Exact solution of the excited-state geminate A*+B <==> C*+D reaction with two different lifetimes and quenching

The authors obtain, in the Laplace transform space, the exact analytic solution for the Green function and survival probabilities for the excited-state diffusion-influenced reversible geminate reaction, A*+B <==> C*+D, with two different lifetimes and in the presence of an added quenching process. This extends a previous investigation by Popov and Agmon [J. Chem. Phys. 117, 5770 (2002)] of the ground-state reaction without quenching. The long-time asymptotic behavior of the survival probabilities is obtained in the time domain. It is found to be different from the equal-lifetime case. This paper also provides a useful short-time approximation for the kinetics.     

Melting and vaporization of salts in a U-LiCl-Li2O system

Summary The electrochemical reduction of uranium oxide in the treatment of spent nuclear fuel requires a characterization of the LiCl-Li₂O salt used as a reaction medium. Physical properties, melting and vaporization are important for the application of the salt and thus they have been investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG), respectively. Experimental data suggest LiCl and Li₂O compound formations, leading to a melting point depression of the LiCl and a co-vaporization of the LiCl-Li₂O salt.     

Preparation of nanoporous MgO-coated TiO2 nanoparticles and their application to the electrode of dye-sensitized solar cells

Sol-gel-derived Mg(OH)(2) gel was coated onto TiO(2) nanoparticles, and the subsequent thermal topotactic decomposition of the gel formed a highly nanoporous MgO crystalline coating. The specific surface area of the electrode that was prepared from the core-shell-structured TiO(2) nanoparticles significantly increased compared with that of the uncoated TiO(2) electrode. The increase in the specific surface area of the MgO-coated TiO(2) electrode was attributed to the highly nanoporous MgO coating layer that resulted from the topotactic reaction. Dye adsorption behavior and solar cell performance were significantly enhanced by employing the MgO-coated TiO(2) electrode. Optimized coating of a MgO layer on TiO(2) nanoparticles enhanced the energy conversion efficiency as much as 45% compared to that of the uncoated TiO(2) electrode. This indicates that controlling the extrinsic parameters such as the specific surface area is very important to improve the energy conversion efficiency of TiO(2)-based solar cells.     

In situ friction study of Ag Underpotential deposition (UPD) on Au(111) in aqueous electrolyte

The electrodeposition of silver on Au(111) was investigated using lateral force microscopy (LFM) in Ag⁺ containing sulfuric acid. Friction force images show that adsorbed sulfate forms structure ( on Au(111) prior to Ag underpotential deposition (UPD) and structure ( ) on a complete monolayer or bilayer of Ag. Variation of friction with normal load shows a non-monotonous dependence, which is caused by increasing penetration of the tip into the sulfate adlayer. In addition, the friction force is influenced by the varying coverage and mobility of Ag atoms on the surface. Before Ag coverage reaches the critical value, the deposited silver atoms may be mobile enough to be dragged by the movement of AFM tip. Possible penetration of the tip into the UPD layer at very high loads is discussed as a model for self-healing wear. However, when the coverage of Ag is close to 1, the deposited Ag atoms are tight enough to resist the influence of the AFM tip and the tip penetrates only into the sulfate adlayer.     

Chemical Constituents of Brown Rice Grain (<Emphasis Type="Italic">Oryza sativa</Emphasis>)

One new compound 3,7,11,15,19-pentamethyl-9α,10α,11α,17α,18α-pentahydroxy-n-tetracosan-1-oxy-p-hydroxycaffeoate (oryzaterpenyl caffeoate) (1), together with three known fatty acids linoleic acid, stearic acid and myristic acid were isolated and identified from the rice grain of Oryza sativa. The structure of the new compound was elucidated by 1D and 2D NMR spectroscopic techniques (¹H-¹HCOSY, ¹H-¹³C HETCOR) aided by EI-MS, and IR spectra. __________ Published in Khimiya Prirodnykh Soedinenii, No. 6, pp. 535–537, November–December, 2005.     

Excitation-energy migration in self-assembled cyclic zinc(II)-porphyrin arrays: a close mimicry of a natural light-harvesting system

The excitation-energy-hopping (EEH) times within two-dimensional cyclic zinc(II)-porphyrin arrays 5 and 6, which were prepared by intermolecular coordination and ring-closing metathesis reaction of olefins, were deduced by modeling the EEH process based on the anisotropy depolarization as well as the exciton-exciton annihilation dynamics. Assuming the number of energy-hopping sites N = 5 and 6, the two different experimental observables, that is, anisotropy depolarization and exciton-excition annihilation times, consistently give the EEH times of 8.0 +/- 0.5 and 5.3 +/- 0.6 ps through the 1,3-phenylene linkages of 5 and 6, respectively. Accordingly, the self-assembled cyclic porphyrin arrays have proven to be well-defined two-dimensional models for natural light-harvesting complexes.     

Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

It has been experimentally established in numerous cases that precipitation of monodispersed colloids from homogeneous solutions is a complex process. Specifically, it was found that in many systems nuclei, produced rapidly in a supersaturated solution, grow to nanosize primary particles (singlets), which then coagulate to form much larger final colloids in a process dominated by irreversible capture of these singlets. This paper describes a kinetic model that explains the formation of dispersions of narrow size distribution in such systems. Numerical simulations of the kinetic equations, with experimental model parameter values, are reported. The model was tested for a system involving formation of uniform spherical gold particles by reduction of auric chloride in aqueous solutions. The calculated average size, the width of the particle size distribution, and the time scale of the process agreed reasonably well with the experimental values. Copyright 1999 Academic Press.     

The first fullerene-metal sandwich complex: an unusually strong electronic communication between two C(60) cages

Reaction of Rh(6)(CO)(9)(dppm)(2)(mu(3)-eta(2),eta(2),eta(2)-C(60)) (1) with C(60) in refluxing chlorobenzene followed by treatment with CNR (R = CH(2)C(6)H(5)) at room temperature affords the first fullerene-metal sandwich complex Rh(6)(CO)(5)(dppm)(2)(CNR)(mu(3)-eta(2),eta(2),eta(2)-C(60))(2) (2). Compound 2 has been characterized by an X-ray diffraction study. Electrochemical study of 2 reveals six well-separated reversible redox couples localized at C(60) cages due to a strong electronic communication between the two C(60) centers via the Rh(6) cluster spacer.     

Synthesis, characterization and protein adsorption behaviors of PLGA/PEG di-block co-polymer blend films

A series of poly(D,L-lactic-co-glycolic acid) (PLGA)/poly(ethyleneglycol) (PEG) di-block copolymers were synthesized by ring-opening polymerization of D,L-lactide and glycolide with different molecular weights of monomethoxy polyethyleneglycol (mPEG) 750, 2000 and 5000 as an initiator. The bulk properties of these co-polymers were characterized by using 1H NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry (DSC). Electron spectroscopy for chemical analysis (ESCA) results, in which the blend films with the di-block copolymers showed increasing surface oxygen atomic percentage with increasing PEG chain length, indicate that PEG chain segment in the di-block copolymers is surface oriented and enriched onto the surface of the blend films. The extent of protein adsorption onto the surface of these blend films was studied, using iodine radio-labeled human serum albumin, gamma globulin and human growth hormone. The protein adsorption amount was reduced for the blend films prepared with PLGA/PEG 750 and 2000 di-block copolymers, but increased to a great extent for PLGA/PEG 5000 di-block copolymer. This is due to the increased water uptake capacity of the blend film, which absorbed more protein molecules into a swollen polymer matrix in addition to surface adsorption.