Effect of oxygen on the formation and decay of stilbene radical cation during the resonant two-photon ionization

Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S.+) during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of O(2) have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns). The transient absorption spectra of S.+ were observed with a peak around 470-490 nm. The formation quantum yield of S.+ (0.06-0.29) increased with decreasing oxidation potential (E(ox)) and increasing fluorescence lifetime (tau(f)) of S, except for trans-4-methoxystilbene which has the lowest E(ox) and longer tau(f) among S. The considerable low yield and fast decay in a few tens of nanoseconds time scale were observed for trans-4-methoxystilbene.+ in the presence of O(2), but not for other S.+ . It is suggested that formation of the ground-state complex between trans-4-methoxystilbene and O(2) and the distonic character of trans-4-methoxystilbene.+ with separation and localization of the positive charge on the oxygen of the p-methoxyl group and an unpaired electron on the beta-olefinic carbon are responsible for the fast reaction of trans-4-methoxystilbene.+ with O(2) or superoxide anion, leading to the considerable low yield and fast decay of trans-4-methoxystilbene.+ . The mechanism based on the transient absorption measurement of S.+ during the TPI is consistent with the relatively high oxidation efficiency of trans-4-methoxystilbene among S based on the product analysis during the photoinduced electron transfer in the presence of a photosensitizer such as 9,10-dicyanoanthracene and O(2) in acetonitrile.     

Decrease in skin permeation and antibacterial effect of parabens by a polymeric additive, poly(2-methacryloyloxyethyl phosphorylcholine-co-butylmetacrylate)

The inhibitory effect of poly(2-methacryloyloxyethyl phosphorylcholine-co-butylmetacrylate) (PMB) on the in vitro skin permeation of p-hydroxybenzoic acid and its esters (parabens; methylparaben, ethylparaben, n-propylparaben and n-butylparaben) as model compounds was evaluated. Solubility of the parabens in distilled water was increased by addition of PMB, and the increasing ratio was dependent on the concentration of PMB. The increment of the ratio was more marked in lipophilic parabens than hydrophilic ones. Skin permeability of parabens from their aqueous suspensions was decreased by addition of PMB, and the decreasing ratio was dependent on the PMB concentration. Then, antibacterial effect of parabens of their aqueous solutions was determined with and without PMB by measuring minimum inhibitory concentration (MIC) against E. coli and S. aureus. As results, MIC and antibacterial effect were increased and decreased, respectively, by addition of PMB. The contribution of PMB was more marked in butylparaben than methylparaben. These decreases in skin permeation and antibacterial effect of parabens by PMB are probably due to a higher solubility in water and a lower partition to the skin and bacterial membranes of parabens by addition of PMB.     

Ultrafast photoinduced intramolecular charge separation and recombination processes in the oligothiophene-substituted benzene dyads with an amide spacer

Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.     

The role of chain transfer agents in the emulsion polymerization of styrene

The effect of chain transfer agents on the nucleation and growth of polymer particles in the emulsion polymerization of styrene were examined extensively. The chain transfer agents used are carbon tetrachloride, carbon tetrabromide, and four primary mercaptans (C₂, n-C₄, n-C₇, and n-C₁₂). It is shown that with an increase in the amount of chain transfer agents charged the rate of polymerization per particle decreases progressively. The number of polymer particles formed, on the other hand, increases initially then decreases. These effects can be enhanced by using a chain transfer agent with higher values of chain transfer constant and solubility in water. It is also demonstrated that with increasing radical desorption from the particles, aided by chain transfer agents, the emulsifier dependence exponent for the number of polymer particles formed increases from 0.6 to 1.0 and the initiator dependence exponent decreases from 0.4 to 0. The effect of chain transfer agents on the nucleation and growth of polymer particles in the emulsion polymerization of styrene can be explained in terms of desorption of chain-transfered radicals from the polymer particles.     

Preparation of boron-doped semiconducting diamond films using BF3 and the electrochemical behavior of the semiconducting diamond electrodes

Boron-doped semiconducting diamond films were prepared using BF₃ by microwave plasma assisted chemical vapor deposition. B-doping was confirmed by SIMS and Raman spectroscopic measurements and the B-doping levels were estimated. Electrochemical behaviors of boron-doped diamond thin-film electrodes prepared using B₂H₆ and BF₃ were studied by measuring cyclic voltammograms for anodic oxidation of 1,4-difluorobenzene in the liquid electrolyte, neat Et₄NF·4HF. The results of the direct thermal interaction of elemental fluorine with hydrogenated and oxidized diamond surfaces are also presented.     

Four-way-branched DNA-porphyrin conjugates for construction of four double-helix-DNA assembled structures

[reaction: see text] DNA-porphyrin conjugates having four DNA strands were designed and synthesized. Four double helices were assembled using two DNA-porphyrin conjugates and their complementary strands, and the formation of the four double-helix assembled structures with the two DNA-porphyrin units was examined by gel electrophoresis and spectroscopic analysis. The interaction between two porphyrin chromophores in the complex was investigated by measurement of fluorescence lifetimes, and the singlet energy transfer between the two different phorphyrin units (Zn-porphyrin and H2-porphyrin) was observed. These results indicate that multiple and different porphyrin chromophores can be integrated into the DNA structures by programming the sequences of the DNA strands.     

Sol-Gel Formation of ITO Thin Film from a Sol Including ITO Powder

Recently, low temperature synthesis of ITO thin film of low resistivity is desired in the field of LCD (Liquid Crystalline Display) production. This paper describes the results of experiment to produce ITO thin film from a sol containing crystalline ITO particles and alkoxides of In and Sn. Crystallization temperature of ITO thin film obtained from the sol mixed with ITO powder was lower than that of ITO from the sol without ITO powder. Furthermore, the resistivity of the former ITO thin film was lower than that of the latter. It was assumed that these results were attributed to the promotion effect of ITO crystalline particles to crystalize the amorphous ITO gel at lower temperature. This low temperature crystallization effect plays an important role in enhancing the crystallinity and electric conductivity of ITO thin film from sol-gel process.     

Ultraviolet-Laser-Induced Crystallization of Sol-Gel Derived Indium Oxide Films

Modification of sol-gel derived indium oxide thin films using ultraviolet lasers was investigated. Irradiation by an ArF excimer (6.4 eV) and the fourth harmonic generation of a Nd : YAG laser (4.7 eV) was found to be effective in crystallization with a loss of hydroxyl groups and a decrease in the sheet resistance of the sol-gel films. Transparent crystalline indium oxide films were successfully obtained by 6.4 eV laser irradiation at fluences below 20 mJ/(cm2·shot), whereas degradation of the films was induced by a relatively high-fluence beam.