Atomically Defined Monocarborane Copper(I) Acetylides with Structural and Luminescence Properties Tuned by Ligand Sterics

The weakly coordinating cluster [CB₁₁H₁₂]⁻ is used as a versatile building block for the preparation of luminescent copper(I) complexes. Treatment of [CB₁₁H₁₁-12-C≡CH]⁻ with CuI and ammonia affords {(CB₁₁H₁₁-12-C≡C)₂Cu₄(NH₃)₃}_n ( 2 ). Addition of selected phosphine and pyridine ligands transforms 2 to a range of homometallic products 3 – 13 . All compounds have been characterized by X-ray crystallography, which reveals a surprising variety of structural complexity featuring cores with two to ten Cu⁺ centers. In the solid state, products 3 – 13 exhibit room-temperature phosphorescence across the visible spectrum in colors from blue to deep red. Lifetimes at room temperature are in the microsecond regime, and quantum yields of up to Φ=0.99 are observed.     

Bernal’s road to random packing and the structure of liquids

Until the 1960s, liquids were generally regarded as either dense gases or disordered solids, and theoretical attempts at understanding their structures and properties were largely based on those concepts. Bernal, himself a crystallographer, was unhappy with either approach, preferring to regard simple liquids as ‘homogeneous, coherent and essentially irregular assemblages of molecules containing no crystalline regions’. He set about realizing this conceptual model through a detailed examination of the structures and properties of random packings of spheres. In order to test the relevance of the model to real liquids, ways had to be found to realize and characterize random packings. This was at a time when computing was slow and in its infancy, so he and his collaborators set about building models in the laboratory, and examining aspects of their structures in order to characterize them in ways which would enable comparison with the properties of real liquids. Some of the imaginative – often time consuming and frustrating – routes followed are described, as well the comparisons made with the properties of simple liquids. With the increase of the power of computers in the 1960s, computational approaches became increasingly exploited in random packing studies. This enabled the use of packing concepts, and the tools developed to characterize them, in understanding systems as diverse as metallic glasses, crystal–liquid interfaces, protein structures, enzyme–substrate interactions and the distribution of galaxies, as well as their exploitation in, for example, oil extraction, understanding chromatographic separation columns, and packed beds in industrial processes.     

Hydrogenic systems,frequency standards and fundamental constants

Hydrogenic (two-body) systems are the only atomic systems for which uncertainties in calculations of the energy levels approach the current state of the art in frequency measurement. This article discusses progress in the theory and measurement of transition frequencies in hydrogenic systems. These studies have relevance to the determination of fundamental constants and the testing of physical theories, especially quantum electrodynamics. A set of high accuracy calculable frequency standards could also be realized by using hydrogenic systems.     

Organocatalyzed anodic oxidation of aldehydes

A method for the catalytic formation of electroauxiliaries and subsequent anodic oxidation has been developed. The process interfaces N-heterocyclic carbene-based organocatalysis with electro-organic synthesis to achieve direct oxidation of catalytically generated electroactive intermediates. We demonstrate the applicability of this method as a one-pot conversion of aldehydes to esters for a broad range of aldehyde and alcohol substrates. Furthermore, the anodic oxidation reactions are very clean, producing only H(2) gas as a result of cathodic reduction.     

Kirkpatrick–Baez mirrors to focus hard X‐rays in two dimensions as fabricated,tested and installed at the Advanced Photon Source

The micro‐focusing performance for hard X‐rays of a fixed‐geometry elliptical Kirkpatrick–Baez (K–B) mirrors assembly fabricated, tested and finally implemented at the micro‐probe beamline 8‐BM of the Advanced Photon Source is reported. Testing of the K–B mirror system was performed at the optics and detector test beamline 1‐BM. K–B mirrors of length 80 mm and 60 mm were fabricated by profile coating with Pt metal to produce focal lengths of 250 mm and 155 mm for 3 mrad incident angle. For the critical angle of Pt, a broad bandwidth of energies up to 20 keV applies. The classical K–B sequential mirror geometry was used, and mirrors were mounted on micro‐translation stages. The beam intensity profiles were measured by differentiating the curves of intensity data measured using a wire‐scanning method. A beam size of 1.3 µm (V) and 1.2 µm (H) was measured with monochromatic X‐rays of 18 keV at 1‐BM. After installation at 8‐BM the measured focus met the design requirements. In this paper the fabrication and metrology of the K–B mirrors are reported, as well as the focusing performances of the full mirrors‐plus‐mount set‐up at both beamlines.     

Metabolism of selenite in human lung cancer cells: X-ray absorption and fluorescence studies

Selenite is an inorganic form of selenium that has a cytotoxic effect against several human cancer cell lines: one or more selenite metabolites are considered to be responsible for its toxicity. X-ray absorption spectroscopy was used to monitor Se speciation in A549 human lung cancer cells incubated with selenite over 72 h. As anticipated, selenodiglutathione and elemental Se both comprised a large proportion of Se in the cells between 4 and 72 h after treatment, which is in accordance with the reductive metabolism of selenite in the presence of glutathione and glutathione reductase/NADPH system. Selenocystine was also present in the cells but was only detected as a significant component between 24 and 48 h concomitant with a decrease in the proportion of selenocysteine and the viability of the cells. The change in speciation from the selenol, selenocysteine, to the diselenide, selenocystine, is indicative of a change in the redox status of the cells to a more oxidizing environment, likely brought about by metabolites of selenite. X-ray fluorescence microscopy of single cells treated with selenite for 24 h revealed a punctate distribution of Se in the cytoplasm. The accumulation of Se was associated with a greater than 2-fold increase in Cu, which was colocalized with Se. Selenium K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed Se-Se and Se-S bonding, but not Se-Cu bonding, despite the spatial association of Se and Cu. Microprobe X-ray absorption near-edge structure spectroscopy (μ-XANES) showed that the highly localized Se species was mostly elemental Se.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.