Anisotropic Proton Conductivity Arising from Hydrogen-Bond Patterns

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Anisotropic Proton Conductivity Arising from Hydrogen-Bond Patterns in Anhydrous Organic Single Crystals, Imidazolium Dicarboxylates Yoshiya Sunairi, Akira Ueda, Junya Yoshida, Keisuke Suzuki, and Hatsumi Mori J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00814 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry

Anisotropic Proton Conductivity Arising from Hydrogen-Bond Patterns in Anhydrous Organic Single Crystals, Imidazolium Dicarboxylates Yoshiya Sunairi, Akira Ueda*, Junya Yoshida, Keisuke Suzuki and Hatsumi Mori* The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan

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ABSTRACT Organic acid-base salts have attracted increasing attention as a promising candidate of anhydrous proton conductors. In this study, we have successfully disclosed the relationship between proton conductivity and hydrogen-bond (H-bond) interactions in such kinds of organic salts, composed of dicarboxylic acid and imidazole. We have grown high-quality single crystals of imidazolium succinate (Im-Suc) or glutarate (Im-Glu) with two-dimensional H-bonding networks and measured the proton conductivity within and perpendicular to the networks. On the basis of the observed “intrinsic” proton conductivities without grain boundary contributions, their relationship to the crystal structure and molecular arrangement was investigated in detail. Importantly, in both materials, the proton conductivities within the H-bonding networks are almost two orders of magnitude higher than those perpendicular to the networks, demonstrating that the proton conduction is highly mediated by the H-bonds. In addition, a suitable combination and arrangement of acid and base molecules for realizing high proton conductivity is discussed in terms of their proton donating/accepting abilities (pKa) and molecular motions. These results provide important insights into the effects of H-bonds on proton conductivity in this kind of anhydrous organic acid-base salts.

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1. INTRODUCTION Proton conduction is a fundamentally important phenomenon not only in living systems but also in solid materials, such as fuel cells. Various kinds of basic and applied studies related to this phenomenon have been performed in a wide range of scientific fields, such as chemistry, physics, biology, and engineering.1–5 In particular, the development of high proton conducting materials and the elucidation of the conducting mechanism are one of pivotal issues in this research field. For example, hydrogen sulfates such as CsHSO4 are known to show a significantly high proton conductivity close to 10–1 S cm–1 at high temperatures (~135 °C),6–10 for which the reorientation of the sulfate ions plays an important role.11–13 In addition to such inorganic acid salts, organic polymers with sulfonic or phosphoric acid side chains, such as Nafion®, are also good proton conducting materials (conductivity () ~ 10–1 S cm–1)14–19 and thus practically used in various devices. In these materials, protons are believed to be effectively transported together with water molecules through the sulfate groups introduced in the polymer side chains; however, the detailed mechanism is still under discussion. Under this situation, recently, as a new class of proton conductors, molecule-based crystalline materials have attracted attention.20–53 Their crystalline nature would allow a more detailed study on the structure-property relationship and the proton conduction mechanism. In addition, unique assembled structures, dynamic behavior, and proton conducting properties derived from a wide variety of molecules are expected. For example, by utilizing the proton donating/accepting abilities and rotational motions of a kind of heterocyclic compounds, imidazole, Kitagawa, S. et al. have realized a significant improvement of proton conductivity in a metal-organic framework (MOF)-based material.54, 55 Furthermore, MacDonald, J. C. et al. and Pogorzelec-Glaser, K. et al. have developed hydrogen-bonded (H-bonded) anhydrous purely organic proton conductors 2 Environment ACS Paragon Plus


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consisting of imidazole and dicarboxylic acids, (HOOC)(CH2)n(COOH) (Figure 1).56–58 Despite the fact that no water molecules are included, these acid-base co-crystals show relatively high proton conductivity ( ~10–7–10–3 S cm–1).57, 58 In particular,  of the succinate (n = 2) salt, ImSuc, reaches 10–3 S cm–1 at around 400 K in the compressed pellets.58

H

N

N

O

H HO

Imidazolium (Im)

O n

O

n = 2: Succinate (Suc) n = 3: Glutarate (Glu)

Figure 1. Chemical structure of the imidazolium dicarboxylates described in this paper.

Our group has studied “electrical” conductivity of H-bonded organic charge-transfer complex crystals.59–69 Among them, we have found that the electrical conductivity can be switched by hydrogen dynamics in the intermolecular H-bonds (i.e., so-called order–disorder transition).70–74 Also, in H-bonded organic dielectric crystals, a dielectric response due to the shift of the Hbonded proton has been investigated.75, 76 These structure-property studies related to the hydrogen dynamics in organic crystals have encouraged us to investigate the effect of H-bonds on the high “protonic” conductivity in the above-mentioned imidazolium dicarboxylates. Therefore, in this study, we have grown high-quality single crystals of imidazolium succinate (Im-Suc) and its analogue salt, imidazolium gutarate (Im-Glu) (Figure 1), and performed the ac conductivity measurements for several directions of the crystals. As a result, “intrinsic” proton


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conductivity of these salts without the contributions of grain boundaries was successfully revealed for the first time. On the basis of these data, we have investigated and discussed the relationship between the proton conductivity and crystal structure in detail, especially focusing on the proton donating/accepting abilities (pKa) and molecular motions of the H-bonded molecules. These results provide important insights into the effects of H-bonds on proton conductivity in this kind of anhydrous organic acid-base salts.

2. EXPERIMENTAL SECTION 2.1. Preparation of Single crystals Single crystals of Im-Suc and Im-Glu were prepared by the following method based on that described in the literature.56, 57 All chemicals were commercially available and used as received. Im-Suc: Imidazole [681 mg, 10 mmol, Wako Pure Chemical Industries (purity > 98 %)] and succinic acid [1181 mg, 10 mmol, Wako Pure Chemical Industries (purity > 99.5 %)] were suspended in a mixed solvent of dehydrated methanol [25 mL, Wako Pure Chemical Industries (purity > 99.8 %)] and dehydrated acetonitrile [25 mL, Wako Pure Chemical Industries (purity > 99.8 %)]. This suspension was stirred at 40 °C until all the materials were dissolved (about 10 min). The resulting solution was slowly evaporated for 3 days in an incubator (35 °C), and then the resulting precipitates were collected by filtration and washed with ethyl acetate, to give the desired material as colorless crystals. Im-Glu: Colorless crystals of Im-Glu were obtained by the same method as Im-Suc, by using imidazole [681 mg, 10 mmol, Wako Pure Chemical Industries (purity > 98 %)] and glutaric acid [1321 mg, 10 mmol, Tokyo Chemical Industry (purity > 99 %)].



2.2. Sample Characterization The crystal structures of Im-Suc and Im-Glu prepared above were identified56, 57 by X-ray diffraction analyses using a Rigaku Mercury II diffractometer with Mo-Kα radiation ( = 0.71073 Å) at room temperature. The crystallographic orientation of Im-Suc and Im-Glu was determined by using a four-circle X-ray diffractometer (Rigaku AFC-7R) with Mo-Kα radiation ( = 0.71073 Å) at room temperature. Thermal stability of the crystals of Im-Suc and Im-Glu was evaluated by means of differential scanning calorimetry (DSC) using a Netzsch DSC 200 F3-T21 Maia calorimeter with an Al as a reference. The sample (about 7 mg) was encapsulated in an Al pan and heated from 20 °C to 300 °C with a rate of 5 °C min–1 under nitrogen. The results are given in the Supporting Information (Figure S1). Impedance spectroscopy measurements were carried out by the two-probe method using a Solartron Impedance Analyzer SI 1260 and Dielectric Interface 1296. As-grown single crystals of Im-Suc and Im-Glu were cut into blocks (typical size of 0.4 × 0.4 × 0.5 mm3 and 0.2 × 1.4 × 1.7 mm3, respectively) to expose the desired planes for the anisotropic measurements (see Figure 3a). The probes were separately attached to the two opposite sides by using silver paste and gold wires (15 m diameter), (Note: This combination (silver paste and gold wires) is one of the typical ones to make the electrical contact for measuring the ac conductivity of solid materials, including proton conductors.77, 78). The complex impedance of the samples was measured from 1 Hz to 1 MHz in the temperature range of 59–94 °C for Im-Glu and 103–116 °C for Im-Suc, where the temperature was controlled by using a homebuilt high-temperature cryostat. The upper limit temperature of each compound (94 °C for Im-Glu and 116 °C for Im-Suc) was set to the


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temperature at which the endothermic reaction (or melting) started to appear in the DSC curve (see Figure S1). In fact, at temperatures over the upper limits, the Debye-type Cole-Cole plots were partially deformed (Figure S2).

3. RESULTS 3.1. Imidazolium Succinate (Im-Suc) In this study, we have successfully grown large, high-quality single crystals of imidazolium succinate (Im-Suc) with a typical size of 0.5 × 0.7 × 1.0 mm3 by modifying the literature method56 (see the experimental section). Then, the single-crystal proton conductivity measurements were performed in three directions, as shown below. Here we note that this kind of single-crystal measurements is known to provide “intrinsic” proton conductivity of the sample itself,24, 29, 33, 34, 37, 79–81 in contrast to the measurements using compressed pellet samples which inevitably include grain boundary effects.82, 83 First, we explain the crystal structure of this salt [Figure 2, at 24 °C (297 K)]. As shown in Figure 2a and b, this crystal is composed of two-dimensional (2D) sheet structures, in which the imidazolium cations (Im) and succinate anions (Suc) are connected with hydrogen bonds (Hbonds).56 Two N–H moieties of the Im molecule are connected to carbonyl oxygen atoms of the Suc molecule with dNO of 2.72 Å (red dashed lines) and 2.87 Å (brown dashed lines) and also form short contacts with the adjacent oxygen atoms of the Suc molecule with dNO ~ 3.05, 3.15 Å (blue dashed lines), like bifurcated H-bonds. In addition, a C-H···O short contact (gray dashed lines, dCO ~ 3.08 Å) is observed between the Im and Suc molecules. Furthermore, there is an OH···O H-bond between the Suc molecules (green dashed lines, dOO ~ 2.49 Å). On the other hand,



although the shortest C···C distance (3.39 Å, black arrows) between the networks is less than the sum of van der Waals radii (3.40 Å), no effective H-bonds are found between these 2D networks (Figure 2c). The shortest N···O distances between the networks are 3.50 and 3.56 Å (blue and red arrows) which are much longer than the sum of van der Waals (vdW) radii of N and O atoms (3.07 Å).84


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a)

b)

c)

Figure 2. Crystal structure of Im-Suc56: a) Two-dimensional (2D) H-bonded sheet structure (red, blue, light blue, and brown dashed lines: N–H···O H-bonds, green dashed lines: O–H···O Hbonds, gray dashed lines: C–H···O contacts), b) N–H···O H-bonds around Im molecules, and c)



stacking arrangement of the 2D H-bonded networks. Black, blue, and red arrows in c) represent short C···O, N···O, and N···O contacts, respectively. Proton conductivity measurements of ImSuc (Figures 3, 4, Table 1) were carried out in the [1 0 0], [1 4 –9], and [0 1 1] directions (shown in a) and c)).

The relationship between the crystal shape and crystallographic axes was determined by X-ray diffraction measurements (Figure 3a), where the [1 0 0] and [1 4 –9] directions are within the 2D network (Figure 2a) and the [0 1 1] is perpendicular to the 2D network (Figure 2c). Therefore, by measuring and comparing the proton conductivity in these three directions, we have revealed the effects of the H-bonding interactions and molecular arrangement on the proton conduction. Figures 3b, c show the results of single-crystal impedance measurements of Im-Suc in the [1 0 0] direction at several temperatures below the melting starting point estimated by DSC (see the experimental section). The complex impedance planes (Cole-Cole plots) (Figure 3b) gave almost perfect semicircle profiles at each temperature, in contrast to the distorted semicircles seen in the pellet measurements by Pogorzelec-Glaser et al.58 and also by us (Figure S3). This result means that a single Debye-type relaxation occurs in our single-crystal measurements and thus the obtained conductivity is the intrinsic proton conductivity without contributions from the grain boundaries. The semicircles contract with increasing temperature (Figure 3b), which corresponds to the shortening of the dielectric relaxation time. Then we estimated the ac conductivity ( ac) of Im-Suc in the [1 0 0] direction from the obtained ac impedances. Figure 3c shows the frequency dependences of  ac at the temperatures. In the low frequency region (below 10 Hz),  ac is frequency-independent, thus corresponding to the dc conductivity. Therefore, we defined the  ac at 1 Hz as the proton conductivity  of Im-Suc. This type of almost perfect semicircle plots was


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also obtained in the measurements along the [1 4 –9] and [0 1 1] directions (Figure S4), which proves that the “intrinsic” proton conductivity was successfully obtained in all the three directions of the Im-Suc single crystals.



a)

b) -12.0x10

9

Z'' ()

105 °C 108 °C 111 °C 113 °C 115 °C

[1 0 0] Rcon

-9.0

Rsam

CPEsam

-6.0

-3.0

0.0 0.0

c)

10 10 -1

ac (S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 10 10 10

-4

3.0

105 °C 108 °C 111 °C 113 °C 115 °C

-5

-6

6.0

Z' ()

9.0

12.0x10

9

[1 0 0]

-7

-8

-9

10

0

10

1

2

10 10 Frequency (Hz)

3

10

4

Figure 3. a) A photograph of a single crystal of Im-Suc showing the relationship between the crystal shape and crystallographic axes. b) Debye-type Cole-Cole plots (inset: the equivalent 11 Environment ACS Paragon Plus

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circuit model used for theoretical fitting) and c) frequency dependence of ac conductivity measured for a single crystal of Im-Suc along the [1 0 0] direction at several temperatures below 116 °C (the melting starting point). Circles in b) and c) represent the experimental data and solid lines in b) denote the theoretical fitting curves obtained by using the equivalent circuit model shown in the inset of b), which consists of one series resistance Rcon and one parallel combination of a single resistance Rsam and constant phase element CPEsam.

Then, we compare the proton conducting properties in these three directions (Figure 4, Table 1). First, the proton conductivity  in the directions parallel to the H-bonding network ([1 0 0] and [1 4 –9]) and that in the direction perpendicular to the network ([0 1 1]) are compared (Note:

 [0 1 1] less than 108 °C was not estimated, owing to the large measurement noise resulting from the large impedance). As summarized in Table 1, at around 115 °C, the former two directions ([1 0 0] = 4.94 × 10–7 S cm–1 and [1 4 –9] = 1.27 × 10–7 S cm–1) provide almost two orders higher conductivity than the latter one ([0 1 1] = 3.55 × 10–9 S cm–1). This large difference is maintained in the present temperature range (Figure 4), indicating that the proton conduction occurs more readily in the H-bonding network than in its perpendicular direction. Here, we note that these  values are certainly low compared to that of water-containing materials, such as Nafion® and some kinds of metal organic frameworks (MOFs) (≥ 10–3 S cm–1)14–19, 26–29, 32, 33; nevertheless, the present  values (up to 5 × 10–7 S cm–1) are ranked as a top-class  in anhydrous organic single crystals.48, 49 In addition, it is of great interest that this high  was realized by its rapid increase (two orders of magnitude) above 108 °C (Figure 4). Although the  values of this material at further high temperatures was not evaluated (owing to the starting of melting), our



results suggest that further high , comparable to that of Nafion® and MOFs (~ 10–3 S cm–1), can be obtained in these anhydrous organic materials by enhancing their thermal stability.

115

-6.0

T (℃)

110

105

100

[1 0 0] [1 4 -9] [0 1 1]

-7.0 -1

log( / S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-8.0

-9.0

-10.0 2.56

2.60

2.64

2.68

-1

1000/T (K )

Figure 4. Arrhenius plots of the proton conductivity () for a single crystal of Im-Suc measured along the [1 0 0] (orange circles), [1 4 –9] (blue circles), and [0 1 1] (black circles) directions, respectively. The theoretical fitting lines for the [1 0 0], [1 4 –9], and [0 1 1] directions are denoted in gray (solid line; 103–108 °C, dashed line; 108–115 °C), black (solid line; 102–108 °C, dashed line; 108–115 °C), and red (108–116 °C), respectively ( [0 1 1] less than 108 °C was not estimated, owing to the large measurement noise resulting from the large impedance).

Table 1. Proton conductivities () and activation energies (Ea) of Im-Suc single crystal measured along the [1 0 0], [1 4 –9], and [0 1 1] directions.


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Direction [1 0 0] (parallel to H-bonding network) [1 4 –9] (parallel to H-bonding network) [0 1 1] (perpendicular to Hbonding network)

Proton conductivity 

Activation energy Ea

(S cm–1)

(eV)

4.94 × 10–7 [115 °C]

1.27 × 10–7 [115 °C]

3.55 × 10–9 [116 °C]

3.32 [103–108 °C], 6.24 [108–115 °C] 3.18 [102–108 °C], 6.79 [108–115 °C]

3.92 [108–116 °C]

Next,  in the two directions parallel to the H-bonding network ([1 0 0] and [1 4 –9]) are compared. At 115 °C, the [1 0 0] direction has about four times higher (4.94 × 10–7 S cm–1) than the [1 4 –9] direction (1.27 × 10–7 S cm–1) (Table 1) and this trend is unchanged in this temperature region (Figure 4). The difference of  between these two directions (about four times) is much smaller than that between the parallel and perpendicular directions (two orders) shown above. However, the following section reveals that this small difference is significant in connection with anisotropy of the proton conduction in the 2D H-bonding network, especially in terms of the intermolecular interactions and molecular arrangement. Also, we estimated the activation energies Ea in these directions (Table 1) by the following Arrhenius equation,

 (T) = 0 exp(–

Ea ) kT

(1)



where is the proton conductivity, is the pre-exponential factor, k is the Boltzmann constant, and T is the temperature. The fitting lines for the [1 0 0], [1 4 –9], and [0 1 1] directions are shown in gray, black, and red, respectively (Figure 4). Here, we note that  in the former two directions parallel to the H-bonding network were fitted with two lines (solid and dashed ones), because of their non-linearity with a significant increase at high temperatures. On the other hand,

 in the perpendicular direction ([0 1 1]) has a linear temperature dependence, fitted with a single line. This behavior is also shown in Arrhenius plots of  (Figure S5). The estimated Ea are 3.32 eV (103–108 °C) and 6.24 eV (108–115 °C) in the [1 0 0] direction and 3.18 eV (102– 108 °C) and 6.79 eV (108–115 °C) in the [1 4 –9] direction (Table1). These values seem to be somewhat high, however, similar high Ea values are seen in imidazole-based salts57, 85, 86 and also imidazole itself.48 In addition, these non-Arrhenius-type profiles with temperature-dependent changes in Ea are probably attributed to some dynamic property changes in the H-bonding network. In addition, focusing on the relationship between the Ea and  values (Table 1), we found that the high conductive [1 0 0] and [1 4 –9] directions have a higher Ea than the low conductive [0 1 1] direction above 108 °C. This kind of materials with a low Ea often exhibit a higher  than those with a high activation energy.1–5 However, according to the Arrhenius equation ( (T) = 0 exp(–Ea / kT)), the Ea value has no direct relationship with the absolute value of , and actually, this nature has been supported by experiments.57 Instead, the Ea value should be related to the dynamics of molecules participating in the proton conduction. In order to obtain information about the molecular dynamics related to the anisotropy and temperature dependence of the Ea values, anisotropic NMR and IR measurements using the single crystal, high-temperature X-ray studies, and theoretical calculations will be performed in the future. 3.2. Imidazolium Glutarate (Im-Glu) 15 Environment ACS Paragon Plus

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In the previous section, we have successfully measured the single-crystal proton conductivity of Im-Suc in three different directions, indicating that the existence of (1) a large anisotropy in the proton conduction in the parallel and perpendicular directions of the H-bonding network and also (2) a small but significant anisotropy in the two parallel directions. In order to gain further information about the relationship between the crystal structure and proton conducting properties in this system, we then focused on an analogue of Im-Suc, imidazolium glutarate (Im-Glu) (Figure 1). Similar to Im-Suc (Figure 2), this material is also composed of 2D H-bonding networks [Figure 5, at 20 °C (293 K)].57 However, the detailed H-bond manner (Figure 5a, b) is significantly different from that of Im-Suc (Figure 2a, b). In the Im-Glu system, one N–H moiety of the Im molecule is connected to carbonyl group of the Glu molecule in a bifurcated manner (brown and blue dashed lines), similar to the case of Im-Suc; however, the other N–H moiety forms a linear-type H-bond with the other carbonyl group (red dashed lines). A C-H···O short contact (gray dashed lines) and an O-H···O H-bond (green dashed lines) are also found in this compound. In addition, there are no effective H-bonds between the 2D networks (Figure 5c). We have also prepared large, high-quality single crystals of Im-Glu (see the experimental section) and measured the single-crystal proton conductivity (Figure 6). Because of the relationship between the crystal shape and crystallographic axes, the proton conductivity measurements were carried out in two directions; one is the [–1 2 1] direction, parallel to the Hbonding network, and another is the [3 0 1] direction, perpendicular to the network (Figure 5 and Figure S6).



a)

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[-1 2 1]

o

b)

c)

o

[3 0 1]

Figure 5. Crystal structure of Im-Glu57: a) Two-dimensional (2D) H-bonded sheet structure (Red, blue, and brown dashed lines: N–H···O H-bonds, green dashed lines: O–H···O H-bonds, gray dashed lines: C–H···O contacts), b) N–H···O H-bonds around Im molecules, and c) stacking arrangement of the 2D H-bonded networks. Black and blue arrows in c) represent short C···O


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and N···O contacts, respectively. Proton conductivity measurements of Im-Glu (Figure 6, Table 2) were carried out in the [–1 2 1] and [3 0 1] directions (shown in a) and c)).

The single crystals of Im-Glu also provided the intrinsic proton conductivity originating from its crystal bulk (Figure S6). Figure 6 shows Arrhenius plots for the [–1 2 1] and [3 0 1] directions and Table 2 summarizes the analysis results. The proton conductivity  in the parallel direction [–1 2 1] at 94 °C is 2.40 × 10–6 S cm–1, which is about 40 times higher than that in the perpendicular direction [3 0 1] (5.92 × 10–8 S cm–1 at 94 °C). This trend is similar to that in ImSuc (Figure 4, Table 1). In fact, such large conductivity differences between the H-bonded and non H-bonded directions have been observed in other systems.24, 29, 33, 34, 37, 79–81 These results demonstrate that the proton conduction in this system is also mediated by the H-bonds.

-5.0

95 90

T (°C) 80

70

-1

60

[-1 2 1] [3 0 1]

-6.0

log ( / S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-7.0 -8.0 -9.0

-10.0 -11.0 2.70 2.75 2.80 2.85 2.90 2.95 3.00 -1

1000 / T (K )

Figure 6. Arrhenius plots of the proton conductivity () for a single crystal of Im-Glu measured along the [–1 2 1] (green diamonds) and [3 0 1] (black diamonds) directions. The theoretical 18 Environment ACS Paragon Plus


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fitting lines for the [–1 2 1] and [3 0 1] directions are denoted in black and red (solid line; 59– 77 °C, dashed line; 77–86 °C, dash–dot line; 86–94 °C), respectively.

Table 2. Proton conductivities () and activation energies (Ea) of Im-Glu single crystal measured along the [–1 2 1] and [3 0 1] directions.

Direction

[–1 2 1] (parallel to H-bonding network)

[3 0 1] (perpendicular to Hbonding network)

Proton conductivity 

Activation energy Ea

(S cm–1)

(eV) 0.90 [59–77 °C],

–6

2.40 × 10 [94 °C]

2.02 [77–86 °C], 4.87 [86–94 °C]

1.63 [59–77 °C], 5.92 × 10–8 [94 °C]

2.50 [77–86 °C], 3.99 [86–94 °C]

4. DISCUSSION 4.1. Comparison of Proton Conductivity between Im-Suc and Im-Glu. As mentioned above, both of Im-Suc and Im-Glu are 1: 1 co-crystals composed of imidazolium and alkyl carboxylate [i.e., succinate (n = 2) and glutarate (n = 3)] (Figure 1) and thus have a similar crystal structure based on the 2D H-bonding networks (Figures 2, 5). The proton conductivity of these salts are, however, remarkably different from each other (Tables 1,


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2): Im-Glu showed 2–3 orders higher conductivity than Im-Suc both in and perpendicular to the H-bonding network [Im-Glu: 2.40 × 10–6 and 5.92 × 10–8 S cm–1 (94 °C), Im-Suc: 3.41 × 10–9 (103 °C) and 2.79 × 10–10 S cm–1 (108 °C)]. In this section, we discuss the reason of the large differences in conductivity, on the basis of differences in the chemical properties of the component dicarboxylic acid molecules, succinic and glutaric acids. In general, proton conduction is believed to occur via proton donor and accepter sites (or molecules). Therefore, the proton donating and accepting abilities, pKa1 [= – log10 (acid dissociation constant)] and pKaH [= – log10 (conjugate acid dissociation constant)], are one of the most important chemical parameters related to the proton conductivity. In fact, the proton conductivity tends to be higher with decreasing the difference of pKa1 and pKaH, that is, pKa.85– 87

This trend can be interpreted that a smaller pKa makes the barrier of proton transfer between

the donor and acceptor lower. Here, the pKa1 values of succinic and glutaric acids are 4.19 and 4.34, respectively,88 and thus the differences (pKa) between the pKaH value of imidazole (6.95)89 are calculated to be 2.76 (= 6.95 – 4.19) and 2.61 (= 6.95 – 4.34), respectively. Our present experiments indicate that Im-Glu with a smaller pKa has a higher proton conductivity than Im-Suc, showing agreement with the above-mentioned trend.85–87 Therefore, from a qualitative viewpoint, one can imagine that the difference in proton donating ability (or pKa1) between succinic and glutaric acids resulted in the difference in the proton conductivity of these salts. In order to gain further insight into this relationship between the difference in pKa and the difference in the proton conductivity, we compare the present results with those of the analogues.85, 86 The 1:1 co-crystals of benzimidazole, an imidazole derivative with pKaH = 5.49,90



and dicarboxylic acid are a similar type of H-bonded proton conductors, showing  = ~10–5 S cm–1 in the glutaric acid (pKa1 = 4.34) salt85 and ~10–4 S cm–1 in the sebacic acid (pKa1 = 4.72) salt in the compressed pellet state at around 77 °C.86 This result indicates that the latter salt with a smaller pKa of 0.77 (= 5.49 – 4.72) shows a higher proton conductivity than the former salt with a pKa of 1.15 (= 5.49 – 4.34). This trend is similar to that in the present imidazole systems, suggesting that the proton conductivity significantly increases with decreasing pKa. However, the degree of the conductivity increase seems to be somewhat different in the benzimidazole and imidazole systems. Namely, the proton conductivity of the former system is increased one order of magnitude by the pKa difference of 0.38 (= 1.15 – 0.77), whereas that of the latter is increased 2–3 orders of magnitude by the pKa difference of 0.15 (= 2.76 – 2.61) as described before. Thus, simply stated, the proton conductivity of the present imidazole system is more sensitive to the pKa change than that of the benzimidazole system. This difference of the pKa effect on the proton conductivity probably originates from the difference in the molecular structure of imidazole and benzimidazole. Although these two systems have a similar overall crystal structure, the molecular structure difference would modulate the details, such as the intermolecular distances, H-bonding manners, and molecular arrangements, which result in the above-described difference in the pKa effect. 4.2. Anisotropy of Proton Conductivity in H-bond Network of Im-Suc. In this study, we observed the difference of proton conductivity in two directions parallel to the 2D H-bonding network in Im-Suc, that is,  = 4.94 × 10–7 S cm–1 in the [1 0 0] direction and

 = 1.27 × 10–7 S cm–1 in [1 4 –9] direction (see Figure 4, Table 1). This result is consistent with the previous theoretical calculations,58 which point out that the potential barrier against proton


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conduction along the [1 0 0] direction is lower than that along the [1 4 –9] direction. Here, we discuss a possible origin of the proton conductivity anisotropy and the mechanism of the proton conduction, by comparing the structures of these two directions, especially focusing on the molecular arrangements and H-bonding manners. In the [1 0 0] direction with a higher proton conductivity, the imidazolium ion (Im) and the succinate ion (Suc) are alternately arranged (···–Im–Suc– Im–Suc–···), as represented by the orange belt in Figure 7a. On the other hand, in the [1 4 –9] direction with a lower conductivity, two succinate ions exist between the imidazolium ions (···–Im–Suc–Suc–Im –···), as represented by the light blue belt in Figure 7b. In addition, although the [1 0 0] direction only has H-bonds between the proton donor (dicarboxylic acid) and acceptor (imidazole) (red dashed lines in Figure 7a), the [1 4 –9] direction has H-bonds not only between the donor and acceptor but also between the donors (black dashed lines in Figure 7b). As mentioned in the section 4.1, proton conduction in this kind of materials arises by proton relay through the H-bonds between proton donor and acceptor. From this perspective, the donor-donor H-bond in the [1 4 –9] direction should be inconvenient for proton relay and thus a lower proton conductivity was obtained. In fact, according to the previous quantum dynamics simulation by Hori, Y. et al.,91 the proton transfer rate in the H-bond between succinic acid and succinate anion (Suc ↔ Suc) is 1.2–1.5 times slower than that between imidazolium cation and succinate anion (Im ↔ Suc), suggesting that the Suc–Suc H-bonds lower the proton conductivity in the [1 4 –9] direction. In addition, the solid-state NMR study for Im-Suc92 indicates the presence of rotation motion of imidazolium ion (Figure 7c), which probably promotes the proton transfer in the [1 0 0] direction, as shown by blue arrows in Figure 7a. Therefore, we conclude that these structural differences in the H-bond



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manners and molecular arrangement caused the difference or anisotropy of the proton conductivity between the [1 0 0] and [1 4 –9] directions in the 2D H-bonding network.

a) [1 0 0] higher 

b)

c) [1 4 -9] lower 

Figure 7. Molecular arrangement along a) the [1 0 0] and b) the [1 4 –9] directions in the Hbonding network of Im-Suc. The orange and light blue belts represent the possible proton conducting paths, respectively. Red dashed lines, black dashed lines, gray dashed lines, and blue arrows denote N-H···O H-bonds, O-H···O H-bonds, C-H···O contacts, and possible proton migration paths accompanied by c) the rotation motion of imidazolium ion.


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4.3. Proton conductivity between H-bond Networks of Im-Suc and Im-Glu Finally, we mention about the proton conduction perpendicular to the H-bonding networks of Im-Suc and Im-Glu. Both of the materials have no H-bonds in this direction (see Figures 2c, 5c), however, our measurements reveal the presence of proton conduction of 10–8 ~ 10–9 S cm–1 [3.55 × 10–9 S cm–1 for Im-Suc (at 116 °C) and 5.92 × 10–8 S cm–1 for Im-Glu (at 94 °C); Tables 1, 2]. Interestingly, these values are comparable to that of single-component imidazole crystals measured along the H-bonding chain.48 Here we focus on the overlapping manner of molecules between the neighboring networks in detail (Figure 8a). There are the carboxy groups of succinate ions up and down imidazolium ions. In addition, the imidazolium ions are expected to show a 180° flip-flop motion (Figure 8b) observed by the solid-state NMR study.92 Therefore, one can imagine that through this dynamic motion the imidazole molecules relay protons from succinic acids in the upper plane to succinate anions in the lower plane (Figure 8c), leading to the above-mentioned proton conductivity. If that’s the case, this will be a unique proton conduction phenomenon based on the characteristic dynamic motion of imidazole. Further detailed studies are required; however, here we emphasize that by the present anisotropic measurements using the single crystals, the unique proton conductivity between the H-bonding networks was discovered.



a)

b) O

N

c)

Figure 8. a) Overlapping of an imidazolium and succinate between the neighboring 2D Hbonding networks, b) possible flip-flop motion around the pseudo two-fold axis of imidazolium, and c) schematic image of a plausible proton conduction perpendicular to the H-bonding networks of Im-Suc (yellow circles: protons, blue dashed lines: the N–O overlapping shown in a)).

5. CONCLUSIONS In this study, we have revealed the “intrinsic” proton conductivity without grain boundary contributions in acid-base type anhydrous organic single co-crystals, imidazolium succinate (ImSuc) and imidazolium glutarate (Im-Glu), with a 2D H-bonding network. The obtained conductivities are 1–5 × 10–7 S cm–1 (Im-Suc) and 2 × 10–6 S cm–1 (Im-Glu) within the 2D H-


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bonding network, which are much higher than those in the perpendicular direction to the 2D network [4 × 10–9 S cm–1 (Im-Suc) and 6 × 10–8 S cm–1 (Im-Glu)]. These results demonstrate that the H-bonds significantly promote proton conduction in this kind of anhydrous materials. Furthermore, we have shown that this proton conduction is 1) more significant in the H-bonds between the acid and base molecules than in those between the acid and its conjugate base and is 2) enhanced by decreasing the difference of pKa between the H-bonded acid and base molecules. These findings about the H-bond effects on the proton conductivity would be intuitively expected; however, we emphasize that this study is a rare example of investigation into such a detailed structure-property relationship of this kind of anhydrous purely organic proton conductors by the single-crystal measurements. In addition, the present results offer the possibility that the molecular motions of imidazole play a crucial role in the proton conduction not only in the 2D H-bond network but also between the networks. The details about the internetwork proton conduction and also the non-Arrhenius-type profile of proton conductivity in the H-bond network are currently under investigation in terms of the molecular dynamics by using solid-state NMR and IR spectroscopies, high-temperature X-ray analysis, and theoretical calculations.

ASSOCIATED CONTENT Supporting Information Differential scanning calorimetry (DSC) profiles, supporting crystal structure figures, and additional proton conductivity measurements data of Im-Suc and Im-Glu. This materials is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION Corresponding Author Phone/Fax: +81-4-7136-3444. E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partly supported by Grants-in-Aid for Scientific Research (Grants Nos. 24340074, , 26610096, 15H00988, 15K17691, 16H04010, 16K05744, 17H05143, and 17K18746) from MEXT and JSPS, by a Grant-in-Aid for Scientific Research on Innovative Areas “π-Figuration” (No. 26102001) and the Canon Foundation

REFERENCES (1) Kreuer, K. D. Proton Conductivity; Materials and Applications. Chem. Mater. 1996, 8, 610– 641. (2) Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M. Transport in Proton Conductors for Fuel-cell Applications: Simulations, Elementary Reactions, and Phenomenology. Chem. Rev. 2004, 104, 4637–4678. (3) Norby, T. The promise of protonics. Nature 2001, 410, 877–878. (4) Proton conductors: Solids, membranes and gels–materials and devices; Colomban, P., Ed.; Cambridge University Press: Cambridge, UK, 1992. (5) Solid State Proton Conductors: Properties and Applications in Fuel Cells; Knauth, P., Di


Page 28 of 40

Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vona, M. L., Eds.; John Wiley & Sons, Ltd.: Chichester, 2012. (6) Baranov, A.; Shuvalov, L.; Shchagina, N. Superion conductivity and phase transitions in CsHSO4 and CsHSeO4 crystals. JETP Letters 1982, 36, 459–462. (7) Pawłowski, A.; Pawlaczyk, Cz.; Hilzcer, B. Electric conductivity in crystal group Me3H(SeO4)2 (Me: NH4+, Rb+, Cs+). Solid State Ionics 1990, 44, 17–19. (8) Haile, S. M.; Lentz, G.; Kreuer, K. D.; Maiser, J. Superprotonic conductivity in Cs3(HSO4)2(H2PO4). Solid State Ionics 1995, 77, 128–134. (9) Haile, S. M.; Calkins, P. M.; Boysen, D. Superprotonic conductivity in Cs3(HSO4)2(Hx(PyS)O4). Solid State Ionics 1997, 97, 145–151. (10) Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Meerle, R. B. Solid acids as fuel cell electrolytes. Nature 2001, 410, 910–913. (11) Münch, W.; Kreuer, K. D.; Traub, U.; Maiser, J. A molecular dynamics study of the high proton conducting phase of CsHSO4. Solid State Ionics 1995, 77, 10–14. (12) Norby, T.; Friesel, M.; Mellander, B. E. Proton and deuteron conductivity in CsHSO4 and CsDSO4 by in situ isotopic exchange. Solid State Ionics 1995, 77, 105–110. (13) Chisholm, C. R. I.; Haile, S. Superprotonic behavior of CsHSO4–CsH2PO4 system. Solid State Ionics 2000, 136–137, 229–241. (14) Sone, Y.; Ekdunge, P.; Simonsson, D. Proton Conductivity of Nafion 117 as Measured by a Four-Electrode AC Impedance Method. J. Electrochem. Soc. 1996, 143, 1254–1259. (15) Heitner-Wirguin, C. Recent advances in perfluorinated ionomer membranes: structure, properties and applications. J. Membr. Sci. 1996, 120, 1–33. (16) Paddison, St. J.; Bender, G.: Kreuer, K. D.; Nicoloso, N.; Zawodzinski, Th.; The microwave region of the dielectric spectrum of hydrated NAFION and other sulfonated membranes. J. New.



Mater. Electrochem. Syst. 2000, 3, 293. (17) Kreuer, K. D. On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells. J. Membr. Sci. 2001, 185, 29–39. (18) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104, 4535–4586. (19) Schmidt-Rohr, K.; Chen, Q. Parallel cylindrical water nanochannels in Nafion fuel-cell membranes. Nat. Mater. 2008, 7, 75–83. (20) Hurd, J. A.; Vaidhyanathan, R.; Thangadurai, V.; Ratcliffe, C. I.; Moudrakovski. I. L.; Shimizu, G. K. H. Anhydrous proton conduction at 150 °C in a crystalline metal-organic framework. Nat. Chem. 2009, 1, 705–710. (21) Sadakiyo, M.; Yamada, T.; Kitagawa, H. Rational Designs for Highly Proton-Conductive Metal–Organic Frameworks. J. Am. Chem. Soc. 2009, 131, 9906–9907. (22) Shigematsu, A.; Yamada, T.; Kitagawa, H. Wide Control of Proton Conductivity in Porous Coordination Polymers. J. Am. Chem. Soc. 2011, 133, 2034–2036. (23) Jeong, N. C.; Samanta, B.; Lee, C. Y.; Farha, O. K.; Hupp, J. T. Coordination-Chemistry Control of Proton Conductivity in the Iconic Metal-Organic Framework material HKUST-1. J. Am. Chem. Soc. 2011, 134, 51–54. (24) Tang, Q.; Liu, Y.; Liu, S.; He, D.; Miao, J.; Wang, X.; Yang, G.; Shi, Z.; Zheng, Z. High Proton Conduction at above 100 °C Mediated by Hydrogen Bonding in a Lanthanide MetalOrganic Framework. J. Am. Chem. Soc. 2014, 136, 12444–12449. (25) Yu, S.-S.; Liu, S.-X.; Duan, H.-B. Dielectric response and anhydrous proton conductivity in a chiral framework containing a non-polar molecular rotor. Dalton Trans. 2015, 44, 20822– 20825.


Page 30 of 40

Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Borges, D. D.; Devautour, S.; Jobic, H.; Ollivier, J.; Nouar, F.; Semino, R.; Devic, T.; Serre, C.; Paesani, F.; Maurin, G. Proton Transport in Highly Conductive Porous Zirconium-Based Metal-Organic Framework: Molecular Insight. Angew. Chem. Int. Ed. 2016, 55, 3919–3924. (27) Mileo, P. G. M.; Devautour-Vinot, S.; Mouchaham, G.; Faucher, F.; Guillou, N.; Vimont, A.; Serre, C.; Maurin, G. Proton-Conducting Phenolate-Based Zr Metal-Organic Framework: A Joint Experimental-Modeling Investigation. J. Phys. Chem. C 2016, 120, 24503–24510. (28) Wei, Y.-S.; Hu, X.-P.; Han, Z.; Dong, X.-Y.; Zang, S.-Q.; Mak, T. C. W. Unique Proton Dynamics in an Efficient MOF-Based Proton Conductor. J. Am. Chem. Soc. 2017, 139, 3505– 3512. (29) Li, R; Wang, S.-H.; Chen, X.-X.; Lu, J.; Fu, Z.-H.; Li, Y.; Xu, G.; Zheng, F.-K.; Guo, G.-C. Highly Anisotropic and Water Molecule-Dependent Proton Conductivity in a 2D Homochiral Copper(II) Metal–Organic Frameworks. Chem. Mater. 2017, 29, 2321–2331. (30) Song, B.-Q.; Chen, D.-Q.; Ji, Z.; Tang, J.; Wang, X.-L.; Zang, H.-Y.; Su, Z.-M. Control of bulk homochirality and proton conductivity in isostructural chiral metal-organic frameworks. Chem. Commun. 2017, 53, 1892–1895. (31) Zhang, G.; Fei, H. Missing metal-linker connectivities in a 3-D robust sulfonate-based metal-organic framework for enhanced proton conductivity. Chem. Commun. 2017, 53, 4156– 4159. (32) Zhang, F.-M.; Dong, L.-Z.; Qin, J.-S.; Wei, G.; Liu, J.; Li, S.-L.; Lu, M.; Lan. Y.-Q.; Su, Z.M.; Zhou, H.-C. Effect of Imidazole Arrangements on Proton-Conductivity in Metal-Organic Frameworks. J. Am. Chem. Soc. 2017, 139, 6183–6189. (33) Joarder, B.; Lin, J.-B.; Romero, Z.; Shimizu, G. K. H. Single Crystal Proton Conduction Study of a Metal Organic Framework of Modest Water Stability. J. Am. Chem. Soc. 2017, 139,



7176–7179. (34) Yoon , M.; Suh, K.; Kim, H.; Kim, Y.; Selvapalam, N.; Kim, K.; High and Highly Anisotropic Proton conductivity in Organic Molecular Porous Materials. Angew. Chem. Int. Ed. 2011, 50, 7870. (35) Dey, C.; Kundu, T.; Banerjee, R. Reversible phase transformation in proton conducting Standberg-type POM based metal organic materials. Chem. Comm. 2012, 48, 266–268. (36) Horike, S.; Umeyama, D.; Inukai, M.; Itakura, T.; Kitagawa, S. Coordination-NetworkBased Ionic Plastic Crystal for Anhydrous Proton conductivity. J. Am. Chem. Soc. 2012, 134, 7612–7615. (37) Umeyama, D.; Horike, S.; Inukai, M.; Itakura, T.; Kitagawa, S. Inherent Proton Conduction in a 2D Coordination Framework. J. Am. Chem. Soc. 2012, 134, 12780–12785. (38) Horike, S.; Chen, W.; Itakura, T.; Inukai, M.; Umeyama, D.; Asakura, H.; Kitagawa, S. Order-to-disorder structural transformation of a coordination polymer and its influence on proton conduction. Chem. Comm. 2014, 50, 10241–10243. (39) Yoshii, Y.; Hoshino, N.; Takeda, T.; Akutagawa, T. Protonic Conductivity and Hydrogen Bonds in (Haloanilinium)(H2PO4) Crystals. J. Phys. Chem. C 2015, 119, 20845–20854. (40) Nagarkar, S. S.; Horike, S.; Itakura, T.; Ouay, B. L.; Demessence, A.; Tsujimoto, M.; Kitagawa, S. Enhanced and Optically Switchable Proton Conductivity in a Melting Coordination Polymer Crystal. Angew. Chem. Int. Ed. 2017, 56, 4976–4981. (41) Yoshizawa-Fujita, M.; Fujita, K.; Forsyth, M.; MacFarlane, D. R. A new class of protonconducting plastic crystals based on organic cations and dihydrogen phosphate. Electrochem. Comm. 2007, 9, 1202–1205. (42) Luo, J.; Jensen, A. H.; Brooks, N. R.; Sniekers, J.; Knipper, M.; Aili, D.; Li, Q.; vanroy, B.;


Page 32 of 40

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wübbenhorst, M.; Yan, F. et al. 1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells. Energy Environ. Sci. 2015, 8, 1276–1291. (43) Rao, J.; Vijayaraghavan, R.; Chen, F.; Zhu, H.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. Porotic organic ionic plastic crystals based on a difunctional cation and the triflate anion: a new solid-state proton conductor. Chem. Comm. 2016, 52, 14097–14100. (44) Mondal, A.; Balasubramanian, S. Proton Hopping Mechanisms in a Protic Organic Ionic Plastic Crystal. J. Phys. Chem. C 2016, 120, 22903–22909. (45) Chandra, S.; Kundu, T.; Kandambeth, S.; Babarao, R.; Marathe, Y.; Kunjir, S. M.; Banerjee, R. Phosphoric Acid Loaded Azo (–N=N–) Based Covalent Organic Framework for Proton Conduction. J. Am. Chem. Soc. 2014, 136, 6570–6573. (46) Chandra, S.; Kundu, T.; Dey, K.; Addicoat, M.; Heine, T.; Banerjee, R. Interplaying Intrinsic and Extrinsic Proton Conductivities in Covalent Organic Frameworks. Chem. Mater. 2016, 28, 1489–1494. (47) Xu, H.; Tao, S.; Jiang, D. Proton conduction in crystalline and porous covalent organic frameworks. Nat. Mater. 2016, 15, 722–726. (48) Kawada, A.; McGhie, A. R.; Labes, M. M. Protonic Conductivity in Imidazole Single Crystal. J. Chem. Phys. 1970, 52, 3121–3125. (49) Yutronic, N.; Merchan, J.; Jara, P.; Manriquez, V.; Wittke, O.; Gonzalez, G. Single-crystal Anisotropic Proton Conductivity in the Clathrate of the Hydrogen-diquinuclidine Ion Inserted in a Polyanionic Thiourea-chloride Matrix. Supramole. Chem. 2004, 16, 411–414. (50) Schuster, M. F. H.; Meyer, W. H.; Schster, M.; Kreuer, K. D. Toward a New Type of Anhydrous Organic Proton Conductor Based on Immobilized Imidazole. Chem. Mater. 2004, 16,



329–337. (51) Pawlaczyk, Cz.; Pawłowski, A.; Połomska, M.; Pogorzelec-Glaser, K.; Hilczer, B. Anhydrous proton conductors for use as solid electrolytes. Phase Transitions 2010, 83, 854–867. (52) Kaur, R.; Perumal, S. S. R. R.; Bhattacharyya, A. J.; Yashonath, S.; Row, T. N. G. Structural Insights into Proton Conduction in Gallic Acid–Isoniazid Cocrystals. Crys. Growth. Des. 2014, 14, 423–426. (53) Kaur, R.; Swain, D.; Dutta, D.; Brajesh, K.; Singh, P.; Bhattacharyya, A. J.; Ranjan, R.; Narayana, C.; Hulliger, J.; Row, T. N. G. Proton Conduction in a Quaternary Organic Salt: Its Phase Behavior and Related Spectroscopic Studies. J. Phys. Chem. C 2015, 119, 20845–20854. (54) Bureekaew, S.; Horike, S.; Higuchi, M.; Mizuno, M.; Kawamura, T.; Tanaka, D.; Yanai, N.; Kitagawa, S. One-dimensional imidazole aggregate in aluminium porous coordination polymers with high proton conductivity. Nat. Mater. 2009, 8, 831–836. (55) Horike, S.; Umeyama, D.; Kitagawa, S. Ion Conductivity and Transport by Porous Coordination Polymer and Metal–Organic Frameworks. Acc. Chem. Res. 2013, 46, 2376–2384. (56) MacDonald, J. C.; Dorrestein, P. C.; Pilley, M. M. Design of Supramolecular Layers via Self-Assembly of Imidazole and Carboxylic Acids. Cryst. Growth. Des. 2001, 1, 29–38. (57) Pogorzelec-Glaser, K.; Garbarczyk, J.; Pawlaczyk, Cz.; Markiewicz, E. Electrical conductivity in new imidazolium salts of dicarboxylic acids. Mater. Sci. Poland. 2006, 24, 1. (58) Pogorzelec-Glaser, K.; Pawlaczyk, Cz.; Pietraszko, A.; Markiewicz, E. Crystal structure and electrical conductivity of imidazolium succinate. J. Power. Sources. 2007, 173, 800–805. (59) Suzuki, H.; Yamashita, K.; Suto, M.; Maejima, T.; Kimura, S.; Mori, H.; Nishio, Y.; Kajita, K.; Moriyama, H. Control of electronic state in the 2D hydrogen-bonded system: ”-(CnDTEDO-TTF)2(PF6)x (n = 5, 6, 7, 8). Synth. Met. 2004, 144, 89–95.


Page 34 of 40

Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(60) Suzuki, H.; Ichikawa, S.; Yamashita, K.; Kimura, S.; Mori, H.; Nishio, Y.; Kajita, K. Crystal structures and conductivities of new hydrogen-bonded system: CnDT-EDO-TTF and BzDT-EDO-TTF. Synth. Met. 2005, 154, 261–264. (61) Lee, S. C.; Ueda, A.; Kamo, H.; Takahashi, K.; Uruichi, M.; Yamamoto, K.; Yakushi, K.; Nakao, A.; Kumai, R.; Kobayashi, K. et al. Charge-order driven proton arrangement in a hydrogen-bonded charge-transfer complex based on a pyridyl-substituted TTF derivative. Chem. Commun. 2012, 48, 8673–8675. (62) Kamo, H.; Ueda, A.; Isono, T.; Takahashi, K.; Mori, H. Synthesis and properties of catechol-fused tetrathiafulvalene derivatives and their hydrogen-bonded conductive chargetransfer salts. Tetrahedron Lett. 2012, 53, 4385–4388. (63) Isono, T.; Kamo, H.; Ueda, A.; Takahashi, K.; Nakao, A.; Kumai, R.; Nakao, H.; Kobayashi, K.; Murakami, Y.; Mori, H. Hydrogen bond-promoted metallic state in a purely organic singlecomponent conductor under pressure. Nat. Commun. 2013, 4, 1344. (64) Yoshida, J.; Ueda, A.; Nakao, A.; Kumai, R.; Nakao, H.; Murakami, Y.; Mori, H. Solidsolid phase interconversion in an organic conductor crystal: hydrogen-bond-mediated dynamic changes of -stacked molecular arrangement and physical properties. Chem. Commun. 2014, 50, 15557–15560. (65) Isono, T.; Kamo, H.; Ueda, A.; Takahashi, K.; Kimata, M.; Tajima, H.; Tsuchiya, S.; Terashima, T.; Uji, S.; Mori, H. Gapless Quantum Spin Liquid in an Organic Spin-1/2 Triangular-Lattice -H3(Cat-EDT-TTF)2. Phys. Rev. Lett. 2014, 112, 177201. (66) Ueda, A.; Kamo, H.; Mori, H. Un-expected Formation of ortho-Benzoquinone-fused Tetraselenafulvalene (TSF): Synthesis, Structures, and Properties of a Novel TSF-based Donoracceptor Dyad. Chem. Lett. 2015, 44, 1538–1540.



(67) Yoshida, J.; Ueda, A.; Kumai, R.; Murakami, Y.; Mori, H. Anion substitution in hydrogenbonded organic conductors: the chemical pressure effect on hydrogen-bond-mediated phase transition. CrystEngComm 2017, 19, 367–375. (68) Higashino, T.; Ueda, A.; Yoshida, J.; Mori, H. Improved stability of a metallic state in benzothienobenzothiophene-based molecular conductors: An effective increase of dimensionality with hydrogen bonds. Chem. Commun., 2017, 53, 3426–3429. (69) Shimozawa, M.; Hashimoto, K.; Ueda, A.; Suzuki, Y.; Sugii, K.; Yamada, S.; Imai, Y.; Kobayashi, R.; Itoh, K.; Iguchi, S. et al. Quantum-disordered state of magnetic and electric dipoles in an organic Mott system. Nat. Commun., 2017, 8, 1821. (70) Ueda, A.; Yamada, S.; Isono, T.; Kamo, H.; Nakao, A.; Kumai, R.; Nakao, H.; Murakami, Y.; Yamamoto, K.; Nishio, Y. et al. Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism: A Phase Transition Caused by Deuterium and Electron Transfer in a Hydrogen-Bonded Purely Organic Conductor Crystal. J. Am. Chem. Soc. 2014, 136, 12184– 12192. (71) Ueda, A.; Hatakeyama, A.; Enomoto, M.; Kumai, R.; Murakami, Y.; Mori, H. Modulation of a Molecular -Electron System in a Purely Organic Conductor that Shows Hydrogen-BondDynamics-Based Switching of Conductivity and Magnetism. Chem. Eur. J. 2015, 21, 15020– 15028. (72) Yamamoto, K.; Kanematsu, Y.; Nagashima, U.; Ueda, A.; Mori, H.; Tachikawa, M. Theoretical study of H/D isotope effect on phase transition of hydrogen-bonded organic conductor -H3(Cat-EDT-TTF)2. Phys. Chem. Chem. Phys. 2016, 18, 29673–29680. (73) Ueda, A. Development of Novel Functional Organic Crystals by Utilizing Proton- and Electron-Donating / Accepting Abilities. Bull. Chem. Soc. Jpn. 2017, 90, 1181–1188.


Page 36 of 40

Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(74) Yamamoto, K.; Kanematsu, Y.; Nagashima, U.; Ueda, A.; Mori, H.; Tachikawa, M. Multicomponent DFT study of geometrical H/D isotope effect on hydrogen-bonded organic conductor, -H3(Cat-EDT-TTF)2. Chem. Phys. Lett. 2017, 674, 168–172. (75) Suzuki, H.; Mori, H.; Yamaura, J.; Matsuda, M.; Tajima, H.; Mochida, T. Proton Migration in –N···H···O– Hydrogen-bonded Complex of (Chloranilic Acid)(1,2-Diazine)2 Studied by Dielectric Response and Infrared Absorption Spectra. Chem. Lett. 2007, 36, 402–403. (76) Ohchi, H.; Takahashi, K.; Yamaura J.; Takaishi, J.; J.; Mori, H. Dielectric Response of Novel One-Dimensional Hydrogen-Bonded Molecular Crystal [4,6-dmpH][Hca]. Physica B 2010, 405, S341-S343. (77) Wang, M.; Luo, H.-B.; Zhang, J.; Liu, S.-X.; Xue, C.; Zou, Y.; Ren, X.-M. An openframework manganese(II) phosphite and its composite membranes with polyvinylidene fluoride exhibiting intrinsic water-assisted proton conductance. Dalton Trans. 2017, 46, 7904–7910. (78) Chen, X.; Zhang, Y.; Ribeiorinha, P.; Li, H.; Kong, X.; Boaventura, M. A proton conductor electrolyte based on molten CsH5(PO4)2 for intermediate-temperature fuel cells. RSC Adv. 2018, 8, 5225–5232. (79) Kreuer, K. D.; Dippel, T.; Baikov, Y. M; Maier, J. Water solubility, proton and oxygen diffusion in acceptor doped BaCeO3: A single crystal analysis. Solid State Ionics 1996, 86-88, 613-620. (80) Bao, S.-S.; Otsubo, K.; Taylor, J. M.; Jiang, Z.; Zheng, L.-M.; Kitagawa, H. Enhanced Proton Conduction in 2D Co-La Coordination Frameworks by Solid-State Phase Transition. J. Am. Chem. Soc. 2014, 136, 9292−9295.



(81) Bao, S.-S.; Li, N.-Z.; Taylor, J. M.; Shen, Y.; Kitagawa, H.; Zheng, L.-M. Co-Ca Phosphonate Showing Humidity-Sensitive Single Crystal to Single Crystal Structural Transformation and Tunable Proton Conduction Properties. Chem. Mater. 2015, 27, 8116–8125. (82) Lavrova, G. V.; Burgina, E. B.; Matvienko, A. A.; Ponomareva, V. G. Bulk and surface properties of ionic salt CsH5(PO4)2. Solid State Ionics 2006, 177, 1117–1122. (83) Düvel, A.; Wilkening, M.; Uecker, R.; Wegner, S.; Sepelak, V.; Heitjans, P. Mechanosynthesized nanocrystalline BaLiF3: The impact of grain boundaries and structural disorder on ionic transport. Phys. Chem. Chem. Phys. 2010, 12, 11251–11262. (84) Bondi, A. van der Waals Volumes and Radii. J. Chem. Phys. 1964, 68, 441–451. (85) Pogorzelec-Glaser, K.; Rachocki, A.; Ławniczak P.; Pietraszko, A.; Pawlaczyk, Cz.; Hilczer, B.; Pugaczowa-Michalska, M. Structure, hydrogen bond network and proton conductivity of new benzimidazole compounds with dicarboxylic acids. CrystEngComm 2013, 15, 1950–1959. (86) Rachocki, A.; Pogorzelec-Glaser, K.; Ławniczak P.; Pugaczowa-Michalska, M.; Łapiński, A.; Hilczer, B.; Matczak, M.; Pietraszko, A. Proton Conducting Compound of Benzimidazole with Sebacic Acid: Structure, Molecular Dynamics, and Proton Conductivity. Crys. Growth. Des. 2014, 14, 1211–1220. (87) Narayanan, S. R.; Yen, S.-P.; Liu, L.; Greenbaum, S. G. Anhydrous Proton-Conducting Polymeric Electrolytes for Fuel Cells. J. Phys. Chem. B 2006, 110, 3942–3948. (88) Brown, H. C.; McDaniel, D. H; Häflinger, O. In Determination of Organic Structures by Physical Methods; Braude, E. A., Nachod, F. C., Eds.; Academic Press: New York, 1955; pp 567−662. (89) Bruice, T. C.; Schemir, G. L. Imidazole Catalysis. II. The Reaction of Substituted Imidazoles with Phenyl Acetates in Aqueous Solution. J. Am. Chem. Soc. 1958, 80, 148–156.


Page 38 of 40

Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(90) Jerez, G.; Kaufman, G.; Prystai, M.; Schenkeveld, S.; Donkor, K. K. Determination of thermodynamic pKa values of benzimidazole and benzimidazole derivatives by capillary electrophoresis. J. Sep. Sci. 2009, 32, 1087–1095. (91) Hori, Y.; Ida, T.; Mizuno, M. Potential energy construction in the diabatic picture for quantum mechanical rate constants of intermolecular proton transfer. Phys. Chem. Chem. Phys. 2017, 19, 16857–16866. (92) Umiyama, T.; Ohashi, R.; Ida, T.; Mizuno, M. Analysis of Molecular Motion of Protonconductive Imidazolium Hydrogen Succinate Crystal Using Solid-state NMR. Chem. Lett. 2013, 42, 1323–1325.



Table of Contents (TOC) Image 2D H-bonding network 115

-6.0

T (℃)

110

105

100

proton conductivity [1 0 0]

-7.0

intra-network [1 0 0]

-1

log( / S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

[1 4 –9]

-8.0

-9.0

inter-network [0 1 1]

-10.0 2.56

[0 1 1]

intra-network [1 4 –9]

2.60 2.64 -1 1000/T (K )


2.68

Anisotropic Proton Conductivity Arising from Hydrogen-Bond Patterns

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