Molecular Assembly and Ferroelectric Response of

Article pubs.acs.org/JPCC

Molecular Assembly and Ferroelectric Response of Benzenecarboxamides Bearing Multiple −CONHC14H29 Chains Yuta Shishido,† Hayato Anetai,† Takashi Takeda,†,‡ Norihisa Hoshino,†,‡ Shin-ichiro Noro,§ Takayoshi Nakamura,§ 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 § Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan ‡

S Supporting Information *

ABSTRACT: Five simple benzenecarboxamide (BC) derivatives bearing multiple −CONHC14H29 chainsN,N′-bis(tetradecyl)-1,4-benzenedicarboxamide (2BC), N,N′,N″-tri( t e t r a d e c y l ) - 1 , 3 , 5 -b e n z e n e t r i c a r b o x a m i d e ( 3 B C ) , N,N′,N″,N″-tetra(tetradecyl)-1,2,4,5-benzenetetracarboxamide (4BC), N,N′,N″,N‴,N⁗-penta(tetradecyl)benzenepentacarboxamide (5BC), and N,N′,N″,N‴,N⁗,N⁗′-hexa(tetradecyl)benzenehexacarboxamide (6BC)were examined in terms of their molecular assemblies in solution, organogels, liquid crystals, and solids as well as their phase transition behavior and dielectric responses. The molecular assemblies of compounds 3BC−6BC were dominated by the intermolecular N−H∼O= hydrogen-bonding interactions along the πstacking directions and formed one-dimensional π-stacking nanofibers. The excellent organogelation characteristics of compound 3BC were observed in common organic solvents such as ethanol, acetonitrile, acetone, and N,N-dimethylformamide, whereas compounds 4BC and 6BC formed organogels in hexane and/or toluene. Mechanical fraying of the three-dimensional entangled nanofibers in the organogel state resulted in a two-dimensional cobweb-like nanofiber network, where the typical height and width of each nanofiber on the substrate surface were ca. 3.5 and 200 nm, respectively. A single nanofiber was constructed by a π-stacking column through intermolecular N−H∼O= hydrogen-bonding interactions, of which the hexagonal arrangement resulted in ordered hexagonal columnar (Colho) discotic liquid crystalline phases for compounds 3BC−6BC. Both of the intercolumnar and intracolumnar distances in the Colho phase were linearly increased according to the number of −CONHC14H29 chains. The temperature- and frequency-dependent dielectric constants of compounds 2BC−6BC in cast-films revealed dielectric anomalies around the solid to Colho phase transition temperatures due to thermally activated molecular motion. Polarization−electric field (P−E) curves of compounds 2BC, 3BC, and 5BC in the mesophases showed hysteretic behavior with ferroelectric ground states, whereas paraelectric behavior with linear P−E dependence was observed for compounds 4BC and 6BC.

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processes of the molecules and the structural flexibility of the assemblies.5 The structural flexibility of amide-type intermolecular N−H∼O= hydrogen-bonding interactions has been applied to construct functional supramolecular assemblies.6,7 In addition, the directionality of the N−H∼O= hydrogen-bonding interactions has been effectively utilized for the construction of low-dimensional supramolecular systems. One of the typical molecular assemblies that has been reported is the discotic liquid crystalline materials with hydrophobic −CONHCnH2n+1 chains, where the effective intermolecular interactions form

INTRODUCTION

Intermolecular hydrogen-bonding interactions have been utilized for the association and dissociation of molecules in flexible and transformable molecular-assembly structures,1 which are essential to realize the specific functions of amino acids and the assembly structures within proteins. Among the various types of hydrogen-bonding interactions, amide-type hydrogen-bonding interactions have been utilized for the construction of biological assembly structures such as α-helix or β-sheets in polypeptides, collagen, and keratin.2−4 The typical bonding energy of amide-type hydrogen-bonding interactions is in the range of 5−10 kJ mol−1; therefore, the structural reconstruction of proteins in biological systems is effectively activated by both of the association−dissociation © 2014 American Chemical Society

Received: June 17, 2014 Revised: August 18, 2014 Published: August 19, 2014 21204

dx.doi.org/10.1021/jp506035h | J. Phys. Chem. C 2014, 118, 21204−21214

The Journal of Physical Chemistry C

Article

Scheme 1. Molecular Structures of Five Benzenecarboxamide Derivatives Bearing Multiple −CONHC14H29 Chains: N,N′Bis(tetradecyl)-1,4-benzenedicarboxamide (2BC), N,N′,N″-Tris(tetradecyl)-1,3,5-benzenetricarboxamide (3BC), N,N′,N″,N″Tetra(tetradecyl)-1,2,4,5-benzenetetracarboxamide (4BC), N,N′,N″,N‴,N⁗-Penta(tetradecyl)benzenepentacarboxamide (5BC), and N,N′,N″,N‴,N⁗,N⁗′-Hexa(tetradecyl)benzenehexacarboxamide (6BC)

one-dimensional hydrogen-bonding π-stacking columns.8 The intermolecular N−H∼O= hydrogen-bonding interactions easily generate one-dimensional nanofibers and have thus also been utilized for the design of organogels.9,10 The three-dimensional entanglement of the one-dimensional fibrous assemblies form pore structures in the organogels. For example, the high organogelation ability of cyclohexane−1,2-dialkylamide derivatives has been reported.11 In consideration of this, alkylamide− CONHCnH2n+1 chains are expected to be an interesting functional unit that will form flexible one-dimensional molecular assemblies such as discotic liquid crystals, organogels, and nanofibers. The formation of organogels in CH2Cl2, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), nitrobenzene, and benzonitrile has been reported in N,N′,N″-tri(octadecyl)1,3,5-benzenetricarboxamide,12,13 where the π-stacking helical nanofibers are formed by intermolecular N−H∼O= hydrogenbonding interactions.14−20 The formation of a hexagonal columnar (Colh) discotic liquid crystalline phase has been reported for the symmetrical three-chain system of N,N′,N″trialkyl-1,3,5-benzenetricarboxamide, whereas two-chain systems such as N,N′-dialkyl-1,4-benzenedicarboxamide and N,N′-dialkyl-1,3-benzenedicarboxamide exhibited only multistep solid−solid phase transitions.8 The introduction of three −CONHCnH2n+1 chains into the benzene core was necessary to generate the intermolecular interactions to form the Colh phase. The π-stacking columns, aided by the intermolecular N− H∼O= hydrogen-bonding interactions, have been characterized through the single crystal X-ray diffraction analysis of N,N′,N″trimethyl-1,3,5-benzenetricarboxamide and N,N′,N″-trimethoxyethyl-1,3,5-benzenetricarboxamide.21−23 The replacement of terminal methyl groups with long alkyl chains was essential to increase thermally activated molecular motion in the Colh phase. Another interesting physical response of the ferroelectric switching was reported for the Colh phases of N,N′,N″-trihexyl1,3,5-benzenetricarboxamide and N,N′,N″-trioctyl-1,3,5-benzenetricarboxamide, which exhibited polarization-electric field (P−E) hysteresis curves.24−27 The one-dimensional intermolecular N−H∼O= hydrogen-bonding interactions along the πstacking direction were inverted by the application of an electric field, which resulted in the dipole inversion. The two types of hydrogen-bonding orientations between the N−H∼O= and =O∼H−N configurations along the π-stacking direction were transformable by the application of an electric field. The detailed mechanism for the dielectric relaxation and motional freedom within the π-stacks of N,N′,N″-trialkyl-1,3,5-benzene-

tricarboxamide derivatives have been discussed by Fitié et al.27 The saturated polarization (Ps) and coercive electric field (Ec) of N,N′,N″-tridecyl-1,3,5-benzenetricarboxamide in the Colh phase at 397 K were 1.8 μC cm−2 and 29.2 V μm−1, respectively.12 N,N′,N″-Trialkyl-1,3,5-benzenetricarboxamide has also been utilized as a structural building block for metal−organic frameworks (MOF),28,29 porous materials with halogen− halogen interactions,30 ion-sensing materials with crown ethers,31 and one-dimensional nanostructures on a substrate surface.32−35 Although a large number of N,N′,N″-trialkyl-1,3,5benzenetricarboxamide derivatives with C3 symmetry have been developed and their functionalities have been examined, other benzenecarboxamide derivatives bearing 2, 4, 5, and 6 −CONHCnH2n+1 chains have not been thoroughly examined yet. The number of −CONHCnH2n+1 chains and their substitution positions in the benzene core are associated with the intermolecular hydrogen-bonding interactions and molecular-assembly structures. Thus, the introduction of multiple −CONHCnH2n+1 chains into the benzene core is expected to increase the intermolecular hydrogen-bonding interactions and form thermally stable π-stacking columnar structures. In addition, there was a question whether N,N′,N″-trialkyl-1,3,5benzenetricarboxamide has intrinsic molecular symmetry for ferroelectric switching in the Colh phase. The ferroelectricity of N,N′,N″-trialkyl-1,3,5-benzenetricarboxamides in the Colh phase was realized by dipole inversion arising from the intermolecular N−H∼O= hydrogen-bonding interaction along the π-stacking column; therefore, both the number and strength of intermolecular hydrogen-bonding interactions are important to determine the ferroelectric parameter. In the four chain system, both the organogelation ability and the hierarchical assemblies have been examined for N,N′,N″,N″tetra(decyl)-1,2,4,5-benzenetetracarboxamide by Tong et al.36 High molecular aggregation abilities in the one-dimensional hydrogen-bonding column have been applied to the fabrication of Eu(III)-doped luminescent organogel.37 Furthermore, the anion-sensitive response of the molecular aggregations has been examined by the solubility control of N,N′,N″,N″-tetra(ethylhexanoate)-1,2,4,5-benzenetetracarboxamide. 38 The tetra(alkyl)-1,2,4,5-benzenetetracarboxamide derivatives have high potential to form functional soft materials for applications such as organogels, liquid crystals, one-dimensional nanostructures, luminescent organogels, anion sensors, drug delivery systems,39 and tissue engineering.40 From this perspective, we examine five benzenecarboxamide (BC) derivativesN,N′-bis21205



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(tetradecyl)-1,4-benzenedicarboxamide (2BC), N,N′,N″-tris(tetradecyl)-1,3,5-benzenetricarboxamide (3BC), N,N′,N″,N″tetra(tetradecyl)-1,2,4,5-benzenetetracarboxamide (4BC), N,N′,N″,N‴,N⁗-penta(tetradecyl)benzenepentacarboxamide (5BC), and N,N′,N″,N‴,N⁗,N⁗′-hexa(tetradecyl)benzenehexacarboxamide (6BC)where the lengths of the −CONHC n H 2n+1 chains were fixed at the tetradecyl (−C14H29) group to evaluate the effects of the number and substitution position of −CONHC14H29 chains (Scheme 1). The molecular assemblies in solution, organogels, liquid crystals, and solids as well as phase transition behavior and dielectric properties of 2BC, 3BC, 4BC, 5BC, and 6BC were investigated.

pentachloride were removed by atomospheric distillation and vacuum distillation, respectively, to give pyromellitic acid chloride. To pyromellitic acid chloride was added a solution of tetradecylamine (17 g, 80 mmol) and triethylamine (13 mL, 94 mmol) in anhydrous THF (30 mL). The mixture was stirred at room temperature for 24 h, and then the resulting suspension was filtrated. Solid was recrystallized twice from ethanol−toluene to yield 4BC as a white powder (4.3 g, 67%). Elemental analysis for compound 4BC: Calcd for C66H122N4O4: C, 76.54; H, 11.87; N, 5.41. Found: C, 76.33; H, 12.02; N, 5.55; mp (dec) ca. 470 K. HRMS (FAB) calcd for C66H123O4N4 1035.9544 [(M + H)+]; found 1035.9541. Compound 5BC. A mixture of benzenepentacarboxylic acid (1.5 g, 5.0 mmol) and phosphorus pentachloride (11 g, 50 mmol) in anhydrous 1,2,4-trichlorobenzene (20 mL) was stirred at 150 °C for 6 h. The solvent and unreacted phosphorus pentachloride were removed by atomospheric distillation and vacuum distillation, respectively, to give benzenepentacarboxylic acid pentachloride. To benzenepentacarboxylic acid pentachloride was added a solution of tetradecylamine (5.5 g, 26 mmol) and triethylamine (5.0 mL, 36 mmol) in anhydrous THF (60 mL). The mixture was stirred at room temperature for 24 h, and the resulting suspension was filtered. Solid was recrystallized twice from ethanol/toluene yielded 5BC as a white powder (7.2 g, yield 39%). Elemental analysis for compound 5BC: Calcd for C81H151N5O5: C, 76.30; H, 11.94; N, 5.49. Found: C, 76.14; H, 12.09; N, 5.51; mp (dec) ca. 470 K. HRMS(FAB) calcd for C 81 H 152 O 5 N 5 1275.1793 [(M + H)+]; found 1275.1802. Compound 6BC. A mixture of mellitic acid (1.8 g, 5.2 mmol) and phosphorus pentachloride (11 g, 52 mmol) in anhydrous 1,2,4-trichlorobenzene (20 mL) was stirred at 150 °C for 15 h. The solvent and unreacted phosphorus pentachloride were removed by atomospheric distillation and vacuum distillation, respectively, to give mellitic acid hexachloride. To mellitic acid hexachloride was added an anhydrous THF solution (60 mL) of tetradecylamine (7.4 g, 35 mmol) and triethylamine (6.0 mL, 43 mmol). The mixture was stirred at room temperature for 24 h, and then the resulting suspension was filtered. Solid was recrystallized twice from ethanol/toluene to yield 6BC as a white powder (1.9 g, yield 24%). Elemental analysis for compound 6BC: Calcd for C96H180N6O6: C, 76.13; H, 11.98; N, 5.55. Found: C, 75.94; H, 12.03; N, 5.58; mp (dec) ca. 470 K. Physical Measurements. Infrared spectroscopy (IR; Thermo Fisher Scientific Nicolet 6700, 400−6000 cm−1) measurements were conducted with a resolution of 4 cm−1 using the KBr pellet method. Solution spectra in CCl4 and toluene were measured in a cuvette with an optical length of 0.5 mm. Thermogravimetry−differential thermal analyses (TGDTA) were conducted using a thermal analysis station (Rigaku Thermo plus TG8120) with Al2O3 as a reference in the temperature range from 293 to 600 K with a heating rate of 5 K min−1 under a nitrogen atmosphere. Temperature-dependent powder pattern X-ray diffraction data were collected using a diffractometer (Rigaku Rint-Ultima III) with Cu Kα (λ = 1.541 87 Å) radiation. Scanning electron microscopy (SEM; Jeol JSEM-5400F) and atomic force microscopy (AFM; Jeol JSPM5200) were conducted for samples on highly ordered pyrolytic graphite (HOPG) and mica substrates, respectively. An acceleration voltage of 5 or 10 kV under a vacuum of less than 10−4 Pa was employed for the SEM measurements. Commercially available Si cantilevers with a force constant of

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EXPERIMENTAL SECTION Preparations. Commercially available terephthaloyl dichloride and 1,3,5-benzenetricarbonyl trichloride were employed for preparation of 2BC and 3BC. In the case of 4BC− 6BC, corresponding acid chlorides were prepared from pyromellic acid, benzenepentacarboxylic acid, and mellitic acid, respectively, by the reaction with phosphorus pentachloride. Condensation between tetradecylamine and the corresponding acid chlorides with triethylamine yielded compounds 2BC−6BC. The elemental analyses of compounds 2BC−6BC were consistent with the calculated values. The compounds 2BC and 3BC were characterized by 1H NMR spectra in CDCl3 (Figure S1), whereas the compounds 4BC, 5BC, and 6BC were not soluble enough to measure 1H NMR spectra. Mass spectra of compounds 2BC, 3BC, 4BC, and 5BC could be obtained by high-resolution FAB mass spectrometry. However, the mass spectrum of compound 6BC was not obtained due to low solubility and large molecular weight. Compound 2BC. To terephthaloyl dichloride (2.1 g, 10 mmol) was added a solution of tetradecylamine (5.4 g, 25 mmol) and triethylamine (4.5 mL, 32 mmol) in anhydrous THF (100 mL). The mixture was stirred at room temperature for 24 h, and then resulting suspension was filtered. The obtained solid was recrystallized twice from 2-propanol/toluene to yield 2BC as a white powder (3.3 g, 60%). Elemental analysis for compound 2BC: Calcd for C36H64N2O2: C, 77.64; H, 11.58; N, 5.03. Found C, 77.74; H, 11.80; N, 5.06; mp 453 K. 1 H NMR (400 MHz, CDCl3, 40 °C) δ: 0.88 (t, J = 7.0, 6H), 1.18−1.43 (m, 44H), 1.58−1.67 (m, 4H), 3.46 (td, J = 6.0, 7.1 Hz, 4H), 6.06 (t, J = 6.0 Hz, 2H), 7.80 (s, 4H). HRMS (FAB) calcd for C36H65N2O2 557.5046 [(M + H)+]; found 557.5051. Compound 3BC. To 1,3,5-benzenetricarbonyl trichloride (2.5 g, 9.4 mmol) was added a solution of tetradecylamine (7.1 g, 33 mmol) and triethylamine (5.0 mL, 36 mmol) in anhydrous THF (50 mL). The mixture was stirred at room temperature for 24 h, and then the resulting suspension was filtered. Solid was recrystallized twice from ethanol to yield 3BC as a white powder (2.3 g, yield 30%). Elemental analyses for compound 3BC: Calcd for C51H93N3O3: C, 76.92; H, 11.77; N, 5.28. Found: C, 76.82; H, 11.85; N, 5.30; mp 467 K. 1 H NMR (400 MHz, CDCl3, 25 °C) δ: 0.88 (t, J = 7.0 Hz, 9H), 1.21−1.43 (m, 66H), 1.58−1.68 (m, 6H), 3.38 (dt, J = 7.6, 5.9 Hz, 6H), 6.40 (t, J = 5.9 Hz, 3H), 8.33 (s, 3H). HRMS (FAB) calcd for C51H94O3N3 796.7295 [(M + H)+]; found 796.7291. Compound 4BC. A mixture of pyromellitic acid (4.6 g, 18 mmol) and phosphorus pentachloride (15 g, 73 mmol) in anhydrous 1,2,4-trichlorobenzene (20 mL) was stirred at 120 °C for 6 h. The solvent and unreacted phosphorus 21206



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4.5 N m−1 were used for AFM measurements. Temperaturedependent dielectric constants were measured using the twoprobe ac impedance method at frequencies from 1 kHz to 1 MHz (Hewlett-Packard, HP4194A) with a liquid crystal cell placed in a temperature control system (Linkam, LTS350). The P−E characteristics were measured with a commercially available ferroelectric tester (Precision LC, Radient Technologies) using a liquid crystalline cell with an electrode gap of 2 μm. The liquid state 2BC and 3BC compounds were introduced into the electrode gap of the liquid crystal cell. Thermal decomposition was observed for compounds 4BC, 5BC, and 6BC in the liquid state, so that cast films were first fabricated on indium tin oxide (ITO) glass substrates (SZA311P6N) that were annealed at 423 K for 10 min and covered with another ITO glass substrate to form an electrode sandwich. Calculations. The optimized molecular structures with −CONHCH3 chains were obtained by density functional theory (DFT) calculations with the M05/6-31G(d,p) basis set using Gaussian 09W.41 The two kinds of model structures for the orientation of −CONHCH3 groups were evaluated in all up configuration and alternate up−down one. The monomer and π-dimer structures were geometry optimized in order to compare the stable intramolecular and intermolecular hydrogen-bonding interactions. After the structural optimizations, single point energies were compared to each other.

Table 1. Melting Point, Solubility, and Organogelation Ability of Compounds 2BC−6BCa solvent

2BC

3BC

4BC

5BC

6BC

mp (K)

453

467

>470

>470

>470

ethanol 2-propanol acetonitrile acetone hexane benzene toluene cyclohexane chloroform 1,2-dichloromethane DMF DMSO

× × × × × × × × × × × ×

OG OG OG OG TG L L L L OG OG OG

× × × × TG TG TG VL VL × × ×

× × × × VL VL VL VL VL × × ×

× × × × TG VL TG TG VL × × ×

a

The solubility and organogelation ability were evaluated for 20 mM solutions of compounds 2BC−6BC in 12 different organic solvents. The 20 mM solution was heated at the boiling point and cooled down to room temperature, and the bulk states were then visually evaluated. The notations of ×, OG, TG, L, and VL denote insoluble (less soluble), opaque organogel, transparent organogel, liquid, and viscous liquid, respectively.

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RESULTS AND DISCUSSION Molecular Assemblies in Organic Solvents and Organogels. The effective intermolecular N−H∼O= hydrogen-bonding interactions between −CONHC14H29 chains in compounds 3BC, 4BC, 5BC, and 6BC become the important driving force to form molecular assemblies, even in the solution phase. The three-dimensional entanglement assemblies of nanofibers and/or microfibers resulted in pore spaces to entrap the organic solvents; therefore, the formation of one-dimensional fibrous molecular assemblies should a necessary requirement to form an organogel. The organogelation ability of compounds 2BC, 3BC, 4BC, 5BC, and 6BC were evaluated for the 12 different organic solvents with a fixed concentration of 20 mM (Table 1). The 20 mM solutions of compounds 2BC−6BC were heated to the boiling point and then cooled to room temperature, and the solubility and organogelation ability were categorized as insoluble (×), opaque organogel (OG), transparent organogel (TG), liquid (L), and viscous liquid (VL). Figure 1 shows photographs of typical OG, TG, and VL states for compounds 3BC and 4BC with a fixed concentration of 20 mM. Compounds 2BC and 3BC have melting points at 453 and 467 K, respectively, whereas thermal decomposition was observed for compounds 4BC, 5BC, and 6BC at ca. 470 K. The increase in the number of −CONHC14H29 chains enhanced the intermolecular interactions and the melting point. Compound 2BC was less soluble in typical organic solvents such as ethanol and DMSO, and formation of an organogel was not confirmed in all solvents (Table 1). In contrast, high solubility and organogelation ability were observed for compound 3BC in various organic solvents such as ethanol, acetonitrile, acetone, and DMSO. Compound 3BC was highly soluble in benzene, toluene, cyclohexane, and chloroform without organogelation, whereas organogels were formed in ethanol, propanol, acetonitrile, acetone, DMF, and DMSO. The TG of compound 3BC was only observed in

Figure 1. Photographs of organogels and viscous liquid states of compounds 3BC and 4BC with a fixed concentration of 20 mM: (a) opaque organogel (OG) of 3BC in ethanol, (b) transparent organogel (TG) of 4BC in hexane, and (c) a viscous liquid (VL) of 4BC in chloroform.

hexane at a concentration of 20 mM, whereas OGs were observed in other solvents. Light is scattered by large size microfibers, so that the TGs and OGs were explained according to the size of the fibrous molecular assemblies within the organogels. Compounds 4BC, 5BC, and 6BC were less soluble in ethanol, 2-propanol, acetonitrile, acetone, 1,2-dichloromethane, DMF, and DMSO, and the formation of organogels and/or VL was observed in hexane, benzene, toluene, and chloroform. Only the VL was observed for compound 5BC with a concentration of 20 mM. The lower molecular symmetry of 5BC than the other derivatives decreased the intermolecular interactions and suppressed the formation of an organogel. The VL and TG could be changed to TG and OG, respectively, by increasing the concentration from 20 mM. In toluene, compounds 3BC, 4BC, 5BC, and 6BC at a concentration of 20 mM formed the L, TG, VL, and TG states, respectively. The hydrogen-bonding molecular aggregation states of compounds 2BC−6BC were evaluated by vibrational IR spectroscopy of the solution and solid. The shift of the N−H stretching mode (νNH) has been observed by the formation of intermolecular N−H∼O= hydrogen bonds. The vibrational energy of free νNH mode was blue-shifted by ca. 200 cm−1 compared with that of the intermolecular N−H∼O= hydrogenbonding energy. Figures 2a and 2b show solution phase IR spectra for compounds 3BC−6BC with a concentration of 1 mM in CCl4 and toluene, respectively. The energies of the νNH modes in compounds 3BC, 4BC, 5BC, and 6BC in CCl4 were observed at 3244, 3244, 3247, and 3250 cm−1, respectively 21207



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Figure 2. IR spectra of compounds 2BC−6BC in solution and as solids. N−H stretching mode (νNH) of compounds 3BC, 4BC, 5BC, and 6BC in (a) 1 mM CCl4 and (b) 1 mM toluene. (c) Solid state IR spectra of compounds 2BC−6BC as KBr pellets in the energy range from 2500 to 3600 cm−1.

(Figure 2a), which are consistent with the vibrational energy of intermolecular N−H∼O= hydrogen bonding, νNH. The IR spectra measured in toluene are slightly different from those measured in CCl4. Compounds 3BC, 4BC, 5BC, and 6BC in 20 mM toluene solution formed L, TG, VL, and TG states, respectively, which suggests that the intermolecular interaction decreased in the order of 6BC ∼ 4BC > 5BC > 3BC. The νNH modes of compounds 4BC, 5BC, and 6BC in 1 mM toluene solution were observed at 3244, 3248, and 3250 cm−1, respectively (Figure 2b), which were consistent with the vibrational energy of the intermolecular hydrogen-bonding νNH modes. In contrast, the νNH mode of compound 3BC was observed at 3452 cm−1, which was blue-shifted ca. 200 cm−1 compared with those the hydrogen-bonding compounds, which suggests the 3BC molecules are isolated in 1 mM toluene solution. The solid state νNH modes of compounds 3BC, 4BC, 5BC, and 6BC in KBr pellets were observed at 3244, 3240, 3250, and 3250 cm−1, respectively (Figure 2c), which are consistent with the intermolecular N−H∼O= hydrogenbonding νNH mode. The νNH band of compound 2BC at 3334 cm−1 showed a blue-shift, which indicates that the intermolecular N−H∼O= hydrogen-bonding interactions of only two −CONHC14H29 chains were less effective than those of the other derivatives. Molecular Assembly Nanofibers on a Substrate Surface. Intermolecular N−H∼O= hydrogen-bonding interactions play an important role to form the molecular assemblies in organogels and viscous liquids. Organogels are obtained by the three-dimensional entanglement assemblies of fibers. Figures 3 shows SEM images of xerogels for compounds 3BC and 4BC on high ordered pyrolytic graphite (HOPG). Compounds 3BC in ethanol and 4BC in toluene with

concentrations of 20 mM formed OG and TG, respectively, which were consistent with the size difference of the fibrous molecular assemblies in the xerogels (Figures 3a and 3b). Helical fibrous assemblies with typical lengths of 100−200 μm and their three-dimensional entanglement structures were observed in the xerogel of compound 3BC, whereas the xerogel of compound 4BC showed relatively uniform surface morphology without the formation of a fibrous assembly. The fine nanofibers of compound 4BC were difficult to observe using SEM. The TG of compound 4BC was changed to an OG and the fractal-like network morphologies were observed in the SEM images (Figure 3b) with an increase in the concentration (∼30 mM in toluene). The main two-dimensional fiber network was accompanied by a branched fibrous network, which was repeated several times. The entangled fibrous molecular assemblies in the organogels of 3BC, 4BC, 5BC, and 6BC were undone using the mechanical force of a spin-coater. Figure 4 presents the surface morphologies of the two-dimensional nanofiber networks of compounds 4BC, 5BC, and 6BC on a hydrophilic mica surface. Each film was fabricated from a dilute solution (0.1 mg mL−1) under control of the rotational speed (ϕ) of the spin coater at 500, 1000, and 2000 rpm, respectively. The surface morphologies of the two-dimensional nanofiber networks were affected by the mechanical force of the spin-coater. The typical dimensions of nanofiber 4BC at ϕ = 500 rpm was 5 × 400 × 1500 nm3 (Figure 4a), whereas those at ϕ = 1000 and 2000 rpm were 5 × 200 × 1000 nm3 (Figures 4b and 4c). The occupancy areas of nanofiber 4BC on the substrate surface at ϕ = 500, 1000, and 2000 rpm were 50, 54, and 33%, respectively. Each nanofiber bundle was undone like a knot to form a twodimensional cobweb-like nanonetwork. Further increase in the rotation speed destroyed the nanofiber network, which resulted in pieces of nanofiber on the substrate surface at ϕ = 6000 rpm. A well-grown nanofiber network of compound 3BC was not observed using the same fabrication conditions as that for compound 4BC at ϕ = 2000 rpm (Figure S3). The fragments of 3BC nanofibers on the mica substrate suggested the 3BC nanofiber has lower mechanical strength than the 4BC nanofiber. Therefore, four −CONHC14H29 chains increase the intermolecular N−H∼O= hydrogen-bonding interactions and the mechanical strength of the nanofiber. Well-grown two-dimensional nanofiber networks were observed for compounds 5BC and 6BC under the same fabrication conditions as that used for 4BC. The typical dimensions of 5BC nanofiber at ϕ = 500 rpm were 5 × 400 ×

Figure 3. SEM images of xerogels of compounds 3BC and 4BC on HOPG. (a) Helical fibrous assembly of 3BC xerogel from ethanol. (b) Fractal-like xerogels of compound 4BC from toluene (∼30 mM). The scale bars in (a) and (b) are 10 and 500 μm, respectively. 21208



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Figure 5. Optimized molecular structures of compounds 4BC, 5BC, and 6BC with multiple −CONHCH3 chains using DFT calculation of M05/6-31G(d,p). (a) Monomer with alternate up−down configuration of multiple −CONHCH3 chains. (b) π-Dimer with alternate up−down configuration of multiple −CONHCH3 chains. The hydrogen atoms were omitted to clarify figures, and the dashed lines were corresponded to the intermolecular N−H∼O= hydrogenbonding interactions.

ΔE (kJ mol−1) was defined as E(all-up) − E(up−down), and the positive and negative ΔE corresponded to the stable molecular conformation of all-up and up−down conformation of −CONHCH3 chains, respectively. Although the all-up conformation of −CONHCH3 groups was utilized for the initial structure of compound 4BC, the optimized molecular structure was converged to the up−down −CONHCH3 conformation (left in Figure 5a), which was consistent with quite small magnitude of energy difference ΔE ∼ 0.15 kJ mol−1. In addition, intramolecular N−H∼O= hydrogen-bonding interactions were observed between neighboring −CONHCH3 groups with an N−O distance of ca. 2.70 Å, which stabilized the up−down alternate −CONHCH3 conformation. The energy of optimized molecular structure of 5BC for the up−down conformation was ∼50 kJ mol−1 lower than that of the all-up conformation (center in Figure 5b and Table 2), whereas that of 6BC for the up−down conformation was ∼110 kJ mol−1 stable than that of up− down one (right in Figure 5b). It should be noted that the allup conformations of 2BC and 3BC were slightly stable than the up−down one. However, the energy differences of ΔE = 1.77 and 10.8 kJ mol−1 for 2BC and 3BC suggested the thermally activated conformational change between the all-up and up− down ones. The optimized molecular structures of π-dimer for 2BC− 6BC with the up−down conformation were stable than those of the all-up one. The magnitude of ΔE was increased in the order of 2BC (ΔE = −5.27 kJ mol−1), 3BC (ΔE = −3.62 kJ mol−1), 4BC (ΔE = −92.7 kJ mol−1), 5BC (ΔE = −128 kJ mol−1), and 6BC (ΔE = −210 kJ mol−1). The intermolecular N−H∼O= hydrogen-bonding energy was effectively interacted to each molecule, resulting in the effective π−π interaction within the column. The alternate up−down conformation of −CONHC14H29 groups in compounds 4BC and 6BC canceled the net dipole moment along the π-stacking column, where the dipole inversions of the neighboring −CONHC14H29 chains were completely suppressed by steric hindrance and the intramolecular hydrogen-bonding interactions. In compound 5BC, the one −CONHC14H29 chain has motional freedom and can contribute to the net dipole moment of the π-stacking column. The π-stacking columnar structures have already been reported in 3BC derivatives, where the intermolecular N−

Figure 4. Two-dimensional nanofiber networks of compounds (a−c) 4BC, (d−f) 5BC, and (g−i) 6BC on mica substrates. The rotation speed of the spin-coater was (a, d, g) 500, (b, e, h) 1000, and (c, f, (i) 2000 rpm. All AFM images were acquired by scanning an area of 10 × 10 μm2.

1000 nm3 (Figure 4d), whereas those at ϕ = 1000 and 2000 rpm were 3.5 × 200 × 1000 nm3 (Figures 4e and 4f). The widths of 5BC nanofiber (ca. 200 nm) at ϕ = 1000 and 2000 rpm were approximately half of that (ca. 400 nm) at ϕ = 500 rpm, and the occupancy areas at ϕ = 500, 1000, and 2000 rpm were 54, 54, and 45%, respectively, which indicates rotationspeed-independent behavior. For compound 6BC, the typical dimensions of nanofiber at ϕ = 500, 1000, and 2000 rpm were 7 × 400 × 500 nm3, 3.5 × 400 × 500 nm3, and 3.5 × 200 × 500 nm3, respectively, and the occupancy areas at ϕ = 500, 1000, and 2000 rpm were 56, 65, and 44%, respectively. The occupancy areas of nanofibers 4BC, 5BC, and 6BC at ϕ = 2000 rpm were 33, 45, and 44%, respectively. The much denser nanofiber networks of compounds 5BC and 6BC on mica than that of compound 4BC were affected by the strength of the intermolecular N−H∼O= hydrogen-bonding interactions. The π-stacking structure formed by the aid of intermolecular N−H∼O= hydrogen-bonding interactions within the nanofiber was evaluated using the optimized molecular structure by DFT calculations with M05/6-31G(d,p) basis set.41 The flexible −CONHC14H29 groups in compounds 2BC, 3BC, 4BC, 5BC, and 6BC were replaced by simple −CONHCH3 groups for the calculations. The neighboring −CONHCH 3 groups in compounds 4BC, 5BC, and 6BC have two possible orientations, up−up and/or up−down conformations due to the steric hindrance between −CONHCH3 groups. Figure 5 shows the calculated molecular structures for compounds 4BC, 5BC, and 6BC with multiple up−down configuration of −CONHCH3 chains. Table 2 summarizes the single point energies for the optimized molecular structures of monomer and π-dimer of 2BC−6BC. Monomer and π-dimer structures were optimized starting from the initial atomic coordinates of all-up −CONHCH3 conformation (all-up) and alternate up− down −CONHCH3 one (up−down). The energy difference of 21209



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Table 2. Single Point Energy of the Optimized Molecular Structures of Monomer and π-Dimer of 2BC−6BCa monomer E(all-up) E(up−down) ΔE,b kJ mol−1 π-dimer E(all-up) E(up−down) ΔE,b kJ mol−1

2BC

3BC

4BC

5BC

6BC

−647.87 407 952 −647.87 340 738 +1.77

−855.781 846 1 −855.777 723 9 +10.82

−1063.685 915 −1063.685 858 +0.15

−1271.565 617 −1271.584 149 −48.66

−1479.444 925 −1479.486 329 −108.7

−1295.773 492 −1295.775 498 −5.27

−1711.596 171 −1711.597 548 −3.62

−2127.389 431 −2127.424 735 −92.69

−2543.173 309 −2543.222 338 −128.7

−2958.954 564 −2959.034 687 −210.4

a

Molecular structures were optimized using DFT calculation of M05/6-31G(d,p). Single point energy was obtained in hartree units. Monomer and π-dimer structures were optimized for the initial atomic coordinates of all-up −CONHCH3 conformation (all-up) and alternate up−down −CONHCH3 one. bEnergy difference ΔE (kJ mol−1) was defined at E(all-up) − E(up−down). The positive and negative signs corresponded to the stable molecular conformation of all-up and alternate up−down −CONHCH3 conformation, respectively.

H∼O= hydrogen-bonding interactions along the π-stack were the driving force to form a one-dimensional molecular assembly. The all-trans-conformation of two −CONHC14H29 chains in compound 2BC resulted in a maximum molecular length of ca. 4.5 nm along the long axis of the molecule. The nanofiber heights of ca. 3.5 nm for compounds 4BC, 5BC, and 6BC on mica at ϕ = 2000 rpm almost corresponded with the diameter of the π-stacking column, whereas the nanofiber width of ca. 200 nm was consistent with the parallel arrangement of ca. 50 π-stacking columns on the mica substrate. The nanofiber length of ca. 1000 nm was constructed from ca. 2000 π-stacking molecules, so that one nanofiber with typical dimensions of 3.5 × 200 × 500 nm3 would be constructed from approximately 50 000 molecules. The outer surface of the π-staking column is constructed of hydrophobic −CONHC14H29 chains; therefore, the nanofiber should be in intimate contact with the hydrophobic substrate surface. The hydrophobic nanofibers were easily shed on the hydrophilic mica surface, which resulted in the knot of nanofiber assemblies and nanonetwork structures. Phase Transition Behavior. The formation of a hexagonal ordered columnar (Colho) discotic liquid crystalline phase has been reported for compound 3BC in the temperature range from 334 to 582 K,8 whereas the liquid crystalline properties of compounds 4BC, 5BC, and 6BC have not yet been examined sufficiently. The weight losses due to the thermal decomposition of compounds 4BC, 5BC, and 6BC were clearly observed with the increase in the temperature up to ca. 470 K (Figure S10). The thermal stabilities of compounds 4BC, 5BC, and 6BC were lower than those of compounds 2BC and 3BC. Figure 6 shows DSC profiles for compounds 2BC−6BC. The thermal cycles of compounds 4BC, 5BC, and 6BC were repeated at temperatures up to 420 K, and the temperatures for the Colho phase to isotropic liquid (I) phase transition should be higher than the thermal decomposition temperatures around 470 K. Three crystal phases (S1, S2, and S3) were observed in compound 2BC without a liquid crystalline phase until the melting point at 453 K. The temperatures for the S1−S2, S2− S3, and S3−I phase transitions during the heating process were observed at 382 K (ΔH = 16.8(2) kJ mol−1), 442 K (ΔH = 5.90(8) kJ mol−1), and 453 K (ΔH = 41.6(2) kJ mol−1), respectively, while those for the I−S3, S3−S2, and S2−S1 phase transitions during the cooling process were observed to be reversible at 449 K (ΔH = 44.1(3) kJ mol−1), 435 K (ΔH = 1.46(6) kJ mol−1), and 381 K (ΔH = 19.4(3) kJ mol−1). The temperatures for the S1−Colho and Colho−I phase transitions in compound 3BC during the heating process were 313 K (ΔH =

Figure 6. DSC profiles for compounds 2BC−6BC. The maximum temperature of the DSC measurements for compounds 4BC, 5BC, and 6BC was ca. 420 K to avoid thermal decomposition.

20.8(2) kJ mol−1) and 468 K (ΔH = 5.47(5) kJ mol−1), respectively, whereas those for the I−Colho and Colho−S1 transitions appeared at 473 K (ΔH = 7.33(5) kJ mol−1) and 308 K (ΔH = 20.0(2) kJ mol−1). The reversible Colho phase of compound 3BC in the temperature cycle could be assigned to the discotic hexagonal ordered columnar phase (Colho), as shown in the X-ray diffraction pattern of Figure 7. The S1− Colho phase transition temperatures for compounds 4BC, 5BC,

Figure 7. Colho liquid crystalline phase of compounds 3BC−6BC. (a) Powder X-ray diffraction patterns of compounds 3BC (T = 373 K), 4BC (T = 393 K), 5BC (T = 403 K), and 6BC (T = 373 K). (b) Lattice parameters d (100) and d (001) in the hexagonal lattice as a function of the number of −CONHC14H29 chains. (c) Schematic model of Colho phase and lattice periodicities of d (100) and d (001). 21210



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reported for N,N′,N″-tris(CnH2n+1)-1,2,3-benzenetricarboxamide (n = 6, 8, 10, and 18).26,27 Ferroelectric switching in the Colho phase was realized by the dipole inversion of the intermolecular hydrogen-bonding direction from the =O∼H− N− to the −N−H∼O conformation along the π-stacking column, which was confirmed from the P−E hysteresis curve associated with the twisting motion of the hydrogen-bonding πstacking column. Figure 8 shows the temperature- and frequency-dependent real part dielectric constants (ε1) and DSC profiles for compounds 2BC, 4BC, 5BC, and 6BC during the heating process.

and 6BC during the heating process were observed at 319 K (ΔH = 46.9(3) kJ mol−1), 299 K (ΔH = 90.0(3) kJ mol−1), and 330 K (ΔH = 84.9(3) kJ mol−1), respectively, which were reversible at 323 K (ΔH = 46.9(3) kJ mol−1), 304 K (ΔH = 75.3(3) kJ mol−1), and 324 K (ΔH = 64.5(3) kJ mol−1) during the cooling process. The S1−Colho transition temperature for compound 5BC was approximately 20 K lower than those of compounds 4BC and 6BC, which indicates that the solid phase is destabilized by the introduction of five −CONHC14H29 chains. The optimized molecular structure based on DFT calculations indicated the presence of one free alkylamide chain among the five chains that has the motional freedom within the molecular assembly. Compounds 3BC, 4BC, 5BC, and 6BC exhibited the same Colho liquid crystalline phase. Figure 7a shows powder X-ray diffraction patterns for the Colho phases of compounds 3BC− 6BC. The sharp diffraction peaks around 2θ ∼ 4° for the Colho phase of compounds 3BC−6BC were assigned to the in-plane (100) reflection of the hexagonal lattice.42 Both the in-plane (110) and (200) reflections were also observed at 2θ ∼ 7.9° and 9.1°, respectively, which are consistent with the Colho phase. The π-stacking columns through intermolecular N− H∼O= hydrogen-bonding interactions are hexagonally arranged. The d (100) distances of compounds 3BC, 4BC, 5BC, and 6BC were 2.24(3), 2.42(3), 2.54(3), and 2.68(3) nm, respectively, which indicate a linear correlation with the number of −CONHC14H29 chains (Figure 6b). The length of d (100) was increased in the order of 3BC < 4BC < 5BC < 6BC. However, the average π-stacking distance within the column was evident from the broad low angle diffraction peak around 2θ ∼ 25°. The average π-stacking distances d (001) for the Colho phase of compounds 3BC, 4BC, 5BC, and 6BC were 0.354(8), 0.398(8), 0.421(8), and 0.452(8) nm, respectively, which also had a linear correlation with the number of −CONHC14H29 chains (Figure 7b). The disk-like molecules within the π-stacking column in the Colho phase are thermally rotated along the direction normal to the π-plane; therefore, the intercolumn distance d (100) in the hexagonal lattice corresponds to the average molecular size of the rotating 3BC, 4BC, 5BC, and 6BC. The maximum molecular length of compounds 3BC−6BC was ca. 4.5 nm, assuming the all-trans conformation of −CONHC14H29 chains; therefore, the average intercolumn distance d (100) was approximately 60% shorter than the ideal molecular length of 4.5 nm. The interdigitated molecular assemblies of −CONHC14H29 chains were assumed in the Colho phase. The ratio of d (100)/d (001) ∼ 6 was commonly observed in compounds 4BC, 5BC, and 6BC, whereas enhanced d (100)/d (001) ∼ 6.2 was observed in compound 3BC. The thermally activated molecular rotation along the director normal to the π-plane for compounds 4BC, 5BC, and 6BC resulted in the average molecular structures, which formed the similar packing structure of d (100)/d (001) ∼ 6. The intensity of broad diffraction around 2θ = 18° clearly depended on the number of −CONHC14H29 chains. Since the intermolecular correlation length was roughly proportional to the inverse of the full width at half-maximum (fwhm) of broad diffraction at 2θ = 18°, the intermolecular correlation and magnitude of disorder were decreased in the order of 3BC, 4BC, 5BC, and 6BC. The motional freedom within the molecular assemblies was decreased in the order of 3BC, 4BC, 5BC, and 6BC. Dielectric Properties. The dielectric relaxation and ferroelectric properties in Colho phase of 3BC have been

Figure 8. Temperature- and frequency-dependent real part dielectric constants ε1 (lower panel) and DSC profiles (upper panel) for compounds (a) 2BC, (b) 4BC, (c) 5BC, and (d) 6BC during the heating process. The measurement frequencies were 0.1, 1, 10, 100, and 1000 kHz using a liquid crystal cell with an electrode gap of 2 μm.

The ε1 ∼ 3.2 for compound 2BC indicates temperature independent behavior below 380 K, where the ε1 values were decreased slightly around the S1−S2 phase transition temperature of ca. 382 K. The dielectric responses at low frequencies of 100 and/or 1000 Hz were enhanced by an increase in the temperatures to above 400 K, where a shoulder-like dielectric peak was observed around the S2−S3 phase transition temperature of 442 K. For compound 3BC, the temperatureand frequency-dependent ε1 value was observed around the S1−Colho transition at 320 K, where the constant ε1 ∼ 3 below 350 K was gradually enhanced in the low-frequency measurements without a dielectric peak (Figure S12). For measurements at 100 Hz, ε1 ∼ 9.4 at 450 K was 3 times larger than ε1 ∼ 2.8 at 300 K. However, no dielectric anomaly was observed for compound 4BC around the S1−Colho phase transition at ca. 330 K. The ε1 values for compound 4BC measured at 100 Hz 21211



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were gradually increased from ε1 ∼ 1.1 at 400 K to ε1 ∼ 1.4 at 460 K. The dielectric enhancement of compound 4BC was significantly lower than that for compound 3BC, which indicates that thermally activated molecular motion is suppressed in the Colho phase of compound 4BC. A slight decrease of ε1 was observed at the S1−Colho phase transition of compound 5BC at around 310 K, and the constant ε1 ∼ 2.5 was observed at temperatures up to 400 K. Above 400 K, the ε1 value measured at 100 Hz was gradually enhanced from ca. 2.5 at 400 K to ca. 3.3 at 465 K, which suggests the occurrence of thermally activated slow molecular motion above 400 K. No dielectric anomaly was observed around the S1−Colho phase transition temperature for compound 6BC, at ca. 330 K, and the small dielectric constant of ε1 ∼ 0.8 was gradually enhanced above 400 K accompanied by a discontinuous change in ε1 around 460 K. Although the no significant ε1 changes or dielectric peak were observed around the S1−Colho phase transition temperatures for compounds 3BC−6BC, thermally activated ε1 enhancements were confirmed in the Colho phases above 400 K. The ε1 enhancement ratios from 300 to 460 K, i.e., ε1(460 K)/ε1(300 K), for compounds 2BC, 3BC, 4BC, 5BC, and 6BC were 1.4, 3.5, 1.2, 1.33, and 1.26, respectively, which correspond to the magnitude of thermally activated molecular motion by the increase in the temperature of the Colho phase. Among these, the 3BC had the largest ε1(460 K)/ε1(300 K) ratio of ca. 3.5, which suggests larger thermally activated molecular motion of 3BC than the other BC derivatives. Large amplitude molecular motion of −CONHC14H29 chains was achieved by the dipole inversion of intermolecular N−H∼O= hydrogenbonding interactions along the π-stacking column. In contrast, compounds 4BC, 5BC, and 6BC had ε1(460 K)/ε1(300 K) ratios in the range of 1.2−1.3, which suggests a small magnitude of thermally activated molecular motion by the increase in temperature. Thus, the thermal motion of −CONHC14H29 chains was suppressed by the intramolecular N−H∼O= hydrogen-bonding interaction and steric hindrance of the neighboring chains. In the P−E response curves, the compound 3BC showed hysterics behavior with a coercive electric field, Vc, of 25 V μm−1 and a remanent polarization, Pr, of 0.8 μC cm−2 at 343 K, which was almost the same as that previously reported for N,N′,N″-tris(CnH2n+1)-1,3,5-benzenetricarboxamide (n = 6, 8, 10, and 18).12 In contrast, no P−E hysteresis behavior was observed for compounds 4BC and 6BC, but instead linear P−E responses of the paraelectric ground state were observed. The compound 3BC has the ideal molecular structure to achieve dipole inversion of the intermolecular N−H∼O= hydrogenbonding π-stacking column. The compounds 2BC and 5BC also showed P−E hysteresis curves for the mesophase and Colho liquid crystalline phases. Figures 9a and 9b show the P−E hysteresis curves and dP/ dE−E profiles for compounds 2BC and 5BC, respectively. Three solid phases of S1, S2, and S3 were observed for compound 2BC. The distinctive P−E hysteresis behavior of compound 2BC was observed at 423 and 433 K, which correspond to the temperatures of the intermediate S2 mesophase. The narrow and relatively high temperature range of the S3 phase (442 < T < 453 K) made it difficult to measure the P−E profiles of compound 2BC, so that only the linear P− E response was observed for the low-temperature S1 phase (T < 382 K). Vc and Pr for 2BC at 423 K were ∼42 V μm−1 and ∼1.33 μC cm−2, respectively, whereas those at 433 K were ∼40

Figure 9. P−E hysteresis curves for compounds 2BC and 5BC. P−E (lower panels) and dP/dE−E (upper panels) profiles for (a) 2BC at 373, 423, and 433 K and (b) 5BC at 393, 443, and 453 K, measured at 1 Hz.

V μm−1 and ∼2.4 μC cm−2. Although the magnitude of Vc was independent of the temperature, Pr gradually increased with the temperature. The dP/dE−E profiles of compound 2BC showed dP/dE peaks at Vc, which is characteristic of a ferroelectric P−E hysteresis curve. Although the P−E hysteresis of compound 5BC was not observed for the Colho phase at 393 and 443 K, the hysteresis appeared for the Colho phase at 453 K. An increase in the temperature enhanced the thermally activated molecular motion in the Colho phase, which enabled inversion of the direction of the intermolecular N−H∼O= hydrogenbonding chain along the π-stacking column. Vc and Pr for compound 5BC at 453 K were ∼15 V μm−1 and ∼1.0 μC cm−2, respectively. Vc for compound 5BC was about half magnitude of that for 2BC, whereas Pr was approximately 3 times larger than that for 2BC. The Pr ∼ 0.8 μC cm−2 of 3BC at 348 K and Pr ∼ 1.0 μC cm−2 of 5BC at 453 K in Colho phase were compared with the theoretical magnitude based on the lattice constants of Colho phase. The conformations of multiple −CONHCH3 groups in 3BC and 5BC were fixed at an angle of 90° between the CO bond and π-plane of benzene, whereas the angle between N− CH3 and C−CO bonds was 180°. The dipole moments of 3BC and 5BC normal to the π-planes were 11.0 and 17.4 D, respectively. The theoretical Pr values for 3BC and 5BC in Colho phase were 1.58 and 1.71 μC cm−2, respectively, which were larger than the experimental Pr values. The disorder of polarized column in Colho phase decreased the magnitude of Pr form ideal values.

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CONCLUSIONS The self-assembly of simple benzenecarboxamide (BC) derivatives bearing multiple −CONHC14H29 chainsN,N′bis(tetradecyl)-1,4-benzenedicarboxamide (2BC), N,N′,N″-tris(tetradecyl)-1,3,5-benzenetricarboxamide (3BC), N,N′,N″,N″tetra(tetradecyl)-1,2,4,5- benzenetetracarboxamide (4BC), N,N′,N″,N‴,N⁗-penta(tetradecyl)benzenepentacarboxamide (5BC), and N,N′,N″,N‴,N⁗,N⁗′-hexa(tetradecyl)benzenehexacarboxamide (6BC)were systematically examined in solution, liquid crystal, organogel, and nanofiber. The benzene core bearing multiple −CONHC14H29 chains have effective intermolecular N−H∼O= hydrogen-bonding interactions that form one-dimensional π-stacking columnar molecular assemblies. The hydrogen-bonding π-stacking 21212



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(4) Stryer, L. Biochemistry; Freeman: New York, 1995. (5) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Truhlar, D. G., Ed.; Oxford University Press: New York, 1997. (6) Gilli, G.; Gilli, P. The Nature of the Hydrogen Bond, Outline of a Comprehensive Hydrogen bond Theory; Oxford University Press: Oxford, 2009. (7) Supramolecular Assembly via Hydrogen Bonds I and II; Mingos, D. M. P., Ed.; Springer: Berlin, 2004. (8) Matsunaga, Y.; Miyajima, N.; Nakayasu, Y.; Sakai, S.; Yonenaga, M. Design of Novel Mesomorphic Compounds: N, N′, N″-Trialkyl1,3,5-benzenetricarboxamides. Bull. Chem. Soc. Jpn. 1988, 61, 207− 210. (9) Molecular Gels; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, 2006. (10) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133−3160. (11) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Prominent Gelation and Chiral Aggregation of Alkylamides Derived from trans1,2-Diaminocyclohexane. Angew. Chem., Int. Ed. 1996, 35, 1949−1951. (12) Yasuda, Y.; Ishii, E.; Inada, H.; Shirota, Y. Novel Low-MolecularWeight Organic Gels: N,N′,N″-Tristearyltrimesamide/Organic Solvent System. Chem. Lett. 1996, 575−576. (13) Hanabusa, K.; Koto, C.; Kimura, M.; Shirai, H.; Kakehi, A. Remarkable Viscoelasticity of Organic Solvents Containing Trialkyl1,3,5-benzenetricarboxamides and Their Intermolecular Hydrogen Bonding. Chem. Lett. 1997, 429−430. (14) Palmans, A. R. A.; Vekemans, J. A. J. M.; Hikmet, R. A.; Fischer, H.; Meijer, E. W. Lyotropic Liquid-Crystalline Behavior in DiscShaped Compounds Incorporating the 3,3′-Di(acylamino)-2,2′-bipyridine Unit. Adv. Mater. 1998, 10, 873−876. (15) Brunsveld, L.; Schenning, A. P. H. J.; Broeren, M. A. C.; Janssen, H. M.; Vekemans, J. A. J. M.; Meijer, E. W. Chiral Amplification in Columns of Self-Assembled N,N′,N″-Tris((S)-3,7-dimethyloctyl)benzene-1,3,5-tricarboxamide in Dilute Solution. Chem. Lett. 2000, 292−293. (16) Roosma, J.; Mes, T.; Leclère, P.; Palmans, A. R. A.; Meijer, E. W. Supramolecular Materials from Benzene-1,3,5-tricarboxamide-Based Nanorods. J. Am. Chem. Soc. 2008, 130, 1120−1121. (17) Stals, P. J. M.; Smulders, M. M. J.; Martín-Rapún, R.; Palmans, A. R. A.; Meijer, E. W. Asymmetrically Substituted Benzene-1,3,5tricarboxamides: Self-Assembly and Odd−Even Effects in the Solid State and in Dilute Solution. Chem.Eur. J. 2009, 15, 2071−2080. (18) Cantekin, S.; Balkenende, D. W. R.; Smulders, M. M. J.; Palmans, A. R. A.; Meijer, E. W. The Effect of Isotopic Substitution on the Chirality of a Self-Assembled Helix. Nat. Chem. 2011, 3, 42−46. (19) Stals, P. J. M.; Korevaar, P. A.; Gillissen, M. A. J.; de Greef, T. F. A.; Fitié, C. F. C.; Sijbesma, R. P.; Palmans, A. R. A.; Meijer, E. W. Symmetry Breaking in the Self-Assembly of Partially Fluorinated Benzene-1,3,5-tricarboxamides. Angew. Chem., Int. Ed. 2012, 51, 11297−11301. (20) Leenders, C. M. A.; Albertazzi, L.; Mes, T.; Koenigs, M. M. E.; Palmans, A. R. A.; Meijer, E. W. Supramolecular Polymerization in Water Harnessing both Hydrophobic Effects and Hydrogen Bond Formation. Chem. Commun. 2013, 49, 1963−1965. (21) Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G.; Warren, J. E. New Supramolecular Packing Motifs: p-Stacked Rods Encased in Triplyhelical Hydrogen Bonded Amide Strands. Chem. Commun. 1999, 1945−1946. (22) Ranganathan, S.; Muraleedharan, K. M.; Rao, C. H. C.; Vairamani, M.; Karle, I. L.; Gilardi, R. D. Stacking of a Benzenehexacarboxylic Acid Core in the Crystal Structure of Benzenehexacarboxylic Acid α-Aminomethyl Isobutyrate Amide (MA-Aib6)−Sodium Nitrate Complex. Chem. Commun. 2001, 2544− 2545. (23) Kristiansen, M.; Smith, P.; Chanzy, H.; Baerlocher, C.; Gramlich, V.; McCusker, L.; Weber, T.; Pattison, P.; Blomenhofer, M.; Schmidt, H.-W. Structural Aspects of 1,3,5-Benzenetrisamides−A

assemblies result in highly viscous liquid and organogel states of compounds 3BC, 4BC, 5BC, and 6BC in various types of organic solvents. A two-dimensional cobweb-like nanofiber network with typical dimensions of 3.5 × 200 × 1000 nm3 was observed in the spin-coated films on mica substrates, of which the occupation area and network connectivity could be controlled by changing the rotational speed of the spin coater. The occupancy area and connectivity of the nanofiber network on mica increased in the order of 3BC < 4BC < 5BC < 6BC, which suggests that the introduction of −CONHC14H29 chains enhances the strength of the one-dimensional self-assembly. The compounds 3BC, 4BC, 5BC, and 6BC exhibited discotic hexagonal ordered columnar (Colho) liquid crystalline phases, where the intercolumn and average π-stacking distances are dependent on the number of −CONHC14H29 chains. No distinct dielectric response was observed for compounds 4BC, 5BC, and 6BC around the solid−Colho phase transition temperatures. P−E hysteresis characteristics were observed for compounds 2BC, 3BC, and 5BC, whereas compounds 4BC and 6BC had linear P−E responses. The former three compounds exhibited typical ferroelectric switching behavior, whereas the latter two compounds had effective intramolecular N−H∼O= hydrogen-bonding interactions between the neighboring two −CONHC14H29 chains, which suppressed rotational inversion of the −CONHC14H29 chain and dipole inversion along the π-stacking column. The temperature- and frequency-dependent dielectric enhancements of compounds 2BC, 3BC, and 5BC were larger than those of compounds 4BC and 6BC; therefore, the molecular motion of −CONHC14H29 chains was thermally activated in the mesophases of compounds 2BC, 3BC, and 5BC. Further chemical design of the ferroelectric switching properties in liquid crystals and organic thin films are planned for aromatic π-frameworks with multiple −CONHCnH2n+1 chains.

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ASSOCIATED CONTENT

S Supporting Information *

SEM and AFM images for xerogels and nanofibers, polarized optical microscopic images, X-ray diffractions, temperaturedependent dielectric constants of compound 3BC, and P−E curves for compounds 4BC and 6BC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone +81-22-217-5653; Fax +81-22-217-5655; e-mail [email protected] (T.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and by Management Expenses Grants for National Universities of Japan.

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REFERENCES

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