Reactivity of activated versus nonactivated 2-(bromomethyl)aziridines with respect to sodium methoxide: a combined computational and experimental study

The difference in reactivity between the activated 2-bromomethyl-1-tosylaziridine and the nonactivated 1-benzyl-2-(bromomethyl)aziridine with respect to sodium methoxide was analyzed by means of DFT calculations within the supermolecule approach, taking into account explicit solvent molecules. In addition, the reactivity of epibromohydrin with regard to sodium methoxide was assessed as well. The barriers for direct displacement of bromide by methoxide in methanol are comparable for all three heterocyclic species under study. However, ring opening was found to be only feasible for the epoxide and the activated aziridine, and not for the nonactivated aziridine. According to these computational analyses, the synthesis of chiral 2-substituted 1-tosylaziridines can take place with inversion (through ring opening/ring closure) or retention (through direct bromide displacement) of configuration upon treatment of the corresponding 2-(bromomethyl)aziridines with 1 equiv of a nucleophile, whereas chiral 2-substituted 1-benzylaziridines are selectively obtained with retention of configuration (via direct bromide displacement). Furthermore, the computational results showed that explicit accounting for solvent molecules is required to describe the free energy profile correctly. To verify the computational findings experimentally, chiral 1-benzyl-2-(bromomethyl)aziridines and 2-bromomethyl-1-tosylaziridines were treated with sodium methoxide in methanol. The presented work concerning the reactivity of 2-bromomethyl-1-tosylaziridine stands in contrast to the behavior of the corresponding 1-tosyl-2-(tosyloxymethyl)aziridine with respect to nucleophiles, which undergoes a clean ring-opening/ring-closure process with inversion of configuration at the asymmetric aziridine carbon atom.     

Stringing the Perylene Diimide Bow

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red‐shifted absorptions but also more facile reductions.     

Chemical functionalization of single-walled carbon nanotube field-effect transistors as switches and sensors

Because of the one-dimensional (1D) nanostructural nature of single-walled carbon nanotubes (SWNTs) and their advantages of chemical flexibility and sensitivity arising from the susceptibility of their active surfaces to interacting species, great effort has been made to integrate carbon nanotube field-effect transistors (NTFETs) into functional optoelectronic devices capable of converting external stimuli to easily detectable electrical signals. In this Review article, we aim to capture recent advances of rational design and chemical functionalization of NTFETs for the purpose of switching or biosensing applications. To provide a deeper understanding of the device responses to analytes, this review will also survey the proposed sensing mechanisms. As demonstrated by these remarkable examples, the concept of combining the proper selection of functional molecular materials and molecular self-assembly with device micro/nanofabrication offers attractive new prospects for constructing NTFET-based molecular optoelectronic devices with desired functionalities.     

Influence of heat and UV light on the coercive field of lithium niobate crystals

Heating and UV illumination reduce the coercive field of lithium niobate crystals. It is found that both effects add up, leading to the conclusion that the underlying mechanisms are different. We present arguments that heating leads to an increased ionic conductivity that changes defect structures which pin domain walls, while UV illumination enables electrical screening of the pinning defects by photo-excited electrons.     

Organization of acenes with a cruciform assembly motif

This study explores the assembly in the crystalline state of a class of pentacenes that are substituted along their long edges with aromatic rings forming rigid, cruciform molecules. The crystals were grown from the gas phase, and their structures were compared with DFT-optimized geometries. Both crystallographic and computed structures show that a planar acene core is the exception rather than the rule. In the assembly of these molecules, the phenyl groups block the herringbone motif and further guide the arrangement of the acene core into higher order structures. The packing for the phenyl-substituted derivatives is dictated by close contacts between the C-H's of the pendant aromatic rings and the carbons at the fusions in the acene backbone. Using thiophene substituents instead of phenyls creates cofacially stacked acenes. In thin films, the thiophene-substituted derivative forms devices with good electrical properties: relatively high mobility, high ON/OFF ratios, and low threshold voltage for device activation. An unusual result is obtained for the decaphenyl pentacene when devices are fabricated on its crystalline surface. Although its acene cores are well isolated from each other, this material still exhibits good electrical properties.     

Blood group B galactosyltransferase: insights into substrate binding from NMR experiments

The biosynthesis of human blood group B antigens is accomplished by a highly specific galactosyltransferase (GTB). On the basis of NMR experiments, we propose a "molecular tweezers mechanism" that accounts for the exquisite stereoselectivity of donor substrate selection. Transferred NOE experiments for the first time reveal the bioactive conformation of the donor substrate UDP-galactose (UDP-Gal) and of its enzymatically inactive analogue, UDP-glucose (UDP-Glc). Both bind to GTB in a folded conformation that is sparsely populated in solution, whereas acceptor ligands bind in a conformation that predominates in solution. The bound conformations of UDP-Gal and UDP-Glc are identical within experimental error. Therefore, GTB must discriminate between the two activated sugars on the basis of a hitherto unknown transition state that can only be formed in the case of UDP-Gal. A full relaxation and exchange matrix analysis of STD NMR experiments reveals that acceptor substrates dissociate significantly faster (k(off) > 100 Hz) from the binding pocket than donor substrates (k(off) approximately 10 Hz). STD NMR experiments also directly show that proper recognition of the hexopyranose rings of the UDP sugars requires bivalent metal cations. At the same time, this analysis furnishes the complete three-dimensional structure of the enzyme with its bound donor substrate UDP-Gal on the basis of a prior crystal structure analysis. We propose that, upon acceptor binding, GTB uses the Asp 302 and Glu 303 side chains as "molecular tweezers" to promote bound UDP-Gal but not UDP-Glc into a transition state that leads to product formation.     

Mirror‐Image Photoswitching of Individual Single‐Walled Carbon Nanotube Transistors Coated with Titanium Dioxide

Dotting the i's : Stimuli‐responsive optoelectronic devices are formed from the title transistors functionalized with photoactive quantum dots. The p‐type semiconducting tubes show a fast current decrease under UV irradiation and reversibility when the UV irradiation is switched off. In contrast, ambipolar tubes show mirror‐image photoswitching effects when negative and positive gate bias voltages are applied.

     

Radiation-damage-assisted ferroelectric domain structuring in magnesium-doped lithium niobate

Irradiation of 5% magnesium-doped lithium niobate crystals (LiNbO₃:Mg) with high-energy, low-mass ³He ions, which are transmitted through the crystal, changes the domain reversal properties of the material. This enables easier domain engineering compared to non-irradiated material and assists the formation of small-sized periodically poled domains in LiNbO₃:Mg. Periodic domain structures exhibiting a width of ≈520 nm are obtained in radiation-damaged sections of the crystals. The ferroelectric poling behavior between irradiated and non-treated material is compared.