2-pyridones from cyanoacetamides and enecarbonyl compounds: application to the synthesis of nothapodytine B

The condensation of an enone or enal with cyanoacetamide derivatives and t-BuOK furnishes either 3-cyano-2-pyridones or 3-unsubstituted-2-pyridones, depending on whether the reaction is carried out in the presence or in the absence of O(2). In the first case, in situ oxidation of Michael-type intermediates takes place; in the second case, the products result from "decyanidative aromatization" of such intermediates. A one-step synthesis of 3-alkyl-2-pyridones has been devised on the basis of decyanative union of an enone/enal and a 2-alkylcyanoacetamide. The new reaction forms the centerpiece of an unusually concise synthesis of nothapodytine B (mappicine ketone).     

New stereoselective reaction of methylglyoxal with 2-aminopyridine and adenine derivatives: formation of imino acid-nucleic base derivatives in water under mild conditions

A remarkable stereoselective reaction of methylglyoxal with 2-aminopyridine, the nucleic base adenine and adenine nucleosides leads in good yield to heterocycles of a new family in water under mild conditions and should be of interest in the understanding of the biological effects of methylglyoxal which is toxic, mutagenic and involved in diabetic complications.     

Molecular control of recombination dynamics in dye sensitised nanocrystalline TiO2 films

Modification of the structure of a porphyrin dye shows a significant change in the rate of charge recombination between injected electrons in the TiO2 and the oxidized dye anchored to it following optical excitation, offering an insight into fundamental understanding of processes occurring at the dye/semiconductor interface.     

Nonlinear effects in acousto-optic imaging

Acousto-optic (AO) imaging is a promising technique that is able to reveal optical properties in the millimeter range inside scattering media by tagging the photon paths with an ultrasonic beam. To increase both the contrast and the resolution of the AO images, we have explored the possibility of using the nonlinear response of the speckle modulation. Variation of the second-harmonic signal as the square of the ultrasonic amplitude has been found, and strong reduction of the tagged zone size has been demonstrated.     

Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

The multiphoton ionization (MPI) spectrum of toluene arising from the ¹B₂ (¹L_b) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν₁(a₁), ν₁₂(a₁) and ν₁₄(b)₂). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν₁₂ is vibronic coupling.     

Relaxation of photoexcited Na$_\mathsf{3}$F

This paper describes the spectroscopy of Na₃F both in the frequency and time domains. The photoionization efficiency curve shows two thresholds, associated to two isomers. The excited electronic states of the C_2v isomer have been probed by photodepletion spectroscopy, and the results are used to analyze a time-resolved study of photoexcited Na₃F, probed by photoionization. The pump-probe signal clearly shows damped oscillations, the period of which is fitted to fs, close to twice the previously measured bending mode of Na₂F [1], while the relaxation time is fs.Received: 31 October 2003, Published online: 6 January 2004PACS: 36.40.Qv Stability and fragmentation of clusters - 33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)     

Electric-field induced capillary interaction of charged particles at a polar interface

We study the electric-field induced capillary interaction of charged particles at a polar interface. The algebraic tail of the electrostatic pressure of each charge results in a deformation of the interface u approximately r(-4), where r is the lateral distance. The capillary interaction of nearby particles is repulsive and varies as rho(-6) with their distance rho. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged poly(methyl methacrylate) particles at an oil-water interface.     

Photoelectron imaging of helium droplets

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.     

Fluorination of the epsilon-Keggin Al13 polycation

Fluorination of the Al13 polycation in epsilon-Keggin (Kepsilon-J) conformation, leads to partial substitution of di-mu2-OH bridges by fluorine and enhances the formation of Al30 polycation.