In Situ Monitoring of Heterogeneous Hydrosilylation Reactions Using Infrared and Raman Spectroscopy: Normalization Using Phase-Specific Internal Standards.

Heterogeneous reaction systems are prevalent in the chemical industry. In situ monitoring of heterogeneous reaction systems by vibrational spectroscopy techniques offers real-time composition and conversion information without sampling and with minimal perturbation. The multiphase nature introduces new challenges which are not typically encountered in the monitoring of homogeneous systems. We investigated the kinetics of the Pt catalyzed hydrosilylation reactions between allyl polyether and SiH containing silicone using both infrared (IR) and Raman spectroscopy. The reaction mixture remains biphasic for the majority of the reaction time due to the low miscibility of polyether and silicone. The results demonstrated that by normalization with appropriate internal standards (phase-specific normalization), more accurate quantitation of the SiH and allyl functional groups can be achieved based on in situ Raman results than that based on in situ attenuated total reflection Fourier transform infrared (ATR FT-IR) results. This is believed to be due to the wavelength-dependent penetration depth of the ATR mode. Raman results were obtained using two immersion optics with different focal lengths. The advantages and disadvantages of these two immersion optics are clearly illustrated in this study.

Tunneling Electrical Connection to the Interior of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are typically poor electrical conductors, which limits their uses in sensors, fuel cells, batteries, and other applications that require electrically conductive, high surface area materials. Although metal nanoclusters (NCs) are often added to MOFs, the electrical properties of these hybrid materials have not yet been explored. Here, we show that adding NCs to a MOF not only imparts moderate electrical conductivity to an otherwise insulating material but also renders it photoconductive, with conductivity increasing by up to 4 orders of magnitude upon light irradiation. Because charge transport occurs via tunneling between spatially separated NCs that occupy a small percent of the MOF's volume, the pores remain largely open and accessible. While these phenomena are more pronounced in single-MOF crystals (here, Rb-CD-MOFs), they are also observed in films of smaller MOF crystallites (MIL-53). Additionally, we show that in the photoconductive MOFs, the effective diffusion coefficients of electrons can match the typical values of small molecules diffusing through MOFs; this property can open new vistas for the development of MOF electrodes and, in a wider context, of electroactive and light-harvesting MOFs.

A metal-organic framework stabilizes an occluded photocatalyst.

Occlusion and confinement of a [Ru(bpy)3 ]Cl2 photocatalyst in the cavities of a γ-cyclodextrin (CD) metal-organic framework (MOF) does not affect the catalyst's activity but prevents its photodegradation. Additionally, the OH(-) ions and/or ROH groups present inside the CD-MOF act as electron donors and complete the catalytic cycle. The occlusion approach is a technically straightforward alternative to the covalent modification of MOF scaffolds with catalytic units.

Transport into metal-organic frameworks from solution is not purely diffusive.

Chemistry in motion: a combination of confocal microscopy and reaction-diffusion modeling provided a powerful toolkit with which solution transport into metal-organic framework crystals was studied. Commonly used pure diffusion models are insufficient to describe this process and, instead, it is necessary to account for the interactions of the guest molecules and the MOF scaffold.

Charged nanoparticles as supramolecular surfactants for controlling the growth and stability of microcrystals.

Microcrystals of desired sizes are important in a range of processes and materials, including controlled drug release, production of pharmaceutics and food, bio- and photocatalysis, thin-film solar cells and antibacterial fabrics. The growth of microcrystals can be controlled by a variety of agents, such as multivalent ions, charged small molecules, mixed cationic-anionic surfactants, polyelectrolytes and other polymers, micropatterned self-assembled monolayers, proteins and also biological organisms during biomineralization. However, the chief limitation of current approaches is that the growth-modifying agents are typically specific to the crystalizing material. Here, we show that oppositely charged nanoparticles can function as universal surfactants that control the growth and stability of microcrystals of monovalent or multivalent inorganic salts, and of charged organic molecules. We also show that the solubility of the microcrystals can be further tuned by varying the thickness of the nanoparticle surfactant layers and by reinforcing these layers with dithiol crosslinks.

How and why nanoparticle's curvature regulates the apparent pKa of the coating ligands.

Dissociation of ionizable ligands immobilized on nanopaticles (NPs) depends on and can be regulated by the curvature of these particles as well as the size and the concentration of counterions. The apparent acid dissociation constant (pK(a)) of the NP-immobilized ligands lies between that of free ligands and ligands self-assembled on a flat surface. This phenomenon is explicitly rationalized by a theoretical model that accounts fully for the molecular details (size, shape, conformation, and charge distribution) of both the NPs and the counterions.

Chromatography in a single metal-organic framework (MOF) crystal.

Millimeter-sized single MOF-5 crystals are used as "chromatographic columns" to effectively separate mixtures of organic dyes. Remarkably, owing to the nanoscopic pore dimensions and the molecular-level interactions between the migrating molecules and the MOF scaffold, the separations occur over a distance of only a few hundred micrometers which is unambiguously confirmed by fluorescence confocal microscopy.

Photoswitchable catalysis mediated by dynamic aggregation of nanoparticles.

Catalytic activity of gold nanoparticles in a hydrosilylation reaction is controlled by irradiation with UV or visible light. When exposed to UV, the particles aggregate and the catalysis is effectively switched "off". When the particles are exposed to visible light, the particles redisperse and catalysis can proceed.