Lewis-acid-catalyzed photodimerization of coumarins and N-methyl-2-quinolone

Abstract The BF(3)-catalyzed photodimerization of parent coumarin (1), three 6-alkylcoumarins (2-4) and N-methyl-2-quinolone (5) in dichloromethane was studied by time-resolved UV-vis spectroscopy. The lowest triplet state properties in the absence and presence of BF(3) were outlined, in particular the effect of self-quenching which initiates dimerization. The quantum yield of intersystem crossing (Phi(isc)) of 1-4 increases with BF(3) concentration, approaching Phi(isc) = 0.3. Phi(isc) and the relative quantum yield of dimerization go along, thereby favoring an overall triplet mechanism in both the direct and BF(3)-catalyzed photodimerization. The product ratio of 5 changes strongly with the BF(3) concentration from 100%anti-hh for the free quinolone to 100%syn-ht for the 1:1 complex.     

Isolation and synthesis of N-methyl-4,7,8-trimethoxy-2-quinolone from Spathelia sorbifolia L

The structure of a minor constituent of Spathelia sorbifolia L has been deduced to be N- methyl-4,7,8-trimethoxy-2-quinolone (2, R=Me) from spectral data and this has been confirmed by synthesis.     

Approaches to the solvation of the molecular probe N-methyl-6-quinolone in its excited state

The molecular probe N-methyl-6-quinolone (MQ) gives experimental access to its local chemical environment, e.g. inside a biomolecule. Using ab initio molecular dynamics (MD), it is possible to simulate the time evolution of the Stokes shift as a function of the actual atomistic coupling to the surrounding hydrogen bond network and thus obtain a comprehensive view of the local environment. In contrast to ground state ab initio MD simulations, the choice of a method for excited state MD is nontrivial. Here, we develop a simple and accurate model for the solvation dynamics of MQ in its first excited state.     

Optimal control of ultrafast laser driven many-electron dynamics in a polyatomic molecule: N-methyl-6-quinolone

We report time-dependent configuration interaction singles calculations for the ultrafast laser driven many-electron dynamics in a polyatomic molecule, N-methyl-6-quinolone. We employ optimal control theory to achieve a nearly state-selective excitation from the S(0) to the S(1) state, on a time scale of a few ( approximately 6) femtoseconds. The optimal control scheme is shown to correct for effects opposing a state-selective transition, such as multiphoton transitions and other, nonlinear phenomena, which are induced by the ultrashort and intense laser fields. In contrast, simple two-level pi pulses are not effective in state-selective excitations when very short pulses are used. Also, the dependence of multiphoton and nonlinear effects on the number of states included in the dynamical simulations is investigated.     

Decoupling the electrical conductivity and Seebeck coefficient in the RE2SbO2 compounds through local structural perturbations

Compromise between the electrical conductivity and Seebeck coefficient limits the efficiency of chemical doping in the thermoelectric research. An alternative strategy, involving the control of a local crystal structure, is demonstrated to improve the thermoelectric performance in the RE(2)SbO(2) system. The RE(2)SbO(2) phases, adopting a disordered anti-ThCr(2)Si(2)-type structure (I4/mmm), were prepared for RE = La, Nd, Sm, Gd, Ho, and Er. By traversing the rare earth series, the lattice parameters of the RE(2)SbO(2) phases are gradually reduced, thus increasing chemical pressure on the Sb environment. As the Sb displacements are perturbed, different charge carrier activation mechanisms dominate the transport properties of these compounds. As a result, the electrical conductivity and Seebeck coefficient are improved simultaneously, while the number of charge carriers in the series remains constant.     

Transformations in the 2-quinolone series

The treatment of 1-substituted-1,2-dihydro-4-hydroxy-2-oxo-3-quinoline carboxylic acid esters and 4-hydroxy-1-methyl-3-nitro-2-(1H)quinolinone with phosphorus oxychloride resulted in the formation of the corresponding 4-chloro-2-quinolones. Their reactions with a variety of carbon, nitrogen, oxygen, and sulfur nucleophiles is described.     

Water droplet charging process in water-in-oil microemulsions: an electrical conductivity study

 The electrical conductivity σ of water-in-oil microemulsions formulated with AOT in pentane, heptane, decane and carbon tetrachloride was investigated in a wide range of composition and temperature. Assuming an Arrhenius-type temperature dependence of the conductivity, from the slopes of the linear portions of the plots ln(σ) vs. 1/T, the equilibrium constant K of the overall kinetic reaction yielding the droplet ionization has been evaluated as a function of the water-to-surfactant molar ratio, W. The activation energies of the whole conductivity process lie in the range 25–40 kJ mol^-1, depending on the characteristics of the oil phase. These values suggest that the ionization of the water droplets, responsible of the observed conductivity, is caused by the exchange of the droplet content (Na⁺ ions in aqueous solution, deriving from surfactant molecule dissociation), when droplets coalesce and then redisperse. Received: 12 December 1997 Accepted: 16 June 1998     

Stereoselective photodimerization of alkyl 3-alkoxyl-2-naphthoates

Photodimerization of methyl 3-alkoxyl-2-naphthoates with chiral auxiliaries in the alkoxyl groups (1a and 1b) in organic solutions results in cubane-like photodimers. Asymmetric induction up to 90% diastereomeric excess (de) has been achieved and the absolute configuration of the pair of the diastereomers has been established.     

Preparation and electrical conductivity of polypyrrole/WS2 layered nanocomposites

Lithium intercalated tungsten disulfide Li(x)WS(2) with x>1 was solvothermally obtained. It was revealed that Li(x)WS(2) can remain mostly unchanged under vacuum (about 4 kPa) for 5 days. The pH dependences of surface zeta potential for single molecular layers of WS(2) were determined, illustrating that its isoeletric point is approximately at pH 3. The polypyrrole/WS(2) layered nanocomposites were prepared via the combination of the exfoliation-restacking technique and in situ oxidative polymerization. The intercalation of PPy into the host galleries resulted in an interlayer expansion of about 3.7 A, which suggested that the intercalated polymeric chains are in a monolayer arrangement with the pyrrole rings lying parallel to the WS(2) host layers. The resulting material shows good thermal stability with the decomposition of the interacted chains within the disulfide galleries occurring at around 400 degrees C. The electrical conductivity of PPy/WS(2) nanocomposite was found to be high in the order of 10(-2) S cm(-1) at ambient temperature.     

Synthesis of unnatural 1-methyl-2-quinolone derivatives

Unnatural 1-methyl-2-quinolone derivatives were synthesized by regioselective C-C bond formation. When 1-methyl-3,6,8-trinitro-2-quinolone (TNQ) was treated with enamines, nucleophilic addition readily occurred at the 4-position, and succeeding hydrolysis of enamine moiety followed by elimination of nitrous acid furnished 4-acylmethyl-1-methyl-6,8-dinitro-2-quinolones. The same products could be prepared by the reaction of TNQ with ketones in the presence of triethylamine. The present reaction enabled the introduction of various kinds of acylmethyl groups substituted with alkyl, aryl or hetaryl groups.     

Enantioselective [2 + 2] photodimerization reactions of coumarins in solution

[reaction: see text] The [2 + 2]photodimerization reactions of coumarin to an optically active anti-head-to-head dimer in the presence of an optically active host compound in cyclohexane solution proceeded with high enantioselectivity.     

Diastereodifferentiating photodimerization of alkyl 2-naphthoates with chiral auxiliaries

Photodimerization of alkyl 2-naphthoates with chiral auxiliaries resulted in cubane-like anti head-to-head photodimers in 93% yield and with up to 59% diastereomeric excess. After removal of the chiral auxiliaries enantiomers of cubane-like photodimer of 2-naphthalene carboxylic acid were obtained in 80% yield.     

A novel reaction model for the electrical conductivity of ultra-thin TiO2 films in H2

Two ultra-thin TiO(2) films with platinum electrodes were prepared on a porous anodic aluminium oxide thick layer and a ceramic plate, respectively. It was found that in both TiO(2) films the conductance was almost proportional to the fourth root of the H(2) concentration in N(2) ambient at 500 °C. A quantitative model based on the spill-over mechanism is established to describe this novel correlation.     

Temperature dependent electrical conductivity of polyaniline/Y2O3 composites

Composites of polyaniline with yttrium oxide (Y₂O₃) nanoparticles have been prepared by chemical polymerizations method by increasing the weight percentage of yttrium oxide. X-Ray diffraction (XRD) and Fourier Transform Infrared Spectra (FTIR) were used to characterize the composites. XRD and FTIR pattern indicate that Polyaniline (PANI) is intercalated into the layers of Y₂O₃ nanoparticles successfully by in situ polymerization and therefore the degree of crystallinity increases due to crystalline of yttrium oxide nanoparticles. The scanning electron micrographs (SEM) also confirm the formation of dual phase of platelet as well as of flaky structure in PANI-Y₂O₃. Temperature dependant DC conductivity showed three dimensional variable ranges hopping (3D VRH) model. Activation energy, density of states and hopping length are calculated and found to be influenced by intercalating PANI into the layers of Y₂O₃ clay.     

Self-assembling bisurea macrocycles used as an organic zeolite for a highly stereoselective photodimerization of 2-cyclohexenone

We report a highly selective 2 + 2 cycloaddition of 2-cyclohexenone in the presence of self-assembled bisurea macrocycles that yields the head-to-tail photodimer. The reaction proceeds with high conversion and with decreased incidence of secondary photorearrangement. Furthermore, the product can be extracted from the assembly, and the solid assembly is readily recovered and reused, much like a zeolite.     

Fluorination and electrical conductivity of BN nanotubes

Fluorination of BN nanotubes has been performed using a catalytic growth method, which leads to the appearance of markedly curved fluorine-doped BN sheets and converts originally insulating BN nanotubes to semiconductors, as confirmed by the comparative electron transport four-probe measurements on doped and undoped individual BN nanotubes.     

The electrical conductivity of colloidal tungstic acid

Colloid and Polymer Science - It has been observed that during the dilution of tungstic acid the pH value varies and the variation becomes slow with increasing dilution and after a certain stage it...     

Increasing mechanical resilience and enhanced electrical conductivity through the incorporation of CNF reinforcing additives in PA6 nanocomposites

Structural Chemistry - In pursuit of strong, tough, and functional advanced composite materials, a series of polymer nanocomposite blends were prepared from the engineering thermoplastic polyamide...