Extended and localized electronic states for the undulatorily-graded band-gap model of amorphous semiconductors

A novel approach is taken regarding the theoretical basis for the electronic structure of amorphous semiconductors. For these materials we propose a model displaying intermediate range order with undulations in composition or structure on a scale within the range of validity of the graded band-gap concept. Electronic states well within the allowed bands are determined by a perturbative analysis starting with Bloch states; states near the band edges, by a modification of an effective mass analysis previously developed for monotonically graded mixed crystals. Modulated extended states are deduced from the first analysis; localized states, by the second. We identify the band edges as the loci of the classical turning points of the effective mass functions for the localized states. Thermal activation from the band edge troughs and tunneling between troughs are considered. From the model and analysis it appears possible to explain field-dependent conductivity, to account for the persistence of the optical band-gap on conversion from crystalline to amorphous semiconductors, and to predict sensitivity of electronic properties to doping in some amorphous semiconductors.     

Characterization of two new zinc(II) complexes with saccharinate and imidazole or benzimidazole as ligands

The crystal structure of the complexes [Zn(sac)₂(im)₂] (1) and [Zn(sac)₂(bzim)₂]₂2Et-OHH₂O (2) (sac = saccharinate anion; im = imidazole; bzim = benzimidazole; EtOH = ethanol) was determined by single crystal X-ray diffractometry. Complex 1 crystallizes in the monoclinic P2 ₁/n space group with a = 9.1585(5), b = 16.4409(6), c = 15.0249(5) Å, = 94.079(1)°, and Z = 4, whereas complex 2 belongs to the triclinic space group P1 with a = 10.8500(2), b = 12.4860(2), c = 13.5640(3) Å, = 115,696(1), = 100.086(1), = 102.169(1)°, and Z = 1. In both complexes, the Zn(II) cations are in a slightly distorted tetrahedral ZnN₄ environment, coordinated to two saccharinate anions and to two imidazole (1) and to two benzimidazole (2) molecules. The complexes were also characterized by means of infrared spectroscopy and their thermal behavior investigated by means of thermogravimetric and differential thermal analytical techniques.     

Sintokamides A to E, chlorinated peptides from the sponge Dysidea sp. that inhibit transactivation of the N-terminus of the androgen receptor in prostate cancer cells

The new chlorinated peptides sintokamides A to E (1-5) have been isolated from specimens of the marine sponge Dysidea sp. collected in Indonesia. Their structures were elucidated by a combination of spectroscopic and single-crystal X-ray diffraction analyses. Sintokamide A (1) is an inhibitor of N-terminus transactivation of the androgen receptor in prostate cancer cells.     

Intermolecular vibrational coherence in the bacteriochlorophyll proteins B777 and B820 from Rhodospirillum rubrum

The low-frequency vibrational coherence in the bacteriochlorophyll (BChl)-containing subunit proteins B777 and B820 from the LH1 light-harvesting complex isolated from Rhodospirillum rubrum G9 exhibits rapidly damped modulation components arising from intermolecular, formally nonbonding interactions between the BChl macrocycle and polar groups in the surrounding detergent or protein. The vibrational coherence observed in the monomeric B777 system resembles that observed previously with BChl in acetone because it contains a pair of broad overlapping line shapes with a mean frequency of 191 cm(-1), but the 10:1 intensity ratio of the librational and translational components is distinctive of the motions of the polar head groups in the nonionic detergent micelle that solvates the BChl macrocycle. In contrast, the vibrational coherence observed with the dimeric B820 complex is almost 20 times weaker in intensity and exhibits narrower line shapes and lower average frequencies than observed in B777. The structure of the B820 complex sterically protects the pair of BChl macrocycles from the surrounding solvent, so modulation components assigned to intrinsic interactions between the BChl and the protein and between the pair of BChl's are revealed. A relatively well-ordered interaction between the BChl macrocycle and a tryptophan residue in each alpha-helical polypeptide accounts for a 28 cm(-1) component with a narrow line shape, but most of the intensity arises from a broader 46 cm(-1) component that is assigned to the interaction between the paired BChl macrocycles. The breadth of the line shape for this component is a measure of the disorder in the ensemble of B820 subunits. The results support the hypothesis that the excited-state vibrational dynamics and the optical and/or Marcus charge-transfer reorganization energies of BChl in photosynthetic light-harvesting proteins and reaction centers are strongly controlled by van der Waals modes with neighboring molecules, with dominant contributions to the intermolecular potential arising from the London dispersion and dipole-dipole interactions.     

Peptide cysteine thiyl radicals abstract hydrogen atoms from surrounding amino acids: the photolysis of a cystine containing model peptide

Peptide cysteine thiyl radicals were generated through UV-photolysis of disulfide precursors, in order to follow intramolecular reactions of those radicals with neighboring amino acids. When reactions were carried out in D(2)O, there was a significant incorporation of deuterium specifically into the C(alpha)-H bonds of glycine residues in positions i+1 and i-1 to the Cys residue, indicating a fast reversible H-atom transfer. This H-atom transfer occurred prior to the formation of final, nonradical products including free thiol, thioaldehyde, and aldehyde. Such fast H-atom transfer is relevant to biologic conditions of oxidative stress and to the stabilization of proteins against oxidation, where the formation of carbon-centered radicals in proteins may lead to fragmentation, intramolecular cross-linking, aggregation and/or epimerization.     

Development of candidate reference materials for the measurement of lead in bone

The production of modest quantities of candidate bone lead (Pb) reference materials is described, and an optimized production procedure is presented. The reference materials were developed to enable an assessment of the interlaboratory agreement of laboratories measuring Pb in bone; method validation; and for calibration of solid sampling techniques such as laser ablation ICP-MS. Long bones obtained from Pb-dosed and undosed animals were selected to produce four different pools of a candidate powdered bone reference material. The Pb concentrations of these pools reflect both environmental and occupational exposure levels in humans. The animal bones were harvested post mortem, cleaned, defatted, and broken into pieces using the brittle fracture technique at liquid nitrogen temperature. The bone pieces were then ground in a knife mill to produce fragments of 2-mm size. These were further ground in an ultra-centrifugal mill, resulting in finely powdered bone material that was homogenized and then sampled-scooped into vials. Testing for contamination and homogeneity was performed via instrumental methods of analysis.

Simultaneous Controllability of PSD and MWD in Emulsion Polymerisation

A sensitivity study of particle size distribution (PSD) and molecular weight distribution (MWD) responses to perturbations in initiator, surfactant, monomer and chain transfer agent in a semi‐batch emulsion polymerisation is presented. The objective is to provide a systematic study on the ability to simultaneously control both PSD and MWD, towards inferential control of end‐use product properties. This would lead towards identification of the practical feasible regions of operability. All inputs appeared to have an intrinsic and simultaneous influence on end‐time PSD and MWD. Trends shown in experimental results have been explained in a mechanistic sense and also compared to simulation results from a combined PSD/MWD population balance model. The preliminary comparison between experiment and simulation highlights areas to be focussed on with respect to model improvement.

     

Light-emitting iridium complexes with tridentate ligands

Once the Cinderella amongst the Platinum Group Metals at the Photochemistry Ball, iridium has become of intense interest since the beginning of the decade. Complexes of iridium(III) can be prepared that are highly luminescent, with emission wavelengths tuneable over the whole of the visible region. Whilst most studies have focused on tris-bidentate complexes, a rich and varied chemistry is also possible using tridentate ligands. In this review, we discuss the synthesis and excited-state properties of such complexes, exploring in particular how the number of cyclometallating carbon atoms in the coordination sphere of the metal ion influences the luminescence. Moving from [IrN6]3+ to [IrN3X3] coordination via [IrN5X]2+ and cis/trans-[IrN4X2]+ complexes, where N is a heterocyclic nitrogen and X is an anionic ligand or cyclometallated carbon, a whole range of luminescence efficiencies are encountered, ranging from the barely detectable to room temperature quantum yields approaching unity. We consider the extent to which these profound differences, arising as a result of subtle changes in molecular structure, can be rationalised in terms of the nature of the frontier orbitals.