Porous organic crystals crosslinked by free-radical reactions.

Two hydrogen-bonded crosslinked organic frameworks (HCOFs) were synthesized via free radical reactions utilizing butadiene and isoprene as crosslinkers. These HCOFs exhibit high crystallinity, enabling detailed structural characterization via single-crystal X-ray diffraction analysis. Subsequently, one of the olefin-rich HCOFs was converted to a hydroxylated framework through hydroboration-oxidation while maintaining the high crystallinity.

Lead-Free Semiconductors: Phase-Evolution and Superior Stability of Multinary Tin Chalcohalides.

Tin-based semiconductors are highly desirable materials for energy applications due to their low toxicity and biocompatibility relative to analogous lead-based semiconductors. In particular, tin-based chalcohalides possess optoelectronic properties that are ideal for photovoltaic and photocatalytic applications. In addition, they are believed to benefit from increased stability compared with halide perovskites. However, to fully realize their potential, it is first necessary to better understand and predict the synthesis and phase evolution of these complex materials. Here, we describe a versatile solution-phase method for the preparation of the multinary tin chalcohalide semiconductors Sn2SbS2I3, Sn2BiS2I3, Sn2BiSI5, and Sn2SI2. We demonstrate how certain thiocyanate precursors are selective toward the synthesis of chalcohalides, thus preventing the formation of binary and other lower order impurities rather than the preferred multinary compositions. Critically, we utilized 119Sn ssNMR spectroscopy to further assess the phase purity of these materials. Further, we validate that the tin chalcohalides exhibit excellent water stability under ambient conditions, as well as remarkable resistance to heat over time compared to halide perovskites. Together, this work enables the isolation of lead-free, stable, direct band gap chalcohalide compositions that will help engineer more stable and biocompatible semiconductors and devices.

Ultradynamic Isoreticularly Expanded Porous Organic Crystals.

Porous organic materials showcasing large framework dynamics present new paths for adsorption and separation with enhanced capacity and selectivity beyond the size-sieving limits, which is attributed to their guest-responsive sorption behaviors. Porous hydrogen-bonded crosslinked organic frameworks (HCOFs) are attractive for their remarkable ability to undergo guest-triggered expansion and contraction facilitated by their flexible covalent crosslinkages. However, the voids of HCOFs remain limited, which restrains the extent of the framework dynamics. In this work, we synthesized a series of HCOFs characterized by unprecedented size expansion capabilities induced by solvents. These HCOFs were constructed by isoreticularly co-crystallizing two complementary sets of hydrogen bonding building blocks to generate porous molecular crystals, which were crosslinked through thiol-ene/yne single-crystal-to-single-crystal transformations. The generated HCOFs exhibit enhanced chemical durability, high crystallinity, and extraordinary framework dynamics. For instance, HCOF-104 crystals featuring a pore diameter of 13.6 Å expanded in DMF to 300 ± 10% of their original lengths within just 1 min. This expansion allows the HCOFs to adsorb guest molecules that are significantly larger than the pore sizes of their crystalline states. Through methanol-induced contraction, these large guests were encapsulated in the fast-contracted HCOFs. These advancements in porous framework dynamics pave the way for new methods of encapsulating guests for targeted delivery.

Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy.

High-field magic angle spinning (MAS) dynamic nuclear polarization (DNP) is becoming a common technique for improving the sensitivity of solid-state nuclear magnetic resonance (SSNMR) by the hyperpolarization of nuclear spins. Recently, we have shown that gamma irradiation is capable of creating long-lived free radicals that are amenable to MAS DNP in quartz and a variety of organic solids. Here, we demonstrate that ball milling is able to generate millimolar concentrations of stable radical species in diverse materials such as polystyrene, cellulose, borosilicate glass, and fused quartz. High-field electron paramagnetic resonance (EPR) was used to obtain further insight into the nature of the radicals formed in ball milled quartz and borosilicate glass. We further show that radicals generated in quartz by ball milling can be used for solid-effect DNP. We obtained 29Si DNP enhancements of approximately 114 and 33 at 110 K and room temperature, respectively, from a sample of ball milled quartz.

Probing the Molecular and Macroscopic Structure of Solid Solutions by Dynamic Nuclear Polarization (DNP) Enhanced 13C and 15N Solid-State NMR Spectroscopy.

Crystallization is a widely used purification technique in the manufacture of active pharmaceutical ingredients (APIs) and precursor molecules. However, when impurities and desired compounds have similar molecular structures, separation by crystallization may become challenging. In such cases, some impurities may form crystalline solid solutions with the desired product during recrystallization. Understanding the molecular structure of these recrystallized solid solutions is crucial to devise methods for effective purification. Unfortunately, there are limited analytical techniques that provide insights into the molecular structure or spatial distribution of impurities that are incorporated within recrystallized products. In this study, we investigated model solid solutions formed by recrystallizing salicylic acid (SA) in the presence of anthranilic acid (AA). These two molecules are known to form crystalline solid solutions due to their similar molecular structures. To overcome challenges associated with the long 1H longitudinal relaxation times (T1(1H)) of SA and AA, we employed dynamic nuclear polarization (DNP) and 15N isotope enrichment to enable solid-state NMR experiments. Results of solid-state NMR experiments and DFT calculations revealed that SA and AA are homogeneously alloyed as a solid solution. Heteronuclear correlation (HETCOR) experiments and plane-wave DFT structural models provide further evidence of the molecular-level interactions between SA and AA. This research provides valuable insights into the molecular structure of recrystallized solid solutions, contributing to the development of effective purification strategies and an understanding of the physicochemical properties of solid solutions.

Accelerated acquisition of wideline solid-state NMR spectra of spin 3/2 nuclei by frequency-stepped indirect detection experiments.

73% of all NMR-active nuclei are quadrupolar nuclei with a nuclear spin I > 1/2. The broadening of the solid-state NMR signals by the quadrupolar interaction often leads to poor sensitivity and low resolution. In this work we present experimental and theoretical investigations of magic angle spinning (MAS) 1H{X} double-echo resonance-echo saturation-pulse double-resonance (DE-RESPDOR) and Y{X} J-resolved solid-state NMR experiments for the indirect detection of spin 3/2 quadrupolar nuclei (X = spin 3/2 nuclei, Y = spin 1/2 nuclei). In these experiments, the spectrum of the quadrupolar nucleus is reconstructed by plotting the observed dephasing of the detected spin as a function of the transmitter offset of the indirectly detected spin. Numerical simulations were used to investigate the achievable levels of dephasing and to predict the lineshapes of indirectly detected NMR spectra of the quadrupolar nucleus. We demonstrate 1H, 31P and 207Pb detection of 35Cl, 81Br, and 63Cu (I = 3/2) nuclei in trans-Cl2Pt(NH3)2 (transplatin), (CH3NH3)PbCl3 (methylammonium lead chloride, MAPbCl3), (CH3NH3)PbBr3 (methylammonium lead bromide, MAPbBr3) and CH3C(CH2PPh2)3CuI (1,1,1-tris(diphenylphosphinomethyl)ethane copper(I) iodide, triphosCuI), respectively. In all of these experiments, we were able to detect megahertz wide central transition or satellite transition powder patterns. Significant time savings and gains in sensitivity were attained in several test cases. Additionally, the indirect detection experiments provide valuable structural information because they confirm the presence of dipolar or scalar couplings between the detected nucleus and the quadrupolar nucleus of interest. Finally, numerical simulations suggest these methods are also potentially applicable to abundant spin 5/2 and spin 7/2 quadrupolar nuclei.

Breaking New Ground: MBene Route toward Selective Vinyl Double Bond Hydrogenation in Nitroarenes.

Doping, or incremental substitution of one element for another, is an effective way to tailor a compound's structure as well as its physical and chemical properties. Herein, we replaced up to 30% of Ni with Co in members of the family of layered LiNiB compounds, stabilizing the high-temperature polymorph of LiNiB while the room-temperature polymorph does not form. By studying this layered boride with in situ high-temperature powder diffraction, we obtained a distorted variant of LiNi0.7Co0.3B featuring a perfect interlayer placement of [Ni0.7Co0.3B] layers on top of each other─a structural motif not seen before in other borides. Because of the Co doping, LiNi0.7Co0.3B can undergo a nearly complete topochemical Li deintercalation under ambient conditions, resulting in a metastable boride with the formula Li0.04Ni0.7Co0.3B. Heating of Li0.04Ni0.7Co0.3B in anaerobic conditions led to yet another metastable boride, Li0.01Ni0.7Co0.3B, with a CoB-type crystal structure that cannot be obtained by simple annealing of Ni, Co, and B. No significant alterations of magnetic properties were detected upon Co-doping in the temperature-independent paramagnet LiNi0.7Co0.3B or its Li-deintercalated counterparts. Finally, Li0.01Ni0.7Co0.3B stands out as an exceptional catalyst for the selective hydrogenation of the vinyl C═C bond in 3-nitrostyrene, even in the presence of other competing functional groups. This research showcases an innovative approach to heterogeneous catalyst design by meticulously synthesizing metastable compounds.

Relativistic DFT Calculations of Cadmium and Selenium Solid-State NMR Spectra of CdSe Nanocrystal Surfaces.

Solid-state NMR spectra have been used to probe the structure of CdSe nanocrystals and propose detailed models of their surface structures. Density functional theory (DFT)-optimized cluster models that represent probable molecular structures of carboxylate-coordinated surface sites have been proposed. However, to the best of our knowledge, 113Cd and 77Se chemical shifts have not been calculated for these surface models. We performed relativistic DFT calculations of cadmium and selenium magnetic shielding tensors on model compounds with previously measured solid-state NMR spectra with (i) the four-component Dirac-Kohn-Sham (DKS) Hamiltonian and (ii) the scalar and (iii) spin-orbit levels within the ZORA Hamiltonian. Molecular clusters with Cd and Se sites in varying bonding environments were used to model CdSe (100) and CdSe(111) surfaces capped with carboxylic acid ligands. Our calculations identify the observed 113Cd isotropic chemical shifts δ(iso) of -465, -318, and -146 ppm arising from CdSeO3, CdSe2O2, and CdSe3O surface groups, respectively, with very good agreement with experimental measurements. The 113Cd chemical shifts linearly decrease with the number of O-neighbors. The calculated spans (δ11 - δ33) encompass the experimental values for CdSe3O and CdSe2O2 clusters but are slightly larger than the measured value for CdSeO3 clusters. Relativistic DFT calculations predicted a one-bond 113Cd-77Se scalar coupling of 258 Hz, which is in good agreement with the experimental values of 250 Hz. With a dense coverage of carboxylic acid ligands, the CdSe (100) surface shows a distribution of Cd-Se bond lengths and J-couplings. Relativistic DFT simulations thus aid in interpretation of NMR spectra of CdSe nanocrystals and related nanomaterials.

Designing complex Pb3SBrxI4-x chalcohalides: tunable emission semiconductors through halide-mixing.

Chalcohalides are desirable semiconducting materials due to their enhanced light-absorbing efficiency and stability compared to lead halide perovskites. However, unlike perovskites, tuning the optical properties of chalcohalides by mixing different halide ions into their structure remains to be explored. Here, we present an effective strategy for halide-alloying Pb3SBrxI4-x (1 ≤ x ≤ 3) using a solution-phase approach and study the effect of halide-mixing on structural and optical properties. We employ a combination of X-ray diffraction, electron microscopy, and solid-state NMR spectroscopy to probe the chemical structure of the chalcohalides and determine mixed-halide incorporation. The absorption onsets of the chalcohalides blue-shift to higher energies as bromide replaces iodide within the structure. The photoluminescence maxima of these materials mimics this trend at both the ensemble and single particle fluorescence levels, as observed by solution-phase and single particle fluorescence microscopy, respectively. These materials exhibit superior stability against moisture compared to traditional lead halide perovskites, and IR spectroscopy reveals that the chalcohalide surfaces are terminated by both amine and carboxylate ligands. Electronic structure calculations support the experimental band gap widening and volume reduction with increased bromide incorporation, and provide useful insight into the likely atomic coloring patterns of the different mixed-halide compositions. Ultimately, this study expands the range of tunability that is achievable with chalcohalides, which we anticipate will improve the suitability of these semiconducting materials for light absorbing and emission applications.

Structural characterization of tin in toothpaste by dynamic nuclear polarization enhanced 119Sn solid-state NMR spectroscopy.

Stannous fluoride (SnF2) is an effective fluoride source and antimicrobial agent that is widely used in commercial toothpaste formulations. The antimicrobial activity of SnF2 is partly attributed to the presence of Sn(II) ions. However, it is challenging to directly determine the Sn speciation and oxidation state within commercially available toothpaste products due to the low weight loading of SnF2 (0.454 wt% SnF2, 0.34 wt% Sn) and the amorphous, semi-solid nature of the toothpaste. Here, we show that dynamic nuclear polarization (DNP) enables 119Sn solid-state NMR experiments that can probe the Sn speciation within commercially available toothpaste. Solid-state NMR experiments on SnF2 and SnF4 show that 19F isotropic chemical shift and 119Sn chemical shift anisotropy (CSA) are highly sensitive to the Sn oxidation state. DNP-enhanced 119Sn magic-angle turning (MAT) 2D NMR spectra of toothpastes resolve Sn(II) and Sn(IV) by their 119Sn chemical shift tensor parameters. Fits of DNP-enhanced 1D 1H → 119Sn solid-state NMR spectra allow the populations of Sn(II) and Sn(IV) within the toothpastes to be estimated. This analysis reveals that three of the four commercially available toothpastes contained at least 80% Sn(II), whereas one of the toothpaste contained a significantly higher amount of Sn(IV).

A focus on detection of polymorphs by dynamic nuclear polarization solid-state nuclear magnetic resonance spectroscopy.

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy has found increasing application as a method for quantification and structure determination of solid forms (polymorphs) of organic solids and active pharmaceutical ingredients (APIs). However, ssNMR spectroscopy suffers from low sensitivity and resolution, making it challenging to detect dilute solid forms that may be present after recrystallization or reaction with co-formers. Cousin et al. (S. F. Cousin et al., Chem. Sci., 2023, https://doi.org/10.1039/D3SC02063K) have demonstrated that dynamic nuclear polarization (DNP) enhanced 13C cross-polarization (CP) saturation recovery experiments can be used to detect dilute polymorphic forms that are present within a mixture of solid forms. Enhancement of the NMR signal by DNP and differences in signal build-up rates for different polymorphs provide the sensitivity and contrast needed to resolve NMR signals from minor polymorphic forms. This method demonstrated by Cousin et al. should aid the discovery of solid drug forms.

Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, which provides an ideal platform for both fundamental and applied studies of interfaces. Good progress has been achieved in the functionalization of MXenes with small inorganic ligands, but relatively little work has been reported on the covalent bonding of various organic groups to MXene surfaces. Here we synthesize a family of hybrid MXenes (h-MXenes) that incorporate amido- and imido-bonding between organic and inorganic parts by reacting halogen-terminated MXenes with deprotonated organic amines. The resulting hybrid structures unite tailorability of organic molecules with electronic connectivity and other properties of inorganic 2D materials. Describing the structure of h-MXene necessitates the integration of concepts from coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic constituents. h-MXenes also exhibit superior stability against hydrolysis.

Spatially Directed Pyrolysis via Thermally Morphing Surface Adducts.

Combustion is often difficult to spatially direct or tune associated kinetics-hence a run-away reaction. Coupling pyrolytic chemical transformation to mass transport and reaction rates (Damköhler number), however, we spatially directed ignition with concomitant switch from combustion to pyrolysis (low oxidant). A 'surface-then-core' order in ignition, with concomitant change in burning rate,is therefore established. Herein, alkysilanes grafted onto cellulose fibers are pyrolyzed into non-flammable SiO2 terminating surface ignition propagation, hence stalling flame propagating. Sustaining high temperatures, however, triggers ignition in the bulk of the fibers but under restricted gas flow (oxidant and/or waste) hence significantly low rate of ignition propagation and pyrolysis compared to open flame (Liñán's equation). This leads to inside-out thermal degradation and, with felicitous choice of conditions, formation of graphitic tubes. Given the temperature dependence, imbibing fibers with an exothermically oxidizing synthon (MnCl2 ) or a heat sink (KCl) abets or inhibits pyrolysis leading to tuneable wall thickness. We apply this approach to create magnetic, paramagnetic, or oxide containing carbon fibers. Given the surface sensitivity, we illustrate fabrication of nm- and µm-diameter tubes from appropriately sized fibers.

Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core-Shell In1-xGaxP/ZnS Colloidal Quantum Dots.

Semiconductors are commonly divided into materials with direct or indirect band gaps based on the relative positions of the top of the valence band and the bottom of the conduction band in crystal momentum (k) space. It has, however, been debated if k is a useful quantum number to describe the band structure in quantum-confined nanocrystalline systems, which blur the distinction between direct and indirect gap semiconductors. In bulk III-V semiconductor alloys like In1-xGaxP, the band structure can be tuned continuously from the direct- to indirect-gap by changing the value of x. The effect of strong quantum confinement on the direct-to-indirect transition in this system has yet to be established because high-quality colloidal nanocrystal samples have remained inaccessible. Herein, we report one of the first systematic studies of ternary III-V nanocrystals by utilizing an optimized molten-salt In-to-Ga cation exchange protocol to yield bright In1-xGaxP/ZnS core-shell particles with photoluminescence quantum yields exceeding 80%. We performed two-dimensional solid-state NMR studies to assess the alloy homogeneity and the extent of surface oxidation in In1-xGaxP cores. The radiative decay lifetime for In1-xGaxP/ZnS monotonically increases with higher gallium content. Transient absorption studies on In1-xGaxP/ZnS nanocrystals demonstrate signatures of direct- and indirect-like behavior based on the presence or absence, respectively, of excitonic bleach features. Atomistic electronic structure calculations based on the semi-empirical pseudopotential model are used to calculate absorption spectra and radiative lifetimes and evaluate band-edge degeneracy; the resulting calculated electronic properties are consistent with experimental observations. By studying photoluminescence characteristics at elevated temperatures, we demonstrate that a reduced lattice mismatch at the III-V/II-VI core-shell interface can enhance the thermal stability of emission. These insights establish cation exchange in molten inorganic salts as a viable synthetic route to nontoxic, high-quality In1-xGaxP/ZnS QD emitters with desirable optoelectronic properties.

Optically Enhanced Solid-State 1H NMR Spectroscopy.

Low sensitivity is the primary limitation to extending nuclear magnetic resonance (NMR) techniques to more advanced chemical and structural studies. Photochemically induced dynamic nuclear polarization (photo-CIDNP) is an NMR hyperpolarization technique where light is used to excite a suitable donor-acceptor system, creating a spin-correlated radical pair whose evolution drives nuclear hyperpolarization. Systems that exhibit photo-CIDNP in solids are not common, and this effect has, up to now, only been observed for 13C and 15N nuclei. However, the low gyromagnetic ratio and natural abundance of these nuclei trap the local hyperpolarization in the vicinity of the chromophore and limit the utility for bulk hyperpolarization. Here, we report the first example of optically enhanced solid-state 1H NMR spectroscopy in the high-field regime. This is achieved via photo-CIDNP of a donor-chromophore-acceptor molecule in a frozen solution at 0.3 T and 85 K, where spontaneous spin diffusion among the abundant strongly coupled 1H nuclei relays polarization through the whole sample, yielding a 16-fold bulk 1H signal enhancement under continuous laser irradiation at 450 nm. These findings enable a new strategy for hyperpolarized NMR beyond the current limits of conventional microwave-driven DNP.

Inductively Coupled Nonthermal Plasma Synthesis of Size-Controlled γ-Al2O3 Nanocrystals.

Gamma alumina (γ-Al2O3) is widely used as a catalyst and catalytic support due to its high specific surface area and porosity. However, synthesis of γ-Al2O3 nanocrystals is often a complicated process requiring high temperatures or additional post-synthetic steps. Here, we report a single-step synthesis of size-controlled and monodisperse, facetted γ-Al2O3 nanocrystals in an inductively coupled nonthermal plasma reactor using trimethylaluminum and oxygen as precursors. Under optimized conditions, we observed phase-pure, cuboctahedral γ-Al2O3 nanocrystals with defined surface facets. Nuclear magnetic resonance studies revealed that nanocrystal surfaces are populated with AlO6, AlO5 and AlO4 units with clusters of hydroxyl groups. Nanocrystal size tuning was achieved by varying the total reactor pressure yielding particles as small as 3.5 nm, below the predicted thermodynamic stability limit for γ-Al2O3.

Acceleration of indirect detection 195Pt solid-state NMR experiments by sideband selective excitation or alternative indirect sampling schemes.

The measurement of the of chemical shift (CS) tensors via solid-state NMR (ssNMR) spectroscopy has proven to be a powerful probe of structure for organic molecules, biomolecules, and inorganic materials. However, when measuring the NMR spectra of heavy spin-1/2 isotopes the chemical shift anisotropy (CSA) is commonly on the order of thousands of parts per million, which makes acquisition of NMR spectra difficult due to the low NMR sensitivity imposed by the breadth of the signals and challenges in uniformly exciting the NMR spectrum. We have recently shown that complete 195Pt NMR spectra could be rapidly measured by using 195Pt saturation or excitation selective long pulses (SLP) with multiple rotor-cycle durations and RF fields less than 50 kHz into 1H{195Pt} or 1H-31P{195Pt} PE S-RESPDOR, TONE D-HMQC-4, J-resolved, and J-HMQC pulse sequences. The SLP only provide signal or dephasing when they are applied on resonance with a spinning sideband. The magic angle spinning 195Pt NMR spectrum is reconstructed in the sideband selective NMR experiments by acquiring 1D NMR spectra at variable 195Pt pulse offsets. In this work, we present a detailed investigation of the specific pulse conditions required for the ideal performance of sideband selective experiments. Sideband selective experiments are shown to be able to accurately reproduce MAS NMR spectra with minimal distortions of relative sideband intensities. It is also demonstrated that a 195Pt NMR spectrum indirectly detected with HMQC can be rapidly obtained by acquiring a single rotor cycle of indirect dimension evolution points. We dub this method One Rotor Cycle of Acquisition (ORCA) HMQC. Sideband selective experiments and ORCA HMQC experiments are shown to provide a one order of magnitude improvement in experiment times as compared to conventional wideline HMQC experiments.

General Strategy for Incorporation of Functional Group Handles into Covalent Organic Frameworks via the Ugi Reaction.

The library of imine-linked covalent organic frameworks (COFs) has grown significantly over the last two decades, featuring a variety of morphologies, pore sizes, and applications. An array of synthetic methods has been developed to expand the scope of the COF functionalities; however, most of these methods were designed to introduce functional scaffolds tailored to a specific application. Having a general approach to diversify COFs via late-stage incorporation of functional group handles would greatly facilitate the transformation of these materials into platforms for a variety of useful applications. Herein, we report a general strategy to introduce functional group handles in COFs via the Ugi multicomponent reaction. To demonstrate the versatility of this approach, we have synthesized two COFs with hexagonal and kagome morphologies. We then introduced azide, alkyne, and vinyl functional groups, which could be readily utilized for a variety of post-synthetic modifications. This facile approach enables the functionalization of any COFs containing imine linkages.

A Crosslinked Ionic Organic Framework for Efficient Iodine and Iodide Remediation in Water.

Iodine is widely used as an antimicrobial reagent for water disinfection in the wilderness and outer space, but residual iodine and iodide need to be removed for health reasons. Currently, it is challenging to remove low concentrations of iodine and iodide in water (≈5 ppm). Furthermore, the remediation of iodine and iodide across a broad temperature range (up to 90 °C) has not previously been investigated. In this work, we report a nitrate dimer-directed synthesis of a single-crystalline ionic hydrogen-bonded crosslinked organic framework (HC OF-7). HC OF-7 removes iodine and iodide species in water efficiently through halogen bonding and anion exchange, reducing the total iodine concentration to 0.22 ppm at room temperature. Packed HC OF-7 columns were employed for iodine/iodide breakthrough experiments between 23 and 90 °C, and large breakthrough volumes were recorded (≥18.3 L g-1 ). The high iodine/iodide removal benchmarks recorded under practical conditions make HC OF-7 a promising adsorbent for water treatment.