Ferroelectric Alkylamide Substituted Helicene Derivative with 2D

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Ferroelectric Alkylamide Substituted Helicene Derivative with 2D Hydrogen-Bonding Lamellar Phase Hayato Anetai, Takashi Takeda, Norihisa Hoshino, Higashi Kobayashi, Nozomi Saito, Masanori Shigeno, Masahiko Yamaguchi, and Tomoyuki Akutagawa J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11222 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Journal of the American Chemical Society

Ferroelectric Alkylamide Substituted Helicene Derivative with 2D Hydrogen-Bonding Lamellar Phase Hayato Anetai,† Takashi Takeda,†, ‡ Norihisa Hoshino,†, ‡ Higashi Kobayashi,⁋ Nozomi Saito,⁋ Masanori Shigeno,⁋ Masahiko Yamaguchi,⁋ and Tomoyuki Akutagawa †, ‡* † Graduate

School of Engineering, Tohoku University, Sendai 980-8579, Japan Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ⁋ Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai 980-8578, Japan ‡

ABSTRACT: Alkylamide (-CONHCnH2n+1) substituted benzene and its pyrene derivatives have shown a discotic hexagonal columnar liquid crystalline phase through an one-dimensional (1D) intermolecular N-H•••O= hydrogen-bonding interaction, the direction of which is inverted through the application of an alternate current (AC) voltage. The polar hydrogen-bonding chains and dipole inversion reveal a ferroelectric polarization–electric filed (P–E) hysteresis curve. Non--planar helicene derivatives bearing two –CONHC14H29 chains also indicate a ferroelectric response. The racemic helicene derivative shows a bilayer lamellar liquid crystal phase within a temperature range of 330–420 K, whereas there is no liquid crystallinity for the optically active derivative owing to the different molecular assembly structure. The racemic phase is constructed through a two-dimensional (2D) N-H•••O= hydrogen-bonding network, which shows ferroelectric P–E hysteresis curves at above 340 K. The collective dipole inversion in the 2D layer contributes to the ferroelectricity in the lamellar phase. The remanent polarization (Pr) of 11.1 C cm-2 is about 6-times higher than those of the -planar benzene- and pyrene-based 1D ferroelectrics. Both the density of the hydrogen-bonding site and the domain orientation in the 2D system are higher than those of the 1D columnar system.

INTRODUCTION Non-volatile ferroelectric memories have been utilized in information technology, such as contactless smart cards.1 Two kinds of ferroelectric mechanisms of order–disorder and atomic displacement types have been well recognized in the typical ferroelectrics, where the latter one has much faster switching speed than that of the former.2 Among a variety of inorganic ferroelectrics, lead zirconate titanate (PZT) and barium titanate (BaTiO3) have been widely utilized as atomic-displacement type ferroelectric memory devices.1, 3 However, to develop environmentally friendly memory devices, the toxicity, scarce elemental resources, high-cost, and other problems need to be resolved in the near future. From this perspective, chemically designable organic ferroelectrics have attracted significant attention in the fabrication of new types of low-cost flexible memory devices. The well-known organic ferroelectric polymer of polyvinylidene difluoride (PVDF) indicates the rotation of the difluoro-ethylene (-CF2CH2-) group and the polarization inversion through the application of an alternate current (AC) voltage.4 In contrast, low-molecular-weight molecular crystals can also demonstrate a ferroelectric response with a polarization inversion of the order-disorder polarization switching through the intermolecular proton-transfer of proton tautomerization and/or molecular rotation of a flip-flop supramolecular rotator in (m-fluoroanilinium)(dibenzo[18]crown-6)[Ni(dmit)2] salt (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate).5-7 Almost all order-disorder type organic ferroelectrics have a uniaxial polarization axis (one-dimensional system) along the hydrogen-

bonding chain and normal to the rotational axis.7, 8 Much more flexible molecular assemblies such as plastic and liquid crystalline materials are promising candidates for the design of a dynamic ferroelectric molecular system.9-12 For instance, the simple benzene derivative of N,N’,N”-trioctadecylbenzene1,3,5-tricarboxamide (3BC) forms a one-dimensional (1D) hydrogen-bonding molecular assembly structure and discotic hexagonal columnar (Colh) mesophases, where the application of AC voltage in the Colh phase changes the direction of the dipole moment in the amide-type N-H•••O= hydrogen-bonding chain, resulting in a hysteresis loop in the polarization–electric field (P–E) curve.13-16 To obtain multi-functional ferroelectrics, we also designed a pyrene derivative bearing four tetradecylamide (-CONHC14H29) chains, which shows Colh mesophase, organogel, nanowire, and concentration-dependent excimer emissions, as well as ferroelectricity.17-20 A planar electronic system such as benzene and pyrene derivatives bearing alkylamide (-CONHCnH2n+1) chains simply forms a ferroelectric 1D columnar mesophase, in which the design of a -electronic system with optical, electrical, and magnetic properties is a useful approach to achieving multi-functionality. However, the present chemical approach using a structural transformable dynamic hydrogen-bonding chain in a Colh mesophase can only achieve a uniaxial ferroelectric response along the columnar direction. In contrast, biaxial (twodimensional (2D) system) and/or triaxial (three-dimensional (3D) system) ferroelectrics are useful in fabricating thin-film memory devices and stable ON/OFF switching behavior in practical applications. To design such 2D or 3D intermolecular interactions, the utilization of a non-planar -electronic system

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is a possible approach to modulating a 1D columnar structure. In a plastic crystalline state, the 3D isotropic rotation or atomic displacement of the polar spherical molecules enable the design of 3D multi-polar axes and a stable operation of thin-film memory devices.10 However, the designs of 2D and 3D polar axes are quite difficult to achieve in liquid crystalline phases. Two-dimensional lamellar and 3D cubic phases can form multidimensional molecular assembly networks, which have the potential to form multi-polar ferroelectric axes. Because -planar molecules such as benzene, pyrene, and coronene bearing hydrophobic alkyl chains have a tendency to form a 1D -stacking columnar molecular assembly, lowering the -planarity is one approach to modulating a 1D columnar structure and achieving 2D or 3D intermolecular interactions. For instance, non-planar 3D -electron compounds such as fullerene derivatives have been widely studied in organic electronic devices to realize an efficient carrier transportation pathway.21 In addition, partial structural units of fullerene, such as sumanene, corannulene, and a carbon belt, have attracted significant attention in the formation of high-performance electronic materials.22-25 Among them, helicene is a well-known and interesting non-planar -electronic system with axial chirality, and has been utilized in nanofiber formations, circular polarized luminescence (CPL), and other applications.26-29 A helicene derivative of 1,12-dimethylbenzo[c]phenanthrene can form P and M isomers with axial chirality owing to the fixed conformation of a non-planar -plane based on steric repulsion in the terminal methyl groups, through which racemization occurs at temperatures above 523 K, accompanied by a thermal decomposition.30, 31 In addition, the permanent dipole moment of a helicene derivative on a gold substrate surface can be modulated through the application of an electric field.32 The molecular assembly using a non-planar π-electronic system has the potential to form a multi-dimensional hydrogen-bonding network structure. Herein, we focus on racemic (Rac-1) and optically active (P-1) helicene derivatives bearing two –CONHC14H29 chains (Scheme 1), and examined their phase transition behavior, liquid crystal formation, molecular assembly structure, and ferroelectric response. The effective formation of an amide-type intermolecular hydrogen-bonding interaction between – CONHC14H29 chains is discussed based on a comparison with the corresponding racemic and optically active alkylester (COOC14H29) derivatives of Rac-2 and (P)-2. A single-crystal Xray structure analysis of a racemic short-chain helicene derivative (Rac-3) bearing two –CONHC3H7 chains was conducted in this study to evaluate the hydrogen-bonding and packing structures, which revealed the formation of a 2D hydrogen-bonding layer of Rac-3. The excellent ferroelectric response of Rac-1 can be explained through the formation of high-density and well-oriented domains in a 2D hydrogenbonding network layer.

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Scheme 1. Molecular structures of Rac-1, Rac-2, Rac-3, (P)-1, and (P)-2.

RESULTS AND DISCUSSION Racemic and optically active helicene derivatives of Rac-1, (P)1, Rac-2, (P)-2, and Rac-3 bearing –CONHCnH2n+1 or – COOCnH2n+1 chains (n = 14 and 3) were synthesized from the corresponding racemic and optically active helicene carboxylic acid derivatives,33 resulting in a white powder. The DSC charts of each derivative indicate a different phase transition behavior despite the resembled molecular structures. The melting points of Rac-1, (P)-1, Rac-2, and (P)-2 were observed at 424, 411, 336, and 324 K, respectively, and the –CONHC14H29 substituted helicene derivatives of Rac-1 and (P)-1 had about 100 K higher melting points than those of the corresponding –COOC14H29 derivatives of Rac-2 and (P)-2. The differences in these thermal properties were associated with the formation of an amide-type intermolecular –N-H•••O= hydrogen-bonding interaction, which generated short-range molecular assemblies even in a liquid state. In the IR spectra, a free N-H asymmetrical stretching vibrational band (N-H) at 3,400–3,500 cm-1 was observed at a larger energy than that of an intermolecular hydrogen-bonding N-H band at 3,060–3,330 cm-1.17 The N-H bands of Rac-1 on KBr pellet were observed at 3,420, 3,260, and 3,070 cm-1, whereas those of (P)-1 were confirmed at 3,320, 3,230, and 3,070 cm-1. The intermolecular hydrogen-bonding N-H band was about 160 cm-1 red-shifted from the free N-H band, suggesting the formation of an effective intermolecular amide-type N-H•••O= hydrogen-bonding interaction in Rac-1 (Figure S1). The temperature-dependent 1H NMR spectra of Rac-1 in CDCl3 also supported the formation of an intermolecular N-H•••O= hydrogen-bonding molecular assembly even in the solution phase, where the chemical shift at  = 6.20 for the N-H protons at 298 K was shifted to  = 6.12 at 323 K owing to the shielding effect of the intermolecular hydrogen-bonding assembly at 298 K (Figure S2). Interestingly, only Rac-1 indicated a mesophase within the temperature range of 330–420 K, and the POM images under the cross-Nicol optical arrangement showed both fluidic and birefringence behaviors with a characteristic texture of the liquid crystalline phase (Figure 1b). On the contrary, the formation of a mesophase was not confirmed in an optically active (P)-1 derivative, indicating only a solid-liquid phase transition at 411 K with H = 20.8 kJ mol-1. The melting point of Rac-1 at T = 424 K with H = 25.3 kJ mol-1 was about 10 K higher than that of (P)-1, suggesting an effective intermolecular hydrogenbonding interaction for Rac-1. The difference in the molecular assembly structures between Rac-1 and (P)-1 was also consistent with the organogelation ability of (P)-1. The organogelation behavior was only observed in the optically active (P)-1 derivative in the mixed solvent of chloroformhexane (Figure 1c), whereas there was no organogelation for racemic liquid crystalline Rac-1. The organogel state of (P)-1 was clearly consistent with the formation of a 1D molecular assembly structure, whereas the liquid crystalline Rac-1 did not form such an assembly structure. These differences can also be confirmed in the solid-state vibrational IR spectra. The intermolecular hydrogen-bonding NH bands of Rac-1 were observed at 3,260 and 3,070 cm-1, whereas those of (P)-1 were confirmed at 3,320, 3,230, and 3,070 cm-1 owing to the different hydrogen-bonding environment (Figure S1). A similar difference in the phase transition behavior for the racemic and optically active compounds was also observed in the –


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Journal of the American Chemical Society COOC14H29 derivatives of Rac-2 and (P)-2. The melting point of Rac-2 at 336 K with H = 52.4 kJ mol-1 was about 12 K higher than that of (P)-2 at 324 K with H = 32.8 kJ mol-1, suggesting a much stronger intermolecular interaction of the racemic compound than that of the optically active compound. The optically active and racemic helicene -cores affected both the liquid crystallinity and the organogelation ability. The molecular assembly structure in the liquid crystalline phase of Rac-1 in the absence of an organogelation ability should be different from a 1D hydrogen-bonding columnar assembly. Non-intermolecular hydrogen-bonding alkylester derivatives did not form a 1D fibrous molecular assembly or a liquid crystalline phase. The approximately 100 K lower melting points of helicene derivatives bearing two –COOC14H29 chains clearly indicate the role of an intermolecular –N-H•••O= hydrogen-bonding interaction between the –CONHC14H29 chains.

Figure 1. Thermal properties and molecular assemblies of liquid crystal and organogel states of helicene derivatives. a) DSC charts of Rac-1 (black), (P)-1 (red), Rac-2 (blue), and (P)-2 (green). b) POM images of Rac-1 under a cross-Nicol polarized optical microscopy arrangement. c) Difference in organogelation behavior for Rac-1 and (P)-1 in chloroform-hexane with a concentration of 52.7 mM.

Figure 2. Crystal structure of Rac-3•CH2Cl2. CH2Cl2 molecules and hydrogen atoms were omitted for clarification. a) Dimeric hydrogen-bonding structure of P and M isomers in the unit cell

view along the c axis, where the layer-type lamellar structure was elongated along the c axis. b) Interdimer (green) and intradimer (yellow) N-H•••O= hydrogen-bonding 2D layer in the ab plane.

Although we tried to prepare single crystals of Rac-1 for a discussion of the molecular assembly structures in a liquid crystalline phase, no high-quality single crystals formed. Instead of Rac-1, we could obtain a short-chain derivative of Rac-3 bearing two –CONHC3H7 chains, whose molecular, hydrogen-bonding, and packing structures were evaluated using a single-crystal X-ray structural analysis of Rac-3•CH2Cl2 (Figure 2). In the racemic crystal, both P- and M- isomers existed at a 1:1 occupation ratio in the unit cell (Figure 2a), which formed an effective amide-type N-H•••O= hydrogenbonding dimer structure with dN-O = 2.864(3) and 2.995(3) Å (Figure 2b). The hydrogen-bonding (Rac-3)2 dimer was stacked at the brick-stone arrangement along the b axis, forming a 2D -stacking layer in the ab plane. Each (Rac-3)2 dimer was further connected using an interdimer N-H•••O= hydrogenbonding interaction with dN-O = 3.005(3) and 2.884(3) Å. Both the intradimer and interdimer hydrogen-bonding distances in the 2D layer reached almost the same magnitude, forming a 2D dipole layer in the ab plane. Along the c axis, each alkyl chain separated the hydrogen-bonding layer to form a lamellar type molecular assembly structure. On the contrary, the independent P and M isomer columns were observed in racemic single crystals of 5,8-bis(aminomethyl)-1,12dimethylbenzo[c]phenanthrene.30 Therefore, the dimensionalities of the intermolecular hydrogen-bonding interactions in Rac-1 and (P)-1 differed from each other, which is consistent with the difference in the organogelation ability and liquid crystal formation.

Figure 3. Lamellar-type molecular assembly structure of Rac-1. a) PXRD patterns of Rac-1 at T = 400 K (black), Rac-3 at 300 K (blue), and a simulated PXRD pattern of Rac-3 (red) based on a single crystal X-ray structural analysis at T = 173 K (red). b)



Possible model structure of Rac-1 with a layer periodicity of d001 = 2.57 nm.

The molecular arrangement of Rac-1 will be evaluated by a PXRD pattern of Rac-1 and single crystal X-ray structural analysis of Rac-3 (Figures 3 and S3). From the PXRD patterns at 300 K, only Rac-1 indicated high crystallinity and sharp Bragg reflections with a d-spacing of 2.57, 1.27, 0.641, 0.489, and 0.422 nm, which corresponded to the formation of a lamellar structure. Also, Rac-1 gradually form the broad peak with the increasing temperature. On the contrary, optically active (P)-1 showed broad diffraction peaks with a d-spacing of 2.51, 1.52, 0.477, and 0.412 nm, the crystallinity of which was lower than that of racemic Rac-1. The simulated PXRD pattern of Rac-3 based on a single-crystal X-ray structural analysis was consistent with the PXRD pattern of Rac-3. Although nonliquid crystalline Rac-3 has the insufficient motional freedom in the molecular assembly, the single crystal X-ray structural analysis revealed the most effective and dominant intermolecular –N-H•••O= hydrogen-bonding interaction in the molecular assembly. Therefore, the fundamental 2D hydrogenbonding layer of Rac-3 and its lamella-type arrangement should kept even in liquid crystalline phase of Rac-1. The Bragg reflection corresponding to the lamellar periodicity in Rac-3 was observed at 2 = 6.84 ° with a d-spacing of 1.29 nm, whereas that of Rac-1 was observed at 2 = 3.432 ° with d001 = 2.50 nm. The d001 periodicity of Rac-1 was consistent with the calculated molecular length assuming all-trans -CONHC14H29 chains, suggesting the formation of an interdigitated lamellar arrangement of the –CONHC14H29 chains (Figure 3b). The PXRD patterns of the ester derivatives of Rac-2 and (P)-2 were quite different from each other (Figure S3). Therefore, the optically active helicene -core in the absence of amide-type hydrogen-bonding interaction drastically affected the molecular assembly structures, phase transition behaviors, and liquid crystallinities. Interestingly, the Bragg diffractions of (P)-2 within a 2range of 10–30° showed an interesting temperature-dependent structural transformation behavior. The PXRD patterns of (P)-2 indicated a broadened temperature increase of 200–300 K, whereas a suddenly change to sharp diffractions was shown at a temperature of around 310 K owing to the enhancement of the crystallinity. The exothermic DSC peak in the heating process was consistent with the recrystallization process from a low-ordered amorphous-like molecular assembly to a highly ordered crystalline assembly at around room temperature (Figure S4). Multiple alkylamide (-CONHC14H29) substituted benzene and pyrene derivatives showed ferroelectricity owing to the dipole inversion of the 1D intermolecular amide-type NH•••O= hydrogen-bonding interaction along the -stacking columnar direction. In Rac-1, the cooperative dipole inversion in the –CONHC14H29 groups in the 2D hydrogen-bonding layer is an origin of the ferroelectricity. Figure 4a shows the temperature- and frequency-dependent real part of the dielectric constants (1) of Rac-1. The 1-value of Rac-1 was observed to be 3.02 at around 300 K, which was suppressed by an increase in temperature, and showed a discontinuous 1-change at a solid-liquid crystal phase transition temperature. The magnitude of the 1-value was gradually enhanced at 410 K (1 ~ 4 for f = 100 Hz), which was noticeable under lower frequency conditions owing to the slow molecular motion and rotation of the amide group in the 2D molecular assembly. The phase transition from ferroelectric lamellar liquid crystal to

paraelectric liquid one existed around 420 K, where the dielectric peak was observed (Figure S14). The 1-1 - T plots indicated the similar behavior to the phase transition in the 1D columnar ferroelectric 3BC derivative. Although no obvious dielectric peak was shown in the 1–T plots of Rac-1, the 1–T behavior resembled those of alkylamide-substituted arene derivatives in a liquid crystalline state (Figure S5).15-17, 20 The 1–T plots of Rac-1 in the ferroelectric phase did not show the Curie-Weiss behavior, which should be observed in the paraelectric isotopic liquid state above the melting point.17 For much the same reason as this, 4PC did not show the Curie-Weiss behavior too. The imaginary part of the dielectric constant (2) also showed an enhancement after the phase transition into liquid crystal (Figure S6). At f = 1 kHz, the magnitude of 2 = 0.01 at 300 K was enhanced to 2 = 0.85 at 409 K, where the low-frequency 2 values were much larger than those of the high frequency ones (2 = 0.22 at 410 K for f = 10 kHz). The small magnitude of the 2-values indicated almost an ignorable electrical conductivity and leak current in the measurement cell of Rac-1.

Figure 4. Ferroelectric behavior of Rac-1. a) Frequency- and temperature-dependent real-part dielectric constants 1 in the cooling process. b) The P-E hysteresis curves of Rac-1 at f = 0.5 Hz within a temperature range of 343–423 K together with the P– E curve of 4PC at T = 393 K and f = 0.5 Hz (central black curve). c) Temperature-dependent Pr (red in the left axis) and Ec values (blue in the right axis).

A characteristic ferroelectric hysteresis loop was observed in the P–E measurement of Rac-1 at above 340 K, where the frequency-dependent dielectric constants were prominently observed (Figure 4b). The ferroelectricity of Rac-1 in the 2D lamellar phase indicated a well-developed P–E hysteresis loop


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Journal of the American Chemical Society in contrast with those of conventional alkylamide-substituted arene derivatives.15-18, 20 The mechanism of ferroelectricity in the amide-type hydrogen-bonding lamellar phase is also consistent with the dipole inversion of the N-H•••O= hydrogenbonding direction in the 2D layer. In contrast, the ferroelectricity was not observed in the non-liquid crystalline (P)-1, Rac-2, or (P)-2 derivatives owing to an insufficient flexibility for the specific molecular motions in a solid state. The magnitude of remanent polarization (Pr) and the coercive electric field (Ec) of Rac-1 were 2.5 Ccm-2 and 22 Vm-1, respectively, at 343 K, which were linearly increased and decreased by a lowering of the temperature from 340 K. Saturation of the Pr and Ec values was observed at around 420 K owing to the fully thermally activated molecular motions of all –CONHC14H29 chains (Figure 4c). Both the Pr and Ec values depended on the magnitude of the measured E and f values (Figures S10 and S11). At T = 413 K and f = 0.5 Hz, the Ec magnitude was gradually enhanced from 5 to 25 Vm-1 accompanied with a well-developed hysteresis loop. Both Pr and Ec values were also saturated at 11.1 Ccm-2 and 20 Vm1, respectively. Insufficient application of the outer E-value to Rac-1 partially rotated the hydrogen-bonding –CONHC14H29 chains in the absence of a complete polarization inversion, whereas the sufficient magnitude of the E-application at above 20 Vm-1 achieved a complete inversion of all polar amide groups in the lamellar liquid crystalline phase. The f-dependent Pr and Ec measurements at 423 K indicated an enhancement under a low-f condition, suggesting that the rotational frequency for the amide groups was consistent with the measurement at around f = 0.1 Hz. The rotational motion of the amide groups could not be followed at a measured f-value of higher than 10 Hz.

amide-substituted arene derivatives was associated with the formation of a 2D hydrogen-bonding molecular assembly structure in the liquid crystalline phase. The following reasons are considered for the excellent ferroelectricity in the 2D lamellar phase (Scheme 2). The first is the high density of hydrogen-bonding polar sites per unit volume. Because a long –CONHC14H29 chain has a large volume against the -core and hydrogen-bonding N-H•••O= site, the hydrogen-bonding density of the N-H•••O= site per unit nm3 for the four – CONHC14H29 chain in the 1D columnar system of 4PC should be smaller than that for the two-chain system of the 2D layer of Rac-1. Therefore, hydrogen-bonding site density of 2.8 NH•••O= sites per unit nm3 for the lamellar structure of Rac-1 was larger than that 2.0 sites per unit nm3 for the 1D columnar structure of 4PC. The 2D lamellar type hydrogen-bonding structure had excellent ferroelectric parameters as compared to the 1D columnar assembly structure. The theoretically maximum Pr = 3.1 C cm-2 for the 2D system of Rac-1 based on the dipole moment in the DFT calculation was 2-times larger than the 1D one of 4PC with Pr = 1.4 Ccm-2. Because a large Pr value is useful to increase the ON/OFF switching ratio in non-volatile memory devices, the high-density 2D hydrogenbonding system should be a useful approach to enhancing the Pr value. The second reason is the existence of highly oriented 2D hydrogen-bonding in the ferroelectric domain. Each 1D column in 4PC was needed to apply the initial DC voltage and align the 1D columns along the sandwich-type electrode prior to the measurement of the ferroelectric P-E hysteresis loop. In contrast, the 2D layer domains of Rac-1 were not needed in the initial poling procedure to observe the well-developed ferroelectric P-E hysteresis responses. The 2D hydrogenbonding lamellar phase indicated a spontaneous domain orientation between the sandwich-type electrodes. The nonplanar helicene -core itself has a permanent dipole moment, which dipole contribution is quite small magnitude based on the theoretical DFT calculations (Figure S15). Two-dimensional hydrogen-bonding organic ferroelectrics are thus promising candidates for a non-volatile high-density memory device. CONCLUSIONS

Scheme 2. Schematic models of 1D column in 4PC and 2D hydrogen-bonding layer in Rac-1. Interestingly, both the ferroelectric parameters of the Pr and Ec values of Rac-1 were larger than those of the 1D hydrogenbonding columnar system of pyrene derivative bearing four – CONHC14H29 chains (4PC).13 For instance, Pr = 10.7 C cm-2 and Ec = 10.8 V m-1 of Rac-1 were larger than Pr = 0.923 C cm-2 and Ec = 2.10 V m-1 of 4PC. In particular, the Pr value of the two-dimensional Rac-1 was about 10-times larger than that of the 1D system of 4PC and 2-times larger than 3BC (Pr ~4 C cm-2).16 The non-linear correlation between the applied E and Pr indicated that the Pr of Rac-1 was reached at 5.67 C cm2 for E = 3.81 Vm-1 (Figure S10), which was almost similar to that of typical 1D columnar system of 3BC of Pr = 6~7 C cm2.16 The higher P value of Rac-1 than those in previous alkylr

Racemic and optically active non-planar π-conjugated dimethyl-helicene derivatives bearing two –CONHC14H29 chains were studied in terms of the phase transition, molecular assembly structure, and ferroelectricity. The optically active and racemic helicene derivatives formed 1D columnar and 2D lamellar-type molecular assembly structures, the phase transition behavior, liquid crystallinity, and organogelation ability of which differed from each other. Although the former optically active derivative indicated only a solid phase and organogel state, the latter racemic derivative formed a 2D hydrogen-bonding lamellar-type liquid crystalline phase. The 2D hydrogen-bonding network in Rac-1 revealed quite excellent ferroelectric parameters of Pr and Er, which were enhanced from a conventional 1D hydrogen-bonding columnar phase. The density of the dipole inversion units per unit volume of Rac-1 was higher than that of a 1D columnar system. In addition, highly oriented 2D ferroelectric domains were observed in the 2D system, which could omit the initial poling procedure. Two-dimensional hydrogen-bonding organic ferroelectrics in the absence of a poling process will be applicable to the development of excellent non-volatile flexible memory devices.



ASSOCIATED CONTENT Supporting Information. Experimental section, IR spectra, 1H NMR spectra of Rac-1 in CDCl3, PXRD patterns of Rac-1, (P)-1, Rac-2, and (P)-2 at 300 K, temperature-dependent PXRD patterns of (P)-2, frequency- and temperature-dependent dielectric constants of Rac-1, (P)-1, Rac-2, and (P)-2, Electric field dependent P-E curves of Rac-1, and frequency-dependent P-E curves of Rac-1. CCDC-1873930. This material is available free of charge via the Internet at http://pubs.acs.org.

Bring Light to Practical Applications. J. Am. Chem. Soc. 2018, 140, 8051−8059. (13) Matsunaga, Y., Miyajima, N., Nakayasu, Y., Sakai, S., Yonenaga, M. Design of Novel Mesomorphic Compounds: N, N′, N″-Trialkyl-1,3,5-benzenetricarboxamides. Bull. Chem. Soc. Jpn. 1988, 61, 207–210. (14) Yasuda, Y, Iishi, E, Inada, H, Shirota, Y. Novel Low Molecular Weight Organic Gels: N, N, N”-Tristearyltrimesamide/Organic Solvent System. Chem. Lett. 1996, 25, 575–576. (15) Fitié, C. F. C., Roelofs, W. S. C., Magusin, P. C. M. M., Wübbenhorst, M., Kemerink, M., Sijbesma, R. P. Polar Switching in Trialkylbenzene-1,3,5-tricarboxamides. J. Phys. Chem. B 2012, 116, 3928–3937.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘-Figuration’ (JP26102007), KAKENHI Kibankenkyu (B) (JP15H03791), JSPS Research Fellow (16J03265), JST CREST Grant Number JPMJCR18I4, and ‘Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials’ from MEXT.

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SYNOPSIS TOC.

8 ACS Paragon Plus Environment

Ferroelectric Alkylamide Substituted Helicene Derivative with 2D

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