Non-Nernstian two-electron transfer photocatalysis at metalloporphyrin-TiO2 interfaces

A long-standing question in the photochemical sciences concerns how to integrate single-electron transfers to catalytic multielectron transfer reactions that produce useful chemical fuels. Here we provide a strategy for the two-electron formation of C-C bonds with molecular catalysts anchored to semiconductor nanocrystallites. The blue portion of the solar spectrum provides band gap excitation of the semiconductor while longer wavelengths of light initiate homolytic cleavage of metal-carbon bonds that, after interfacial charge transfer, restore the catalyst. The semiconductor utilized was the anatase polymorph of TiO(2) present as a nanocrystalline, mesoporous thin film. The catalyst was cobalt meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride, Co(TCPP)Cl. For this catalyst and iron protoporphyrin IX chloride, Fe(PPIX)Cl, two distinct and sequential metal-based M(III/II) and M(II/I) reductions were observed under band gap illumination. Spectroelectrochemical characterization indicated that both reductions were non-Nernstian, behavior attributed to an environmentally dependent potential drop across the molecule-semiconductor interface. Reaction of Co(I)(TCPP)/TiO(2) with organobromides (RBr = 1-Br-hexane or benzyl bromide) resulted in the formation of Co(III)-R(TCPP)/TiO(2). Visible light excitation induced homolytic cleavage of the Co-C bond and the formation of C-C-bonded products. The reactions were catalytic when band gap excitation or an electrochemical bias provided TiO(2) electrons to the oxidized catalyst. Sustained photocurrents were quantified in photoelectrosynthetic solar cells under forward bias.     

Diastereoselective synthesis of 1,3,5-trisubstituted 4-nitropyrrolidin-2-ones via a nitro-Mannich/lactamization cascade

A versatile one-pot nitro-Mannich/lactamization cascade for the direct synthesis of 1,3,5-trisubstituted 4-nitropyrrolidin-2-ones has been developed. The reaction is easy to perform and broad in scope, and high levels of diastereoselectivity can be achieved.     

Treadmilling swimmers near a no-slip wall at low Reynolds number

The motion of a circular treadmilling low Reynolds number swimmer near a no-slip wall is studied analytically. First, the exact solution of Jeffrey and Onishi [Q. J. Mech. Appl. Math., 34 (1981)] for a translating and rotating solid cylinder near a no-slip wall is rederived using a novel conformal mapping approach that differs from the original derivation which employed bipolar coordinates. Then it is shown that this solution can be combined with the reciprocal theorem, and the calculus of residues, to produce an explicit non-linear dynamical system for the treadmilling swimmer's velocity and angular velocity. The resulting non-linear dynamical system governing the swimmer motion is used to corroborate the qualitative results obtained by an approximate model of the same swimmer recently presented in Crowdy and Or [Phys. Rev. E., 81 (2010)].     

Rank numbers for some trees and unicyclic graphs

A ranking on a graph is an assignment of positive integers to its vertices such that any path between two vertices of the same rank contains a vertex of strictly larger rank. The rank number of a graph is the fewest number of labels that can be used in a ranking. In this paper we determine rank numbers for some trees and unicyclic graphs.     

Tuning percolation speed in layer-by-layer assembled polyaniline�Cnanocellulose composite films

Polyaniline of low molecular weight (ca. 10?kDa) is combined with cellulose nanofibrils (sisal, 4?C5?nm average cross-sectional edge length, with surface sulphate ester groups) in an electrostatic layer-by-layer deposition process to form thin nano-composite films on tin-doped indium oxide (ITO) substrates. AFM analysis suggests a growth in thickness of ca. 4?nm per layer. Stable and strongly adhering films are formed with thickness-dependent coloration. Electrochemical measurements in aqueous H₂SO₄ confirm the presence of two prominent redox waves consistent with polaron and bipolaron formation processes in the polyaniline?Cnanocellulose composite. Measurements with a polyaniline?Cnanocellulose film applied across an ITO junction (a 700?nm gap produced by ion beam milling) suggest a jump in electrical conductivity at ca. 0.2?V vs. SCE and a propagation rate (or percolation speed) two orders of magnitude slower compared to that observed in pure polyaniline This effect allows tuning of the propagation rate based on the nanostructure architecture. Film thickness-dependent electrocatalysis is observed for the oxidation of hydroquinone.     

SiO2 coated pure and doped titania pigments: low temperature CVD deposition and quantum chemical study

We report the in-flight CVD coating with smooth 1-2 nm thick SiO(2) of pure and doped rutile particles via the oxidation of SiCl(4) vapour introduced in the high temperature zone of a purpose built thermal reactor. The effectiveness of the coatings was determined by a combination of electron microscopy, surface analysis and photocatalytic measurements. No excess Cl was detected on the coated pigment particles indicating the complete oxidation of the SiCl(4) precursor. In conjunction with the experimental outcomes of this optimised deposition process, we use first-principles density functional and semi-empirical quantum chemical calculations to examine the underlying electronic processes which determined the morphology and photocatalytic properties of the coated titania. We highlight the presence of low lying valence electronic states which reduce photocatalytic activity, and as a consequence decrease the population of photo-excited titania electrons which transfer to the surrounding matrix. Born-Oppenheimer molecular dynamics (MD) simulations indicate that the coating process is completed within the order of 10 ps.     

Expedient Route to the functionalized calyciphylline A-type skeleton via a Michael addition-RCM strategy

An efficient, robust, and scalable strategy to access the functionalized core of calyciphylline A-type alkaloids has been developed starting from commercially available 3-methylanisole. Key features of this approach are an intramolecular Michael addition/allylation sequence and a ring-closing metathesis step.     

Highly NIR-emissive lanthanide polyselenides

Ln(SePh)(3) (Ln = Ce, Pr, Nd), prepared by reduction of PhSeSePh with elemental Ln and Hg catalyst, reacts with excess elemental Se to give (py)(11)Ln(7)Se(21)HgSePh, an ellipsoidal polyselenide cluster. The molecular structure contains two square arrays of eight- or nine-coordinate Ln fused at one edge to form a V shape that is also capped on the concave side by a centrally located nine-coordinate (Se(3))pyLn(Se(3)) and on the convex side by a 2-fold disordered SeHgSePh. The central Ln coordinates to selenido, triselenido, and pyridine ligands, while all other Ln coordinate to selenido, diselenido, triselenido, and pyridine ligands. Thermal treatment of the Pr compound at 650 °C gave Pr(2)Se(3) and Pr(3)Se(4). NIR emission studies of the Nd compound show four transitions from the excited-state (4)F(3/2) ion to (4)I(9/2), (4)I(11/2), (4)I(13/2), and (4)I(15/2) states. The (4)F(3/2) ion to (4)I(11/2) transition (1075 nm emission) exhibited 43% quantum efficiency. This is the highest quantum efficiency reported for a 'molecular' Nd compound and leads a group of selenide-based clusters that has shown extraordinary quantum efficiency. In terms of efficiency and concentration, these compounds compare favorably with solid-state materials.     

Enhancing gas adsorption and separation capacity through ligand functionalization of microporous metal-organic framework structures

Hydroxyl- and amino- functionalized [Zn(BDC)(TED)(0.5)]·2DMF·0.2H(2)O leads to two new structures, [Zn(BDC-OH)(TED)(0.5)]·1.5DMF·0.3H(2)O and [Zn(BDC-NH(2))(TED)(0.5)]·xDMF·yH(2)O (BDC=terephthalic acid, TED=triethylenediamine, BDC-OH=2-hydroxylterephthalic acid, BDC-NH(2)=2-aminoterephthalic acid). Single-crystal X-ray diffraction and powder X-ray diffraction studies confirmed that the structures of both functionalized compounds are very similar to that of their parent structure. Compound [Zn(BDC)(TED)(0.5)]·2DMF·0.2H(2)O can be considered a 3D porous structure with three interlacing 1D channels, whereas both [Zn(BDC-OH)(TED)(0.5)]·1.5DMF·0.3H(2)O and [Zn(BDC-NH(2))(TED)(0.5)]·xDMF·yH(2)O contain only 1D open channels as a result of functionalization of the BDC ligand by the OH and NH(2) groups. A notable decrease in surface area and pore size is thus observed in both compounds. Consequently, [Zn(BDC)(TED)(0.5)]·2DMF·0.2H(2)O takes up the highest amount of H(2) at low temperatures. Interestingly, however, both [Zn(BDC-OH)(TED)(0.5)]·1.5DMF·0.3H(2)O and [Zn(BDC-NH(2))(TED)(0.5)]·xDMF·yH(2)O show significant enhancement in CO(2) uptake at room temperature, suggesting that the strong interactions between CO(2) and the functionalized ligands, indicating that surface chemistry, rather than porosity, plays a more important role in CO(2) adsorption. A comparison of single-component CO(2), CH(4), CO, N(2), and O(2) adsorption isotherms demonstrates that the adsorption selectivity of CO(2) over other small gases is considerably enhanced through functionalization of the frameworks. Infrared absorption spectroscopic measurements and theoretical calculations are also carried out to assess the effect of functional groups on CO(2) and H(2) adsorption potentials.     

Semi-targeted analysis of metabolites using capillary-flow ion chromatography coupled to high-resolution mass spectrometry

This work describes a novel application of capillary-flow ion chromatography mass spectrometry for metabolomic analysis, and comparison of the technique to octadecyl silica and hydrophilic interaction chromatography (HILIC)-based mass spectrometry. While liquid chromatography/mass spectrometry (LC/MS) is rapidly becoming the standard technique for metabolomic analysis, metabolomic samples are extremely heterogeneous, leading to a requirement for multiple methods of analysis and separation techniques to perform a 'global' metabolomic analysis. While C18 is suitable for hydrophobic metabolites and has been used extensively in pharmaceutical drug metabolism studies, HILIC is, in general, efficient at separating polar metabolites. Phosphorylated species and organic acids are challenging to analyse and effectively quantitate on both systems. There is therefore a requirement for an MS-compatible analytical technique that can separate negatively charged compounds, such as ion-exchange chromatography. Evaluation of capillary flow ion chromatography with electrolytic suppression was performed on a library of metabolite standards and was shown to effectively separate organic acids and sugar di- and tri-phosphates. Limits of detection for these compounds range from 0.01 to 100 pmol on-column. Application of capillary ion chromatography to a comparative analysis of energy metabolism in procyclic forms of the parasitic protozoan Trypanosoma brucei where cells were grown on glucose or proline as a carbon source was demonstrated to be more effective than HILIC for detection of the organic acids that comprise glucose central metabolism and the tricarboxylic acid (TCA) cycle.