Tuning of redox properties of iron and iron oxides via encapsulation within carbon nanotubes

We report the tuning of the redox properties of iron and iron oxide nanoparticles by encapsulation within carbon nanotubes (CNTs) with varying inner diameters. Raman spectroscopy was employed to investigate the interaction of the encapsulated nanoparticles with the CNTs. A red shift of the Fe-O mode is observed in the nanoparticles deposited on the outer CNT surfaces with respect to bulk Fe2O3. However, this mode is found to be stepwise blue-shifted with decreasing inner diameter in the CNT-encapsulated Fe2O3 nanoparticles, suggesting an enhanced interaction of Fe2O3 with the inner CNT surface as its curvature increases. The autoreduction of the encapsulated Fe2O3 is significantly facilitated inside CNTs with respect to the outside nanoparticles. Interestingly, it becomes more facile with decreasing CNT channel diameter as evidenced by temperature programmed reaction, in situ XRD, and Raman spectroscopy. The oxidation of encapsulated metallic Fe nanoparticles on the other hand is retarded in comparison to that of the outside Fe particles as shown by in situ XRD and gravimetrical measurements with an online microbalance. We attribute this tunable redox behavior of transition metal nanoparticles inside CNTs to a particular electronic interaction of the encapsulates with the interior CNT surface, which stabilizes the metallic state of Fe.     

Entire solutions of eiconal type equations

The paper characterizes entire solutions to partial differential equations for polynomials p in . Received: 14 August 2006     

Necessary Conditions in Multiobjective Optimization with Equilibrium Constraints

We study multiobjective optimization problems with equilibrium constraints (MOPECs) described by parametric generalized equations in the form

Simple bis-thiocarbono-hydrazones as sensitive, selective, colorimetric, and switch-on fluorescent chemosensors for fluoride anions

Bis-thiocarbono-hydrazones are found to be a class of sensitive, selective, ratiometric, and colorimetric chemosensors for anions such as fluoride (F(-)) or acetate (Ac(-)). The sensitivities, or the binding constants of the sensors with anions, were found to be strongly dependent on the substituents appended on the pi-conjugation framework, the delocalization bridge CH==N, the aromatic moiety, and the hetero atom in the C==X group (X=O, S) of the sensors. Single-crystal structures and (1)H NMR titration analysis shows that the --CH==N-- moiety is a hydrogen-bond donor, and it is proposed that an additional CHF hydrogen bond is formed for the sensors in the presence F(-). A sensor bearing anthracenyl groups is demonstrated as a switch-on fluorescent chemosensor for F(-) and Ac(-). The recognition of F(-) in acetonitrile (MeCN) by a sensor with nitrophenyl substituents is tolerant to MeOH (MeCN/MeOH=10:1, v/v) and water (MeCN/H(2)O=30:1, v/v); at these solvent ratios the absorption intensity of the sensor-F(-) complex solution at maximal absorption wavelength was attenuated to half of the original value in pure MeCN.     

Assessing the mechanical properties of a molecular spring

We report on the dramatic effect of increasing helix diameter on the hybridization of oligopyridine-dicarboxamide strands into double helices. Upon replacing a single pyridine by a 1,8-diazaanthracene unit within an oligomeric strand, a 4.7 A enlargement of the helix diameter occurs parallel to the long anthracene axis. This structure change results in a spectacular stabilization of the double helical hybrids derived from these strands (factors of over 10(7)). Detailed investigations of the hybridization process using X-ray crystallography, NMR, fluorescence measurements and molecular mechanics calculations allowed us to assign the duplex stabilization to two enthalpic effects. First, the increase in diameter results in an augmented surface, involved in intermolecular pi-pi stacking. Second, the enlarged diameter leads to a lower tilt angle of the helical strand, with respect to the helix axis, which in turn results in smaller dihedral angles at the aryl-amide linkages and thus a considerably lowered enthalpic cost of the spring-like extension of the strands during the hybridization process. These results provide novel insights into how subtle tuning of molecular components may result in considerable and rationalizable changes in double helical supramolecular architectures.     

A beta-keto ester as a novel, efficient, and versatile ligand for copper(I)-catalyzed C-N, C-O, and C-S coupling reactions

Employing ethyl 2-oxocyclohexanecarboxylate as a novel, efficient, and versatile ligand, the copper-catalyzed coupling reactions of various N/O/S nucleophilic reagents with aryl halides could be successfully carried out under mild conditions. A variety of products including N-arylamides, N-arylimidazoles, aryl ethers, and aryl thioethers were synthesized in good to excellent yields.     

CuI-catalyzed domino process to 2,3-disubstituted benzofurans from 1-bromo-2-iodobenzenes and beta-keto esters

CuI-catalyzed coupling of 1-bromo-2-iodobenzenes with beta-keto esters in THF at 100 degrees C leads to 2,3-disubstituted benzofurans. This domino transformation involves an intermolecular C-C bond formation and a subsequent intramolecular C-O bond formation process. Benzofurans with different substituents at the 5- and 6-position are accessible by employing the corresponding 1-bromo-2-iodobenzenes.     

Solution-assisted assembly of organic semiconducting single crystals on surfaces with patterned wettability

Two efficient approaches to assembling organic semiconducting single crystals are described. The methods rely on solvent wetting and dewetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. Substrates were functionalized with different self-assembled monolayers (SAMs) to achieve the desired wettabilities. The assembly of different organic crystals over centimeter-squared areas on Au, SiO2, and flexible plastic substrates was demonstrated. By designing line features on the substrate, the alignment of crystals, such as CuPc needles, was also achieved. As a demonstration of the potential application of this assembly approach, arrays of single-crystal organic field-effect transistors were fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes.