Distinct effects of homogeneous weak disorder and dilute strong scatterers on phase competition in manganites

We study the two orbital double-exchange model in two dimensions including antiferromagnetic (AFM) superexchange, Jahn-Teller coupling, and substitutional disorder. At hole doping x = 0.5 we focus on phase competition between the ferromagnetic metal (FMM) and the charge-ordered (CO) and orbital-ordered (OO) CE state and compare the impact of weak homogeneous disorder to that of a low density of strong scatterers. Even moderate homogeneous disorder suppresses the CE-CO-OO phase and leads to a glass with nanoscale correlations, while dilute strong scatterers of comparable strength convert the CE-CO-OO phase to a phase separated state with ferromagnetic metal and AFM-CO-OO clusters.     

Solvation dynamics in protein environments: comparison of fluorescence upconversion measurements of coumarin 153 in monomeric hemeproteins with molecular dynamics simulations

The complexes of the fluorescence probe coumarin 153 with apomyoglobin and apoleghemoglobin are used as model systems to study solvation dynamics in proteins. Time-resolved Stokes shift experiments are compared with molecular dynamics simulations, and very good agreement is obtained. The solvation of the coumarin probe is very rapid with approximately 60% occurring within 300 fs and is attributed to interactions with water (or possibly to the protein itself). Differences in the solvation relaxation (or correlation) function C(t) for the two proteins are attributed to differences in their hemepockets.     

Relativistic coupled-cluster calculations of parity nonconservation in Ba+ by the sum-over-states approach

The authors present the results of their calculation for the parity nonconserving 5p(6)6s(1/2)-->5p(6)5d(3/2) transition in Ba+ using the relativistic coupled-cluster theory in the singles, doubles, and partial triples approximation. The contributions from the leading intermediate states are explicitly considered. It is found that the largest contribution comes from the |5p(6)6p(1/2)> state. Their results are in reasonable agreement with other calculations.     

One photon two electron excitations between doubly excited states of helium

Variational calculations using Hylleraas coordinates have been performed for the first time for estimating the energies of 3dnf((1,3)D(o)) state of helium for n=4,5,6. We predict absorption peaks at 12.219, 12.647, and 12.857 eV for the (3)D(o) series converging to N=3 ionization threshold of He(+) which can be expected to be observed in the experiment of single photon double excitation of lowest (3)P(e) state of helium placed in synchrotron radiation.     

Organic-inorganic nanocomposites via directly grafting conjugated polymers onto quantum dots

Nanocomposites of poly(3-hexylthiophene)-cadmium selenide (P3HT-CdSe) were synthesized by directly grafting vinyl-terminated P3HT onto [(4-bromophenyl)methyl]dioctylphosphine oxide (DOPO-Br)-functionalized CdSe quantum dot (QD) surfaces via a mild palladium-catalyzed Heck coupling, thereby dispensing with the need for ligand exchange chemistry. The resulting P3HT-CdSe nanocomposites possess a well-defined interface, thus significantly promoting the dispersion of CdSe within the P3HT matrix and facilitating the electronic interaction between these two components. The photophysical properties of nanocomposites were found to differ from the conventional composites in which P3HT and CdSe QDs were physically mixed. Solid-state emission spectra of nanocomposites suggested the charge transfer from P3HT to CdSe QDs, while the energy transfer from 3.5 nm CdSe QD to P3HT was implicated in the P3HT/CdSe composites. A faster decay in lifetime further confirmed the occurrence of charge transfer in P3HT-CdSe nanocomposites.     

A dual detector capillary waveguide biosensor for detection and quantification of hybridized target

We describe a novel technique for improving the sensitivity of analytical instruments based on the measurement of fluorescent intensity. Independent measurement of the Rayleigh scattered intensity component by means of a second photodetector leads to normalized data, which are independent of various experimental parameters. Incorporation of this technique into a fully automated capillary waveguide biosensor improved the instrument sensitivity by a factor of three. The technique enables quantification, as well as detection, of the hybridized target molecules.     

Self-assembly of a Mn9 nanoscopic mixed-valent cluster: synthesis, crystal structure, and magnetic behavior

A rare Mn9 micro3-oxo-centered mixed-valent cluster [Mn9O7(O2CPh)11(thmn)(py)2 (H2O)3] (1) is prepared by assembling an oxo-centered MnIIMnIII2 triangle, [Mn3O(O2CPh)6(py)2(H2O)].0.5MeCN, as the secondary building unit in the presence of a tripodal alcohol, 1,1,1-tris(hydroxymethyl)nitromethane (H3thmn), as the capping ligand. Complex 1 was formed along with a minor byproduct, [Mn6O2(O2CPh)10(MeCN)4] (2). Complex 1 was characterized by X-ray single-crystal structure analysis and was crystallized in a monoclinic system, space group P2(1)/n, a=16.214(6) A, b=25.874(10) A, c=26.497(10) A, and beta=94.214(7) degrees. The Manganese-oxo-carboxylate core in 1 looks like a funnel. Variable-temperature magnetic studies down to 2 K reveal the existence of dominant ferromagnetic interaction within the cluster. Alternating current susceptibility data of the cluster show strong frequency dependence of both the real and imaginary parts of susceptibility chi' and chi' below 5 K. Moreover, the calculated relaxation time, tau0=1.2x10(-7) s, and the energy barrier, DeltaE=25 K, are consistent with the single-molecule magnetic behavior of 1.     

A tetracopper(II)-tetraradical cuboidal core and its reactivity as a functional model of phenoxazinone synthase

The coordination chemistry of the tridentate ligand N-(2-hydroxy-3,5-di-tert-butylphenyl)-2-aminobenzylalcohol H3L has been studied with the copper(II) ion. The ligand is noninnocent in the sense that it is readily oxidized in the presence of air to its o-iminobenzosemiquinonato [L*]2- radical form. The crystal structure of the synthesized tetracopper(II)-tetraradical complex [CuII4(L*)4] (1), has been determined by X-ray crystallography at 100 K. Variable-temperature (2-290 K) magnetic susceptibility measurements of complex 1 containing eight paramagnetic centers establish the spin ground state to be diamagnetic (St=0) arising from the antiferromagnetic interactions. Electrochemical measurements (cyclic voltammograms and square wave voltammograms) indicate four one-electron reductions of the ligand prior to the reduction of the metal center. Complex 1 is found to catalyze the aerial oxidation of 2-aminophenol to 2-amino-phenoxazine-3-one, thus modeling the catalytic function of the copper-containing enzyme phenoxazinone synthase. Kinetic measurements together with electron paramagnetic resonance and electronic spectral studies have been used to decipher the complex six-electron oxidative coupling of 2-aminophenol. An "on-off" mechanism of the radicals together with redox participation of the metal center is proposed for the catalytic oxidation processes.     

Compartmentalization of reactants in different regions of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate/heptane/water reverse micelles and its influence on bimolecular electron-transfer kinetics

Sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micellar medium has been used to study the photoinduced electron-transfer (ET) reactions between some coumarin derivatives and amines, namely, aniline (AN) and N,N-dimethylaniline (DMAN) at different w(0) (w(0) = [water]/[AOT]) values, to explore the appearance of Marcus inversion and also the possible role of w(0), if any, on the Marcus correlation curves. The coumarin derivatives are found to partition between the heptane-like and the water-like phases of the reverse micelles, and their locations have been confirmed by time-resolved anisotropy measurements. Fluorescence quenching is found to depend both on the location of the coumarin molecules and on the hydrophobicity of the amine donors. Various aspects such as the effect of differential partitioning of the quenchers, the location of the probes in the two phases, the diffusion of the reactants in the micellar phase, etc. have been considered to rationalize the fluorescence quenching rates in reverse micelles. Rotational relaxation times and the diffusion parameters estimated from the anisotropy results do not show good correlation with the observed quenching rates indicating that the diffusion of reactants has no role in the quenching kinetics in reverse micelles. Marcus inversion behavior has been observed for the coumarin-amine systems in the water-like phase at a relatively high exergonicity of approximately 1.2 eV suggesting that the solvent reorganization energy contributes fully to the free energy of activation for the ET reactions in the present systems. This is in accordance with the fast solvent relaxation dynamics reported in reverse micelles. Quenching rates in the water-like phase are found to decrease or increase marginally with increasing w(0) for the coumarin-DMAN and coumarin-AN systems, respectively. This is explained on the basis of the changing solubility of these amines in the water-like phase with changing w(0) values of the reverse micelles. In the heptane-like phase, no clear inversion in the quenching rate versus free energy plot could be observed because the study could not be extended to higher exergonicity due to nonsolubility of the dye C151 in this phase. Present results, especially in the water-like phase, suggest that the confinement of reactants in micellar media can effectively remove the influence of reactant diffusion on bimolecular ET rates and thus make the systems more conducive for the observation of the Marcus inverted region.     

Liquid chromatographic analysis of certain commercial formulations for non-opioid analgesics

A simple and sensitive method for separation and quantitative determination of non-opioid analgesics from pharmaceutical preparations has been developed and validated. Commercial formulations of three non-opioid analgesics, viz. paracetamol, ibuprofen and diclofenac, were chosen for present studies. These were extracted, isolated, purified and recrystallized and were characterized by melting point, lambda(max) and IR. Quantitative determination was carried out using HPLC and TLC supplemented with UV spectrophotometry.