Molecular Insights into Carbon Dioxide Sorption in Hydrazone-Based Covalent Organic Frameworks with Tertiary Amine Moieties

Tailorable sorption properties at the molecular level are key for efficient carbon capture and storage and a hallmark of covalent organic frameworks (COFs). Although amine functional groups are known to facilitate CO2 uptake, atomistic insights into CO2 sorption by COFs modified with amine-bearing functional groups are scarce. Herein, we present a detailed study of the interactions of carbon dioxide and water with two isostructural hydrazone-linked COFs with different polarities based on the 2,5-diethoxyterephthalohydrazide linker. Varying amounts of tertiary amines were introduced in the COF backbones by means of a copolymerization approach using 2,5-bis(2-(dimethylamino)ethoxy)terephthalohydrazide in different amounts ranging from 25 to 100% substitution of the original DETH linker. The interactions of the frameworks with CO2 and H2O were comprehensively studied by means of sorption analysis, solid-state NMR spectroscopy, and quantum-chemical calculations. We show that the addition of the tertiary amine linker increases the overall CO2 sorption capacity normalized by the surface area and of the heat of adsorption, whereas surface areas and pore size diameters decrease. The formation of ammonium bicarbonate species in the COF pores is shown to occur, revealing the contributing role of water for CO2 uptake by amine-modified porous frameworks

Understanding Atomic-Scale Compositions, Structures, and Properties of Semi-Crystalline Inorganic Nanomaterials

Many important material properties are determined by the surface layer (0.5-100 nm); these include optoelectronic properties like conductivity, absorptivity, and reflectance as well as physiochemical properties such as molecular adsorption, hydrophobicity, and surface diffusivity. Furthermore, the crystallization or assembly processes of technologically-important nanomaterials are mediated by surface interactions and influence the surface compositions and properties of the resultant material, including zeolites, colloidal semiconductors, carbon-based electrocatalysts, and mesoporous inorganic oxides. Catalytic reaction properties of diverse porous heterogeneous materials are determined by molecular interactions of adsorbates, reactants, or product species at pore or exterior surface sites. Despite the broad importance of surface interactions in determining material properties, fundamental questions remain regarding the physiochemical interactions at surfaces that determine crystallization, adsorption, optoelectronic, and/or reaction properties. This is because such properties often depend on dilute surface moieties, defect species, and/or molecular adsorbates that occupy distributions that are partially- or non-ordered and are therefore challenging or impossible to characterize by conventional scattering techniques. Developing atomic-level insights into the types, interactions, and distributions of such dilute non-ordered species is crucial to elucidate the molecular-level origins of the properties and synthesis pathways of materials such as zeolites, semiconductor nanoparticles, and mesoporous electrocatalysts. By understanding the crystallization, synthesis, and assembly processes of these materials, as well as the resulting structures and active species, the resulting insights can be applied to develop new synthetic or post-synthetic treatments to generate more effective, stable, and/or active materials with desirable properties. The objective of this dissertation is to measure, understand, and correlate the atomic-scale compositions, structures, and properties of heterogenous materials with diverse applications, including heterogeneous catalysis and solid-state lighting. Recently-developed solid-state nuclear magnetic resonance (NMR) techniques with complementary X-ray diffraction and electron microscopy analyses are applied to elucidate the structures and compositions of dilute surface, defect, or heteroatom species, which are correlated to the macroscopic properties of interest. These techniques are applied to analyze diverse heterogeneous inorganic nanomaterials, including aluminosilicate zeolites, cementitious solids, precious-metal-free electrocatalysts, and nanocrystalline semiconductors. Though the material systems vary in composition and application, in each case the important optical, electronic, and/or catalytic properties arise from dilute partially- or non-ordered defect or heteroatom species in a semi-crystalline lattice. The overall unifying themes are: (1) analysis of order and disorder in semi-crystalline inorganic solids using state-of-the-art diffraction and spectroscopic characterization techniques; (2) determining the distributions and structures of non-stoichiometric species, particularly at surfaces and interfaces; and (3) correlating atomic-level structures and compositions with macroscopic material properties. The insights provided are of broad importance and relevance to diverse material systems of technological interest for sustainable energy storage, conversion, and utilization

Understanding Atomic-Scale Compositions, Structures, and Properties of Semi-Crystalline Inorganic Nanomaterials

Many important material properties are determined by the surface layer (0.5–100 nm); these include optoelectronic properties like conductivity, absorptivity, and reflectance as well as physiochemical properties such as molecular adsorption, hydrophobicity, and surface diffusivity. Furthermore, the crystallization or assembly processes of technologically-important nanomaterials are mediated by surface interactions and influence the surface compositions and properties of the resultant material, including zeolites, colloidal semiconductors, carbon-based electrocatalysts, and mesoporous inorganic oxides. Catalytic reaction properties of diverse porous heterogeneous materials are determined by molecular interactions of adsorbates, reactants, or product species at pore or exterior surface sites. Despite the broad importance of surface interactions in determining material properties, fundamental questions remain regarding the physiochemical interactions at surfaces that determine crystallization, adsorption, optoelectronic, and/or reaction properties. This is because such properties often depend on dilute surface moieties, defect species, and/or molecular adsorbates that occupy distributions that are partially- or non-ordered and are therefore challenging or impossible to characterize by conventional scattering techniques. Developing atomic-level insights into the types, interactions, and distributions of such dilute non-ordered species is crucial to elucidate the molecular-level origins of the properties and synthesis pathways of materials such as zeolites, semiconductor nanoparticles, and mesoporous electrocatalysts. By understanding the crystallization, synthesis, and assembly processes of these materials, as well as the resulting structures and active species, the resulting insights can be applied to develop new synthetic or post-synthetic treatments to generate more effective, stable, and/or active materials with desirable properties. The objective of this dissertation is to measure, understand, and correlate the atomic-scale compositions, structures, and properties of heterogenous materials with diverse applications, including heterogeneous catalysis and solid-state lighting. Recently-developed solid-state nuclear magnetic resonance (NMR) techniques with complementary X-ray diffraction and electron microscopy analyses are applied to elucidate the structures and compositions of dilute surface, defect, or heteroatom species, which are correlated to the macroscopic properties of interest. These techniques are applied to analyze diverse heterogeneous inorganic nanomaterials, including aluminosilicate zeolites, cementitious solids, precious-metal-free electrocatalysts, and nanocrystalline semiconductors. Though the material systems vary in composition and application, in each case the important optical, electronic, and/or catalytic properties arise from dilute partially- or non-ordered defect or heteroatom species in a semi-crystalline lattice. The overall unifying themes are: (1) analysis of order and disorder in semi-crystalline inorganic solids using state-of-the-art diffraction and spectroscopic characterization techniques; (2) determining the distributions and structures of non-stoichiometric species, particularly at surfaces and interfaces; and (3) correlating atomic-level structures and compositions with macroscopic material properties. The insights provided are of broad importance and relevance to diverse material systems of technological interest for sustainable energy storage, conversion, and utilization

Solid-state 183W NMR spectroscopy as a high-resolution probe of polyoxotungstate structures and dynamics

Polyoxometalates such as ammonium paratungstate (APT) are an important class of metal oxides with applications for catalysis, (opto)electronics, and functional materials. Structural analyses of solid polyoxometalates mostly rely on X-ray or neutron diffraction techniques, which are limited to compounds that can be isolated with long-range crystallographic order. While 183W NMR has been shown to probe polyoxotungstate structures and dynamics in solution, its application to solids has been extremely limited. Here, state-of-the-art methods for the detection of solid-state 183W NMR spectra are tested and compared for APT in different hydration states. The highly resolved solid-state spectra distinguish each crystallographically distinct site in the tungstate structure. Furthermore, the 183W chemical shifts are shown to be highly sensitive to the local structure, dynamics, and symmetry of APT, establishing solid-state 183W NMR spectroscopy as a potent probe for analysis of polyoxotungstates and other tungsten-derived materials to complement solution NMR and diffraction-based techniques

Spatially correlated distributions of local metallic properties in bulk and nanocrystalline GaN

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Classifying and understanding the reactivities of Mo based alkyne metathesis catalysts from 95Mo NMR chemical shift descriptors

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3MoCR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the con-text of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of potent catalysts. Analysis by first principles calculations of 95Mo chemical shift tensors ex-tracted from solid-state 95Mo NMR spectra show a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2+2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronega-tivity (σ-donation) and electron delocalization through Mo-O π-interactions, thus explaining the unique reactivity of the silanolate canopy catalysts. These results further motivate exploration of transition-metal NMR signatures and their relations to electronic structure and reactivity

Leveraging Surface Siloxide Electronics to Enhance Relaxation Properties of a Single-Molecule Magnet

Single-molecule magnets (SMMs) hold promise for unmatched information storage density as well as applications in quantum computing and spintronics. To date, the most successful SMMs are organometallic lanthanide complexes. However, their surface immobilization, one of the requirements for device fabrication and commercial application, remains challenging due to sensitivity of magnetic properties to small changes in the electronic structure of the parent SMM. Thus, finding controlled approaches to SMM surface deposition is a timely challenge. In this contribution we apply the concept of isolobality to identify siloxides present at the surface of partially dehydroxylated silica as a suitable replacement for archetypal ligand architectures in organometallic SMMs. We demonstrate theoretically and experimentally that isolated siloxide anchorages not only enable successful immobilization, but also lead to two-orders-of-magnitude increase in magnetization relaxation times.<br /

Regularized dynamical decoupling noise spectroscopy – a decoherence descriptor for radicals in glassy matrices

Decoherence arises from a fluctuating spin environment, captured by its noise spectrum S(ω). Dynamical decoupling (DD) with n π pulses extends the dephasing time if the associated filter function attenuates S(ω). Inversely, DD noise spectroscopy (DDNS) reconstructs S(ω) from DD data by approximating the filters pass band by a δ-function. This restricts application to qubit-like spin systems with inherently long dephasing times and/or many applicable pulses. We introduce regularized DDNS to lift this limitation and thereby infer S(ω) from DD traces of paramagnetic centers in glassy o-terphenyl and water–glycerol matrices recorded with n ≤ 5. For nitroxide radicals at low temperatures, we utilize deuteration to identify distinct matrix- and spin center-induced spectral features. The former extends up to a matrix-specific cut-off frequency and characterizes nuclear spin diffusion. We demonstrate that rotational tunneling of intramolecular methyl groups drives the latter process, whereas at elevated temperatures S(ω) reflects the classical methyl group reorientation. Ultimately, S(ω) visualizes and quantifies variations in the electron spins couplings and thus reports on the underlying spin dynamics as a powerful decoherence descriptor.ISSN:1463-9084ISSN:1463-907

Leveraging Surface Siloxide Electronics to Enhance the Relaxation Properties of a Single-Molecule Magnet

International audienceSingle-molecule magnets (SMMs) hold promise for unmatched information storage density as well as for applications in quantum computing and spintronics. To date, the most successful SMMs have been organometallic lanthanide complexes. However, their surface immobilization, one of the requirements for device fabrication and commercial application, remains challenging due to the sensitivity of the magnetic properties to small changes in the electronic structure of the parent SMM. Thus, finding controlled approaches to SMM surface deposition is a timely challenge. In this contribution we apply the concept of isolobality to identify siloxides present at the surface of partially dehydroxylated silica as a suitable replacement for archetypal ligand architectures in organometallic SMMs. We demonstrate theoretically and experimentally that isolated siloxide anchoring sites not only enable successful immobilization but also lead to a 2 orders of magnitude increase in magnetization relaxation times

Revisiting Edge-Sites of -Al2O3 Using Needle- Shaped Nanocrystals and Recoupling-Time Encoded {27Al}-1H D-HMQC NMR Spectroscopy

Despite being widely used in numerous catalytic applications, our understanding of reactive surface sites of high surface-area -Al2O3 remains limited to date. Recent contributions have pointed towards the potential role of highly reactive edge-sites contained in the high-field signal of the 1H-NMR spectrum of -Al2O3 materials. This work combines the development of needle-shaped -Al2O3 nanocrystals having a high relative fraction of edge sites with the use of state of art solid-state NMR – 1H-1H Single-Quantum (SQ) Double-Quantum (DQ) and Arbitrary- Indirect-Dwell (AID) dipolar Heteronuclear Multiple Quantum Coherence (D-HMQC) – to significantly deepen our understanding of this specific signal. We identify two distinct hydroxyl sites which possess altered isotropic chemical shifts, different positions within the dipole-dipole network and distinct proximities to different aluminum surface sites. Moreover, the use of recoupling-time encoded D-HMQC data allows us to partially revise previous assignments of D- HMQC data of -Al2O3 materials. While previous work has ascribed the high-field signal to be correlated to a single four-coordinate Al-site with substantial quadrupolar broadening we can identify the presence of two four-coordinate Al-sites with similar isotropic chemical shifts but different quadrupolar coupling constants. Recoupling-time-encoded data are thus able differentiate sites that would otherwise only be achievable with access to multiple fields or usage of highly advanced NMR techniques