Photocatalyzed Hydrogen Evolution from Water by a Composite Catalyst of NH2‐MIL‐125(Ti) and Surface Nickel(II) Species

A composite of the metal–organic framework (MOF) NH₂‐MIL‐125(Ti) and molecular and ionic nickel(II) species, catalyzed hydrogen evolution from water under UV light. In 95 v/v % aqueous conditions the composite produced hydrogen in quantities two orders of magnitude higher than that of the virgin framework and an order of magnitude greater than that of the molecular catalyst. In a 2 v/v % water and acetonitrile mixture, the composite demonstrated a TOF of 28 mol H₂ g(Ni)^?1 h^?1 and remained active for up to 50 h, sustaining catalysis for three times longer and yielding 20‐fold the amount of hydrogen. Appraisal of physical mixtures of the MOF and each of the nickel species under identical photocatalytic conditions suggest that similar surface localized light sensitization and proton reduction processes operate in the composite catalyst. Both nickel species contribute to catalytic conversion, although different activation behaviors are observed.     

Nonlinear equations with unbounded heat conduction and integrable data

We consider a class of quasi-linear diffusion problems involving a matrix A(t,x,u) which blows up for a finite value m of the unknown u. Stationary and evolution equations are studied for L ¹ data. We focus on the case where the solution u can reach the value m. For such problems we introduce a notion of renormalized solutions and we prove the existence of such solutions.      

Preparation,physical properties,on-bead binding assay and spectroscopic reliability of 25 barcoded polystyrene-poly(ethylene glycol) graft copolymers

Here we describe the preparation of 25 beaded polystyrene-poly(ethylene glycol) graft copolymers from six spectroscopically active styrene monomers: styrene, 2,5-dimethylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 4-tert-butylstyrene, and 3-methylstyrene. These polymers were thoroughly characterized by Raman, infrared, and (1)H/(13)C NMR spectroscopies, and differential scanning calorimetry. Determination of the swelling properties, peptide synthesis, and on-bead streptavidin-alkaline phosphatase (SAP) binding assay further established that their physical and chemical properties where not significantly altered by the diversity of their encoded polystyrene core. Each of the 25 resins displayed a unique Raman and infrared vibrational fingerprint, which was converted into a "spectroscopic barcode". The position of each bar matches the peak wavenumber in the corresponding spectrum but is independent of its intensity. From this simplified representation similarity maps comparing 35 000 resin pairs were generated to establish the spectroscopic barcoding as a reliable encoding methodology. In effect, in 99% of the cases, the highest similarity coefficients were obtained for resin pairs prepared from the same styrene derivatives even after SAP binding assay. We have also shown that a small but unique combination of a resin's vibrations (30-40%) is sufficient for its identification. However, in rare cases where a resin's vibrational signature has been severely compromised, both the Raman and infrared barcodes were synergistically and reliably utilized to unequivocally identify its chemical make up.     

Helical rosette nanotubes with tunable chiroptical properties

On the basis of transmission electron microscopy (TEM), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and circular dichroism (CD) studies, compound 1 was shown to exist mainly in two states: (a) At high concentration (> or =1 mM, in methanol), 1 undergoes hierarchical self-assembly to generate rosette nanotubes with approximately 4 nm diameter and a concentration-dependent hydrodynamic radius in the range 10-100 nm. Under these conditions, addition of a chiral amino acid promoter (L-Ala), that binds to the crown ether moiety of 1 via electrostatic interactions, promotes a rapid transition (k(0) approximately equal 0.48 s(-1), for [1] = 0.046 mM, [L-Ala] = 2.8 mM) from racemic to chiral rosette nanotubes with predefined helicities as indicated by the resulting induced circular dichroism (ICD). (b) At low concentration (< or =0.04 mM, in methanol), 1 exists mainly in a nonassembled state as shown by TEM and DLS. Addition of L-Ala in this case triggers a relatively slow (k(0) approximately equal 0.07 s(-1) for [1] = 0.04 mM, [L-Ala] = 2.4 mM) sequence of supramolecular reactions leading to the hierarchical self-assembly of rosette nanotubes with predefined helicities. Under both conditions a and b, the kinetic data unveiled the intrinsic ability of the rosette nanotubes to promote their own formation (autocatalysis). The degree of chiral induction was found to depend dramatically upon the chemical structure of the promoter. This process appears also to follow an all-or-none response, as the vast majority of the crown ether sites must be occupied with a promoter for a complete transition to chiral nanotubes to take place. Finally, both supramolecular pathways a and b offer an efficient approach for the preparation of helical rosette nanotubes with tunable chiroptical properties and may also be viewed as a process by which a predefined set of physical and chemical properties that characterizes a molecular promoter is expressed at the macromolecular level.     

The origin of virgin argan oil's high oxidative stability unraveled

To prepare either virgin edible or beauty argan oil, roasted or unroasted argan kernels are cold-pressed, respectively. Comparing the physicochemical parameters of edible and beauty argan oil immediately after preparation and after a two-year delay has led to the suggestion that phospholipids are a new and essential type of oil component participating in the excellent oxidative stability of edible argan oil, in addition to the already suggested Maillard-reaction products, phenols, and tocopherols.     

High quality GaAs quantum wires grown by flow rate modulation epitaxy

Accepted: 14 October 1997     

Probing the reaction pathways of DL-proline on TiO2 (001) single crystal surfaces

The reaction of DL-Proline on O2-annealed (stoichiometric) and O-defected (sub-stoichiometric) TiO2 (001) single-crystal surfaces has been investigated. This is of significance in trying to understand the concept of how biomolecules interact with the surfaces of biomedical implants (molecular recognition). On an O2-annealed TiO2 surface, proline is found to largely adsorb then desorb intact at approximately 350 K. DFT (B3LYP) calculations of proline bound to a Ti(OH)4 cluster suggest a binding through the carboxylate functional group rather than through the NH group of the ring. In contrast, proline reaction was considerably different on the O-defected surface. First, proline was further stabilized, evidenced by a shift of its desorption temperature (during temperature-programmed desorption) to approximately 530 K. Along with proline desorption, two distinctive sets of reaction processes occurred at 530 and 630 K, respectively. The first pathway (alpha) at 530 K shows desorption of large amounts of m/e 55 (attributed to 1-azetine) and m/e 42 (attributed to ketene). At still higher temperature, 630 K, a pathway (beta) dominated by the appearance of low masses, mainly m/e 28, 27, and 26, is seen. These masses are tentatively attributed to desorption of HCN, ethylene, and/or acetylene as they represent the logical further decomposition of the different fragments of proline.     

A reusable polymer‐supported copper(I) catalyst for triazole click reaction on water: An experimental and computational study

In the search for establishing a clickable copper‐catalysed (3 + 2) Huisgen azide–alkyne cycloaddition (CuAAC) reaction under strict conditions, in particular in terms of preventing the presence of copper particles/traces in reaction products and using an environmentally benign medium such as water, we describe here the synthesis of an aminomethyl polystyrene‐supported copper(I) catalyst (Cu(I)‐AMPS) and its characterization by means of Fourier transform infrared and energy‐dispersive X‐ray spectroscopies and scanning electron microscopy. Cu(I)‐AMPS was found to be highly active in the CuAAC reaction of various organic azides with alkynes affording the corresponding 1,4‐disubstituted 1,2,3‐triazoles in a regioselective manner in air at room temperature and using water as solvent. The insolubility and/or partial solubility of the organic azide and alkyne precursors as well as the heterogeneous Cu(I)‐AMPS catalytic system points to the occurrence of the cycloaddition at the organic–water interface ‘on water’ affording quantitative yields of water‐insoluble 1,2,3‐triazoles. A mechanistic study was performed using density functional theory aiming at explaining the observed reactivity and selectivity of the Cu (I)‐AMPS catalyst in CuAAC reactions.     

Synthesis of rhenium chelated MAG3 functionalized rosette nanotubes

Rosette nanotubes (RNTs) are discrete nanostructures self-assembled from a guanine–cytosine hybrid motif (G∧C) under aqueous conditions. These materials have substantial design flexibility and a range of applications, which are partly attributed to their diverse surface functionalization. Given the potential for interesting properties resulting from a metal-RNT construct, here we describe an oxorhenium-functionalized RNT. More specifically, we present the synthesis of a twin G∧C motif expressing the mercaptoacetyl triglycine (MAG₃) ligand. We then examine the chelation reaction of the MAG₃ with ReOCl₃(PPh₃)₂ and self-assemble the resulting ReO-MAG₃-G∧C conjugate into RNTs under DMSO and aqueous conditions.     

Tin-based hybrid materials with a two-level structural hierarchy

Self-assembled tin-based hybrid materials with two levels of hierarchy were obtained spontaneously from the hydrolysis of hexaalkynylorganoditins bridged with an aromatic spacer, while in the case of an aliphatic spacer this phenomenon occurred only in the presence of a surfactant.