Liquid Crystalline Phase Induced by Molecular Rotator and Dipole

Nov 12, 2015 - Although the molecular rotators in single crystals partially fuse the periodic crystal lattice, a dramatic phase transition accompanyin...
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Liquid Crystalline Phase Induced by Molecular Rotator and Dipole Fluctuation Qian-Chong Zhang,† Takashi Takeda,† Norihisa Hoshino,† Shin-ichiro Noro,‡ Takayoshi Nakamura,‡ and Tomoyuki Akutagawa*,† †

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 (RIES), Hokkaido University, Sapporo 001-0020, Japan S Supporting Information *

ABSTRACT: The thermal properties, crystal structures, dielectric relaxations, and rotational potential energy curves were examined for new rod-like molecules 1 and 2 bearing three aromatic rings connected by two -CONH- linkage groups to clarify the dynamic molecular behavior and phase transition behavior of the molecular assemblies. The molecular structures of 1 and 2 differed in that the central aromatic ring was phenyl (-C6H4-) in 1 and pyridyl (-C5NH3-) in 2, which affected the phase transition behavior owing to the permanent dipole moment without the center of inversion in molecule 2. Although the crystal structures of 1 and 2 were isostructural, the melting point of crystal 2 was approximately 43 K lower than that of crystal 1, and a smectic A mesophase was reversibly observed in crystal 2. A broad endothermic thermal anomaly of crystal 2 was observed in the heating process on the differential scanning calorimetry chart because of thermally activated dipole fluctuation, which was consistent with the frequency- and temperature-dependent dielectric relaxations. Double- and single-minimum-type potential energy curves were observed in the rotations of -C6H4- and -C5NH3- rings, respectively, from density functional theory calculations. The difference in rotational symmetry affected the crystal lattice energy and appearance of the mesophase.

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INTRODUCTION Various types of motional freedom in molecular assemblies are essential to generate their hierarchical ordered structure and physical properties.1,2 Relatively small-amplitude motional freedom such as rotation of methyl groups and intramolecular proton transfer has been easily allowed in the closest-packed molecular assemblies of single crystals,3−7 whereas largeamplitude motional freedom has typically been prohibited by steric hindrance and effective intermolecular interactions. Among various molecular assemblies, the periodic lattice order in plastic and liquid crystalline phases is lower than that in single crystals, where thermally activated molecular rotations occurred in the molecular assemblies.8−11 Free molecular rotation with a fixed center of molecular gravity is achieved in plastic crystalline materials such as spherical adamantine and C60, and the phase transition from the lowtemperature ordered phase to the high-temperature disordered rotator phase has been well characterized.12−18 In single crystals, the design of molecular rotation environments in the closest-packed structure becomes a specific approach in molecular gyroscopes, supramolecular rotators, and metal− organic frameworks.19−28 For instance, the covalently bonding rotator and stator structure in molecular gyroscope molecules has been crystallized in the closest-packed structure, while the motional freedom of the rotator unit was maintained, even in single crystals. The supramolecular rotator system of anilinium([18]-crown-6), m-fluoroanilinium(dibenzo[18]crown-6) and © XXXX American Chemical Society

adamantylammonium([18]crown-6) in [Ni(dmit)2] crystals (dmit2− = 2-thioxo-1,3-dithiole-4,5-dithiolate) enables us to provide a relatively conventional technique for introducing a rotational environment of the rotator unit into single crystals.22−25 The molecular symmetries of anilinium and adamantylammonium yielded 2- and 3-fold molecular rotation symmetry, respectively, in single crystals. Although the molecular rotators in single crystals partially fuse the periodic crystal lattice, a dramatic phase transition accompanying the physical responses has not been observed in the symmetrical molecular rotator system of anilinium and adamantylammonium because the initial and rotated states have the same molecular structure. In contrast, the motional freedom of the dipole unit could possibly demonstrate the paraelectric− ferroelectric phase transition as a physical response.25 The designs of rotation environments in single crystals are not simple or conventional, whereas molecular rotations have been easily achieved in plastic and liquid crystalline states according to a simple molecular structure design. A rod-like molecule bearing both a rigid core and a flexible long alkyl chain forms the thermotropic liquid crystalline phases of nematic- and smectic-type molecular assembly order.10,11,27−29 Because the order in the liquid crystalline phase is associated Received: May 20, 2015 Revised: October 28, 2015

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molecular assemblies such as the α-helix and β-sheet.40−42 Molecules 1 and 2 can both support the intermolecular N−H··· O hydrogen-bonding interaction in molecular assemblies, whereas the association−dissociation process of the hydrogenbonding interaction is affected by the molecular motion and phase transition behavior. Complete dissociation of intermolecular N−H···O hydrogen bonds resulted in liquid crystal and/or isotropic liquid states. Crystal structural analyses, dielectric spectroscopy, and theoretical calculations of the rotational potential energy were performed to clarify the difference in the phase transition behaviors of crystals 1 and 2.

with the thermal motion of each molecule, the molecular structural correlation between the molecular motion and phase transition behavior is an interesting consideration for constructing new molecular rotation systems in which the molecular rotation is coupled with the physical properties. Anisotropic molecular rotations and order parameter of rod-like liquid crystalline molecules in smectic mesophases have been extensively examined using dielectric and 13C NMR spectroscopies.10,11,30−34 For instance, rapid molecular rotation over 100 MHz has been observed in the typical rod-like molecule such as p-decyloxybenzylidene-p′-amino-2-methylbutyl cinamate along the long axis of the molecule.30 The formation of liquid crystalline state and phase transition to isotropic liquid are governed by the intermolecular multipole attractive interaction between the central aromatic cores and/or excluded-volume effect.10,11 One of the notable liquid crystalline systems has been achieved in the binary hydrogenbonding liquid crystalline molecules.35−37 The intermolecular hydrogen-bonding complex between proton donor of 4butoxybenzoic acid and proton acceptor of trans-4-[(4ethoxybenzoyloxy)-4′-stilbazole increased the thermal stability of nematic liquid crystalline phase through the formation of intermolecular hydrogen-bonding interaction along the long axis of the molecule, which extended the aromatic core to increase the attractive intermolecular interaction.35 On the contrary, the rod-like liquid crystalline molecules bearing the hydrogen-bonding -CONH- linkage group between aromatic cores have been examined to form the lateral intermolecular -N−H···O hydrogen-bonding interaction in smectic mesophase, which formed the hydrogen-bonding anchoring and spinning parts within the mesophase.38,39 Actually, the rod-like molecule could be considered as a rapid rotating rigid cylinder in mesophase, where the intermolecular hydrogen-bonding interaction played an important role to dissociate and associate each molecule. The thermally activated molecular motion modifies the crystal lattice and sometimes reveals the liquid crystalline phases before melting to an isotropic liquid. Herein, we designed two types of rod-like molecules bearing three aromatic rings connected by two -CONH- amide linkage groups and two flexible -C5H11 chains (1 and 2 in Scheme 1). Although the

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EXPERIMENTAL SECTION

Preparation of Crystals. Molecules 1 and 2 were synthesized by a one-step condensation reaction between terephthalic acid chloride (or pyridine-2,5-dicarboxylic acid chloride) and p-pentylaniline under triethylamine. Single crystals of 1 and 2 were grown by slow bilayer solvent diffusion between dimethyl sulfoxide (DMSO) and CH3OH. Molecule 2 (2.8 mg, 0.0061 mmol) in DMSO (0.7 mL) was placed at the bottom of a test tube, and methanol (0.9 mL) was slowly applied to the DMSO layer. After 2 days, single crystals of 2 were obtained as colorless needles. Single crystals of 1 were obtained by the same method as that for single crystals of 2. Elemental analysis of molecule 1. Calc. for C30H36N2O2: C, 76.12; H, 7.71; N, 9.18. Found: C, 76.24; H, 7.82; N, 9.19. Molecule 2. Calc. for C29H34N2O3: C, 78.91; H, 7.95; N, 6.13. Found: C, 78.92; H, 8.06; N, 6.27. NMR of molecule 1. 1H NMR (400 MHz, DMSO-d6) δ 0.87 (t, J = 6.4 Hz, 6H), 1.24−1.34 (m, 8H), 1.54−1.61 (m, 4H), 2.56 (t, J = 7.2 Hz, 4H), 7.18 (d, J = 8.4 Hz, 4H), 7.68 (d, J = 8.4 Hz, 4H), 8.08 (s, 4H), 10.31 (s, 2H); 13C NMR (100 MHz, DMSO-d6) δ 14.4 (2C), 22.4 (2C), 31.2 (2C), 31.3 (2C), 35.0 (2C), 121.0 (4C), 128.1 (4C), 128.8 (4C), 137.1 (2C), 137.9 (2C), 138.4 (2C), 165.1 (2C). Molecule 2. 1H NMR (400 MHz, DMSO-d6) δ 0.87 (t, J = 6.4 Hz, 6H), 1.26−1.34 (m, 8H), 1.54−1.61 (m, 4H), 2.56 (t, J = 7.6 Hz, 4H), 7.20 (dd, J = 4.4 Hz, J = 8.0 Hz, 4H), 7.68 (d, J = 8.4 Hz, 2H), 7.82 (d, J = 8.4 Hz, 2H), 8.28 (d, J = 8.0 Hz, 1H), 8.54 (dd, J = 2.0 Hz, J = 8.4 Hz, 1H), 9.18 (s, 1H), 10.55 (s, 1H), 10.69 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 14.1 (2C), 22.7 (2C), 29.18 (2C), 31.47 (1C), 31.51 (1C), 31.8 (2C), 119.8 (4C), 120.5 (1C), 122.2 (1C), 129.0 (2C), 129.1 (2C), 134.8 (1C), 135.0 (1C), 136.3 (1C), 139.6 (1C), 146.9 (2C), 152.1 (1C), 160.9 (1C), 162.5 (1C). Physical Measurements. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a Bruker Avance III 400 NMR spectrometer. Chemical shifts (δ) are expressed in ppm relative to tetramethylsilane (1H 0.00 ppm) or residual nondeuterated solvent (CDCl3; 13C 77.0 ppm) as an internal standard. Infrared (IR, 400− 4000 cm−1) spectra were measured on a KBr pellet using a Thermo Fisher Scientific Nicolet 6700 spectrophotometer with a resolution of 4 cm−1. Thermogravimetric differential thermal analysis and differential scanning calorimetry (DSC) were conducted using a Rigaku Thermo plus TG8120 thermal analysis station with an Al2O3 reference and a heating rate of 5 K min−1 under nitrogen. The temperaturedependent dielectric constants were measured using the two-probe AC impedance method at frequencies from 1 kHz to 1 MHz (HewlettPackard, HP4194A) using the temperature controller of a Linkam LTS-E350 system. The liquid crystal state of sample 2 was investigated using a liquid crystal cell (KSSZ-05/A111P6NSS05 from E. H. C. Co., Ltd.) with an indium tin oxide electrode 1 cm2 in area with an electrode gap of 5 μm. The experimental data were calibrated using the previous method to eliminate the effect of the electrode capacitor.43 Crystal Structure Determination. Temperature-dependent crystallographic data (Table 1) were collected using a Rigaku RAPID-II diffractometer equipped with a rotating anode fitted with a multilayer confocal optic using Cu−Kα (λ = 1.54187 Å) radiation from a graphite monochromator. Structural refinements were conducted using the full-matrix least-squares method on F2. Calculations were performed using the Crystal Structure software packages.44,45 The parameters were refined using anisotropic temper-

Scheme 1. Molecular Structures of Rod-like Molecules 1 and 2

molecular structures of 1 and 2 are quite similar, except that the central aromatic ring is phenyl (-C6H4-) in 1 and pyridyl (-C5NH3-) in 2, the absence of an inversion center in molecule 2 gives rise to a permanent dipole moment from the central pyridyl ring. Another important structural feature is the existence of two hydrogen-bonding -CONH- amide linkage groups, which affect the molecular assembly structures. Intermolecular N−H···O hydrogen-bonding interactions play an important role in forming biological polypeptide B

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Table 1. Crystal Data, Data Collection, and Reduction Parameters of Crystals 1 and 2

a

molecule

1

2

chemical formula formula weight space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K Dcalc, g·cm−1 μ, cm−1 refls meas indep refls refls used Ra Rw(F2)a GOF

C30H36N2O2 456.62 P1̅ (#2) 5.3488(2) 6.6508(1) 17.1429(2) 85.881(2) 82.614(2) 88.902(1) 603.20(3) 1 173 1.257 6.099 6476 2168 2168 0.1309 0.2670 0.947

C29H35N3O2 457.61 P1̅ (#2) 5.3499(1) 6.6535(2) 17.155(2) 85.881(1) 82.623(1) 88.910(1) 604.00(3) 1 100 1.264 6.230 6468 2169 2169 0.1116 0.2516 0.954

Article

RESULTS AND DISCUSSION

Phase Transition Behavior. The phase transition behavior of crystals 1 and 2 was characterized by DSC, vibrational infrared (IR) spectra, powder X-ray diffraction (PXRD), dielectric spectroscopy, and theoretical calculations. Both crystals were thermally stable up to approximately 570 K in the thermogravimetry measurements (Figure S3). Figure 1a summarizes the DSC charts of crystals 1 and 2 from 300 K. Crystal 1 exhibited only one reversible phase transition peak, at 585 K, from solid (S1) to isotropic liquid (IL), and there was no evidence of mesophase formation between 300 and 585 K. The decomposition of molecule 1 was observed at the IL state around 580 K. In the IL state, In contrast, reversible phase transitions peaks from solid to smectic A (SmA) and from SmA to IL were observed in crystal 2 at 501 and 542 K, respectively. Batonnets texture was observed in a polarized optical microscopy (POM) image of crystal 2 taken with a crossNicol optical arrangement in the temperature range from 501 to 542 K (SmA phase in Figure 1b). The first heating process of crystal 2 showed an endothermic thermal anomaly with a broad baseline curve (red trace in Figure 1a). Thermally activated structural relaxation before the phase transition to the SmA phase occurred in the heating process of crystal 2, where two types of solid phase, S1 (low-T phase) and S2 (high-T phase) existed in the molecular assembly before the SmA phase. In the cooling process from the IL state, the stable sold S2 phase was observed in crystal 2 in the absence of S1 phase (Figure S12). The as-grown crystal was metastable crystal form S1, which was transformed to the stable crystal form S2 after the thermal treatment. Because the melting point of crystal 2 was 43 K lower than that of crystal 1, the change of the central part from a phenyl (-C6H4-) ring to a pyridyl (-C5NH3-) ring decreased the thermal stability of the crystal lattice and generated the SmA phase. Figure 1c shows the variable temperature PXRD patterns of crystal 2 in the S1 phase at 295 K, the S2 phase at 460 K, and the SmA phase at 523 K. Sharp low-angle diffraction peaks in the S1, S2, and SmA phases were observed at 2θ = 2.78, 3.70, and 2.94°, respectively, corresponding to periodicities of 31.78, 23.88, and 30.05 Å, respectively. Although the periodicity of

R = Σ||Fo| − |Fc||/Σ|Fo|, and Rw = (Σω(|Fo| − |Fc|)2/ΣωFo2)1/2.

ature factors, except for the hydrogen atom. Temperature-dependent powder X-ray crystallographic data were collected using a Rigaku RintUltima III diffractometer employing Cu−Kα (λ = 1.54187 Å) radiation. Calculation. The calculations were performed using the GAUSSIAN 09W package and a density functional theory (DFT) method with the B3LYP hybrid functional and the 6-31G(d, p) basis sets.46 The calculated structures of molecules 1 and 2 were obtained by single-crystal structural analyses. The single-point energy was obtained at 30° rotation intervals of the central phenyl (-C6H4-) and pyridyl (-C5NH3-) rings of molecules 1 and 2, respectively. The dipole moment of molecule 2 (2.6 D) was also obtained by the same procedure.

Figure 1. Phase transition behavior of crystals 1 and 2. (a) DSC chart of crystals 1 (upper) and 2 (lower). The red trace was the first heating scan of crystal 2. (b) POM image of with batonnets texture in SmA phase of 2 taken with cross-Nicol optical arrangement at 520 K. The arrow was the direction of two polarizers. (c) PXRD patterns of crystal 2 in (i) S1 phase at 295 K, (ii) S2 phase at 460 K, and (iii) SmA phase at 523 K. C

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were found to be 3.1473(4) and 3.171(3) Å, respectively. Each hydrogen-bonding chain formed the π-stacking structure along the 4b + c axis at the central phenyl and outer phenyl rings in crystal 1 and the central pyridyl and outer phenyl rings of crystal 2. The dihedral angle of two π-planes and average π−π distance are 4.27° and 3.62 Å in crystal 1, which were almost the same as those of crystal 2, 4.70° and 3.79 Å. The crystal lattice energies of 1 and 2 originate mainly from the hydrogenbonding and π-stacking interactions and van der Waals interactions. IR and Dielectric Spectra. The difference in the thermal behavior of crystals 1 and 2 was further evaluated using the temperature-dependent IR spectra and dielectric spectroscopy. Figure 3 shows the temperature-dependent IR spectra of

31.78 Å in the S1 phase was similar to that of 30.05 Å in the SmA phase, the value of 23.88 Å in the S2 phase represents 75% of that of the S1 phase. Thermally activated structural relaxation was expected in the high-temperature S2 phase before the transition to the SmA phase. The molecular lengths of 1 and 2 were approximately 31.3 Å assuming the all-trans conformation of two -C5H11 chains, which was almost consistent with the layer spacing of the SmA phase. Crystal 1 exhibited no dramatic changes in the PXRD pattern. Crystal Structures. Figure 2 shows the crystal structures of isostructural crystals 1 and 2. The similar unit cell volumes of

Figure 2. Crystal structures of isostructural crystals 1 and 2. (a) Unit cell of crystal 1 viewed along the a axis, and alternating layered arrangement of rigid π-plane and flexible alkyl chain elongated along the 4b + c axis. The intermolecular N−H···O hydrogen-bonding layer of (b) crystal 1 and (c) crystal 2 along the a axis. Figure 3. Temperature-dependent IR spectra in the heating process of crystals (a) 1 and (b) 2 in the energy range from 3200 to 3500 cm−1 on KBr pellets.

crystals 1 (603.20 Å3) and 2 (604.00 Å3) was consistent with the same packing structure in the S1 phase. Half of the molecule was involved in the crystallographically asymmetric unit and the center of inversion located at the central phenyl and pyridyl rings for molecules 1 and 2, respectively (Figure S5). Two C5H11- chains in crystals 1 and 2 adopted the all-trans conformation with a total molecular length of 31.3 Å. One nitrogen atom in the central -C5NH3- ring was difficult to assign because of a subtle difference in the electron density and the existence of the inversion center. Owing to the absence of information on the dipole orientation of the pyridyl ring in the S1 phase in crystal 2 (the dipole moment of molecule 2 was 2.6 D from the DFT calculation), several possibilities can be assumed, that is, ferroelectric, antiferroelectric, and random glass arrangement of each dipole moment arising from the pyridyl ring. We could not determine the dipole structures of S1 phase form the electric filed−polarization meausrements. Thermally activated dynamic relaxation of the dipole arrangement in the solid is closely associated with the phase transition behavior and emergence of a high-temperature S2 phase in crystal 2. The πplanes of three phenyl and/or pyridyl rings in crystals 1 and 2 were parallel to each other. An alternating layered arrangement of the π-plane and -C5H11 chains was elongated along the 4b + c axis without the interdigitated structure of the alkyl chains, which was consistent with the layered molecular arrangement in the SmA phase. Amide-type intermolecular N−H···O hydrogen-bonding interactions were observed along the a axis in both crystals 1 and 2, forming a one-dimensional hydrogenbonding chain through the two -NHCO- sites (Figure 2, panel b and c, respectively). The N−O distances in crystals 1 and 2

crystals 1 and 2 on KBr pellets in the heating process. The intermolecular hydrogen-bonding N−H stretching mode (νN−H) of crystal 1 was observed at 3315 cm−1 at 323 K, which was slightly blue-shifted to 3331 cm−1 at 523 K owing to the decrease in the bonding energy of N−H···O intermolecular hydrogen bonds due to thermal fluctuation.47,48 The behavior of the temperature-dependent νN−H mode of crystal 2 differed from that of crystal 1. Two types of νN−H modes were observed in crystal 2 at 3319 and 3361 cm−1 at 323 and 393 K (S1 phase), indicating the existence of two types of N−H···O hydrogen-bonding interaction. The potential energy calculation indicated the orientation of pyridyl ring affected the relative energy and also energy of νN−H mode (see the section on DFT calculation). The two νN−H bands merged into one νN−H band at 3319 cm−1 at temperatures above 433 K (S2 phase), where an endothermic broad baseline appeared in the DSC chart owing to the thermally activated relaxation process. The νN−H band at SmA phase showed the red-shift at 3348 cm−1 due to rapid molecular rotation of molecule 2. The temperature-dependent IR spectral change in crystal 2 was caused by the thermal transformation from the low-temperature S1 phase with two types of N−H···O hydrogen-bonding environment to the high-temperature S2 phase with one type. Therefore, thermally induced structural transformation should occur in crystal 2, which was consistent with the DSC and XRD measurements. To evaluate the thermally activated molecular motion of molecule 2 in the S2 phase in the crystal, the dielectric spectra D

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maximum frequency of the broad ln ε2 peak (f max) versus T−1 show a linear correlation with a single relaxation process in crystal 2 (Figure S11), yielding an activation energy of Ea = 33.5 kJ·mol−1 and a relaxation frequency of ω0 = 2.97 × 1010 s−1.50−52 Theoretical DFT calculations to evaluate the rotational freedom of the phenyl and pyridyl rings in molecules 1 and 2 were performed to obtain the potential energy curves and energy barrier using the B3LYP/6-31+G(d, p) basis set. Figure 5 summarizes the rotation angle (ϕ1) dependence of the

were measured in the liquid crystal cell with an electrode gap of 5 μm. The dielectric measurements were sensitive to the molecular motions of the polar structural unit in the measurement frequency (103−106 Hz) and temperature (298 < T < 550 K) ranges.49 Thermally activated molecular motion of the polar pyridyl ring can be detected in the dielectric relaxation measurements. In contrast, the motional freedom of the symmetric phenyl ring cannot be detected in the dielectric spectra. Before the measurements, samples 1 and 2 in the liquid crystal cell were prepolarized in the cooling process from an IL to a solid under an applied electric field at 20 kV cm−1. Figure 4a shows the temperature-dependent real part of the complex

Figure 4. Dielectric properties of crystals 1 and 2. (a) Temperaturedependent real part of dielectric constant (ε1) of crystals 1 (red) and 2 (black) obtained using liquid crystal cell. (b) Frequency- and temperature-dependent imaginary part of dielectric constant (ε2) of crystal 2.

dielectric constants (ε1) of crystals 1 and 2 at a measurement frequency of 100 kHz. A phase transition temperature of 585 K from IL to S1 of crystal 1 was observed, and the ε1 value of 5.6 at 550 K decreased gradually with decreasing temperature, finally reaching ε1 = 3.7 at 450 K. There was no specific anomaly in the ε1−T plots of crystal 1, suggesting that no thermally activated molecular motions of the polar structural unit occurred in the intermolecular N−H···O hydrogenbonding interaction. In contrast, a dielectric anomaly was observed in crystal 2 as step-like behavior around 390 and 460 K, where the endothermic broad peak in the DSC chart and structural change in the XRD pattern were confirmed in the S2 phase. The thermally activated molecular motion of the pyridyl ring contributed to the dielectric anomaly of crystal 2 in the S2 phase. Figure 4b shows the frequency- and temperature-dependent imaginary part of the complex dielectric constant (ε2) of crystal 2. The ε2 values decreased monotonically in all the ε2−f plots of crystal 1 at different temperatures (Figures S7 and S9). In contrast, a broad dielectric relaxation peak was observed in crystal 2 at 400−460 K, and the ε2−f plots below 380 K showed frequency-independent behavior. The ε2−f plots at 400 K showed a broad peak at f = 1.9 × 105 Hz, which shifted to higher frequency with increasing temperature, suggesting the thermally activated structural relaxation process in the S2 phase. Around this temperature range, the endothermic broad baseline in the DSC chart also supported the phase transition from S1 to S2 accompanying the molecular motion of the polar structural unit. From a comparison of the molecular structures of 1 and 2, the relaxation process found in crystal 2 was assigned to the thermally activated rotation of the polar pyridyl ring in the solid, whereas dielectric relaxation of the symmetrical phenyl ring was not observed in crystal 1. Arrhenius plots of the

Figure 5. Potential energy curves for rotations of phenyl and pyridyl rings. (a) Rotation of central phenyl ring at rotation angle (ϕ1) of 0, 60, and 90° in molecule 1. (b) Rotation of central pyridyl ring at ϕ1 of 0, 90, and 150° in molecule 2. (c) Rotational potential energy (ΔE) of molecules 1 (black) and 2 (red) was plotted versus ϕ1 using a hybrid function of the B3LYP/6-31+G(d, p) basis set.

normalized rotational potential energy (ΔE). The relative energy at ϕ1 = 0° was defined as ΔE = 0 kJ·mol−1, and the two central C−C bonds of -C-(C6H4)-C- and -C-(C5NH3)-C- were rotated in 30° intervals with a fixed molecular conformation of the outer two CH3−C6H4−NHCO- moieties (Figure 5a,b). Molecules 1 and 2 exhibited different ΔE−ϕ1 profiles (Figure 5c). A typical double-minimum-type potential energy curve was observed in molecule 1 for the rotation of the phenyl ring, where the potential energy maxima appeared at ϕ1 = 90 and 210°, with a potential energy barrier of 30 kJ·mol−1. In contrast, a single-minimum-type potential energy curve was confirmed in molecule 2 with three potential energy maxima at ϕ1 = 90, 150, and 210°. The potential energy barrier of 60 kJ·mol−1 at ϕ1 = 150° in molecule 2 was twice that of molecule 1, which was close to the activation energy of 33.5 kJ·mol−1 obtained in the dielectric measurements. Unfortunately, the activation energy for the 2-fold flip-flop motion of phenyl ring could not be detected in the dielectric measurements due to the absence of change in dipole moment. The 2-fold flip-flop motion of the pyridyl ring was completely suppressed by the potential energy barrier at ϕ1 = 150°. One steric repulsion between hydrogen atoms was observed at ϕ1 = 0° in molecule 2, whereas two H··· E

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H steric repulsions existed at ϕ1 = 150° (Figure 5b). The different rotational symmetries of the phenyl and pyridyl rings in molecules 1 and 2 affected the lattice energy in the solids. Although the same molecular rotation mode was expected in the isostructural crystals, the potential energy curves of molecules 1 and 2 exhibited double- and single-minimumtype profiles, respectively. The rotation of the phenyl ring in molecule 1 occurred at every 180° with 2-fold symmetry, whereas that of the pyridyl ring in molecule 2 occurred every 360° with 1-fold left. The rotation energy can be represented by 1/2Iω2, where I and ω are moment of inertia and the angular velocity, respectively. Assuming the same I, ω, and mass for rotations of the phenyl and pyridyl units, the energy for every 180° rotation was half the magnitude of that for the 360° one, which affected the lattice energy and decreased the melting point of crystal 2. The difference in the rotation symmetry influenced the thermal properties and appearance of the liquid crystalline phase of crystal 2.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +81 22 217 5653. 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. We thank the Chinese Scholarship Council for its support.

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REFERENCES

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CONCLUSIONS The phase transition behavior of two types of rod-like molecules bearing central phenyl (-C6H4-) and pyridyl (-C5NH3-) rings was examined by single-crystal X-ray structural analysis and dielectric measurement, and the potential energy curves for both molecules were calculated on the basis of DFT calculations. The replacement of the phenyl ring by a pyridyl one decreased the melting point by approximately 43 K and newly generated the SmA phase before melting. A broad endothermic thermal anomaly in the heating process on the DSC chart was observed in the temperatures before the phase transition from the solid to the SmA phase, suggesting that molecular rotation with the dipole structural unit of the central pyridyl group contributed to the thermal relaxation process in the solids. The temperature- and frequency-dependent dielectric relaxations were also consistent with the thermally activated molecular motions of the polar structural unit. The rotational potential energy curves for the phenyl and pyridyl rings had double- and single-minimum-type profiles, respectively, with energy barriers of approximately 30 and 60 kJ· mol−1, which indicated possible thermally activated molecular rotations. The rotational freedom of the pyridyl ring decreased the lattice energy and generated the SmA phase before melting. Controlling the motional freedom of the dipole unit within molecular assemblies, which is associated with the phase transitions and physical properties, is useful for fabricating new materials.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00695. Crystallographic information files for 1 and 2 are also available from the Cambridge Crystallographic Data Center via http:// www.ccde.cam.ac.uk/data_request/cif (CCDC deposition numbers 1063977−1063978). The preparations, TG diagram, IR spectra, atomic numbering scheme, crystal structure of 1, and dielectric measurements (PDF) Crystallographic information files for 1 and 2 (CIF) F

DOI: 10.1021/acs.cgd.5b00695 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.5b00695 Cryst. Growth Des. XXXX, XXX, XXX−XXX