Carbon fibre electrodes in flow potentiometric stripping analysis

The construction of carbon fibre flow electrodes suitable for use in connection with potentiometric and constant-current stripping is described, and the fibre electrodes are compared with a glassy carbon disc thin layer cell. The signal-to-background ratio is approximately 1.6 times higher for an 8–10 μm carbon fibre compared to the glassy carbon disc electrode. If an Ag/AgCl tube is used as both counter and reference electrode, the signal-to-noise ratio of the fibre electrode is approximately five times better than for a glassy carbon disc electrode with a calomel reference; the latter electrode design, however, gives slightly better precision. The dead volume and internal potential drop of the fibre electrodes are more than one order of magnitude smaller than for the glassy carbon disc electrode. Because of the simplicity of the manufacturing process and low material cost, the fibre cells can be used as disposable electrodes and the polishing process necessary in connection with glassy carbon disc electrodes can be omitted.     

Aminonaphthalic anhydrides as red-emitting materials: electroluminescence, crystal structure, and photophysical properties

The red and orange emitters (ANA-1-3) consisting of a 4-amino-1,8-naphthalic anhydride group were synthesized. The lowest absorption band of these ANA molecules centered at approximately 450 nm is assigned to be a charge-transfer transition with emission at 514-536 nm in nonpolar solvents such as n-hexane and at approximately 590-640 nm in polar solvents such as THF and CH(2)Cl(2) and in the solid states. Emission lifetimes are measured with time-correlated single photon counting. Shorter lifetimes are observed for the ANA molecules when dissolved in polar solvents compared with those in nonpolar solvents. Strong dipole-dipole interaction of ANA molecules with solvents is indicated. At high concentrations the measured emission lifetimes, generally shortened from self-quenching, are found to remain about the same order of magnitude in ANAs. This implies that the exciton states of aggregates are formed and they exhibit a relatively long lifetime. Crystallographic data of 4-(phenyl antracen-9-yl) (ANA-2) and 4-(phenyl-2-naphthyl) amino-1,8-naphthalic anhydrides (ANA-3) show that the molecules exist as dimeric structures with antiparallel head-to-tail stacking of naphthalic anhydride planes in addition to other pi-pi stacking. The strong dipole-dipole interactions and the pi-pi stacking account for the observed red-shifted emissions of ANAs in the powders. For films prepared from vacuum sublimation, a structure similar to that in the crystal but with less crystalline order is expected based on the emission wavelength. Several electroluminescent devices based on these ANAs are reported here; they emit orange-red light at 602-628 nm with high brightness and steady external quantum efficiency.     

Study on the Thermodynamic Stabilities of Te-Substituted Di-n-butyltellurium Ylide

SincethediscoveryofphosphoniumylidebyWittig',carbanionsstabilizedbyadjacentfifthorsixthgrouponiumionshavebeendevelopedasavaluablereactiveintermediate.However,despitethegreateffortsinylidechemistryinthepastfewdecades',relativelylittleisknownaboutthethermodynamicstabilitiesofylidecarbanions.Itiswellknowthatthequantitativemeasuresofthecarbanionstabilityhavecontributedagreatdealtothedevelopmentoforganicchemistry,thereforeitshouldbeexpectedthattheknowledgeaboutylidestabilitywillalsoprovidavaluableg…     

Determination of lead in whole blood with a simple flow-injection system and computerized stripping potentiometry

An automated (24 samples/hour) procedure is described for the determination of lead (0–1000 μg l^?1) in human blood based on flow-injection stripping potentiometry. The samples are diluted 20-fold with 0.5 M hydrochloric acid containing 100 mg l^?1 mercury and 40 μg l^?1 cadmium (II), and a 1.1 ml aliquot is injected into the flow system. With a mercury-coated carbon fibre as working electrode, lead (II) is determined by using cadmium (II) as internal standard and a calibration graph prepared from bovine blood. Analyses of two human blood reference samples yielded results of 335±37 and 691±24 μg l^?1 lead, the certified values being 332 and 663 μg l^?1, respectively.     

Tocopherols and Triterpenoids from Sida acuta

A new tocopherol derivative, 7_a‐methoxy‐α‐tocopherol ( 1 ), and a new taraxastane triterpene, taraxast‐1,20(30)‐dien‐3‐one ( 5 ), together with four known compounds, β‐tocopherol ( 2 ), α‐tocopherol ( 3 ), α‐tocospiro B ( 4 ) and taraxasterone ( 6 ) were isolated from the whole plant of Sida acuta. Their structures were elucidated by spectral analysis including MS, 1D and 2D‐NMR spectroscopy. Among those compounds, compounds 1 , 2 , and 3 showed significant antioxidant effect (EC₅₀ = 86.9, 68.2, and 70.9 μM, respectively) in the DPPH radicals scavenging activity assay.     

Thermotropic Liquid Crystals Based on Extended 2,5‐Disubstituted‐1,3,4‐Oxadiazoles: Structure–Property Relationships,Variable‐Temperature Powder X‐ray Diffraction,and Small‐Angle X‐ray Scattering Studies

A class of extended 2,5‐disubstituted‐1,3,4‐oxadiazoles R¹‐C₆H₄‐{OC₂N₂}‐C₆H₄‐R² (R¹=R²=C₁₀H₂₁O 1 a , p‐C₁₀H₂₁O‐C₆H₄‐C?C 3 a , p‐CH₃O‐C₆H₄‐C?C 3 b ; R¹=C₁₀H₂₁O, R²=CH₃O 1 b , (CH₃)₂N 1 c ; F 1 d ; R¹=C₁₀H₂₁O‐C₆H₄‐C?C, R²=C₁₀H₂₁O 2 a , CH₃O 2 b , (CH₃)₂N 2 c , F 2 d ) were prepared, and their liquid‐crystalline properties were examined. In CH₂Cl₂ solution, these compounds displayed a room‐temperature emission with λ_max at 340–471 nm and quantum yields of 0.73–0.97. Compounds 1 d , 2 a – 2 d , and 3 a exhibited various thermotropic mesophases (monotropic, enantiotropic nematic/smectic), which were examined by polarized‐light optical microscopy and differential scanning calorimetry. Structure determination by a direct‐space approach using simulated annealing or parallel tempering of the powder X‐ray diffraction data revealed distinctive crystal‐packing arrangements for mesogenic molecules 2 b and 3 a , leading to different nematic mesophase behavior, with 2 b being monotropic and 3 a enantiotropic in the narrow temperature range of 200–210 °C. The structural transitions associated with these crystalline solids and their mesophases were studied by variable‐temperature X‐ray diffractometry. Nondestructive phase transitions (crystal‐to‐crystal, crystal‐to‐mesophase, mesophase‐to‐liquid) were observed in the diffractograms of 1 b, 1 d , 2 b, 2 d , and 3 a measured at 25–200 °C. Powder X‐ray diffraction and small‐angle X‐ray scattering data revealed that the structure of the annealed solid residue 2 b reverted to its original crystal/molecular packing when the isotropic liquid was cooled to room temperature. Structure–property relationships within these mesomorphic solids are discussed in the context of their molecular structures and intermolecular interactions.     

Synthesis and Structure of MnCl2(HDpyF) (HDpyF = N,N′‐di(2‐pyridyl)formamidine): New Crystallographic Disorder and Bonding Mode of the HDpyF Ligand

The reaction of N,N′‐di(2‐pyridyl)formamidine (HDpyF) with MnCl₂‐4H₂O afforded the complex MnCl₂(HDpyF), which was characterized by X‐ray crystallography. The HDpyF ligand chelates to the Mn(II) center through the first and the third nitrogen atoms to form a six‐membered ring, leaving the second and the fourth nitrogen atoms uncoordinated. The HDpyF ligand is crystallographically disordered such that two different molecules can be solved. The neutral HDpyF ligand adopts the new s‐cis‐syn‐s‐trans conformation.     

PHOTOCYCLOADDITION REACTION OF 5,7-DIMETHOXYCOUMARIN TO THYMIDINE

Abstract— The photocycloaddition reaction of 5,7-dimethoxycoumarin to thymidine on direct irradiation (λ > 300 nm) is studied as a model for photosensitization reaction of furocoumarins. The major photoadducts were isolated by silica gel column and gel permeation chromatography. Each component of the photoadducts was further separated by reverse phase, paired-ion high performance liquid chromatography. The structure of these photoproducts isolated is consistent with 1:1 C₄-cycloadducts in accordance with characteristics of their UV, IR, NMR and mass spectra and elemental analysis data. The stereochemistry of each isomer was studied by Fourier transform NMR, UV and IR spectra. The fraction C has the anti head-to-tail configuration and the fraction D has the configuration of anti head-to-head. The fractions A and B probably have the syn configuration.