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Nematic-to-columnar mesophase transition by in situ supramolecular polymerization

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Science  11 Jan 2019:
Vol. 363, Issue 6423, pp. 161-165
DOI: 10.1126/science.aan1019

Mating disks and rods into an ordered phase

Disk-shaped molecules tend to stack in columns, whereas rod-shaped ones tend to align parallel to each other. When the two types of molecules are mixed, they tend to phase separate. Yano et al. found the right recipe that allowed enough affinity between the disks and rods so that they formed a blended twisted columnar phase. The phase could be stabilized by polymerizing the disks. The orientation of the twisted columns could be altered using electric fields, whereas optical stimuli could lead to a second ordering transition.

Science, this issue p. 161

Abstract

Disk- and rod-shaped molecules are incompatible in coassembly, as the former tend to stack one-dimensionally whereas the latter tend to align in parallel. Because this type of incompatibility can be more pronounced in condensed phases, different-shaped molecules generally exclude one another. We report that supramolecular polymerization of a disk-shaped chiral monomer in nematic liquid crystals comprising rod-shaped molecules results in order-increasing mesophase transition into a single mesophase with a core-shell columnar geometry. This liquid crystalline material responds quickly to an applied electric field, resulting in unidirectional columnar ordering. Moreover, it can be modularly customized to be optoelectrically responsive simply by using a photoisomerizable rod-shaped module. The modular strategy allows for cooperative integration of different functions into elaborate dynamic architectures.

Polymerization in organized media has attracted attention because of the potential of the media to promote higher-order structures and the resulting physical properties (15). Conversely, supramolecular polymerization (68) in organized media has been explored very little to date (9, 10), because the resultant noncovalent structures (if any) conferred by the media cannot be isolated.

We focused on supramolecular polymerization in liquid crystalline (LC) media (Fig. 1A). If the affinity between the resulting supramolecular polymer and the LC medium is large, then they are miscible, affording a uniform LC dispersion of the polymer (Fig. 1A, b). Conversely, if their affinity is poor, the polymer molecules are bundled and phase-separated from the LC medium (Fig. 1A, d). In the latter case, the insoluble polymer bundles may form a cross-linked three-dimensional network, which possibly compartmentalizes the LC phase and enhances its dynamic properties, as seen with LC physical gels (9). We find that supramolecular polymerization in nematic LC media induces an order-increasing mesophase transition (Fig. 1A, c), allowing highly elaborate, core-shell columnar coassembly.

Fig. 1 Phase transition of LC media by supramolecular polymerization.

(A) Schematic representations of possible modes of supramolecular polymerization in LC media. A monomer can form its one-dimensional supramolecular polymer in a nematic LC medium. (a to d) Depending on the affinity between the monomer or the resultant supramolecular polymer and the LC medium, four different modes are expected. (B) Molecular structures of disk-shaped chiral monomers derived from BTA: C10DiskNH*, OCBDiskNH*, and OCBDiskNMe*. Me, methyl. (C) Molecular structures of rod-shaped LC molecules affording a nematic order: OCBRod and AZORod. Upon trans-cis isomerization, AZORod undergoes a reversible phase transition between its nematic LC phase and isotropic liquid. Vis, visible. (D) Phase diagram of OCBDiskNH*-OCBRod. Iso, SC, Colr, Colh, Col, Ch, N, and Cr denote isotropic, soft crystalline, rectangular columnar, hexagonal columnar, columnar (unidentified), cholesteric, nematic, and crystalline phases, respectively.

Our initial motivation was to investigate whether a chiral dopant, if supramolecularly polymerized into a helical strand, could geometrically twist a nematic mesophase into a cholesteric mesophase to a greater extent than its monomeric form could (11). For this purpose, we chose a chiral benzenetricarboxamide (BTA) analog (12) as a supramolecularly polymerizable chiral dopant whose polymerization in solution has been investigated by Meijer and co-workers (7, 13).

In general, different-shaped molecules, such as rods and disks, tend to exclude one another despite the large entropic penalty of the resulting phase separation (14). The combination of OCBRod, a rod-shaped LC molecule affording a nematic order (Fig. 1C), and a BTA derivative monomer such as C10DiskNH* (Fig. 1B, left) is not an exception to this general tendency. C10DiskNH*, which carries three chiral hydrocarbon side chains, noncovalently polymerizes via triple hydrogen-bonding (H-bonding) interactions at its amide groups. We heated a mixture of C10DiskNH* and OCBRod (C10DiskNH*/OCBRod ratio = 1/30) (figs. S13B, S17C, and S21C) and then allowed the resulting isotropic melt to cool so that C10DiskNH* could polymerize noncovalently in the LC medium of OCBRod. Polarized optical microscopy (POM) revealed bundled fibers that were phase-separated from the LC medium (fig. S14, A and C). To address this incompatibility issue, we synthesized OCBDiskNH* (Fig. 1B, center, and figs. S15A and S32) (12), whose hydrocarbon side chains are appended with an oxycyanobiphenyl (OCB) terminus as a potential compatibilizer with OCBRod (15, 16). A mixture of OCBDiskNH* and OCBRod (molar ratio, 1/30), treated thermally in a similar fashion, showed a fan-shaped POM texture (fig. S14, B and D) characteristic of columnar LC phases rather than cholesteric LC phases. Figure 1D shows a full phase diagram of the OCBDiskNH*-OCBRod system, where a columnar mesophase at an OCBDiskNH*/OCBRod molar ratio of 1/6 develops entirely as a single mesophase over the temperature range from 117 to –6°C (Fig. 2D and fig. S11E). When this mixture was diluted with OCBRod, an exothermic peak assignable to its isotropic-to-nematic phase transition appeared at ~68°C, indicating that excess OCBRod phase-separated (figs. S12 and S24). Unlike OCBDiskNH* and OCBRod alone (figs. S11, S16, and S19), OCBDiskNH*-OCBRod (molar ratio, 1/6) in its synchrotron x-ray diffraction (XRD) profile at 100°C presented sharp and intense peaks in a low-angle region, whereas the wide-angle region presented only a broad diffraction peak with a d-spacing of 3.4 Å (Fig. 2G). We confirmed that this columnar LC material adopts a centered rectangular c2mm symmetry (lattice parameters, a = 75.2 Å; b = 35.0 Å), where each column contains tilted π-stacks of polymeric OCBDiskNH* in its core (17), which is wrapped by a polymeric shell comprising self-assembled OCBRod (Fig. 2A). Molecular dynamics (MD) simulations (12) also indicated the possibility of the formation of a core-shell columnar structure from OCBDiskNH* and OCBRod (figs. S54 to S57 and S60). Upon being cooled from 100 to 60°C, this columnar LC material changed its fundamental geometry from rectangular to hexagonal (a = 40.8 Å) without an exotherm (figs. S11E and S20). This phase sequence is opposite to those commonly observed (18) and has been observed in only rare cases (19).

Fig. 2 Noncovalent modular approach to hierarchically ordered core-shell columnar LC phases.

(A to C) Schematic representations; (D to F) POM images under crossed polarizers; (G to I) synchrotron XRD patterns; and (J and K) CD spectra of [(A), (D), (G), and (J)] OCBDiskNH*-OCBRod at 100°C, [(B), (E), (H), and (K)] OCBDiskNH*-AZORod at 140°C, and [(C), (F), and (I)] OCBDiskNMe*-OCBRod at 30°C at disk/rod molar ratios of 1/6. Supramolecular polymerization of OCBDiskNH* in (A) OCBRod and (B) AZORod results in mesophase transition into a columnar LC phase with a core-shell geometry. Each column bears a helical polymeric core composed of H-bonded OCBDiskNH*, which is surrounded by a polymeric shell comprising (A) OCBRod and (B) AZORod with a complementary helical handedness. (C) Nonpolymerizable OCBDiskNMe* in OCBRod forms a cholesteric LC phase. [(D) to (F)] For the POM observations, a sandwich-type glass cell (5 μm of separation) was employed. Scale bars represent 100 μm. [(G) to (I)] Synchrotron XRD patterns in a capillary were obtained upon cooling from isotropic melts. Miller indices are given in parentheses. a.u., arbitrary units; q, scattering vector. (Insets) Diffraction profiles in the wide-angle region. [(J) and (K)] For the CD spectroscopy, each of the samples was introduced into a sandwich-type quartz cell. The CD spectra were normalized by the thicknesses of their quartz cells. mdeg, millidegrees.

Fourier-transform infrared (FTIR) spectroscopy (figs. S30A and S31) of an isotropic state of OCBDiskNH*-OCBRod (molar ratio, 1/6) showed stretching vibrations due to free amide C=O and N–H moieties at 1673 and 3401 cm–1, respectively (13). When this isotropic mixture was cooled to 100°C, allowing the phase transition into the columnar LC phase, these vibrational bands shifted abruptly to the lower wave numbers of 1640 and 3243 cm–1, respectively, indicating that the amide moieties of OCBDiskNH* H-bonded to form its supramolecular polymer. The supramolecular polymerization of OCBDiskNH* is essential for the mesophase to adopt a columnar geometry. Chiral OCBDiskNMe* (Fig. 1B, right, and figs. S13D, S15C, and S22), which is an N-methylated version of OCBDiskNH*, cannot form a H-bonded supramolecular polymer. This modified version, when doped into the nematic phase of OCBRod (OCBDiskNMe*/OCBRod ratio = 1/6), gave rise to the formation of only a cholesteric mesophase (Fig. 2, C, F, and I) and did not induce the nematic-to-columnar mesophase transition.

As described above, chiral OCBDiskNH* carries a stereogenic center in each of its side chains that is in proximity to the H-bonding amide unit (Fig. 1B, center). We confirmed that the supramolecular polymers of (R)-OCBDiskNH (OCBDiskNHR) and (S)-OCBDiskNH (OCBDiskNHS) in methylcyclohexane were optically active and presented circular dichroism (CD) spectra that are mirror images (figs. S28A and S34). Heating to 80°C or mixing with 5 volume % methanol allowed the supramolecular polymers to depolymerize, and their characteristic CD bands virtually disappeared (fig. S28B). We also found that polymeric OCBDiskNHR in the columnar mesophase of OCBDiskNHR-OCBRod (molar ratio, 1/6) at 100°C adopts a helical geometry, displaying a distinct chiroptical feature (Fig. 2J, red, and fig. S29C). The MD simulation of OCBDiskNH*-OCBRod (molar ratio, 1/6) again indicated the formation of a helical BTA assembly in the core (fig. S58). As expected, when the opposite enantiomer OCBDiskNHS was coassembled with OCBRod (molar ratio, 1/6), the resultant CD spectrum was a mirror image of that observed for OCBDiskNHR-OCBRod (Fig. 2J, blue, and fig. S29E). Upon heating to 160°C to induce the phase transition into its isotropic melt, OCBDiskNH*-OCBRod became virtually CD silent (fig. S29A).

Columnar LC phases are known to be rigid and poorly responsive to electrical stimuli (2022). We introduced OCBDiskNH*-OCBRod (molar ratio, 1/6) into a comb-type electrical cell, enabling in-plane application of the electric field (E-field) (Fig. 3A) (12). Before the application of the E-field, the sample at 100°C in POM was birefringent and displayed a fan texture characteristic of randomly oriented multidomain columnar LC phases. The two-dimensional (2D)–XRD pattern, obtained by irradiation with an x-ray beam along the z axis of the sample (Fig. 3A), was isotropic with a concentric diffraction feature (fig. S46A). However, when a direct-current (DC) E-field (40 V μm–1) was applied to this sample along the arrow shown in Fig. 3A, the fan texture disappeared within 1 s (fig. S36 and movie S1). Upon being rotated around the z axis, the resulting material in POM showed bright- and dark-field images alternately at 45° intervals (fig. S35, A and B), indicating that the LC columns uniformly align unidirectionally for up to several hundred micrometers. In 2D-XRD, this oriented sample showed two distinct diffraction arcs, indexed to the (200) and (400) planes of the rectangular geometry, only in the equatorial direction (Fig. 3C), indicating parallel orientation of the core-shell columns relative to the direction of the applied DC E-field. The order parameter of the columnar orientation, as determined by scattering intensity plots, was 0.91 (fig. S47A). The observed response time of the columnar LC sample in its electrical orientation was shorter than most reported (table S1). Despite the rapid response of OCBDiskNH*-OCBRod, the unidirectional order remained unchanged for several days at 100°C after the E-field was turned off. This slow structural relaxation is not expected for the parent nematic LC material of OCBRod. In polarized FTIR spectroscopy at 100°C, this unidirectional order presented a dichroic feature (Fig. 3D and fig. S51), where the polar plots of the H-bonded amide N–H (3239 cm–1) and C≡N (2224 cm–1) groups showed their absorption maxima in the meridional and equatorial directions, respectively. Together with the 2D-XRD profiles (Fig. 3C), the FTIR results make it apparent that the amide N–H and C≡N groups preferentially align parallel and perpendicular to the columnar axis, respectively, in accordance with the MD simulation (fig. S59). By switching the applied E-field from DC to AC (alternating current) (10 kHz, 24 V μm–1), the columnar direction was changed from parallel to perpendicular and vice versa relative to the direction of the applied E-field (Fig. 3, B, E, and F; figs. S37 to S39; and movie S2). Such frequency-dependent behavior is reasonable considering that the C≡N and H-bonded amide groups, which are arranged orthogonally to each other in the LC column, tend to align parallel to the applied AC and DC E-fields, respectively (22). The orientational change profile, obtained from the scattering intensity–azimuthal angle (θ) plots, showed that two peaks (θ = 0° and 180°; perpendicular orientation) emerged at the expense of the original peaks (θ = ±90°; parallel orientation) (figs. S48 and S49). Thus, the orientational change of OCBDiskNH*-OCBRod occurred through electrical reconstruction, that is, electrical disassembly and subsequent reassembly (Fig. 3G), rather than rotation of the core-shell columns. The possible energy diagram is shown in Fig. 3G, a, where the intermediate state, featuring dissociated OCBDiskNH* and OCBRod, substantially lowers the energetic barrier for the electrical reconstruction. Such an M-shaped energy diagram is not expected when the core and shell parts of the column are covalently connected (Fig. 3G, b).

Fig. 3 Electrical reconstruction of the core-shell columnar LC material and its energy diagram.

(A) Schematic representation of a comb-type electrical cell comprising two parallel glass plates (20 μm of separation) for in-plane E-field application. (B) Electrical reassembly of OCBDiskNH*-OCBRod (molar ratio, 1/6), where the core-shell LC columns can be oriented parallel and perpendicular to the directions of applied DC and AC E-fields, respectively. (C to F) [(C) and (E)] Synchrotron 2D-XRD profiles and [(D) and (F)] polar plots of the polarized IR absorption intensities of OCBDiskNH*-OCBRod at 100°C oriented by applying [(C) and (D)] a DC E-field (40 V μm–1) and [(E) and (F)] an AC E-field (square wave, 10 kHz, 24 V μm–1). Arel, relative absorbance. [(C) and (E)] Miller indices are given in parentheses. [(D) and (F)] Polarized IR absorption intensities, arising from the H-bonded amide N–H (3239 cm–1; purple) and C≡N (2224 cm–1; orange) groups, were recorded upon rotation of the polarizer at 5° intervals. These intensities were normalized relative to the minimum value among them. (G) Energy diagrams for the electrical reconstruction of columnar LC materials. (a) The noncovalent core-shell column bears an intermediate state featuring disassembled core and shell modules, whereas (b) the covalent one does not.

In general, high levels of structural integrity and stimulus responsiveness are mutually exclusive. However, Zhang et al. recently succeeded in addressing this issue by integrating a protein crystal into a polymer network (23). In this work, we showed that a rod (OCBRod), when combined with a disk (OCBDiskNH*), properly operates for achieving the high levels of structural integrity and stimulus responsiveness. Furthermore, as described below, our material is customizable by modifying the rod- and disk-shaped components. An example made use of photoisomerizable AZORod (Fig. 1C), instead of OCBRod, for the coassembly with OCBDiskNH*, which resulted in the formation of a core-shell columnar mesophase with a rectangular geometry (Fig. 2, B, E, and H, and fig. S23), similar to the case of OCBDiskNH*-OCBRod. The isotropic-to-columnar phase transition, which was observed for OCBDiskNH*-AZORod (molar ratio, 1/6) at 155°C (fig. S11G), was accompanied by the H-bonding–mediated supramolecular polymerization of OCBDiskNH* (figs. S30B and S33, A to C). The columnar LC material of OCBDiskNH*-AZORod was optically active, displaying distinct CD bands at 320 and 370 nm (Fig. 2K) originating from the OCB moieties in OCBDiskNH* [wavelength of maximum absorption (λmax) = 300 nm] and AZORod (λmax = 347 nm), respectively (fig. S25B). Thus, not only the core but also the shell in the core-shell columnar assembly adopts a helical geometry with a complementary helical handedness (Fig. 2B). Just like OCBDiskNH*-OCBRod, OCBDiskNH*-AZORod was electrically alignable (figs. S40 and S50). Furthermore, it was optically responsive (24): The LC material, upon exposure to 365-nm ultraviolet (UV) light in a sandwich-type glass cell at 140°C (Fig. 4A) (12), underwent a phase transition into an isotropic melt, where the photoirradiated portion changed its appearance from birefringent to dark (Fig. 4C, f and g). As shown in movie S3, this reversible change occurred within seconds upon turning the UV light irradiation off and on (fig. S41). We found that this phase transition is due to the depolymerization of columnarly assembled OCBDiskNH* accompanied by the trans-to-cis photoisomerization of AZORod in the shell (Fig. 4B). Upon irradiation with UV light, OCBDiskNH*-AZORod showed an enhancement of its electronic absorption intensity at 445 nm due to the cis form of AZORod at the expense of the absorption intensity at 347 nm due to its trans form (figs. S26 and S27). Concomitantly, the stretching vibration due to the H-bonded amide C=O moiety (1640 cm–1) shifted to the higher wave number of 1671 cm–1, whereas that due to the N–H moiety (3243 cm–1) became highly obscure (fig. S33, D and E). The thermodynamic stability of polymeric OCBDiskNH* is highly sensitive to the polarity of the medium. From the FTIR spectral features described above, we believe that the dissociation of the polymeric OCBDiskNH* upon irradiation with UV light is caused by the enhanced polarity of the medium upon photochemical generation of highly polar cis-AZORod (Fig. 4B).

Fig. 4 LC material–based optoelectrically rewritable and logic gate operation by optical and electrical stimuli.

(A) Schematic representation of a sandwich-type glass cell comprising two parallel glass plates (5 μm of separation) for vertical E-field application and UV light irradiation. (B) Schematic representations of the photoinduced depolymerization and repolymerization of OCBDiskNH*-AZORod (molar ratio, 1/6) between the columnar LC and its isotropic liquid upon UV light irradiation (365 nm) and backward thermal relaxation. (C) POM images of OCBDiskNH*-AZORod (molar ratio, 1/6) upon application of a power-tuned DC E-field (5 V μm–1) and/or upon irradiation with 365-nm UV light with a lattice-patterned photomask. Dashed squares represent areas exposed to UV light, whereas the entire area was exposed to the E-field. Scale bars represent 200 μm.

A wide variety of operating principles have been reported for the realization of molecular information processors (2527). As LC materials can provide an amplified optical output because of their long-range molecular ordering, the development of LC material–based logic gates is an interesting challenge. Such logic gates require LC materials that can respond differently to multiple stimuli. We developed an LC material–based optoelectrically rewritable device by using OCBDiskNH*-AZORod (Fig. 4C), exploiting its optically and electrically responsive properties. The strategy uses a power-tuned DC E-field that cannot solely orient the LC columns but can only assist the photodissociated monomer to reassemble into a unidirectionally oriented columnar order. The as-received columnar LC phase of OCBDiskNH*-AZORod (molar ratio, 1/6) at 140°C provided a birefringent multidomain sample (Fig. 4C, a) in a sandwich-type glass cell (Fig. 4A). This POM image could be darkened by the application of a DC E-field with a power above 5 V μm–1; otherwise, it remained birefringent (Fig. 4C, b and c; fig. S42; and movie S4). By contrast, when this sample was exposed to UV light, its POM image became nonbirefringently dark by the photoinduced columnar-to-isotropic phase transition (Fig. 4C, f). However, when the UV irradiation ceased, the POM image became birefringent again because the columnar mesophase, thus reformed, presented a randomly oriented multidomain feature (Fig. 4C, g) analogous to the initial state (Fig. 4C, a). Thus, the LC logic gate device can initialize written information by light (movie S6). Meanwhile, when the DC E-field application (5 V μm–1) and UV irradiation were performed simultaneously, the birefringent image quickly turned dark within a few seconds (Fig. 4C, d and e; fig. S43; and movie S5), because monomeric OCBDiskNH*, generated by the photoinduced phase transition, reassembled into a unidirectionally ordered column with the assistance of the applied DC E-field. The nonbirefringently dark image remained unchanged for several hours at 140°C after both the external inputs were turned off (fig. S44). Although this pointwise rewritable capability can be realized by using photoisomerizable AZORod alone, the written image became blurry because of the high fluidity of AZORod (fig. S45 and movie S7). As shown in Fig. 4C, the whole process can be regarded as a rewritable and logic gate because the nonbirefringently dark output appears only when the electrical and optical inputs are applied simultaneously.

Supplementary Materials

www.sciencemag.org/content/363/6423/161/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S60

Table S1

References (2857)

Movies S1 to S7

References and Notes

  1. Materials and methods are available as supplementary materials.
Acknowledgments: The synchrotron radiation experiments were performed on BL44B2 and BL45XU at the Super Photon Ring (SPring-8) with the approval of RIKEN (proposal nos. 20140045, 20150022, 20160024, 20170046, and 20180044). The computations were performed by using the Research Center for Computational Science (Okazaki, Japan). We acknowledge K. Kato for generous support for the synchrotron radiation experiments. Funding: This work was financially supported by a JSPS Grant-in-Aid for Scientific Research (S) (18H05260) on “Innovative Functional Materials based on Multi-Scale Interfacial Molecular Science” for T.A. Y.I. is grateful for a JSPS Grant-in-Aid for Young Scientists (A) (16H06035). K.Y. thanks the Program for Leading Graduate Schools (MERIT) and the JSPS Young Scientist Fellowship. Author contributions: K.Y. designed and performed all experiments. Y.I. and T.A. co-designed the experiments. K.Y., Y.I., F.A., G.W., and T.A. analyzed the data and wrote the manuscript. G.W. performed MD simulations. T.H. supported the XRD measurements at SPring-8. Competing interests: The authors have no competing interests. Data and materials availability: All data are available in the main text or in the supplementary materials.
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