Photochemical reactions of aromatic compounds XLV: controlling factors for reactivities and mechanisms in photoelectron transfer reactions of some selected diarylcyclobutanes with electron acceptors

We have attempted to explore mechanistic aspects of the photosensitized ring-cleavage reactions of cis-1,2-diphenylcyclobutane (1), cis-transoid-cis-cyclobutal[1,2-a:4,3-a′] diindene (2) and r-1,c-2-dimethyl-t-3,t-4-di(4-methoxyphenyl)cyclobutane (3) by electron acceptors (A) in acetonitrile. The experimental results demonstrate that the ring cleavage of 1 and 2 occurs as a consequence of the rapid geminate recombination of ion-radical pairs occurring at a rate of well over 10⁹ s⁻¹ without ionic dissociation. In the case of 3, however, the photoreactions proceed by way of a chain-reaction mechanism involving the free cation radical of 3 which undergoes ring cleavage at much less than 10⁷ s⁻¹. The rapid ring cleavage of 1⁺ and 2⁺ is attributed to significant perturbations of the cyclobutane ring by the population of positive charge on the orbital array of the two π-electron systems and the cyclobutane-ring σ framework because of strong through-bond couplings. It is presumed that the cyclobutane ring of 3⁺ is much less distorted since the positive charge is mostly localized on the aryl group. The rapid geminate recombination of the A⁻⁻−1⁺ and A⁻⁻−2⁺ pairs is discussed in terms of a very efficient transition from the “distorted” and “ring-opened” minima of the A⁻⁻−D⁺ surface to the A–D surface. In the case of 3, this mechanism cannot be expected to operate in the geminate recombination.     

Spectrophotometric determination of antimony with vanillylfluorone in the presence of poly(vinyl alcohol)

Antimony in the range up to 2.5 mug/ml in the final solution is determined spectrophotometrically with vanillylfluorone in the presence of poly(vinyl alcohol) in acidic media. The method is compared with that using o-hydroxyhydroquinonephthalein (Qnph). It is simple, rapid and sensitive, without need for heating or solvent extraction, and the apparent molar absorptivity () is 5.0 x 10(4) 1.mole(-1).cm(-1) at 545 nm; for the Qnph method is 2.8 x 10(4) 1.mole(-1).cm(-1) at 520 nm. Tests with an artificial wastewater gave 99-103% recovery.     

Resonance Raman spectra of the TCNE radical anion dimer in ethanol

Inter-and intra-radical vibrational Raman lines are observed for the TCNE radical anion dimer. Counter-ion independence of the spectrum shows that the dimer forms practically no association with counter ions in ethanol. The intra-radical lines are assigned to the out-of-phase combination modes of the monomer vibrations.     

Ruthenium-catalyzed carbon-carbon bond formation between propargylic alcohols and alkenes via the allenylidene-ene reaction

A novel ruthenium-catalyzed carbon-carbon bond formation between propargylic alcohols and alkenes via the allenylidene-ene reaction has been found to afford the corresponding 2,4-disubstituted-1-hexen-5-ynes in moderate yields. The finding described here discloses a new reactivity of allenylidene complexes. As a synthetic application, intramolecular cyclization of propargylic alcohols bearing an alkene moiety has been developed to give the corresponding syn-substituted chromanes in high yields with an excellent diastereoselectivity.     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Ion optical evaluation of a miniature double-focusing mass spectrograph

A new miniature double-focusing mass spectrograph has been designed and constructed. The ion optical system was designed based on Mattauch-Herzog geometry. The mass spectrograph employs a focal plane detector consisting of a microchannel plate, a phosphor layer, a fiber-optic plate and a charge-coupled device. For the evaluation of the ion optics of the instrument, the energy and angular focal planes were investigated both experimentally and by simulation. Double focusing was satisfactorily achieved along a straight line over a wide mass range, and the experimental and simulated results were mutually consistent. A second-order element of the transfer matrix was also measured experimentally and proved to be in good agreement with the simulated result.     

Sub-10-fs Ti:sapphire Kerr-lens Mode Locked Oscillator

Assembly of a sub-10-fs Ti:sapphire oscillator using only optical components commercially available in catalogues is reported. It was demonstrated that stable sub-10-fs optical pulses near 800 nm can be generated from a Kerr-lens mode locked oscillator equipped with a pair of prisms to compensate group delay dispersions (GDD) in the cavity. For the GDD control in a 10fs-pulse oscillator it was concluded that chirped mirrors are not always necessary, and a pair of prisms is still available.     

Guest‐responsive excimer emission in an α‐helix peptide bearing γ‐cyclodextrin and two naphthalene units

An α‐helix peptide (17 amino acids) bearing γ‐cyclodextrin (γ‐CD) and two naphthyl units (γ‐N₂17) was designed and prepared as a new type of chemosensor. The α‐helix peptide with γ‐CD sandwiched between two naphthyl moieties exhibits excimer emission by inserting the two naphthalene moieties into the γ‐CD cavity from the opposite sides in the side chain of the peptide. The two reference peptides, which have one naphthalene moiety and one γ‐CD unit, exhibit only monomer fluorescence and have larger binding constants for the examined guests than γ‐N₂17.