NSF's ChemMatCARS

NSF's ChemMatCARS operates three experimental stations in the areas of advanced small-molecule crystallography, liquid surface and interface scattering, and small to wide-angle scattering at the Advanced Photon Source (APS), the premier undulator-based synchrotron source of high-brilliance high-energy x-rays in the U.S.A. The instrumentation at NSF's ChemMatCARS provides information that addresses a broad range of issues in chemistry and materials research.

Science Highlights

► Large anomalous Hall effect in the chiral-lattice antiferromagnet CoNb₃S₆

Abstract: An ordinary Hall effect in a conductor arises due to the Lorentz force acting on the charge carriers. In ferromagnets, an additional contribution to the Hall effect, the anomalous Hall effect (AHE), appears proportional to the magnetization. While the AHE is not seen in a collinear antiferromagnet, with zero net magnetization, recently it has been shown that an intrinsic AHE can be non-zero in non-collinear antiferromagnets as well as in topological materials hosting Weyl nodes near the Fermi energy. Here we report a large anomalous Hall effect with Hall conductivity of 27 Ω⁻¹ cm⁻¹ in a chiral-lattice antiferromagnet, CoNb₃S₆ consisting of a small intrinsic ferromagnetic component (≈0.0013 μ_B per Co) along c-axis. This small moment alone cannot explain the observed size of the AHE. We attribute the AHE to either formation of a complex magnetic texture or the combined effect of the small intrinsic moment on the electronic band structure.

Nirmal J. Ghimire, A.S. Botana, J.S. Jiang, Junjie Zhang, Y.-S. Chen, J.F. Mitchell, Large anomalous Hall effect in the chiral-lattice antiferromagnet CoNb₃S₆.  Nat. Commun. 9, 3280-1-3280-6 (2018). doi: 10.1038/s41467-018-05756-7 (2018).

► Topology-Guided Stepwise Insertion of Three Secondary Linkers in Zirconium Metal-Organic Frameworks 

Abstract: We report a topology-guided, precise insertion of three distinct secondary linkers into a zirconium-based metal–organic framework, NPF-300. Constructed from a tetratopic linker L and Zr₆ cluster, NPP-300 exhibits a unique scu topology and certain flexibility along the crystallographic a axis, and in conjunction with the conformation change of the primary ligand, is able to accommodate the stepwise insertion of three different secondary linkers along the a and c axes. Size-matching and mechanic strain of the resulting framework are two important factors that determine the chemical stability of the inserted linkers. Secondary linker insertion in NPF-300 significantly enables not only its porosity but also potentials to install up to three different functional groups for the construction of multivariate MOFs with homogeneity.

Xin Zhang, Brandon L. Frey, Yu-Sheng Chen, and Jian Zhang (2018) Topology-Guided Stepwise Insertion of Three Secondary Linkers in Zirconium Metal-Organic Frameworks.  J. Am. Chem. Soc., 140 (40), 7710-7715. doi: 10.1021/jacs.8b04277

► Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptors Accessible with Red/Near-Infrared Irradiation

Abstract: A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh₂(μ-DTolF)₂(μ-L)₂][BF₄]₂, where DTolF = N,N′-di(p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and from the electron donor p-phenylenediamine. Photoinduced charge transfer in the latter was inaccessible with previous Rh₂(II,II) complexes. These results underscore the fact that the excited state of this class of molecules can be readily tuned for electron-transfer reactions upon simple synthetic modification and highlight their potential as excellent candidates for p- and n-type semiconductor applications and for improved harvesting of low-energy light to drive useful photochemical reactions.

Tyler J. Whittemore, Agustin Millet, Hannah J. Sayre, Congcong Xue, Brian S. Dolinar, Eryn G. White, Kim R. Dunbar, and Claudia Turro (2018) Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptor Accessible with Red/Near-Infrared Irradiation. J. Am. Chem. Soc. 140 (15) 5161-5170. doi: 10.1021/jacs.8b00599

► Dissecting Porosity in Molecular Crystals: Influence of Geometry, Hydrogen Bonding, and [π···π] Stacking on the Solid-State Packing of Fluorinated Aromatics

Abstract: Porous molecular crystals are an emerging class of porous materials that is unique in being built from discrete molecules rather than being polymeric in nature. In this study, we examined the effects of molecular structure of the precursors on the formation of porous solid-state structures with a series of 16 rigid aromatics. The majority of these precursors possess pyrazole groups capable of hydrogen bonding, as well as electron-rich aromatics and electron-poor tetrafluorobenzene rings. These precursors were prepared using a combination of Pd- and Cu-catalyzed cross-couplings, careful manipulations of protecting groups on the nitrogen atoms, and solvothermal syntheses. Our study varied the geometry and dimensions of precursors, as well as the presence of groups capable of hydrogen bonding and [π···π] stacking. Thirteen derivatives were crystallographically characterized, and four of them were found to be porous with surface areas between 283 and 1821 m² g^–1. Common to these four porous structures were (a) rigid trigonal geometry, (b) [π···π] stacking of electron-poor tetrafluorobenzenes with electron-rich pyrazoles or tetrazoles, and (c) hydrogen bonding between the terminal heteroaromatic rings.

Mohamed I. Hashim, Ha T. M. Le, Teng-Hao Chen, Yu-Sheng Chen, Olafs Daugulis, Chia-Wei Hsu, Allan J. Jacobson, Watchareeya Kaveevivitchai, Xiao Liang, Tatyana Makarenko, Ognjen Å . MiljaniÄ, Ilja Popovs, Hung Vu Tran, Xiqu Wang, Chia-Hua Wu, and Judy I. Wu (2018) Dissecting Porosity in Molecular Crystals: Influence of Geometry, Hydrogen Bonding, and [π···π] Stacking on the Solid-State Packing of Fluorinated Aromatics. J. Am. Chem. Soc. 140 (18) 6014-6026. doi: 10.1021/jacs.8b02869

► Ordering of Molecular Rotor Monolayer on Aqueous Surface

Abstract: In situ grazing-incidence X-ray scattering shows that a monolayer of artificial rod-shaped dipolar molecular rotors produced on the surface of an aqueous subphase in a Langmuir trough has a structure conducive to a 2D ferroelectric phase. The axes of the rotors stand an average of 0.83 nm apart in a triangular grid, perpendicular to the surface within experimental error. They carry 2,3-dichlorophenylene rotators near rod centers, between two decks of interlocked triptycenes installed axially on the rotor axle. The analysis is based first on simultaneous fitting of observed Bragg rods and second on fitting the reflectivity curve with only three adjustable parameters and the calculated rotor electron density, which also revealed the presence of about seven molecules of water near each rotator. Dependent on preparation conditions, a minor and variable amount of a different crystal phase may also be present in the monolayer.

Jiří Kaleta, Jin Wen, Thomas F. Magnera, Paul I. Dron, Chenhui Zhu, and Josef Michl (2018) Structure of a Monolayer of Molecular Rotors on Aqueous Subphase from Grazing-Incidence X-ray Diffraction. PNAS. 115 (38) 9373-9378. https://doi.org/10.1073/pnas.1712789115

► Nanoscale View of Assisted Ion Transport Across the Liquid-Liquid Interface

Abstract: During solvent extraction, amphiphilic extractants assist the transport of metal ions across the liquid–liquid interface between an aqueous ionic solution and an organic solvent. Investigations of the role of the interface in ion transport challenge our ability to probe fast molecular processes at liquid–liquid interfaces on nanometer-length scales. Recent development of a thermal switch for solvent extraction has addressed this challenge, which has led to the characterization by X-ray surface scattering of interfacial intermediate states in the extraction process. Here, we review and extend these earlier results. We find that trivalent rare earth ions, Y(III) and Er(III), combine with bis(hexadecyl) phosphoric acid (DHDP) extractants to form inverted bilayer structures at the interface; these appear to be condensed phases of small ion–extractant complexes. The stability of this unconventional interfacial structure is verified by molecular dynamics simulations. The ion–extractant complexes at the interface are an intermediate state in the extraction process, characterizing the moment at which ions have been transported across the aqueous–organic interface, but have not yet been dispersed in the organic phase. In contrast, divalent Sr(II) forms an ion–extractant complex with DHDP that leaves it exposed to the water phase; this result implies that a second process that transports Sr(II) across the interface has yet to be observed. Calculations demonstrate that the budding of reverse micelles formed from interfacial Sr(II) ion–extractant complexes could transport Sr(II) across the interface. Our results suggest a connection between the observed interfacial structures and the extraction mechanism, which ultimately affects the extraction selectivity and kinetics.

Zhu Liang Wei Bu, Karl J. Schweighofer, David J. Walwark Jr., Jeffrey S. Harvey, Glenn R. Hanlon, Daniel Amoanu, Cem Erol, Ilan Benjamin, and Mark L. Schlossman (2018) Nanoscale View of Assisted Ion Transport Across the Liquid-Liquid Interface. PNAS. https://doi.org/10.1073/pnas.1701389115

Scientific Focus

The scientific focus of user activities at ChemMatCARS is the study of structure and dynamics over the range of length scales from atomic and molecular to mesoscopic. The experimental techniques available to the scientific community span a spatial resolution of sub-angstrom to micrometer and a time resolution from 50 ns to minutes. These include:

​High Precision Crystallography to study charge (i.e., electron) densities, bonding, microcrystals, resonant diffraction, high-pressure (up to 10 GPa) single crystal diffraction, and transient state/photo-crystallography.

Scattering From Liquid Surfaces and Liquid-Liquid Interfaces to measure atomic, molecular, and mesoscopic ordering at interfaces using resonant and non-resonant reflectivity, grazing-incidence diffraction and small angle scattering, surface fluorescence, surface diffuse scattering, and fast techniques, such as grazing incidence diffraction in the 1D pinhole geometry.

Anomalous Small Angle X-ray scattering (ASAXS) to probe element specific structural information from materials in the length scales of few nanometers to hundreds of nanometers.

NSF's CHEMMATCARS IS SUPPORTED BY THE DIVISIONS OF CHEMISTRY AND MATERIALS RESEARCH,
NATIONAL SCIENCE FOUNDATION
UNDER GRANT NUMBER NSF/CHE-1834750.