Supramolecular Polymerization in Liquid Crystalline Media: Toward

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Supramolecular Polymerization in Liquid Crystalline Media: Toward Modular Synthesis of Multifunctional Core–Shell Columnar Liquid Crystals Keiichi Yano, Takahiro Hanebuchi, Xu-Jie Zhang, Yoshimitsu Itoh, Yoshiaki Uchida, Takuro Sato, Keisuke Matsuura, Fumitaka Kagawa, Fumito Araoka, and Takuzo Aida J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03961 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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Journal of the American Chemical Society

Supramolecular Polymerization in Liquid Crystalline Media: Toward Modular Synthesis of Multifunctional Core–Shell Columnar Liquid Crystals Keiichi Yano,1† Takahiro Hanebuchi,1† Xu-Jie Zhang,1 Yoshimitsu Itoh,1,* Yoshiaki Uchida,3 Takuro Sato,2 Keisuke Matsuura,2 Fumitaka Kagawa,2,4 Fumito Araoka,2 and Takuzo Aida1,2,* 1

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

2

RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.

3

Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan.

4

Department of Applied Physics, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Supporting Information Placeholder ABSTRACT: Recently, we discovered a modular synthetic approach for constructing core–shell columnar liquid crystals (LCs) by supramolecular polymerization in LC media. In the present work, we successfully confirmed that our modular synthetic approach has the potential to be widely extended to the development of multifunctional columnar LCs. Herein, we constructed the first core–shell columnar LC that was proved to be orientable by both electric and magnetic fields by the supramolecular polymerization of NODiskNH* in a nematic LC medium of 4-cyano-4'-pentyloxybiphenyl (5OCB). NODiskNH* is a chiral benzenetricarboxamide derivative bearing 2,2,6,6-tetramethylpiperidine 1-oxyl termini, which is known to form a helical supramolecular polymer via a triple hydrogen-bonding array. NODiskNH* alone formed a hydrogenbonded liquid phase without any long-range structural ordering. However, a nematic LC medium of 5OCB, when mixed with NODiskNH* at a molar ratio of 1/3, underwent a "structural order-increasing" mesophase transition, affording an optically active single LC phase with a hexagonally arranged core–shell columnar geometry in a temperature range from 113 °C to 51 °C. Unprecedentedly, this core–shell columnar LC can orient its columns both electrically and magnetically, resulting in unidirectional columnar ordering.

INTRODUCTION Supramolecular polymerization has attracted particular attention because it is not only a noncovalent alternative to conventional covalent polymerization but is also applicable to the elaboration of functional soft materials.1 Historically, the field of supramolecular polymerization has made considerable progress by addressing a fundamental question of "how easy or difficult it is to noncovalently realize what conventional polymerization can do". Successful examples reported in the last two decades include stereoselective or helical supramolecular polymerization,2a–d seeded or chain-growth supramolecular living polymerization including noncovalent block copolymer synthesis,2e–i sequence-specific supramolecular polymerization,2j and so on. However, more recently, we and some other groups have started to redirect the focus of the research in this field to the question of "how easy or difficult it is to noncovalently achieve what conventional polymerization cannot do".3 In the beginning of this year, we reported supramolecular polymerization of disk-shaped OCBDiskNH*, a chiral benzenetricarboxamide (BTA) derivative (Figure 1a), in a typical nematic liquid crystalline (LC) medium comprising a rod-

Figure 1. Molecular structures of (a) disk-shaped monomers NODiskNH * and OCB DiskNH* and (b) rod-shaped LC molecules 7TB and 5OCB.

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shaped molecule of 4-cyano-4'-pentyloxybiphenyl (5OCB) (Figure 1b). This supramolecular polymerization unprecedentedly induces a "structural order-increasing" mesophase transition, affording a core–shell columnar LC with a rectangular geometry.4 BTA monomers with chiral side chains are known to form one-handed helical supramolecular polymers by triple hydrogen bonds (Hbonds) at their amide moieties.5 In our system, such a noncovalent helical polymer constitutes the core part of each LC column that is wrapped by a helical shell consisting of 5OCB. Although columnar LCs are generally rigid and barely orientable by electric fields (Efields) due to their high structural order,6 this core–shell columnar LC rapidly responds electrically, since its constituent columns can dynamically dissociate into the core- and shell-forming modules, thereby facilitating their electrical reorganization. Furthermore, when this supramolecular polymerization was carried out in a nematic LC medium composed of a rod-shaped azobenzene derivative, a core–shell columnar LC with a photoisomerizable shell was formed. This modularly obtained columnar mesophase was successfully elaborated into a LC-based "AND" logic gate, where the columnar structure was quickly reorganized in an E-field under the irradiation with UV light. The above successful example prompted us to examine the potential of our modular synthetic approach to multifunctional core–shell columnar LCs. Recently, Uchida and coworkers developed a particular type of LC molecules containing nitroxyl radicals, the mesophases of which showed an interesting magnetoLC effect.7,8 Hence, we decided to pursue the synthesis of a nitroxyl radical-appended version of the above core–shell columnar LC using our newly developed modular approach.4 After initial struggles, we eventually found that the supramolecular polymerization of nitroxyl radical-appended NODiskNH* in 5OCB gave a core–shell columnar LC (Figure 2). Although the resulting LC material did not show a magneto-LC effect, we found that its columnar orientation can be redirected both electrically and magnetically (Figures 6a, b). It should be noted that only a few magnetically orientable columnar LC materials have been reported to date,9 none of which was claimed to be electrically orientable. Our previous core–shell columnar LC using OCBDiskNH* as the monomer (Figures 1a, b) was not an exception to this

Figure 2. Schematic representations of the supramolecular polymerization of NO DiskNH* in a LC medium of 5OCB, resulting in a “structural order-increasing” mesophase transition into a hexagonal columnar LC phase with a helical core–shell geometry.

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understanding because it was orientable electrically4 but not magnetically (Figure S7). RESULTS AND DISCUSSION Design of an Organic Radical-Appended Disk-Shaped Monomer. Following our previous work,4 we initially attempted the supramolecular polymerization of OCBDiskNH* (Figure 1a) in a nematic LC comprising 7TB (Figure 1b) that bears a thermally stable organic radical 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) unit.10 However, OCBDiskNH* and 7TB, upon mixing, gave a dark image in polarized optical microscopy (POM) (Figure S2), suggesting the absence of any ordered structure. Therefore, we covalently attached TEMPO to the monomer component to obtain NODiskNH* in 9 steps (Figure 1a).11 The DSC profile of NO DiskNH* alone on cooling suggested the presence of a mesophase over a wide temperature range from 126 °C to 22 °C (Figure S1a). Using Fourier-transform infrared (FTIR) spectroscopy, we confirmed that the amide N–H moieties were H-bonded with each other in the mesophase temperature range (Figure S16). However, its POM image was dark (Figure S3) with a featureless X-ray diffraction (XRD) profile (Figure S9). Thus, unlike semicrystalline OCB DiskNH*, nitroxyl radical-appended NODiskNH* alone does not form a long-range structural order but is essentially a viscous-liquid mesophase. Modular Synthesis of a Core–Shell Columnar LC bearing Organic Radicals. We eventually found that NODiskNH* and 5OCB, upon mixing, affords a core–shell columnar mesophase (Figure 2): NODiskNH*/5OCB at a molar ratio of 1/3 was heated once to its isotropic phase (Iso) and then cooled in order to facilitate the supramolecular polymerization of NODiskNH* in 5OCB, giving rise to a birefringent POM texture (Figure 3b). The XRD profile of NODiskNH*/5OCB (1/3) at 110 °C featured sharp and intense diffraction peaks in its lower angle region that can be assigned to a hexagonal columnar mesophase (Colh; a lattice parameter, a = 41.4 Å) (Figure 3d). Its intercolumnar distance agreed well with that predicted from the molecular dimensions of NO DiskNH* upon hexagonal packing (Figure S28).11 In the FTIR spectrum of NODiskNH*/5OCB (1/3) in its isotropic phase, a vibrational band due to amide N–H was observed at 3347 cm–1 (Figure S15), indicating that the core amide units of NODiskNH* were not H-bonded. When the isotropic mixture was allowed to cool to 110 °C in order to induce the phase transition into its columnar mesophase, this vibrational band abruptly shifted toward a lower wavenumber of 3244 cm–1 (Figure 3g) characteristic of Hbonded amides. These results indicate that the Iso-to-Colh phase transition of NODiskNH*/5OCB (1/3) is accompanied by the Hbonding-mediated supramolecular polymerization of NODiskNH*. We note that NODiskNH* is chiral, having in each of its side chains a stereogenic center in proximity to the amide unit (Figure 1a). In the circular dichroism (CD) spectrum of NODiskNH*/5OCB (1/3) (Figures 3f, S14), its Colh mesophase showed remarkable Cotton effects at 339 nm and 297 nm due to the oxycyanobiphenyl (OCB) units (Figure S13). Upon being heated to induce a phase transition into its isotropic phase, NODiskNH*/5OCB (1/3) became CD-silent. These results suggest that chiral NODiskNH* forms a helical polymer in 5OCB by the H-bonding-mediated supramolecular polymerization. Additionally, we note that NODiskNH*/5OCB (1/3) was optically active in the absorption range of TEMPO at around 500 nm (Figure 3f), suggesting that the organic radicals are


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Journal of the American Chemical Society located in the vicinity of the helical strand of the columnar core. Upon cooling to 80 °C (Figures 3c, e), the XRD profile in its lower angle region displayed a number of peaks that were successfully indexed as a 3D-ordered columnar phase (3DColh) consisting of hexagonally assembled core–shell columns (a = 63.2 Å) with an average persistent length of 17.8 nm (Figures S10c, S28).12 Next, we investigated the mesophase behaviors of NODiskNH*/5OCB prepared at different mixing molar ratios of 1/3, 1/6, and 1/9. For

1/3 (Figures 3b, c, S10) and 1/6 (Figures S5, S11), DiskNH*/5OCB upon cooling showed a phase sequence of Isoto-Colh-to-3DColh (Figures 3a), where the Iso-to-Colh phase transition temperature was lower as the content of 5OCB increased, with 113 °C obtained for 1/3 (Figure S1b) and 108 °C for 1/6 (Figure S1c). NODiskNH*/5OCB (1/9), upon cooling from its isotopic phase, showed tetragonal and rectangular columnar mesophases (Colt and Colr) at 85 °C and 70 °C, respectively (Figures S6, S12). NO

Magnetic Properties. First of all, we characterized DiskNH*/5OCB (1/3) using electron paramagnetic resonance (EPR) spectroscopy upon heating and cooling in a temperature range between 25 °C and 130 °C. Fitting analysis of the obtained NO

Figure 4. Temperature dependences of (a, b) cpara’, (c, d) ΔHpp L, and (e, f) ΔHpp G of NO DiskNH*/5OCB (1/3), obtained by EPR spectroscopy at 0.33 T (a, c, e; open circles) upon heating and (b, d, f; closed circles) cooling between 25 °C and 130 °C. (a, b) The obtained values were normalized relative to each corresponding maximum value.

Figure 3. (a) Phase behaviors of NO Disk NH*, 5OCB, and their mixtures (NODiskNH */5OCB) at different molar ratios. Iso, VL, Colh, 3DColh, Colt, Colr, and N denote isotropic liquid, viscous liquid, hexagonal columnar LC, 3D-ordered hexagonal columnar LC, tetragonal columnar LC, rectangular columnar LC, and nematic LC phases, respectively. (b, c) POM images under crossed polarizers and (d, e) synchrotron XRD patterns of NODiskNH*/5OCB (1/3) at (b, d) 110 °C and (c, e) 80 °C. (b, c) Scale bars represent 100 μm. (d, e) Miller indices are given in parentheses. (f) CD spectra of NODiskNH*/5OCB (1/3) at 115 °C (red) and OCB Disk NH*/5OCB (1/6) at 100 °C (black), each adopting a columnar LC mesophase. (g) Variable-temperature FTIR spectral profile upon cooling of NO DiskNH*/5OCB (1/3) derived from amide N–H (3390–3190 cm–1).

Figure 5. Temperature dependences of cM of NO DiskNH*/5OCB (1/3), obtained by SQUID magnetometry at 0.05 T (a; open circles) upon heating and (b; closed circles) cooling between 4 K and 400 K. Fitting curves, shown in red, were obtained using the Curie–Weiss equation with a temperature-dependent Weiss constant.9h,11



EPR spectra at different temperatures allowed us to obtain normalized molar paramagnetic susceptibilities χpara’, Lorentzian and Gaussian peak-to-peak line widths ΔHppL and ΔHppG, and g-values.7f Upon both heating and cooling, χpara’ (Figures 4a, b), ΔHppL (Figures 4c, d), and ΔHppG (Figures 4e, f) changed abruptly in a temperature range for the phase transition between the 3DColh and Colh phases (orange-shaded areas). The observed changes in ΔHppL and ΔHppG indicate that the 3DColh-to-Colh phase transition is accompanied by the attenuation of the spin–spin exchange interaction and enhancement of the inhomogeneity of intermolecular contacts, respectively, between the TEMPO radicals.11,13 Then, we measured the molar magnetic susceptibilities χM of NODiskNH*/5OCB (1/3) using a superconducting quantum interference device (SQUID) over a wide temperature range between 4 K and 400 K (Figure 5). Both upon heating (Figure 5a, inset) or cooling (Figure 5b, inset), the temperature dependence of χM was monotonic in the mesophase temperature range, thereby excluding the possibility

that a magneto-LC effect may emerge.7 By fitting analysis of the χM curves as a function of temperature upon heating (Figure 5a) and cooling (Figure 5b), we also found that the Weiss constants change continuously over the entire examined temperature range,11 suggesting that the TEMPO units in NODiskNH*/5OCB (1/3) remain mobile even at very low temperatures. Columnar Orientation by Electric Field. As described above, disk-shaped OCBDiskNH* in our previous work4 was a semicrystalline substance. However, newly developed NODiskNH* was a viscous liquid without any long-range structural ordering. Hence, the fact that the newly obtained core–shell columnar LC has a higher structural order (p6mm hexagonal symmetry) than the previously reported LC (c2mm rectangular symmetry) is intriguing. Having this notion in mind, we investigated whether our columnar LC is electrically orientable. For this purpose, we prepared a glass cell with a comb-type electrode11 and filled its interior with NO DiskNH*/5OCB (1/3). Then, this sample was kept at the mesophase (Colh) temperature of 110 °C and was subjected to an in-plane direct-current (DC) E-field (40 V µm–1). Its POM image remained birefringent (Figure S8), indicating that in contrast to that of our previously reported OCBDiskNH*/5OCB, the Colh mesophase is not sufficiently dynamic to reorganize electrically. However, when the E-field was applied during the cooling process (1 °C min–1) from its isotropic phase, the resulting Colh mesophase at 110 °C was unidirectionally ordered, as confirmed by the fact that the sample showed bright- and dark-field POM images alternately at 45° intervals (Figure 6c). Accordingly, its polarized FTIR spectral profile was anisotropic for H-bonded amide N–H at 3239 cm–1 and 5OCB C≡N at 2224 cm–1, and their absorption maxima appeared in the directions parallel and perpendicular to the applied E-field, respectively (Figures 6e, S17). This orientational preference is the same as that of our previously reported OCB DiskNH*/5OCB.4,14 Columnar Orientation by Magnetic Field. In contrast with DiskNH*/5OCB, nitroxyl radical-appended NODiskNH*/5OCB provides a rare example of magnetically orientable columnar LCs.8 When NODiskNH*/5OCB (1/3) was sandwiched by calcium fluoride (CaF2) plates and cooled slowly at 1 °C min–1 from its isotropic phase to 110 °C in a 9-T magnetic field (Figure 6b), the resulting Colh mesophase in POM (Figure 6d) and polarized FTIR (Figures 6f, S18) displayed a dichroic feature. Since the heating– cooling process was necessary for this magnetic orientation, it is likely that the nucleation event is magnetically perturbed in an anisotropic manner. As shown in Figure 6f, the vibrational band due to H-bonded amide N–H (3239 cm–1; purple) displayed an absorption maximum in a direction perpendicular to the applied magnetic field, whereas those due to C≡N (2224 cm–1; orange) and aryl C–O (1249 cm–1; light green) showed their absorption maxima in a direction parallel to the applied magnetic field. By contrast, the vibrational band due to the nitroxyl N–O• (1363 cm–1; red) at the floppy termini of the side chains displayed essentially an isotropic feature. These results allowed us to conclude that in contrast with the E-field (Figures 6a, 6c, 6e), the magnetic field preferentially orients the core–shell columns to its perpendicular direction (Figure 6b). This orientational preference may be reasonable, provided that the TEMPO-appended biphenyl units in NO DiskNH*/5OCB play a major role in generating magnetically perturbed nucleation species. OCB

Figure 6. (a, b) Schematic representations, (c, d) POM images under crossed polarizers, and (e, f) polar plots of the polarized FTIR absorption intensities of NODiskNH */5OCB (1/3) at 110 °C. (c, e) The sample was introduced into a comb-type electrical cell (20 μm of separation),11 and subjected to a DC E-field (40 V µm–1) upon cooling at 1 °C min–1 from its isotropic phase. (d, f) The sample was sandwiched by CaF2 plates and placed in a 9-T magnetic field upon cooling at 1 °C min–1 from its isotropic phase. (c, d) Scale bars represent 100 μm. (e, f) Polarized FTIR absorption intensities due to H-bonded N–H (3239 cm–1; purple), C≡N (2224 cm–1; orange), N–O• (1363 cm–1; red), and aryl C–O (1249 cm–1; light green), were plotted at every 5° interval and were normalized relative to each corresponding minimum value.


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Journal of the American Chemical Society CONCLUSIONS In the present work, we succeeded in expanding the scope of our modular synthetic approach to multifunctional core–shell columnar LCs by using supramolecular polymerization of diskshaped monomers in nematic mesophases of rod-shaped LC molecules. The material we developed here is the first core–shell columnar LC that was confirmed to be orientable both electrically6,15 and magnetically.8 Such dual-responsive columnar LCs may be elaborated into magnetoelectric materials16 for which the magnetic properties can be electrically modulated and vice versa; additionally, this approach may be further expanded to multiferroic soft materials.17 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, DSC charts, POM images, XRD profiles, Spectroscopic data, SQUID profiles (PDF)

AUTHOR INFORMATION Corresponding Authors [email protected] (Y.I.); [email protected] (T.A.)

Author Contributions † K. Yano and T. Hanebuchi contributed equally to this work.

Notes No competing financial interests have been declared.

ACKNOWLEDGMENTS The synchrotron radiation experiments were performed on BL44B2 at the Super Photon Ring (SPring-8) with the approval of RIKEN (Proposal Nos. 20170046, 20180044). We acknowledged Dr. K. Kato from RIKEN SPring-8 center for his generous support for the synchrotron radiation experiments. We thank Dr. T. Akita, Mr. Y. Sugiyama, and Prof. N. Nishiyama from Osaka University for their help with EPR spectroscopy. This work was financially supported by a JSPS Grant-inAid 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 Scientist (A) (16H06035). K.Y. thanks the Program for Leading Graduate Schools (MERIT) and the JSPS Young Scientist Fellowship. X.-J. Z. thanks the MEXT scholarship and the JSPS Young Scientist Fellowship.

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