Resonant energy transfer and cross relaxation between Sm3+ ions in LiYF4 crystals

Luminescence decay curves of Sm³⁺ ions in LiYF₄ crystals doped with 1, 5 and 10 mol% Sm³⁺ are multi-exponential, whereas that in a LiYF₄ crystal doped with 0.1 mol% Sm³⁺ is well approximated by a single exponential function with a decay time of 4.8 ms. The average luminescence decay times decrease from 4.8 to 0.60 ms with the increasing Sm³⁺ concentrations between 1 and 10 mol%. The decay curves for all crystals are found to be almost independent of the temperature between 15 and 300 K. The decrease of the decay times for the higher Sm³⁺ concentrations indicates energy transfer between two Sm³⁺ ions. Taking the crystal structure of LiYF₄ into account, it is deduced that a single-step energy transfer process for the 1 and 5 mol% Sm³⁺ concentrations occurs from a Sm³⁺ ion at the origin of (0 0 0) to one of the Sm³⁺ ions substituting for the first nearest neighbor Y³⁺ sites and beyond within a sphere with an approximate radius of less than 0.7 nm. On the other hand, a multi-step energy transfer process dominates for the highest concentration (10 mol%) because the calculated average distance between two Sm³⁺ ions in the 10 mol% Sm³⁺ sample is comparable with the migration length of the single-step energy transfer process estimated from the 1 and 5 mol% Sm³⁺ samples.     

Real-time measurement of temporal waveforms of a terahertz pulse using a probe pulse with a tilted pulse front

We propose a novel method that uses a femtosecond pulse with a tilted pulse front as a probe pulse to perform real-time measurement of temporal waveforms of a terahertz pulse. This method can operate on a single shot in principle. Real-time measurement of the terahertz pulse was achieved using the dynamic subtraction method with a synchronized high-speed complimentary metal-oxide semiconductor sensor. A time window of 2.6 ps was achieved. This value agrees well with results that were calculated theoretically and measured by an interferometric inverted field autocorrelator.     

Study of ion desorption induced by a resonant core-level excitation of condensed NH3 using Auger-electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Photostimulated ion desorption at the 4a₁ ← N 1s resonant transition of condensed NH₃ was studied using electron emission spectroscopy and Auger-electron photoion coincidence (AEPICO) spectroscopy. The total ion yield divided by the Auger-electron yield exhibited a threshold peak at hν = 399 eV which is ascribed to the resonant transition from the N 1s to the N---H antibonding 4a₁ orbital. The electron emission spectrum at the 4a₁ ← N 1s resonance is decomposed into three components: a valence photoelectron emission spectrum, and normal- and resonant-Auger-electron emission spectra. We ascribe the resonant-Auger-electron emission spectrum mainly to spectator-Auger transitions on the basis of the peak assignment. A series of AEPICO spectra at the 4a₁ ← N 1s resonance was also measured as a function of the Auger-electron kinetic energy. The electron kinetic energy dependence of the H⁺ AEPICO yield displays a shape approximately similar to that of the mixed spectrum of normal- and spectator-Auger-electron emission spectra. Based on this result the H⁺ desorption at the 4a₁ ← N 1s resonance is concluded to originate from the spectator-Auger transitions and from the normal-Auger transitions following the delocalization of the excited electron.     

Structure of ophiorines A and B; novel type gluco indole alkaloids isolated from ophiorrhiza spp.

Two novel type gluco indole alkaloids, ophiorines A and B, were found in Rubiaceous plants, Ophiorrhiza japonica B1. and O. kuroiwai Mak., and their structures were elucidated by chemical and spectroscopical methods.     

Samarium(II)-promoted radical spirocyclization onto an aromatic ring

Samarium(II)-mediated spirocyclization onto an aromatic ring was achieved by the reaction of methyl 4-(4-oxoalkyl)benzoates with SmI(2) in the presence of i-PrOH and HMPA, yielding methyl 1-alkyl-1-hydroxyspiro[4.5]dec-6-ene-8-carboxylates in moderate to high yields. Utilizing this chemistry, spiro[3.5] and -[5.5] systems, and sterically congested spiro[4.5] systems, were easily synthesized. For the successful conversion, appropriate activation of the aromatic ring has proven to be extremely important: while an ester or amide functionality on the aromatic ring can promote the spirocyclization, a sulfonamide substituent causes ortho cyclization.     

Synthesis of 1,3-dialkylurea from ethylene carbonate and amine using calcium oxide

Several basic metal oxide catalysts were tested for the synthesis of 1,3-disubstituted urea from ethylene carbonate (EC) and amine. Among the catalysts used, CaO has been found to be an excellent recyclable catalyst for the reaction. It has been suggested that strongly basic property of CaO results in its high activity. Disubstituted ureas are obtained from propylamine and butylamine with high yields at 100 °C. Slightly higher reaction temperatures are necessary for obtaining good yields from amines having larger molecular weights and urea is not produced from dibutylamine as a secondary amine. Propylene carbonate can be used instead of EC for the reaction. A reaction mechanism was proposed, which involves reaction between EC and amine giving a carbamate followed by catalytic reaction between the carbamate and amine, yielding 1,3-disubstituted urea. It is suggested that the latter reaction is the rate-determining step. On the basis of this reaction mechanism, the synthesis of unsymmetric urea was also examined. 2-Hydroxyethyl butylcarbamate is selectively produced from EC with butylamine in the absence of the catalyst at a low temperature and reacts with benzylamine producing 1-butyl-3-benzylurea along with symmetric dialkyl ureas.     

Synthesis and crystallographic characterization of low-dimensional and porous coordination compounds capable of supramolecular aromatic interaction using the 4,4'-azobis(pyridine) ligand

Coordination compounds with a 4,4'-azobis(pyridine) (azpy) ligand, {[M2(azpy)6(H2O)5] x 4PF6 x azpy x H2O}n (M = Ni(II) (1) or Co(II) (2)) (0-dimensional (0-D) dimer), {[Zn(azpy)3(H2O)2] x 2PF6 x 2azpy x 4H2O}n (3) (1-dimensional (1-D) fishbone-type chain), {[Ag(azpy)] x PF6}n (4) (1-D linear chain), {[Mn(NCS)2(azpy)2] x azpy}n (5) (2-dimensional (2-D) grid sheet), and {[Ni(NCS)2(azpy)2] x 3toluene}n (6) (2-D grid sheet), were synthesized and structurally characterized. Compounds 1 and 2 have a 0-D dimer motif, in which one M(II) (M(II) = Ni(II) or Co(II)) coordination site is shared by unidentate azpy and H2O ligands, each with half-occupancy, i.e., exhibiting static disorder. Compounds 3 and 4 afford 1-D fishbone-type and 1-D linear chain motifs, respectively. Azpy mediates pi-pi and pi-p interactions between these low-dimensional structures. Compounds 5 and 6 possess a 2-D grid sheet motif. These sheets assemble to form microporous frameworks that incorporate aromatic guests, such as coordination-free azpy (5) and toluene (6). There exist not only pi-pi and pi-p interactions but also CH-pi interactions between the framework azpy ligands and guests. It should be noted that the azpy ligand is a good candidate for the construction of new assembling systems of coordination compounds through its aromatic interactions.     

An environmentally benign and practical synthesis of sugar orthoesters promoted by potassium fluoride

An extremely practical method for synthesis of sugar orthoesters has been developed without using any organic amines or heavy metals as additives. Various sugar orthoesters were prepared in good yields by the reaction of an acylated glycosyl bromide and an alcohol in the presence of potassium fluoride.     

Asymmetric synthesis of macrocyclic binaphthol dimers using a Sonogashira coupling reaction

Asymmetric synthesis of 24- and 26-membered macrocyclic binaphthol dimers was achieved by assembling a 3,3′-diethynyl-1,1′-bi-2-naphthol unit and 1,2- and 1,3-phenylene units with Sonogashira coupling reaction.