A positron annihilation lifetime measurement system with an intense positron microbeam

An intense positron microbeam was formed using an electron linear accelerator. The beam is pulsed to apply positron lifetime spectroscopy to very small samples and to obtain positron lifetime images by scanning it. Positron lifetimes are measured with time resolution of <300 ps and with lateral spatial resolution of 30–100 μm. A counting rate of the γ-ray to measure positron lifetime is about 10³ s⁻¹ which is 10 times higher than that achieved by the radioisotope based microbeam.     

Proton‐Induced Generation of Remote N‐Heterocyclic Carbene–Ru Complexes

The proton‐induced Ru?C bond variation, which was previously found to be relevant in the water oxidation, has been investigated by using cyclometalated ruthenium complexes with three phenanthroline (phen) isomers. The designed complexes, [Ru(bpy)₂(1,5‐phen)]⁺ ([ 2 ]⁺), [Ru(bpy)₂(1,6‐phen)]⁺ ([ 3 ]⁺), and [Ru(bpy)₂(1,7‐phen)]⁺ ([ 4 ]⁺) were newly synthesized and their structural and electronic properties were analyzed by various spectroscopy and theoretical protocols. Protonation of [ 4 ]⁺ triggered profound electronic structural change to form remote N‐heterocyclic carbene (rNHC), whereas protonation of [ 2 ]⁺ and [ 3 ]⁺ did not affect their structures. It was found that changes in the electronic structure of phen beyond classical resonance forms control the rNHC behavior. The present study provides new insights into the ligand design of related ruthenium catalysts.     

Aromaticity‐Controlled Thermal Stability of Photochromic Systems Based on a Six‐Membered Ring as Ethene Bridges: Photochemical and Kinetic Studies

Three photochromic compounds—2‐butyl‐5,6‐bis[5‐(4‐methoxyphenyl)‐2‐methylthiophen‐3‐yl]‐1 H‐benzo[de]isoquinoline‐1,3(2 H)‐dione (BTE‐NA), 4,5‐bis[5‐(4‐methoxyphenyl)‐2‐methylthiophen‐3‐yl]benzo[c][1,2,5]thiadiazole (BTA), and BTTA, which contain naphthalimide, benzothiadiazole, and benzobisthiadiazole as six‐membered ethene bridges with different aromaticities—were systematically studied in solution, sol–gel, and single‐crystal states. They exhibit typical photochromic performance with considerably high cyclization quantum yields. BTE‐NA, BTA, and BTTA form a typical donor–π–acceptor (D –π–A) system with significant intramolecular charge transfer (ICT) between HOMO and LUMO upon excitation, thus realizing the fluorescence modulation by both photochromism and solvatochromism. The three ethene bridges with different degrees of aromaticity can provide a systematic comparison of the thermal stability evolution for their corresponding closed forms (c‐BTE‐NA, c‐BTA, and c‐BTTA). c‐BTE‐NA shows first‐order decay in various solvents from cyclohexane to acetonitrile. c‐BTA only shows first‐order decay in polar solvents such as chloroform, whereas it is stable in nonpolar solvents like toluene. In contrast, the less aromatic property of BTTA gives rise to its unprecedented thermal stability in various solvents even at elevated temperatures in toluene (328 K). Moreover, the small energy barrier between the parallel and antiparallel conformers allows the full conversion from BTTA to c‐BTTA. In well‐ordered crystal states, all three compounds adopt a parallel conformation. Interestingly, BTTA forms a twin crystal of asymmetric nature with interactions between the electron‐rich oxygen atom of the methoxy group and the carbon atom of the electron‐deficient benzobisthiadiazole moiety. This work contributes to the understanding of aromaticity‐controlled thermal stability of photochromic systems based on a six‐membered ring as an ethene bridge, and a broadening of the novel building blocks for photochromic bisthienylethene systems.     

Tandem arrays of TEMPO and nitronyl nitroxide radicals with designed arrangements on DNA

Herein we describe one-dimensional electron-spin arrays consisting of two different organic radicals with the designed arrangement based on the DNA sequence. Two mismatch-binding ligands carrying 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) and nitronyl nitroxide selectively bind to the predetermined sites on double stranded DNA. By using the two mismatch-binding ligands carrying the organic radicals as the glue for DNA, electron-spin assembly could be successfully synchronized with the hybridization. Periodically and tandemly arranged, two kinds of organic radical molecules at designed positions might be useful for an approach to build up scalable qubits of electron-spin-based quantum computing. The approach using DNA nanostructures as a scaffold to assembly functional small molecules can afford one of the promising ways for the future application of DNA nanostructures and nanotechnology.     

Permeability control of metal ions using temperature- and pH-sensitive gel membranes

Temperature- and pH-sensitive copolymer gels were synthesized by the simultaneously occurring radiation-induced polymerization and self-bridging of acryloyl- -proline methyl ester (A-ProOMe) with acrylic acid (AAc) in aqueous solutions. The gel swelling behavior and the metal permeation characteristic of its gel membrane were investigated with regard to very slight changes of temperature and pH. The pH threshold of the swelling of a copoly(A-ProOMe/AAc, 70/30 mol%) gel in the range of 5–30°C lay in the region between pH 4.0 and 5.0. The permeability results of metal ions showed that at 40°C the gel membrane blocks the permeation of lithium (Li) and cesium (Cs) ions at pH values lower than 4.75 and 4.60, respectively. The permselectivity (P_Li/Cs value) of the two metal ions at 30°C was also studied and, as a result, its value was obtained to be 1.33 at pH 4.65 and 30°C. This permeation study indicates that the selective metal separation of copoly(A-ProOMe/AAc) gel membranes can be controlled by changing temperature and pH values.     

All-optical orientation of photoisomerizable octupolar zinc(II) complexes in polymer films

A new type of 4,4'-bis(styryl)-2,2'-bipyridine functionalized by a dialkylamino-azobenzene group has been prepared. This ligand has allowed the preparation of photoisomerizable octupolar tris(bipyridyl)zinc(II) complexes and the corresponding star-shaped polymer by atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). The photoisomerization properties of such new metallo-chromophores have been studied. The macroscopic molecular orientation of the corresponding doped and grafted NLO-polymer films is reported for the first time.     

Simultaneous detection of monohydroxybenzo[a]pyrene positional isomers by reversed-phase liquid chromatography coupled to electrospray ionization mass spectrometry

A liquid chromatographic (LC) method has been developed for the separation of 11 monohydroxybenzo[a]pyrenes (OH BaPs) positional isomers, and for their detection using electrospray ionization mass spectrometry (ESI-MS). All OH BaPs isomers were separated on an octadecylsilyl (C(18))-bonded amorphous organosilica column utilizing gradient elution with acetonitrile-water and triethylamine (TEA) at pH 11.0 and determined by MS, except 2- and 8-OH BaPs which were coeluted. The lower detection limits were in the range from 1.6 micro g/L for 12-OH BaP to 12 micro g/L for 5-OH BaP without any sample enrichment. The relative standard deviations of area response were in the range from 1.8% (9-OH BaP) to 4.9% (12-OH BaP) except for 9.4% (7-OH BaP). The developed method was successfully applied to incubation mixtures of BaP and CYP1A1/epoxide hydrolase. This method identified 1-, 3- and 9-OH BaPs as the major metabolites, and 2- (and/or 8-) and 12-OH BaPs as the minor metabolites in the incubation mixture.