Search results for "RADIATION"

Evidence for reverse pathways and equilibrium in singlet energy transfers between an artificial special pair and an antenna

2013

A dyad, 1, built on an artificial special pair (bis(meso-nonyl)zinc(II)porphyrin), [Zn2], a spacer (biphenylene), a bridge (1,4-benzene), and an antenna (di-meso-(3,5-di(t-butyl)phenyl)porphyrin free base), FB, is prepared by Suzuki coupling and is analyzed by absorption and steady state, and time-resolved emission spectroscopy at 298 and 77 K. Using bases from the Förster theory, evidence for two pathways for S 1 energy transfer, FB* → [Zn2], and [Zn2]* → FB, along with their respective rates, k ET ( S 1)1 and k ET ( S 1)-1, are extracted from the comparison of the fluorescence decays monitored at the emission maximum. At 77 K, the unquenched (1.79 ([Zn2]) and 10.6 ns (FB)) and quenched c…

Effect of N-substitution in multinuclear complexes allows interplay between magnetic states and multistability investigated by Mössbauer spectroscopy

2006

A series of pentadentate ligands N-X-5LH2 (X=H, Methyl, Benzyl)=N-X-saldptn (4-X-N,N′-bis(l-hydroxy-2-benzylidene)-1,7-diamino-4-azaheptane) has been prepared by a Schiff base condensation between 1,7-diamino-4-X-azaheptane and salicylaldehyde. Complexation with Fe(III) yields a series of high-spin (S=5/2) complexes of [FeIII(N-X-5L)Cl]. Such precursors were combined with [Mo(CN)8]4− and a series of blue nonanuclear cluster compounds [MoIV(CN)FeIII(N-X-5L)8]Cl4 resulted. Such star-shaped nonanuclear compounds are high-spin systems at room temperature. On cooling to 10K some of the iron(III) centers switched to the low-spin state as proven by Mossbauer spectra, i.e. multiple electronic trans…

Microwave-Assisted Synthesis of Polysubstituted 4-Quinolones from Deprotonated α-Aminonitriles

2010

The α-alkylation of deprotonated N-aryl-α-aminonitriles with α-bromoesters furnishes intermediates that can be cyclized to 4-quinolones upon microwave irradiation. Alternatively, base-induced dehydrocyanation of the alkylation products furnishes enaminoesters, which can, for example, be converted into quinoline-3-carboxylates.

ChemInform Abstract: Cyclocondensation of α-Aminonitriles and Enones: A Short Access to 3,4-Dihydro-2H-pyrrole 2-Carbonitriles and 2,3,5-Trisubstitut…

2010

The reaction of α,β-unsaturated carbonyl compounds with aminoacetonitrile hydrochloride furnishes 3,5-disubstituted 3,4-dihydro-2H-pyrrole-2-carbonitriles in a one-pot reaction sequence. While these products can serve as starting materials for the preparation of polysubstituted pyrrolizidines, they are kinetically stable against the base-induced elimination of HCN. In contrast, their 2-substituted analogues obtained from α-substituted α-aminonitriles can be readily converted to the corresponding 2,3,5-trisubstituted pyrroles under microwave irradiation. The key step presumably involves the thermal electrocyclization of a stabilized 2-azapentadienyl anion formed by condensation of the reacta…

A new interpretation for the structure of the VN bands of ethylene

1971

Abstract Potential surfaces for CC stretch and CH2 twisting of ethylene obtained by the combined SCF CI method lead to the prediction that non-vertical transitions are responsible for the absorption maximum in the VN bands and that the broad diffuse nature of the spectrum in this region could well be caused by the proximity of two states of the same symmetry of the twisted molecule.

Efficient synthesis of 2-bromo-1-[18F]fluoroethane and its application in the automated preparation of 18F-fluoroethylated compounds

2002

An efficient synthesis of 2-bromo-1-[18F]fluoroethane from commercially available 1,2-dibromoethane and its integration into an automated preparation device was developed for the routine synthesis of 18F-fluoroethylated compounds. The 1,2-dibromoethane was reacted with the [18F]fluoride/Kryptofix 2.2.2./carbonate complex in acetonitrile at 70 degrees C for 3 min resulting in 60-70% radiochemical yields. The crude reaction mixture was diluted with water, loaded on a LiChrolute EN-cartridge, eluted with acetonitrile and passed through an AluminaB-cartridge. This method provides 2-bromo-1-[18F]fluoroethane with 98% radiochemical purity and <0.1 micromol of 1,2-dibromoethane within 10 min, thus…

Effect of Mn Doping on the Optical Properties of Chalcogenide Compounds ZnS and ZnTe

2020

In this paper, the influence of the addition of Mn on the optical properties of the binary compounds ZnS and ZnTe deposited by a close-spaced vacuum sublimation method onto nonoriented glass at different substrate temperatures was considered. The spectral dependences of the transmittance T (λ), reflectance R (λ) and the absorption α (λ), as well as the bandgap for each material (Eg) were measured and calculated.

Optical properties of lithium niobate single crystals

2005

Studies of thermal and γ-irradiation effects on the optical properties in congruous lithium niobate single crystals containing Y, Mg, Gd, B, and Zn dopants including samples with double dopants Y, Mg and Gd, Mg are reported. Formation of defects at irradiation and thermal treatment of the samples is explored by electron absorption spectra. Considerable increase of absorption with the dose of γ-radiation is observed at 500 nm. The changes of absorption examined under different conditions are explained by creation and destruction of Nb 4 + defects.

The Effects of Admixtures on Resistance to Radiation of Lithium Niobate Crystals

2015

The studies of optical absorption and transmission of crystalline lithium niobate compounds modified by rare-earth and alkali-earth elements: LiNbO3: Y (0.46 wt %), LiNbO3: Y (0.32 wt %), Mg (0.24 wt %), LiNbO3: Mg (0.27 wt %), LiNbO3: Gd (0.004, 0.04, 0.26, and 0.43 wt %), and ostensibly pure LiNbO3 is reported. The features of absorption and transmission are examined with respect to the dosage of γ-irradiation, the annealing temperature, and the type and concentration of modifying admixtures. The features revealed in different ways of bleaching γ-irradiated and annealed in vacuum ostensibly pure lithium niobate crystals are used to refine the mechanisms of developing electron and point de…

1987

Diffusion coefficients D of singly and doubly end-labeled polystyrenes (PS) with molecular weights in the range 6500 ≤ M ≤ 75400 were measured in a PS matrix of molecular weight M′ = 111000, and in matrices with M′ = M. At 212°C, we find no influence of the label. In a power-law description D = D0 · M−α · M′−β we find α = 2,0 for M′ = 111000 and α + β = 2,4 for M = M′. The crossover to Rouse-like behavior for short-M chains is at Me ≈ 18000 for the former (high M′), and at Mc ≈ 33000 in the latter (M′ = M) case. At 185°C, we find a more complex scaling behavior and different D values for singly and doubly labeled PS.