Highly Anisotropic and Water Molecule-Dependent Proton

tivity of 1.43 × 10−3 S cm−1 of 1·3H2O at 80 °C and 95% relative humidity was observed among the reported 2D MOF crystals ... ACS Paragon Plus ...
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Highly Anisotropic and Water Molecule-Dependent Proton Conductivity in a 2D Homochiral Copper(II) Metal−Organic Framework Rong Li,†,‡ Shuai-Hua Wang,† Xu-Xing Chen,† Jian Lu,†,‡ Zhi-Hua Fu,† Yan Li,§ Gang Xu,† Fa-Kun Zheng,*,† and Guo-Cong Guo*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China § Department of Chemistry, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: Proton conductivity research on single crystals is essential to elucidate their conduction mechanism and guide the unidirectional crystal growth to improve the performance of electrolyte materials. Herein, we report a highly anisotropic proton-conductive 2D metal−organic framework (MOF) [Cu2(Htzehp)2(4,4′-bipy)]·3H2O (1·3H2O, H3tzehp = N-[2-(1Htetrazol-5-yl)ethyl]-L-hydroxyproline) with definite crystal structures showing single-crystal to single-crystal transformation between the anhydrate (1) and trihydrate (1·3H2O) phases. The hydrogen bonded chains consisted of well-defined lattice water molecules and hydroxyl functional groups of the Htzehp2− ligand array inside the 2D interlayer spaces along the crystallographic a-axis ([100] direction) in 1·3H2O. Temperature- and humidity-dependent proton conductivity was achieved along the [100] and [010] directions, respectively. The anisotropic proton conductivity of σ[100]/σ[010] in a single crystal of 1·3H2O was as high as 2 orders of magnitude. The highest proton conductivity of 1.43 × 10−3 S cm−1 of 1·3H2O at 80 °C and 95% relative humidity was observed among the reported 2D MOF crystals. The relation between the proton conductivity and structure was also revealed. The hydrogen bonded chain in 1·nH2O plays a significant role in the proton transport. The time-dependent proton conductivity and single-crystal X-ray diffraction measurements demonstrated that 1·3H2O is temperature- and humidity-stable and acts as an underlying electrolyte material for fuel cell applications.

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INTRODUCTION In response to increasing green energy demand, fuel cell technology as an alternative energy conversion mode has drawn extensive attention.1−6 New proton (ion)-conductive materials play critical roles in achieving better conductive performance for fuel cell applications and in understanding the nature of complex biological mechanisms.7−10 However, their limited operating temperature (< 80 °C) and the amorphous phase of commercially used materials (such as Nafion) hinder the further development of fuel cell applications.11,12 In contrast to traditional materials such as polymers,13,14 solid acids,15 and perovskite oxides,16 metal−organic frameworks (MOFs) have well-defined structures that can be designed through the purposeful selection of metal ions and ligands.17−19 Several MOFs as proton conductors with excellent proton-conduction ability have been reported.20−23 Structurally designable and definable MOFs as new potential conducting materials will bring new breakthroughs.24−28 To date, two prevailing approaches have been adopted to improve the proton conductivity of MOFs. The first approach is to directly introduce the proton carriers into the pores to act as counterions (e.g., NH4+, H3O+, HSO4−)29−31 and to expand © 2017 American Chemical Society

their application to higher temperatures (e.g., imidazole molecules, H3PO4).32,33 The second method is to modify the organic ligands by substitution of their functional groups (e.g., −COOH, −OH) to enhance their acidity and hydrophilicity.34,35 The latter method is the most effective way to comprehensively satisfy the acidity and structural varieties of MOFs to achieve high conductivity and develop new electrolyte materials. Proton conductivity research on single crystals provides a better understanding of the potential conduction mechanism and guides the unidirectional crystal growth to improve the performance of electrolyte materials,20,36−38 which will help to build up a “visible” proton-conducting pathway. Despite the fact that the single-crystal structure of MOFs can be determined by X-ray crystallography, the positions of guest molecules in the pores are often not easy to exactly identify because of their disorder.39 As a result, the relationship between structure and proton conduction has been ambiguous and Received: December 30, 2016 Revised: February 10, 2017 Published: February 10, 2017 2321

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group array inside the 2D interlayer interval along the crystallographic a-axis ([100] direction). The highest proton conductivity of the single crystal of 1·3H2O was 1.43 × 10−3 S cm−1 among the reported 2D MOFs at 80 °C and 95% relative humidity. Highly anisotropic proton conductivity was observed, and the σ[100]/σ[010] value was as high as 2 orders of magnitude. The single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD) analyses of 1·nH2O at different temperatures and under various relative humidity conditions demonstrate that 1·3H2O exhibits excellent stability with good endurance at higher temperature and moisture levels and is a potential electrolyte material for fuel cell applications.

insufficiently investigated. Although microcrystalline samples are usually used to estimate proton conductivity through alternating current (AC) impedance analysis, the characterization of anisotropic effects is precluded because of the random orientations.36 To solve these problems, single crystals with ordered and well-defined guest molecules are necessary. The single-crystal to single-crystal (SC−SC) transformation leads to a visible structural change. Unfortunately, to date, most conductive MOFs have lacked a large enough size, sufficiently regular shape, or adequate stability (durability at higher working temperatures and moisture levels).40−42 The single crystals used to study proton conduction are scarce. Chirality is inherent in biological systems, such as in assembly of protein bundles and DNA, and plays a vital role in various chemical and biological processes.43−45 Amino acids and their derivatives with chiral centers have attracted broad attention and have been widely used to fabricate homochiral MOFs or chiral bioMOFs.43−48 N-heterocycle−amino acid derivatives, especially tetrazoles are, outstanding choices over pure natural amino acids because most of the former provide more coordination modes to expand the structural dimensionality to form diverse structures with better biocompatibility. Nevertheless, only a handful of MOFs prepared using amino acid derivatives simultaneously exhibit chirality and proton conductivity.49,50 Proton-conductive homochiral MOFs constructed using amino acid−tetrazole ligands have not been reported thus far. The use of amino acid−tetrazole chiral ligands provides an opportunity to achieve various homochiral MOFs for proton-conduction applications and with ion channels for biological system simulations. On the basis of the aforementioned considerations, we synthesized a new chiral amino acid−cyano ligand, N-(2cyanoethyl)-L-hydroxyproline (L-cehp), from purchased Lhydroxyproline. N-[2-(1H-tetrazol-5-yl)ethyl]-L-Hydroxyproline (H3tzehp) was obtained via an in situ [2 + 3] cycloaddition reaction. The H3tzehp ligand features two chiral carbon atoms in its rigid azole skeleton and is thus unlikely to undergo racemization.51 The hydroxyl on the framework enhances its acidity and hydrophilicity. As a result, we obtained a new homochiral MOF, [Cu2(Htzehp)2(4,4′-bipy)]·3H2O (1·3H2O; Scheme 1). The 2D framework of 1·nH2O (n = 0, 3) displays an SC−SC transformation between the anhydrate 1 and trihydrate 1·3H2O phases by adjusting the temperature and humidity. The hydrogen bonded chains of 1·3H2O consist of well-ordered lattice water molecules and a hydroxyl functional

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EXPERIMENTAL SECTION

Materials and Instruments. All chemicals except L-cehp were analytical grade and were obtained from commercially available sources and used as received without further purification. The N-(2cyanoethyl)-L-hydroxyproline (L-cehp) was synthesized by the reaction of L-hydroxyproline and acrylonitrile in distilled water under 0−4 °C for 2 d. PXRD patterns at room temperature were collected on a Rigaku Miniflex II diffractometer using Cu Kα radiation (λ = 1.540598 Å) at 40 kV and 40 mA in the range of 5° ≤ 2θ ≤ 65°. In situ temperature-dependent PXRD patterns were recorded on an UltimaIV diffractometer with Cu Kα radiation (λ = 1.54056 Å) at a scanning speed of 2°/min. Simulated PXRD patterns were obtained from the Mercury Version 1.4 software (http://www.ccdc.cam.ac.uk/products/ mercury). Thermogravimetric analysis (TGA) experiments were carried out on a METTLER TOLEDO thermogravimetric analyzer in N2 atomosphere with the sample heated in an Al2O3 crucible at a heating rate of 10 K min−1. Elemental analyses were measured on an Elementar Vario EL III microanalyzer. The FT-IR spectra were obtained on a PerkinElmer Spectrum One Spectrometer using KBr pellets in the 4000−400 cm−1 range. Solid-state circular dichroism (CD) spectra were measured on a Bio-Logic MOS-450 CD spectrometer (France) in a KCl matrix at 25 °C. A PerkinElmer Lambda 950 spectrophotometer was utilized to achieve UV−vis spectra. Calculation of the Density of States (DOS). X-ray crystallographic data for 1·3H2O were used to calculate the density of states (DOS). Calculation of the DOS was carried out using density functional theory (DFT) with one of the three nonlocal gradientcorrected exchange-correlation functionals (GGA-PBE) and performed with the CASTEP code in the Materials Studio v5.5 software package,52,53 which uses a plane wave basis set with Vanderbilt ultrasoft pseudopotentials for the core electrons. The number of plane waves included in the basis set was determined by a cutoff energy of 340.0 eV for 1·3H2O, and numerical integration of the Brillouin zone was performed by a 1 × 1 × 1 Monkhorst−Pack κ-point sampling for accurate calculations of the optical properties of the compounds. Other parameters in the calculations were set to the CASTEP code default values. Circular Dichoism Properties. Circular dichroism (CD) spectroscopy, based on the differential absorption of right and left circularly polarized light, plays a vital role in characterization of chiral compounds to confirm its absolute configuration through the positive or negative CD signals. The intrinsic mechanism of CD signals has been investigated by theoretical advances in quantum chemical calculation methods.54−56 A Gaussian function is generally used as the broadening function for CD spectra with the width of the band (σ), excitation energies (ΔEi), and rotatory strengths (Ri) as follows:

Scheme 1. Hydrothermal Synthesis of 1·3H2O

Δε(E) =

1 2.297 × 10−39

1 2πσ

n

2

∑ ΔEiR ie[−(E −ΔEi /2σ)] i

Proton Conductivity Measurements. Proton conductivity measurements were performed using a quasi-four-electrode AC impedance technique with a Solartron 1260 impedance/gain-phase analyzer. The microcrystalline samples were compressed to 0.71 mm 2322

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Chemistry of Materials Table 1. Crystallographic Parameters and Structure Refinement Details for 1·3H2O and 1 compound conditions formula formula mass space group a/Å b/Å c/Å V/Å3 Z Dc/g cm−3 μ/mm−1 F(000) reflns collected unique reflns Rint flack GOF R1e [I > 2σ(I)] wR2f (all data) CCDC

1·3H2Oa

1b

1·3H2Oc

1·3H2Od

as-synthesized C26H36Cu2N12O9 787.75 P21212 18.710(5) 7.807(2) 10.452(3) 1526.7(7) 2 1.714 1.468 812.0 14226 3448 0.0376 −0.009(7) 1.047 0.0347 0.0973 1512462

dry N2, 80 °C C26H30Cu2N12O6 731.68 P21212 18.7672(10) 7.6504(4) 10.1885(5) 1462.83(13) 2 1.666 1.519 752 5044 3290 0.0264 0.00(2) 1.079 0.0430 0.0940 1512463

80 °C, 95% RH C26H36Cu2N12O9 787.75 P21212 18.656(4) 7.7891(17) 10.402(2) 1511.6(5) 2 1.731 1.483 812 16024 3451 0.0245 0.006(15) 1.077 0.0293 0.0834

100 °C, H2O C26H36Cu2N12O9 787.75 P21212 18.6728(3) 7.79480(17) 10.4341(2) 1518.69(6) 2 1.723 1.476 812 17533 4202 0.0216 0.008(7) 1.034 0.0324 0.0864

a The SCXRD data of the as-synthesized crystals of 1·3H2O were collected at 25 °C under an air atmosphere (RH > 70%). bThe SCXRD data of 1 were in situ collected at 80 °C under a dried N2 atmosphere (RH < 5%). cThe SCXRD data were collected after the as-synthesized crystals were treated at 80 °C under 95% RH for 48 h. dThe SCXRD data were collected after the as-synthesized crystals were heated and refluxed in H2O at 100 °C for 48 h. eR1 = Σ(Fo − Fc)/ΣFo. fwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.

in thickness and 2.5 mm in diameter under a pressure of ∼0.1 GPa. Two sides of the pellet were connected to gold wires using gold paste. For single-crystal measurements, crystal sizes were determined by indexing the crystals of 1·3H2O on a Rigaku Pilatus CCD diffractometer; their dimensions were 1.152 × 0.167 × 0.089 mm3 and 0.334 × 0.327 × 0.091 mm3, respectively. Gold wires were connected to both ends of the longer axis of each crystal. The sample pellet and single crystals were both measured at frequencies ranging from 107 to 0.1 Hz as the temperatures were varied from 30 to 80 °C and/or as the relative humidities (RH) were varied from 40 to 98%. The conductivity of the samples was deduced from the Debye semicircle in the Nyquist plot. Syntheses of [Cu2(Htzehp)2(4,4′-bipy)]·3H2O (1·3H2O). A mixture of L-cehp (0.5 mmol), NaN3 (0.5 mmol), CuCl2·H2O (0.5 mmol), and 4,4′-bipy (0.25 mmol) was added into a 25 mL Teflonlined stainless steel vessel with 7.0 mL of distilled water and kept heating for 3 days at 140 °C and then cooled to room temperature. Blue prismatic crystals suitable for X-ray analyses were obtained, washed with distilled water, and dried in air. Yield: 85% (based on Cu) for 1·3H2O. Anal. Calcd for C26H36Cu2N12O9: C, 39.64; H, 4.60; N, 21.34%. Found: C, 40.28; H, 4.53; N, 22.16%. IR (KBr pellet, cm−1): 3743 w, 3449 w, 3061 w, 3008 w, 2952 w, 2894 w, 2074 w, 1651 s, 1539 w, 1500 m, 1453 w, 1413 m, 1365 s, 1312 w, 1278 w, 1215 w, 1112 m, 1083 m, 2054 m, 1010 m, 956 w, 912 w, 869 w, 821 m, 761 w, 648 w, and 502 s. Caution. Sodium azide and tetrazolate compounds are energetic materials, which might explode under certain conditions and should be used only a small quantities. Caution is required during the preparation and handling of these compounds. Crystal Structure Determination. The SCXRD measurements were performed on a Rigaku Pilatus CCD diffractometer for the assynthesized crystals (1·3H2O) at 25 °C and under an air atmosphere. The SCXRD data of 1 were in situ collected on a SuperNova device at 80 °C under a dried N2 atmosphere. The as-synthesized crystals were treated at 80 °C under 95% RH and heated and refluxed at 100 °C, respectively. After the treated crystals were encapsulated in capillaries, their SCXRD data were collected on the Rigaku Pilatus diffractometer. The two CCD diffractometers were both equipped with a graphite-

monochromatic Mo Kα radiation source (λ = 0.71073 Å) by the ω scan mode. These structures were solved by methods using the SHELXTL version 5 package.57 Subsequent successive differences of Fourier syntheses yielded other non-hydrogen atoms. The final structures were refined using a full-matrix least-squares refinement on F2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of ligands were added geometrically and refined using the riding model. Hydrogen atoms of all water molecules were located in the idealized positions and refined with O−H distances restrained to a target value of 0.85 Å, the H···H distance to 1.34 Å, and Uiso(H) = 1.5 Ueq(O). All of the calculations were carried out by the SHELXTL-2014 program package of crystallographic software.58 Crystallographic parameters and structural refinement details for 1·3H2O and 1 obtained under different treatment conditions are listed in Table 1. The selected bond distances and angles are given in Tables S1 and S2.

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RESULTS AND DISCUSSION

Structural Description and Discussion. Since a safe, convenient, and environmentally friendly procedure to synthesize tetrazole ligands by [2 + 3] cycloaddition of an azide to a nitrile was reported by Demko and Sharpless,59 many tetrazole-based complexes with special properties have been obtained through in situ ligand syntheses.60−62 We have synthesized a series of tetrazole-based complexes with cyano ligands using this method in our lab.63,64 In this study, N-(2cyanoethyl)-L-hydroxyproline (L-cehp) was used to synthesize tzehp3−-based Cu(II) homochiral MOFs (1·3H2O). The synthetic route to 1·3H2O is illustrated in Scheme 1. The IR spectra denote the occurrence of the cycloaddition reaction. The diagnostic peak of the cyano group at approximately 2200 cm−1 disappeared, and the peaks of the tetrazolate group (at ca. 1500 and 1419 cm−1) emerged (Figure S1). The experimental PXRD pattern of the samples shows good agreement with the pattern simulated on the basis of the crystal structure, affirming the phase purity of 1·3H2O (Figure S2). 2323

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Figure 1. (a) The coordination environment around the Cu(II) atoms, Htzehp2−, and 4,4′-bipy ligands in 1·3H2O, with hydrogen atoms omitted for clarity. Symmetry codes: (A) −x + 1/2, y + 1/2, −z + 1; (B) −x + 1/2, y − 1/2, −z + 1; (C) −x, −y + 1, z; (b) 2D network of 1·3H2O with the hydroxyproline and ethyl functional groups omitted for clarity; (c) 3D supramolecular framework of 1·3H2O formed through hydrogen bonds with a green dotted line along the a-axis; (d) helical hydrogen bonds with green and black dotted lines along the b-axis.

Figure 2. A diagrammatic sketch of the reversible single-crystal to single-crystal transition between 1·3H2O (right) and 1 (left).

tetrazole ligands.48,65 The Cu(II) atom is coordinated by four N atoms (N12, N21, N14A, and N15A) and one O atom (O12A) with a distorted tetragonal pyramid coordination geometry (τ = 0.19).66 The O/N−Cu−N angles vary from 93.14(17) to 172.44(17)°. The N12 atom from the other Htzep2− ligand is located at the axial position with a bond distance of 2.285(5) Å. The Cu−N lengths in the range from 1.981(3) to 2.297(4) Å are consistent with those reported for penta-coordinated Cu(II) complexes.67 The Cu−O distance is 1.960(3) Å, which is within the rational bond length range. The Htzep2− ligand chelates the Cu(II) center to fabricate 1D helix chains along the b-axis (Figure S4a). The hydroxy group of the Htzep2− ligand does not coordinate to the Cu(II) atom. The

Structural analyses reveal that 1·3H2O crystallizes in the orthogonal space group P21212 and features a 2D framework constructed by Htzehp2− ligands, 4,4′-bipy ligands, and Cu(II) atoms. Each asymmetric unit comprises one Cu(II) atom, one Htzep2− ligand, one-half of a 4,4′-bipy ligand, and one-and-onehalf lattice water molecules (Figure 1a). All the atoms in the structure were exactly defined with no disorder, except the hydrogen atoms. Each Htzehp 2− ligand adopts a μ 4 kN12:kN14:kN15:kO12 coordination mode to bind one Cu(II) center into a tridentate fac mode (by the amine, carboxylate, and tetrazole groups) (Figure S3). The resulting five- and sixmembered chelate rings agree with those observed in other compounds featuring tridentate amino acid and amino acid− 2324

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Chemistry of Materials adjacent 1D chains are bridged through centro-symmetric 2connected 4,4′-bipy ligands to generate a 2D network parallel to the ab plane with a total solvent-accessible volume of approximately 9.5%, as calculated by the PLATON software (Figures 1b and S4b).68 The hydrogen bonds of 1·3H2O consist of well-ordered lattice water molecules and hydroxyl functional groups array inside the 2D interlayer spaces along the crystallographic a-axis ([100] direction) and construct the supramolecular 3D framework (Figures 1c and S5, Table S3). The hydrogen bond lengths along the b-axis are longer (>5.0 Å, represented with a black dotted line) (Figure 1d) than those along the a-axis, resulting in weaker hydrogen bonds. The helical hydrogen-bond chain has potential values for simulating the ion channels of biological systems.49 Interestingly, the 1·3H2O compound can undergo reversible SC−SC transitions (Figure 2). A new phase of 1 with a total solvent-accessible volume of approximately 7.4% and no lattice water molecules were obtained through the treatment of 1· 3H2O at 80 °C under a purged N2 atmosphere. The 1 phase maintains the same space group P21212, implying almost no structural change. However, the b- and c-axes in the unit cell are slightly shortened (Table 1).69 The 1 phase can transform to the 1·3H2O phase under a high humidity atmosphere. Reversible shrinking/swelling of frameworks through desorption/adsorption of H2O molecules has also been observed in the flexible MOFs.63,70,71 Additionally, the L-cehp ligands as homochiral molecular building blocks (MBBs) are prone to utilization as homochiral MOFs because of their two chiral carbon atoms in the rigid azole skeleton.51 The chiral centers of the ligand can be shifted into the framework of the MOFs while maintaining the configuration, which is referred to as “chirality conservation”. Notably, the structural analyses indicate that 1·3H2O maintains the original chiral configuration of the L-cehp. To verify the chiral configuration and inheritance of 1·3H2O, solid-state CD measurements of 1·3H2O and the L-cehp ligand were performed, respectively. The 1·3H2O compound shows two positive CD signals at ∼320 and ∼650 nm (Figure 3a, black line), and L-cehp displays one positive CD signal at ∼250 nm (Figure S6). To clarify the predominant mechanism of the solid-state CD spectra, additional calculations were carried out using the GAUSS software.72 The calculated transition energy (eV), oscillator f, rotational strength R, and contributions of 1· 3H2O are listed in Table S4. The dinuclear molecular fragment ([Cu2(Htzehp)2(4,4′-bipy)2]) of 1·3H2O was selected for calculation at the TDHF/6-31G** level, which was generated by the SpecDis software.73 The calculated CD spectra values of 1·3H2O are 470 and 790 nm, respectively, which exhibit a redshift of ∼145 nm compared with the experimental values (Figure 3a). In the case of the UV−vis spectra, there is about a ∼164 nm redshift between the theoretical and experimental values (Figure 3b). All of the red-shift values are in an appropriate range, reflecting the reasonableness of our selected calculation modes. The main contributions of the molecular orbitals are shown in Table S4 and Figure S7. The first positive Cotton effect (at ∼320 nm) is mainly attributed to the transition from state 22 to 24, including the LMCT interactions from the 4,4′-bipy ligands to the Cu2+ atoms and π−π* transition of Htzehp2− ligand. By analysis of the states 4 and 8, the LMCT transition from 4,4′-bipy and Htzehp2− ligands to the Cu2+ atoms and d−d transition from Cu2+ atoms are responsible for the second positive Cotton effect at ∼650 nm.

Figure 3. Comparison of (a) the experimental solid-state CD spectra and (b) UV−vis spectra of 1·3H2O to the calculated data. The theoretical CD and UV−vis spectra of 1·3H2O are red-shifted by 145 and 164 nm, respectively.

Thermal and Water Stability. The TGA curve (Figure S9) illustrates that 1·3H2O incurred a weight loss of 6.32% as the temperature increased up to 106 °C, in accordance with the release of three lattice water molecules (calcd 6.85%), and no further weight loss occurred until approximately 200 °C. Under continuous heating, the framework of 1·3H2O collapsed. Figure 4 shows the in situ PXRD patterns of 1·nH2O samples in a dried air atmosphere (RH < 5%). When heated to 80 °C, the 1·3H2O phase is converted into a new phase 1. As the temperature increases further, 1 remains almost unchanged until 200 °C. The diffraction peaks of 1·nH2O shift gradually to higher 2θ values, suggesting that, with increasing temperature, the unit cells shrink because of decreasing pore size.63,70,71 Meanwhile, the shifted peaks in the PXRD pattern return to their initial positions measured at 30 °C under an air atmosphere (RH > 70%), demonstrating that the shrinking/ swelling of the frameworks is reversible via the removal and readsorption of guest water molecules. These results are all consistent with the simulated SCXRD data for the two phases. The experimental SCXRD and in situ PXRD data confirm the thermal stability and the flexible framework with the breathing effect of the 1·nH2O samples.63,70,71 In addition, PXRD measurements under an air atmosphere (RH > 70%) were performed after the same samples were heated for 1 h in an oven at each temperature point, the resulting patterns show approximately no change in all of the peaks of the PXRD pattern and demonstrate that the samples are thermally stable to 200 °C (Figure S9). Furthermore, the SCXRD and PXRD data demonstrate the high moisture durability of the samples after treatment in water, such as immersion in water at room temperature for several weeks or refluxing in boiling water for 48 h (Figure S2). Water Vapor Adsorption/Desorption Isotherms. To illuminate the relationship between the number of adsorbed 2325

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Figure 4. Variable-temperature PXRD patterns of 1·nH2O from 30 to 200 °C (left) and a graphic description of the correlative structure transformation (right).

Figure 6 shows the Nyquist plots composed of a regular semicircle and capactive tail, which are typical for proton

water molecules and the RH, the water vapor adsorption/ desorption isotherms of 1 were measured at 25 °C with 1· 3H2O samples pretreated at 100 °C under vacuum overnight (Figure 5). Compound 1 exhibits relatively rapid adsorption of

Figure 6. Nyquist plots for pellets of the microcrystalline samples of 1· 3H2O at 30 °C at various RH levels.

Figure 5. Water vapor adsorption/desorption isotherms of 1 (at 25 °C). Closed and open squares represent the adsorption and desorption processes, respectively. A P/P0 value of 1 corresponds to 100% RH.

conductive materials.75,76 The conductivities of 1 and 1·3H2O were deduced from the semicircles. The crystal of 1 shows almost no conductivity (out of instrument’s detection range) at low RH and 30 °C. As the RH was increased, the conductivity increased and reached a maximum of 1.13 × 10−5 S cm−1 at 98% RH (Figure S11) with the formation of the 1·3H2O phase (Figures 2 and 4), which is comparable to those of previously reported 2D MOFs (Figure S12, Table S5). The conductivity increases with increasing RH, because the adsorbed water molecules can act as the conductive media.77,78 The low conductivity of 1 suggests that the hydrogen bonded chain in 1· nH2O plays an important role in the proton transport. Conductivity measurements involving microcrystalline samples compacted into pellets are often unable to reveal the intrinsic proton conductivity of the materials because of the grain boundaries. More importantly, characterization of anisotropic conductivity is precluded. Although a number of proton-conducting single-crystal inorganic materials have been reported,79 conductive single-crystal MOFs are still rare because of undefined guest molecules in the permanent pores or insufficient large size.20,80 In this work, the lattice water

the H2O molecules at low humidity and adsorbs 2.92 molecules per formula unit at 95% RH, approaching saturation adsorption. An obvious hysteresis is observed in the adsorption/desorption process at low humidity, which further confirms the flexible framework of 1·nH2O. The adsorbed water molecules were almost desorbed, indicating the reversible transformation between the two phases (1·3H2O and 1). We also investigated the N2 adsorption isotherms of 1 at 77 K (Figure S10). No significant N2 adsorption was observed, suggesting insufficient space within the interlayer for N2 uptake with a relative larger kinetic diameter of 3.64 Å.74 The strong adsorption of water molecules might be attributable to their smaller size (2.64 Å)74 and strong hydrogen bond interactions. Proton Conduction. To elucidate the relationship between the proton conductivity and the two phases, AC impedance measurements were performed using compacted pellets of microcrystalline samples obtained after vacuum treatment of 1· 3H2O at 100 °C (the formation of the 1 phase, Figure 4). 2326

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Figure 7. Nyquist plots for single crystals of 1·3H2O along the [100] and [010] directions under different RH levels at 30 °C (a, b) and under various temperatures at 95% RH (c, d).

molecules of 1·3H2O were accurately determined as mentioned above, which provides an opportunity to further study the conductivity of 1·3H2O in single crystals. The orientations of the single crystal were determined by a SuperNova, four-circle diffractometer, indexed as the [100], [010], and [001] directions (Figure S13). Compared with other directions, the (001) face (ab plane), corresponding to the crystallographic 2D layer in the structure, grows slowest, resulting in a rather thin thickness in size along the [001] direction. The [100] and [010] directions were selected to investigate the proton conductivity. As expected, higher proton conductivities of 1·3H2O were observed along the [100] direction in the range from 50 to 95% RH (Figure 7a). At 30 °C and 95% RH, the value of proton conductivity is 1.39 × 10−4 S cm−1. The value of the proton conductivity along the [010] direction is 1.52 × 10−6 S cm−1 under the same environmental conditions (Figure 7b). These results indicate that 1·nH2O is a highly anisotropic proton conductive MOF, and the σ[100]/σ[010] value is as high as 2 orders of magnitude (Figure S11). The temperature-dependent conductivity was also investigated under 95% RH (Figure 7c,d). According to the structure analysis and the water adsorption/ desorption isotherms, the hydrogen bonded chain remains stable at 80 °C and 95% RH and the phase of 1·nH2O is 1· 3H2O. The activation energy (Ea) values along [100] and [010] were as 0.48 and 0.56 eV, respectively (Table S6 and Figure 8). The relative weaker proton conductivity and higher Ea values along the [010] direction compared to the [100] direction are consistent with weaker hydrogen bonding along the b-axis than along the a-axis. The relatively high Ea value along the [010] direction suggests that the proton migration of 1·3H2O predominantly follows the Vehicle mechanism,81,82 which

Figure 8. Plots of ln(σT) vs 1000/T for the single crystals of 1·3H2O at 95% RH, as measured along the [100] and [010] directions, respectively. The red line is the fitted curve.

involves the proton carriers of free water molecules forming H3O+ to carry the protons,83−85 while hydrogen bonding along the [100] direction is in a confined space and the proton migration may follow the Grotthuss-type mechanism,86 which is consistent with the reported work by Kitagawa and coworkers.39 Therefore, the conducting mechanism in 1·3H2O includes both Vehicle-type and Grotthuss-type mechanisms. Increasing the temperature may enhance the acidity of the water molecules and increase the proton mobility to improve the proton conductivity. At 80 °C and 95% RH, the singlecrystal conductivity is 1.43 × 10−3 S cm−1 along the [100] direction. To date, only three 2D MOFs that exhibit singlecrystal proton-conduction behavior have been reported (compounds S11, S12, and S13) (Table S5). Remarkably, a single crystal of 1·3H2O shows the highest conductive value among them.37,41,42 Furthermore, 1·nH2O with structure2327

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Chemistry of Materials

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dependent proton conductivity may lead to the development of a new type of electrical switch and humidity sensor.80 For practical applications in fuel cells, time-dependent proton conductivity was recorded along the [100] and [010] directions at 80 °C under 95% RH (Figure 9). The proton conductivity of

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05497. Selected lengths and angles for the as-synthesized crystals of 1·3H2O and for 1; the main hydrogen bonds for 1· 3H2O; properties of transitions and their contributions of 1·3H 2O; proton conductivities and activation energies; parameters for the best fit of the data, activation energies (Ea), and proton conduction mechanism of 1· 3H2O; FT-IR spectra; PXRD patterns; tridentate fac mode; 1D helix chain of 1·3H2O; stacking direction of the supramolecular 3D framework; solid-state experimental CD spectra; molecular orbitals; TGA curve; adsorption/desorption isotherms; humidity-dependence proton conductivity; comparison of proton conductivity of microcrystalline sample 1·3H2O; single crystal directions of [100], [010], and [001] of 1·3H2O (PDF) C26H36Cu2N12O9 (CIF)

Figure 9. Time-dependent proton conductivity of the single crystals of 1·3H2O, as measured along the [100] and [010] directions at 80 °C and 95% RH.

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the single crystals of 1·3H2O was maintained up to 48 h with a negligible loss. The crystal structure of 1·3H2O was stable at 80 °C and 95% RH and was verified by both SCXRD and PXRD measurements (Table 1 and Figure S2). These results establish the high working temperature and high humidity durability of the 2D MOF, which is essential for practical applications.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fa-Kun Zheng: 0000-0002-7264-170X Guo-Cong Guo: 0000-0002-7450-9702 Notes

CONCLUSIONS

The authors declare no competing financial interest.

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We prepared a highly anisotropic proton conductive 2D homochiral copper(II) MOF with a high working temperature and high humidity durability. The reversible SC−SC transformation between the anhydrate (1) and trihydrate (1·3H2O) phases was induced by thermal treatment and humidity alteration and was clarified by SCXRD. The ordered and exactly defined water guest molecules in single crystals of 1· 3H2O provide the feasibility of building up “visible” protonconducting pathways. Single-crystal conductivity measurements revealed that the preferred proton-conduction path is along the [100] direction in 1·3H2O, consistent with the direction of the hydrogen bonded chain array. The anisotropic proton conductivity σ[100]/σ[010] was as high as 2 orders of magnitude. Among the reported 2D MOFs, the highest proton conductivity of 1·3H2O in single crystals was observed with 1.43 × 10−3 S cm−1 at 80 °C and 95% RH. Time-dependent measurements show the proton conductivity remained almost unchanged after being kept at 80 °C and 95% RH for 48 h, indicating good durability under moisture conditions and high thermal stability. This work is the first example and investigation demonstrating the proton conduction of MOFs based on amino acid−tetrazole ligands. This work illustrates a 2D MOF as an underlying proton-conductive material, providing guidance for unidirectional crystal growth to improve the performance of electrolyte materials and potential value for simulating and understanding the essence of complex biological mechanisms.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21371170, 21301059, and 21601186) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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