Conformational control of excited-state dynamics in highly distorted Ru(II) polypyridyl complexes

Tris(bipyridyl)ruthenium(II) complexes modified such that one of the bipyridines is appended with a crown ether display luminescence that is responsive to complexation with metal ions. The parent species, Ru(bpy)3(2+), is moderately luminescent, with an emission lifetime of about 1 micros in fluid solution at room temperature. The modified complexes are much less emissive, with lifetimes near 1 ns. Conformational flexibility and distortion in the crown-ether complexes enhance nonradiative decay. Noncovalent binding of metal ions, however, restores luminescence intensity by reducing nonradiative decay and increasing the lifetime 10- to 100-fold. Reported here are the syntheses and steady-state and time-resolved luminescence measurements in addition to other supporting spectroscopic characterization. Seven metals were investigated; significant luminescence enhancements occur in the presence of Mg2+, Ca2+, and Pb2+. Effective concentrations of metal ions range from tens of microM to hundreds of mM. The steady-state enhancements are readily measured, but they are less than would be expected from the lifetime changes, partly because only a portion (not more than 50%) of the fast (1 ns) decay in Ru(bpy)2(bpy-crown) is capable of converting to the conformation possessing the longer lifetime. A photophysical model is proposed to explain these and other observations.     

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.     

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.     

Correlated atomic motions in Pd79Si21 alloy glass observed by inelastic pulsed neutron scattering

The dynamical structure factor, , for Pd₇₉Si₂₁ alloy glass was measured with a fine Q and ω resolution by inelastic neutron scattering using the chopper spectrometer, MARI, installed at the ISIS facility, Rutherford Appleton Laboratory. Fine structure is clearly observed in over the dynamic range

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, which indicates the existence of well defined modes caused by correlated atomic motions even in the disordered structure of metallic alloy glasses.     

The structure of liquid tetrahydrofuran

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the structural correlation functions of liquid tetrahydrofuran at room temperature. Empirical potential structure refinement (EPSR) has been used to build a three-dimensional model of the liquid structure that is consistent with the experimental data. Analysis to the level of the orientational correlation functions shows that the liquid displays a preference for T-like configurations between the tetrahydrofuran molecules, a local structure that results in void-like regions of approximately 1.25 angstroms radius within the bulk liquid. The surface chemistry of these voids suggests a slightly positive electrostatic character. These findings are consistent with the known propensity of the liquid to solvate free electrons.     

Excess entropy in alcohol-water solutions: a simple clustering explanation

We show that the anomalous negative excess entropy of mixing characteristic of aqueous lower alcohols containing hydrophobic groups is quantitatively consistent with a model exploiting only the experimentally observed molecular-scale segregation of the components across the entire concentration range. The simple model presented here, which uses plausible interatomic distances as its only free parameters, obviates the need to invoke "iceberg" or other water restructuring concepts which, though frequently postulated in explaining the hydrophobic interaction, are unsupported by recent experiments.     

In situ monitoring of hydrogen loss during pyrolysis of wood by neutron imaging

Hydrogen is an element of fundamental importance for energy but hard to quantify in bulk materials. Neutron radiography was used to map in situ loss of elemental hydrogen from beech tree wood samples during pyrolysis. The samples consisted of three wood cylinders (finished dowel or cut branch) of approximately 1 cm in length. The samples were pyrolyzed under vacuum in a furnace vessel that was placed inside a cold neutron imaging beamline using a temperature ramp of 5 °C/min from ambient up to 400 °C. Neutron radiographs with exposures of 30 s were sequentially recorded with a charge-coupled device over the course of the experiment. Relative absorbance/scattering of the neutron beam by each sample was based on intensity (or brightness) values as a function of pixel position. The much larger neutron cross section for hydrogen compared to carbon and oxygen enables almost direct conversion of neutron attenuation into sample hydrogen content for each time step during the pyrolysis experiment. Target and vessel temperatures were recorded concurrently with collection of the radiographs so that changes could be directly correlated to different states of pyrolysis. The most visible change appeared at the initial phase of the 400 °C plateau as evidenced by strong hydrogen loss and primarily diametric shrinking of the samples. The loss of elemental hydrogen between initial and final states of pyrolysis was estimated to be about 70%.