Switchable Metal Sites in Metal-Organic Framework MFM-300(Sc): Lewis Acid Catalysis Driven by Metal-Hemilabile Linker Bond Dynamics.

Uncommon reversible guest-induced metal-hemilabile linker bond dynamics in MOF MFM-300(Sc) was unraveled to switch on/switch off catalytic open metal sites. The catalytic activity of this MOF with non-permanent open metal sites was demonstrated using a model Strecker hydrocyanation reaction as a proof-of-concept. Conclusively, the catalytic activity was evidenced to be fully reversible, preserving the conversion performance and structure integrity of MFM-300(Sc) over multiple cycles. These experimental findings were corroborated by quantum-calculations that revealed a reaction mechanism driven by the Sc-open metal sites. This discovery paves the way towards the design of new effective and easily regenerable heterogeneous MOF catalysts integrating switchable metal sites.

Coordinative Reduction of Metal Nodes Enhances the Hydrolytic Stability of a Paddlewheel Metal-Organic Framework.

Enhancement of hydrolytic stability of metal-organic frameworks (MOFs) is a challenging issue in MOF chemistry because most MOFs have shown limitations in their applications under a humid environment. Meanwhile, inner sphere electron transfer has constituted one of the most intensively studied subjects in contemporary chemistry. In this report, we show, for the first time, a new conceptual coordinative reduction of Cu2+ ion, which is realized in a paddlewheel MOF, HKUST-1, with a postsynthetic manner via inner sphere "single" electron transfer from hydroquinone (H2Q) to Cu2+ through its coordination bond. H2Q treatment of HKUST-1 under anhydrous conditions leads to the single charge (1+) reduction of approximately 30% of Cu2+ ions. Thus, this coordinative reduction is an excellent reduction process to be self-controlled in both oxidation state and quantity. As described below, once Cu2+ ions are reduced to Cu+, the reduction reaction does not proceed further, in terms of their oxidation state as well as their amount. Also, we demonstrate that a half of the Cu+ ions (about 15%) remains in paddlewheel framework with pseudo square planar geometry and the other half of the Cu+ ions (about 15%) forms [Cu(MeCN)4]+ complex in a small cage in the fashion of a ship-in-a-bottle after dissociation from the framework. Furthermore, we show that the coordinative reduction results in substantial enhancement of the hydrolytic stability of HKUST-1 to the extent that its structure remains intact even after exposure to humid air for two years.

A Chemical Role for Trichloromethane: Room-Temperature Removal of Coordinated Solvents from Open Metal Sites in the Copper-Based Metal-Organic Frameworks.

Open coordination sites (OCSs) in metal-organic frameworks (MOFs) have shown potential in applications such as molecular separation, sorption, catalysis, and sensing. Thus, the removal of coordinated solvent has been viewed as an essential step that needs to be performed prior to the use of the MOFs in the above applications. To date, a thermal method that is normally performed by applying heat and vacuum has been the most commonly employed activation method despite its negative influence on the structural integrity of the MOFs. In this report, we demonstrate that commonly inert trichloromethane (TCM) can activate OCSs; the TCM treatment process serves as an alternative chemical route to activation that does not require the external thermal energy. On the basis of the Raman study, we suggest a possible mechanism for the chemical activation process where TCM may weakly coordinate to the OCSs and then spontaneously dissociate. In addition, we prove that the chemical activation behavior is substantially boosted when a small amount of external heat energy (55 °C, 2.6 meV) is supplied during the TCM treatment. Using an HKUST-1-polyvinylidene fluoride (PVDF) mixed matrix (MM), we also demonstrate that this chemical activation strategy is a safe way to activate thermally deformable MOF-polymer mixed matrices.

A Chemical Route to Activation of Open Metal Sites in the Copper-Based Metal-Organic Framework Materials HKUST-1 and Cu-MOF-2.

Open coordination sites (OCSs) in metal-organic frameworks (MOFs) often function as key factors in the potential applications of MOFs, such as gas separation, gas sorption, and catalysis. For these applications, the activation process to remove the solvent molecules coordinated at the OCSs is an essential step that must be performed prior to use of the MOFs. To date, the thermal method performed by applying heat and vacuum has been the only method for such activation. In this report, we demonstrate that methylene chloride (MC) itself can perform the activation role: this process can serve as an alternative "chemical route" for the activation that does not require applying heat. To the best of our knowledge, no previous study has demonstrated this function of MC, although MC has been popularly used in the pretreatment step prior to the thermal activation process. On the basis of a Raman study, we propose a plausible mechanism for the chemical activation, in which the function of MC is possibly due to its coordination with the Cu(2+) center and subsequent spontaneous decoordination. Using HKUST-1 film, we further demonstrate that this chemical activation route is highly suitable for activating large-area MOF films.

A metal-organic framework-based material for electrochemical sensing of carbon dioxide.

The free primary hydroxyl groups in the metal-organic framework of CDMOF-2, an extended cubic structure containing units of six γ-cyclodextrin tori linked together in cube-like fashion by rubidium ions, has been shown to react with gaseous CO2 to form alkyl carbonate functions. The dynamic covalent carbon-oxygen bond, associated with this chemisorption process, releases CO2 at low activation energies. As a result of this dynamic covalent chemistry going on inside a metal-organic framework, CO2 can be detected selectively in the atmosphere by electrochemical impedance spectroscopy. The "as-synthesized" CDMOF-2 which exhibits high proton conductivity in pore-filling methanolic media, displays a ∼550-fold decrease in its ionic conductivity on binding CO2. This fundamental property has been exploited to create a sensor capable of measuring CO2 concentrations quantitatively even in the presence of ambient oxygen.

Core-shell strain structure of zeolite microcrystals.

Zeolites are crystalline aluminosilicate minerals featuring a network of 0.3-1.5-nm-wide pores, used in industry as catalysts for hydrocarbon interconversion, ion exchangers, molecular sieves and adsorbents. For improved applications, it is highly useful to study the distribution of internal local strains because they sensitively affect the rates of adsorption and diffusion of guest molecules within zeolites. Here, we report the observation of an unusual triangular deformation field distribution in ZSM-5 zeolites by coherent X-ray diffraction imaging, showing the presence of a strain within the crystal arising from the heterogeneous core-shell structure, which is supported by finite element model calculation and confirmed by fluorescence measurement. The shell is composed of H-ZSM-5 with intrinsic negative thermal expansion whereas the core exhibits a different thermal expansion behaviour due to the presence of organic template residues, which usually remain when the starting materials are insufficiently calcined. Engineering such strain effects could have a major impact on the design of future catalysts.

Solvent-Driven Dynamics: Crafting Tailored Transformations of Cu(II)-Based MOFs.

Metal-organic frameworks (MOFs), a sort of crystalline porous coordination polymers composed of metal ions and organic linkers, have been intensively studied for their ability to take up nonpolar gas-phase molecules such as ethane and ethylene. In this context, interpenetrated MOFs, where multiple framework nets are entwined, have been considered promising materials for capturing nonpolar molecules due to their relatively higher stability and smaller micropores. This study explores a solvent-assisted reversible strategy to interpenetrate and deinterpenetrate a Cu(II)-based MOF, namely, MOF-143 (noninterpenetrated form) and MOF-14 (doubly interpenetrated forms). Interpenetration was achieved using protic solvents with small molecular sizes such as water, methanol, and ethanol, while deinterpenetration was accomplished with a Lewis-basic solvent, pyridine. Additionally, this study investigates the adsorptive separation of ethane and ethylene, which is a significant application in the chemical industry. The results showed that interpenetrated MOF-14 exhibited higher ethane and ethylene uptakes compared to the noninterpenetrated MOF-143 due to narrower micropores. Furthermore, we demonstrate that pristine MOF-14 displayed higher ethane selectivity than transformed MOF-14 from MOF-143 by identifying the "fraction of micropore volume" as a key factor influencing ethane uptake. These findings highlight the potential of controlled transformations between interpenetrated and noninterpenetrated MOFs, anticipating that larger MOF crystals with narrower micropores and higher crystallinity will be more suitable for selective gas capture and separation applications.

Light-harvesting and ultrafast energy migration in porphyrin-based metal-organic frameworks.

Given that energy (exciton) migration in natural photosynthesis primarily occurs in highly ordered porphyrin-like pigments (chlorophylls), equally highly ordered porphyrin-based metal-organic frameworks (MOFs) might be expected to exhibit similar behavior, thereby facilitating antenna-like light-harvesting and positioning such materials for use in solar energy conversion schemes. Herein, we report the first example of directional, long-distance energy migration within a MOF. Two MOFs, namely F-MOF and DA-MOF that are composed of two Zn(II) porphyrin struts [5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphinato]zinc(II) and [5,15-bis[4-(pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II), respectively, were investigated. From fluorescence quenching experiments and theoretical calculations, we find that the photogenerated exciton migrates over a net distance of up to ~45 porphyrin struts within its lifetime in DA-MOF (but only ~3 in F-MOF), with a high anisotropy along a specific direction. The remarkably efficient exciton migration in DA-MOF is attributed to enhanced π-conjugation through the addition of two acetylene moieties in the porphyrin molecule, which leads to greater Q-band absorption intensity and much faster exciton-hopping (energy transfer between adjacent porphyrin struts). The long distance and directional energy migration in DA-MOF suggests promising applications of this compound or related compounds in solar energy conversion schemes as an efficient light-harvesting and energy-transport component.

Engineering Catalysis within a Saturated In(III)-Based MOF Possessing Dynamic Ligand-Metal Bonding.

Metal-organic frameworks have developed into a formidable heterogeneous catalysis platform in recent years. It is well established that thermolysis of coordinated solvents from MOF nodes can render highly reactive, coordinatively unsaturated metal complexes which are stabilized via site isolation and serve as active sites in catalysis. Such approaches are limited to frameworks featuring solvated transition-metal complexes and must be stable toward the formation of "permanent" open metal sites. Herein, we exploit the hemilability of metal-carboxylate bonds to generate transient open metal sites in an In(III) MOF, pertinent to In-centered catalysis. The transient open metal sites catalyze the Strecker reaction over multiple cycles without loss of activity or crystallinity. We employ computational and spectroscopic methods to confirm the formation of open metal sites via transient dissociation of In(III)-carboxylate bonds. Furthermore, the amount of transient open metal sites within the material and thus the catalytic performance can be temperature-modulated.

Coordination-chemistry control of proton conductivity in the iconic metal-organic framework material HKUST-1.

HKUST-1, a metal-organic framework (MOF) material containing Cu(II)-paddlewheel-type nodes and 1,3,5-benzenetricarboxylate struts, features accessible Cu(II) sites to which solvent or other desired molecules can be intentionally coordinated. As part of a broader investigation of ionic conductivity in MOFs, we unexpectedly observed substantial proton conductivity with the "as synthesized" version of this material following sorption of methanol. Although HKUST-1 is neutral, coordinated water molecules are rendered sufficiently acidic by Cu(II) to contribute protons to pore-filling methanol molecules and thereby enhance the alternating-current conductivity. At ambient temperature, the chemical identities of the node-coordinated and pore-filling molecules can be independently varied, thus enabling the proton conductivity to be reversibly modulated. The proton conductivity of HKUST-1 was observed to increase by ~75-fold, for example, when node-coordinated acetonitrile molecules were replaced by water molecules. In contrast, the conductivity became almost immeasurably small when methanol was replaced by hexane as the pore-filling solvent.

Effective panchromatic sensitization of electrochemical solar cells: strategy and organizational rules for spatial separation of complementary light harvesters on high-area photoelectrodes.

Dye-sensitized solar cells, especially those comprising molecular chromophores and inorganic titania, have shown promise as an alternative to silicon for photovoltaic light-to-electrical energy conversion. Co-sensitization (the use of two or more chromophores having complementary absorption spectra) has attracted attention as a method for harvesting photons over a broad spectral range. If implemented successfully, then cosensitization can substantially enhance photocurrent densities and light-to-electrical energy conversion efficiencies. In only a few cases, however, have significant overall improvements been obtained. In most other cases, inefficiencies arise due to unconstructive energy or charge transfer between chromophores or, as we show here, because of modulation of charge-recombination behavior. Spatial isolation of differing chromophores offers a solution. We report a new and versatile method for fabricating two-color photoanodes featuring spatially isolated chromophore types that are selectively positioned in desired zones. Exploiting this methodology, we find that photocurrent densities depend on both the relative and absolute positions of chromophores and on "local" effective electron collection lengths. One version of the two-color photoanode, based on an organic push-pull dye together with a porphyrin dye, yielded high photocurrent densities (J(SC) = 14.6 mA cm(-2)) and double the efficiency of randomly mixed dyes, once the dyes were optimally positioned with respect to each other. We believe that the organizational rules and fabrication strategy will prove transferrable, thereby advancing understanding of panchromatic sensitization as well as yielding higher efficiency devices.

Glass-encapsulated light harvesters: more efficient dye-sensitized solar cells by deposition of self-aligned, conformal, and self-limited silica layers.

A major loss mechanism in dye-sensitized solar cells (DSCs) is recombination at the TiO(2)/electrolyte interface. Here we report a method to reduce greatly this loss mechanism. We deposit insulating and transparent silica (SiO(2)) onto the open areas of a nanoparticulate TiO(2) surface while avoiding any deposition of SiO(2) over or under the organic dye molecules. The SiO(2) coating covers the highly convoluted surface of the TiO(2) conformally and with a uniform thickness throughout the thousands of layers of nanoparticles. DSCs incorporating these selective and self-aligned SiO(2) layers achieved a 36% increase in relative efficiency versus control uncoated cells.

Metal-organic framework materials with ultrahigh surface areas: is the sky the limit?

We have synthesized, characterized, and computationally simulated/validated the behavior of two new metal-organic framework (MOF) materials displaying the highest experimental Brunauer-Emmett-Teller (BET) surface areas of any porous materials reported to date (~7000 m(2)/g). Key to evacuating the initially solvent-filled materials without pore collapse, and thereby accessing the ultrahigh areas, is the use of a supercritical CO(2) activation technique. Additionally, we demonstrate computationally that by shifting from phenyl groups to "space efficient" acetylene moieties as linker expansion units, the hypothetical maximum surface area for a MOF material is substantially greater than previously envisioned (~14600 m(2)/g (or greater) versus ~10500 m(2)/g).

Crystalline hydrogen bonding of water molecules confined in a metal-organic framework.

Hydrogen bonding (H-bonding) of water molecules confined in nanopores is of particular interest because it is expected to exhibit chemical features different from bulk water molecules due to their interaction with the wall lining the pores. Herein, we show a crystalline behavior of H-bonded water molecules residing in the nanocages of a paddlewheel metal-organic framework, providing in situ and ex situ synchrotron single-crystal X-ray diffraction and Raman spectroscopy studies. The crystalline H-bond is demonstrated by proving the vibrational chain connectivity arising between hydrogen bond and paddlewheel Cu-Cu bond in sequentially connected Cu-Cu·····coordinating H2O·····H-bonded H2O and by proving the spatial ordering of H-bonded water molecules at room temperature, where they are anticipated to be disordered. Additionally, we show a substantial distortion of the paddlewheel Cu2+-centers that arises with water coordination simultaneously. Also, we suggest the dynamic coordination bond character of the H-bond of the confined water, by which an H-bond transitions to a coordination-bond at the Cu2+-center instantaneously after dissociating a previously coordinated H2O.

Effect of water on the behavior of semiconductor quantum dots in zeolite Y: aggregation with framework destruction with H-Y and disaggregation with framework preservation for NH4-Y.

Treatment of dry M(2+)-exchanged zeolite Y (M(2+) = Cd(2+), Mn(2+), and Zn(2+)) with dry H(2)S leads to the formation of isolated, ligand-free, subnanometer MS quantum dots (QDs) in zeolite Y with no framework destruction and with H(+) as the countercation. Treatment of the dry H(+)/CdS QD-incorporating zeolites Y with dry NH(3) leads to the neutralization of H(+) to NH(4)(+). During this process, the framework structure remains intact. However, small amounts of interconnected CdS QDs were formed within the zeolite Y by coalition of isolated CdS QDs at the windows. With H(+) as the countercation, isolated CdS QDs rapidly aggregate into interconnected and mesosized QDs with accompanying destruction of ∼50% of sodalite cages leading to the framework rupture. With NH(4)(+) as the countercation, however, the isolated QDs and zeolite framework remain intact even after exposure to the moist air for 4 weeks. Interestingly, the interconnected QDs that were formed during neutralization of H(+) with NH(3) disintegrate into isolated QDs in the air. Similar results were obtained from ZnS and MnS QDs generated in zeolite Y. Thus, ligand-free, naked, subnanometer QDs can now be safely preserved within zeolite pores under the ambient conditions for long periods of time. This finding will expedite the generation and dispersion of various QDs in zeolite pores, their physicochemical studies, and applications.

Kinetic separation of propene and propane in metal-organic frameworks: controlling diffusion rates in plate-shaped crystals via tuning of pore apertures and crystallite aspect ratios.

A series of isostructural, noncatenated, zinc-pillared-paddlewheel metal-organic framework materials has been synthesized from 1,2,4,5-tetrakis(carboxyphenyl)benzene and trans-1,2-dipyridylethene struts. Substantial kinetic selectivity in the adsorption of propene over propane can be observed, depending on the pore apertures and the rectangular-plate morphology of the crystals.

Photocurrent enhancement by surface plasmon resonance of silver nanoparticles in highly porous dye-sensitized solar cells.

Localized surface plasmon resonance (LSPR) by silver nanoparticles that are photochemically incorporated into an electrode-supported TiO(2) nanoparticulate framework enhances the extinction of a subsequently adsorbed dye (the ruthenium-containing molecule, N719). The enhancement arises from both an increase in the dye's effective absorption cross section and a modest increase in the framework surface area. Deployment of the silver-modified assembly as a photoanode in dye-sensitized solar cells leads to light-to-electrical energy conversion with an overall efficiency of 8.9%. This represents a 25% improvement over the performance of otherwise identical solar cells lacking corrosion-protected silver nanoparticles. As one would expect based on increased dye loading and electromagnetic field enhanced (LSPR-enhanced) absorption, the improvement is manifested chiefly as an increase in photocurrent density ascribable to improved light harvesting.

A convenient route to high area, nanoparticulate TiO2 photoelectrodes suitable for high-efficiency energy conversion in dye-sensitized solar cells.

Ethanol-soluble amphiphilic TiO(2) nanoparticles (NPs) of average diameter ∼9 nm were synthesized, and an α-terpineol-based TiO(2) paste was readily prepared from them in comparatively few steps. When used for fabrication of photoelectrodes for dye-sensitized solar cells (DSSCs), the paste yielded highly transparent films and possessing greater-than-typical, thickness-normalized surface areas. These film properties enabled the corresponding DSSCs to produce high photocurrent densities (17.7 mA cm(-2)) and a comparatively high overall light-to-electrical energy conversion efficiency (9.6%) when deployed with the well-known ruthenium-based molecular dye, N719. These efficiencies are about ∼1.4 times greater than those obtained from DSSCs containing photoelectrodes derived from a standard commercial source of TiO(2) paste.

Photovoltaic effects of CdS and PbS quantum dots encapsulated in zeolite Y.

Zeolite Y films (0.35-2.5 µm), into which CdS and PbS quantum dots (QDs) were loaded, were grown on ITO glass. The CdS QD-loaded zeolite Y films showed a photovoltaic effect in the electrolyte solution consisting of Na(2)S (1 M) and NaOH (0.1 M) with Pt-coated F-doped tin oxide glass as the counter electrode. In contrast, the PbS QD-loaded zeolite Y films exhibited a negligible PV effect. This contrasting behavior was proposed to arise from the large difference in driving force for the electron transfer from S(2-) in the solution to the hole in the valence band of QDs, with the former being much larger (~2 eV) than the latter (~1 eV). In the case of CdS QD-loaded zeolite Y with a loaded amount of CdS of 6.3 per unit cell, the short circuit current, open circuit voltage, fill factor, and efficiency were 0.3 mA cm(-2), 423 V, 28, and 0.1%, respectively, under the AM 1.5, 100 mW cm(-2) condition. This cell was stable for more than 18 days of continuous measurements. A large (3-fold) increase in overall efficiency was observed when PbS QD-loaded zeolite Y on ITO glass was used as the counter electrode. This phenomenon suggests that the uphill electron transfer from ITO glass to S in the solution is facilitated by the photoassisted pumping of the potential energy of the electron in ITO glass to the level that is higher than the reduction potential of S by PbS QDs. Under this condition, the incident-photon-to-current conversion efficiency (IPCE) value at 398 nm was 42% and the absorbed-photon-to-current conversion efficiency (APCE) value at 405 nm was 82%. The electrolyte-mediated interdot charge transport within zeolite films is concluded to be responsible for the overall current flow.

Control of mode of crystal networking during monolayer assembly of microcrystals on water.

A series of hydrocarbon (HC)-coated cubic zeolite microcrystals (1.7 microm) was prepared. The HCs were n-octyl, n-dodecyl, methyl n-undecanoate, n-octadecyl, and n-heptadecafluorodecyl. The measured water contact angles (theta) of the corresponding HC-coated glass plates were 64, 77, 82, 102, and 105 degrees, respectively, indicating that the hydrophobicity of the surface-tethered hydrophobic chain (HC) increased in the above order. The HC-coated zeolite microcrystals readily formed closely packed monolayers at the air-water interface through interdigitation of surface-tethered HCs, and on glass plates after transferring onto glass plates by dip coating. Interestingly, while the mode of networking was face-to-face (FTF) contacting with n-octyl or n-dodecyl (theta < or =77 degrees) as HC, it changed to edge-to-edge (ETE) contacting mode with n-octadecyl or n-heptadecafluorodecyl (theta > or = 102 degrees) as HC. With methyl n-undecanoate (theta = 82 degrees) as HC, both modes appeared in the monolayers, with about equal populations. The resulting monolayers of cubic zeolite microcrystals with their three-fold axes oriented perpendicular to substrates would be useful for application of the zeolite monolayers for advanced materials.