63 research outputs found
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Hydrogen bonding structure of confined water templated by a metal-organic framework with open metal sites.
Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate. Here, we combine infrared spectroscopy and many-body molecular dynamics simulations to probe the structure and dynamics of confined water as a function of relative humidity within a metal-organic framework containing cylindrical pores lined with ordered cobalt open coordination sites. Building upon the agreement between experimental and theoretical spectra, we demonstrate that water at low relative humidity binds initially to open metal sites and subsequently forms disconnected one-dimensional chains of hydrogen-bonded water molecules bridging between cobalt atoms. With increasing relative humidity, these water chains nucleate pore filling, and water molecules occupy the entire pore interior before the relative humidity reaches 30%. Systematic analysis of rotational and translational dynamics indicates heterogeneity in this pore-confined water, with water molecules displaying variable mobility as a function of distance from the interface
Rapid and Precise Determination of Zero-Field Splittings by Terahertz Time-Domain Electron Paramagnetic Resonance Spectroscopy
Zero-field splitting (ZFS) parameters are fundamentally tied to the
geometries of metal ion complexes. Despite their critical importance for
understanding the magnetism and spectroscopy of metal complexes, they are not
routinely available through general laboratory-based techniques, and are often
inferred from magnetism data. Here we demonstrate a simple tabletop
experimental approach that enables direct and reliable determination of ZFS
parameters in the terahertz (THz) regime. We report time-domain measurements of
electron paramagnetic resonance (EPR) signals associated with THz-frequency
ZFSs in molecular complexes containing high-spin transition-metal ions. We
measure the temporal profiles of the free-induction decays of spin resonances
in the complexes at zero and nonzero external magnetic fields, and we derive
the EPR spectra via numerical Fourier transformation of the time-domain
signals. In most cases, absolute values of the ZFS parameters are extracted
from the measured zero-field EPR frequencies, and the signs can be determined
by zero-field measurements at two different temperatures. Field-dependent EPR
measurements further allow refined determination of the ZFS parameters and
access to the g-factor. The results show good agreement with those obtained by
other methods. The simplicity of the method portends wide applicability in
chemistry, biology and material science.Comment: 36 pages, 30 figures, 1 tabl
Observing Nearby Nuclei on Paramagnetic Trityls and MOFs via DNP and Electron Decoupling
Dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement
technique that mediates polarization transfer from unpaired electrons to
NMR-active nuclei. Despite its success in elucidating important structural
information on biological and inorganic materials, the detailed
polarization-transfer pathway-from the electrons to the nearby and then the
bulk solvent nuclei, and finally to the molecules of interest-remains unclear.
In particular, the nuclei in the paramagnetic polarizing agent play significant
roles in relaying the enhanced NMR polarizations to more remote nuclei. Despite
their importance, the direct NMR observation of these nuclei is challenging
because of poor sensitivity. Here, we show that a combined DNP and electron
decoupling approach can facilitate direct NMR detection of these nuclei. We
achieved an ~80 % improvement in NMR intensity via electron decoupling at 0.35
T and 80 K on trityl radicals. Moreover, we recorded a DNP enhancement factor
of ~ 90 and ~11 % higher NMR intensity using electron decoupling on
a paramagnetic metal-organic framework, magnesium hexaoxytriphenylene (MgHOTP
MOF)
Million-Fold Electrical Conductivity Enhancement in Fe<sub>2</sub>(DEBDC) versus Mn<sub>2</sub>(DEBDC) (E = S, O)
Reaction of FeCl[subscript 2] and H[subscript 4]DSBDC (2,5-disulfhydrylbenzene-1,4-dicarboxylic acid) leads to the formation of Fe[subscript 2](DSBDC), an analogue of M[subscript 2](DOBDC) (MOF-74, DOBDC[superscript 4–] = 2,5-dihydroxybenzene-1,4-dicarboxylate). The bulk electrical conductivity values of both Fe2(DSBDC) and Fe[subscript 2](DOBDC) are ~6 orders of magnitude higher than those of the Mn[superscript 2+] analogues, Mn[subscript 2](DEBDC) (E = O, S). Because the metals are of the same formal oxidation state, the increase in conductivity is attributed to the loosely bound Fe[superscript 2+] β-spin electron. These results provide important insight for the rational design of conductive metal–organic frameworks, highlighting in particular the advantages of iron for synthesizing such materials.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)Alfred P. Sloan FoundationResearch Corporation for Science Advancement3M CompanyEuropean Research Council (Grant 277757
Robust Chemiresistive Behavior in Conductive Polymer/MOF Composites
Metal-organic frameworks (MOFs) are promising materials for gas sensing but
are often limited to single-use detection. We demonstrate a hybridization
strategy synergistically deploying conductive MOFs (cMOFs) and conductive
polymers (cPs) as two complementary mixed ionic-electronic conductors in
high-performing stand-alone chemiresistors. Our work presents significant
improvement in i) sensor recovery kinetics, ii) cycling stability, and iii)
dynamic range at room temperature. We demonstrate the effect of hybridization
across well-studied cMOFs based on 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP)
and 2,3,6,7,10,11-hexaiminotripphenylene (HITP) ligands with varied metal nodes
(Co, Cu, Ni). We conduct a comprehensive mechanistic study to relate energy
band alignments at the heterojunctions between the MOFs and the polymer with
sensing thermodynamics and binding kinetics. Our findings reveal that hole
enrichment of the cMOF component upon hybridization leads to selective
enhancement in desorption kinetics, enabling significantly improved sensor
recovery at room temperature, and thus long-term response retention. This
mechanism was further supported by density functional theory calculations on
sorbate-analyte interactions. We also find that alloying cPs and cMOFs enables
facile thin film co-processing and device integration, potentially unlocking
the use of these hybrid conductors in diverse electronic applications
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High Electrical Conductivity in Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 , a Semiconducting Metal–Organic Graphene Analogue
Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni2+ in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metalorganic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 Scm -1 and 40 Scm -1, respectively, both records for MOFs and among the best for any coordination polymer.Chemistry and Chemical Biolog
Investigation of the synthesis, activation, and isosteric heats of CO₂ adsorption of the isostructural series of metal-organic frameworks M₃(BTC)₂ (M = Cr, Fe, Ni, Cu, Mo, Ru)
The synthesis, activation, and heats of CO₂ adsorption for the known members of the M₃(BTC)₂ (HKUST-1) isostructural series (M = Cr, Fe, Ni, Zn, Ni, Cu, Mo) were investigated to gain insight into the impact of CO₂–metal interactions for CO₂ storage/separation applications. With the use of modified syntheses and activation procedures, improved BET surface areas were obtained for M = Ni, Mo, and Ru. The zero-coverage isosteric heats of CO₂ adsorption were measured for the Cu, Cr, Ni, Mo, and Ru analogues and gave values consistent with those reported for MOFs containing coordinatively unsaturated metal sites, but lower than for amine functionalized materials. Notably, the Ni and Ru congeners exhibited the highest CO₂ affinities in the studied series. These behaviors were attributed to the presence of residual guest molecules in the case of Ni₃(BTC)₂(Me₂NH)₂(H₂O) and the increased charge of the dimetal secondary building unit in [Ru₃(BTC)₂][BTC].Massachusetts Institute of Technology. Energy Initiative (Seed Fund
Cation-Dependent Intrinsic Electrical Conductivity in Isostructural Tetrathiafulvalene-Based Microporous Metal-Organic Frameworks
Isostructural metal–organic frameworks (MOFs) M[subscript 2](TTFTB) (M = Mn, Co, Zn, and Cd; H[subscript 4]TTFTB = tetrathiafulvalene tetrabenzoate) exhibit a striking correlation between their single-crystal conductivities and the shortest S···S interaction defined by neighboring TTF cores, which inversely correlates with the ionic radius of the metal ions. The larger cations cause a pinching of the S···S contact, which is responsible for better orbital overlap between pz orbitals on neighboring S and C atoms. Density functional theory calculations show that these orbitals are critically involved in the valence band of these materials, such that modulation of the S···S distance has an important effect on band dispersion and, implicitly, on the conductivity. The Cd analogue, with the largest cation and shortest S···S contact, shows the largest electrical conductivity, σ = 2.86 (±0.53) × 10[subscript –4] S/cm, which is also among the highest in microporous MOFs. These results describe the first demonstration of tunable intrinsic electrical conductivity in this class of materials and serve as a blueprint for controlling charge transport in MOFs with π-stacked motifs.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)National Science Foundation (U.S.). Graduate Research Fellowship Program (Award 1122374)David & Lucile Packard Foundation (Fellowship
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