Ultrashort-pulse fiber ring lasers

Received: 18 February 1997/Revised version: 22 April 1997     

Metal insulator transition characteristics of macro-size single domain VO2 crystals

The metal insulator transition (MIT) characteristics of macro-size single-domain VO₂ crystal were investigated. At the MIT, the VO₂ crystal exhibited a rectangular shape hysteresis curve, a large change in resistance between the insulating and the metallic phases, in the order of ～10⁵, and a small transition width (i.e. temperature difference before and after MIT) as small as 10^?3°C. These MIT characteristics of the VO₂ crystals are discussed in terms of phase boundary motion and the possibility of controlling the speed of the phase boundary, with change in size of crystal, is suggested.     

Synthesis of (-)-aphanorphine using aryl radical cyclization

[structure: see text] The synthesis of (-)-aphanorphine was achieved by using Bu(3)SnH-mediated aryl radical cyclization of 1-benzyloxycarbonyl-2-(2-bromo-4-methoxyphenylmethyl)-2-methoxycarbonyl-4-(phenylthiomethylene)pyrrolidine, leading to exclusive formation of the 6-exo cyclization product.     

A facile preparation of o-acelated β-d-ribofuranosyl cyanide,an important intermediate of c-nucleoside synthesis

Tri-O-benzoyl and tri-O-acetyl derivatives of the title compound are obtained from the corresponding 1-O-acetates in good yields by treatment with cyanotrimethylsilane.     

The nature of marbled Terra Sigillata slips: a combined μXRF and μXRD investigation

In addition to the red terra sigillata production, the largest Gallic workshop (La Graufesenque) made a special type of terra sigillata, called “marbled” by the archaeologists. Produced exclusively at this site, this pottery is characterized by a surface finish made of a mixture of yellow and red slips. Because the two slips are intimately mixed, it is difficult to obtain the precise composition of one of the two constituents without contamination from the other. In order to obtain very precise correlation at the appropriate scale between the color aspect and the elemental and mineralogical phase distributions in the slip, combined electron microprobe, X-ray micro spectroscopies and micro diffraction on cross-sectional samples were performed. The aim of this study is to discover how potters were able to produce this unique type of terra sigillata and especially this particular slip of an intense yellow color. Results show that the yellow component of marbled sigillata was made from a titanium-rich clay preparation. The color is due to the formation of a pseudobrookite (TiFe₂O₅) phase in the yellow part of the slip, the main characteristics of that structure being considered nowadays as essential for the fabrication of stable yellow ceramic pigments. Its physical properties such as high refractive indices and a melting point higher than that of most silicates widely used as ceramic colorants are indeed determinant for this kind of application. Finally, the red parts have a similar composition (elementary and mineralogical) to the one of standard red slip.     

Novel ring transformation of thiazolidine to oxazolidine: acyl halide-promoted intramolecular rearrangement of 4(R)-hydroxymethyl-3-methyl-1,3-thiazolidine

An interesting transformation occurs during acylation of 4(R)-hydroxymethyl-3-methyl-1,3-thiazolidine when 4(R)-acylthiomethyl-3-methyl-1,3-oxazolidines are yielded: the reaction competing with O-acylation is controlled by the bulkiness of the acyl group.     

Short Radiative Lifetime and Non-Triplet Sensitization in Near-Infrared-Luminescent Yb(III) Complex with Tripodal Schiff Base

We prepared Ln(III) (Ln=Eu, Gd, and Yb) complexes with a tripodal Schiff base, tris[2-(5-methylsalicylideneimino)ethyl]amine (H₃L) and studied their photophysical properties. Upon ligand excitation, YbL showed Yb(III)-centered luminescence in the near-infrared region. While the overall quantum yield (0.60(1)%) of YbL in acetonitrile was moderate among the reported values for Yb(III) complexes, its radiative lifetime (0.33(2) ms) was significantly shorter than those reported previously. We propose that the ligand-to-metal charge-transfer (LMCT) state mediated the sensitization in YbL. The emission and excitation spectra of EuL indicated the participation of the LMCT state in the sensitization. The radiative lifetime (0.84(7) ms) for EuL in the solid state was rather short compared to those of reported Eu(III) complexes. Our results show that the Yb(III) complex with the Schiff base ligand has two features: the short radiative lifetime and the non-triplet sensitization path.     

Solubility of Anthraquinone Derivatives in Supercritical Carbon Dioxide: New Correlations

Solubility of several anthraquinone derivatives in supercritical carbon dioxide was readily available in the literature, but correcting ability of the existing models was poor. Therefore, in this work, two new models have been developed for better correlation based on solid–liquid phase equilibria. The new model has five adjustable parameters correlating the solubility isotherms as a function of temperature. The accuracy of the proposed models was evaluated by correlating 25 binary systems. The proposed models observed provide the best overall correlations. The overall deviation between the experimental and the correlated results was less than 11.46% in averaged absolute relative deviation (AARD). Moreover, exiting solubility models were also evaluated for all the compounds for the comparison purpose.     

Toward uniform polymer architecture through stereospecific living polymerization and chromatographic separation technique

Isotactic (it-) and syndiotactic (st-) poly(methyl methacrylate)s (PMMAs) were fractionated into uniform PMMAs (without molecular weight distribution) by supercritical fluid chromatography (SFC). The SFC technique was applied to the isolation of uniform it- and st-PMMAs with a hydroxy group (it- and st-PMMA-OH) at the chain end. Equimolar amounts of uniform it- and st-PMMA-OHs were coupled with sebacoyl dichloride to form uniform stereoblock PMMA. The reaction of uniform st-PMMA-OH with methacryloyl chloride gave uniform PMMA macromonomer with methacryloyl group at the chain end. The resulting uniform macromonomer was polymerized radically and the products were fractionated into uniform comblike polymers (1mer to 4mer) by means of gel-permeation chromatography (GPC). The uniform st-PMMA-OH was reacted with 1, 3, 5-benzenetricarbonyl trichloride to form uniform st-tri-armed star polymer. Some of the properties of these uniform stereoregular polymer architectures were studied.