Efficient photocyclization of dithienylethene dimer, trimer, and tetramer: quantum yield and reaction dynamics

Multi-dithienylethene arrays, in which two, three, or four 1,2-bis(2,4-dimethylthiophen-3-yl)perfluorocyclopentenes are ethynylene-bridged, were synthesized. Upon irradiation with ultraviolet light the hexane solutions of the arrays turned violet-blue and the color disappeared by irradiation with visible light. The quantum yields of photocyclization reactions successively increased from 0.21 to 0.40 by increasing the number of the dithienylethene moieties in the arrays from one to four. Picosecond laser photolysis as well as the fluorescence depolarization experiment confirmed that efficient excited energy migration in the arrays from the photochemically inactive parallel conformer to the photoactive antiparallel conformer resulted in the high quantum yields.     

636 nm room temperature CW operation by heterobarrier blocking structure InGaAlP laser diodes

636 nm room temperature CW operation has been achieved by heterobarrier blocking structure InGaAlP laser diodes with a quaternary active layer. This structure was fabricated by two-step metal-organic chemical vapour deposition. The threshold current was 102 mA at 20 degrees C and CW operation of 3 mW was attained at up to 48 degrees C.<>     

Association of hydrophobic counterions in aqueous solution of poly(allylammonium) chloride: Comparison between p-n-propylbenzenesulfonate and p-iso-propylbenzenesulfonate ions

The hydrophobic association behavior of p-propylbenzenesulfonate ions (n-PBS^? and iso-PBS^?) around poly(allylammonium) cation (PAAH⁺ was studied by absorption and ¹H nuclear magnetic resonance (NMR) spectroscopy. In the presence of PAAH⁺Cl^?, the broadening (hypochromism) of absorption band of PBS^? were observed. In addition, all ¹H NMR signals of PBS^? exhibited up-field shift which resulted from the intermolecular ring current shift. These results indicate the hydrophobic association of negative PBS^? around PAAH⁺. The hydrophobic association arises from the accumulation of counterions around the polyion, and is stabilized by the hydrophobic interaction between propyl groups and the stacking interaction between benzene rings. The association of iso-PBS^? ions is rather weaker than that of n-PBS^?, suggesting that the steric hindrance of isopropyl groups prevents an effective association of hydrophobic counterions. Furthermore, through viscosity and Cl^? activity measurements, it was found that the binding of associated PBS^?,s to PAAH⁺ causes the change of its surroundings to hydrophobic character and its conformation. © 1994 John Wiley & Sons, Inc.     

Optical frequency spacing tunable four-channel integrated 1.55 mu m multielectrode distributed-feedback laser array

A four-channel integrated 1.55- mu m multielectrode distributed-feedback (DFB) laser array was fabricated using the metallorganic vapor-phase deposition/liquid-phase epitaxy (MOVPE/LPE) hybrid method. Simultaneous single-longitudinal mode operation was achieved in each multielectrode DFB laser on a single chip. Utilizing the frequency tunability of multielectrode DFB lasers, optical frequency spacings were controlled and set to within a few gigahertz. The drift of frequency spacings due to temperature fluctuation was in the range of +or-50 MHz for temperature control of +or-0.1 degrees .<>     

Narrow-beam divergence of the emission from low-threshold GaInAsP/InP double-heterostructure lasers

The angles of beam divergence perpendicular to the junction plane have been studied for low-threshold GaInAsP/InP double-heterostructure lasers with a thin active layer emitting at 1.31 ?m. The narrowest beam divergence obtained was 23° for a laser diode with an active-layer thickness of 0.05 ?m. At an active-layer thickness of 0.13 ?m, the beam divergence was 40° with a threshold current density of 770 A/cm2.     

Studies towards the synthesis of the fluorescent bases of phenylalanine transfer ribonucleic acids: synthesis of 7-methylwye isolated from extremely thermophilic archaebacteria

Acid- or base-catalyzed acylation of 1-benzylwye (7) provided the 7-substituted derivatives 9, 10, and 11 in poor yields. Although the reactions of lithiated 7 with electrophiles gave the 2-substituted derivatives 14, 15, 17, 20, 21, and 22, lithiation of 1-benzyl-7-bromo-2-chlorowye (23) followed by treatment with Me2CHCH2CHO (13) successfully introduced a side chain at the 7-position to afford 1-benzyl-2-chloro-7-(1-hydroxy-3-methylbutyl)wye (24). Cyclization of 1-benzyl-3-methylguanine (5) with 3-bromo-2-butanone followed by catalytic hydrogenolysis afforded 7-methylwye (2b), the hypermodified base isolated from archaebacterial transfer ribonucleic acids. A more efficient route for the synthesis of 2b has been developed via a series of reactions: the Vilsmeier-Haack reaction of 7, reduction with NaBH4, and catalytic hydrogenolysis over Pd-C.     

Adsorption of formaldehyde on Pt(111) and Pt(100) electrodes: cyclic voltammetry and scanning tunneling microscopy

The adsorption of formaldehyde (HCHO) on Pt(111) and Pt(100) electrodes was examined by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in 0.1 M HClO(4). The extent of HCHO adsorption at both Pt electrodes was evaluated by comparing the CVs, particularly for the hydrogen adsorption and desorption between 0.05 and 0.4 V, obtained in 0.1 M HClO(4) with and without HCHO. The adsorption of HCHO on these Pt electrodes was significant only when [HCHO] >/= 10 mM. Adsorbed organic intermediate species acted as poisons, blocking Pt surfaces and causing delays in the oxidation of HCHO. Compared to Pt(111), Pt(100) was more prone to poisoning, as indicated by a 200 mV positive shift of the onset of HCHO oxidation. However, Pt(100) exhibited an activity 3 times higher than that of Pt(111), as indicated by the difference in peak current density of HCHO oxidation. Molecular resolution STM revealed highly ordered structures of Pt(111)-( radical7 x radical7)R19.1 degrees and Pt(100)-( radical2 x radical2) in the potential region between 0.1 and 0.3 V. Voltammetric measurements further showed that the organic poisons produced by HCHO adsorption behaved differently from the intentionally dosed CO admolecules, which supports the assumption for the formation of HCO or COH adspecies, rather than CO, as the poison. On both Pt electrodes, HCHO oxidation commenced preferentially at step sites at the onset potential of this reaction, but it occurred uniformly at the peak potentials.     

Synthesis and structure of the hypermodified nucleoside of rat liver phenylalanine transfer ribonucleic Acid

The first synthesis of (alphaS,betaS)-beta-hydroxy-alpha-[(methoxycarbonyl)amino]-4,6-dimethyl-9-oxo-3-beta-D-ribofuranosyl-4,9-dihydro-3H-imidazo[1,2-a]purine-7-butanoic acid methyl ester [(alphaS,betaS)-11] has been achieved by OsO(4) oxidation of [S-(E)]-4-[4,6-dimethyl-9-oxo-3-[2,3,5-tris-O-(tert-butyldimethylsilyl)-beta-D-ribofuranosyl]-4,9-dihydro-3H-imidazo[1,2-a]purin-7-yl]-2-[(methoxycarbonyl)amino]-3-butenoic acid methyl ester (13) followed by successive gamma-deoxygenation through the cyclocarbonates, separation from the (alphaS,betaR)-isomer by means of flash chromatography, and deprotection. On the other hand, the minor nucleoside of rat liver tRNA(Phe) was isolated on a scale of 100 microg by partial digestion of unfractionated tRNA (1 g) with nuclease P(1), followed by reverse-phase column chromatography, complete digestion with nuclease P(1)/alkaline phosphatase, and reverse-phase HPLC. Comparison of this nucleoside with the synthetic one has unambiguously established its structure to be (alphaS,betaS)-11.     

Effect of underlying coronene and perylene adlayers for [60]fullerene molecular assembly

Supramolecularly assembled layers of C60 on coronene- and perylene-modified Au(111) surfaces were investigated and the structure of the C60 adlayer was found to be strongly influenced by the underlying organic layers, suggesting that the latter underlying organic adlayers play an important role in the process of formation of the C60 molecular adlayer.     

Electrorheological toners for electrophotography

The electrorheological (ER) behavior of pigment suspensions dispersed in a nonaqueous solvent was examined for their application as liquid toners for electrophotography. In electric fields, particles can align into chains along the field vector by dielectric polarization forces and the suspensions undergo a rapid transition from Newtonian fluids to Bingham bodies. However, the migration and deposition of particles can take place by the electrophoretic effect, because charge control agents are added to liquid toners for fast development. The combined effects of dielectric polarization forces, electrophoretic forces, and hydrodynamic forces make rheological behavior very complicated. To simulate the ER behavior of liquid toners in reprographic processes, viscosity measurements were carried out in electrodes with a honeycomb pattern. Nonuniform electric fields enhance the dipole-dipole interactions between particles and give rise to a striking ER effect. Based on measurements in honeycomb pattern electrodes, new ER toners were developed which can reproduce images with high quality.