Journal of Photochemistry最新文献

The excited state behavoir of 2-phenylbenzthiazole and 3-phenyl-1,2- benzisothiazole was examined by laser flash photolysis techniques in order to determine which reaction intermediates preceded their subsequent photochemistry. Both compounds, together with related derivatives, form longlived readily detectable triplet states. Quenching constant with representatives olefins have been determined and the energy transfer equilibrium between 3-phenyl-1,2-benzisothiazole and 1-methylnaphthalene has been examined. The results suggest that olefins and alkynes react with excited states of 2-phenylbenzthiazole and 3-phenyl-1,2-benzisothiazole by 1,2 addition, and no evidence for prior ring opening was obtained.

Quantum yields for the formation of hydrated electrons (Φ_e−) and single-strand breaks (Φ_ssb) were measured for polyadenylic acid (poly A), polycytidylic acid (poly C), polyuridylic acid (poly U) and single-stranded (ss) DNA in deoxygenated aqueous solution at room temperature upon nanosecond laser excitation at 248 nm. The values Φ_e⁻ = (1.7 − 3) × 10⁻² and Φ_ssb = (0.5 − 5) × 10⁻⁴ at intensities of (0.5 − 1.5) × 10⁷ W cm⁻² were obtained. The results show that base radical cations of these polynucleotides and of ssDNA lead to ssb formation. The large difference between Φ_e⁻ and Φ_ssb (a factor of 60 – 380) is explained as being the result of back reactions of geminate pairs of radicals and/or radical ions. Conductivity changes after the laser pulse, representing ssb formation, reveal a fast increase and a consecutive show increase for all nucleic acids studied. The activation energies and pre-exponential factors, calculated from the rate constant k_obs of the slow conductivity increase at pH 7.3, are reported. Dithiothreitol (DTT) inhibits ssb formation of poly C (rate constant k_DTT = 1.7 × 10⁶ M⁻¹ s⁻¹) and poly U whereas for poly A and ssDNA a smaller effect was found (k_DTT < 0.7 × 10⁶ M⁻¹ s⁻¹. The pH dependence of k_obs is very similar for poly A and ssDNA, but different from that of poly C and poly U. This indicates that the slow strand break formation in ssDNA upon laser excitation is related to the purine moiety.

The triplet—triplet absorption spectra of three diaminopyridines have been assigned from variable-temperature flash photolysis studies. In all cases the lowest triplet state is ππ* with a phosphorescence lifetime of about 2 s at 77 K in ethyl ether—pentane—ethyl alcohol. Complete neglect of differential overlap—configuration interaction calculations were performed to supplement the interpretation of the fluorescence and phosphorescence data and to provide a satisfactory correlation of the observed and predicted behavior of these molecules.

Measurementds of photoconductivity on thin polymer films are described using an electrostatic recordimg device built in our laboratory from components actually utilized in a commercial photocopier. The voltage applied to the surface of thin films may be measured using an electrostatic voltmeter and photodecays may be easily monitored by a digital-type storage oscilloscope which when coupled to a BBC microcomputer, via the RS423 input, gives an accurately stored decay profile of the charge with time on the millisecond scale. In this paper, the construction of the equipment and its operation are described and some measurements are presented which support its practical viability under various conditions

Photolyses of p-, m- and o-chlorotoluene in methanol gave formation quantum yields of methylanisoles of the order of 10⁻², which are substantially smaller than that for chlorobenzene. The values for the para and ortho isomers are smaller than that for the metal isomer. This result can be explained in terms of the effect of the methyl substituent, which induces negative charges at the ortho and para positions. The effect of biacetyl addition was also examined and the results indicated that the triplet route is significant, as in the case of chlorobenzene. However, about half the methylanisole was formed without any dependence on the biacetyl concentration, indicating the presence of an intermediate state with a very short lifetime.

Decarboxylation and oxidation reactions of carboxylic acids in the presence of various light receivers are reviewed from the experimental and mechanistic standpoints. The homogeneous photoreceivers studied are the ions UO₂²⁺, Fe²⁺ and Cu²⁺. They differ in activity and in selectivity as far as the nature of the hydrocarbons formed by decarboxylation is concerned. The activity of the heterogeneous photocatalyst TiO₂ (anatase) in oxidation is shown to be greatly increased in the presence of dissolved substances, e.g. Fe³⁺ and Cu²⁺.