Science Highlights

Exciton Migration and Amplified Quenching on Two-Dimensional Metal–Organic Layers

May 3, 2017
J. Am. Chem. Soc., 2017, 139 (20), pp 7020–7029

The dimensionality dependence of resonance energy transfer is of great interest due to its importance in understanding energy transfer on cell membranes and in low-dimension nanostructures. Light harvesting two-dimensional metal–organic layers (2D-MOLs) and three-dimensional metal–organic frameworks (3D-MOFs) provide comparative models to study such dimensionality dependence with molecular accuracy.  The scientists at the Xiamen University, China and the University of Chicago, USA report the construction of 2D-MOLs and 3D-MOFs from a donor ligand and a doped acceptor ligand. These 2D-MOLs and 3D-MOFs are connected by similar hafnium clusters, with key differences in the topology and dimensionality of the metal–ligand connection. Energy transfer from donors to acceptors through the 2D-MOL or 3D-MOF skeletons is revealed by measuring and modeling the fluorescence quenching of the donors. We found that energy transfer in 3D-MOFs is more efficient than that in 2D-MOLs, but excitons on 2D-MOLs are more accessible to external quenchers as compared with those in 3D-MOFs. These results not only provide support to theoretical analysis of energy transfer in low dimensions, but also present opportunities to use efficient exciton migration in 2D materials for light-harvesting and fluorescence sensing.

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Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S = 1 quantum antiferromagnet

April 20, 2017
Phys. Rev. B., 2017, 95, 134435

The magnetic ground state of the quasi-one-dimensional spin-1 antiferromagnetic chain is sensitive to the relative sizes of the single-ion anisotropy (D) and the intrachain (J) and interchain (J′) exchange interactions. The ratios D/J and J′/J dictate the material's placement in one of three competing phases: a Haldane gapped phase, a quantum paramagnet, and an XY-ordered state, with a quantum critical point at their junction. Density-functional theory calculations result in similar couplings (J = 9.2 K,  J′ = 1.8 K) and predict that the majority of the total spin population resides on the Ni(II) ion, while the remaining spin density is delocalized over both ligand types. The general procedures outlined in this paper permit phase boundaries and quantum-critical points to be explored in anisotropic systems for which single crystals are as yet unavailable.

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Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by Photocrystallography

March 1, 2017
J. Am. Chem. Soc., 2017, 139 (8), pp 2912–2915

Metal-oxygen and metal-nitrogen multiply bonded complexes are ubiquitous intermediates in synthetic as well as biological oxidation chemistry. While the effect of π-π* interaction between filled ligand orbital and vacant d-orbitals of early transition metals tend to stabilize the metal-ligand multiply bonded complexes of early transition metals, the same effect destabilizes the metal-ligand multiple bonds in mid-to-late transition metals due to π-π* interaction between filled ligand orbital and filled d-orbitals. These complexes are thus difficult to crystallographically characterize. Synthetic tuning of the ligands can stabilize these reactive complexes for characterization, but it typically renders the resulting complexes unreactive. The highlight of my research so far is demonstrating direct characterization of a reactive Ru2-nitride intermediate without altering its reactivity or stability, using photo-crystallography. The results obtained from photo-crystallographic experiments were in good agreement with the results obtained from EPR and EXAFS experiments, confirming the reliability of this method to characterize reactive molecules.

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Molecular Structure of Canonical Liquid Crystal Interfaces

February 8, 2017
J. Am. Chem. Soc., 2017, 139, pp 3841-3850

Liquid crystals (LCs) have been studied extensively, particularly in the context of display technologies and optical devices. The control of molecular orientation at the interface (LCs) is critical for these applications.  It is therefore of considerable interest to develop a detailed understanding of LC interfaces.  Researchers from the University of Chicago Institute for Molecular Engineering (including members of ChemMatCARS), the University of Illinois at Chicago Department of Physics, and the University of Wisconsin−Madison Department of Chemical Engineering have reconstructed a detailed picture of interfacial molecular organization at the air-liquid crystal interface. Synchrotron X-ray reflectivity measurements, accompanied by large-scale atomistic molecular dynamics simulations, are used to explore the interfacial molecular structure in two of the most widely used nematic (5CB) and smectic (8CB) LCs. Quantitative agreement is found between experiment and simulation. Results show that at the air interface both 5CB and 8CB adopt a homeotropic orientation, and exhibit well-defined surface-induced layers at the surface. The nematic system, 5CB, shows two surface-induced smectic layers at the 5CB-air interface. The smectic forming material, 8CB, induces multiple distinct surface-induced smectic layers with much longer penetration depth. In the isotropic phase of both 5CB and 8CB, however, only a single dominant layer is formed. 

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Robust Gold Nanoparticle Sheets by Ligand Cross-Linking at the Air–Water Interface

January 13, 2017
ACS Nano, 2017, 11, pp 1292-1300

Self-assembly of nanoparticles (NPs) into ordered two and three-dimensional (2D and 3D) structures gives access to materials with useful electronic, optical, and magnetic properties.  Researchers from University of Massachusetts and The University of Chicago have characterized the cross-linking of 2D Au-NPs at the air-water interface in situ by using techniques including AFM, XR, and GIXD. Highly elastic and robust films were obtained when an aqueous soluble PC-pyridine-substituted catalyst was introduced to the water subphase. The cross-linked NP monolayers enables the film to preserve its integrity upon compression and expansion, features that are absent in its non-cross-linked NP counterparts.


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Imparting amphiphobicity on Metal-Organic Framework materials

October 31, 2016
Nature Communications 7, 2016, pp 13300

Single-crystalline Metal-Organic Framework (MOF) materials show great promise for practical applications and have been extensively investigated for applications in gas storage and separation, carbon capture, catalysis etc.  However, in order to be practically applicable, they need to maintain their properties over long spans of time, under a variety of environments, including high levels of humidity and presence of organic vapors.  For this purpose, scientists from the Zhejiang University in China and from the University of South Florida developed a procedure for the functionalization of the exterior crystal surfaces with perfluoroalkyl groups, rendering them amphiphobic (i.e. both superhydrophobic and oleophobic).  The treated crystals are structurally identical to the untreated ones, as revealed by single crystal X-ray diffraction measurements performed at the ChemMatCARS Crystallography facility on 15-IDB.  Following the perfluoroalkyl treatment, the crystals withstood prolonged exposure to 100% relative humidity CO2 atmosphere, showing no degradation.

Understanding Peptide Oligomeric State in Langmuir Monolayers of Amphiphilic 3-Helix Bundle-forming Peptide-PEG Conjugates

October 26, 2016
Biomacromolecules, 2016, 17, pp 3964-3972

Coiled-coil peptide-polymer conjugates are an emerging class of biomaterials.  Combining the results from Langmuir isotherm, XR, and GIXD measurements, researchers from the University of California, Berkeley have investigated the peptide structure and the effect of PEGylation in governing the oligomeric state and self-assembly process of coiled-coil peptide-polymer conjugates.  PEGylated 3-helix amphiphiles exhibited a surface pressure dependent transition from a mixture of dimers and trimers at intermediate pressure to complete trimers at high pressure. Furthermore, the PEGylated 3-helix amphiphile was able to preserve the inter-helical distance at high surface pressure.

Basing Earth-abundant metal catalysts at Metal-Organic frameworks

October 10, 2016
Nat. Commun., 2016, 7, pp 12610

Studies of Earth-abundant metal catalysts have been performed by University of Chicago Chemistry department researchers, led by Prof. Wenbin Lin.  Recent results of these studies were published in the Nature Communications Aug. 2016 issue.  Using a straightforward metalation of Metal-Organic Framework (MOF) building units with cobalt and iron salts the researchers produced highly active and reusable single-site solid catalysts for a variety of organic reactions.  The molecular structure of the resulting catalysts was studied using single crystal diffraction, at the ChemMatCARS crystallography facility.  These results are of potential great economic significance showing that, using widely abundant metals, MOFs provide a platform for the development of highly active and affordable catalysts for the sustainable synthesis of fine chemicals. 

The Model Catalyst

September 30, 2016
J. Am. Chem. Soc., 2016, 138 (38), pp 12432–12439

Researchers used ChemMatCARS (Sector 15 at APS) to carry out detailed structural characterization of the catalyst binding sites in situ, such as single site catalysts on silica supports.  Octadecyltrioxysilane (OTOS) monolayers formed from octadecyltrimethoxysilane (OTMS) at the air−liquid interface after hydrolysis and condensation at low pH were chosen as a model system of surface binding sites in silica-supported Zn2+ catalysts. The results show that OTOS monolayers may serve as a platform for studying silica surface chemistry or hydroxyl-mediated reactions.

This work has been featured in APS Science Highlights

Surprising Ground State Structure of Crystalline Layered Nickelate

June 21, 2016
PNAS, 2016, 113 (32), pp 8945-8950

The quasi-2D nickelate, La4Ni3O8 (La-438), is a transition metal oxide (TMO) belonging to a family of materials of significant interest to researchers.  The nickelates are structurally and electronically related to high Tc superconductors and may serve as potential alternatives to cuprates.  La-438 undergoes a phase transition on cooling through 105 K, accompanied by a dramatic change in resistivity.  Diffraction studies of single crystals of La-438 at the ChemMatCARS crystallography facility, published in PNAS entitled “Stacked charge stripes in the quasi-2D trilayer nickelate La4Ni3O8”, showed that this phase transition is associated with charge stripe ordering, where the charge stripes are stacked directly on top of one another within each nickel oxide trilayer, but staggered between successive trilayers.  This result is surprising since, from simple local Coulomb energy considerations, a staggered arrangement within each trilayer was expected.

Metal−Organic Frameworks Stabilize Solution-Inaccessible Cobalt Catalysts for Highly Efficient Broad-Scope Organic Transformations

February 11, 2016
J. Am. Chem. Soc., 2016, 138 (9), pp 3241–3249

Researchers, led by Prof. Wenbin Lin from the University of Chicago, used ChemMatCARS crystallography facility in 15-ID-B highly to determined the structure of a robust, active, and reusable cobaltbipyridine- and cobalt-phenanthroline-based metal−organic framework (MOF) catalysts that the group has designed for alkene hydrogenation and hydroboration, aldehyde/ketone hydroboration, and arene C−H borylation. In alkene hydrogenation, the MOF catalysts tolerated a variety of functional groups and displayed unprecedentedly high turnover numbers of ∼2.5 × 106 and turnover frequencies of ∼1.1 × 105 h−1 .  MOFs thus provide a novel platform for discovering new base-metal molecular catalysts and exhibit enormous potential in sustainable chemical catalysis.

Surface Mechanical and Rheological Behaviors of PLGA–PEG Monolayers at the Air–Water Interface

January 2016
Langmuir, 2015, 31 (51), pp 13821-13833

Researchers using the ChemMatCARS 15-ID-C beamline at the APS are investigating PLGA-PEG monolayers due to the exhibition of protein resistance and reasonable chemical stability at the air-water interface. The results show that PLGA-PEG monolayers seem to have the potential to be used in lung surfactant applications.

Precise Molecular Fission and Fusion: Quantitative Self-Assembly and Chemistry of a Metallo-Cuboctahedron

September 10, 2015
Angew. Chem., 2015, 54 (32), pp 9224–9229

Scientists from University of Akron created the largest molecular spheres (a cuboctahedron) that were unequivocally characterized by synchrotron X-ray analysis performed at ChemMatCARS Advanced Crystallography facility.

Electric Field Effect on Phospholipid Monolayers at an Aqueous−Organic Liquid−Liquid Interface

July 23, 2015
J. Phys. Chem. B, 2015, 119 (29), pp 9319–9334

The electric potential difference across cell membranes, known as the membrane potential, plays an important role in the activation of many biological processes.  Using ChemMatCARS liquid surface scattering facility, researchers from University of Illinois at Chicago have investigated the effect of the membrane potential on the molecular ordering of lipids within a biomimetic membrane at an electrified oil/water interface.  Measurements at higher positive potentials illustrate a monotonic decrease in the lipid interfacial density and accompanying variations in the interfacial configuration of the lipid. 

Humidity-dependent compression-induced glass transition of the air–water interfacial Langmuir films of a polymer poly(D,L-lactic acid-ran-glycolic acid) (PLGA)

May 28, 2015
Soft Matter, 2015, 11 (28), pp 5666-5677

Scientists from Purdue University, led by Prof. You-Yeon Won, performed liquid surface x-ray reflectivity using ChemMatCARS Liquid Surface Scattering facility to elucidate the molecular mechanisms responsible for the transient glass transition behavior observed with highly compressed PLGA monolayers.  Combining with other experimental methods including surface pressure-area isotherm and interfacial rheological measurements, they have concluded, when the monolayer is compressed at a sufficiently fast speed, it produces dry regions on the air side of the monolayer at a rate higher than the re-moisturization rate of the monolayer.

Doughnut-like nanostructures

May 18th, 2015
Chem. Commun., 2015​, 51 (44), pp 9223-9226

Researchers using the ChemMatCARS 15-ID-B beamline at the APS are exploring a new family of anionic organic–inorganic hybrid doughnut-like nanostructures. The team’s research has been published in the Royal Society of Chemistry’s Chemical Communications Journal.

A versatile environmental control cell for in situ guest exchange single-crystal diffraction

April 2015

Scientists, led by Prof. Jason Benedict from University at SUNY, Buffalo, Natural Sciences Complex, designed and commissioned successfully a versatile environmental control cell for in situ guest exchange single-crystal diffraction measurements at ChemMatCARS Advanced Crystallography facility.  The first experiment performed under dynamic gas-flow conditions revealed that the cell was capable of stabilizing a novel metastable intermediate in the dehydration reaction of a previously reported metal-organic framework.

Reversible Crystallization at the Air-Water Interface

February 17, 2015
Sci. Rep., 2015, 5, pp 8497

Experiments performed at the ChemMatCARS liquid surface scattering facility provided evidence of a reversible phase transition process, between a 2-dimensional monolayer and a 3-dimensional crystalline structure.  The transition is driven by changes in surface pressure and, since crystalline phase normally represents the lowest energy state of condensed matter, the reversibility of this process is counterintuitive.  The existence of 3-dimensional crystals in the compressed phase was confirmed by atomic force microscopy measurements and the results of the study were published in Scientific Reports.

The Temperature Dependent Photoswitching of a Classic Diarylethene Monitored by in Situ X-ray Diffraction

January 9, 2015
J. Phys. Chem. A, 2015, 119 (5), pp 884–888

Researchers from University of SUNY Buffalo have examined temperature dependence of the light-driven photocyclization reaction in a classic diarylethene the using ChemMatCARS Advanced Crystallography facility (15ID-B).  These organic photochromic molecules including diarylethenes are of particular interest for their numerous potential applications including high-density optical data storage and light-activated switches.  Understanding how these systems behave under nonstandard conditions is critical to developing guiding principles that can be used to design and engineer photochromic materials with tailor-made properties.