Electronic interplay on illuminated aqueous carbon nanohorn-porphyrin ensembles

Tetracationic water-soluble porphyrin (H(2)P(4+)) has been immobilized by pi-pi stacking interactions onto the skeleton of carbon nanohorns (CNH), without disrupting their pi-electronic network. The stable aqueous solution of the CNH-H(2)P(4+) nanoensemble was examined by both electron microscopy and spectroscopic techniques. The efficient fluorescence quenching of the H(2)P(4+) moiety in the CNH-H(2)P(4+) nanoensemble was probed by steady-state as well as time-resolved fluorescence emission spectroscopy, suggesting charge separation from the photoexcited H(2)P(4+) to CNH. In the presence of methyl viologen dication (MV(2+)) and a hole trap, accumulation of the reduced species of methyl viologen was observed by the photoillumination of CNH-H(2)P(4+), suggesting that the electron migration from the initially formed charge-separated state takes place. Transient absorption spectroscopy gave further insights on the transient species such as the charge-separated state (CNH(*-))-(H(2)P(4+))(*+), which was consumed in the presence of MV(2+) and hole shifter, leaving the reduced methyl viologen.     

Carboxylations of alkali metal phenoxides with carbon dioxide

The reaction mechanism of the Kolbe-Schmitt reaction of phenol and 2-naphthol has been investigated. An alkali metal phenoxide-CO2 complex is not an intermediate that can be easily transformed into a carboxylic acid, such as salicylic acid (SA) and p-hydroxybenzoic acid (pHBA). A direct carboxylation of phenoxide with CO2 takes place even at room temperature, and is competitive with the formation of the CO2 complex. The resulting complex decomposes thermally (above ca. 100 degrees C) to phenoxide, which then undergoes further competitive reactions. Experiments using a carbon-13 labeled complex support a mechanism of direct carboxylation, and not the mechanism via a CO2 complex. The reactivity, C-13 NMR and MOPAC/PM3 calculations suggest a new carbonate-like structure for the CO2 complex.     

Selective matching of catalyst element and carbon source in single-walled carbon nanotube synthesis on silicon substrates

We have studied the compatibility of various catalysts for ethylene and ethanol chemical vapor deposition (CVD) syntheses of single-walled carbon nanotubes (SWNTs) on Si substrates. A strong selectivity between the catalyst elemental species and carbon source was found; SWNT yield for Fe (Co) catalysts was much higher for ethylene (ethanol) CVD than for ethanol (ethylene) CVD. This strong and completely opposite selectivity implies significantly different SWNT growth mechanisms for ethanol and ethylene CVD on Si substrates.     

Amperometric measurement of ds-DNA content using a peroxidase-modified electrode

An enzyme electrode with a chemically amplified response for methylene blue (MB) was constructed from a glassy carbon electrode and a layer containing immobilized horseradish peroxidase (HRP). MB is reduced on the electrode but regenerated through the HRP-catalyzed reaction in the presence of H(2)O(2). The electroreduction/regeneration cycle for MB resulted in an amplified electrode response. The enzyme electrode was applied to the highly sensitive measurement of ds-DNA. The current for MB decreased in association with its complexation with DNA, and the current response caused by DNA was also amplified through the recycling processes. The detection limit of ds-DNA (from salmon testes) was as low as 5 ng ml(-1).     

Thermomechanical analysis of calcium alginate hydrogels in water

Spherical hydrogels of Ca alginate (CaAlg) were prepared by conversion of Na alginate (NaAlg) to CaAlg in CaCl₂ aqueous solution. The degree of conversion was varied by changing the conversion times. NaAlg samples with mannuronic and guluronic component ratios (M/G ratio) 0.21 and 0.88 were used. The degrees of conversion and water contents (W_c=mass of water in CaAlg/mass of dry CaAlg) were measured by weighing. The degrees of conversion from NaAlg to CaAlg increased with increasing immersion time and the content of G. W_c of CaAlg hydrogels decreased with increasing immersion time and levelled off at 75 g g^-1 when immersion time exceeded about 10 min. Viscoelastic properties of CaAlg hydrogels in water were measured using a thermomechanical analyzer (TMA) equipped with a quartz compression probe. Dynamic modulus (E') of CaAlg with M/G=0.88 increased with increasing immersion time when immersion time is less than 5 min. When immersion time exceeded 5 min, E' reached a constant value at 4.5*10⁴ Pa. In contrast, E' of CaAlg with M/G=0.21 increased with increasing immersion time. This suggests that densely crosslinked hydrogels are formed when the guluronate sequence of the samples is rich and Na ions are fully converted to Ca ions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interlayer interaction of two-dimensional layered spin crossover complexes [FeIIH3L(Me)][FeIIL(Me)]X (X-=ClO4-, BF4-, PF6-, AsF6-, and SbF6-; H3L(Me)=tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine)

A series of two-dimensional (2D) spin crossover complexes, [FeIIH3L(Me)][FeIIL(Me)]X (X-=ClO4-, BF4-, PF6-, AsF6-, SbF6-) 1-5, have been synthesized, where H3L(Me) denotes an hexadentate N6 tripodlike ligand containing three imidazole groups, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. Compounds 1-5 exhibit a two-step (HS-[FeIIH3L(Me)](2+) + HS-[FeIIL(Me)]-) <--> (HS-[FeIIH3L(Me)](2+) + LS-[FeIIL(Me)]-) <--> (LS-[FeIIH3L(Me)](2+) + LS-[FeIIL(Me)]-) spin-transition. The crystal structure of [FeIIH3L(Me)][FeIIL(Me)]PF6 (3) was determined at 295, 200, and 100 K. The structure consists of homochiral extended 2D puckered sheets, in which the complementary [FeIIH3L(Me)](2+) and [FeIIL(Me)]- capped tripodlike components, linked together by imidazole-imidazolate hydrogen bonds, are alternately arrayed in an up-and-down mode. The Fe-N bond distances and angles revealed that the FeII sites of both constituting units are in the high-spin (HS) state at 295 K; at 200 K, the FeII sites of [FeIIH3L(Me)](2+) and [FeIIL(Me)]- are in the HS and low-spin (LS) states, respectively. The FeII sites of both constituting units are in the LS state at 100 K. The size of the counteranion affects significantly the intra- and interlayer interactions leading to modifications of the spin crossover behavior. The onset of the second spin-transition of the ClO4- (1) and BF4- (2) salts adjoins the first spin-transition, while a mixed (HS-[FeIIH3L(Me)](2+) + LS-[FeIIL(Me)]-) spin-state spans a temperature range as wide as 70 K for salts 3-5 with larger counteranions, PF6-, AsF6-, and SbF6-, respectively. Compounds 1 and 2 showed remarkable LIESST (light induced excited spin state trapping) and reverse-LIESST effects, whereas 3-5 showed no remarkable LIESST effect. The interlayer interaction due to the size of the counteranion is an important factor governing the spin crossover behavior and LIESST effect.     

A novel aminomethylation of silyl enol ethers with aminomethyl ethers catalyzed by iodotrimethylsilane or trimethylsilyl trifluoromethanesulfonate

The iodotrimethylsilane-catalyzed reaction of silyl enol ethers with aminomethyl ethers in acetonitrile gives aminomethylation products of the corresponding ketones readily. The reaction can slso be catalyzed by trimethylsilyl trifluoromethanesulfonate in dichloromethane.     

Formation of Giant Liposomes from Crystalline Complexes of Monoalkylammonium Surfactants and 4-Hydroxybiphenyl

A parallel rather than a perpendicular alignment of aromatic compounds with respect to surfactant molecules (see schematic representation) is preferred for the formation of a liposome structure, because the perpendicular alignment would reduce the hydrophilicity of the aggregate. This is the result of studies on crystalline complexes of monoalkylammonium halides and various aromatic compounds.     

Dicarba-closo-dodecaboranes as a pharmacophore. Novel potent retinoidal agonists.

The synthesis and biological evaluation of the dicarba-closo-dodecaborane (carborane) derivatives of retinoids are described. Retinoidal activity was examined in terms of the differentiation-inducing ability toward human promyelocytic leukemia HL-60 cells. High retinoidal activity (agonist or antagonist for the retinoid receptor RAR) requires a carboxylic acid moiety and an appropriate hydrophobic group located at a suitable position on the molecule. 4-[4-(1,2-Dicarba-closo-dodecaboran-1-yl)phenylamino]b enzoic acids and 4-[3-(1,2-dicarba-closo-dodecaboran-1-yl)phenylamino]b enzoic acids showed potent agonistic activity at concentrations of 10(-8)-10(-9) M. The results indicate that carboranes are applicable as the hydrophobic moiety of biologically active molecules.