Mesophases and Ionic Conductivities of Simple Organic Salts of M

Dec 26, 2014 - ABSTRACT: Simple organic salts such as (Li+)(m-IBA) (1),. (Na+)(m-IBA) (2), (K+)(m-IBA) (3), (Rb+)(m-IBA) (4), and. (Cs+)(m-IBA) (5) (m...
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Mesophases and Ionic Conductivities of Simple Organic Salts of M(m‑Iodobenzoate) (M = Li+, Na+, K+, Rb+, and Cs+) Manami Endo,† Yuta Nakane,† Kiyonori Takahashi,† Norihisa Hoshino,†,‡ Takashi Takeda,†,‡ 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: Simple organic salts such as (Li+)(m-IBA) (1), (Na+)(m-IBA) (2), (K+)(m-IBA) (3), (Rb+)(m-IBA) (4), and (Cs+)(m-IBA) (5) (m-IBA = m-iodobenzoate) were shown to form a mesophase before crystal melting or decomposition. The crystals were obtained in the hydrated form, e.g., 1·(H2O), 2· (H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O); they were then converted into dehydrated forms by increasing the temperature to ∼450 K. Optically anisotropic-layered mesophases were observed in unhydrated crystals 2, 3, 4, and 5, whereas an optically isotropic mesophase (e.g., rotator phase) was found for crystal 1. The single-crystal X-ray structural analysis of the hydrated crystals revealed an inorganic−organic alternate layer structure, which is consistent with the average molecular orientation in the layered mesophase. The m-IBA anions formed a π-stacking columnar structure in the hydrated crystals, while one- or two-dimensional M+∼O networks were observed in the inorganic layers. Our results showed that the M+∼O interactions and their connectivity are strongly influenced by the size of the cations. The reconstruction of the M+∼O networks by removing H2O molecules was crucial for the formation of the mesophases. A strong response of both the real and imaginary parts of the dielectric constant was observed around the solid-mesophase phase-transition temperatures of crystals 1−5, with the ionic conductions playing a critical role.

I. INTRODUCTION Soft molecular assemblies such as micelles, vesicles, gels, and lyotropic liquid crystals can be obtained using amphiphilic molecules by systematically changing the energy balance of the hydrophilic−hydrophobic and/or hydrogen-bonding interactions.1−3 Structurally flexible hydrophobic long alkyl chains are among the most important structural units that can be used to stabilize specific soft molecular-assembly structures in solution. The ability of amphiphilic molecules to easily associate and dissociate at room temperature has been effectively utilized for the thermodynamic reconstruction of biological assemblies.4−6 The mesophases between solid and liquid states are interesting dynamic molecular assemblies that have several types of motional freedoms such as rotation, diffusion, and translation.7−10 The plastic- and liquid-crystalline phases are among the best known mesophases. The orientational disorder of each molecule has been observed in plastic crystals such as adamantane, cyclohexane, and CCl4 through thermally activated molecular rotations by keeping their molecular centers of gravity fixed.10−14 Because the molecular orientations were fixed in the low-temperature ordered phase, the order− disorder phase transition was reversibly observed in the © XXXX American Chemical Society

temperature cycles. In particular, the order−disorder phase transitions of adamantane and cyclohexane were observed at 208 and 186 K, respectively.11−14 In contrast, the molecular centers of gravity were randomly distributed in the liquid crystalline states by keeping the average molecular orientation of the rodlike and disklike molecules fixed, leading to the formation of calamitic and discotic liquid crystals, respectively.7−9 The characteristic molecular framework of the calamitic liquid-crystalline materials has a rigid-rod-like core and long flexible alkyl chains, with the latter destabilizing the three-dimensional crystal lattice through thermally activated molecular motions. Therefore, typical calamitic liquid-crystalline materials have been designed to have both the rigid core and long flexible alkyl chains; these chemical modifications can strongly influence the thermal properties of liquid-crystalline materials. Relatively simple 1:1 organic salts of alkali metal ions (M+ = + Li , Na+, K+, Rb+, and Cs+) with m-substituted benzoate (mReceived: November 8, 2014 Revised: December 24, 2014

A

DOI: 10.1021/jp5112026 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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and related to their mesophase properties, phase-transition behaviors, and dielectric responses.

XBA) showed a smectic liquid-crystalline phase at relatively high temperatures (above 500 K).15−17 The smectic A (SmA) liquid-crystalline properties of these salts have been reported for the first time by Voländer in 1927, who employed polarized optical microscopy;15,16 their phase-transition behavior was then reexamined by Binnemans.17 For instance, the simple 1:1 (Na+)(benzoate) salt formed a smectic phase in the temperature range of 704−724 K; however, this was not observed for (Li+)(benzoate), (K+)(benzoate), (Rb+)(benzoate), and (Cs+)(benzoate). Thus, the Na+ ion is crucial for the formation of the SmA phase of the salt containing the benzoate anion. In contrast, replacement of the benzoate anion with m-XBA such as m-fluorobenzoate (m-FBA), m-chlorobenzoate (m-ClBA), mbromobenzoate (m-BrBA), m-iodobenzoate (m-IBA), mmethylbenzoate (m-MeBA), and m-methoxybenzoate (mMeOBA) expanded the liquid-crystalline materials.17 The fact that simple M+(m-XBA) salts (with M+ = Na+, K+, Rb+, and Cs+ and X = F, Cl, Br, I, and OCH3) can generate the SmA phase suggested that the asymmetric molecular structures of the mXBA anions may be critical for the formation of the layer structure. Because of the high solid−liquid crystalline-phase transition temperatures (above 500 K), the phase-transition behavior, molecular-assembly structures, and physical properties of these salts have not been sufficiently examined. Only the X-ray diffraction of Na+(ClBA) at mesophase has been characterized as the layer periodicity of the SmA phase.17 The single-crystal X-ray structural analysis of 1:2 salts such as K+(m-ClBA), Rb+(m-ClBA), and Rb+(m-BrBA) has been successfully performed;17 in these crystals, the neutral benzoic acid was introduced to stabilize the three-dimensional crystal lattice. The crystal structure of 1:1 salts has been reported only for Na+(m-IBA)(H2O);17 in this system, the crystal lattice consists of two-dimensional alternate layers of m-IBA anions, Na+∼O networks, and crystallization H2O molecules. Such organic−inorganic alternate-layer structure is consistent with the formation of a layered mesophase, although the long alkyl chains were not introduced into the molecular system. Herein, we successfully prepared single crystals of Li+(m-IBA)·(H2O), Na + (m-IBA)·(H 2 O), K +(m-IBA)·0.5(H 2 O), Rb + (m-IBA)· (H 2 O), and Cs + (m-IBA)·(H 2 O) (Scheme 1). All the

II. EXPERIMENTAL SECTION Commercially available m-iodobenzoic acid and the alkali metal hydroxides, e.g., LiOH·H2O, NaOH, KOH, RbOH, and CsOH, were used without further purification. An equimolar amount of m-iodobenzoic acid (2.48 mg) and LiOH·H2O (0.420 mg) was solved in H2O−C2H5OH (50 mL) and stirred for 1 h at room temperature.17 After evaporation of the solvent, white powder was collected and redisolved in H2O−CH3OH (1:1). CH3CN was slowly dropped onto the H2O−CH3OH layer to form a bilayer-separation state. After the slow diffusion between upper and lower layers, hydrated crystal 1·(H2O) was obtained as colorless needles. Single crystals of 2·(H2O), 3·0.5(H2O), 4· (H2O), and 5·(H2O) were also prepared by 1:1 mixing of miodobenzoic acid and its corresponding alkali metal hydroxide in H2O−CH3OH. Colorless needles were obtained by slow evaporation of CH3OH−H2O. Elemental analysis for crystals 1· (H2O): Calcd for C7H6O3LiI: C, 30.91; H, 2.22. Found: C, 30.71; H, 2.28. 2·(H2O): Calcd for C7H6O3NaI: C, 29.19; H, 2.10. Found: C, 29.16; H, 2.13. 3·0.5(H2O): Calcd for C7H5O2.5KI: C, 28.49; H, 1.71. Found: C, 28.26; H, 1.76. 4· (H2O): Calcd for C7H6O3RbI: C, 23.99; H, 1.73. Found: C, 23.87; H, 1.75. 5·(H2O): Calcd for C7H6O3CsI: C, 21.13; H, 1.52. Found: C, 21.63; H, 1.56. B. Physical Measurements. Infrared (IR, 400−4000 cm−1) spectra were measured on KBr pellets using a Thermo Fisher Scientific Nicolet 6700 spectrophotometer with a resolution of 4 cm−1. The TG-DTA analysis was carried out with a Rigaku Thermo plus TG8120 thermal analysis station using an Al2O3 reference in the temperature range of 293−600 K with a heating rate of 5 K min−1 under nitrogen. Temperature-dependent dielectric constants were measured using the two-probe ac impedance method at frequencies ranging between 100 and 1 × 106 Hz (Hewlett-Packard, HP4194A). Temperature control was carried out on the heating stage of Linkam LTS350 from 300 to 600 K. The cast films were fabricated on an indium tin oxide (ITO) glass using 1·(H2O), 2·(H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O), which were heated up to the H2O-elimination temperature (around 450 K). The ITO glass was then covered with the cast film via a ∼2 μm insulating spacer. A sandwich ITO−electrode configuration (with an electrode area and gap of 16 mm2 and 2 μm, respectively) was used for the dielectric measurements. C. X-ray Structural Analysis. Temperature-dependent Xray 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 radiation (λ = 1.541 87 Å). Structure refinements were carried out using the full-matrix least-squares method on F2. Calculations were performed using the Crystal Structure and SHELEX software packages.18,19 Parameters were refined using the anisotropic temperature factors, except for the hydrogen atoms. Since all the as-grown single crystals included the hydrated H2O molecule, the crystals were not stable for the picking up procedure from crystallization solvents. Temperature-dependent powder X-ray crystallographic data were collected using a Rigaku Rint-Ultima III diffractometer employing Cu Kα radiation (λ = 1.541 87 Å).

Scheme 1. Molecular Structure of M+(m-IBA) Salts with M+ = Li+, Na+, K+, Rb+, and Cs+ and Number of Crystallization H2O Molecules (n) in the Crystals and the Optical Anisotropy

dehydrated crystals Li+(m-IBA) (1), Na+(m-IBA) (2), K+(mIBA) (3), Rb+(m-IBA) (4), and Cs+(m-IBA) (5) showed a phase transition from solid to mesophase. The optically isotropic (M1) mesophase was identified for Li+(m-IBA), while the optically anisotropic (M2) mesophases were observed in the other dehydrated crystals. The molecular arrangements and intermolecular interactions of these crystals were evaluated B

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The Journal of Physical Chemistry B Table 1. Crystal Data, Data Collection, and Reduction Parameters 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 reflections measured independent reflections reflections used Rint R1a Rw(F2)a GOF a

1·(H2O)

2·(H2O)

3·0.5(H2O)

4·(H2O)

5·(H2O)

C7H6O3LiI 271.97 P-1 (#2) 6.6506(2) 6.8955(2) 18.4951(4) 81.559(2) 84.290(1) 84.975(2) 832.52(3) 4 110 2.170 299.052 9005 2964 2593 0.1543 0.0762 0.0932 1.301

C7H6O3NaI 288.02 P-1 (#2) 6.8620(2) 7.0267(5) 18.4262(4) 84.110(1) 82.710(8) 89.611(1) 876.62(4) 4 107 2.182 289.135 10111 3149 3149 0.1687 0.0825 0.2141 0.993

C7H5O2.5KI 295.12 P-1 (#2) 6.3689(2) 7.4984(2) 18.6977(4) 83.832(2) 83.446(2) 84.606(2) 878.93(4) 4 123 2.230 325.376 9445 3131 3131 0.1647 0.0709 0.1974 1.071

C7H6O3RbI 350.49 P21/c (#14) 19.6879(3) 4.0010(2) 12.0832(2)

C7H6O3CsI 397.93 P21/c (#14) 19.6351(3) 4.1386(1) 12.4789(2)

105.8050(10)

106.518(1)

915.82(2) 4 110 2.542 336.288 14345 1684 1684 0.1505 0.0559 0.1445 1.088

972.22(4) 4 120 2.718 543.871 9274 1756 1756 0.1433 0.1028 0.2480 1.063

R1 = ∑∥Fo| − |Fc∥/∑|Fo| and Rw = (∑ω(|Fo| − |Fc|)2/∑ωFo2)1/2.

III. RESULTS AND DISCUSSION The mesophase formation for 2−5 was confirmed by polarized optical microscopy. Although the mesophase of 5 has not been previously reported,17 our data showed its formation in a narrow temperature range. 1 showed an M1 mesophase under the cross-Nichol optical arrangement before crystal melting. The mesophase formation in these simple 1:1 organic salts is associated with the elimination of the crystallization H2O molecules from the hydrated crystals. The thermal properties of the H2O elimination, crystal structures, and dielectric responses of 1−5 were discussed in terms of the size effect of the alkali metal ions on their intermolecular interactions. A. Thermal Stability and Phase Transition. The H2O molecules were crystallized in all the studied crystals. The crystal formula and the thermal stability of 1·(H2O), 2·(H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O) were evaluated by TG and DSC measurements; the results are shown in Figure 1. A weight loss was observed in all crystals in the temperature above 300 K. The weight loss of 6.80 and 6.14% at 450 K of 1· (H2O) and 2·(H2O), respectively, is consistent with the calculated weight of one H2O molecule (6.60 and 6.14%). The weight loss of 3.45% of 3·0.5(H2O) at 470 K is also in agreement with the calculated weight of 0.5 H2O (3.5%). The calculated weights of 5.14 and 4.52% corresponding to one H2O molecule of 4·(H2O) and 5·(H2O) are in line with the observed values of 5.33% (at T = 450 K) and 4.40% (at T = 430 K), respectively. An increase in the temperature (up to ∼470 K) resulted in H2O elimination in all the hydrated crystals. Among these, the thermal stability of 3·0.5(H2O) was the highest because of the difference in the H2O coordination environment around the K+ ion. Hydrated crystals 1·(H2O), 2· (H2O), 4·(H2O), and 5·(H2O) showed a one-step H2O elimination around 450, 450, 450, 450, and 430 K, respectively, whereas a two-step H2O elimination was observed for 3· 0.5(H2O) around 470 K. This suggests that two types of H2O coordination environments exist for 3·0.5(H2O). In particular, the H2O elimination temperature of 3·0.5(H2O) of around 470

Figure 1. Thermal properties of M+(m-IBA) salts (M+ = Li+, Na+, K+, Rb+, and Cs+). (a) TG diagram of hydrated crystals 1·(H2O), 2· (H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O) up to 600 K. (b) DSC diagram of dehydrated crystals 1 (i), 2 (ii), 3 (iii), 4 (iv), and 5 (v) in the second heating and cooling cycles. Crystallization H2O molecules were completely eliminated from the crystals. M1 and M2 indicate the optically isotropic and anisotropic mesophases, respectively.

K is about 20 K higher than that of the other crystals, indicating a strong H2O coordination to the K+ cation. The decomposition temperatures of unhydrated crystals 1, 2, 3, 4, and 5 were 650, 630, 610, 590, and 527 K, respectively, indicating that the thermal stability of the M+(m-IBA) salts decreases in the following order: Li+, Na+, K+, Rb+, and Cs+. The phase-transition behavior of dehydrated crystals 1−5 was examined by DSC measurements; the first heating process showed endothermic peaks corresponding to the H2O elimination (Figure S3). Table 2 summarize the phasetransition temperatures of unhydrated crystals 1−5. The DSC C

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and S2 phases of unhydrated crystal 1 under the crossed Nichol optical arrangement. In contrast, the M1 phase of crystal 1 did not show birefringence, but the crystal shape was clearly confirmed without the polarizer (Figure 2d). Because the M1 phase could not be assigned to the typical liquid-crystalline phase, the mesophase of crystal 1 was assumed to be an optically isotropic crystalline rotator phase; i.e., the optical birefringence in the M1 phase of crystal 1 could not be observed because of the molecular rotation of the m-IBA anions. B. Crystal Structures. Single-crystal X-ray structural analyses were successfully carried out for hydrated crystals 1· (H2O), 2·(H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O). Crystals 1·(H2O), 2·(H2O), and 3·0.5(H2O) were isostructural, with the space group being triclinic P-1; crystals 4·(H2O) and 5·(H2O) were also isostructural, showing a monoclinic P21/c space group. As a result of the replacement of the alkali metal ions, from Li+, Na+, and K+, the unit cell volumes (V) of crystals 1· (H2O), 2·(H2O), and 3·0.5(H2O) increased in the following order: 1·(H2O) (V = 832.52(3) Å3), 2·(H2O) (V = 874.74(6) Å3), and 3·0.5(H2O) (V = 878.93(4) Å3). A further increase in the V values was observed for crystals 4·(H2O) (V = 915.82(2) Å3) and 5·(H2O) (V = 972.22(4) Å3). Figure 3 displays the crystal structure of 1·(H2O). Although the liquid-crystalline state of unhydrated crystal 1 could not be established before crystal melting, we assumed it to be an isotropic rotator phase from the optical microscopic images. Two types of m-IBA (A and B), two lithium ions (Li1+ and Li2+), and two H2O molecules (O5 and O6) constitute the crystallographically independent structural units of crystal 1· (H2O). Figure 3a shows the unit cell of crystal 1·(H2O) along the b-axis. Alternate layers of m-IBA anions and Li+∼O cations elongate along the c-axis. The m-IBA anions form a head-tohead arrangement along the c-axis. Figure 3b shows the πstacking of the anion layer within the a + b + c plane. The πplanes of the m-IBA anions are stacked along the a-axis in an ∼A∼B∼A∼B∼ arrangement, with the mean interplanar distances of the C6-planes for A∼B and B∼A′ pairs being 3.89 and 3.00 Å, respectively. The m-IBA anions A and B form a twisted π-stacked dimer at an angle of 76.1° along the b-axis. Although the number of H2O molecules in 3·0.5(H2O) is a half of that in 2·(H2O), their crystal structures were found to be isostructural. The M2 mesophase was observed in unhydrated crystals 2 and 3, which show M2−I phase-transition temperatures above 630 and 610 K, respectively. Considering that the single crystal X-ray structural analysis of 2·(H2O) has been already reported,17 we focused on the crystal structure of 3· 0.5(H2O) (Figure 4). Both the b- and c-axes of 3·0.5(H2O) are

Table 2. Phase-Transition Temperatures of Unhydrated Crystals 1−5 salt

M+

1

Li+

2

Na+

3

K+

4

Rb+

5

Cs+

TS−S, Ka ΔH, kJ mol−1

TS−M, Ka ΔH, kJ mol−1

TM−I, Ka ΔH, kJ mol−1

463 (463) 1.07

530 (503) 5.28 553 (533) 11.6 539 (530) 9.54 515 (501) 10.3 510 (483) 13.4

608 (599) 1.51 630 (dec) 610 (dec) 590 (dec) 527 (528) 2.46

a

In parentheses, the transition temperature during the cooling processes. The increase in the temperature of the liquid-crystalline phase led to the decomposition of crystals 2−4. Average transition enthalpy of the heating and cooling processes.

peaks of dehydrated crystals 1−5 were reversibly observed in the second temperature cycle (Figure 1b). Although the mesophases of 2−4 were identified as SmA phase in previous studies,17 we did not observed the typical textures of the SmA phase, such as fan-shape, focal conic, and batonnets for dehydrated crystals. These may form a more highly ordered mesophase than the SmA phase under dehydrated conditions. The M1 mesophase of dehydrated crystal 1 did not show optical anisotropy; in contrast, dehydrated crystals 2, 3, 4, and 5 showed a phase transition from solid to M2 mesophase at 551, 534, 515, and 505 K, respectively, during the heating process (Figure 2a). Because the phase-transition temperatures from solid to M2 mesophase are higher than those of H2O elimination, dehydrated crystals 2−5 formed the M2 mesophase. The phase-transition temperatures from liquid crystal to isotropic liquid (I) of 2, 3, 4, and 5 were observed at >630 (decomposition), >610 (decomposition), >590 (decomposition), and 527 K, respectively, following the decreasing order of Na+, K+, Rb+, and Cs+. The strength of the intermolecular interactions in these molecular assemblies decreased as the size of the alkali metal ion increased. Unhydrated crystal 1 showed a three-step phase transition at 463 K (S1−S2), 530 K (S2−M1), and 608 K (M1−I) during the heating process; this was reversibly observed at 599 K (I− M1), 503 (M1−S2), and 463 K (S2−S1) during the cooling process. A large thermal hysteresis was observed at the S2−M1 and M1−I phase transitions. Figures 2b and 2c show the polarized optical microscopy images of the S2 and M1 phases of crystal 1, respectively. Birefringence was observed in the S1

Figure 2. Polarized optical microscopy images of unhydrated crystals 1 and 2. (a) Optically anisotropic (M2) mesophase of crystal 2 at 558 K. (b) S2 phase of crystal 1 at 483 K. (c) Optically isotropic (M1) mesophase of crystal 1 at 550 K under the crossed Nichol optical arrangement and (d) that without a polarizer. D

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Figure 3. Crystal structure of hydrated crystal 1·(H2O). (a) View of the unit cell along the b-axis. Green, red, and blue atoms are Li+, O, and I, respectively. (b) Two-dimensional π-stacked layer of m-IBA anions within the a + b + c plane.

Figure 4. Crystal structure of hydrated crystal 3·0.5(H2O). (a) View of the unit cell along the a-axis. Crystallographically independent m-IBA ligands (A and B) are stacked along the b-axis. (b) π-Stacked cation layer within the ab plane. Green, red, and blue atoms are K+, O, and I, respectively.

Figure 5. Crystal structure of hydrated crystal 4·(H2O). (a) View of the unit cell along the b-axis. (b) π-Stacked cation layer within the bc plane. Pink, red, and blue atoms are Rb+, O, and I, respectively.

longer than those of 2·(H2O) due to the larger cation size of K+ compared to that of Na+. However, the a-axis (6.3689(2) Å) of 3·0.5(H2O) is 0.3−0.4 Å shorter than that of 1·(H2O) and 2· (H2O) (a = 6.6506(2) Å and a = 6.8673(3) Å, respectively). Two types of m-IBA anions (A and B), two potassium ions (K1+ and K2+), and one H2O molecule (O5) constitute the crystallographically independent structural unit of 3·0.5(H2O). Figure 4a shows the unit cell of 3·0.5(H2O) along the a-axis. Alternate layer structures of m-FBA anions and inorganic K+∼O networks elongate along the c-axis. The m-IBA anions form a head-to-head arrangement along the c-axis. Figure 4b shows the π-stacking arrangement of the m-IBA anions within the ab plane. The π-planes of the m-IBA anions are stacked along the b-axis according to an ∼A∼B∼A∼B∼ arrangement. The mean interplanar distances of the C6-planes for A∼B and A∼B′ pairs are 3.55 and 3.36 Å, respectively.

One H2O molecule is included in isostructural hydrated crystals 4·(H2O) and 5·(H2O), with the space group being P21/c. While V of crystal 5·(H2O) is larger than that of 4· (H2O), its a-axis is slightly shorter, i.e., a = 19.6351(3) Å to compare with a = 19.6879(3) Å. Alternate organic−inorganic layer structures were found in both the crystals along the a-axis. The smaller size of the Rb+ ion (compared to that of Cs+) led to a decrease in the interlayer cation−anion spacing along the a-axis (Figure 5a). Figure 5b shows the m-IBA arrangement within the bc plane of 4·(H2O); in addition, a regular π-stacked structure was observed along the b-axis with a mean interplanar distance of the C6-planes of 3.644 and 3.661 Å in 4·(H2O) and 5·(H2O), respectively. The iodine atoms of the m-IBA anions are oriented along the same direction within the π-stacking column and the bc plane. The zigzag arrangement of the m-IBA anions in 4·(H2O) and 5·(H2O) differs from that observed in 1· E

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Figure 6. Temperature-dependent powder X-ray diffraction patterns. (a) Crystal 1·(H2O) at 300 K (i), simulated diffraction pattern from singlecrystal X-ray structural analysis (ii), dehydrated S1 phase at 450 K (iii), S2 phase at 500 K (iv), and M1 phase at 570 K (v). (b) Diffraction patterns of optically anisotropic mesophases of 2 at 616 K (i), 3 at 593 K (ii), 4 at 603 K (iii), and 5 at 525 K (iv).

(H2O), 2·(H2O), and 3·0.5(H2O). The Rb+∼O and Cs+∼O interactions led to the formation of the inorganic layers within the bc plane of 4·(H2O) and 5·(H2O). C. High-Temperature Mesophases. While dehydrated crystals 2−5 showed a phase transition from solid to M2 mesophase, the M1 mesophase was observed in crystal 1 at the temperature range of 530−610 K. The increase in the temperature during the dehydration processes was crucial for the formation of the mesophases. Figure 6a displays the temperature-dependent powder X-ray diffraction (PXRD) patterns of hydrated crystal 1·(H2O) at 300 K and those of its dehydrated form 1 at 450, 500, and 570 K. The PXRD pattern of 1·(H2O) only slightly differs from the one calculated from the single-crystal X-ray structural analysis (ii in Figure 6a). The intense diffraction at 2θ = 4.82° with a periodicity of 18.35 Å is consistent with c = 18.4951(4) Å of 1·(H2O) at T = 110 K. Although the PXRD pattern of dehydrated crystal 1 at 450 K showed a broadening of the diffraction peaks at 2θ ∼ 25° (iii in Figure 6a), the low-angle weak diffraction at 2θ = 5.30° corresponds to the interlayer spacing of c = 16.68 Å. The cation−anion alternate-layer structure is expected to be maintained even in dehydrated crystal 1. A similar interlayer spacing was observed in the dehydrated S2 phase (iv in Figure 6, T = 500 K) and M1 phase (v in Figure 6a, T = 570 K) at 2θ = 5.72° and 5.70°, respectively, which corresponds to c values of 15.45 and 15.40 Å, respectively. The difference between the PXRD pattern of the M1 phase of crystal 1 and that of the smectic liquid-crystalline phase (i.e., multiple reflections onto the broad diffraction peak around 2θ = 22°) indicates an optically isotropic plastic-crystalline phase. Figure 6b shows the PXRD patterns of the M2 mesophase of crystals 2 at 616 K (i), 3 at 593 K (ii), 4 at 603 K (iii), and 5 at 525 K (iv). Hydrated crystals 2·(H2O) and 3·0.5(H2O) show the layer structure along the c-axis, whereas the layer of isostructural 4·(H2O) and 5·(H2O) elongates along the a-axis. The layer periodicities of 2·(H2O), 3·0.5(H2O), 4·(H2O), and 5·0.5(H2O) at 100 K were determined to be 18.4262(4), 18.6977(4), 19.6879(3), and 19.6351(3) Å, respectively. After H2O elimination, the cation−anion layer periodicities were

virtually unchanged due to the appearance of a sharp (001) reflection (Figure S4). The XRD patterns of mesophases 2−5 differ from the typical smectic liquid-crystalline state. All PXRD patterns showed relatively sharp diffraction patterns, which were assigned to the interlayer periodicity of (00l). The interlayer periodicities of mesophases 2, 3, 4, and 5 were determined to be 16.23, 17.01, 16.67, and 16.67 Å, respectively; these are clearly shorter than those of the hydrated and/or dehydrated crystals. In contrast, the in-plane diffraction peaks of (hk0) were not observed in the PXRD patterns of the M2 mesophase; this suggests that the disordered state of the m-IBA ligands, which are characterized by a relatively high molecular motion, is thermally activated. The crystal periodicity of the layer structures was maintained in the high-temperature mesophases of 2−5, which showed a highly ordered molecular-assembly state as compared to the typical smectic liquid-crystalline state. While the mesophases of the M+(mIBA) salts have been identified as SmA phase by Voländer in 1927, the corresponding dehydrated salts form an optically anisotropic-layered mesophase. The phase-transition temperature is one of the most important parameters to evaluate the magnitude of intermolecular interactions. The phase-transition temperatures from solid to mesophase (M1 or M2) in crystals 1, 2, 3, 4, and 5 were determined to be 530, 553, 539, 515, and 510 K, respectively, whereas those from mesophase to I are 608 (melt) > 630 (decomposition), > 610 (decomposition), > 590 (decomposition), and 527 K (melt). The phase-transition temperatures of the salts containing Li+, Rb+, and Cs+ were found to be lower than those with Na+ and K+, confirming the strength of the Na+∼O and K+∼O interactions within the crystals. The formation of the M+∼O network structures upon H2O elimination is crucial to understand the phase-transition behavior of the mesophases. Given the difficulty with which the single-crystal X-ray diffraction data for the dehydrated crystals could be obtained, their M+∼O network structures found in the hydrated crystals are expected to be one of the fundamental structural units of the dehydrated crystals. F

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The Journal of Physical Chemistry B Figure 7 shows the M+∼O network structures of the hydrated crystals 1·(H2O), 2·(H2O), 3·0.5(H2O), and 4·

to form the M+∼O networks. In crystal 1·(H2O), both the Li1+ and Li2+ cations are coordinated by four oxygen atoms resulting in a tetrahedron-coordination environment (Figures S5 and S6 and Table S1). Because the Li1+ and Li2 + tetrahedrons are connected via the water-molecule O5 and O6 oxygen atoms along the a-axis, a one-dimensional Li+∼O network chain was obtained (Figure 7a). H2O elimination from this one-dimensional Li+∼O chain resulted in the formation of an isolated Li+∼m-IBA unit (Scheme 2). The pentacoordination of the Na+ ion with three oxygen atoms of m-IBA and two water-molecule oxygen atoms led to the formation of two types of pentacoordinated units, e.g., Na1+−(O)5 and Na2+−(O)5 (Figures S7 and S8 and Table S2). The one-dimensional Na+∼O network chains between the oxygen atoms of the m-IBA anions and Na+ ion are connected via crystallization H2O molecules in 2·(H2O) along the a-axis, forming a two-dimensional Na+∼O layer (Figure 7b). Upon H2O elimination, a two-dimensional Na+∼O layer was reconstructed that maintains the layer periodicity of ∼16.2 Å (Figure 6b and Scheme 2). The K+ ion is coordinated by four oxygen atoms of m-IBA and one water-molecule oxygen atom, leading to the formation of pentacoordinated K1+−(O)5 and K2+−(O)5 units (Figures S9 and S10 and Table S3). Because the electrostatic K+∼O interactions were mainly observed in the m-IBA ligands, the thermal stability of crystal 3·0.5(H2O) is relatively higher than those of 1·(H2O) and 2·(H2O). A double K+∼O layer structure was observed in crystal 3·(H2O), with the upper and lower twodimensional K+∼O layers being connected to both the oxygen atoms of m-IBA and H2O (Figure 7c). This suggests that this layer structure should be stable even after H2O elimination. The high thermal stability of dehydrated crystal 3 is consistent with the structurally stable two-dimensional bilayer structure, showing a layer periodicity of ∼17.0 Å (Scheme 2). The bridging H2O molecules allowed the observation of the double layer structures of Rb+∼O and Cs+∼O in crystals 4· (H2O) and 5·(H2O) (Figure 7d), our data confirming that these are isostructural. The Rb+ ion, which is connected along the b- and c-axis to form a two-dimensional Rb+∼O network within the bc plane, is coordinated by four oxygen atoms of the m-IBA anions and four H2O molecules. Figure 7d shows the

Figure 7. M+∼O network structures of (a) 1·(H2O), (b) 2·(H2O), (c) 3·0.5(H2O), and (d) 5·(H2O). Crystal structures of 5·(H2O) and 6· (H2O) are isostructural. Bright-green, blue, purple, dark-green red, and red atoms are Li+, Na+, K+, Cs+, and O, respectively.

(H2O) along a parallel (upper figure) and normal (lower figure) direction to the inorganic layer. The H2O molecules, which were eliminated from the initial M+∼O networks as a result of the temperature increase above ∼450 K, are necessary

Scheme 2. Schematic Illustration of the Fundamental Structural Units before (Left Figure for the Hydrated Crystal) and after H2O Elimination (Right Figure for the Dehydrated Crystal)

G

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Figure 8. Temperature- and frequency-dependent real part of the dielectric constant (ε1) of unhydrated crystals (a) 1, (b) 2, and (c) 4 during the heating process. The DSC diagram during the heating process is shown in the upper figure to clarify the phase-transition behaviors.

Figure 9. Nyquist plots of conductance (G/S) and susceptance (B/S) of (a) 1, (b) 2, and (c) 4 unhydrated crystals.

300 K, increases to ∼7.2 at 500 K. Although an endothermic peak was detected at 465 K in the DSC diagram, no dielectric anomaly around the S1−S2 phase transition was observed. In contrast, a significant increase of ε1 was observed in the S2−M1 phase transition of crystal 1 at ∼530 K. The ε1 values at 0.1, 1, 10, 100, and 1000 kHz for the M1 phase at 550 K were determined to be 44 000, 33 000, 16 000, 3400, and 530, respectively, i.e., about 5000 times larger than those of the S1 and S2 phases. According to the DSC diagram, the dielectric responses are reversible in the temperature cycles. The ε1 increase at low frequency is significantly larger than that at high frequency, suggesting slow-motional freedoms within the M1 phase. In addition, a large increase in the dielectric loss (ε2) was observed in the M1 phase, indicating a contribution from the conducting properties to the M1 rotator phase. Figure 9a shows a Nyquist plot of crystal 1 at 310, 552, and 596 K. Semicircular traces in the conductance−susceptance (G−B) plots were observed in the M1 phase with ionic conductivities of 1.0 × 10−5 S cm−1 (T = 552 K) and 3.0 × 10−5 S cm−1 (T = 596 K). The M1 phase of crystal 1 shows the long-range translational motion of Li+. Unhydrated crystal 2 showed the S1−M2 phase transition around 550 K; ε1 was determined to be ∼3.4 and ∼30 at about 300 and 550 K, respectively. A discontinuous and strong frequency-dependent ε1 enhancement was observed at the S1− M2 phase-transition temperature during the heating process. The reversible ε1 response in the temperature cycles is consistent with the DSC diagram. Inflection points appeared in the ε1−T plots at 560 K, and the ε1 values at measuring frequencies of 0.1, 1, 10, 100, and 1000 kHz are 57 000, 19 000,

two-dimensional Rb+∼O layer along the a-axis (upper figure) and b-axis (lower figure). Because each Rb+ ion within the bc plane is coordinated by m-IBA anions to form the twodimensional Rb+∼O network, this should not be affected by H2O elimination (Scheme 2). Similar Cs+∼O interactions were observed in crystal 5·(H2O), suggesting that the corresponding Cs+∼O networks have similar thermal properties to those of crystal 4 (Figures S11−S14 and Tables S3 and S4). While the intralayer Rb+∼O interactions are a result of the electrostatic interactions between the oxygen atoms of the m-IBA anions and Rb+, the interlayer interactions are stabilized through bridging H2O molecules. This suggests that the Rb+∼O double layers may be reconstructed after H2O elimination from the interlayer space, thereby causing no changes in the layer periodicity of ∼16.7 Å (Scheme 2). D. Dielectric Properties. Dielectric measurements are sensitive to the molecular motions of the polar structural units within molecular assemblies. The motional frequencies that range between 102 and 106 Hz can be easily detected by the ac impedance method.20 Because 1·(H2O), 2·(H2O), 3·0.5(H2O), 4·(H2O), and 5·(H2O) were unstable during the heating process due to H2O elimination, the cast films of the corresponding unhydrated crystals after the thermal annealing at 450 K were utilized for the dielectric measurements using the sandwich ITO electrode. Figures 8 displays the DSC diagram (upper figure) and the frequency-dependent real part of the dielectric constant (ε1, lower figure) of crystals 1, 2, 4, and 5 in the temperature range of 300−600 K during the heating process. The ε1 value of 2.1, which was determined for crystal 1 at a temperature of about H

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were obtained as hydrated crystals, e.g., 1·(H2O), 2·(H2O), 3· 0.5(H2O), 4·(H2O), and 5·(H2O). Crystallization H2O molecules were eliminated by increasing the temperature to 350−400 K. Unhydrated crystals 2, 3, 4, and 5 showed a phase transition from solid to M2 layered mesophase at 553, 539, 515, and 510 K, respectively; in contrast, unhydrated crystal 1 changed to a M1 mesophase above 463 K. The melting points of crystals 1 and 5 were observed at 608 and 527 K, whereas a thermal decomposition occurred for crystals 2−4 before the crystal melting point. The intermolecular interactions in the layered mesophases decreased in the order of Na+, K+, Rb+, and Cs+. The single-crystal X-ray crystal structural analysis showed a common structural property among the studied hydrated crystals, i.e., alternate cation−anion layers. The organic m-IBA anions formed a two-dimensional π-stacked layer, whereas the inorganic layers consisted of one- and/or two-dimensional M+∼O networks between the M+ ions and the oxygen atoms of m-IBA and/or H2O molecules. A systematic change of the metal cation from Li+, Na+, K+, Rb+ to Cs+ strongly influenced the resulting M+∼O network connectivity and thermal properties. A reconstruction of the inorganic M+∼O network was thus necessary to form the layered mesophase after H2O elimination from the hydrated M+∼O network structures. The temperature- and frequency-dependent ε1 and ε2 parts of the dielectric constants showed a large response to the temperatures of the phase transition from solid to mesophases; in particular, both ε1 and ε2 at 100 Hz were about 104 times larger than those at 1 MHz. The slow-motional freedom was thus confirmed in these mesophases. The semicircular traces of the Nyquist plots between the conductance and susceptance were related to the ionic conduction in the mesophases. The simple 1:1 M+(m-IBA) organic salts formed mesophases, despite the long alkyl chains. The M+(m-IBA) salts were located in the boundary between the optically isotopic crystals (M+ = Li+) and anisotropic crystals (M+ = Na+, K+, Rb+, and Cs+). Finally, our results clearly showed that the simple M+(m-IBA) organic ionic salts reported by Voländer in 1927 (without alkyl chains) can still provide critical information about the formation of mesophases.

4000, 700, and 20, respectively. The significant increase at low frequencies (e.g., 100 and 1000 Hz) is consistent with the slowmotional freedoms of the polar unit in the M2 phase. In addition, ε2 showed a significant increase around the S1−M2 phase-transition temperature; semicircular G−B traces were observed with ionic conductivities of 3.2 × 10−6 S cm−1 (T = 548 K) and 1.1 × 10−5 S cm−1 (T = 559 K), respectively (Figure 9b). Unhydrated crystals 3, 4, and 5 also showed S1− M2 phase transitions at 534, 513, and 508 K, respectively, with a large increase of the ε1 values occurring during the heating process. The ε1 value of crystal 3 was determined to be ∼2.2 and ∼2.4 at 300 and 520 K, respectively; this is a result of the rigid structural backbone of the K+∼O bilayer network. However, a strong frequency- and temperature-dependent response of ε1 was observed at the S1−M2 phase transition (temperature above 520 K); i.e., the ε1 values at frequencies of 0.1, 1, 10, 100, 1000 kHz are 24 000, 17 000, 9900, 4000, and 20, respectively. The semicircular G−B traces of the M2 phase in crystal 3 showed ionic conductivities of 3.7 × 10−6 S cm−1 (T = 548 K) and 4.7 × 10−5 S cm−1 (T = 560 K). In crystal 4, ε1 showed a temperature-independent behavior. In particular, ε1 at around 300 and 515 K is ∼1.3 and ∼1.4, respectively. However, following the inflection points at 535 K of the ε1−T plots, the ε1 values of crystal 4 around the S1−M2 phase transition (at 513 K) at frequencies of 0.1, 1, 10, 100, and 1000 kHz are 71 000, 29 000, 6500, 1100, and 180, respectively (T = 560 K). The semicircular G−B traces in the M2 phase correspond to the ionic conductivities of 3.2 × 10−5 S cm−1 (T = 524 K) and 2.6 × 10−5 S cm−1 (T = 535 K). A sudden ε1 increase was also observed in the S1−SmA phase transition (at 510 K) in crystal 5. The ε1 values at frequencies of 0.1, 1, 10, 100, and 1000 kHz are 72 400, 23 000, 1500, 600, and 1, respectively (T = 522 K), with the semicircular G−B traces in the M2 phase corresponding to an ionic conductivity of 6.2 × 10−5 S cm−1 at 507 K. Large dielectric responses were observed for the M1 and M2 mesophases of all studied crystals together with a large dielectric loss and ionic conduction. The low-frequency dielectric increase, which was typically larger than that at high frequency, confirmed the existence of slow motional freedoms. The magnitude of the frequency-dependent dielectric increase was evaluated by the ratio of ε1 values at 100 Hz and 1 MHz, e.g., ε1 (100 Hz)/ε1 (1 MHz). The ε1 (100 Hz)/ε1 (1 MHz) ratio for crystals 1, 2, 3, 4, and 5 in the mesophase is 80 (T = 550 K), 2800 (T = 590 K), 1200 (T = 555 K), 950 (T = 550 K), and 380 (T = 560 K), respectively. For the M2 phase, the ratio decreases following a cation-size order, i.e., Na+, K+, Rb+, and Cs+. This can be explained by the fact that the transport properties of the alkali metal ions in the molecular assembly depend on the mass of the cations. Although the mass of Li+ is smaller than that of Na+, the ε1(100 Hz)/ε1 (1 MHz) ratio was found to be ∼80 in the M1 phase of crystal 1, which is significantly lower than that determined for the M2 phase of crystal 2 (2800). The motional freedom of Li+ in the optically isotropic rotator phase is thus reduced in contrast to that of Na+ in the layered mesophase.

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ASSOCIATED CONTENT

S Supporting Information *

Atomic-numbering scheme, structural analysis, IR spectra, DSC chart, temperature-dependent PXRD, and temperature-dependent polarized optical microscopy images of crystals 1−5. 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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IV. CONCLUDING REMARKS The phase-transition behaviors, crystal structures, and dielectric properties of simple 1:1 organic salts of alkali metal ions (M+ = Li+, Na+, K+, Rb+, and Cs+) and m-IBA anions were examined in terms of the size effect of the M+ ions on the molecularassembly structures and physical properties. All single crystals

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 the Management Expenses Grants for National Universities of Japan. I

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REFERENCES

(1) Wittem, T. A.; Pincus, P. Structured Fluids, Polymers, Colloids, Surfactants; Oxford University Press: Oxford, 2004. (2) Supramolecular Organization and Materials Design; Jones, W., Rao, C. N. R., Eds.; Cambridge University Press: Cambridge, 2002. (3) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Recent progress in understanding hydrophobic interactions. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15739−15746. (4) Stryer, L. Biochemistry; Freeman: New York, 1995. (5) Advances in Protein Chemistry ; Baldwin, R. L., Baker, D., Eds.; Elsevier Academic Press: Amsterdam, 2006; Vol. 72. (6) The Amide Linkage; Greenberg, A., Breneman, C. M., Liebman, J. F., Eds.; Wiley-Interscience: Hoboken, NJ, 2003. (7) Collings, P. J.; Hird, M. Introduction to Liquid Crystals, Chemistry and Physics; Gray, G. W., Goodby, J. W., Fukuda, A., Eds.; Taylor & Francis: London, 1997. (8) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: New York, 1992. (9) Kumar, S. Chemistry of Discotic Liquid Crystals; CRC Press: New York, 2011. (10) The Plastically Crystalline State; Sherwood, J. N., Ed.; John Wiley & Sons: Chichester, 1979. (11) Chang, S.-S.; Westrum, E. R., Jr. Heat capacities and thermodynamic properties of globular molecules. I. Adamantane and hexamethylenetetramine. J. Phys. Chem. 1960, 64, 1547−1551. (12) Nordman, C. E.; Schmitkons, D. L. Phase transition and crystal structures of adamantine. Acta Crystallogr. 1965, 18, 764−767. (13) Suga, H.; Seki, S. Frozen-in states of orientational and positional disorder in molecular solids. Faraday Discuss. Chem. Soc. 1980, 69, 221−240. (14) Kahn, R.; Fourme, R.; Andri, D.; Renaud, M. Crystal structure of cyclohexane I and II. Acta Crystallogr., Sect. B 1973, 29, 131−138. (15) Vorländer, D. Verhalten der Salze Organischer Säuren beim Scmelzen. Ber. Dtsch. Chem. Ges. 1910, 43, 3120−3135. (16) Bruce, D. W.; Heyns, K.; Vill, V. Vorlander’s wheel. Liq. Cryst. 1997, 23, 813−819. (17) Deun, R. V.; Ramaekers, J.; Nockemann, P.; Hecke, K. V.; Meervelt, L. V.; Binnemans, K. Alkali-metal salts of aromatic carboxylic acids: Liquid crystals without flexible chains. Eur. J. Inorg. Chem. 2005, 563−571. (18) Crystal Structure: Single-Crystal Structure Analysis Software, Ver. 3.6; Rigaku Corporation and Molecular Structure Corporation, 2004. (19) Sheldrick, G. M. SHELX97 Programs for Crystal Structure Analysis; Universität Göttingen: Göttingen, Germany, 1998. (20) Kao, K. C. Dielectric Phenomena in Solids; Elsevier: Amsterdam, 2004.

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