Understanding the congener-specific toxicity in polychlorinated dibenzo-p-dioxins: chlorination pattern and molecular quadrupole moment.

It is well known that the biological activities and toxicities of planar polychlorinated aromatic compounds are extremely sensitive to chlorination pattern. Although their toxic responses have been correlated with the relative affinity for the receptor, the origin of this congener specificity is not well understood. We present a general interpretation of the congener-specific activity in polychlorinated dibenzo-p-dioxins, which concludes that molecular electrostatics is the principal factor determining the structure-activity relationship in this highly controversial environmental pollutant even though this electrostatic interaction represents only a part of the total interaction energy. Through calculations of the molecular charge distribution in the complete set of 76 dioxin congeners, we show that all active congeners share a unique charge distribution pattern, which is quantitatively described in terms of the molecular quadrupole moment (QM). The QM of dioxins changes sensitively and systematically with chlorination pattern. The three-dimensional electrostatic interactions at the receptor-binding site, which are optimized at a specific QM pattern represented by that of 2,3,7,8-tetrachlorodibenzo-p-dioxin, could explain the congener specificity in the binding affinity and toxicity. Although the polarizability also changes systematically with chlorination, it can only account for the effect of the degree of chlorination, not the congener specificity.     

Hopping transport in molecularly doped polymers: Joint modelling of positional and energetic disorder

Hole transport in molecularly doped polymers (MDPs) is modelled as random walks in a bias field E over organic donors D embedded in a polymer matrix. Positional disorder for donor fraction p < 1 is represented by randomly placing donors at sites in a fcc lattice, while energetic disorder is given by a Gaussian distribution of site energies with width and spatial correlations in a sphere of radius R c . Random walks generated by Marcus or small polaron rates for steps between nearby donors yield the mobility w( E , T ). In addition to and R c , the rates depend on the parameters nand ufor the distance dependence and reorganization energy respectively. With tritolylamine (TTA) in polystyrene as the paradigm, a procedure is presented for fixing the interdependent parameters , u, nand R c that reproduce the field and temperature dependences of w( E , T ) over a wide range of p that includes dilute systems with different TTA packings enforced by saturated bonds. Positional disorder exceeds energetic disorder in dilute systems and yields constant w( E , T 0 ) near room temperature. Joint modelling of TTA and related systems accounts for the characteristic w( E , T ) of MDPs and substantially extends the picture of hopping between localized states, with nincreased by about 15% and reduced by about 25% from conventional analysis using the Gaussian disorder model. Similar parameter changes are expected in other MDPs based on the compensation temperature T 0 and on scaling TTA results.     

The concerted and stepwise chemisorption mechanisms of isothiazole and thiazole on Si(100)?2?��?1 surface

The surface reaction pathways of isothiazole and thiazole on Si(100)?2?×?1 surface were theoretically investigated using multireference wavefunctions. In the case of isothiazole, the Si?CN dative adduct turned out to be the major surface product. In contrast, a direct reaction competition between a concerted [4?+?2]_CC cycloaddition and Si?CN dative adduct was found in the adsorption of thiazole. Therefore, it is concluded that the particular geometric arrangements of heteroatoms exhibit distinctly different initial surface reaction mechanisms.     

Carter–Payne homomorphisms and Jantzen filtrations

We prove a q-analogue of the Carter–Payne theorem in the case where the differences between the parts of the partitions are sufficiently large. We identify a layer of the Jantzen filtration which contains the image of these Carter–Payne homomorphisms and we show how these homomorphisms compose.     

Readily Accessible and Predictable Naphthalene‐Based Two‐Photon Fluorophore with Full Visible‐Color Coverage

Herein we report 22 acedan‐derived, two‐photon fluorophores with synthetic feasibility and full coverage of visible wavelength emission. The emission wavelengths were predicted by computational analysis, which enabled us to visualize multicolor images by two‐photon excitation with single wavelength, and to design a turn‐on, two‐photon fluorescence sensor for endogenous H₂O₂ in Raw 264.7 macrophage and rat brain hippocampus ex vivo.     

Room-temperature and solution-processed vanadium oxide buffer layer for efficient charge injection in bottom-contact organic field-effect transistors

We introduce a room temperature and solution-processible vanadium oxide (VO_x) buffer layer beneath Au source/drain electrodes for bottom-contact (BC) organic field-effect transistors (OFETs). The OFETs with the VO_x buffer layer exhibited higher mobility and lower threshold voltages than the devices without a buffer layer. The hole mobility with VO_x was over 0.11 cm²/V with the BC geometry with a short channel length (10 μm), even without a surface treatment on SiO₂. The channel width normalized contact resistance was decreased from 98 kΩ cm to 23 kΩ cm with VO_x. The improved mobility and the reduced contact resistance were attributed to the enhanced continuity of pentacene grains, and the increased work function and adhesion of the Au electrodes using the VO_x buffer layer.     

New parallel optimal‐parameter fast multipole method (OPFMM)

The three key translation equations of the fast multipole method (FMM) are deduced from the general polypolar expansions given earlier. Simplifications are introduced for the rotation‐based FMM that lead to a very compact FMM formalism. The optimum‐parameter searching procedure, a stable and efficient way of obtaining the optimum set of FMM parameters, is established with complete control over the tolerable error (ε). This new procedure optimizes the linear scaling with respect to the number of particles for a given ε. In addition, a new parallel FMM algorithm, which requires virtually no internode communication, is suggested that is suitable for the parallel construction of Fock matrices in electronic structure calculations. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1484–1501, 2001