Enhanced proton conductivity in a flexible metal–organic framework promoted by single-crystal-to-single-crystal transformation

MFM-722(Pb)-DMA undergoes a single-crystal-to-single-crystal (SCSC) transformation to give MFM-722(Pb)-H 2 O via ligand substitution upon exposure to water vapour. In situ single crystal impedance spectroscopy reveals an increase in proton conductivity due to this structural transition, with MFM-722(Pb)-H 2 O showing a proton conductivity of 6.61 (cid:2) 10 (cid:3) 4 S cm (cid:3) 1 at 50 8 C and 98% RH. The low activation energy ( E a = 0.21 eV) indicates that the proton conduction follows a Grotthuss mechanism.

exhibit a conductivity of 1.27 Â 10 À1 S cm À1 at 80 1C under 100% RH. 8 In contrast, improving the proton conductivity of non-porous MOFs can be highly challenging due to limitations in their design to enable flexible chemical modification or doping. 15,16 Phase transition has been reported to be an effective approach to tune the property of flexible MOFs, 17 and this can be triggered by ligand substitution, 18 guest uptake, 19 or by changes in temperature 20 and/or pressure. 21 However, studies on the impact of single-crystal-to-singlecrystal (SCSC) transformations on proton conductivity in MOFs have been reported rarely. 22,23 We report herein the SCSC transformation via ligand substitution in a nonporous Pb(II)-based MOF, MFM-722(Pb)-DMA, and the enhancement of proton conductivity in the resultant MFM-722(Pb)-H 2 O. The diverse coordination environment of Pb(II) ions naturally endows Pb(II)-based MOFs with abundant structural diversity and flexibility. Single crystal impedance spectroscopy can reduce the impact of grain boundaries that are inherent in bulk-pellet measurements, 24 and has been employed here to evaluate the change of proton conductivity of MFM-722(Pb)-DMA during the SCSC transformation on exposure to water vapour.
MFM-722(Pb)-DMA was synthesised by solvothermal reaction of Pb(NO 3 ) 2 and biphenyl-3,3 0 ,5,5 0 tetracarboxylic acid (H 4 L) 25 in DMA at 90 1C for 3 days and isolated as colourless rod-shaped single crystals. The single crystals were collected on cooling by filtration and dried in air. Single crystal X-ray diffraction revealed that MFM-722(Pb)-DMA, [Pb 2 (L)(DMA) 2 ], crystallises in the triclinic space group P% 1, featuring a three dimensional framework comprised of extended Pb(II) oxide chains [Pb 2 O 10 ] N bridged by two crystallographically-independent ligands L 4À (Fig. 1d and Fig. S1, Table S1, ESI †). This yields a narrow channel along the c axis, which is filled with two coordinated DMA molecules, resulting in a nonporous structure (Fig. 1d). There are two crystallographically-independent Pb atoms, both of which are 7 coordinated by oxygen donors (Fig. 1c). Pb (1)  According to the VSEPR model, 23 the geometry of both [PbO 7 ] polyhedron can be described as a distorted c-trigonal bipyramid (c-TBP) with a vacant vertex in the equatorial plane. The powder X-ray diffraction (PXRD) patterns of MFM-722(Pb)-DMA confirm its phase purity (Fig. S13, ESI †). TGA shows that the coordinated DMA molecules can be removed at 125-220 1C (weight loss of 19%, calc. 19%), followed by a framework decomposition at B400 1C (Fig. S15, ESI †).
We sought to monitor the change of proton conductivity of MFM-722(Pb)-DMA in situ during the SCSC transformation using single crystal AC impedance spectroscopy (Fig. 2). The measurements were carried out on two single crystals of MFM-722(Pb)-DMA (sizes of 468 Â 64 Â 54 mm, Fig. 2a, and 325 Â 41 Â 31 mm, Fig. 2b-d) using a conventional two-contact wire-paste method. 24 The single crystals were rested on an insulating glass substrate in a humidity chamber. Soft gold wires (F = 25 mm) connected to Pt foil electrodes were contacted with single crystals using gold paste to enable measurement of the proton conductivity (Fig. S18a, ESI †). Analysis of the face index confirms that the proton conductivity was measured along the crystallographic c axis in MFM-722(Pb)-DMA, which remained the same for MFM-722(Pb)-H 2 O on SCSC transformation ( Fig. S18b and c, ESI †). Nyquist plots show a typical semi-circle in the high frequency region indicative of the intrinsic conductivity of the material, with the tail at low frequency representing the blocking of protons at the electrode interface. 26 At 25 1C and 98% RH, the proton conductivity of the single crystals increased gradually from 3.64 Â 10 À5 S cm À1 (at 0 h) to 8.09 Â 10 À5 S cm À1 (at 9 h), which then stabilised to 1.33 Â 10 À4 S cm À1 over 30 h (Fig. 2a). The increase in conductivity originates from the phase transition from MFM-722(Pb)-DMA to MFM-722(Pb)-H 2 O (Fig. S19, ESI †), and Fig. 2b, c show the temperature dependence of the single crystal proton conductivity of MFM-722(Pb)-H 2 O. The conductivity increases with increasing temperature and reaches 6.61 Â 10 À4 S cm À1 at 50 1C at 98% RH. This value is comparable to a Cu-MOF system with the proton conductivity of 5.48 Â 10 À3 S cm À1 at 60 1C and 95% RH after the phase transition (Table S8, ESI †). 22 At temperatures above 50 1C, the proton conductivity of MFM-722(Pb)-H 2 O decreases slightly to 3.55 Â 10 À4 S cm À1 at 75 1C, most likely due to the partial loss of surface/interstitial water molecules. No apparent structural change is observed for MFM-722(Pb)-H 2 O between 25 and 75 1C at 98% RH (Fig. S20, ESI †), and the proton conductivity returns to 1.16 Â 10 À4 S cm À1 on cooling to 25 1C (Fig. S21, ESI †). The activation energy (E a ) for MFM-722(Pb)-H 2 O was calculated from the variable temperature impedance spectra to be 0.21 eV (Fig. 2d), suggesting that the proton diffusion is governed by the Grotthuss mechanism, where protons hop along the hydrogen bonding networks.
The proton conductivity of bulk MFM-722(Pb)-DMA has also been measured using compressed pellets at 21 1C at 98% RH (Fig. S22, ESI †). The proton conductivity of the pellet of MFM-722(Pb)-DMA (3.47 Â 10 À8 S cm À1 , Fig. S23, ESI †) increases slowly over time, and reaches 1.20 Â 10 À4 S cm À1 at 21 1C and 98% RH over B7 days linked to the phase transition and formation of MFM-722(Pb)-H 2 O (Fig. S22, ESI †). Given the nonporous structure of this material, additional time is required to allow diffusion of water through the compressed pellet to drive the phase transition compared to the single crystal. Distinct to systems reported in literature 9 where the single crystal impedance measurements often yield a B100-fold increase in proton conductivity compared to that obtained from measurements on bulk sample (due to the elimination of resistance from the crystallite boundaries and the judicious utilisation of the anisotropic conductivity), single crystals and pellets of MFM-722(Pb)-H 2 O exhibit similar values of conductivity.
A detailed examination of the hydrogen bonding network in MFM-722(Pb)-H 2 O identified the crystallographic a axis as the potential pathway for proton hopping (dashed red lines in Fig. 3c). Six adjacent oxygen centres (O2, O7, O9, O2*, O7*, O9*) from two neighbouring [Pb] 4 clusters form a supra-octahedron [OÁ Á ÁO = 2.837(34)-3.504(43) Å, Table S3, ESI †] that enables proton hopping (Fig. 3d). A continuous hydrogen bonding pathway is established by packing these hexa-oxygen supra-octahedra along the a axis with a gap of B4.5 Å, which can be bridged by interstitial water molecules (Fig. 3e). Indeed, the water adsorption isotherm of MFM-722(Pb)-H 2 O at 25 1C shows a type-II profile with a total uptake of 3.32 mmol g À1 (Fig. S12, ESI †), attributed to the adsorption of surface/interstitial water. Thus, the a axis is predicted to be the conducting axis of MFM-722(Pb)-H 2 O. However, due to the rod-shaped single crystal, accurate proton conductivities can only be measured along the c axis, which has a much greater barrier for proton hopping due to the ligands (Fig. S8, ESI †). This explains the similar conductivity observed in the single crystal and powder impedance measurements.
In summary, a new flexible Pb(II)-based nonporous MOF has been synthesised and its framework flexibility characterised. On exposure to water, MFM-722(Pb)-DMA undergoes a SCSC transformation to MFM-722(Pb)-H 2 O, induced by the substitution of coordinated DMA by water ligands at room temperature. The single crystal proton conductivity of MFM-722(Pb)-H 2 O reaches 6.61 Â 10 À4 S cm À1 at 50 1C under 98% RH, a four-fold enhancement compared with MFM-722(Pb)-DMA. The low activation energy (E a = 0.21 eV) is consistent with the Grotthuss mechanism with the protons hopping along the hydrogen bonding network. This study will promote future design of flexible nonporous MOFs showing enhanced proton conductivity on phase transition.
We thank EPSRC (EP/I011870, EP/P001386), China Scholarship Council (CSC) and the University of Manchester for funding. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 742401, NANOCHEM). We are grateful to Diamond Light Source for access to the Beamline I11.