(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

High-power diode-pumped Yb3+:CaF2 femtosecond laser

We report what is believed to be the first demonstration of a high-power passively mode-locked diode-pumped femtosecond laser based on an Yb3+:CaF2 single crystal, directly pumped by a 15-W fiber-coupled laser diode. With a 5-at. % Yb3+ -doped sample and prisms for dispersion compensation we obtained pulses as short as 150 fs, with 880 mW of average power and up to 1.4-W average output power, with a pulse duration of 220 fs, centered at 1049 nm. The laser wavelength could be tuned from 1040 to 1053 nm in the femtosecond regime. Using chirped mirrors for dispersion compensation, the oscillator provided up to 1.74 W of average power, with a pulse duration of 230 fs, corresponding to a pulse energy of 20 nJ and a peak power of 85 kW.     

Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

Latest developments of bulk crystals and thin films of rare-earth doped CaF2 for laser applications

Several CaF₂ single crystals doped with trivalent rare-earth ions have been grown in the recent years in the form of bulk crystals by using the Bridgman method and in the form of thin films by using the MBE and LPE techniques. The spectroscopic, gain and laser properties of these crystals doped with Pr³⁺, on the one hand, and with Yb³⁺, Tm³⁺ or Er³⁺ ions, on the other hand, have been studied and are reviewed here for their laser potentials in the red and in the infrared spectral domains, respectively.     

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.     

Dependence of pulser driving responses on electrical and motional characteristics of NDE ultrasonic probes

Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results.     

Nd3+ → Yb3+ energy transfer in a codoped metaphosphate glass as a model for Yb3+ laser operation around 980 nm

A complete spectroscopic investigation of a metaphosphate glass with composition Pb(PO₃)₂ doped with various amounts of Nd³⁺ and Yb³⁺ (1 up to 10 at.%) is reported. Efficient Nd³⁺ → Yb³⁺ energy transfers occur both radiatively and non-radiatively, the latter being dominant and partly resonant and partly phonon-assisted by phonons of the order of 950 cm⁻¹, which fits well with the reported Raman spectrum of the material. These transfers mainly concern the ⁴F_3/2 → ⁴I_9/2 emission and the ²F_7/2 → ²F_5/2 absorption transitions of the Nd³⁺ and Yb³⁺ ions around 900 nm, respectively. They are analysed both via spectral and temporal data. The results show that about 5% Nd³⁺ and 5% Yb³⁺ ions have to be incorporated to reach energy transfers exceeding about 65%, which is in agreement with data recently reported in the case of a YAl₃(BO₃)₄ crystal. Simulations based on the obtained data show that laser thresholds of a few tens of mW should be easily attainable by operating the materials in a channel waveguide configuration.     

Evolution of Yb environment and luminescence properties under ionizing irradiation in aluminoborosilicate glasses

Yb-doped aluminoborosilicate glasses were irradiated with 2.3 MeV electron and gamma rays at different doses ranging between 10⁴ and 2.6 × 10⁹ Gy and the local structure around the Yb³⁺ ions has been studied using a combination of NMR, EPR and photoluminescence spectroscopic techniques. The spectroscopic results indicate the presence of two distinct Yb³⁺ sites in these glasses and their relative fractions depend on both the Yb concentration and the irradiation dose. These two sites can be attributed to Yb³⁺ ions with and without Yb next-nearest neighbors. The evolution of the shape of the ⁷F_5/2 → ⁷F_7/2 infrared emission band under irradiation is explained by a preferential reduction of the Yb³⁺ ions with Yb next-nearest neighbors at high integrated doses (> 10⁸ Gy). This interpretation is supported by a strong decrease of the visible cooperative luminescence under irradiation due to the reduction into the Yb pairs. In other terms, it shows an interesting effect of ionizing irradiation on the Yb clusters. A low intensity emission band appears in irradiated samples at 390 nm that can be attributed to Yb²⁺. Finally, the decrease of the Yb³⁺ fluorescence lifetime observed under irradiation appears to be primarily due to electronic interaction of these ions with Non-Bridging Oxygen Hole Centre defects created by the ionizing radiation.     

Pr<Superscript>3+</Superscript>-doped Tl<Subscript>3</Subscript>PbBr<Subscript>5</Subscript>: a non-hygroscopic,non-linear and low-energy phonon single crystal for the mid-infrared laser application

This letter presents the optical and spectroscopic properties of a new low-energy phonon, moisture-resistant and non-linear Pr³⁺:Tl₃PbBr₅ single crystal. Though only weakly doped with Pr³⁺ ions, centimetre-size and good-quality single crystals could be grown and analyzed. Absorption and emission spectra as well as fluorescence kinetics were registered in the mid-infrared spectral domain. Strong luminescence at around 4–5.5 μm was observed and assigned to the ³H₅→³H₄ transition of the Pr³⁺ ions. The high value of the resulting emission cross section associated with the long lifetime of the ³H₅ emitting level indicates that this material could be a good candidate for the development of a broadly tunable mid-infrared solid-state laser source.