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Optically Isotropic Homochiral Structure Produced by Intercalation of Achiral Liquid Crystal Trimers Atsushi Yoshizawa, Yusuke Kato, Haruna Sasaki, Yoichi Takanishi, and Jun Yamamoto J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b01242 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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The Journal of Physical Chemistry

Optically Isotropic Homochiral Structure Produced by Intercalation of Achiral Liquid Crystal Trimers Atsushi Yoshizawa,*, a Yusuke Kato,a Haruna Sasaki,a Yoichi Takanishi,b and Jun Yamamotob a

Department of Frontier Materials Chemistry, Graduate School of Science and

Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan E-mail: [email protected]; Phone: +81-172-39-3002. b

Department of Physics, Graduate School of Science, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto, 606-8562, Japan

Abstract Dark conglomerates of domains with opposite handedness, which are designated dark conglomerate phases (DC phases), have attracted much attention. We prepared an achiral

liquid

crystal

trimer,

4,4’-bis{7-[4-(5-octyloxypyrimidin-2-yl)phenyloxy]nonyloxy}biphenyl

(I–9),

and

investigated the physical properties. A droplet of trimer I–9 formed a conventional nematic phase on cooling from the isotropic liquid, and then changed to an optical isotropic phase with homochirality. X-ray diffraction measurements reveal that the isotropic phase has an intercalated layer structure with a correlation length of 95 nm. We

prepared

binary

mixtures

with

a

nematic

liquid

crystal,

4’-hexyloxy-4-cyanobiphenyl (6OCB). The mixtures containing 30–75 mol% of 6OCB exhibited smectic phases above the isotropic phase. We investigated mesogenic properties of trimer I–n (n=5–9) depending on the parity of the linking group. Only trimer I–9 possessing the longest odd-numbered spacers showed the chiral isotropic phase, suggesting that a rigid bent structure is not necessary for the appearance of the isotropic phase. The experimental results reveal that trimer I–9 exhibits a soft crystalline DC phase representing a new modification of chiral symmetry breaking in lamellar liquid crystal phases.

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INTRODUCTION Spontaneous chiral symmetry breaking in soft matter has attracted much attention.1–7 Chiral conglomerates consisting of domains with opposite handedness have been observed recently not only in liquid-crystalline phases8–19 but also in isotropic liquids.20 Among the chiral conglomerates, dark conglomerate phases (DC phases) have been observed in many bent-core liquid crystals. Depending on the local structure of these DC phases, they are classifiable as liquid-crystalline sponge phases,21–26 formed by strongly deformed fluid layers, or as helical nanofilament phases (HNF phases, also assigned as B4 phases).27–32 The sponge phases, usually formed during cooling of the isotropic liquid, have little or no birefringence. The texture under crossed polarizers is nearly dark. Such phases are macroscopically isotropic sponge-like soft solids. Application of high voltage transforms some of such phases to B2 phase.21,26 The B2 phase is formed from the stacking of fluid layers of bent-core molecules, where the molecular long axis is tilted relative to the layer normal. The HNF phase is apparently a solid phase, with in-plane hexatic positional ordering of the layers and short-range layer order.27 Although no electro-optical switching can be found in the HNF phase, it is active in second harmonic generation (SHG),7 indicating the existence of polar order in the phase. Results of earlier studies have demonstrated that smectic layers in both the sponge phases and HNF phases of bent-core liquid crystals tend to have saddle splay curvature.21,27 Although chiral domains with opposite handedness in DC phases have usually been observed to be equal in proportion, formation of homogeneous chiral domains is often observed instead of conglomerate formation. The long range homochiral domain formation is not only for the DC phases but also for the chiral isotropic liquids20 and especially the chiral cubic phases.2,11 Various methods have been developed to control of the chirality in DC phases.33–36 Recently, the enantioselective formation of helical nanofilaments was obtained using twisted-nematic cells.37 On the other hand, DC phases of new types have been observed.38–41 Therefore it is an attractive issue to explore a novel DC phase exhibiting spontaneous generation in chiral asymmetry. Furthermore, the spontaneous formation of homochirality can provide a new aspect on the discussion of emergence of uniform chirality in prebiotic systems.2,42,43 Liquid crystal trimers have a super molecular structure. By designing intermolecular interactions, they can exhibit various molecular packing structures in the 2 ACS Paragon Plus Environment

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liquid-crystalline phases.43,44 We have reported achiral liquid crystal trimers stabilizing DC phases as follows: 1) an equimolecular mixture of a nematic trimer and a smectc trimer,46 2) binary mixtures of a nematic trimer possessing a central biphenyl unit and 4’-hexyloxy-4-cyanobiphenyl (6OCB),47 and 3) a single substance of an asymmetric trimer.48 For the study described in this report, we prepared a homologous series of achiral trimer I–n possessing a central biphenyl unit as shown in Figure 1. We found trimer I–9 exhibiting an optically isotropic homochiral droplet on cooling from the nematic (N) phase. We propose a new structure for the DC phase and discuss how the chiral asymmetry generates in the molecular assembly consisting of achiral trimers.

Figure 1. Molecular structure of trimer I–n. EXPERIMENTAL SECTION Materials. Materials were prepared using a similar method for trimer I–7 described in our previous report.47 Purity of each final compound was confirmed using elemental analysis (EA 1110; CE Instruments Ltd.). Infrared (IR) spectroscopy (FTS-30; Bio-Rad Laboratories Inc.) and proton nuclear magnetic resonance (1H NMR) spectroscopy (JNM-ECA500; JEOL) elucidated the structure of the final product. Analytical data for trimer I–9 are presented below. 4,4’-Bis{7-[4-(5-octyloxypyrimidin-2-yl)phenyloxy]nonyloxy}biphenyl (I–9) 1

HNMR (500 MHz, CDCl3, TMS): δH/ppm: 8.41 (s, 4H, Ar-H), 8.26 (d, 4H, Ar-H,

J=8.6 Hz), 7.45 (d, 4H, Ar-H, J = 8.6 Hz), 6.97 (d, 4H, Ar-H, J = 9.2 Hz), 6.94 (d, 4H, Ar-H, J = 9.2 Hz), 4.07 (t, 4H, -OCH2-, J = 6.6 Hz), 4.02 (t, 4H, -OCH2-, J = 6.6 Hz), 3.98 (t, 4H, -OCH2-, J = 6.3 Hz), 1.85–1.77 (m, 12H, aliphatic-H), 1.47–1.30 (m, 40H, aliphatic-H), 0.89 (t, 6 H, -CH3-, J = 6.9 Hz). IR (KBr) ν cm-1: 2938, 2852 (C-H str), 1607 (Ar-H str), 1246 (C-O str). Elemental analysis (%): Calc. for C66H90O6N4: C, 76.60; H, 8.76; N, 5.41. Found: C, 76.90; H, 8.55; N, 5.40. Liquid-Crystalline and Physical Properties. The initial phase assignments and corresponding transition temperatures for each final compound were determined using a



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polarizing optical microscope (POM, BX-51; Olympus Optical Co. Ltd.) equipped with a temperature control unit (LK-600PM; Japan High Tech Co. Ltd.). The temperatures and enthalpies of transition were investigated using differential scanning calorimetry (DSC, DSC 6200; Seiko Instruments Inc.). The X-ray diffraction patterns of the homeotropically aligned sample during cooling were obtained using a real-time X-ray diffractometer (D8 Discover; Bruker AXS GmbH) equipped with a hot stage and a temperature-control processor. A sample was put on a convex lens, which was then placed in a custom-made temperature-stabilized holder (stability within ±0.1 °C). The phase transition of the sample under the X-ray beam was monitored by observing the texture simultaneously using polarized light microscopy with a CCD camera. The X-ray apparatus was equipped with a cross-coupled Göbel mirror on a platform system with a two-dimensional position-sensitive proportional counter (PSPC) detector (HI-Star; Bruker AXS GmbH). Then X-rays were generated at 45 kV and 20 mA. A parallel Cu Kα X-ray beam was used to irradiate the sample. The irradiation time was 10 min. Each diffraction pattern was obtained using the 2D PSPC detector at a camera distance of 300 mm. The correlation length along the layer normal (ξ) was determined using the Ornstein−Zernike expression as follows. First, the X-ray profile as a function of 2θ was converted to a scattering function of q according to the following equation. q = (4π/λ) sin θ

(1)

By fitting the X-ray profiles using the following Lorentzian equation, correlation length ξ was determined.

=

+ background (2)

Therein, I0 and q0 respectively signify the peak height and the peak position of q. Electro-optical properties were measured using standard electro-optical techniques. The ITO-coated glass sandwich cells purchased from EHC Corp. were constructed with 2 µm spacers. The ITO electrodes were covered with a polymer alignment layer, rubbed unidirectionally. Atomic force microscopy (AFM: NanoNavi2/E-Sweep) was used in the tapping mode at room temperature. The sample on a glass slide was heated to the isotropic liquid on a hot plate, and cooled to room temperature. Scanning tunneling electron microscopy (STEM: Hitachi SU9000) was used. The sample was heated to the isotropic 4 ACS Paragon Plus Environment

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liquid, and then cooled to 26 °C. It was deposited with a thin layer Pt–C to give the replica for STEM measurements. RESULTS AND DISCUSSION Liquid-Crystalline and Physical Properties. We prepared a homologous series of trimer I–n (n=5–9), in which two phenylpyrimidine units and a central biphenyl unit were connected via flexible methylene spacers. Only trimer I–9 showed a DC phase. Phase transition temperatures obtained by optical microscopy and enthalpies of transition for trimer I–9 were the following: Iso 147.2 °C (8.5 kJ mol–1) N 131.6 °C (97.1 kJ mol–1) DC. When heated, the crystal melted at 136 °C to the N phase. The DSC thermogram is portrayed in Figure 2. The N–DC transition temperature observed by the thermal analysis was 3.5 K lower than that observed using optical microscopy.

Figure 2. DSC thermogram of trimer I–9. The rate of heating and cooling was 5 °C min-1. The Cry–N–Iso transitions during heating and the Iso–N–DC transitions during cooling were observed. Iso: isotropic liquid; N: nematic phase; DC: dark conglomerate phase; Cr: crystal. Figure 3 portrays the optical textures of trimer I–9 on a glass slide with a cover glass in



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the DC phase at 130.4 °C under crossed and uncrossed polarizers. The texture of the DC phase under crossed polarizers is nearly dark, suggesting that it is optically isotropic. The optical activity can be recognized as follows.2,23 By observing the sample in the DC phase under slightly uncrossed polarizers (10°), the texture was split into darker and brighter domains. Transmittance of light from one domain is increased by a clockwise rotation of the analyzer (Figure 3, left), while the transmittance from the other domain is increased by a counter-clockwise rotation (Figure 3, right). The two domains cannot be distinguished under crossed polarizers (Figure 3, center). The domain brightness did not change by rotation of the sample between the polarizers. Therefore the two domains separated by a clear boundary were found to have optical activity with opposite senses. The coalescence of two domains with the same sense of optical activity yielded no boundary, while the coalescence of domains with opposite sense gave a clear boundary. These results reveal the formation of a helix in the DC phase of trimer I–9. With respect to the N phase above the DC phase, no chiral nature was detected. Chiral aggregation occurs at the N to DC phase transition. Each chiral domain in the DC phase has large area of several millimetres square. The actual length scale of chiral domains in DC phases is dependent on the number of nuclei and the growth rate,23,30 and it ranges from being below the wave length of light to square centimetres. The formation of large chiral domains was observed in mixtures of achiral bent-core and rod-like molecules49 or in hockey-stick liquid crystals.32,41 Cooperativity provided by multicontact interactions is thought to play an important role in the growth rate.2 Then we observed a phase transition behavior of isotropic liquid droplets of trimer I–9. Figure 4 shows the polarized optical textures of trimer I–9 on a glass slide without a cover glass. Each droplet was exhibited individually as the isotopic liquid. During cooling, the droplets became the nematic phase, and then they changed to the DC phase. Transmittance of light from the large droplet is increased by a clockwise rotation of the analyzer (Figure 4, left), while the transmittance from the small droplet is increased by a counter-clockwise rotation (Figure 4, right). The domain brightness did not change by rotation of the sample between the polarizers. Therefore the two droplets have opposite handedness, indicating that each droplet consists exclusively of right-handed or left-handed chiral structure. Such a homochiral structure has never been formed for a sample of trimer I–9 in a sandwich cell. Although we have investigated the dependence of the formation of a homochiral structure on experimental conditions, e.g., cooling rate 6 ACS Paragon Plus Environment

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and sample thickness, we do not establish a method to control the formation at present. In some cases, a single droplet was divided by two homochiral domains with opposite handedness (Figure S1). However, we can say that trimer I–9 forms a macroscopic homochiral structure spontaneously via the N to DC phase transition. The formation of the chiral domain can be prepared by relatively fast cooling at a rate of 5 °C min-1 from the N phase, indicating the existence of cooperative multi contact interactions in the DC phase.

Figure 3. Polarized optical textures of trimer I–9 on a glass slide with a cover glass in the DC phase at 130.4 °C with uncrossed and crossed polarizers. The cooling rate was 5 °C min-1.

Figure 4. Polarized optical textures of trimer I–9 on a glass slide without a cover glass in the DC phase at 125.3 °C with uncrossed polarizers. The cooling rate was 5 °C min-1. We investigated the electro-optical response in the DC phase by application of an AC



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field to the sample in a homogeneously aligned cell with a 2 µm gap using a triangle wave with an AC field of ±25 V µm-1 at a frequency of 10 Hz. Electro-optical switching was not observed in the DC phase. X-ray Diffraction. To elucidate the phase structure, X-ray measurements were conducted. Alignment was performed by slow cooling of a small drop of the sample on a convex lens. The N to DC phase transition was confirmed by the texture change. Figure 5 shows X-ray diffraction profiles of trimer I–9 in the N and DC phases in the small angle region (Figure 5 (a)) and those in the wide angle region (Figure 5 (b)). A diffuse diffraction with an intensity of about 41 a.u. was detected around 2θ = 2.07° in the N phase. It is too weak to be seen in Figure 5(a). Therefore trimer I–9 exhibits a conventional nematic phase possessing only positional order. A sharp diffraction peak at 2θ = 2.02° was observed in the DC phase, revealing that the DC phase has a layer structure with a periodicity length of 43.6 Å. The correlation length along the layer normal is 948 Å, which corresponds to about 22 layers. The correlation length in typical HNF phases corresponds to about 5–7 layers. The fourth diffraction peak was detected, indicating that the DC phase has a distinguished layer structure. With respect to the wide-angle region, a broad diffraction around 2θ = 19.5° with an intensity of about 27 a.u. was observed in the N phase. On the other hand, the four peaks at 2θ = 17.8°, 19.1°, 20.6°, and 23.4° were observed in the DC phase as portrayed in Figure 5(b), indicating that it has positional order within the layer, just as HNF phases for bent-shaped LCs have. This pattern excludes fluid sponge phases, which would show one completely diffuse wide-angle diffraction profile aside from the layer reflection. We discuss the layer structure in the DC phase. An extended molecular length for trimer I–9 with all trans conformation of the spacers is estimated by semi-empirical calculations using MOPAC-6/PM3 to be 69 Å. The conformation is shown in Figure S2 (a). The layer spacing in the DC phase of 43.6 Å is much shorter than the molecular length. If the DC phase has a monolayer structure, the difference between layer spacing and molecular length is possible by molecular tilt with respect to the layer normal, conformational chain disorder, and chain interdigitation. If the trimer with the extended structure is tilted with the layer normal, the tilt angle based on the XRD measurements is estimated as 51°, which is unrealistic. If the trimer forms a twisted structure, the molecular length is estimated as 63 Å. The conformation is shown in Figure S2 (b). It is much longer than 8 ACS Paragon Plus Environment

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the layer spacing. Furthermore, the chain interdigitation cannot explain the short layer spacing. Therefore the trimer molecules are not though to form a monolayer structure in the DC phase. We infer an intercalated layer structure for the DC phase as shown in Figure 6. (a)

(b)

Figure 5 (a). X-ray diffraction profiles of trimer I–9 in the N (137 °C) phase and the DC phase (128 °C) in the small angle region and (b) those in the wide angle region. Blue lines show diffraction profiles in the N phase. Red lines show those in the DC phase.



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Figure 6. Model for the molecular organization of trimer I–9 in the DC phase. Miscibility Studies. Trimer I–9 exhibited several peaks in the wide angle region and showed no electrooptical switching in the DC phase, suggesting that the DC phase is an HNF phase. It can be swollen in a nematic liquid crystal if the DC phase is an HNF phase.49–51 On the other hand, we reported that binary mixtures of trimer I–7 showing only a nematic phase and a nematic liquid crystal 6OCB exhibited a DC phase.47 We prepared binary mixtures of trimer I–9 and 6OCB. Then we investigated their phase transition behavior depending on a concentration of 6OCB. Figure 7 shows a phase diagram of trimer I–9 and 6OCB. The phase transition temperatures were ascertained using polarized optical microscopy. Although the DC phase appeared even in the mixture containing 90 mol% of 6OCB, the stability discontinuously decreased in the mixture containing 30 mol% of 6OCB. It is particularly interesting that smectic phases were induced above the DC phase in the binary mixtures containing 30–75 mol% of 6OCB. In the cooling DSC thermogram of a mixture of trimer I–9 (75 mol%) and 6OCB (25 mol%), a shoulder peak aside from the large peak for the N–DC transition was observed at 124.5 °C (Figure S3). However, no texture change 10 ACS Paragon Plus Environment

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corresponding to the shoulder peak was detected using polarized optical microscopy. The optical textures of the DC phase of the mixture containing of 25 mol% of 6OCB (Figure S4) resemble those of trimer I–9. On cooling a mixture of trimer I–9 (70 mol%) and 6OCB (30 mol%) in the almost homeotropically aligned N phase (Figure 8(a)), textures with different defects appeared at 127 °C (Figure 8(b)). Each small domain of the needle-like texture (the upper part of Figure 8(b)) consisted of periodic lines, which resemble helical pitch lines in a chiral smectic C phase (Figure 8(c)). The distance between lines is about 1–2 µm. The phase structure is not identified at present. Then the birefringence began to decrease at 103 °C. The needle-like defects remained in the DC phase (Figure 8(d)). The SmA and SmC phases of the mixtures containing 40–75 mol% of 6OCB were identified by polarizing optical microscopy. In a mixture of trimer I–9 (50 mol%) and 6OCB (50 mol%), defects resulting from the texture of the high– temperature phase were observed across the whole surface in the DC phase. The phase transition behaviors of those mixtures give useful information to discuss the structure of the DC phase of trimer I–9. The DC phase of trimer I–7 and 6OCB has a monolayer structure as typical HNF phases have,47 but XRD measurements of trimer I–9 suggest that the DC phase has an intercalated layer structure. There might be difference in layer structure of a DC phase between trimer I–9 and its mixture with 6OCB. Competitive formation between those different layer structures might occur in the mixture containing 30 mol% of 6OCB. Defects resulting from the competitive formation of layer structures are thought to be memorized in the low–temperature DC phase. We infer that the DC phase of trimer I–9 has a distinct layer structure instead of a saddle splay curvature. If the needle-like texture consisting of periodic lines originates in the DC phase of trimer I–9, the DC phase may have a layer structure in which the helical axis exists along the layer normal. A cyanobiphenyl unit is known to interact with a phenylpyrimidine unit via a specific core–core interaction to induce a smectic A phase.52 The intermolecular interactions between a phenylpyrimidine moiety of trimer I–9 and a cyanobiphenyl moiety of 6OCB might induce the SmA and SmC phases above the DC phase in the mixtures. Because HNF phases are crystalline fibers, these HNF phases can be swollen in a nematic LC where the chirality of the fibers is transferred to the nematic phase filling the space between them.2 Induction of the smectic phases of the mixtures with 6OCB is difficult to explain if the DC phase of I–9 has an HNF structure. On the other hand, binary mixtures of trimer I–7 and 6OCB exhibited N and DC phases without a 11 ACS Paragon Plus Environment


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smectic phase.47 The optical textures of the DC phase of the mixture containing 10 mol% of 6OCB (Figure 9) are different from neither those of the DC phase of trimer I–9 nor those of the mixture of trimer I–7 (10 mol%) and 6OCB (90 mol%). In a binary system of nematic trimer I–7 and 6OCB, the optical texture of the induced DC phase is independent of the content of 6OCB.47 The origin of the appearance of the DC phase for the system is intermolecular interactions between a phenylpyrimidine moiety of trimer I–7 and a cyanobiphenyl moiety of 6OCB. With respect to the binary system of trimer I–9 and 6OCB, with increasing 6OCB concentration in the mixtures with trimer I–9, the driving force of the appearance of DC phase is thought to change from intermolecular interactions between phenylpyrimidine moieties in adjacent trimer molecules to those between a phenylpyrimidine moiety of trimer I–9 and a cyanobiphenyl moiety of 6OCB. Recently, Lee and Araoka reported that a mixture of an odd-membered dimer exhibiting a DC phase and a nematic liquid crystal shows transformation of architectures as increasing the contents of the nematic LC.53 The DC phase of a mixture of trimer I–9 and 6OCB can transform the structure depending on the 6OCB concentration.

Figure 7. Phase diagram of trimer I–9 and 6OCB.


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(a)

(c)

(b)

(d)

Figure 8. Polarized optical textures of a binary mixture of trimer I–9 (70 mol%) and 6OCB (30 mol%) on a glass slide with a cover glass: (a) N phase at 142.5 °C with crossed polarizers; (b) unidentified phase at 125.2 °C with crossed polarizers; (c) expansion of the upper part of Figure 8 (b) for clearance; (c) DC phase at 99.0 °C with uncrossed polarizers. The cooling rate was 5 °C min-1.

Figure 9. Polarized optical textures for a mixture of trimer I–9 (10 mol%) and 6OCB (90 mol%) in the nematic phase at 80.8 °C.



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Structure–Property Relationship. We investigated mesomorphic properties of trimer I–n depending on the parity of the linking group. The results are presented in Figure 10. Phase transition temperatures and enthalpies are listed in Supporting Information (Table S1). As reported previously,47 trimer I–7 showed only a nematic phase, but trimer I–9 exhibited nematic and DC phases. Both trimers I–6 and I–8 exhibited nematic and smectic A phases. The biphenyl-based dimers54 and the azobenzene-based dimers55,56 reportedly exhibit HNF phases depending on the parity of their linking chain. In each series, the dimers possessing the shortest odd-numbered spacers showed the most stable HNF phase. These observations indicate that the appearance of the HNF phase for the dimers can be attributed to the rigid bent-shaped conformers. Marked differences exist in the dependence of the chiral conglomerate stability on the spacer length between the previously reported dimers54,55 and trimer I–n, suggesting that the DC phase of trimer I–9 is different from typical HNF phases of bent-core LCs.

Figure 10. Phase transition behavior of trimer I–n as a function of methylene number in the linking chain. Model for Spontaneous Generation of Chiral Asymmetry. We next discuss how chirality arises in the DC phase of trimer I–9. Typical HNF phases observed for bent-shaped molecules have a monolayer structure. However, the DC phase of trimer I–


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9

has

an

intercalated

layer

structure.

Dipole–dipole

interactions

between

phenylpyrimidine units are thought to play an important role in the formation of the intercalated layer structure as shown in Figure 6. The trimer is thought to combine two bent-shaped dimers. We assume that core–core interactions between phenylpyrimidine units induce the twist orientation between the pseudodimers within trimer I–9, which can stabilize a twist conformation of the central biphenyl. The cooperativity provided by multicontact interactions can evolve the chirality originating in the twisted biphenyl over large distances, leading to large homochiral domains.2 The helical axis can exist parallel and/or perpendicular to the layer normal. The question of why the phase is optically isotropic remains. The structure of both the sponge phases and HNF phases is dominated by the saddle-splay curvature of the layers. The saddle-splay curvature is probably driven by an intra-layer structural mismatch21,27 or by a denser and more ordered packing of twisted molecules.39,40 Although trimer I–9 has a zig-zag bent-shaped structure, it might be much less rigid compared to typical bent-shaped molecules exhibiting DC phases. Therefore, no intra-layer mismatch exits. The observed short layer spacing of the DC phase of trimer I–9, which is 63% of the molecular length, cannot be explained in terms of such a twisted conformation. The intercalated model in which inter-layer correlation might be strong is consistent with the fact that the DC phase has a longer correlation length (948 Å) than typical HNF phases. Such an intercalated organization has difficulty producing the layer deformation to drive the saddle-splay curvature. The experimentally obtained results indicate that the DC phase of trimer I–9 is neither a sponge phase nor an HNF phase. Miscibility studies between trimer I–9 and 6OCB reveal that the DC phase is a soft crystalline phase. Furthermore, there is a possibility that it has a distinct layer structure in which the helical axis exists parallel to the layer normal as discussed previously. Scanning tunneling electron microscopy (STEM) and atomic force microscopy (AFM) were preliminarily carried out to investigate the structure of the DC phase of trimer I–9. Figure 11 shows the result of STEM of the sample. Many tubercles can be seen. The distance between neighboring tubercles is about 200 nm, which is equivalent to twice of the correlation length of the layer periodicity in the DC phase. Each tubercle is surrounded by a hollow, and fan-shaped blocks are located around the hollow as indicated by yellow broken lines. Figure 12 shows AFM images of the sample on a glass slide. Toroidal pits can be seen clearly. The toroidal pit consists of about ten 15 ACS Paragon Plus Environment


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blocks with a width of about 40 nm, and the blocks form a spiral structure. The phase structure is porous. Therefore it can produce nanostructured composites. Figure 13 shows a model for structure of the DC phase. Competition between twisting attributable to the twisted biphenyl of trimer I–9 and the tendency for a uniform intercalated layer structure is accompanied by creation of blocks of smectic layers. The twisted biphenyl of trimer I–9 can induce helical structure parallel and/or perpendicular to the layer normal. The smectic layer blocks forming a helical superstructure are seen in the twist grain boundary (TGB) phase of chiral rod-like molecules.57 We propose a TGB-like soft crystalline structure for the DC phase as follows. The smectic block possessing a helical axis along the layer normal is twisted with respect to the neighboring block as similar to TGB phases, and goes around the pit to form a helical toroidal structure. Finally, we infer the spontaneous formation of a macroscopic homochiral structure as follows. Either a right-handed or left-handed structure is induced at the first stage during the N– DC phase transition. The handedness is stochastic. Consequently, produced chirality is inevitably amplified to the large homochiral area through the cooperative formation of the intercalated layer structure. Then the layer structure with homochirality breaks up into blocks and forms a twisted toroidal pits, leading to macroscopic homochiral structure with optical isotropy.

Figure 11. STEM image of the DC phase of trimer I–9 at room temperature.


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(a)

(b)

Figure 12. (a) Two dimensional and (b) three dimensional AFM images of the DC phase of trimer I–9 at room temperature.

Figure 13. Model for the DC phase of trimer I–9.

CONCLUSIONS



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We prepared a homologous series of trimer I–n (n=5–9) in which two phenylpyrimidine units and a central biphenyl unit were connected via flexible methylene spacers. The longest odd-membered trimer I–9 showed N and DC phases. The other trimers showed no DC phase. Trimer I–9 was found to form a homochiral structure cooling from the nematic droplet. X-ray diffraction measurements reveal that the DC phase has an intercalated layer structure. We conclude that cooperativity provided through multicontact interactions in the intercalated structure can evolve the chirality originating in the twisted biphenyl over large distances, leading to the spontaneous generation of chiral asymmetry. We propose a TGB-like soft crystalline structure for the DC phase of trimer I–9 representing a new modification of the chiral symmetry breaking in lamellar liquid crystal phases. Supporting information Table S1 showing phase transition temperatures (°C) and enthalpies (kJ mol-1) for trimer I–n, Figure S1 showing polarized optical texture of a droplet consisting of two chiral domains with opposite handedness for trimer I–9, Figure S2 showing MOPAC models for trimer I–9 with (a) extended conformation and (b) twisted conformation, Figure S3 showing DSC thermogram of a mixture of trimer I–9 (75 mol%) and 6OCB (25 mol%) on cooling, Figure S4 showing polarized optical textures for a mixture of trimer I–9 (75 mol%) and 6OCB (25 mol%) in the DC phase. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENTS We thank Mr. Natsuki Ujiie of Hiroraki University for AFM measurements. REFERENCES 1. Takezoe, H. Spontaneous Achiral Symmetry Breaking in Liquid Crystalline Phases.

Top. Curr. Chem. 2012, 318, 303–330. 2. Tschierske, C.; Ungar, G. Mirror Symmetry Breaking by Chirality Synchronisation in Liquids and Liquid Crystals of Achiral Molecules. ChemPhysChem, 2016, 17, 9– 26. 3. Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Körblova, E.; Walba, D. M. Spontaneous Formation of Macroscopic Chiral Domains in a Fluid 18 ACS Paragon Plus Environment

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