Conformational and photophysical studies on porphyrin-containing donor-bridge-acceptor compounds. Charge separation in micellar nanoreactors

Photophysical studies with semi-rigid, 1, and flexible, 2, donor-bridge-acceptor (D-b-A2+) molecules with D a porphyrin and A2+ a methyl viologen moiety, were performed in neat polar solvents as well as included in surfactant (DTAB) aqueous and in reverse AOT/n-alkane micelles. The micelles acted as nanoreactors for the photoinduced electron transfer reaction upon laser excitation. In spite of the longer lifetime of the charge separated (CS) state in the semi-rigid tetrad 1(ca. 200 ns vs. ca. 100 ns for the flexible dyad 2), the CS formation quantum yield, for example in acetonitrile, was lower for the former (phi(CS) = 0.13) than for the latter (0.58). Comparison of the time-resolved fluorescence data in neat solvent and in the micelles yielded the phi(CS) values in the dilute micellar solutions. Application of laser-induced optoacoustic spectroscopy at various temperatures to 1 dissolved in a polar organic solvent (benzonitrile, BZN) included in aqueous DTAB nanoreactors afforded structural volume changes for the production in hundreds of ps of the CS state upon excitation of a polar molecule. The contraction during CS formation upon excitation of the collapsed conformer in BZN is attributed to the entering of solvent into the open molecular cavity. The opening upon formation of the CS state due to photoinduced electron transfer in the 1 collapsed conformation arises from the repulsion of the two positively charged ends in this state, as previously calculated. Inclusion of 1 in reverse AOT micelles in various n-alkanes also led to a contraction upon excitation, but the data had much more error due to the limited range of variability of the ratio of thermoelastic parameters. The data obtained with the more flexible "supermolecule" 2 showed the predicted large conformation flexibility of these molecules.     

The formation of highly fluorescent zinc phosphate in the presence of uranium and its application to the direct fluorimetry of uranium in aqueous media

The formation and composition of highly fluorescent zinc phosphates in the presence of traces of uranium were investigated and a direct fluorimetric determination of uranium in aqueous media was developed, based on the formation of finely dispersed and stabilized fluorescent precipitates. The standard deviation for 20 μg of uranium, in 25 ml is 5% ; with the fluorimeter described, 10^-2 μg of uranium per ml can be determined with an error of 50%. The method is very selective and can be employed for analysis of uranium in ores; it is less sensitive than conventional fluorimetry of fused alkali fluorides but is much simpler and faster. Interferences can often be eliminated by suitable masking reactions.     

Homolytic reductive alkylation of methylvinyl ketone with ethers by Ti(III) decomposition of t-butyl hydroperoxide

Reductive alkylatin of methylvinylketone has been accomplished by hydrogen abstraction from cyclic ethers with the redox couple: t-butyl hydroperoxide-titanous chloride. A redox radical mechanism is proposed and the selectivity of the hydrogen abstraction by t-butoxy radicals and reduction of α-ketoalkyl radical by titanous ions is discussed.     

Comparison between adsorption isotherm determination techniques and overloaded band profiles on four batches of monolithic columns

The adsorption isotherms of 4-tert.-butyl phenol were measured on four different monolithic columns, using three different techniques, classical frontal analysis (FA), the perturbation on a plateau method (PP) and the recently introduced numerical procedure known as the inverse numerical method (IN). This last approach requires only the recording of a few overloaded profiles and has the potential advantage of affording a dramatic decrease of the amounts of compounds, solvent, and time needed to determine accurate estimates of the coefficients of the isotherm. The reproducibility of the adsorption data measured on the four columns is discussed with reference to the specific techniques used for obtaining these data and to the most suitable equation used for modeling them. The data obtained for the different columns were highly consistent. The inverse numerical approach was confirmed to provide a powerful, accurate, and economic method for measuring single component adsorption data.     

A convenient enantioselective synthesis of (S)-alpha-trifluoromethylisoserine

[reaction: see text] This report describes two straightforward synthetic methodologies to obtain alpha-CF3-isoserine, a new alpha,alpha-disubstituted beta-amino acid, from alpha-(trifluoromethyl)acrylic acid. The routes involve the synthesis of five-membered cyclic sulfates (using sulfuryl chloride) or sulfamidates (using the Burgess reagent) from the corresponding chiral diols, which are obtained by a catalytic asymmetric dihydroxylation (AD) reaction.     

Unsymmetrically substituted furoxans. Part 16 . Reaction of benzenesulfonyl substituted furoxans with ethanol and ethanethiol in basic medium

The use of benzenesulfonyl substituted furoxans as flexible intermediates for the synthesis of new functionalized furoxans interesting for their potential biological properties is discussed. Reaction of benzenesul-fonylphenylsulfonylfuroxan isomers 7a and 7b with ethanol and ethanethiol in basic medium affords the expected ethers and sulphides respectively. Reaction of bis(benzenesulfonyl)furoxan ( 1 ) with ethanol in basic medium gives 3-benzenesulfonyl-4-ethoxyfuroxan ( 2 ) or diethoxyfuroxan (3), according to the experimental procedure. In contrast the reaction of 1 with ethanethiol gives a mixture of substitution compounds and the 4-benzenesulfonyl-3-ethylthiofurazan ( 11 ). The structure of the compounds has been assigned by nmr spectroscopy and, in the case of 3-benzenesulfonyl-4-ethylthiofuroxan ( 9b ), confirmed by X-ray analysis.     

In situ spectroscopic characterization of rectifying molecular monolayers self-assembled on gold.

We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.     

Characterization of dendrimer properties by capillary electrophoresis and their use as pseudostationary phases

The general properties of dendrimers and in particular their electrolytic characteristics that are relevant in electrokinetic separations, are described. In order to confirm theoretical considerations on commercial dendrimer charge and hydrodynamic radius, several capillary zone electrophoresis (CZE) experiments were performed. Electrophoretic mobilities measured at different pH values indicated a sensible increase of dendrimer hydrodynamic radius at pH values lower than 2.5. This was probably due to the Coulombic repulsion of charged amine groups of the inner dendrimer shells. The principal reasons that should address the use of dendrimers as pseudostationary phases in micellar electrokinetic chromatography (MEKC) are discussed. Moreover, a survey of different separations performed utilizing dendrimers in MEKC as well as of several future plausible uses of various classes of dendrimers is presented.     

Biomedical applications of functionalised carbon nanotubes

The organic functionalisation of carbon nanotubes can improve substantially their solubility and biocompatibility profile; as a consequence, their manipulation and integration into biological systems has become possible so that functionalised carbon nanotubes hold currently strong promise as novel systems for the delivery of drugs, antigens and genes.     

Type-D vacuum solutions with cosmological constant

All type-D vacuum (nonnull orbit and null orbit) solutions with are exhibited in canonical coordinates. The nonnull orbit metrics with contain four families of solutions: the static Levi-Cività metrics, the nondivergingD's, the divergingD's, and the diverging and twisting solutions. The null orbit metrics subdivide into two subclasses of solutions: the divergenceless null orbitD's, and the diverging and twisting null orbit solution.