Picosecond time-resolved emission spectra: techniques and examples

This paper describes a laser spectrometer that yields picosecond time-gated emission spectra which are characterized by a high signal-to-noise ratio and a broad spectral coverage. The crucial experimental methods are presented, with particular emphasis given to the experimental techniques that are necessary to achieve broad spectral coverage. Examples are given of the application of the apparatus in the investigation of the torsional isomerization of 2-vinylanthracene and the transient solvation of electronically excited coumarin 440 in alcoholic solutions.     

超重现毛细管电泳的理论与例证

毛细管电泳经常遭受结果不稳定、不重现的困扰。该文从理论上推导了一些新模式并以例证说明,这些模式能在一定条件下抵抗条件变化,获得超重现电泳谱图。它们是加权淌度谱、迁移电量谱、电荷密度谱、摩尔电荷密度谱、扩散系数谱、液相质量谱,以及它们各自的比例谱等,其中前4种为实时测量模式,其余的为实验后模式。这些模式需要发展新的仪器或算法,但都有发展潜力,值得深入研究。     

Nano-enabled microtechnology: polysulfone nanocomposites incorporating cellulose nanocrystals

Microchannel devices hold the potential to transform many separation processes. This preliminary study investigated the feasibility of incorporating cellulose nanocrystals (CNXLs) into polysulfone, a commonly used ultrafiltration membrane polymer. Incorporating CNXLs into non-water soluble polymers without aggregation has been problematic. A solvent exchange process was developed that successfully transferred an aqueous CNXL dispersion into the organic solvent N-methylpyrrolidone (NMP), which is a solvent for polysulfone (PSf). Films were prepared from the solution of PSf in NMP with dispersed CNXLs by a phase inversion process. Films were then examined by scanning electron microscopy and tested for their transport and mechanical properties. The interaction between the polymer matrix and the CNXL filler was studied by means of thermogravimetric analysis (TGA), which suggested a close interaction between the polymer and filler at the 2% filler loading. The tensile modulus showed a large increase beyond 1% filler loading, which could be due to a percolation effect. The water vapor transport rate increased with increase in filler loading. Agglomeration of the CNXLs seemed to be taking place at filler loadings >7%.     

Bis antennae amphiphilic cyclodextrins: the first examples

Methylated 6A,6D-diamino 6A,6D-dideoxy β-cyclodextrin 4 was coupled with three cholesterol substituted glutaric or succinic acids 5-7 to give amphiphilic cyclodextrins 1-3 with good yields. The succinic acid derivative 1, a typical member of this new family, has shown good compatibility with phosphatidyl choline monolayer and bilayer as confirmed by deuterium magnetic nuclear resonance (²H NMR).     

Single step,solvent-free processes: examples and prospects

An outline is given of some of the options now available — and likely to be of growing importance — for various of ways in which inorganic catalysts may be developed to effect industrially important chemical reactions in environmentally more acceptable means. One major goal is to devise ways of producing in situ (within the sphere of reaction) aggressive oxidants especially those that are environmentally hazardous. Two specific examples are cited: 1) one involves hydroxylamine, generated in a benign fashion, so as to effect the ammoximation of cyclohexanone to its oxime and -caprolactam; 2) the other involves the Baeyer—Villiger reaction (for converting cyclic ketones to lactones) via perbenzoic acid. The role of supported bimetallic catalysts in solvent-free hydrogenations (especially of polyenes) is also highlighted.     

RuII complexes incorporating tetrathiamacrocycles: synthesis and conformational analysis

The synthesis of a series of RuII complexes incorporating thiacrown ligands ([12]ane-S4, [14]ane-S4, [16]ane-S4), as well as 2,2'-bipyridine (bpy) or pyridine, is reported. Structural studies on these complexes have been carried out using a variety of techniques. Detailed 1H NMR spectroscopic studies on the previously reported [Ru([12]ane-S4)(bpy)]2+ (1) reveal that-contrary to earlier reports-the observed fluxional 1H NMR behavior is not due to chemical exchange involving cleavage of the bpy Ru--N bond but is, in fact, due to lone-pair inversion of coordinated macrocyclic sulfur donor atoms. This phenomenon is also observed for the [14]ane-S4 and [16]ane-S4 analogues of 1. For the first time, using a combination of X-ray crystallography, more detailed 1H NMR experiments, and computational methods, an in-depth study on the energetics and dynamics of invertomer formation and conversion for macrocyclic coordination complexes has been carried out. These studies reveal that the steric constraints of assembling each sulfur macrocycle and bpy ligand around the octahedral Ru(II) center lead to close intramolecular contacts. These contacts are largely dependent on the orientation of the electron lone pairs of equatorial sulfur donor atoms and correlate with the comparative stability of the different invertomeric forms. Thus, the conformational preferences of the three macrocyles in [Ru([n]ane-S4)(bpy)]2+ complexes are determined by steric rather than electronic effects.     

MP6方法在分子构型优化中的应用

用多体微扰方法MP6对三个非氢原子组成的分子体系,分别在cc-pVDZ、cc-pVTZ、和cc-pVQZ基组下,进行了构型优化计算,并利用Dunning提出的经验公式把结果外推到CBS极限。由于各种误差的相互抵消,MP2的构型优化结果较MP6或MP4的好。用耦合簇方法(CCSD(T))在同样条件下进行了优化计算,结果比微扰方法更具有优势。     

Dual-mode high-speed counter-current chromatography: retention, resolution and examples

Counter-current chromatography (CCC) is a very versatile technique offering high resolution power in recovering very pure compounds from complex matrices. Dual-mode CCC where the phase role is reversed during the separation is investigated here; it ensures elution of all the injected species from the column while, unlike backflush, the separation is still progressing after phase reversal; equations giving retention and retention factor are derived from the basic equations of chromatography. Compared to normal-mode CCC, it is shown that enhanced resolution in dual-mode CCC can be obtained in conditions derived from a theoretical model. The experimental section provides the validation of the retention prediction while resolution is also proven to be enhanced in dual-mode CCC. However, equations given in the theoretical section cannot fully explain the results obtained for resolution because they do not deal with kinetics. Dual-mode CCC has also been applied to separation of polyoxypropylene glycol polymers; separation can be achieved with a small number of experiments because all the injected compounds are eluted by reversing the phases. Dual-mode CCC also gave improved yields in the purification of antibiotics compared with previous results using normal-mode CCC.     

Nanoparticles in energy technology: examples from electrochemistry and catalysis

Nanoparticles are key components in the advancement of future energy technologies, thus, strategies for preparing nanoparticles in large volume by techniques that are cost-effective are required. In the substitution of fossil-fuels by renewable energy resources, nanometer-sized particles play a key role for synthesizing energy vectors from varying and heterogeneous biomass feedstocks. They are extensively used in reformers for the production of hydrogen from solid, liquid, or gaseous energy carriers. Catalyst activities depend critically on their size-dependent properties. Nanoparticles are further indispensable as electrocatalysts in fuel cells and other electrochemical converters. The desire to increase the activity per unit area, and decrease the necessary amount of the expensive catalytic standard, platinum, has spurred innovative approaches for the synthesis of platinum-alloy nanoparticles by wet chemistry, colloidal routes, or physical techniques such as sputtering.     

Santiagonamine: a new aporphinoid alkaloid incorporating a phenanthridine skeleton

Santiagonamine (6), the first phenanthridine alkaloid, has been isolated from $\text{Berberis darwinii}$ Hook (Berberidaceae).     

Silylenes and disilenes: examples of low coordinated silicon compounds

This report summarises some aspects of silicon compounds of coordination numbers two and three. As examples for such molecules, generation and trapping reactions of the sterically encumbered silylenes: dimesitylsilylene, di-tert-butylsilylene, and diadamantylsilylene are discussed. Some more recent results from the chemistry of stable and marginally stable disilenes are also considered.     

meso-Tetraaryl-7,8-dihydroxydithiachlorins: first examples of heterochlorins

Osmium tetraoxide-mediated dihydroxylation of meso-tetraaryldithiaporphyrins generates the corresponding meso-tetraaryldithia-7,8-dihydroxychlorins and meso-tetraaryldithia-7,8,17,18-tetrahydroxybacteriochlorins. The products are spectroscopically characterized and the substitution positions were unequivocally determined. The UV-vis of the heterochlorins and heterobacteriochlorins are chlorin- and bacteriochlorin-like, respectively. However, the chlorin spectra are surprisingly hypsochromically shifted as compared to the corresponding all-aza chlorins, whereas the bacteriochlorin is bathochromically shifted. meso-Tetraaryldithia-7,8-dihydroxychlorins are susceptible to oxidative ring-opening reactions to form the corresponding meso-tetraaryldithia-7-oxo-8-oxa-dithiaporphyrin (dithiaporpholactone). These derivatives are the first examples of heterochlorins and β-modified dithiaporphyrins.     

Extraction and stirring integrated techniques: examples and recent advances

Extraction techniques, which focus on selectivity and sensitivity enhancement by isolation and preconcentration of target analytes, are essential in many analytical methods. Because many extraction techniques occur under diffusion-controlled conditions, stirring of the sample solution is required to accelerate the extraction by favoring diffusion of the analytes from the bulk solution to the extractant phase. This stirring may be performed by use of an external device or by integrating extraction and stirring in the same device. This review focuses on the latter techniques, which are promising methods for sample treatment. First, stir-bar-sorptive extraction, the most widely used method, is considered, paying special attention to the development of new coatings. Finally, a general overview of novel integrated techniques in both solid-phase and liquid-phase microextraction is presented; their main characteristics and marked trends are reported.     

ESR spectra and electronic structure: some recent examples.

Results of ESR studies are presented for a number of radical ions selected from several classes of compounds of current interest. The strikingly different hyperfine data for the radical cations of 1,6:8,13-propane-1,3-

and 1,6:8,13-ethane-diylidene-[14]annulene

demonstrate the effect of an interplay of homoconjugative, inductive and hyperconjugative perturbations. The unusually stable radical cations of N,N′-dimethyl-

and N,N′-trimethylene-1,6:8,13-bisimino[14]annulene

should be regarded as N-centered radicals with an N-N 3-electron σ-bond. Despite similar geometries of [2.2]paracyclophane

, its 1,1,2,2,9,9,10,10-octa-fluoro-derivative

and its 1,9,-diene

, the corresponding radical anions strongly differ in their electronic structure. An unexpected conformatinal interconversion is observed for the radical anion of

. Electron acceptors, such as tetracyanoquinodimethane (TCNQ;

) and its areno analogues, and electron donors, represented by tetrathiafulvalene (TTF;

) and compounds containing the TTF system, form salts exhibiting high electronic conductivity (“organic metals”). ESR spectra of the radical anions and trianions of these acceptors and the radical cations of these donors provide detailed information about the spin distribution in the respective species.     

Supported catalysts incorporating BINOL units: towards heterogeneous asymmetric catalysis

Optically pure BINOL species were immobilized on two different types of support: organic (cross linked polystyrene) and inorganic (silica). The synthesis of the reticulated polystyrene gels was achieved via the radical crosslinking polymerization of a protected divinyl-(R)-BINOL derivative. The deprotection resulted in polystyrene networks incorporating free (R)-BINOL entities. Silica hybrid materials containing (R)-binaphthyl units were synthesized by sol–gel processing of a trialkoxysilylated (R)-BINOL-bis-carbamate precursor. Both materials were tested as chiral auxiliaries in enantioselective catalysis. The polystyrene-BINOL gels show similar catalytic properties compared to molecular (R)-BINOL and appears as a classical heterogenized catalyst system. Contrarily, the silica hybrid materials incorporating BINOL entities show significantly increased catalytic properties compared to the molecular catalyst in homogeneous solution. A supramolecular effect of the silica matrix on the stability and the selectivity of the catalyst system can be concluded. To cite this article: P. Hesemann, J.J.E. Moreau, C. R. Chimie 6 (2003).     

Meta-B-entacenes: new polycyclic aromatics incorporating two fused borepin rings

The synthesis of new boron-containing acenes (meta-B-entacenes) is reported. These compounds exhibit slightly non-planar core geometries with blue-shifted spectral properties and more negative electrochemical reduction potentials relative to known para isomers. Polarizable π-extended architectures were realized via cross-coupling procedures with chloro-functionalized precursors.     

Electrochemical examples of multiple-electron transfer

Various electroactive reactants that undergo multipleelectron transfer reactions at electrodes are divided into two broad classes. The first class exhibits a series of single electron steps at different electrode potentials. The second class exhibits multiple-electron transfer in a single voltammetric step. Examples from each class are offered and the reasons for their electrochemical behavior are described.     

Symmetrical electron-deficient materials incorporating azaheterocycles

To investigate the potential of di- and tri-azaheterocycles as building blocks for pi-conjugated materials with high electron affinity, linear oligomers incorporating pyrazine and a C(3)-symmetric discotic molecule based on triazine were synthesized. The tridodecyloxyphenyl end-capped ethynylene pyrazinylene oligomers showed remarkable solvatochroism in absorption and emission in solution. The oligomers containing one and two pyrazine rings displayed liquid crystallinity in the solid state. The largest ethynylene pyrazinylene oligomer containing three pyrazine rings had the lowest first reduction potential at -1.08 V. The triazine-derived discotic molecule exhibited UV/Vis and fluorescence behavior comparable to that of the linear oligomers and featured a first reduction potential at -1.49 V, somewhat lower than expected.