Masahiko Sekine

Photoinduced Hydrogen-Atom Eliminations of 6-Hydroxyquinoline and 7-Hydroxyquinoline Studied by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density-Functional-Theory Calculations

Photoreaction mechanisms of 6-hydroxyquinoline (6-HQ) and 7-hydroxyquinoline (7-HQ) in low-temperature argon matrixes have been investigated by Fourier transform infrared (IR) spectroscopy and density-functional-theory (DFT) calculations. A comparison of the observed IR spectra of reactants with the corresponding calculated spectral patterns obtained by the DFT method led to the conclusion that the hydrogen atoms in the O-H group of 6-HQ and in that of 7-HQ are selectively located at the outer position against the quinoline ring. When the matrix samples were irradiated upon UV light around 300 nm, IR spectra of unknown chemical species were observed; they were assigned to the photoreaction intermediates, quinolinoxyl radicals and ketene compounds, produced by eliminations of a hydrogen atom and a hydrogen molecule, respectively. In the photoreaction of 7-HQ, a small amount of keto form was also produced by intramolecular hydrogen-atom transfer from oxygen to nitrogen in an argon cage. Kinetic analyses were made by assuming that 5-ketene and 6-ketene were produced from 6-HQ, while 6-ketene and 7-ketene were produced from 7-HQ. The effective rate constants estimated from the absorbance changes of IR bands against irradiation time revealed that the reaction pathway to produce 6-ketene was minor in both HQs, leading to the conclusion that the conformation of reactants, HQs, plays an important role in the photoproduction of ketenes through biradicals in the Wolff rearrangement.

Thermal luminescence spectra of polyamides and their Maillard reaction with reducing sugars

Thermal luminescence (TL) spectra of polyamides were measured with a Fourier-transform chemiluminescence spectrometer to elucidate the emission mechanism. A TL band of ε-polylysine with a peak at 542 nm observed at 403 K was assigned to the emission due to the interaction of the -CO-NH- group with oxygen molecules by comparison with nylon-6, polyglycine, and polyalanine. When the sample was kept at 453 K, the intensity of the TL band decreased and the wavelength of the peak shifted to 602 nm, which was assigned to the emission due to the interaction of the NH2 group on the side chain with oxygen molecules by comparison with monomeric lysine. A weak emission with a peak at 668 nm was assigned to the advanced glycosylation end products (AGEs) yielded by the Maillard reaction with a catalytic amount of water. To understand this reaction and to examine the TL emission of AGEs, we measured TL spectra of mixtures of polylysine and reducing sugars such as glucose, maltose, lactose, and dextrin. The minimum temperature for TL emission, wavelength of the peak and the relative intensities of the TL emission were found to depend on the size of the sugars.

Infrared and Electronic Spectra of Radicals Produced from 2-Naphthol and Carbazole by UV-Induced Hydrogen-Atom Eliminations

The photoreaction mechanisms of 2-naphthol and carbazole in low-temperature argon matrices have been investigated by infrared and electronic absorption spectroscopy with aids of density-functional-theory (DFT) and time-dependent DFT (TD-DFT) calculations. When the matrix samples were irradiated upon UV light, 2-naphthoxyl and N-carbazolyl radicals were produced by the elimination of the H atom in the O-H group of 2-naphthol and in the N-H group of carbazole, respectively. The observed IR and electronic absorption spectra of the radicals were reproduced satisfactorily by the quantum chemical calculations. To understand a role of the radicals in the excited-state proton transfer (ESPT), the fluorescence and excitation spectra of 2-naphthol and carbazole were measured in aqueous solution at room temperature as well as in the low-temperature argon matrices. It was found that the intensity of the fluorescence emitted from carbazole anion in aqueous solution decreased when oxygen gas was blown into the solution.