Syntheses, structures, and photoisomerization of (E)- and (Z)-2-tert-butyl-9-(2,2,2-triphenylethylidene)fluorene

"Sterically geared" 9-(2,2,2-triphenylethylidene)fluorene (1) is of potential interest as a photoactive moiety in molecular devices, and the 2-tert-butyl derivative (6) has been synthesized to investigate photoisomerization. E and Z stereoisomers of 6 were separated and identified by X-ray crystallography. The tert-butyl group does not introduce additional strain, and its close proximity to the trityl group in the Z isomer suggests an attractive van der Waals interaction. The UV spectra of (E)-6 and (Z)-6 are nearly identical, showing absorption bands that are similar to those of fluorene occurring at wavelengths longer than 240 nm. Photoisomerization of 6 was investigated at 266, 280 and 320 nm. Solutions initially containing only (E)-6 or (Z)-6 were irradiated with pulsed laser light, monitoring isomerization by 1H NMR spectroscopy. Negligible photodecomposition was observed when the solutions were agitated by N2 ebullition. Experimental data were fitted to theoretical curves to obtain photoisomerization quantum yields (phi(ZE) and phi(EZ)) ranging from 0.04 to 0.09. This first photoisomerization study of a dibenzofulvene reveals significant quantum yields, despite theoretical prediction of inefficient or negligible isomerization of the parent hydrocarbon, fulvene. Thermal isomerization of 6 at 270 degrees C (t(1/2) = 120 min) was also followed by 1H NMR spectroscopy, resulting in an estimated activation energy (deltaG(double dagger)) of 43 kcal/mol.     

New insights into the molecular architecture of hardwood lignins by chemical degradative methods

Lignins are composed of phenylpropane units interconnected by labile and resistant bonds. A two-step degradation, thioacidolysis, provides detailed information on these network polymers. The first step involves lignin depolymerization with ethanethiol and BF₃ etherate. The determination of the recovered thioethylated monomers provides an estimation of lignin units only involved in labile ether bonds. The ligninderived dimers, representative of resistant interunit bonds, are determined after a further desulfurization step. Results obtained for native and industrial hardwood lignins underline their structural differences. Native hardwood lignins are typified by a high proportion of linear fragments linked bifunctionally by β-O-4 bonds.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part V. Case of a charged adsorbed reactant. Trichloroacetate electroreduction in the presence of Br−, l−, SCN−, N3−, Ti+ and C(NH2)3+

The rate constant k_f for CCl₃COO^? electroreduction at constant applied potential, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density q_i due to specifically adsorbed supporting ions. Thus, in the presence of the adsorbed anions Br^?, I^?, SCN^? and N₃^?, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with |q_i|, albeit slightly. In the presence of the adsorbed cations Tl⁺ and C(NH₂)₃⁺, Φ increases with the positive charge density q_i due to the cation at a rate which is 3 to 8 times greater than the corresponding rate of decrease in the presence of the adsorbed anions. This behaviour has been interpreted on the basis of the theoretical treatment of ref. 22 by considering that the time of adsorption of the electroactive anion CCl₃COO^? is long enough to permit a perturbation in the distribution of the adsorbed supporting ions in the immediate vicinity of the adsorbed reactant.     

Adsorbed monolayer of H-bonded water molecules with and without neutral polymer molecules: A statistical mechanical treatment accounting for local order

A statistical mechanical treatment of a monolayer consisting both of H-bonded solvent molecules adsorbed in an unspecified number of orientations and of polymeric molecules of a neutral solute is provided. The different size of solvent and solute molecules is accounted for using Flory—Huggins statistics, whereas local order within the monolayer is accounted for using the quasi-chemical approximation. The above treatment is applied to a hexagonal array of adsorbed water molecules oriented in such a way as to be in a condition to be singly or double H-bonded laterally in the monolayer; a further water orientation characterized by full alignment of the dipole moment in the direction away from the electrode and simulating chemisorbed water monomers is included in the molecular model treatment. An adsorption isotherm is derived upon generalizing the molecular model at hand so as to include the presence of polymeric neutral solute molecules adsorbed in a single orientation. The model accounts satisfactorily for a number of salient features of experimental capacity curves at metal—water interphases in the absence of adsorbed solute species, as well as for the adsorption behaviour of aliphatic compounds on mercury, provided that the doubly H-bonded water molecules are excluded from the molecular model. A justification for this exclusion, based on the existence of H-bonds between the first and second layer of water molecules, is provided.     

Fluoropolyether coatings: Relationships of electrochemical impedance spectroscopy measurements,barrier properties,and polymer structure

Ten model coatings, selected and obtained from a family of fluorinated resins synthesized by the reaction of perfluoroether oligomeric diols of different molecular weights with polyisocyanurates of hexamethylenediisocyanate (HDI) and isophoronediisocyanate (IPDI), were characterized with differential scanning calorimetry, mechanical testing, and electrochemical impedance spectroscopy measurements. The electrochemical and chemico‐physical measurements show that the glass‐transition temperature of the starting isocyanate trimers greatly influences the properties of the final urethane coatings; the IPDI trimer gives harder coatings with lower water permeabilities than the corresponding HDI‐based materials. Moreover, for each class of materials (from IPDI or HDI), the fluorine content plays a relevant role: the higher the fluorine percentage, the lower the water absorption into the coatings. Furthermore, the chain length of the polyols used for the synthesis of the prepolymers is a variable that exhibits great influence on the coating properties: coatings containing shorter perfluoropolyether segments show better barrier properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 52–64, 2002     

Fragmentations induced by ion-atom reactions

Fragmentation of electron-impact-generated organic molecular cations, submitted to low-energy collisions with hydrogen in a MS/MS/MS multiquadrupole mass spectrometer, was promoted or significantly increased in the presence of atomic hydrogen generated from a microwave discharge. Loss of hydrogen alpha to a heteroatom and bond cleavage alpha to a carbonyl group are the major H-induced fragmentation pathways.     

Chemistry and biology of glycosaminoglycans in blood coagulation

Glycosaminoglycans (GAGs) are widely distributed in animal tissues where they are usually associated with proteins. Six types are commonly recognized: heparin (Hep), heparan sulfate (HS), dermatan sulfate (DS), chondroitin sulfate (Ch-S), keratan sulfate (KS) and hyaluronic acid (Hyal). They are structurally related with a carbohydrate backbone consisting of alternating hexuronic acid (L-iduronic acid and/or D-glucuronic acid) or galactose units and hexosamine (D-glucosamine or D-galactosamine) residues. All GAGs, except Hyal, show sulfate groups along their chains. Certain sulfate glycoaminoglycans have the ability to interfere with blood coagulation, as demonstrated by the extensive clinical use of Hep as an anticoagulant agent. HS and DS show a good anticoagulant activity, although weaker than that of Hep. In contrast, Ch-S has a low ability to inhibit plasma serine proteases, and KS and Hyal are devoid of any effect on coagulation cascade. The interaction between blood coagulation serine proteases and GAGs can be found to have two principle mechanisms: the specific “lock and key” binding and the nonspecific cooperative electrostatic association. This different ability of GAGs to interact with coagulation cascade proteins depends on the molecular weight, the ratio of iduronic/glucoronic acid and the sulfation degree. Many attempts have been made to improve or induce anticoagulant activity of natural GAGs-by chemical modification. Increasing sulfation degree of DS and Ch-S is followed by their biological activity increasing. Hyal, which is devoid of any anticoagulant effect, acquires a good ability to inactivate plasma serine proteases, i.e. thrombin and Factor Xa, when it is sulfated. This ability increases by increasing the number of sulfate groups per disaccharide unit, although the mechanism of action is different from that of Hep, but seems to be independent of its molecular weight.     

Metal‐ion complexes in the angiogenetic effect

The complex formation between two polysaccharides, hyaluronic acid (Hyal) and its sulphated derivative (HyalS), and two metal ions, Cu²⁺ and Zn²⁺ was investigated in aqueous solution by thermodynamic and spectroscopic techniques. A stoichiometry for the complex species in solution was obtained. The bioactivity of the metal‐polysaccharide complexes was then evaluated in terms of their influence on endothelial cell migration and adhesion. The biological response of the complex species was found to be dependent of both the polysaccharide (Hyal or HyalS) and the metal ion (Cu²⁺ or Zn²⁺).     

Effects of Volume Changes on the Thermal Performance of PCM Layers Subjected to Oscillations of the Ambient Temperature: Transient and Steady Periodic Regimes

The Stefan problem regarding the formation of several liquid–solid interfaces produced by the oscillations of the ambient temperature around the melting point of a phase change material has been addressed by several authors. Numerical and semi-analytical methods have been used to find the thermal response of a phase change material under these type of boundary conditions. However, volume changes produced by the moving fronts and their effects on the thermal performance of phase change materials have not been addressed. In this work, volume changes are incorporated through an additional equation of motion for the thickness of the system. The thickness of the phase change material becomes a dynamic variable of motion by imposing total mass conservation. The modified equation of motion for each interface is obtained by coupling mass conservation with a local energy–mass balance at each front. The dynamics of liquid–solid interface configurations is analyzed in the transient and steady periodic regimes. Finite element and heat balance integral methods are used to verify the consistency of the solutions to the proposed model. The heat balance integral method is modified and adapted to find approximate solutions when two fronts collide, and the temperature profiles are not smooth. Volumetric corrections to the sensible and latent heat released (absorbed) are introduced during front formation, annihilation, and in the presence of two fronts. Finally, the thermal energy released by the interior surface is estimated through the proposed model and compared with the solutions obtained through models proposed by other authors.