Highly stable superprotonic conductivity (>10–2 S cm–1) has been achieved through the unprecedented solvent-free-coordinative urea insertion in MOF-74 M2(dobdc), M = Ni2+, Mg2+; dobdc = ...2,5-dioxido-1,4-benzenedicarboxylate without an acidic moiety. The urea is bound to open metal sites and alters the void volume and surface functionality, which triggers a significant change in proton conductivity and diffusion mechanism. Solid-state 2H NMR revealed that the high conductivity was attributed to the strengthening of the hydrogen bonds between guest H2O induced by hydrogen bonds in the interface between H2O and the polarized coordinated urea.
Proton conductivity has been traditionally investigated with various materials such as organic polymers, metal oxides, and other inorganic and organic compounds because of their potential application ...in the electrochemical devices. In particular, during the last decade, crystalline porous coordination polymers (PCPs) or metal‐organic frameworks (MOFs) have received considerable attention in recent years, as solid‐state proton conductors (SSPCs). To date, proton‐conductive MOFs have achieved high performance in proton conductivity (>10−2 S cm−1) with rational design strategies. In addition, there are dedicated efforts to define the conduction pathway and mechanism using various experimental tools. In this review, we focus on the characterization of proton conductivity and molecular dynamics in hydrated MOFs, with selected examples to provide an understanding of the overall conduction mechanism.
This review aims to provide the interpretation and characterization of proton dynamics derived from the molecular motions in proton conductive metal‐organic frameworks. Overall conduction mechanisms related to conducting media are introduced.
Attractive interactions between ions of like charge remain an elusive concept. Observing and quantifying this type of interaction in liquids and solutions is still a major challenge. Recently, we ...have shown that cation–cation interactions are present in hydroxyl-functionalized ionic liquids and that they can be controlled by the shape, charge distribution and functionality of the ions. In the present study, we demonstrate that cationic cluster formation does not only change the local structures of the ionic liquids but also influences the dynamics of the cations in a characteristic way. We show that solid-state 2H NMR spectroscopy is well suited for the study of molecular motion, even if the hydrogen bonded species of interest are indistinguishable due to fast deuteron exchange. We also provide valuable information about the applicability of well-accepted relaxation models.
The desolvated (3,24)-connected metal–organic framework (MOF) material, MFM-160a, Cu3(L)(H2O)3 H6L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid), exhibits excellent high-pressure ...uptake of CO2 (110 wt% at 20 bar, 298 K) and highly selective separation of C2 hydrocarbons from CH4 at 1 bar pressure. Henry’s law selectivities of 79:1 for C2H2:CH4 and 70:1 for C2H4:CH4 at 298 K are observed, consistent with ideal adsorption solution theory (IAST) predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C2H2:CO2. Solid-state 2H NMR spectroscopic studies on partially deuterated MFM-160-d 12 confirm an ultra-low barrier (∼2 kJ mol–1) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 × 106 Hz cf. 1011–1013 Hz). Upon introduction of CO2 or C2H2 into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO2 and C2H2 around the triazine core are entirely consistent with the 2H NMR spectroscopic observations.
Modulation and precise control of porosity of metal-organic frameworks (MOFs) is of critical importance to their materials function. Here we report modulation of porosity for a series of isoreticular ...octacarboxylate MOFs, denoted MFM-180 to MFM-185, via a strategy of selective elongation of metal-organic cages. Owing to the high ligand connectivity, these MOFs do not show interpenetration, and are robust structures that have permanent porosity. Interestingly, activated MFM-185a shows a high Brunauer–Emmett–Teller (BET) surface area of 4,734 m² g−1 for an octacarboxylate MOF. These MOFs show remarkable CH₄ and CO₂ adsorption properties, notably with simultaneously high gravimetric and volumetric deliverable CH₄ capacities of 0.24 g g−1 and 163 vol/vol (298 K, 5–65 bar) recorded for MFM-185a due to selective elongation of tubular cages. The dynamics of molecular rotors in deuterated MFM-180a-d16 and MFM-181a-d16 were investigated by variable-temperature ²H solid-state NMR spectroscopy to reveal the reorientation mechanisms within these materials. Analysis of the flipping modes of the mobile phenyl groups, their rotational rates, and transition temperatures paves the way to controlling and understanding the role of molecular rotors through design of organic linkers within porous MOF materials.
Observing and quantifying the like-charge attraction in liquids and solutions is still challenging. However, we showed that elusive cation–cation hydrogen bonding may govern the structure and ...interaction in hydroxyl-functionalized ionic liquids. Therefore, cationic cluster formation depends on the shape, charge distribution, and functionality of the ions. We demonstrated by means of solid-state 2H NMR spectroscopy that cationic clusters change the structure and dynamics of ionic liquids. With increasing alkyl chain length, we observed two deuteron quadrupole coupling constants for the OD groups, differing by about 30 kHz. The lower value was assigned to the cation–cation interaction, indicating that the average (c–c) hydrogen bonds are stronger than the (c–a) hydrogen bonds between the cation and the anion despite the repulsive and attractive Coulomb interaction in the first and latter cases. Ion mobility could be studied by 2H NMR spectroscopy, although the deuterons in the hydrogen-bonded clusters underwent fast exchange. Our results also showed that simple relaxation models are not applicable anymore and that anisotropic motion must be considered.
We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3‐octylphosphonium bis(trifluoromethylsulfonyl)imide C8H17PD3NTf2 by means of solid‐state NMR spectroscopy. ...Combined line shape and spin relaxation studies of the deuterons in the PD3 group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD3 deuterons. A transition enthalpy of about 12 kJ mol−1 from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge‐enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded P−D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD3 group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces.
Tunneling driven dynamics of the d3‐octyl phosphonium cation in an ionic liquid is observed by means of solid‐state NMR spectroscopy. Deuteron line‐shape analysis and relaxation time measurements provide information about the structure, mobility and rearrangements of the cations for the broad temperature range between 73 and 343 K. Differently strong hydrogen bonds are reported for the solid, supercooled and liquid state of the IL and map the change in molecular ordering upon phase transformation.
We measured the deuteron quadrupole coupling constants (DQCCs) for hydroxy-functionalized ionic liquids (ILs) with varying alkyl chain length over the temperature range between 60 and 200 K by means ...of solid-state NMR spectroscopy. For all temperatures, the 2H spectra show two DQCCs representing different types of hydrogen bonds. Higher values, ranging from 220 to 250 kHz, indicate weaker hydrogen bonds between cation and anion (c-a), and lower values varying from 165 to 210 kHz result from stronger hydrogen bonds between the OD groups of cations (c-c), in agreement with recent observations in infrared, neutron diffraction, and NMR studies. We observed different temperature dependencies for (c-a) and (c-c) hydrogen bonding. From the static pattern of the 2H spectra at the lowest temperatures, we derived the true DQCCs being up to 20 kHz larger than recently reported values measured at the glass transition temperature. We were able to freeze the librational motions of the hydrogen bonds in the ILs. The temperature dependence of the (c-a) and (c-c) cluster populations in the glassy state is opposite to that observed in the liquid state, partly anticipating the behavior of ILs tending to crystallize.
We report strong isotope effects for the protic ionic liquid triethylammonium methanesulfonate TEAOMs by means of deuterium solid‐state NMR spectroscopy covering broad temperature ranges from 65 K to ...313 K. Both isotopically labelled PILs differ in non‐deuterated and fully deuterated ethyl groups of the triethyl ammonium cations. The N−D bond of both cations is used as sensitive probe for hydrogen bonding and structural ordering. The 2H NMR line shape analysis provides the deuteron quadrupole coupling constants and the characteristics of a broad heterogeneous phase with simultaneously present static and mobile states indicating plastic crystal behavior. The temperatures where both states are equally populated differ by about 80 K for the two PILs, showing that deuteration of the ethyl groups in the trialkylammonium cations tremendously shifts the equilibrium towards the static state. In addition, it leads to a significant less cooperative transition, associated with a significantly reduced standard molar transition entropy.
Isotope effects matter! Strong isotope effects for the protic ionic liquid triethylammonium methanesulfonate are observed by means of deuterium solid‐state NMR spectroscopy. The 2H NMR line shape analysis provides the deuteron quadrupole coupling constants and the characteristics of a broad heterogeneous phase with simultaneously present static and mobile states indicating plastic crystal behavior.
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