we used homeotropically oriented cells, the cells were fully dark in the N phase (Figure S2 b). Upon transition to the SmC phase, a slight brightness was observed because of the tilt of molecules with respect to the smectic layer, and the stripe pattern emerged within a very narrow temperature range just above the transition to the SmC phase. ChemPhysChem 2015, 16, 95 – 98
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Figure 3. POM textures, indicating a texture change from the stripe texture to a schlieren texture upon irradiation of UV light (365 nm, 5.0 mW cm 2). Photoirradiation was performed in 4 at 153 8C. See also the Supporting Information movie.
In low-molecular-weight compounds, the temperature range of the stripe LC phase is much narrower. It is known that a blue phase, which is a frustrated LC phase, appears in a narrow temperature range (ca. 1 8C) between a chiral N phase and an Iso LC phase. Previously, Kikuchi et al. demonstrated the polymer-stabilized blue phase with a broad temperature range of more than 60 8C.[16] This result inspired us to try to design and synthesize the polymerized azobenzene derivative of 5 (Figure 4 a) for the stabilization of the stripes. In
Figure 2. POM textures of the stripe pattern in 4. Sample prepared by slow evaporation of chloroform solution. The image was taken at 153 8C.
sults mentioned above; that is, 1) the emergence of the stripe pattern in the vicinity of the N–Sm transition and 2) the square root dependence of the stripe period on cell thickness, we suggest the same mechanism as in Ref. [5b], that is, large elastic anisotropy for the spontaneous emergence of the stripe pattern, although some differences in the detailed experimental conditions exist. The observation when using a homeotropic cell (see above) suggests that, in addition to a high anisotropy of elastic constants, at least a slight molecular tilt from the surface is necessary for the stripes to form, as suggested in Ref. [5b] and shown in Figure S2 b. To support the mechanism based on large elastic anisotropy, we observed textures in the vicinity of the N–Sm transition in analogous compounds, in which the azo linkage of 3 is replaced by an ester or a tolan linkage.[15] Both compounds showed no stripes. We also confirmed through XRD measurements that the ester and tolan compounds showed no cybotactic clusters and all azo homologues 1–4 showed cybotactic clusters.[15] In these results, the ester and tolan compounds have normal K33 values and normal elastic anisotropy.[15] Next, we investigated the photoresponsive behavior of the stripe pattern induced by trans–cis photoisomerization of azobenzenes. It is well known that cis-azobenzene destabilizes the phase, owing to the decrease in the order parameter, and sometimes a photoinduced phase transition occurs, for instance, from the N to the Iso phase.[8] To induce a trans–cis photoisomerization, we used UV light at 365 nm of 5.0 mW cm 2. Upon UV irradiation to the N phase of 4 with a stripe pattern, the periodic pattern disappeared immediately, and then changed to a schlieren texture (Figure 3, see also the Supporting Information movie). This means that the stripe pattern is in a more ordered state and this order is disturbed by photoisomerized cis state to form a schlieren texture. After the illumination ceased, the schlieren texture reverted back to the stripe pattern. This process was repeatable. The result indicates that the state with the stripe pattern is not metastable, but is thermodynamically stable in a certain temperature range. ChemPhysChem 2015, 16, 95 – 98
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Figure 4. a) Chemical structure of polymerized azobenzene of 5. b) POM texture of 5 prepared by slow evaporation from chloroform and annealing at 210 8C for 5 h. c) DSC curve for the compound (recording rate: 10 8C min 1).
one of the low-molecular-weight azobenzenes, the temperature range in which the stripe pattern can be seen is narrow. In the next trial, we designed and synthesized a polymerized azobenzene derivative that had previously been successful in the polymer-stabilization of unstable phases that can be seen in the blue phases, which are frustrated LC phases and appear 97
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Communications in a narrow temperature range (ca. 1 8C) between the chiral N phase and the Iso phase. Kikuchi et al. showed that the blue phase is stabilized over a broad temperature range of more than 60 8C through polymerization.[16] By using this concept, we succeeded in expanding the temperature range, showing the stripe pattern (Figure 4 b) over the whole N phase between 215 and 206 8C in a cooling process (Figure 4 c). In summary, we synthesized a homologous series of achiral azobenzene compounds and found a stripe pattern over a narrow temperature range (1–2 8C) in the N phase of all of the compounds in the vicinity of the N–Sm phase transition. These compounds possess cybotactic Sm clusters in the whole N phase, and thus have large elastic anisotropy, and K33/K11 100, which could be the origin of the stripe texture. This interpretation is supported by the fact that the stripe pattern is not observed in analogous compounds with a tolan or an ester group instead of the azo group, which show no cybotactic clusters and have normal K33/K11 values. We also found that the stripe pattern is erased by UV light irradiation to show a schlieren texture and it reappears after UV light termination. In addition, we succeeded in stabilizing the stripe texture through polymerization over the whole N phase. In this study, we proposed a concept of construction of periodic structures in the N phase with cybotacticity, destabilization by UV light, and polymer stabilization, which will give a novel insight into self-assembling systems.
Keywords: cybotactic cluster · liquid crystals · nematic phase · photochemistry · stripe pattern [1] C. Tschierske, Angew. Chem. Int. Ed. 2013, 52, 8828; Angew. Chem. 2013, 125, 8992. [2] K. Akagi, Chem. Rev. 2009, 109, 5354. [3] R. Williams, J. Chem. Phys. 1963, 39, 384. [4] I. Dierking, Textures of Liquid Crystals, WILEY-VCH Verlag GmbH & CO.KGaA, Weinheim, 2003. [5] a) I. Lelidis, G. Barbero, Europhys. Lett. 2003, 61, 646; b) V. M. Pergamenshchik, I. Lelidis, V. A. Uzunova, Phys. Rev. E 2008, 77, 041703. [6] a) K. Ichimura, Chem. Rev. 2000, 100, 1847; b) W. M. Gibbons, P. J. Shannon, S.-T. Sun, J. Swetlin, Nature 1991, 351, 49. [7] a) T. Ikeda, J. Mater. Chem. J. Mater. Chem 2003, 13, 2037; b) A. Natansohn, P. Rochon, Chem. Rev. 2002, 102, 4139. [8] a) S. G. Odulov, Yu. A. Reznikov, M. S. Soskin, A. I. Khizhnyak, Sov. Phys. JETP 1983, 58, 1154; b) T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873. [9] a) T. Ikeda, T. Sasaki, K. Ichimura, Nature 1993, 361, 428; b) T. Moriyama, J. Kajita, Y. Takanishi, K. Ishikawa, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 1993, 32, L589. [10] a) K. Okano, Y. Mikami, T. Yamashita, Adv. Funct. Mater. 2009, 19, 3804; b) K. Okano, Y. Mikami, M. Hidaka, T. Yamashita, Macromolecules 2011, 44, 5605. [11] The XRD data of the Sm-N phase transition of 1and 2 will be shown in the future manuscript, those of 3 were discussed in detail in Ref. [15]. [12] a) V. M. Pergamenshchik, Phys. Rev. E 2000, 61, 3936; b) G. Barbero, V. M. Pergamenshchik, Phys. Rev. E 2002, 66, 051706. [13] a) F. Lonberg, R. B. Meyer, Phys. Rev. Lett. 1985, 55, 718; b) P. Cludis, S. Torza, J. Appl. Phys. 1975, 46, 584. [14] S. Aya, H. Obara, D. Pochiecha, F. Araoka, K. Okano, K. Ishikawa, E. Gorecka, T. Yamashita, H. Takezoe, Adv. Mater. 2014, 26, 1918. [15] S. Aya, S. Ogino, Y. Hayashi, K. Okano, D. Pochiecha, K. V. Le, F. Araoka, S. Kawauchi, E. Gorecka, N. Vaupotic, H. Takezoe, K. Ishikawa, submitted. [16] H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, T. Kajiyama, Nat. Mater. 2002, 1, 64.
Acknowledgements We thank Rigaku for the XRD and DSC measurements.
ChemPhysChem 2015, 16, 95 – 98
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Received: August 8, 2014 Published online on October 2, 2014
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