Dynamics of Linkers in Metal-Organic Framework Glasses.

Metal-organic framework (MOF) glasses have emerged as a new class of organic-inorganic hybrid glass materials. Considerable efforts have been devoted to unraveling the macroscopic dynamics of MOF glasses by studying their rheological behavior; however, their microscopic dynamics remain unclear. In this work, we studied the effect of vitrification on linker dynamics in ZIF-62 by solid-state 2H nuclear magnetic resonance (NMR) spectroscopy. 2H NMR relaxation analysis provided a detailed picture of the mobility of the ZIF-62 linkers, including local restricted librations and a large-amplitude twist; these details were verified by molecular dynamics. A comparison of ZIF-62 crystals and glasses revealed that vitrification does not drastically affect the fast individual flipping motions with large-amplitude twists, whereas it facilitates slow cooperative large-amplitude twist motions with a decrease in the activation barrier. These observations support the findings of previous studies, indicating that glassy ZIF-62 retains permanent porosity and that short-range disorder exists in the alignment of ligands because of distortion of the coordination angle.

Cooperative Proton and Li-ion Conduction in a 2D-Layered MOF via Mechanical Insertion of Lithium Halides.

Ionic conduction in highly designable and porous metal-organic frameworks has been explored through the introduction of various ionic species (H+ , OH- , Li+ , etc.) using post-synthetic modification such as acid, salt, or ionic liquid incorporation. Here, we report on high ionic conductivity (σ>10-2  S cm-1 ) in a two-dimensionally (2D)-layered Ti-dobdc (Ti2 (Hdobdc)2 (H2 dobdc), H4 dobdc: 2,5-dihydroxyterephthalic acid) via LiX (X=Cl, Br, I) intercalation using mechanical mixing. The anionic species in lithium halide strongly affect the ionic conductivity and durability of conductivity. Solid-state pulsed-field gradient nuclear magnetic resonance (PFG NMR ) verified the high mobility of H+ and Li+ ions in the temperature range of 300-400 K. In particular, the insertion of Li salts improved the H+ mobility above 373 K owing to strong binding with H2 O. Furthermore, the continuous increase in Li+ mobility with temperature contributed to the retention of the overall high ionic conductivity at high temperatures.

Rational Construction of Molecular Electron-Conducting Nanowires Encapsulated in a Proton-Conducting Matrix in a Charge Transfer Salt.

Insulated molecular wires have gained significant attention owing to their potential contribution in the fields of nanoelectronics and low-dimensional chemistry/physics. Based on molecular charge transfer salts, we demonstrate, for the first time, the rational construction of molecular electron-conducting wires encapsulated in a proton-conducting matrix, which possibly paves the way to ionoelectronics. As expected from the molecular structure of the newly designed complex anion (i.e., propeller-shaped structure with hydrogen-bonding sites at four edges), a three-dimensional hydrogen-bonded framework was constructed within the crystal, which contains a one-dimensional array of an electron donor, tetrathiafulvalene (TTF). From the single-crystal crystallographic and spectroscopic studies, it was clarified that the nonstoichiometric deprotonation of anions and partial oxidation of TTFs occur, whereas the anion is electronically inert. Moderate conductivities of electron and proton were confirmed by dc and ac conductivity measurements. In addition, the electronic isolation of TTF wires was confirmed by the magnetic susceptibility data.

High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid.

We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C8 H17 PD3 ][NTf2 ] 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.

Various Stacking Patterns of Two-Dimensional Molecular Assemblies in Hydrogen-Bonded Cocrystals: Insight into Competitive Intermolecular Interactions and Control of Stacking Patterns.

Control over the stacking patterns in 2D molecular assemblies is demonstrated using chemical modification. A target system is a hydrogen-bonded cocrystal (2:1) composed of 2-pyrrolidone (Py) and chloranilic acid (CA) (PyCA). X-ray crystallography showed that weak intersheet interactions give rise to a variety of metastable overlapping patterns comprised of the 2D assemblies mainly formed via hydrogen bonds, affording reversible and irreversible structural phase transitions. We prepared cocrystals of Py and anilic acids bearing different halogens, in which 2D assemblies isostructural with those observed in PyCA exhibit various overlapping patterns. The order of stability for each overlapping pattern estimated using calculations of the intermolecular interactions did not completely coincide with those indicated by our experimental results, which can be explained by considering the entropic effect: the molecular motion of Py as detected using nuclear quadrupole resonance spectroscopy.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion.

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)2 ](BPh4 )2 ⋅3ac (1⋅3 ac; FPh-terpy=4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 180° flip-flop motion of the ligand's polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state 19 F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

Freezing the Motion in Hydroxy-Functionalized Ionic Liquids-Temperature Dependent NMR Deuteron Quadrupole Coupling Constants for Two Types of Hydrogen Bonds Far below the Glass Transition.

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.

The effect of amorphization on the molecular motion of the 2-methylimidazolate linkers in ZIF-8.

We investigated the effect of amorphization on the mobility of the 2-methylimidazolate linkers in ZIF-8. Solid-state 2H NMR studies revealed that amorphization significantly affects the mobility of the linkers, including the rotational angle and speed. Furthermore, a new intermediate librational mode appeared in amorphous ZIF-8.

Drastic rearrangement of self-assembled hydrogen-bonded tapes in a molecular crystal.

A 2 : 1 hydrogen-bonded crystal of 2-pyrrolidone and chloranilic acid shows structural phase transitions accompanied by the drastic rearrangement of hydrogen-bonded tapes. Such a phenomenon is attributed to the selective and directional character of hydrogen bonds.