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Superradiance and Directional Exciton Migration in Metal–Organic Frameworks
Crystalline metal–organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multilinker molecular excitons display superradiance–a hallmark of coupled oscillators seen in LHCs–with radiative rate constant (k rad ) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely, PCN-222(Zn), NU-1000, and SIU-100, with similar topology but varying linkers suggest that the size of such molecular excitons depends on the electronic symmetry of the linker. This multilinker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. Furthermore, the understandings will provide design principles, especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.
In Situ Probes of Capture and Decomposition of Chemical Warfare Agent Simulants by Zr-Based Metal Organic Frameworks
Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials.
Enhanced Out-of-Plane Conductivity and Photovoltaic Performance in n = 1 Layered Perovskites through Organic Cation Design
Layered perovskites with the formula (R–NH 3 ) 2 PbI 4 have excellent environmental stability but poor photovoltaic function due to the preferential orientation of the semiconducting layer parallel to the substrate and the typically insulating nature of the R–NH 3 + cation. Here, we report a series of these n = 1 layered perovskites with the form (aromatic-O-linker-NH 3 ) 2 PbI 4 where the aromatic moiety is naphthalene, pyrene, or perylene and the linker is ethyl, propyl, or butyl. These materials achieve enhanced conductivity perpendicular to the inorganic layers due to better energy level matching between the inorganic layers and organic galleries. The enhanced conductivity and visible absorption of these materials led to a champion power conversion efficiency of 1.38%, which is the highest value reported for any n = 1 layered perovskite, and it is an order of magnitude higher efficiency than any other n = 1 layered perovskite oriented with layers parallel to the substrate. Furthermore, these findings demonstrate the importance of leveraging the electronic character of the organic cation to improve optoelectronic properties and thus the photovoltaic performance of these chemically stable low n layered perovskites.
Phenanthro[9,10- d ]triazole and imidazole derivatives: high triplet energy host materials for blue phosphorescent organic light emitting devices
A class of wide bandgap host materials is introduced as an alternative to carbazole-based hosts to enhance the efficiency and transport properties of organic light emitting diodes (OLEDs). We have synthesized and investigate the photophysical and electrochemical properties of a series of phenanthrene derivatives incorporating fused triazole or imidazole rings. Here, the resulting phenanthro[9,10-d]triazoles and phenanthro[9,10-d]imidazoles are suitable host materials for blue phosphors due to their high triplet energies and conductivities. Incorporation of bulky substituent groups leads to retention of the high triplet energies in the solid state. The most promising materials are incorporated in blue phosphorescent OLEDs that achieve external quantum efficiencies >20%.
Separating hyperfine from spin-orbit interactions in organic semiconductors by multi-octave magnetic resonance using coplanar waveguide microresonators
Separating the influence of hyperfine from spin-orbit interactions in spin-dependent carrier recombination and dissociation processes necessitates magnetic resonance spectroscopy over a wide range of frequencies. For this, we have designed compact and versatile coplanar waveguide resonators for continuous-wave electrically detected magnetic resonance and tested these on organic light-emitting diodes. By exploiting both the fundamental and higher-harmonic modes of the resonators, we cover almost five octaves in resonance frequency within a single setup. The measurements with a common π-conjugated polymer as the active material reveal small but non-negligible effects of spin-orbit interactions, which give rise to a broadening of the magnetic resonance spectrum with increasing frequency.
Influences of drizzle on stratocumulus cloudiness and organization [Influences of drizzle on cloudiness and stratocumulus organization]
Large-eddy simulations are used to study the influence of drizzle on stratocumulus organization, based on measurements made as part of the Second Dynamics and Chemistry of the Marine Stratocumulus field study-II. Cloud droplet number concentration ( N c ) is prescribed and considered as the proxy for different aerosol loadings. Our study shows that the amount of cloudiness does not decrease linearly with precipitation rate. An N c threshold is observed below which the removal of cloud water via precipitation efficiently reduces cloud depth, allowing evaporation to become efficient and quickly remove the remaining thin clouds, facilitating a fast transition from closed cells to open cells. Using Fourier analysis, stratocumulus length scales are found to increase with drizzle rates. Raindrop evaporation below 300 m lowers the cloud bases and amplifies moisture variances in the subcloud layer, while it does not alter the horizontal scales in the cloud layer, suggesting that moist cold pool dynamic forcings are not essential for mesoscale organization of stratocumulus. Furthermore, the cloud scales are greatly increased when the boundary layer is too deep to maintain well mixed.
Engineering Fe–Fe3C@Fe–N–C Active Sites and Hybrid Structures from Dual Metal–Organic Frameworks for Oxygen Reduction Reaction in H2–O2 Fuel Cell and Li–O2 Battery
Dual metal-organic frameworks (MOFs, i.e., MIL-100(Fe) and ZIF-8) are thermally converted into Fe-Fe3C-embedded Fe-N-codoped carbon as platinum group metal (PGM)-free oxygen reduction reaction (ORR) electrocatalysts. Pyrolysis enables imidazolate in ZIF-8 rearranged into highly N-doped carbon, while Fe from MIL-100(Fe) into N-ligated atomic sites concurrently with a few Fe-Fe3C nanoparticles. Upon precise control of MOF compositions, the optimal catalyst is highly active for the ORR in half-cells (0.88 V in base and 0.79 V versus RHE in acid in half-wave potential), a proton exchange membrane fuel cell (0.76 W cm(-2) in peak power density) and an aprotic Li-O-2 battery (8749 mAh g(-1) in discharge capacity), representing a state-of-the-art PGM-free ORR catalyst. In the material, amorphous carbon with partial graphitization ensures high active site exposure and fast charge transfer simultaneously. Macropores facilitate mass transport to the catalyst surface, followed by oxygen penetration in micropores to reach the infiltrated active sites. Further modeling simulations shed light on the true Fe-Fe3C contribution to the catalyst performance, suggesting Fe3C enhances oxygen affinity, while metallic Fe promotes *OH desorption as the rate-determining step at the nearby Fe-N-C sites. These findings demonstrate MOFs as model system for rational design of electrocatalyst for energy-based functional applications.
Water‐Based Assembly of Polymer–Metal Organic Framework (MOF) Functional Coatings
Metal organic frameworks (MOFs) have gained attention for their porosity, size selectivity, and structural diversity. There is a need for MOF‐based coatings, particularly in applications such as separations, electronics, and energy; yet forming thin, functional, conformal coatings is prohibitive because MOFs exist as a powder. Layer‐by‐layer assembly, a versatile thin film coating approach, offers a unique solution to this problem, but this approach requires MOFs that are water‐dispersible and bear a surface charge. Here, these issues are addressed by examining water‐based dispersions of MIL‐101(Cr) that facilitate the formation of robust polymer–MOF hybrid coatings. Specifically, the substrate to be coated is alternately exposed to an aqueous solution of poly(styrene sulfonate) and nano MIL‐101(Cr) dispersion, yielding linear film growth and coatings with a MOF content as high as 77 wt%. This approach is surface‐agnostic, in which the coating is successfully applied to silicon, glass, flexible plastic, and even cotton fabric, conformally coating individual fibers. By contrast, prior attempts at forming MOF‐coatings are severely limited to a handful of surfaces, require harsh chemical treatment, and are not conformal. The approach presented here unambiguously confirms that MOFs can be conformally coated onto complex and unusual surfaces, opening the door for a wide variety of applications.
Inter‐Fullerene Electronic Coupling Controls the Efficiency of Photoinduced Charge Generation in Organic Bulk Heterojunctions
Photoinduced charge generation (PCG) dynamics are notoriously difficult to correlate with specific molecular properties in device relevant polymer:fullerene organic photovoltaic blend films due to the highly complex nature of the solid state blend morphology. Here, this study uses six judiciously selected trifluoromethylfullerenes blended with the prototypical polymer poly(3‐hexylthiophene) and measure the PCG dynamics in 50 fs–500 ns time scales with time‐resolved microwave conductivity and femtosecond transient absorption spectroscopy. The isomeric purity and thorough chemical characterization of the fullerenes used in this study allow for a detailed correlation between molecular properties, driving force, local intermolecular electronic coupling and, ultimately, the efficiency of PCG yield. The findings show that the molecular design of the fullerene not only determines inter‐fullerene electronic coupling, but also influences the decay dynamics of free holes in the donor phase even when the polymer microstructure remains unchanged.
A highly stable zeotype mesoporous zirconium metal-organic framework with ultralarge pores.
Through topological rationalization, a zeotype mesoporous Zr-containing metal-organic framework (MOF), namely PCN-777, has been designed and synthesized. PCN-777 exhibits the largest cage size of 3.8nm and the highest pore volume of 2.8cm(3)g(-1) among reported Zr-MOFs. Moreover, PCN-777 shows excellent stability in aqueous environments, which makes it an ideal candidate as a support to incorporate different functional moieties. Through facile internal surface modification, the interaction between PCN-777 and different guests can be varied to realize efficient immobilization
Spontaneous Stepwise Self-Assembly of a Polyoxometalate-Organic Hybrid into Catalytically Active One-Dimensional Anisotropic Structures
An inorganic–organic hybrid surfactant with a hexavanadate cluster as the polar head group was designed and observed to assemble into micelle structures, which further spontaneously coagulate into a 1D anisotropic structure in aqueous solutions. Such a hierarchical self-assembly process is driven by the cooperation of varied noncovalent interactions, including hydrophobic, electrostatic, and hydrogen-bonding interactions. The hydrophobic interaction drives the quick formation of the micelle structure; electrostatic interactions involving counterions leads to the further coagulation of the micelles into larger assemblies. This process is similar to the crystallization process, but the specific counterions and the directional hydrogen bonding lead to the 1D growth of the final assemblies. Since most of the hexavanadates are exposed to the surface, the 1D assembly with nanoscale thickness is a highly efficient heterogeneous catalyst for the oxidation of organic sulfides with appreciable recyclability.
Selective Host-Guest Interaction between Metal Ions and Metal-Organic Frameworks Using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy
The host–guest interaction between metal ions (Pt²⁺ and Cu²⁺) and a zirconium metal–organic framework (UiO-66-NH₂) was explored using dynamic nuclear polarization-enhanced ¹⁵N{¹H} CPMAS NMR spectroscopy supported by X-ray absorption spectroscopy and density functional calculations. The combined experimental results conclude that each Pt²⁺ coordinates with two NH₂ groups from the MOF and two Cl⁻ from the metal precursor, whereas Cu²⁺ do not form chemical bonds with the NH₂ groups of the MOF framework. Density functional calculations reveal that Pt²⁺ prefers a square-planar structure with the four ligands and resides in the octahedral cage of the MOF in either cis or trans configurations.
Reaction Pathway to the Only Open-Shell Transition-Metal Keggin Ion without Organic Ligation: Reaction Pathway to the Only Open-Shell Transition-Metal Keggin Ion without Organic Ligation
Aqueous metal-oxo clusters without organic ligands (including polyoxometalates and group 13 polycations) are useful for energy, environmental and biomolecular applications. While ligands stabilize clusters of different elements, such as open-shell transition metals, they inhibit reactivity and solubility. Therefore a grand challenge is to conceive and execute synthetic strategies to isolate metal-oxo clusters of any composition, without ligation. Here we crystallize and structurally characterize a Cr 3+ dodecamer Keggin ion without organic ligation. Detailed characterization of its formation leads to understanding its reaction pathway, which may translate to isolation of other open-shell transition metal-oxo clusters. We employed electrospray ionization mass spectrometry, Raman spectroscopy and X-ray scattering (small-angle and total) to identify important reaction intermediates and processes. The aqueous synthesis presented here is simple to implement and scalable, producing pure phase materials in gram quantities.
A History of Molecular Level Analysis of Natural Organic Matter by FTICR Mass Spectrometry and The Paradigm Shift in Organic Geochemistry
Natural organic matter (NOM) is a complex mixture of biogenic molecules resulting from the deposition and transformation of plant and animal matter. It has long been recognized that NOM plays an important role in many geological, geochemical, and environmental processes. Of particular concern is the fate of NOM in response to a warming climate in environments that have historically sequestered carbon (e.g., peatlands and swamps) but may transition to net carbon emitters. In this review, we will highlight developments in the application of high-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) in identifying the individual components of complex NOM mixtures, focusing primarily on the fraction that is dissolved in natural waters (dissolved organic matter or DOM). We will first provide some historical perspective on developments in FTICR technology that made molecular-level characterizations of DOM possible. In conclusion, a variety of applications of the technique will then be described, followed by our view of the future of high-field FTICR MS in carbon cycling research, including a particularly exciting metabolomic approach.
Bio-reduction of ferrihydrite-montmorillonite-organic matter complexes: Effect of montmorillonite and fate of organic matter
Organic matter (OM) is often associated with Fe (hydr)oxides such as ferrihydrite (Fh) in soils and sediments, forming binary Fh-OM complexes. Microbial reduction of Fh results in destabilization of the complexes and mineral/OM transformation. However, little is known about the role of clay minerals in such processes, despite their common co-existence with Fh and OM in natural environments. Here Fh-OM complexes were synthesized in the presence of montmorillonite (SWy-2), forming ternary Fh-(SWy-2)-OM complexes. A metal-reducing bacterium Geobacter sulfurreducens was used to reduce Fh in the complexes under circumneutral pH and anoxic conditions with or without H 2 as extra electron donor. Various spectroscopy and mass spectrometry methods were used to monitor the progress of Fh bio-reduction and mineral/OM transformation. Results showed that G. sulfurreducens utilized mineral-bound OM as electron donor and/or carbon source to couple with Fh reduction. Relative to Fh-OM complex, Addition of SWy-2 to Fh-OM complex enhanced the bio-reduction extent of Fh by increasing the proportion of bioavailable OM that was weakly bound to SWy-2. However, its effect on the bio-reduction rate was variable. SWy-2 initially decreased the rate, because it spatially separated OM (electron donor) from Fh (electron acceptor). During later incubation, SWy-2 increased the reduction rate by sorbing biogenic Fe 2+ that would otherwise passivate the Fh and cell surfaces. Bio-reduction transformed mineral-bound OM to microbial products (e.g. necromass, extracellular polymeric substances), but organic compounds with aromatic structures, carboxyl groups and large molecular weight were more resistant to desorption and oxidation. The persistence of these compounds against bio-reduction induced transformation is likely due to their stronger binding with minerals and/or lower nominal oxidation states of carbon relative to other compounds. Our results provide new insights into the role of clay minerals in regulating biogeochemical cycling of solid-phase Fe and transformation of mineral-associated OM in anoxic soil environments.
Does dissolved organic matter or solid peat fuel anaerobic respiration in peatlands?
Not Available
Synthesis of porous organic cage CC3 via solvent modulated evaporation
We demonstrate a Humidity Modulated Solvent Evaporation (HMSE) approach to promote the nucleation and growth of CC3 crystals. This approach relies on the gradual evaporation of dichloromethane (solvent) from a diluted concentration of CC3 precursors deposited on aluminum foil. The slow solvent evaporation allowed enough time for the organization and formation of CC3 porous organic cage. The solvent diffusion rate was modulated by the relative humidity in the system. The slow kinetics implied during the development of CC3 phase, allowed the formation of CC3 crystals. HRTEM, SAED, SEM, and XRD patterns were used as pivotal characterization techniques to follow and confirm the formation of CC3 crystals.