Synthesis and characterizations of novel spindle-like terphenyl-type chromophores for non-linear optical materials

A series of novel spindle-like terphenyl-type chromophores, based on 2,5-diphenyl-1,4-distyrylbenzene π-conjugating bridge, N,N-dimethyl and triphenyl amino donors, and Tricyanovinyldihydrofuran(TCF), 1,3,3-trimethyl-5-dicyanovinyl-1-cyclohexene (TDC) acceptors, have been synthesized successfully for the first time. And the non-linear optical properties were evaluated by using the finite-field (FF) method. The results show that, the first-order hyperpolarizability of the chromophores increase with the increase of the withdraw ability of the substituent group on the π-conjugating bridge.     

A new phosphine oxide host based on ortho-disubstituted dibenzofuran for efficient electrophosphorescence: towards high triplet state excited levels and excellent thermal, morphological and efficiency stability

An efficient host for blue and green electrophosphorescence, 4,6‐bis(diphenylphosphoryl)dibenzofuran ( o‐ DBFDPO ), with the structure of a short‐axis‐substituted dibenzofuran was designed and synthesised. It appears that the greater density of the diphenylphosphine oxide (DPPO) moieties in the short‐axis substitution configuration effectively restrains the intermolecular interactions, because only very weak π–π stacking interactions could be observed, with a centroid‐to‐centroid distance of 3.960 Å. The improved thermal stability of o‐ DBFDPO was corroborated by its very high glass transition temperature (T_g) of 191 °C, which is the result of the symmetric disubstitution structure. Photophysical investigation showed o‐ DBFDPO to be superior to the monosubstituted derivative, with a longer lifetime (1.95 ns) and a higher photoluminescent quantum efficiency (61 %). The lower first singlet state excited level (3.63 eV) of o‐ DBFDPO demonstrates the stronger polarisation effect attributable to the greater number of DPPO moieties. Simultaneously, an extremely high first triplet state excited level (T₁) of 3.16 eV is observed, demonstrating the tiny influence of short‐axis substitution on T₁. The improved carrier injection ability, which contributed to low driving voltages of blue‐ and green‐emitting phosphorescent organic light‐emitting diodes (PHOLEDs), was further confirmed by Gaussian calculation. Furthermore, the better thermal and morphological properties of o ‐DBFDPO and the matched frontier molecular orbital (FMO) levels in the devices significantly reduced efficiency roll‐offs. Efficient blue and green electrophosphorescence based on the o ‐DBFDPO host was demonstrated.     

Influence of acidification on the optical properties and molecular composition of dissolved organic matter

Acidification is a common method for preserving dissolved organic matter (DOM) in natural water samples until sophisticated laboratory analyses can be performed. However, little is known about the effects of this practice on the composition and optical properties of DOM. In this study, the effects of acidification on DOM in porewater samples collected from the RL IV bog system of the Glacial Lake Agassiz Peatlands in northern Minnesota were characterized. Molecular composition was determined by ultrahigh resolution mass spectrometry and optical properties by UV absorption and three-dimensional fluorescence spectroscopy. Excitation–emission matrix fluorescence spectroscopy results indicate that the fluorescence properties of the peatland porewater DOM were sensitive to pH and that the observed changes were fluorophore dependent. Ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry revealed the appearance of newly formed, oxygen-rich compounds upon acidification. The extent to which these oxygen-rich compounds were formed was also dependent on the composition of the DOM.     

A finite‐volume scheme for the multidimensional quantum drift‐diffusion model for semiconductors

A finite‐volume scheme for the stationary unipolar quantum drift‐diffusion equations for semiconductors in several space dimensions is analyzed. The model consists of a fourth‐order elliptic equation for the electron density, coupled to the Poisson equation for the electrostatic potential, with mixed Dirichlet‐Neumann boundary conditions. The numerical scheme is based on a Scharfetter‐Gummel type reformulation of the equations. The existence of a sequence of solutions to the discrete problem and its numerical convergence to a solution to the continuous model are shown. Moreover, some numerical examples in two space dimensions are presented. © 2010 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 27: 1483–1510, 2011     

Morphology and physical properties of poly(ethylene oxide) loaded graphene nanocomposites prepared by two different techniques

Organic–inorganic hybrids are artificially created structures presenting novel properties not exhibited by either of the component materials alone. In this contribution one addresses processing, morphology and properties of polymer nanocomposites reinforced graphene. First, synthesis routes to graphite oxide (GO) and foliated graphene sheets (FGS) are illustrated. Physical characterization of these graphene sheets were conducted using atomic force microscopy and X-ray diffraction techniques. Processing, structure and properties of graphene/poly(ethylene oxide) (PEO) nanocomposites are discussed. FGS was dispersed into PEO via two different composite manufacturing techniques: melt compounding and solvent mixing. Morphology of dispersed graphene and properties from different blending routes are compared. TEM showed that graphene distributed parallel to the composite surface using solvent method, while distributed randomly in melt blended method. Optical measurements indicated that the transparency of PEO/graphene prepared by solvent method is higher than that of melt blended method in the visible region. Electrical conductivity measurements are employed to evaluate threshold concentration for rigidity and connectivity percolation. The percolation concentration of the composites prepared by solvent method is less than those of melt blended method. The mechanical performance of the composites prepared by solvent method is higher than melt blended. Halpin–Tsai model has been used to confirm the distribution of the graphene into PEO by the two different processing techniques.     

Methacrylate monolithic stationary phases for gradient elution separations in microfluidic devices

Methacrylate monolithic stationary phases were produced in fused-silica chips by UV initiation. Poly(butyl methacrylate-co-ethylene dimethacrylate) (BMA) and poly(lauryl methacrylate-co-ethylene dimethacrylate) (LMA) monoliths containing 30, 35 and 40% monomers were evaluated for the separation of peptides under gradient conditions. The peak capacity was used as an objective tool for the evaluation of the separation performance. LMA monoliths of the highest density gave the highest peak capacities (≈40) in gradients of 15 min and all LMA monoliths gave higher peak capacities than the BMA monoliths with the same percentage of monomers. Increasing the gradient duration to 30 min did not increase the peak capacity significantly. However, running fast (5 min) gradients provides moderate peak capacities (≈20) in a short time. Due to the system dead volume of 1 μL and the low bed volume of the chip, early eluting peptides migrated over a significant part of the column during the dwell time under isocratic conditions. It was shown that this could explain an increased band broadening on the monolithic stationary phase materials used. The effect is stronger with BMA monoliths, which partly explains the inferior performance of this material with respect to peak capacity. The configuration of the connections on the chip appeared to be critical when fast analyses were performed at pressures above 20 bar.     

Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV-Visible diode array and excitation emission matrix fluorescence

Flow field-flow fractionation (FlFFF) with on-line UV/Visible diode array detector (DAD) and excitation emission matrix (EEM) fluorescence detector has been developed for the characterization of optical properties of aquatic dissolved organic matter (DOM) collected in the Otonabee River (Ontario, Canada) and Athabasca River (Alberta, Canada). The molecular weight (MW) distribution of DOM was estimated using a series of organic macromolecules ranging from 479 to 66,000 Da. Both the number-average (M_n) and weight-average (M_w) molecular weights of Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) determined using these macromolecular standards were comparable to those obtained using polystyrenesulfonate (PSS) standards, suggesting that organic macromolecules can be used to estimate MW of natural organic colloids. The MW of eight river DOM samples determined by this method was found to have an M_n range of 0.8–1.1 kDa, which agrees with available literature estimates. The FlFFF-DAD-EEM system provided insight into the MW components of river DOM including the optical properties by on-line absorbance and fluorescence measurement. A red-shift in emission and excitation wavelength maxima associated with lower spectral slope ratios (S_R = S_275–295:S_350–400) was related to higher MW DOM. However, DOM of different origins at similar MW also showed significant difference in optical properties. A difference of 47 and 40 nm in excitation and emission peak C maxima was found. This supports the hypothesis that river DOM is not uniform in size and optical composition.     

Foldamer-tuned switching kinetics and metastability of [2]rotaxanes

Slip sliding away: Foldamers can function as modular stoppers to regulate the slippage and de-slippage of pseudorotaxanes and the switching kinetics and metastability of bistable rotaxanes. By simply changing the solvent or the length of the hydrogen-bonded foldamer, the lifetime of the metastable co-conformation state can be increased dramatically, from several minutes to as long as several days.